Naked Science Forum
Non Life Sciences => Technology => Topic started by: Karsten on 27/06/2009 22:12:43
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I want to win this race and I want to milk someone's brain about hydrodynamics for very small scale sail boats.
Here is the situation:
Boat max. Length 35cm
Max width: 12cm
Power: Wind from a window box fan
Length of race track around 4 meters
Max. sail surface: 256 square cm
No min. weight
First question:
Does waterline length matter at this scale? I know it matters with bigger boats in displacement mode. Such small boats too?
Second question:
At this scale, should I worry about water resistance more than about water surface tension?
Third question:
What is the ideal shape at that scale and at such speeds (maybe 0.8m/s)
Can't wait to hear your comments and ideas!
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If you can get the hull out of the water on hydrofoils it will be quicker ...
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Make it a hovercraft!
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Make it a hovercraft!
If you are allowed a motor make it a ground effect vechicle. (http://en.wikipedia.org/wiki/Ground_effect_vehicle) (a.k.a. "sea skimmer")...
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You'll have no problems winning.
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An interesting question, and an interesting competition too.
I see that you've clearly specified that it's to be a sailboat, so most of the suggestions about using motors are nonstarters (unless the rules allow you to use wind-turbines to generate electricity, to drive an electric motor, and even then it would be questionable whether you could generate/capture more energy that way - there's only a finite amount of energy in the wind to be used).
I don't think you'll be able to capture enough energy to get your boat hull out of the water, either by planing or in hydrofoil mode, so I'll just stick to conventional designs.
With a conventional type boat i.e. where its hull stays in the water, its speed is limited by the length of the hull. This is largely due to the bow-wave created by the hull as it passes through the water; the water displaced by the hull cannot simply be pushed aside because water is not easily compressible, so it has to be displaced upwards with the result that the boat ends up traveling up a 'hill' of water. The length of the hull becomes significant then, because the length, and therefore the gradient of the bow-wave is proportional to the length of the hull; the longer the hull, the longer the bow-wave.
So the first thing you need to do is maximise the hull-length.
Next you need to reread the rules carefully again. You've said that the max width (beam) is 12 cm but I strongly suspect that this actually says the min width. Once you've clarified that point, you then need to establish if that is the minimum waterline beam or just the overall min beam.
Along with the length of the hull, the waterline beam is important regarding speed because this dictates the cross-section of the hull below the waterline, and a lower cross-section will require less power than a greater cross-section. Because it would be difficult in practice for people to ensure and measure the waterline beam, I'm guessing that the rules only specify the overall beam, and this gives us some scope for a bit of cleverness. The best below the waterline shape for a hull, for optimum speed, is circular. This is because it has least surface area for volume, and it's the volume that dictates buoyancy. However, if you simply design a circular shaped hull where the diameter of the circle is equal to the min beam it will be more buoyant than you need and most of the hull will be out of the water, so what you do is design a narrower hull that is circular below the waterline but with flat sides above the waterline, extending outward to meet the min beam requirement; think in terms of a 'V' shaped hull but with a circular rounded bottom, or point of the 'V', with only the rounded part in the water. This will take some time and effort on your part to get right because you don't want the flattened sides to be touched by the water - they have a large surface area, and therefore surface friction.
Because you're going for speed, and not maneuverability, you don't want much 'rocker' on the hull; this is the curvature along the length of the hull. So although the below-waterline hull cross section will be circular at the widest parts of the hull it will need to get progressively narrower and more elliptical towards the bow and stern. Oh, and don't bother sloping or angling the leading edge of the hull - keep it vertical to maximise hull length.
There's one big problem with circular hulls though; they're unstable if the Center of Gravity (CoG) is below the Center of Buoyancy (CoB) - think in terms of trying to balance when sitting on a floating telegraph pole. Using out-riggers is one way around this, if allowed, but otherwise you'll have to get the CoG below the hull's CoB. However, you don't want to add unnecessary ballast because that'll reduce your acceleration and require more buoyancy, which mean more hull in the water, which in turn means more surface friction.
The last thing I can think of are the sails. If you know that the wind is coming from behind you, you just need a spinnaker type sail, but I doubt that will be the case, so that means you could consider using a 'wing' type sail. With these you replace the normal low-aspect ratio cloth sails with a rigid glider wing type sail. These long and narrow wings have a very high aspect ratio and are more efficient, but they are also more critical, so you'd need some sort of active control system to keep them at the optimum angle to the wind.
If you're capable of designing and implementing an active wing sail, then you also ought to consider a wing type keel too. This would mean that you could use a longer and thinner keel, which would lower the size of the counterweight and reduce the below the waterline cross section. The boat would then only need to be marginally stable when stationary - it would become more stable as it moved and as the wing became effective.
If there are any real geeks in this competition, I would expect to see micro-processor controlled wing type entries.
(Edited to replace a couple of incorrect references to drag with surface friction)
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Hovercraft/skimmer: Cannot do. No additional power is available. Only the box fan.
Hydrofoil: Interesting idea. I will have to create underwater "wings" to lift the hull out of the water. Will they create more resistance than the hull itself (which can be VERY light and does not need to sit in the water very deeply at all)? I am afraid that the speeds the boat in the youtube link need to reach to lift out of the water is far above the speed of the air created by the box fan. My race is a downwind drag race. The boats don't even reach planing speed.
Don't get distracted from my issue but here is a cool video of a hydrofoil trimaran that holds the world record for sailboats.
Or this one for the opposite on the size spectrum: Only windsurfers and kite surfers can go faster.
But don't get distracted! I only have wind available from directly behind the boat.
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Thanks Lee for the long post.
First of all, the maximum width of the boat can be around 12 cm. It is a basic 7th grade student competition but some teachers are competing as well and I have a reputation to lose. So, the boat hulls have to fit in a box of 35 cm by 12cm.
I am actually thinking multi hull. Trimaran or catamaran. Maximum stability with least amount of hull touching the water. Buoyancy is not much of an issue since you can make the boat as light as possible. I appreciate you suggestion to make the hulls circular in cross section. I worry that with such a design I will deal with a lot of water surface tension issues. My previous thinking was to create almost knife-like hulls (two). Something that is big enough to keep the boat afloat (close to the fans where there is a lot of wind & further down the "lake" when there is much less) but sharp enough to cut though the water surface easily.
What is better: trimaran (one large hull with two shorter outriggers in the front to deat with the almost violent winds at the start) or catamaran (two hull equal in length)? If I go with long waterline I would have to think that the shorter outriggers will slow the boat down more than two longer hulls. Right?
Rocker and bow shape: What you say is just what I thought. Maximum water line length is critical.
Send me some of your thoughts regarding multi hull design and shape in this case.
Keel: Necessary? On down wind courses small sailboats pull the centerboard out of the water. To reduce water resistance I assume. My previous boat has raced even without a rudder. It is only 4 meters straight.
Sail: Way in the back to deal with the violent wind at the start. Don't want to bow to plow under the water. The wind is coming exactly from behind and it does not change. Like I said, basic competition. Will it be best to have a sail that is shaped to capture the wind (like a spinnaker)? What does that do in comparison to a flat sail that has the same cross section? I may have to vacuum form a thin plastic film sail.
Microelectronics: Nah. Not that much of a geek. Or better: Not smart enough to do this. And not enough time to learn how to. I would love to see this though. And I don't think the sails would need adjustment since the wind always comes from behind.
I assume that making the hull as smooth as possible is important as this scale. I wonder if I should shape the hull from foam and vacuum form a thin plastic layer over it.
Thanks so far. Good and helpful info. Send me some thoughts on multihulls and downwind sail design!
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Aha - downwind only, using an artificial wind-source [;D]
In that case you just want a single spinnaker sail. I don't know anything in particular about their design though, so you'll have to research that yourself.
If multi-hulls are allowed then it's a tossup between tri and catamarans, and as you say, a keel won't be needed with either of these types as they have a Plane of Buoyancy, not just a point, so you don't need to keep the CoG beneath a Point of Buoyancy; as long as the CoG stays within the area of the Plane of Buoyancy it'll be stable even though the CoG is above the Plane of buoyancy.
Because of the bow-wave issues, I'd make each hull full-length, but as to whether you use two or three smaller equal-dimension hulls, or go for one main hull with two less buoyant outrigger hulls is up to you. Using a single main hull with less buoyant outriggers may give optimum buoyancy for the wetted surface area, but using less buoyant outriggers will reduce stability.
For simplicity and ease of construction, and for maximum stability, I'd go for an equal-length twin-hulled catamaran design.
Using a multi-hulled vessel also means you can forget about complex hull profiles and just use circular hulls, because you just need to work out the hull width needed to supply the required buoyancy and place them as far apart as you can within the max beam limit.
Circular hulls are definitely the way to go. Water tension isn't the issue here; it's the wetted surface area and the resulting friction from it that's important. Circular hulls give maximum volume, and therefore maximum buoyancy, for minimum surface area and cross section, so for any given buoyancy requirement, a circular hull will give least friction and cross section. Knife, or pure 'V' section hulls will result in both greater cross section and surface friction to provide the same buoyancy. Although boat hulls are sometimes described as 'cutting' through the water, they actually do no such thing; they have to push the water aside.
I'm guessing that there's no requirement for the hulls to be convex only (this is a rule in many types of racing boat design) and this means you have some options regarding your hull planiform i.e. when viewed from the top.
In general, using straight lines is a bad idea in aero/hydro-dynamics because, iirc, they increase turbulence along the straight surfaces and don't give the best gradients in terms of how quickly the medium, in this case water, is moved aside. As I said earlier, a convex hull is a rule for some vessels, and for larger non-racing vessels introducing concavities in the design requires more strength, and while they should be avoided in the hull cross section, as they'll only increase surface area and cross section, they can be useful along the length of the boat, in its plan shape.
Therefore, consider a planiform where you have a very long and slender bow, which then flares out to the max beam, which in turn should be located aft of the mid-point, and which then converges more steeply at the stern. The trade-off here is between reducing the bow volume, which in turn reduces the bow-wave gradient that the boat is trying to climb (and with less buoyancy in the bow, the boat should stay more level - ideally, the boat should sit slightly bow-down when stationary) against slightly increasing the drag from the water flowing back into the space left by the boat as it moves forward, as it now has to do this over a shorter distance.
This trade-off is hard to get exactly right though, so don't try anything too drastic; in any case, you want to avoid radical changes of direction in the surface of the hull and smooth flowing lines are the order of the day.
One thing about the sail: You'll need to experiment for the best place to anchor it. The sail will tend to push the bow down into the water, so if you're using a slender bow design consider placing the mast a long way back, and not at the mid-point. In general, iirc, I think the mast should be a little aft of the max beam location.
Hopefully sophiecentaur, being a bit of a sailor, will see this thread and comment on the sails.
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I just lost my post before I could post it.
Anyhow, much shorter now:
I am not sure I understand the the hull shape details you describe, especially the "flaring out" part. I have seen some concave designs in racing canoes, but maybe you can look at the pictures I linked and tell me what you think comes the closest:
1) http://www.kayakcentre.co.za/images/mustang.jpg
2) http://blog.mlive.com/lcn/2008/07/CANOE3.jpg
3) http://img.nauticexpo.com/images_ne/photo-g/rowing-shell-touring-single-scull-130942.jpg
And what do you think about this rowing catamaran I just found:
4) http://www.fotothing.com/photos/0b4/0b48d42c02c342c69bbbdca51e9919d2_adc.jpg
Lee, I love all this information you post. This will be a great boat!
I will wait for Sophiecentaur to talk about sail design for downwind only.
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If the rules allow it, and it's downwind, you're theoretically better off with a propeller and a windmill- these devices can outrun the wind.
The other thing to think about is reynolds number. If you're moving at 0.8 m/s I think it's about 10^5, so it's fully turbulent flow around the boat; I would think normal boat hulls would work reasonably well at this scale.
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If the rules allow it, and it's downwind, you're theoretically better off with a propeller and a windmill- these devices can outrun the wind.
The rules do not allow to use an engine on board. If that is what you mean. I do not understand how a propeller would work without an engine.
How about the stern end of this boat?:
http://farm1.static.flickr.com/115/273686080_9ac118f85c.jpg?v=0
Or the bow of this one?:
http://upload.wikimedia.org/wikipedia/commons/c/c4/Seacycle.jpg
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You stick a windmill on the boat and use that to turn the propeller. If the gearing is right, it's very, very counterintuitive, but it can in principle outrun the wind.
I must admit though I've never heard of people doing it with a boat, but it definitely has been done with land yachts type things. Normal land yachts don't do that though, they just go at an angle to the wind and can go faster than the wind that way with only a relatively simple sail; but the sail type land yachts can't do that directly downwind, whereas the windmill version can.
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You stick a windmill on the boat and use that to turn the propeller. If the gearing is right, it's very, very counterintuitive, but it can in principle outrun the wind.
I must admit though I've never heard of people doing it with a boat, but it definitely has been done with land yachts type things. Normal land yachts don't do that though, they just go at an angle to the wind and can outrun the wind that way with only a relatively simple sail; but the sail type land yachts can't do that directly downwind, whereas the windmill version can.
You are saying I am using the wind created by the box fan to run a wind turbine that moves a propeller that pushes forward the boat faster than the wind blows in the same direction?
I must admit I never heard of anyone doing this with anything and would love to see a picture of anything moving faster than the wind blows directly from behind. How can anything moved by the wind move faster than the wind in the same direction the wind blows? Where does the energy gain come from?
This will turn into a completely different discussion. I can already feel it. I want to discuss my problem a bit longer please.
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For example for the land yacht type thing:
Since your race is downwind you could theoretically do the same (normal sailing is rarely directly downwind).
If you can engineer this kind of thing (it should be possible I think), your boat should absolutely slaughter the others [;D]
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Post this in a thread. I would love to see a discussion of this. I want a peer review so to say.
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I just lost my post before I could post it.
Anyhow, much shorter now:
I am not sure I understand the the hull shape details you describe, especially the "flaring out" part. I have seen some concave designs in racing canoes, but maybe you can look at the pictures I linked and tell me what you think comes the closest:
1) http://www.kayakcentre.co.za/images/mustang.jpg
2) http://blog.mlive.com/lcn/2008/07/CANOE3.jpg
3) http://img.nauticexpo.com/images_ne/photo-g/rowing-shell-touring-single-scull-130942.jpg
And what do you think about this rowing catamaran I just found:
4) http://www.fotothing.com/photos/0b4/0b48d42c02c342c69bbbdca51e9919d2_adc.jpg
Lee, I love all this information you post. This will be a great boat!
I will wait for Sophiecentaur to talk about sail design for downwind only.
Heh - the kayaks in the first two pics appear to be sprint K1s. These are the class of boat I used to race (along with a bit of K2), and although the hull may seem to have some concavity in the hull planiform, the actual hull shape is actually entirely convex (or flat). This type of hull uses the rounded 'V' cross section at the max beam point (in the second pic this is just behind they guy's head) to meet the minimum beam requirement and the reason that it looks as though the hull is concave around that region is because the sides of the 'V' are extended higher than the rest of the hull. I think this pic of a K2 (nothing to do with me - just something I googled) shows it quite well:
(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fwww.kayak.plus.com%2F200m%2Fpics%2Frupert-gavin.jpg&hash=ec21e04a4f75f18f858da1e5cbecd789)
The sides of the 'V' section hull are flat, but that's ok here because they're out of the water; the hull below the waterline will be rounded. The trade-off in doing this is that being higher up out of the water, the hull can be adversely affected by crosswinds - not what you want in an inherently unstable design. You can also see how the bow wave has piled up around the front of the boat as the water is pushed aside and up, and how you get a trough at the back of the boat as the opposite happens - the water is being 'sucked' in to fill the gap left by the hull, producing drag and resulting in a 'trough' which accentuates the hill-climbing effect.
Re the hull planiform I was referring to: I couldn't find any good pics, but think of a reversed and very elongated teardrop shape, but pointed at both ends.
The cat rowing boat is weird [:D]
Incidentally, I believe that a minimum beam has never been a requirement for racing rowing boats, so they don't need 'V' hulls.
...and oh yes, you might want to consider using a bulbous bow. These are normally only found on quite large vessels but they can also be effective with very long narrow hulls running near their max speed, so they might work. The convex hull requirement for many racing boats normally precludes their use.
http://en.wikipedia.org/wiki/Bulbous_bow (http://en.wikipedia.org/wiki/Bulbous_bow)
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If the rules allow it, and it's downwind, you're theoretically better off with a propeller and a windmill- these devices can outrun the wind.
This cannot work. As the boat starts to move, the wind speed over the boat decreases; wind speed over the boat, and therefore its 'windmill' will be absolute wind speed - boat speed. If the boat's speed is equal to the wind speed there is zero wind over the boat, meaning there's nothing to drive the windmill. If the boat travels faster than the wind then the wind speed over the boat will be negative, effectively slowing the boat down.
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If the rules allow it, and it's downwind, you're theoretically better off with a propeller and a windmill- these devices can outrun the wind.
This cannot work. As the boat starts to move, the wind speed over the boat decreases; wind speed over the boat, and therefore its 'windmill' will be absolute wind speed - boat speed. If the boat's speed is equal to the wind speed there is zero wind over the boat, meaning there's nothing to drive the windmill. If the boat travels faster than the wind then the wind speed over the boat will be negative, effectively slowing the boat down.
I see the logic in that!
If your sail surface must not exceed 256 sq cms, and you will have a permanent tail wind, I wonder if your sail should be 13cms high by 20cms wide giving a total of 260 sq cms. then cut 5 x 1cm sq holes in the centre keeping you within the allowed surface area and creating the same effect as a parachute.
[diagram=478_0]
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Is it cheating to freeze the water and mount the "boat" on skates?
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If your sail surface must not exceed 256 sq cms, and you will have a permanent tail wind, I wonder if your sail should be 13cms high by 20cms wide giving a total of 260 sq cms. then cut 5 x 1cm sq holes in the centre keeping you within the allowed surface area and creating the same effect as a parachute.
[diagram=478_0]
That is a VERY interesting idea. I will check the rules.
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Is it cheating to freeze the water and mount the "boat" on skates?
I think so. And the competitor in the other lane might have something to say too.
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...and oh yes, you might want to consider using a bulbous bow. These are normally only found on quite large vessels but they can also be effective with very long narrow hulls running near their max speed, so they might work. The convex hull requirement for many racing boats normally precludes their use.
http://en.wikipedia.org/wiki/Bulbous_bow (http://en.wikipedia.org/wiki/Bulbous_bow)
Like this one?:
http://upload.wikimedia.org/wikipedia/commons/c/c4/Seacycle.jpg
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Hmm... not really. That just has a reverse raked bow and I'm not sure that it offers any advantages over a non-raked vertical bow. The idea behind bulbous bows is that they protrude ahead of the vertical bow, below the waterline, so that the bow wave they produce is out of phase with the bow wave produced by the vertical bow at the waterline and so tend to cancel or reduce it.
To be honest, I probably wouldn't bother about trying to incorporate bulbous bows unless I had an excess of free time, both for the designing and making them, and the subsequent testing of them.
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Hmm... not really. That just has a reverse raked bow and I'm not sure that it offers any advantages over a non-raked vertical bow. The idea behind bulbous bows is that they protrude ahead of the vertical bow, below the waterline, so that the bow wave they produce is out of phase with the bow wave produced by the vertical bow at the waterline and so tend to cancel or reduce it.
To be honest, I probably wouldn't bother about trying to incorporate bulbous bows unless I had an excess of free time, both for the designing and making them, and the subsequent testing of them.
I thought it looked a bit like a bulb but it sure is not as much as shown in your link. So, OK a vertical bow it is. Adding a bulbous nose also increases the length of the boat without adding length to the waterline.
About the hull design: If I understand you right its planiform should look somewhat like the image below (bow on the left). Much skinnier though. And definitely slightly concave after maximum beam. As much as possible, all cross sections below the waterline should be half circles.
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Yup, that looks good to me. You won't be able to use a circular cross section throughout the hull, of course, without also making the hull very curved along its length i.e. when you look at it from the side it would have a very curved bottom. You don't want this; you actually want the hull to be very flat along the hull centerline, from bow to stern, so the cross section will have to become more (vertically) elliptical as the hull becomes narrower.
So when you look at your hulls from the top they'll look just like your picture but from the side they should look like a low but wide rectangle. This curvature along the centerline of the hull is called 'rocker' and the degree of rocker affects how easily the boat can turn; you want it to run straight though, so zero rocker. Some sea kayaks have been designed with negative rocker i.e. the bow and stern are deeper in the water than the middle of the hull, to help prevent the kayak turning when paddling obliquely across waves/swell, but you don't need or want this either.
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Yup, that looks good to me. You won't be able to use a circular cross section throughout the hull, of course, without also making the hull very curved along its length i.e. when you look at it from the side it would have a very curved bottom. You don't want this; you actually want the hull to be very flat along the hull center line, from bow to stern, so the cross section will have to become more (vertically) elliptical as the hull becomes narrower.
So when you look at your hulls from the top they'll look just like your picture but from the side they should look like a low but wide rectangle. This curvature along the center line of the hull is called 'rocker' and the degree of rocker affects how easily the boat can turn; you want it to run straight though, so zero rocker. Some sea kayaks have been designed with negative rocker i.e. the bow and stern are deeper in the water than the middle of the hull, to help prevent the kayak turning when paddling obliquely across waves/swell, but you don't need or want this either.
I was wondering about that. What if I continue to have the circular cross sections but add a fin? The hull could still be in the water with its full length but less deep at the bow and stern. Would adding a fin be worth the reduced water resistance of the hull? You know what I mean? Rocker only to keep the hull uncompromisingly hydrodynamic but adding a fin to get stability for straight running.
Also, would it be beneficial to have a slightly concave planiform at the bow (first third), transition into a convex middle section (second third) and transition into a concave stern section (third third)? Is that better than convex for the first 66% and then concave at the stern?
Like so (bow to the left):
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If you use a circular cross section throughout, then your side view would look like your top view (if you made the deck as rounded as the hull i.e. a mirror image of it. Imagine your top view drawing with the upper half removed). A consequence of this is that because there's less hull in the water at the bow and stern they'll contribute less buoyancy, which will mean that the hull will have to sit deeper in the water to achieve the same amount of buoyancy, which will in turn increase the wetted cross section, which is bad; you want to minimise the cross section.
Another consequence will be that the water will tend to be forced beneath the hull instead of around it. I think this will tend to lift the bow out of the water, shortening the waterline length and exacerbating the problems with bow lift and stern squat; the hull will pitch up even more than it would just due to the bow wave. You need to remember that the bow wave is a result of displacing the water, so just sticking a very thin fin underneath a circular hull won't help because it won't displace a significant volume of water.
When I initially suggested considering a very slightly concave planiform I was actually thinking in terms of applying this to the front of the hull, not the rear. Baring in mind that you're going to have to form these hulls yourself (I don't know if you've got access to any design s/w that would allow to to produce drawings of cross sections, which you could then cut out and use as shaping templates) and that they really need to be as symmetrical as possible along the centerline, you need to keep your ambitions within your abilities; if you think you can make a hull incorporating complex curves, then give it a try, but otherwise go for a less risky and more simple shape that is convex throughout. Any degree of concavity should be tiny in any case.
I've had a think about the sail too, although remember that I'm not a sail scientist [;)]
First of all, I don't think that the sail area will be terribly critical. Sure, more sail should be better than less sail, but as the wind is coming from directly behind you, you won't be able to go any faster than the wind no matter how much sail you have (in oblique winds this isn't so, but that's not what we're dealing with here). So what I think you want to aim for is max acceleration, which means keeping the weight of your boat as low as possible, which also reduces your buoyancy requirements, which then lowers the wetted cross section. The idea I had for a sail was to use clingfilm/food-wrap film. I'll leave the shape up to you, but I think a taller slimmer sail might be better than a lower wider sail, and you could even consider trapezoidal shapes.
But to start with, try this:
Base the height of your sail on the width of your roll of clingfilm/wrap and work out the width of sail you need to comply with the rules. Then unroll about twice this much clingfilm/wrap onto a smooth flat surface and cut it off with a scalpel. Next either glue or tape the film down, at the two ends (but not at what will be the top or bottom of the sail) so that it won't move.
Now you should carefully stick some sellotape/scotch tape across the film to mark out the sides of your sail. You can actually make the sail a little wider than needed when you're applying the tape, so you don't have to worry too much about precision while your trying to apply the tape, which will be difficult enough as it is (if you have access to a vacuum table it would be of immense help [:)]). Once you've managed to successfully apply the two strips of tape, you can then trim the sail to size in the areas covered by the tape. The tape will not only reinforce the film enough so that it can be anchored to the boat but it should also make the sail much more manageable when you're rigging it - it'll still be fragile, of course - lol - but I think it should be strong enough for one or two races, provided that you've kept the overall weight of your boat down. Dusting the film with talc, after you've got the tapes on, should stop it from sticking to itself.
As for rigging, weeelll... I'd start by anchoring each of the bottom two corners of the sail to the extreme tips of the two bows. Next, you're obviously going to need a mast. Another point needs to be clarified here; can any 'yards' (the cross-beams you see on square-rigged sailing ships) be wider than the maximum beam for the vessel?
Whether it can be wider than the vessel or not, you'll need a yard somewhere up your mast, not only to raise it, but to help spread it too. Anyway, the top two corners of the sail should be attached to each end of the yard. Don't attach the sails directly to the bows and yard, of course; you should attach pretty long lines to them and then run the lines through eyelets to a convenient anchoring point on the hull. This way you can experiment with the sail rigging. I don't think you'll need additional lines from the sides of the sail to control it, but the option's there, either tying them to the hull or to another yard on the mast.
You'll need some standing rigging to keep the mast up, of course, and these should be run to the bows and sterns of each hull, as well as being cross-braced to each hull, but remember that the sail will need to run ahead of these lines, not inside them. The sail should sit on these lines when there's no wind and they'll help keep the sail open until it's inflated. As you can imagine, there's a lot of scope for experimentation regarding how you tension your sail and how high up the mast you anchor it.
How are you planning to build your hulls? As weight is of paramount importance, because it dictates the buoyancy required, which then dictates the hull cross section etc, they need to be as light as possible. If you feel up to it, try this:
Carve a single hull from something easily workable, such as balsa, trying to make it as perfect as possible. Then use that as a 'plug' to make a couple of negative molds in plaster-of-paris. Next fill the two molds with some of that two-part foam stuff to produce two copies of your hull. Now you aren't going to use these two foam copies of your hull as the hulls themselves but use them as formers to build very light weight glass-fibre hulls. You'll probably need to cover the two foam formers in clingfilm/wrap, or foil if the resin eats through the film (never tried, so don't know) because you want to get the cores out in one piece for later use. Then lay-up a single layer of very lightweight glass-fibre cloth over the foam plugs. Because the hull will have complex curves, even if it's entirely convex, you'll need to trim the cloth to shape and even cut little bits out of it so that it'll follow the shape of the hull without wrinkling. Don't worry about small gaps; you can fill them. Don't worry about durability either; just concentrate on getting the smoothest shape.
Once you've made the two hulls and removed the foam cores, cut thin slices, say 4-5 mm thick, out of the foam cores and glue them back in the hull to act as strengthening frames. You should consider cutting the centers out of these frames, once again leaving them about 4-5 mm deep.
Now that the hull has been stiffened and strengthened by the frames you can fill any small holes and then seal the hull with a sanding-sealer, and then sand it smooth; try not to sand too deeply into the fibre glass cloth, and watch out for dust - it's an irritant. Finally, paint the hulls to ensure they're watertight; any exposed fibre glass cloth will soak up water.
Don't forget to add a small adjustable fin/skedge beneath one of the hulls so you can trim the boat to go straight; unless you've made absolutely perfect hulls they're bound to have a little bias one way or the other. Just use one of these; you don't need two and if you have two, they've got to be perfectly aligned otherwise they'll fight each other and produce unnecessary drag.
Umm... I think that's all I can think of for now.
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I would just add to LeeE's post, make your hull/rigging as light as possible, regardless of stability, remember, you can always add ballast, but taking it away would not prove easy! Also make sure your hull has a good, well hardened coat of beeswax to reduce friction.
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You want to avoid adding ballast of course; weight -> buoyancy -> cross section + surface area. By all means polish the hull, but it's more important that it's smooth, with no little bumps or ripples. There has been debate about matt hulls Vs. shiny hulls; part of the reasoning, iirc, was that a boundary layer of water sticks to a matt hull and the friction is then water-to-water, rather than water-to-hull. Didn't seem to make any difference when we tried it with racing kayaks though. I've no idea whether anything has been concluded since then.
Part of the reasoning behind making two hulls from a single template is that their buoyancy should be the same and so should not need lateral ballasting.
Oh, another thought; consider using an 'A' frame mast instead of a single central mast. You'll only need fore and aft standing rigging then, and you can then make each mast lighter (heh [;)] i.e. more flimsy)
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How are you planning to build your hulls? As weight is of paramount importance, because it dictates the buoyancy required, which then dictates the hull cross section etc, they need to be as light as possible.
I would just add to LeeE's post, make your hull/rigging as light as possible, regardless of stability, remember, you can always add ballast, but taking it away would not prove easy! Also make sure your hull has a good, well hardened coat of beeswax to reduce friction.
Yesterday I went to the Biodome in Montreal and spend a long time studying the Mackerel. Those are some fast fish I believe and their body shape is similar to what you suggested (planiform).
I will use rigid insulation foam to shape several hulls and test them for drag. Maybe I will make single hulls that I test in unison with outriggers to see which one is least resistant. I like the idea of making a mold and copies with plaster and spray foam. It would make catamaran construction easier and more reliable. Once I am happy I will vacuum form a thin layer of plastic over the chosen ones to get them slick. I built a vacuum forming machine many years ago for my classroom and it could work like a charm for this. Depending on the shape it also may result in the plastic not behaving as I want it. In that case I will cover the foam in liquid epoxy and carefully sand once it is hard. I will check what is better: matte or shiny. It might make a difference at this small scale. Water gets thick when your boat is less than a foot long. Wax is a good idea I will try.
I will build the hulls with no rocker (as you suggested). I may add a fin if one is necessary (transparency film again). It is a short race and some of my earlier boats went straight enough without a fin.
The sail will probably be made from plastic film (overhead transparency). It is heavier than Seran wrap but it carries itself and I can skimp on standing rigging much more. I may try to vacuum form this material into a not-flat shape. The wind is also so violent near the fans that I want some give in the rigging or less sail surface higher up. I will have to check with a stick and short pieces of yarn where the wind really is. Many light boats fall over at the start. Better to get going with a little less acceleration but keep accelerating until the end. The sail will be at the stern end of the boat. Less chances to lever over the boat and closer to the energy source at all times during the race.
This is a long term project. Next year's 7th grade will compete sometime next year, but I want to be ready and prepared early. Good things take a while to complete. I will post a picture of the race ready boat, but it will be a while. All of your advice was very much appreciated. Thanks a bunch!
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Vacuum forming for the hulls sounds great, and much easier too [;D]
Dunno about using OHP film for the sail - it strikes me as being a bit heavy and inflexible but you know the conditions it'll have to deal with. The standing rigging is just to hold the mast up but regardless of what you use for a sail I think you'll have to use the same number of lines to attach and control it i.e. four, one at each corner. You should be able to get some thin rubber/elastic from a modeling shop, but if not, get some thin but long rubber bands and cut them in half and attach the sails to these, and then tie them to the running lines to provide a bit of shock absorption.
If there's a strong risk of the boat tipping over at the start (I was wondering if the fans are just switched on full at the start or if they're ramped up over a short period of time) it might be worth adding ballast after all; it's not be ideal, of course, but a capsized boat is even less ideal.
I'd love to see some pictures of the boat, and even some video, if you can possibly post some to you-tube. It's handy that the race isn't imminent as it gives you plenty of time to make a good job of it, but it'll be a frustrating wait to see how well it does.
Oh re the mackerel: there's no harm in consulting the experts [;)] but go more by their planiform than their side view; for a given length, thinner is better than wider. You don't have to worry about accommodating internal organs and muscles or providing thrust. Also, they don't 'operate' in the underwater equivalent to downwind conditions but move their tail from side to side, equivalent to oblique winds, and so function much more like 'wings'.
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I don't think you would need standing rigging - except to look nice. In a small model, the spars will be easily strong enough. Stresses scale down faster than sizes - to the fourth power! You will never manage to blow a mast down before the boat is laid over it it's less than a metre highmast which is stepped well down into the hull.
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Well, it has been a while but the race did not happen again until this last week and here are the two fastest boats. Mine (red) was second (grrr...) but it was close! Mass mattered a lot and the winning boat weighed in at 10g versus mine at 26g. Next time - lighter hulls, better sail shape. Both boats have the sail aft.
Just so you all know. I am sure you were anxiously waiting for the outcome of this thread. It helped a lot.
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If the rules allow it, and it's downwind, you're theoretically better off with a propeller and a windmill- these devices can outrun the wind.
This cannot work. As the boat starts to move, the wind speed over the boat decreases; wind speed over the boat, and therefore its 'windmill' will be absolute wind speed - boat speed. If the boat's speed is equal to the wind speed there is zero wind over the boat, meaning there's nothing to drive the windmill. If the boat travels faster than the wind then the wind speed over the boat will be negative, effectively slowing the boat down.
Wrong! The trick is the difference in speed between the water and the wind is the same at all speeds, and you can tap that to make energy.
http://en.wikipedia.org/wiki/Sailing_faster_than_the_wind
The record is 1.7 times the wind speed.
Whether it would help in this case is unclear, it depends on things like acceleration and the extra mass of the mechanism; but the windmill, for the same area of 'sail' generates a lot more power since power is force times speed and it's spinning around, but the conventional sail would be barely moving.
I think it would give, much, much better performance than a conventional sail.
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I haven't read all of the posts, so apologies if this has been said before.
Make it as light as possible. This will help with every other problem. It will accelerate faster. It will ride higher in the water and have less water to displace, it will have less surface friction to contend with. A twin hull might be a good idea in conjunction with a squared rigged sail but make it light.