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Author Topic: How do I build a very fast model boat?  (Read 29086 times)

Offline LeeE

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How do I build a very fast model boat?
« Reply #25 on: 01/07/2009 18:04:17 »
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.
 

Offline Karsten

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How do I build a very fast model boat?
« Reply #26 on: 03/07/2009 02:21:10 »
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):

 
« Last Edit: 03/07/2009 02:22:51 by Karsten »
 

Offline LeeE

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How do I build a very fast model boat?
« Reply #27 on: 03/07/2009 18:49:26 »
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.
« Last Edit: 03/07/2009 18:51:06 by LeeE »
 

Offline Don_1

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How do I build a very fast model boat?
« Reply #28 on: 04/07/2009 15:19:49 »
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.
 

Offline LeeE

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How do I build a very fast model boat?
« Reply #29 on: 04/07/2009 19:16:50 »
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)
 

Offline Karsten

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How do I build a very fast model boat?
« Reply #30 on: 04/07/2009 23:22:29 »
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!
« Last Edit: 04/07/2009 23:24:31 by Karsten »
 

Offline LeeE

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How do I build a very fast model boat?
« Reply #31 on: 05/07/2009 12:12:12 »
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'.
« Last Edit: 05/07/2009 12:21:11 by LeeE »
 

lyner

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How do I build a very fast model boat?
« Reply #32 on: 06/07/2009 23:20:04 »
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.
 

Offline Karsten

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Re: How do I build a very fast model boat?
« Reply #33 on: 31/01/2012 20:41:41 »
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.
 

Offline wolfekeeper

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Re: How do I build a very fast model boat?
« Reply #34 on: 31/01/2012 23:21:55 »
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.
« Last Edit: 31/01/2012 23:43:20 by wolfekeeper »
 

Offline MikeS

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Re: How do I build a very fast model boat?
« Reply #35 on: 01/02/2012 13:42:36 »
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.
 

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Re: How do I build a very fast model boat?
« Reply #35 on: 01/02/2012 13:42:36 »

 

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