[mofoemofoe (OP)]

I recently brought a bow and have been wondering more about the design. I see long and short bows. And different shapes. There must be reasons for this.

[Colin2B]

Long bows eg English longbow are actually straight until strung and tensioned. They are easy to manufacture in quantity, but need to be long in order to get power. They are also best made from woods with particular qualities. English longbows were made from yew which can be very straight grained and has a well defined sapwood layer. Sapwood is good in tension whereas heartwood is better in compression, so the sapwood goes on the back of the bow - facing away from the archer.
A longbow is not helpful on horseback when you need to move the bow from one side to the other, or when hunting in close forest. The recurve bow gets around this by making the arms curve away from the archer - like springs - giving greater power for shorter length. Some recurve bows are composite eg the Mongul bow which was made with a bamboo core, horn on the front (compression) and sinew on the back (tension).

[evan_au]

The crossbow is very short, and achieves very high tension, and very high velocity on the projectile (the quarrel).

But you need a mechanical lever or high gear ratio to pull the string into tension.

In some battles, it was found that the extra time to tension the string outweighed the extra velocity of the projectile.
See: https://en.wikipedia.org/wiki/Crossbow
https://en.wikipedia.org/wiki/Bow_and_arrow

[Colin2B]

In some battles, it was found that the extra time to tension the string outweighed the extra velocity of the projectile.

A well trained archer could loose 12 arrows/min for a short time or 6/min for consistent rate of fire. A crossbow could maintain 3-4 bolts/min.
In open battle the longbow had greater range, but the crossbow was most often used as a defensive weapon in castles - the English outpost at Calais kept a stock 53,000 bolts.

[evan_au]

An account of the battle of Crécy is described here, where a small army with longbows defeated a much larger army equipped with crossbows:
http://historyofenglishpodcast.com/2018/12/13/episode-119-the-road-to-war/
Starts at minute 50, duration 15 minutes (with 5 minutes on the etymology of armour, army, alarm and similar words).

...it was also the first major battle using the cannon, although in those early days, apparently the noise had a bigger impact than the rocks it fired!
Also see: https://en.wikipedia.org/wiki/Battle_of_Cr%C3%A9cy#Longbow_versus_crossbow

[Petrochemicals]

They are pre tensioned as it enables the arrow to be located easier. They come in different sizes deoending on application, just like other weapons. The roman infantry sword is short for fighting in close quarters. The pole axe was designed for open combat to destroy knights.

[Colin2B]

An account of the battle of Crécy is described here, where a small army with longbows defeated a much larger army equipped with crossbows:

Thanks for the link @evan_au I’ll have a listen. I remember reading a book on the longbow which said there were 2 problems for the crossbow at Crécy: it was raining so the crossbow strings became slack - not a problem for the longbow as the string could easily be detached to keep it dry, also the Crossbow men had left their large protective shields back at the baggage train and had no defense against the longbow.

They are pre tensioned as it enables the arrow to be located easier.

That’s not the reason. It is still possible to nock an arrow with an untensioned string, but the draw distance is then increased for a particular draw weight, unlikely the arrow would go far.

[Petrochemicals]

They are pre tensioned as it enables the arrow to be located easier.

That’s not the reason. It is still possible to nock an arrow with an untensioned string, but the draw distance is then increased for a particular draw weight, unlikely the arrow would go far.

You would just have a thicker bow. The compound bow was designed to get around the problems you are envisioning. It also is a good illustration of why bows can be pre tensioned, the draw weight being very loose at the beginning. Bows i believe where not stored tensioned.