Hi!

I need a final judgement about the glide. I had a full blown discussion with my mates and we clashed about it. So before we continue tomorrow I need to know the truth.

I watched videos of fast-flowing racing catamarans (I mean the small ones, e.g. Tornado) and I wonder - is what I see a slide or buoyancy swimming?

The monotypes were simple - a sliding yacht has a narrow bow, and wide, flat stern.

In catamarans - narrow hulls, and round bottom (at least Tornado). OK, I understand - a semi-circular hull can also glide at high speed, but a Cat with a hull shaped into a vertical fin not necessarily?

So how is it? It has always been said that buoyancy swimming has its limits, further acceleration - only in glide. Catamarans aren't bound by this?



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I am a guy that does pozycjonowanie!

More recent hull shapes on beach cats (like F18's) are wider and flatter at the bottoms. The are described as planing hulls.

Boats with planing hulls are designed to rise up and glide on top of the water when enough speed is supplied. These boats may operate like displacement hulls when at rest or at slow speeds but climb toward the surface of the water as they move faster.
•Boats with planing hulls can are riding more on top of the water rather than pushing it aside.

paddington

So how is it? It has always been said that buoyancy swimming has its limits, further acceleration - only in glide. Catamarans aren't bound by this?

If I understand, you are debating the difference in planing vs. displacement hulls. With "traditional" monohulls they are limited by displacement speed because of the bow wave. Planing or semi-planing hulls overcome this by getting on top of the wave created by pushing through the water.

Yes indeed, beachcat catamarans and other lightweight cats are not affected by this in the same way. Even the older design cats like the original Tornado and Hobie 16 and 18 can sail way faster than "displacement speed" by multiples despite never planing. It's one of the great things about beachcats!

Where are you from Paddington? I'm a little afraid you are just here to spam you link, so please reassure us you are a sailor interested in discussion and introduce yourself.

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Damon Linkous
1992 Hobie 18
Memphis, TN

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Yves Parlier built an ORMA 60 planing cat, Hydraplaneur. He made it even more complicated with masts on each hull. I'd have to dig real deep into the Multihulls Magazine collection to find more details.

http://parlier.org/en/project/boats/

I assume by “glide” what you are referring to is planing (or specifically hydroplaning). I believe Steve Clarke put it bluntly once stating simply that “catamarans don’t plane.”

The ability for a hull to plane is directly related to the hull’s width to length ratio, wherein greater width (with respect to length) significantly increases the ability to plane. Modern windsurfing boards are a perfect example of this, in some case the width approaches 40% of the lenght. Boards with that profile can fully plane in windspeeds below 10MPH.

Catamarans, on the other hand, derive their speed in exactly the opposite way, having a very slender hull and therefore a very small width to length ratio, more likely on the order of 5%. I would say that some modern catamaran hull shapes have flatter bottoms to help lift the hull and reduce wetted surface at speed. I would not necessarily call that planing though; quasi-planing at best.

sm

Dogboy

I would say that some modern catamaran hull shapes have flatter bottoms to help lift the hull and reduce wetted surface at speed. I would not necessarily call that planing though

It is a blend of even more factors. Scientific design has made catamaran hulls thinner, the bow attack angle more aggressive, the after portions flatter, and with more buoyancy up front. This allows easier knifing through the water, either over a wave or through it, and the buoyancy helps keep you from pointing your hulls down while powered up. Up is up. and down is down. The hulls will go where you aim them.

Additionally, while cats can't plane in the same way a flat-bottomed Laser can, they do heel, and most cat designs present a better profile to the water when heeled. When the heel increases to flying a hull, you have now reduced the friction on your cat's wetted surface by about 45%, which gives a grand improvement to your boat speed. This is why racers are always trying to get the windward hull "just kissing" the water.

I agree with Dogboy about cats not truly planing. Catamarans do utilize a blend of hydrodynamic design and physics elements, along with technique, to make the best of their unique situation.

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Bob
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Prindle 18-2 #244 "Wakizashi"
Prindle 16 #3690 "Pegasus" Sold (sigh)
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