Hull resistance

Discussion in 'Hydrodynamics and Aerodynamics' started by Leo Ambtman, Aug 4, 2021.

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Leo AmbtmanJunior Member

Can anybody tell me what is the difference in hull resistance (speed) sailing on one hull of a catamaran (for example a Tornado) instead of on two. At windforce three and flat water. I should think between 10 and 15 percent. Is there any scientific research done about this?

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bajansailorMarine Surveyor

Welcome to the Forum Leo.
I cannot tell you what the difference might be, but my gut feeling is that it could be more than 15% in some cases.
I am thinking that the optimum way of sailing might be to have the windward hull just clear of the water, so no drag from it, yet minimal heel angle, hence maximum sail area available.
OK, then all the weight is on the leeward hull, so it will sink deeper in the water, and hence have more resistance.
But if it has very little reserve buoyancy, then you might well find that it is no advantage in flying the windward hull just clear of the surface, especially if you need the buoyancy of both hulls just to keep the weight of the cat and you afloat.
So the answer is probably "it depends....." - on many different factors.

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jehardimanSenior Member

This is so win on many different levels.

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Leo AmbtmanJunior Member

The resistance is proportional to the wetted surface and increases quadratic with speed. Weight is cubic. So the full weight on one hull gives less wetted surface as on two. You can calculate that the gain is 12,5 percent in speed. All roughly.

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Mr EfficiencySenior Member

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Leo AmbtmanJunior Member

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Mr EfficiencySenior Member

As was indicated earlier it "depends", I guess if you have loaded all the weight on to the one hull, you are going to reduce wetted area, so frictional resistance may decrease, but the form resistance may increase, it isn't simple problem, and could vary with different boats and conditions. An on-water test with GPS should help you decide, but exactly how you would go about that test might not be that simple, I mean you can hardly contrive a situation where everything is identical except the hull being in or out of the water.

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gonzoSenior Member

You would also need to calculate the effect of dynamic lift. The underwater shape of the hull changes with heel.

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Leo AmbtmanJunior Member

The problem is about a catamaran. The hull is very slender so form resistance will not increase against both hulls in the water.

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Leo AmbtmanJunior Member

The underwater shape of the hull will not change much as the hull is more or less round. For sure the Tornado hull at the last part.

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containerJunior Member

I cant answer your question scientifically but i can say that there is a BIG difference in boat speed when sailing a tornado on one hull compared to two, especially downwind in light breeze. The crew sits on the lee hull where the main beam attaches, helmsman sits on the tramp amidships and as far forward as they can, once the windward hull flies clear of the water you are away. Very noticeable difference when racing

Last edited: Aug 5, 2021
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BlueBell. . . _ _ _ . . . _ _ _

Not the same thing.

There is also wind resistance which effects speed.

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Care to provide a citation for this.

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gonzoSenior Member

The leeward hull will have all the weight of the boat and crew applied on it. That means that its displacement will increase greatly, therefore changing the underwater shape of the hull.

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Leo AmbtmanJunior Member

No. The hull will be deeper in the water but as the hull is more or less round the shape does not change much. One hull more deep is more efficient than two hulls less deep.

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