# Comparing two hull shapes.

Discussion in 'Hydrodynamics and Aerodynamics' started by Alwaysthinking., Nov 7, 2019.

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### Alwaysthinking.Junior Member

If I were designing a sailing yacht for a specific rule, such as 12 meter, the task would be very difficult, but the goal very simple. That is, design the fastest boat I can that doesn't break the rules.
I want to make two crude hull models, of differing design, for a sit on top kayak, and do "tank testing", to compare performance. This would be for myself, so the only rules are ones imposed by me.
If I make both models the same overall length, one will have a longer waterline. If I give them both the same waterline, one will be longer overall.
What if I give them both the same waterline length, but the one that is longer overall, the one that employs my pet theory, ends up having less resistance? Have I accomplished anything? Maybe if I gave them both the same overall length, my pet theory design would be slower. Do I need to make four models?
And what about width? They could have the same width, but different stabilities. What if I build both with the same width, but the hull with the least resistance has less stability? In order to give them the same stability, I'd have to make the faster hull wider and would probably lose the performance advantage.
Tough questions.

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### BlueknarrSenior Member

Only three models needed.
1st of pet theory
2nd of same length as 1st
3rd of same WL as 1st

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### BlueBellAhhhhh...

The shape is of little difference compared to the weight.

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### Eric ruttanJunior Member

Perhaps...
As suggested, model pet theory.
Then solve for a better hull than it, optomizing for <= drag and or <= stability.

But this is kind of silly. Assuming some relatively optomized boat shape, usefull to the domain, the construction weight and practicality become dominate variables.

For example, perhaps in theory round hulls dont suck, but in reality they do.

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### gonzoSenior Member

It depends on what you want to compare: static waterline length, overall length, displacement, etc. In general, you should start by changing only one parameter for simplicity.

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### rnlockJunior Member

If you really want to minimize resistance with a sit on top boat, make it a catamaran, or maybe a trimaran, so the hull shapes don't have to provide stability by themselves. Someone has probably figured out how to calculate the optimum shape in this simpler situation. If I'm not mistaken, someone has already done this for minimum wake, which I'll bet is close to but slightly thinner than a minimum drag shape. Significant waves, though, will make it all complicated again.

When figuring out the optimum, you need to define what you mean by optimum first. Least resistance at least cost? Least resistance per pound of hull weight? Some multi factored figure of merit that includes stability?

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### tspeerSenior Member

In his book, "High Performance Sailing," Frank Bethwaite describes doing tests with dinghy hulls. He used a beam across a power boat, towing a dinghy from each end. The reference dinghy was on one side and the experimental dinghy on the other side. The rotation of the beam showed which one had more drag. He didn't need to actually measure the drag - it was enough to know which hull was better.

One problem with sailboat hulls is they don't operate in a symmetrical condition - they always have leeway unless sailing dead downwind. Not only is there drag due to side force itself, but the flow around the hull is different compared to sailing with zero leeway. You might be able to use the same beam towing technique with the hulls toed in to simulate leeway, but the moment due to differences in side force is likely to swamp any moment due to drag. You might be able to get around this by towing two identical hulls and measuring the net force on the beam. That would give you double the drag of a hull, and the two hulls would be self-compensating for side force.

Or, instead of rigidly attaching the hulls to the beam, the towline to the hulls could be attached near the center of lateral resistance and the rudder used to sail the boat out from the end of the beam like a water skier moving outside the wake. Because no moment would be transmitted by the towline, you might be able to use the same differential drag technique as for the straight-ahead case.

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