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Old 25-06-2016, 05:43   #1
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Rudder foils

I have a medium weight cruising cat 37ftx20ftx 2ft 9500lbs cruising wgt.
The boat tends to squat slightly at crush ping speed (6-10kts).
I'm wondering if adding foils to the rudders, say about 2ft wide would help reduce the squat ??
If this mite be helpful are there any suggestions for the foil profile ??

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Old 25-06-2016, 06:07   #2
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Re: Rudder foils

I can't help you with that because my cat doesn't get to crush ping speed. But in reality, the sterns should start coming up if you are talking about sailing and not motoring.

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Old 25-06-2016, 10:48   #3
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Re: Rudder foils

I honestly couldn't tell you. But my guess is they won't help much. The primary use of rudder foils so far is to provide automatic pitch responce to fast boats. As the bows drive down the AOA of the rudder foils pulls the sterns down, and when the bows lift the AOA changes to lift the stern as well.

Technically you could set them to always have a positive AOA and try to lift the stern, but it would create a lot of drag. How much? I have no idea. I know that the crossover speed on maxi tri's to switch to lifting foils is around 20kn, below that the foils add more drag than they prevent. After that they are magical.

Even foiling cats like the A's don't set the rudder foils to directly create lift. As the dagger boards lift the bow the rudders just gain the AOA necessary to lift as well.

- If animals weren't meant to be eaten then they wouldn't be made of food.
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Old 25-06-2016, 11:28   #4
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Re: Rudder foils

I'd be surprised if they did anything other than slow you down whilst not providing the change in aspect you're looking for. They would need to provide an upthrust of maybe 3-400 lbs? That will be a lot of drag. You might be better to shift weight around inside the boat or, come the winter, expand the sterns with lightweight filler and a glass skin to provide buoyancy. That might also improve waterflow if done with some thought, and you'd be better off all round.
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Old 29-06-2016, 11:50   #5
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Re: Rudder foils

What about changing the rake of the mast? Lean it more up right would move the Centerline of thrust forward, correct?
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oil, rudder

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