High Speed Rudder issue....

as interest here is the rudder plan (NACA0013)



And the rudder actual:

 

Gday Pax

Whilst we are all getting forensic with your time and money I am interested in the leading edge radius of the foil. The plans shows a blunter leading edge than your rudder looks like it has. This could be because both rudders and boards are made in a steel mould. You get some steel sheet and fold it to about 90 degrees. (Like you are going to fold a sheet of paper down the middle and stop before you get far). Then you lay up the laminate inside the mould and afterwards pull the trailing edge around the I beam you laid up.

I looked into using a mould for my 38 ft cat for new daggerboards. I didn't go that way but did use the Tennant mould for the motor.

If your leading edge profile is narrower then it may do something. Before I read Frank Bethwaite's Higher performance sailing I would have said it would make it less tolerant to stall. But you may like to check if you have a 5mm radius on the leading edge of the foil too.

cheers

Phil

 

Catsketcher Phil is right - the leading edge is way too sharp. Just rounding that slightly ... you will immediately notice helm difference.
Malcolm's whole idea with the larger section foils, 13% in your case, was to get away from foil stalling - but your sharp, very sharp leading edge destroys his philosophy.

 

Please post photos as attachments. Including them in the text makes the thread very difficult to read.

 

Sharp leading edge

I didn't look at the images, so maybe I am not accurate here, but the comment about having a sharp leading edge (as compared to the NACA profile) raises a red flag for me.

I suspect that you are getting leading edge separation at speed, specifically because you don't have a shock-free entry. The rounded leading edge of the NACA section is actually quite important. Similarly, you could look at the E-P-H (Elliptical - Parabolic - Hyperbolic) profile that is specified for the cutwater on USN ships....same issue, avoiding leading edge cavitation / separation at 15+ knots.

 

bah! - Blew up another cassette Saturday! Again, moving 12 knots+ rounding a downwind mark (no jibe) and BLAM! The tiller had no undue pressure either way before the break. I think the lower forward rake of the rudder, which is likely in an overbalanced state, led to the rudder suddenly taking over. This blew out the back and side of the cassette....

So now I've had a chance to rebuild both cassettes.

I intend to run my rudders at half height from now on and see what we get. I haven't had a case where either rudder threatened to elevate with that 20 foot beam, and at speed, I'm thinking half rudders are sufficient. And the full 1.2 meters of leverage that the fully deployed rudder provides is killing me (and my cassettes!)

I also intend to round the leading edge of the rudder based on your suggestions above.

This failure was a bit tricker. It was a lee shore and winds rising, but we managed to get a hook down in 5 meters of water. In a rising seaway, I removed the undamaged blade from the shattered cassette and from there motored to the pier on the remaining rudder. Hoo-Yah!

Building new rudders might be in the cards next year....

 

Nice pic of the TJ bridge, and the rec center, in the background.

 

Stitch it

I probably said it before but when you do rebuild the boxes stitch both sides of the composite gudgeon together with some uni strands BEHIND the pin. Otherwise the laminate is only being held together by glue shear. About 4 passes of tow does a huge job in resisting delam. (My 25 hp outboard sled is held on by about 40 passes of tow through the bridgedeck floor. Weighs all of 500gm)

You really do seem to be in the wars with the rudders. When you do a rebuild of the box you could try repositioning the pins. Try motoring around with the good rudder and checking whether it is overbalanced at low speeds too.

cheers

Phil

 

Just make a new rudder half the depth and twice the chord of the present one. Make it's section symmetrical and 13% thickness.
Same as the one shown in post 31.

 

Run you rudders without rake and see if you can helm it.
If you then need to balance them build your cassettes with offset pintals like a nacra carbon 20 foiler has
Nice neat solution

 

I don't have anything useful to contribute regarding the balance but on the ventilation issue I understand you can't put fences on the blades but watching the video I would say you could put a fence on the bottom of the cassettes ?

 

I was going to suggest that too, Red - but the cassette case is usually clear of the water - fence needs to be just below the surface.

 

true

That's why I hung back, but if you watch the rudder vid it shows the bottom of the case well and truly immersed.
Just thinking out loud. Bit of an extension maybe ?
Thats the direction I would head anyway.

 

I appreciate the thought that both you and Gary have put into that, and I agree - at speed, the bottom of the cassette is submerged. When the boat is at rest, it is about 2 inches above the water.

I would imagine the angle of the proposed plate is important. Any suggestions on how determine what it should be? How about it's width? Maybe 1.25"/ 3cm or so? I've got some 3/16" plexi laying around the shop....

 

Try this.

http://www.farriermarine.com/pdf/Rudderfence.pdf