Shared Lift between rudder & Daggerboard

I've got nothing like the experience of others in this thread, but as a comparison a Taipan 4.9 has a centreboard chord of 320-290mm, and word on the 'net is that new
F16s are running with considerably deeper foils - something like 1000mm depth and 250mm chord.

The Taipan weighs about 100kg rigged, has low windage, carries an 8.5m mast, has a very high speed potential and yet it has bigger foils than your design and is apparently beaten by similar boats with still bigger foils.

As another comparison from low-windage high-performance classes that are very much into drag reduction and yet use larger foils, Formula Windsurfer fins are deeper than your centreboard (700mm) and even Raceboard windsurfers use fins of up to 520mm and 850mm deep centreboards... seems like you may well be under-foiled. Catsketcher's little cat has big foils and it's therefore very forgiving and manoeuvrable.

 

I reckon you need to design sailing foils for the lowest rather than the top speed. Many accidents in cruising multis occur because the boats don't answer the helm in close quarter maneuvering. There was terrible crash at Pittwater when two Twiggys crunched. One ended up ploughing through the other's float and up to the main hull. I love being able to sail my little cat with no jib and tack it through the moorings at very slow speed - something uncommon in a multi.

I will tell a story about helm here too. I was sailing my own Twiggy in Middle Harbour with about 10 people on board. We had had a great day and we saw Verbatim on its mooring by the Spit Bridge. I had only the main up and we sailed on a broad reach to go view the boat. Once I entered the moorings there was no way out but for gybing - Twiggys are 29ft wide. Then I got hit by a gust - only main up. So the boat didn't answer the helm. We were making a very wide turn and Verbatim was on its mooring with Ian Johnston on the bow looking at us (we knew each other). Still the boat was not turning well then the gust eased and I could turn a bit more helm (too much and shes ventilates) and could throw the main over. I look up and think we are going to hit - me running straight downwind and them facing us on the mooring - we are doing about 8 knots. I want to run up the front to push off - I can see the two boats crashing in my mind - two fab trimarans destroyed by my silliness and cocky nature. But as soon as the main flung over she turned faster and I cleared Verbatims float by about 10ft doing about 8 knots and Ian gave me a wave - I never told him how close we had come.

I was a good sailor then and really knew my boat and the place but a gust changed my boat's manouvrability markedly. The Twiggy had severe issues in this regard and made my hyper sensitive to steering in my next boats. Kankama has big rudders and I love them. If I were to replace Kankama with another boat she would be almost identical save for a few things - one small change would be to make the boards almost twice as big. Then she would behave as well as a mono in crosswinds under power and I wouldn't have to speed into my mooring in a brisk westerly or race into a marina berth to stop flying sideways. My little cat is almost mono like in her low speed ability and it is all down to foil size.

Farriers boats set the standard as far as I can see. Have a look at his F22 and check out the foil size and rudder volume. Farriers are incredibly nimble and much of this has to do with foil design.

Phil

 

The reason I was asking about speeds is the fact that the chord of 200 mm appears to be too small for this boat.

If the top upwind speed is 10 kts, then these daggerboards and rudders are not good. The chord is too small. At 9-10 kts these foils will work at Reynolds number just below 10^6, hence at the edge of laminar-to turbulent transitional range. At lower speeds the flow will be laminar over a good part of the chord, and hence these foils will easily stall. If the daggerboard stalls, no big deal, you will see a huge leeway and bad upwind behaviour. But if the rudder stalls, that's a safety problem.

Besides that, foils will have a maximum thickness of just 22 mm, so you have to verify if they can carry bending moments of the rudder force at max speed and max tiller angle. I suspect that deflections will be very big, and also the tension stress in the rudder outer skin.

Cheers

 

Dac

Would a board ever really be laminar in real world conditions? I also sail a Tasar dinghy designed by Frank Bethwaite and it was supposed to be laminar flow board. I never could tell the difference between a board that was beautiful and a board with a chip in its front edge (and hence turbulent flow). Does it make much of a real world difference in other boats?

cheers

Phil

 

Good question, but a new thread would be necessary to give a complete answer. One such thread was already started in a brilliant manner by Remmlinger: http://www.boatdesign.net/forums/hy.../turbulence-level-ocean-47143.html#post630991

It is possible to have a laminar flow even in real-world conditions, if the sailing speed is not high and sea is in a sufficiently calm conditions.
But for a very narrow-chord foils like the one in this case, it is possible to have laminar flow even in a much wider range of conditions, imo.

Cheers

 

"The reason I was asking about speeds is the fact that the chord of 200 mm appears to be too small for this boat."

Doing some more investigations and playing with designfoil I see your point.

Using "How to dimension a sailing catamaran" I get a dagger-board area required of 0.2m2, based on the sail area (no wind age allowance)

A MK2 version is attached. These foils will work above 5.6 Knots (2.8m/s).

Thoughts?

Alex