Twin keel placement

[Moss]

Greetings. Is there anyone out there with experience building a flat-bottomed sail boat with twin keels? A friend and I have lofted and have started framing a 20ft hard-chined ply yacht, which will take a junk rig. The plan is to build lead-ballasted, bolt-on, asymetrical twin keels, but we're unsure how to work out precisely where they should be placed on the bottom (which I believe is critical to performance) and how to calculate the optimum camber. Any advice or reference material recommendations?

[LP]

Here's a shot in the dark. Your lateral plane area needs to be balanced against your sail area CE. If you are limited by sail placement, then your center of lateral resistance needs to be 12-14% behind the Center of Effort of the sailplan. However, if your ballast keels are entirely lead(ballast) then their position will determine where the Center of Gravity of the vessel will be. That will need to coincide with the Center of Buoyancy. In this case, your lateral plane area is predetermined and the sailplan will be placed to have the appropriate lead over the lateral plane area. If you are going to built accomodations on your sailboat and probably even if not, you need to do a center of gravity calculation for the approximate location of the keels.

I only see a benefit to assymetrical keels if you plan on sailing with one of them airborne all of the time. Otherwise, they will both be producing lift that counters the other with the net result being increase induced drag. i.e. running before the wind. With symmetrical foils(keels), they will both be producing productive lift if they are both waterborne.

Maybe someone can jump in with foil information.

Good luck.

[zerogara]

I think the most important issue of twin keel boats is that they are balanced on them when they are aground on a tide and don't tip fore/aft.
They produce so much drag for their advantages that only in shallow tidal waters and rivers would people prefer them.

[PAR]

I think the true issue here is why are you beginning the build when the fundamental elements of the vessel have not been ironed out.

Very early in the design process the keel configuration is arranged and balanced under the hull and sail plan. Making an "on the fly" change of this magnitude generally isn't wise without considerable experience in design. Judging from the basic nature of your request, this aspect of the design isn't your forte. You'd be well advised to seek profession help or a difficult to handle, unbalanced craft will result. You are quite correct in it's critical importance, if you'd like to continue the modification, drop me an email and we can work out the boat's needs per your requirements.

[Moss]

Thanks, Par. Will do

[BOATMIK]

Originally Posted by zerogara

I think the most important issue of twin keel boats is that they are balanced on them when they are aground on a tide and don't tip fore/aft.
They produce so much drag for their advantages that only in shallow tidal waters and rivers would people prefer them.

many pics of British boats show the boat balancing on the rudder too when grounded. Normally the CLR ends up being in front of the CG so the boat will want to tilt back. Having the rudder the same length stops it. Maybe have the rudder an inch or so shorter so it is a little bit protected from hitting something hard before the keels do.

Michael Storer

[Howgegoboat]

Hi,
Just a few thoughts
Assymetrical keels, don't work westerly yachts UK (probably the experts on these keels) tried it in the early sixties, but soon reverted to symetrical keels, 10% would be about right. NACA section would do, although need to keep strngth in training edge.
Position needs to be in line with center of sail area, to be balanced, alittle back will give weather helm, and alittle forward will give lee helm, lee helm is prefered but only a small amount.
What are you building it out of?
What size is it, beam, freeboard, what size are twin keels?
YOurs James