Hello Phil and welcome.

Forget the angle of the keel for the moment. The only variable that is going to determine the correct wateline level is the longitudinal position of the centre of gravity and this needs to be on the same vertical line as the centre of buoyancy when the boat is on its correct waterline. It is a quite difficult to calculate unless you have naval architecture experience. It is very easy to find though by practical methods once the hull is built. If you have a look at my Galileo thread you can follow how it can be done.

Are you building direct from a plan?

You can do an approximation of the centre of effort by finding the geometric centre of each of the sails and using moments using the area of each sail and the distance of the geometric centre from a chosen datum. The centre of lateral resistance needs to be behind the centre of effort an on your size of boat I would think somewhere around 40 mm.

Can you post a picture of the plan if you have one.

Tim R

Nice subject Phil

If you look up this thread on the Thames Barges you will find some useful stuff about keels and Ray Wood also used one very successfully on a boat called Wild Duck.

Ray is on holiday at the moment so I bet he will get into trouble if he starts posting but he has lots of experience and is very helpful.

Once you have some pictures in a gallery you can always post them in your thread.

Looking forward to seeing them.

Tim R

Hello Phil

You are well advanced so you will need to think what sort of keel (fin type or long). Fin with a bulb weight will be more efficient but possibly harder to engineer into your existing structure. The stresses can be quite high.

A long detachable keel will probably be easier to fit but won't be anything like so efficient and may make turning a bit sluggish. You will need to increase the rudder area. You could do this be having a larger deeper rudder for sailing keeping the scale version for display, of have a detachable extension which some people do in clear Perspex so it is invisible in the water.

Nice job by the way and looking very sturdy.

Tim R

60cm is on the small side for a radio control sailboat, so it will need all the help it can get.

The "racing fin and torpedo shape bulb" is probably simplest. It will need a threaded metal rod to supply the rigidity (and if it pokes out the bottom of the fin, is handy for mounting the weight), and will need to go up to deck level via a tube. Threaded so you can get a nut on the top. This arrangement spreads the load beteen keel and deck beams, more or less eliminating the twisting forces. About 6" -9" of fin with the bulb on the end should work, try for about 5-10% of the sail area so the fin can help with reducing leeway. Positioning the fin – it needs to be just astern of the line of the center of effort of the sailplan, as it determines the center of lateral resistance ad whether or not it will ever steer straight. The fore and aft position of the torpedo on the end of the fin determines the center of gravity, and thus the static trim in water.

A friend, many years ago, built a 3 foot version. Looked great, sailed very fast, was unable to noticeably steer unless he used his auxiliary power, so a rudder with plenty of authority is good. His was big enough to use internal ballast, as was another boat (Bristol Channel pilot cutter about 4 feet long), whose owner didn't reckon that it had been a good days sailing unless he managed to get water into the on deck lifeboat.

I would have thought M3 or M4 for the rod if single, M3 if 2 of them. It will need some form of location to stop it trying to twist. Most yachts have a socket for the top of the fin to sit in for this. The thinner the rod, the slimmer the fin, and the less drag generated. Also, the narrower the tube can be, so that much less doubling on the main backbone where it passes through. My preferred metod of mounting the mast is to sit it in another tube going from deck to keel. This arrangement transmits the load between sails and fin without stressing the hull between them. A deck stepped mast relies on the rigging to do this, and can be a bit critical on a gaff rig if it needs to be erected at the pool side if it can't be transported ready rigged.

The construction looks solid and roomy enough for a high toque servo with an arm for sail control – if space proves to be tight for 3" of arm to swing about in, one of the new breed of 2 turn winches (about 6" travel) should be very adequate for the amount of sail area it needs to handle. I've tried using a standard servo in the past when it was the only thing available at a sensible price – despite the kit manufacturers claims, it wouldn't hold if there was enough wind to make the boat go. Both high torque servos and small inches are wonderful.