Keel Construction and Flotation

When building a keel (carbon Fibre), is it preferable to have a solid keel or have a hollow keel? I know that some expansion allowances need to be made via a vent hole at top.
I have assumed that it would be stronger to have the leading and trailing edges solid, with a thin rectangular insert in the centre.

The questions are :-

  1. Would a hollow keel that is very light and would float on its own be a disadvantage?
  2. Would a solid heavier keel be better?

IMHO the less weight you can out in the boat means you can put more in the bulb but weight in the keel is almost in the bulb so it’s sort of a toss-up.

every ounce of weight you can get out of the fin and into the bulb is additional righting moment for your boat. That being said, I would not kill myself to make the fin rediculously light. As said by Don, its close enough to the bulb, that the added benefit of getting that weight just a little lower isnt worth spending 5 to 10 X on the fin. At the top of a 65 inch mast, now thats another story!

We have found that medium densit foam cores work quite well. Having some material in there is a good idea for crush resistance and shape stability over time. We have had great luck with 2 part pour foams. We first pour a foam “blank” in the keel mold that is used to do the carbon work. We then pull the blank out once cured, rough sand it, wrap it in carbon and resin, and then insert it back into the mold. Sure that is simplified more than a little bit, but the same none the less.


How many layers of carbon would you use, is there also a layer or two of regular fibers as well?

I use 1 lay of uni layed vertically and 1 lay of 5.8 oz(it.s the lightest I can get) on the diagonal over a balsa core. No other glass. This seems to handle a 5 lb. bulb no problem.

guys whats the difference between a ton of feathers and a ton of lead… the feathers need a bigger bag.

Air in the foil is weight out of the bulb.

I am unsure I flollow your analogy here. Are you advocating for a more dense fin?

I use approximtely the same lay-up schedule as described by Don, it works well for US one meters but is a hair soft for IOM’s. I think I would use 2 layers of uni in the IOM fin to deal with the heavier bulb.


Steve said " Air in the foil is weight OUT of the bulb". In the IOM class this does not compute. The foil AND bulb, as a unit, MUST be able to be readily removed from the hull for weighing, and cannot be more than 2500 grams. Therefore the lighter the foil can be, mass wise, then the heavier the bulb can be.
So, light fin = heavier bulb = greater leverage = more potential ‘va va voom’!

Go for it.



Does anyone have a formula for calculating the righting moment taking into consideration the depth and weight of the keel and bulb ?

This page of Lester’s excellent site talks about righting moment.

To interested persons:

I suggest to go and read the Tread " Fin Experimental Work" published some time ago in this forum ‘How To’ in july 2006.

My Fin are composed of balsa and carbon /epoxy. See photos on above tread
Generally I use 4 crossed layers of 160 g/m² carbon . Fin dimensions (trapezoidal shape) 85 x 520 x 55 weight 170/180 g
Bending 40mm with 3kg bulb