Need help designing bilge keels for 70ft X 20ft ferro-cement motor sailor. Boat is currently 8ft draft, would like to eliminate some draft if possible. Would like to know how to size keels for a given boat (L-B-D). Also keels might help with rolling (first mate does not like rolling). THANK YOU.

It sounds like if your first mate does not like rolling, you're in the wrong hobby. That aside, a catamaran would be a better option than a mono for her.

The primary advantages of bilge keels r to reduce draft & to enable a boat to sit upright when the tide goes out. They also have a few down sides. Firstly they do not do much to reduce roll. Another huge disadvantage l found was that when u r trying to sail to weather, they make some of the worst leeway I've ever seen.

You did not mention what type of keel u currently have now. My suspicions r the the percentage of keel weight does not offset the weight of the ferro-cement hull. One form of shoal draft keel is to design the base of the keel so that it is wider than the root at the hull. This allows u to place additional weight further down, thereby lowering the boats overall center of gravity. A lower center means a more stable boat.

By decreasing ur draft u will lose some of ur righting moment. To retain it would require additional ballast but that would sit u lower in the water. Bilge keels add drag due to more surface area. But u could use them anyway, & make up ur righting moment by decreasing weight aloft, as in a shorter lighter mast, etc. The higher up, the more difference it will make- Moment arm = length X weight as in foot pounds of force making ur boat heel or the keel counter balancing that force.

Well, you already have a big ol beast there. Cant fit in most slips just end ties or side ties or anchor. So make it even bigger and funner. Put a big ol out rigger on it(another sailing hull), with a removable deck or trampoline for weather. modify keels to suit. you get into skinny water and roll less too..------------- and name it ISLAND.

Larry and Carlo have given you the most complete , l would go for adding more ballast and staying in deeper water

l am afraid Im not a marine architect & can not answer ur question, but as someone who owned a bilge keel boat for years, would like to address a few comments above which l believe r misleading.

First, A bilge keel boat will fit in a slip just as easily as a fin keel boat. The keels typically do not stick out beyond the beam of the boat. If anything the lower draft, may allow u to use slips to deep for a fin keel.

The dynamics counteracting heeling r different than with single keel boats, & one can not just look at the weight & length. When heeled, the upwind keel will be far to windward of the centerline thus giving a greater righting motion. One simply can not talk about two spaced keels as if they were a single keel.

l will also note, my boat had substantial hull design features to accomodate the bilge keels, they were not simply bolted onto a uniform hull shape.

You will loose a bit of hull speed to a larger wetted surface area & bilge keel boats tend to not point as well as fin keel boats, but l felt mine pointed just as well as a comparable heavier full keel or 3/4 keel boat.

While l love many of the atttributes of bilge keels, l think u will be disappointed if ur goals is to notably decrease rolling. Consider a trawler with stabalizers if that is important to you.

l loved the opporunities the lower draft of bilge keels opened up to me. In addition to being able to anchor in shallower water becasue of reduced draft, it does not matter if u bottom out & sit on the bottom. In fact that is contributing factor the design & selection of many bilge keel boats.