Question: Need some help with bilge keels?

(Posted by: joeysranger on 2008-09-11 10:11:16)

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 doesn't like rolling). THANK YOU.

Posted by: Cunning Linguist on 2008-09-11, 10:43:26

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

Posted by: Larry M on 2008-09-11, 11:27:35

The primary advantages of bilge keels are to reduce draft and to enable a boat to sit upright when the tide goes out. They also have a few down sides. Firstly they don't do much to reduce roll. Another huge disadvantage i found was that when you are trying to sail to weather, they make some of the worst leeway I've ever seen. You didn't mention what type of keel you currently have now. My suspicions are 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 you to place additional weight further down, thereby lowering the boats overall center of gravity. A lower center means a more stable boat.

Posted by: Carlo C on 2008-09-11, 11:54:17

By decreasing your draft you will lose some of your righting moment. To retain it would require additional ballast but that would sit you lower in the water. Bilge keels add drag due to more surface area. But you could use them anyway, and make up your 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 your boat heel or the keel counter balancing that force.

Posted by: Bandett on 2008-09-11, 13:01:55

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.

Posted by: lombokmarine on 2008-09-11, 16:54:36

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

Posted by: david on 2008-09-12, 09:05:42

I'm afraid Im not a marine architect and can't answer your question, but as someone who owned a bilge keel boat for years, would like to address a few comments above which I believe are 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 you to use slips to deep for a fin keel. The dynamics counteracting heeling are different than with single keel boats, and one can not just look at the weight and 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. I'll 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 and bilge keel boats tend to not point as well as fin keel boats, but I felt mine pointed just as well as a comparable heavier full keel or 3/ 4 keel boat. While I love many of the atttributes of bilge keels, I think you will be disappointed if your goals is to notably decrease rolling. Consider a trawler with stabalizers if that is important to you. I 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 doesn't matter if you bottom out and sit on the bottom. In fact that is contributing factor the design and selection of many bilge keel boats.