Keel length vs bulb weight - RC Groups
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Dec 13, 2009, 04:41 PM
Registered User

Keel length vs bulb weight

Maybe somebody knows the answer and maybe these not an answer, but is there a way to tell keel length vs bulb weight. What I mean is, that if lets say you have a 5inch keel and a 4oz weight on the bottom of the keel. If I make a 10inch keel do I then need a 2oz weight. In this formula I doubled the length and cut the weight in half.
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Dec 13, 2009, 09:33 PM
Damp and Dizzy member
Brooks's Avatar
Righting moment = lever arm * weight. lever arm=keel length for boats. You calculated correctly in your example.
Dec 13, 2009, 09:34 PM
Registered User
captmsr's Avatar
I am not sure what you want to do, but I will give it a shot.
If your boat has 50% ballast of displacement, lets say 8lbs, 4 lbs is ballast on a 4" fin keel.
If you make the lenght or depth of the fin keel 10", then you would still need 4 lbs of ballast.
When you lenghten the keel, or make it deeper, you are only making the distance between the center of bouyancy and center of gravity farther apart, which in most cases, depending on beam, ( a wider boat will make a fulcrum and help lift the ballast during heel) will make the boat stiffer.
If your keel has rake or is swept aft any, then it will move your lateral resistance aft a little and trim your boat by the stern.
Hope this helps,
Dec 13, 2009, 10:25 PM
Registered User
Well, it depends on what you are after. Both the previous replies are "correct", but from different viewpoints. The first poster is adjusting the keel length and weight to give the same torque = the same righting moment, but with a lighter boat. I'm guessing this is probably what you are after. The formula is pretty simple - half the keel weight requires twice the keel length for the same righting moment. The advantage here will be that the boat will sail roughly the same angle, but the lighter keel will help the boat float higher, so there's less wet-surface drag, and less inertial mass to get moving. Of course if you go too long, then the extra keel drag will be a factor, especially if your keel hits the bottom, so bear this in mind.

The 2nd poster is assuming you have the boat floating where you want it, and lengthening the keel will increase the righting moment without changing the weight, making the boat sail more upright without changing it's bouyancy. You'd do this if your original keel was just not effective enough, and the boat was being blown over further than you would like, but the waterline (and overall weight) was about right.
Dec 14, 2009, 09:27 AM
Registered User
This is my thinking. Im just going to use these made up numbers but you will get the pic (I hope). If the total weight of your boat is 8lbs and 3lbs is made up of the keel weight with a 5inch keel. If you make a 10inch keel and a 1.5lb keel weight add in the 5lbs the boat weights you are down to 6.5lbs total weight. witch should sail faster and have the same righting moment as before.
Dec 14, 2009, 11:13 AM
Big Boats Rule!
boater_dave's Avatar
Don't forget that increasing the fin length will increase drag, as surfdabbler pointed out. So if you do go longer, try to reduce the cord length to minimize the drag.
And don't forget the top half! An ounce saved at the masthead is worth a quarter pound in the keel, for exactly the same reasons.

Dec 14, 2009, 01:12 PM
Registered User


there seems to be a point missed the keel is part of the lateral plane of the boat so in addition to holding the bulb for righting moment it also helps to stop the leaway if this was not so we could hang the bulb on a stiff wire both reducing wetted surface and drag but the boat would go sideways thru the water so we are dealing with righting moment lateral plane against drag and weight its all a big trade off and the designer is the final judge if he is right the boat goes fast and does not make much leaway if he is wrong you have a nice large planter for flowers casy
Dec 15, 2009, 08:03 PM
yachtie2k4's Avatar
the heavier boat theoretically will be slower if you could keep the same waterline length. but if taking 1.5lb out your are loosing waterline length, thus you will also loose speed and with your length the keel is going to you are increasing the drag which will slow you down, but you are also taking 1.5lb of weight out of the boat so the sail area to weight is going to be better. the heavier boat will also go through lulls better because of the momentum.
you could go the other route and double the depth and keep the same amount of weight and put bigger sails on the boat!!!!!
Dec 16, 2009, 06:44 AM
Registered User
A lighter boat with the same sail area will accelerate faster. It will also slow down faster when tacking, possibly to the point where it is unable to tack.
The surface area of the fin also acts as a damper, slowing down the rate of heeling, and that energy translates into drive. Everything about yacht design winds up being a compromise - its no good having a fast boat that only goes in one direction, equally its no use having a boat with ultimate steering that wont go anywhere.
Dec 17, 2009, 11:07 AM
Big Boats Rule!
boater_dave's Avatar
...and yes, if you could support a tiny bulb far below the hull you would then need additional fins to be the lateral resistance. Many canting keel boats use dagger boards to replace the loss of keel fin area when canted. The boards are easy to see next to the mast on the Open 60 Skandia.
My comment a few posts back with regards to reducing the cord length was so that a deeper, narrower keel would have the same area as the shorter, wider original.


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