how much lead can i loose?

If i was to have a custom fin and bulb keel with all of the ballast in a bulb and no reduction in draft - how much weight can i leave on the dock?

The boat is an IOR 50, alloy construction with empty displacement of about 11000 kg of which the keel comprises about 4400 kg. The keel is about 6'8" in span and about 7" in thickness at it widest point.

Guessing the CG (for the keel) to be about 0.75 below the hull to keel joint ive calculated that a weight of about 1650 kg acting at a point 2m below the hull to keel joint would provide the same righting moment.

Dont worry im not about to build anything based on these calculations, im simply interested to know practically what the limit would be in efforts to shed the pounds. Im no engineer or architect and i can see many examples of light displacement racers with a comparitively high ballast to displacement ratio ie light weight hulls with heavy bulbs - but what about going the other way ie a heavy hull and (comparatively) light weight bulb but set deeply enough to produce the same RM?

 

You should know also the CF and the CG of the hull to get the RM calculated right..

 

You calculated the bending moment of the keel at the hull, not the righting moment of the boat, and they are not directly related. Righting moment = displacement x righting arm. At small angles of heel up to about 5 degrees righting moment = displacement x metacentric height x sin heel angle.

If you reduce displacement by removing ballast you need to lower the center of gravity of the whole boat enough to increase the righting arm at the heel angle being considered to keep the same righting moment.

 

You have to add the stability of the hull to get a correct figure

 

Definitely there is a lot more to this than the simple calculations ive listed! Im not planning to replace the keel any time soon as im already part way through a major refit thats already replaced the rudder, rigging and engine etc etc and my pocket is low on boat bucks..

Already ive replaced the home built keel the boat came with for its original which fortunately was still around. The previous owner fitted a fin and bulb of his own design, he lost some draft (and 1000 kg) in the process which was good for Lake St Clair. Although the boat sailed ok with the shoal draft keel Im happier with the original - i will post some pics below.

One thing though, the reduced weight did help the boat downwind, hence my interest in just how low you can go in terms of ballast and whether reducing the ballast/displacement ratio will have a negative effect on other things - assuming the righting moment can be kept the same.

The first pic is the previous owners keel to his own design, the second is of the original keel the boat came with going back on.

 

Boy that prop shaft is at a steep angle.
Steve.

 

First limiting factor would be safety related:

In 90 degrees heel condition, weight of new keel still need to balance the weight of rig. "Piviot point" could be taken approx. at 1/2 the freeboard. It is quite easy to calculate, and will serve as first approximation of scale of weight gains/losses.
If old keel gave, say 1200kgm of positive moment against rig, new one should give no less.

In more precise terms, righting arm at 90 degrees of heel have to be the same. For this, a real hydrostatic calculation is necessary.

Once this condition is satisfied, loss or gain in "normal sailing" (up to 30 degrees) righting moment is to be analyzed: to have the same moment? to have the same righting arm? how much loss of moment or arm would be acceptable?

 

permanent stress: yes this is what i was thinking - that my primitive analysis via "pivot point" could act as a first aproximation for the scale of gains and losses.

It seems from such an analysis that it should be possible to reduce the amount of ballast quite a lot - what i am wondering is what other effect a low ballast to displacement ratio will have on a boats motion assuming the righting moment is kept the same as before.