A friend was talking to a person who has built a couple of big boats and was told that as you make the boat wider the keel has to movr farther back. He said not doing so would affect boat speed negatively. Anything in this?
Thanks
Don
Vancouver Island
does that mean that wider hulls have the clr towards the back?
ed
<blockquote id=“quote”><font size=“1” face=“Verdana, Arial, Helvetica” id=“quote”>quote:<hr height=“1” noshade id=“quote”>Originally posted by greg V
…Don, since no one answered this yet, I’ll just state that this conclusion is just to generic. So many design factors come in to play on keel location. I personally have not heard this theory, but I?m no designer, [:-captain] I just sail’em.
There are many here that surely can answer this question with good facts.
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Greg
That was my opinion also but I wasn’t sure. I suppose that on a particular boat if you widened the hull in a particular spot it might need the keel moved back. I just thought tere might be some rule that this person was generalizing.
Thanks
Don
Vancouver Island
Don,
The only think I can think of on this is that if you make the boat wider in the back (skiff) then the center of boyancy moves aft and the keel should move aft to compensate so that it still floats on its lines. The center of boyancy probably shifts further aft as the boat heels since the wider aft sections would come into play so again the keel would need to be aft to keep the thing sailing on its intended heeled waterline. If you don’t do that, the bow would tend to dig in and produce a sizable bow wave which is of course slow…
That could be the effec that your freind was referring to. But it is overly simplistic since you cannot just take a given design and widen it into a skiff. You would need to start from scratch. So the question is kind of irrelavant becuase the boat will be designed with both a static and heeled waterline and that will determine the center of boyancy which in tern will determine where the center of ballast needs to be (and thus the keel)…
Hope that helps a little…
Will Gorgen
Most big boats are designed to function at their best at a particular Froude number.(FN of .4 equals a speed -length ratio of 1.33) That directly affects Prismatic Coeficient(fineness or fullness of ends of hull). And the range of acceptable CB position for moderate and heavy displacement boats is extremely narrow: from 52.6% aft to 53.7% aft.
So making the boat wider will not necessarily have any bearing on whether the keel should move aft or not. Prismatic , CB position and displacement would be the primary factors that would decide keel position. Larrson and Eliasson specify ranges for medium to heavy displacement yachts and say that ,in general, the numbers for lighter displacement yachts are bigger: Prismatic .6 to .7 ;CB up to 55% aft.
Doug Lord
–High Technology Sailing/Racing
I like to see the centre of the fin and ballast on the LCB.
There has been plenty written over the years on optimum LCB abd cp for big boats…does anyone know of any data showing similar information for our models?
cheers
Brett
Generally, I’ll put the quarter chord point at the LCB and move the ballast fore and aft slightly if I have to.(“L” or “T” keel or somewhere in between)
I read something recently and I’m trying to remember where; I’ll post it when I find it.
Much of the currently used information concerning hull resistance was derived from model testing. In the 70’s and 80’s a Professor J Gerritsma at Delft University of Technology in the Netherlands did extensive testing of 39 different models including light displacement models. One of his papers was published in International Shipbuilding Progress in 1981 “Geometry,Resistance and Stability of the Delft Systematic Yacht Series”. It is referenced in Larsson and Eliasson…
Doug Lord
–High Technology Sailing/Racing
I am well aware of the Delft tests,they form most of what is written on the subject today.
However I assume these model tests were corrected for scale effect.
I have a hunch that optimum Cp for one metre sized models may be somewhat higher than recommended in these texts and that optimum LCB could be further aft.
What say other model yacht designers?
cheers
Brett
But if the keel is moved aft that means the centre of lateral resistance is moved and that means the centre of effort is gunna have to moved aft too or other wise you’ll have a lee helm.