Clr

I'm in the course of estimating the sailing balance of a yacht, and I have two questions:
1.Does anybody have any information on the calculation of the Centre of Latelar Resistance of a bulbed keel? To do this properly you would need tanktest or some very good CFD. What I need is a heuristic, first approximation, like the 25% chord on a wing. I know the bulb is in a part of the keel that is not very loaded, and also that it produces some lift itself, but if anybody has an opinion I would like to hear it.
2. I'm also looking for some statistic data on the amount of lead (longitudinal dist. from Centre of Effort of the sailplan to the CLR)used on modern yachts - if anybody knows of any info sources could you tell me?
thanks,
Iason

 

I would ignore the bulb if it is not generating lift. Larsson, Gutelle, Skenes, Garrett "The Symetry of Sailing" and Thornton "The Modern Yacht" all quote leads, some use different methods, i.e. a proportion of rudder area.

Be careful if you average these out that they are not republishing from the same source!

Went to a RINA lecture the other day which convinces me that CFD is still not a serious threat to empirical methods.

Bear in mind that quoted lead values are a rule of thumb, and have no resemblence to the force equilibrium equation to a heeled yacht.

 

thanks dave

The bulb in question is pretty much foil shaped, L-type. So what I did was not completely igoring it, but extending the keel foil to the maximum draft of the bulb and calculating CLR using this foil - this way I took in account some of the bulbs' surface. It's not sophisticated but I think it will do for a start. The error is obviously towards the stern of the yacht since the aft-of-the-keel part of the bulb is ignored, but I can't really estimate it, so I used this method and kept in mind that this certain error exists instead of risking something else.

iason

 

Here's what I posted in another forum back in July on this subject:

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If the graphs in PRINCIPLES OF YACHT DESIGN are right, LCB should be at 53.1% - 53.6% of DWL. I've heard it said that an LCF 1.0% - 1.5% aft of LCB helps damp pitching, which would put LCF at 54% - 55% of DWL. I don't see these values as related to keel placement except as ballast location effects LCG (which must, of course, match LCB). As a rule-of-thumb I like to see the geometric CLR (taken as the centroid of the underbody profile including rudder) aft of LCF for good tracking, but this is mainly something I check when sizing a rudder or skeg or when contemplating radical solutions like a canards.

It's a rule-of-thumb at Sparkman & Stephens to begin a preliminary design with the mast at 40% of DWL. To give you some numbers off the top of my head, I'd say the CE should be at 38% - 41% of DWL for a masthead sloop and 40% to 43% for a fractional sloop, and that the 25% chord of the keel should be at 46% to 49% of DWL at 1/2 the boat's draft. These numbers are a little arbitrary, and I invite discussion if others believe different.

LCB = longitudinal center of buoyancy = the center of immersed volume
LCF = longitudinal center of flotation = the centroid of the datum water plane
DWL = datum waterline
LCG = longitudinal center of gravity
CE = center of effort = centroid of the sail area defined as the foretriangle + maintriangle on CL in profile
CLR = center of lateral resistance
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Larsson recommends using the 25% chord of the keel at 45% of the boat's draft as the CLR. In the above I suggest using 1/2 the boat's draft, which would actually be more accurate in Larsson's own illustrations. In my opinion one problem with the data collected on lead (the distance, not the heavy metal) is that we've been expressing this distance relative to waterline length, when in my opinion it should be expressed as a percentage of heeling arm. Think about it...

Unfortunately, I don't have any data collected this way. If anyone wants to collect some I'd be very interested!

I don't have any data on bulbs, but I think you could use the 25% estimate there, too.

 

thanks Stephen

I presume heeling arm = vertical distance from CE of the sails to the CLR.
This assumption makes a lot of sense, since the lead aims to counteract the weatherhelm caused by the yacht heeling. A good start collecting data based on this percentage would be on a case by case basis. I'll try to find out some data on similar well-balanced yachts to the one I'm studying, and when I have some conclusions and numbers I'll let you know.

 

I think that's right. The sources I've consulted tend to recommend lower leads for ULDBs. It strikes me that length being the denominator in the fraction, when the denominator should be heeling arm, is actually what's causing this. So I, too, have been thinking that, using this revised method, one should be able to extrapolate to a greater degree from knowledge of a single boat.

Good Luck! I'd love to know what you learn, and I may try to do some calculations on a J/41 once it's warmer here in New York.

 

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What is the SAIL DSWL software - I don't recognize this title?

 

dswl

go to vacanti yacht design softvare.

program for sail bal...etc.

dos based program run under windows .
I dont no isthis program stil on vacanti web site.
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zlatko

zlatko256@cs.com