Reverse Engineering (conversions And Modifications)

The center of gravitiy changes with angle of heel. It wouldn't change only in the case that they are all in one single line. Because they are spread out, they align and add up differently as the boat heels.

 

TAD,know the two are different, just knee jerk reaction to an earlier post. Re CLR and the pin head trick, don't know if you are familiar with James Rosborough, Halifax Naval Architect, 1950 to 1990 showed me the idea on my first build,There goes my back yard engineering.Re the naval designers using it to check, you are correct, left out a word, should read aspiring naval designers, but with todays computer programs guess even the back yard builders have no need of such basics anymore, correction made on the post. Just have to gather a bit more info on distributing the ballast for and aft of the CB/CG static line and i think i will have a good handle on things.I'm comfortable with CLR & CE, Aware of weight verses inches of emersion, top hamper, thru the build. Rudder area,Position of the mast, rigging, sail area,drive systems, superstructure, interior all old hat to me now having done it so many times thru refitting and repairs of customers boats.In hindsight i now think i should have tried to find more of a builders forum as the members here are much too qualified to waste their time on such basics. I'm just assembling the rocket and here they are exploring Mars.It was a tought go but almost thru it and really appreciate the patience and informative input from those who took their valuable time to do so. Just need a little more info over the next few days and i'll be out of everyones hair, TNX.Geo.

 

"This will approximate the geometric CLR (center of lateral resistance) which is part of calculations involved in sailing balance"

What worried me about using the knife edge to calculate the CLR on a cutout hull silhouette , is that the hull form must have a profound effect on the CLR. Also, doing the knife edge on the complete hull silhouette cant be accurate. It should be just the hull below the waterline.

For example, at the bow - a nearly vertical side would provide a lot of lateral resistance, while at the stern, although the sillhouette of the hull would appear to be a similar volume, the rounded hull section would surely represent a much reduced lateral resistance.

The only physical method of calculating CLR of a hull seems to me to be by floating a scale model, and then push it sideways with, say, a nail - and when you find a point that the whole hull moves sideways at the same time, you have a CLR.

I guess you could then do the same experiment with the hull weighted at various angles of heel to see the CLR change at different heel angles.

I will be building a scale model soon, so it would be an interesting experiment.

 

herm.........

To try an clear some muddy points.........

Yes, as Gonzo says, as the hull heels and trims, the forces acting through CG and CB change, as it's all rotating around a mythical point called M, the metacentre, and there are separate longitudinal and transverse metacentres......But CG or G does not physically move unless all the bean cans roll to the low side (more likely the liquids (fuel & water) flow to the low side).

And Yes, rwatson, the balancing of shapes to ascertain CLR is a crude approximation only useful when comparing like with like. For instance some folks would include the rudder, some would include half the rudder, and some included no rudder. As you say it all becomes academic when the boat heels......

The paper cut out balance gives you a geometric center not a hydrodynamic center. See Larsson & Eliasson (Principals of Yacht Design) for some data on actual CLR....generally it's far forward of the geometric center.

Try making a full model and launching her in your bath, heel it 10 degrees and give a straight ahead push.....some go one way, others turn out of the heel, others (few) go straight, but it changes with speed. Heel the model to 20 and then 30 degrees and give it the same push test, it will probably turn in different directions each time.......We used to do this in the test tank at Webb, at the time it was called the "unbalance arm". Some hulls turn all the same way no matter the speed or heel, others not.......

 

Tad are you introducing free flow effect and the need for baffles? I like the scaled model idea but how do I calculate for the fiberglass thickness??? how do I represent that in a modle??? Anyone ??

 

Ok, i thought i had some things straight in my head but now my head has been moved. Am i still ok on my prev. assumption, I.E. under a static state i use the longitudional CB as the reference in dividing my ballast weight longitudionally in half, from which i then use this reference to calculate and distribute the ballast fore and aft to maintain the hull on it's working water line. Take into consideration i am working with a full keel shell and using loose auto wheelweights that will be distributed as decribed above installed in layers and encapsulated in polyester resin. I understand to be accurate in the distribution the shell should be divided into stations that reflect the volume/displacement of that particular location along the hull but is this the hands on application of working with CB. in the building stage. It is when i'm working old school GT(gut feeling), I'm hoping CB replaces GT, it would make my job easier. Logic tells me yes but you engineering guys are light years ahead of me in the dynamic effects and you might recommend a new location to compensate for say the average 10 to 15 deg. of heel.(smiley face) Geo.
P.S. P.S. Mr.X is no more, this is the real build info and once i've gathered enought if appropiate for this forum i can present my hull lines that will i'm sure create even more debate, then move over to the "Boatbuilding" catagory as a build in progress.

 

It's that simple only if:
1) The CG of the boat including all gear, liquids and occupants without ballast is over the CB.
2) The ballast is distributed so the it's CG is over the CB. For example, if half the ballast is distributed 6" forward of the CB, and half of the ballast is distributed 24" aft the CB, the CG of the ballast will be 9" aft of the CB.

Some study of the basics of hydrostatics may be in order. There are a number of good texts available.

 

The actual location of the center of the hydrodynamic forces when a hull is moving sideways with no forward speed will be different than when it is moving forward at a reasonble speed.

 

The usual rules for balancing a hull by finding the center of the sail area with the sails on the centerline of the boat (so called Center of Effort) and the center of the sideview of the submerged portion of the boat (so called Center of Lateral Resistance) and then putting the CE ahead of the CLR by fraction of the waterline length (so called Lead) have no direct basis in physics. But given enough information on similar boats to select the Lead it is generally "close enough".

The alternatives are much more complicated and without tank test and wind tunnel tests of the design or similar boats, or good Computational Fluid Mechanics analysis, may not be much better.

 

Thankyou, Thankyou, Thankyou, MR. Cockey, re the ballast location, this is exactly as i have pictured it including my GF days. I realize there is more to it but you have turned on the light with your explaination and better yet you example, this confirmed info allows me to progress. I can now go on to my second enquirey, Somewhere in my past i recall that it is better to concentrate the mass of the ballast closer to for and aft of the CB rather than say spreading it out over the entire lenght of the full keel shell, I suspect this could be for two reasons, #1 It is more effective as this is the most volumus(new word) part of the hull to control transversely. and #2 By less weight at the hull ends there will be less sea saw (pitching) induced into the dynamics of the boat. Am i thinking right here in terms of the basics. Geo.

 

Spreading out the ballast or concentrating close to the center produces different boat behaviours. Depending on the hull type you may want it more or less lively.

 

How does this work for power driven vessels?

 

For power driven vessels, the shaft should be roughly aligned with the CG. It depends also on whether it is a displacement or planing hull. Another thing to consider is if you have skegs, keels or hard chines that affect turning. The propeller location affects the boat in some ways like the CE of sails do.

 

Right on, Gonzo, don't have to make that decision right now thank God, once i present my hull lines i'm sure alot of good info will flow in. Thanks again for your time and input, Geo.

 

Often the ballast CG is tuned to counter other "fixed" weights in the boat......usually the engine and tanks are aft of LCB, and the rig may or may not be forward, batteries may be way aft....etc.

It's easy to establish all structural and equipment weights first, then move the CG of the ballast (by changing it's shape) to bring the entire boat's LCG in line with the LCB. Then she'll float close to "as designed".