Centre of Lateral Resistance

In order to calculate lead, I need to determine the centre of lateral resistance.
Both computer programs that I have give (amongst much other stuff!) the centre(s) of the wetted surface.....same thing ??

 

Nope! Probalby close, but.... If you "draw a line" around the underbody (or project to Cplane), and find the centre of that area, it is usually taken as good enough. I tend to use 50% only of the rudder, and ignore the roach, or full rudder and full roach, just to even things out.

Steve

 

Thanks Steve....as usual, quite elementary.

 

der hang on a tick - not so elementary after all....
You're talking Rhino.....and I do have that, but to say that I've mastered using it would be a GROSS overstatement! Can you do the same thing in AutoCad? (I had to draw criss-cross lines the old-fashioned way to find the CE, so finding the centre of the sail(s) would be handy too....) I've sent a 2D dxf file to AutoCad with the preliminary sailplan....

 

Will,
In autocad, draw a line around the bottom (i.e. make the underbody a closed polyline) then make it a region (you know, type "region", select closed underbody poly, hit "enter" or space as preferred), then type "massprop", "L" for "last", and return. It will give you a nice little pop-up with more info than you could ever use, but will have centre of Area listed. And Bob's your uncle/father/ distant cousin, whatever.
Steve "let me know if you can't understand that"

 

Aha!!....region....I've been trying to figure how to make the massprop feature work, when I've tried before its always come up with no object selected or some other such message......as always....thanks Steve!!

 

Hth

 

Steve -

When finding center of lateral resistance, do you ever "weight" areas of the underbody and appendages differently to account for differences in efficiency in lift generation from the foils vs. the canoe body? Would you calculate CLR differently for a traditional, full keel vessel than for a "skimming dish" with a hi-aspect ratio keel? Just curious about methods...

Chris Krumm

 

Chris,
"No" would be the short answer. But "Yes" in some ways.
I don't weight the areas, except as stated above for roach/no roach. The amount of klead applied does vary, though, which amounts to the same thing. 10% for masthead, 5% for fractional (7/8 or below).
So, no, I wouldn't calculate CLR any differently, but the lead would vary. How much for a traditional full-keel boat? Dunno yet... No-one has asked recemtly, but I would probably defer to Skene's, Chapelle or Dixon Kemp for that one.
Steve

 

There is a method for fin keel yachts in Larsson & Eliasson's Principles of Yacht Design.

 

Steve,

Can you explain "roach", my dictionary gives me either 'type of bug' or 'type of fish'. I suppose naval terms not included.

Peter

 

In the case of a marconi-rigged mainsail, roach is the extra sail area on the leach - the part of the sail most supported by battens.

An IACC boat has a huge amount of roach.

If you have no roach, your mainsail would simply be a triangle. A Chesapeake bay skipjack has no roach.

 

Thanks, Redcoopers.

Peter

 

John Leather, Howard Chappelle, Herreshoff, and an old copy of
Skenes all discuss the topic of lead on different traditional hulls and rigs.

 

And - as always - so does Larsson & Eliasson in "Principles of Yacht Design".

I have an idea: How about contacting the publishers and ask for a donation to this great website? I mean, I cannot count the times this book has been recommended here...

That's all for now - back to the (hopefully peaceful) Olympics!