Hull displacement/shape relative to sail size/confiquration

[Salmoneyes]

I have been looking for some guides, rules of thumb, etc. for sizing sails to specific hull shapes and displacements. My gut says boat A designed to work optimally with sail plan B, will perform less than if changed to sail plan C. To be more specific, changing a boats shape and displacement would require a change in sail plan. But where would one start, to avoid buying or borrowing numerous sails to experiment with..

[TrentePieds]

Quote: "But where would one start, to avoid buying or borrowing numerous sails to experiment with.."

I can't know just what you might already know, but based on the way you phrase your question, I think there might be wisdom in reading (and understanding) "Skeene's Elements of Yacht Design by Francis Kenney and then progressing to Sail Performance : Design and Techniques to Maximize Sail Power by Czeslaw A. Marchaj.

TP

[billknny]

The short answer is there is a reason that naval architecture is a profession, and why there are so few people good enough to make a steady living at it.

It sounds like you might be trying to design your own boat. That's a bad idea. The chances of you getting a good boat are slim, and the chances of getting a great one virtually zero.

You're asking a question that is subtle and complex and has hundreds of variables. There are no real "Rules of Thumb" that solve your problem. Boat design is something that people study for a very long time, and STILL sometimes get things very wrong. There are equations, formulae, and very complex interactions between hull, rig and sails.

There are many textbooks on yacht design that you should be studying, not asking questions here. If you can't handle the math in the text books, than you are in over your depth, and looking for "rules of thumb" is not going to solve your problem.

[Salmoneyes]

Dangerous assumptions... There is a boat for sale where the owner added 5 feet aft. Mostly above waterline. No changes apparently to keel or Sails. If A/R is changed below, it follows it should change above. I dont design boats, I just use em and fix em. Sometimes that gives a certain perspective that book learnin dont, you know what I mean? And no, I do not speak in that manner. Just making a point.

[TrentePieds]

Quote "If A/R is changed below..."

What A/R is that, then? Using symbolic language is never a good idea unless you establish first that your AUDIENCE uses the symbols in the same manner that you do. Asking questions obtusely is almost guaranteed to elicit misleading answers.

You are quite right to think that if a boat owner "adds on" a lump of hull "mostly above waterline", the boat is not what it was before, and it has, in all likelihood, been
injured thereby. But perhaps you can explain to us what the significant design parameters are, exactly, that are affected thereby and whether "A/R" is among them.

TP

[billknny]

Originally Posted by Salmoneyes

Sometimes that gives a certain perspective that book learnin dont, you know what I mean? And no, I do not speak in that manner. Just making a point.

And what exactly that point would be?

When you talk about A/R do you mean aspect ratio? If so, that is only one of a great many factors to consider. And adding length to a hull does not change the aspect ratio of the keel, which is the depth of the keel squared divided by the area of the keel. The length of the hull really doesn't come into it. Not that it is unimportant, but it is not the aspect ratio...

Rather than using marginally understood jargon, just say what you mean so everybody can understand.

Do you have a real question you can ask in a manner we can understand and give a useful answer to?

[michaelratinter]

Ted Brewer Yacht Design

[ferrailleur]

Cruising Sailboat Kinetics, The Art, Science and Magic of Cruising Boat Design, by Danny Greene, N.A.

[Cadence]

Originally Posted by billknny

The short answer is there is a reason that naval architecture is a profession, and why there are so few people good enough to make a steady living at it.

It sounds like you might be trying to design your own boat. That's a bad idea. The chances of you getting a good boat are slim, and the chances of getting a great one virtually zero.

You're asking a question that is subtle and complex and has hundreds of variables. There are no real "Rules of Thumb" that solve your problem. Boat design is something that people study for a very long time, and STILL sometimes get things very wrong. There are equations, formulae, and very complex interactions between hull, rig and sails.

There are many textbooks on yacht design that you should be studying, not asking questions here. If you can't handle the math in the text books, than you are in over your depth, and looking for "rules of thumb" is not going to solve your problem.

Well stated.

[Rhwins]

The center of effort of the sails should be just ahead of the CE of the keel to give enough weather helm to be safe. Adding to the waterline behind the CE of the keel might add some leverage against the keel, moving it back a bit; that will add pressure to the rudder, but what are you going to do? Nothing with a sail ! Change the boom length changes everything but that's not happening, right?

[Salmoneyes]

Originally Posted by michaelratinter

Ted Brewer Yacht Design

Perfect.. What Im looking for...Thanks. I also use this one often to settle disagreements. Now I can zero in a bit better.. I like to get facts and prefer to understand how they work if Im going to use them.

Sail Calculator Pro v3.54 - 3200+ boats

BTW, I appreciate a simple response... Have you noticed how the news is not news anymore but just some talking heads giving their opinion and presenting them as fact...

[denverd0n]

Originally Posted by billknny

The short answer is there is a reason that naval architecture is a profession, and why there are so few people good enough to make a steady living at it.

This ^^^^

Originally Posted by TrentePieds

You are quite right to think that if a boat owner "adds on" a lump of hull "mostly above waterline", the boat is not what it was before, and it has, in all likelihood, been injured thereby.

And this ^^^


It amazes me how many people think they can just add this or take away that, and imagine that they are making the boat better somehow, when in reality they are almost certainly doing exactly the opposite. Naval architecture is a fairly complicated subject, with hundreds of interacting variables, which is not at all easy to get right. This is one reason that I studiously stay away from boats that have any sort of significant structural modifications by previous owners -- they are almost NEVER really done right.