CLR of Hull VS CE of sail

[Boston]

fortunately I wont be contending with that problem as Im bying plans from a well known and very accomplished source

the torture is choosing the boat for the most likely conditions

Im experienced in one small area of the world
Cape Cod area
and with plank on frame only
I dont know jack about other construction methods

thus the questions about stuff that seems to be more intuition than science

so far I have three serious contenders for the coaster I want to build
all 39~46" and all three different configurations

and Im still up in the air

[marshmat]

Originally Posted by messabout

And one more variable attached to the CE. When you are on an offwind leg the CE of the main swings forward. It may move forward a considerable amount depending on the aspect ratio of the main. If the layout is configured to avoid lee helm when reaching, then a squatty main may very well set up some serious weather helm when close hauled. When reaching or running the presumed center of pressure is well outboard which will amount to a moment arm.

A valid point. And I would think this would be more of a problem with high-performance sloops and such, than with a multi-masted type.... perhaps part of the reason we see so many ketch rigs on long-term cruisers? Or am I totally off base here?

If the boat is a planing type, or even a semi planing type, then the CLA is someplace other than where it was when operating in full displacement mode. Thus the helm may change its mood. Way too many indeterminant variables it would seem.

That's why the good NAs cost big bucks..... not sure I want to know what Farr or Juan K. charge for all the analysis on a one-off racer!

Eric, Par, Matt and other clever guys use some science and ,methinks, a whole lot of art when laying out sailplans. In my own silly efforts, I just rebuild the damned boat as many times as it takes to get it right. My method not recommended for professional builders.

I think Eric has the science down pretty well, Par seems to me to be the type who treats it as an art derived from lots of experience. Don't lump me in with those two.... my sailing experience is very limited and I have yet to build one of my own designs.

[PAR]

I have no issue (okay, I'm lying) with the science of it, but it makes ugly boats Matt. Initially, we all start out with what we believe is a "sound" base in the fundamental concepts, principles, theory, etc., then reality bites us in the butt. We design a narrow focus craft (for example), with certain expectations and damn if it doesn't exceed or not quite measure up in some or multiple regards. This is a perplexing phenomenon and often the case, other wise we wouldn't bother with trials, we'd know precisely how things are going to "act", trust the math and "go with it".

Reality shoves experience down our throat and eventually, if you've been paying attentions, patterns and similarities appear. We tuck a line in here, increase deadrise there, etc. all because we've learned from a previous model, not necessarily the science, rather literally our own butt.

This may be age induced cynicism or it may just be a learned response to a set of variables. Figuring out which it is, possibly is the art aspect. An example is a little sloop I'm working on. It'll be fast, in fact faster then the previous model it's based on, because it's lighter and slightly narrower. Will it be faster then the model I've targeted to whip up on, well I don't know, I hope so. Maybe this is the "art" of it.

[Brent Swain]

I once met a French couple in an aluminium centreboarder,who had daggerboards at the stern angled outward, like twin rudders. They said that with both boards down they couldn't get the boat to round up or broach, regardless of how hard they drove her. The had to pull the boards up to even be able to manually steer anywhere but dead downwind. Maybe a good idea on ocean cruisers designed for good downwind control in strong following winds.
Brent
Brent

[Boston]

so why is it on the basic friendship sloop type the CLR is so far behind the CE
were as on the basic Bermuda rig it seems closer

[PAR]

There are many, most quite huge differences between a typical Bermudian rig/hull form combination and a Friendship. The CE is actually closer to the CLP (less lead) on the Friendship, then a usually proportioned Bermudian sloop, for many reasons. The 26 foot "in the sprit of" Friendship I'm working on carries about 7% lead. She's a fairly burdened hull form and drags along a 266 D/L, with an 18.6 SA/D, using a Cp of .56. A 26' modern sloop would be half her weight (and D/L) have possibly a higher SA/D, particularly down wind a higher Cp and the lead would be in the 15% to 18%+ range, especially if carrying lofty, big roach sails.

There's very little in common between the two, except they both sail and float.

[ancient kayaker]

I hesitate to rush in where NA's fear to tread, but all the discussions I have seen on this topic so far seem to assume it can be illustrated in 2 dimensions in the profile view. Looking at the thrust vector in the plan view it is immediately clear that it crosses the centerline aft of the assumed CE, especially on a broad reach.

[marshmat]

Exactly, kayaker.
I think folks like to look at the lead of sail CE over hull CLP in 2D plan view simply because it is simple. It's been done that way for long enough that it's possible to compare new designs to known ones.
As you point out, the actual lift vector in 3D space looks very different. The sketch in post #22 illustrates rather nicely why, even though the CE of the sails is ahead of the CLP, the boat will want to round up into the wind... there is a net moment, in this case counterclockwise, if you release the helm.

[Boston]

why I asked
they seem to be so completely different
yet both have there benifits for what I want a boat to do
coast in the pacific northwest

given the rear sweeping aspect of the friendship hull design
I would have never guessed it had a closer balance than the Bermuda style rig on say a fin and skeg hull

[PAR]

Much of the differences are rig related, though the hull and appendages do play a role. The Friendship's much lower aspect ratio, linked to the gaff's trait to not move it's CE as far outboard, combine to keep the lead lower. There are smaller or shorter couples created in comparison to a higher aspect Bermudian sloop.

You have much to learn grasshopper . . .

[Boston]

Lmao

ok
I prefer to think of myself as a small fish in a big pond
but for now
grasshopper it is

my take on boat design is its one of those things that takes a life time to learn
and yet its not something you really learn its more of an art than that
I have a short period of time before I start sawing and pounding something together
about enough time to maybe make an decent decision as to what will suet my needs best
certainly not enough to design anything

its called delegating the responsibilities
someone designs
someone builds
someone sails
few dew all well
me
I drink beer and steer particularly well
I got a pretty good grip on who to invite to the party
and have a knack for vacations

its like a job
otherwise who would buy all those designs
eh

cheers mate
B

[ancient kayaker]

For me boat design is one of those "find out what works then keep on doing it until you're bored" things. I get bored easily so I have to look for a new way of making a fool of myself on almost every boat. Thus design is the brief interval between figuring out what went wrong on the last one and the joyous anticipation of the next and perfect boat.

I used to use grasshoppers to catch small fish in the town pond when I was a kid, then I'd take the small fish over to the river and try to catch a biggger one. That part never worked though. Think it's called business development now; I never did get the hang of it.

[PAR]

You can learn enough about design in a reasonably short time to fully develop a design, but not without instruction. I think you could do the WestLawn course in a year, which will provide enough education to successfully pen up a boat.

[Boston]

Im workin on modern yacht design and a few others that have been suggested
but I think you guys are selling yourselves short
the intricacies of a truly exceptional design are not the result of a few years study

[PAR]

Your quite correct Boston, but most designers tend to be modest (except for me of course as I'll "flash" just about anyone), particularly in the shadow of the many greats of recent past and a few still working. It's not unlike a carpenter of 30 years experience. Fresh out of trade school, they had a fair idea of what they were doing, but in comparison to a wood grain weary veteran, they're mostly thumbs. You have to start some where. Now is a good time my friend.

It's truly a sin we couldn't all be like Olin Stevens. His grace, poise, understated expertise and particularly his love of life was a joy to all who met him and honestly infectious. Even those who only met him briefly, like me. In these shadows, it's pretty easy to be humble.