Sailplan leads

[Corso]

OK, the search function didnt give me anything, all the books that i compared (and actual plans) end not giving the same parameters, sometimes they even contraddict eachother.
What are the normal percentages of lead for different kind of sailplans? (xpecially on a full keeled, sort of plumb bow, double-ender, gaff cutter). How do you treat the overlaps in counting the sail area, they conventionally count or have to be left out?

PS: consider our same double-ender of above and stretch it (just in lenght) of a 10%: the sail area should be increased of?10%?5%?15%?left as it is?

[ 08-19-2004, 07:54 PM: Message edited by: Corso ]

[imported_Steven Bauer]

Have you read chapter 4 of John Gardner's 'Classic Small Craft You Can Build'? It is titled 'Balancing The Rig Of A Small Sailboat', originally an article in National Fisherman. He talks mostly about smaller boats but he covers the issues you are interested in.

Steven

[Corso]

have lots of books at hand and percentages vary too much: 10 to 20% sometimes with no reference to full keels

[Venchka]

It's a smoke and mirrors kind of thing. Some designers get it right. Some don't. Some hull shape/sailplan combinations are better than others. Boats that handle like dogs can be made right with minor or major changes. Some perfectly good boats are turned into dogs when a "clever" owner decides to make some "improvements."

Case in point: according to the John Gardner article Steven mentioned (which I own), my boat won't sail. On paper the CLE & CLR are all wrong according to John Gardner. In his defense, he was talking about much smaller boats than mine. On the water in the real world, the boat sails just fine.

My advice: find a boat with proven good sailing qualities and get one. Leave it alone and enjoy it!

If it ain't broke, don't fix it.

Redneck proverb :D Wayne
In the Swamp. :D

[mmd]

What are the normal percentages of lead for different kind of sailplans? (xpecially on a full keeled, sort of plumb bow, double-ender, gaff cutter).

Long-keel, short over-hang yachts ("fisherman profile") - 0.0 to 0.10 of the LWL aft of CLR. The taller the rig, the more aft the lead. Gaff rigs require more lead than marconi rigs. Average lead is around 0.12, and gaff rigs range between 0.10 and 0.30 aft. Also, the narrower and deeper the hull, the more the lead has to move aft. Bluff bows and broad, straight buttocks force the lead forward.

Precise, eh? This is where experience and past examples come into play - the "black magic" of design.

Good luck. Plan your rig for easy modification - you may need to.

How do you treat the overlaps in counting the sail area, they conventionally count or have to be left out?

Leave them out; count only the visible profile area. In traditional cruising and working craft it is common to omit topsails from the area. For the CLR, don't count the rudder.

PS: consider our same double-ender of above and stretch it (just in lenght) of a 10%: the sail area should be increased of?10%?5%?15%?left as it is?

Calculate the new displacement of the stretched hull; calculate the SA/Displ. ratio of the original hull & sail plans, increase the new sailplan to arrive at the same SA/D ratio.

[Venchka]

Michael is too modest. I don't suffer from said affliction.

Hire mmd, or any designer whose work you admire AND whose work has been proven on the water, to design exactly what you want. The results will be predictable and far less expensive than trial and error.

Wayne
In the Swamp. :D

[Corso]

Thanks guys,
being a designer im afflicted by the "nicebutidretouchalittlehereandthere"-ite tipical of the trade, being a pilot in my spare time doesnt help either when it comes to not putting my sticky fingers on sails & C. (on the other hand both job and hobby taught me to know what im doing before doing it for real).
All the experiments are on CAD and before being put into practice they will end in the hands of the NA with a 'can i do this to your boat?' :rolleyes: note, some other little things are real on little stuff where there is no harm trying (canoe and a lil cat). But there is still the fact that im at the beginning with all this stuff, questions come in mind, and its not clear where science ends and the experienced eye, or the habit and personal taste, starts. Trying on paper its still the right way to learn to me, and who knows... one day it might be a real boat (if its good) or it will end being just a pond model, its still gained experience right?
For now i just keep bugging you guys in the forum, i know that some questions can be stupid but after all i dont think im the only one with those kind of doubts.

[ 08-21-2004, 01:02 AM: Message edited by: Corso ]

[mmd]

Buy old books of published designs (I'm partial to old "Yachting World" hardcovers) that show linesplans and sailplans. Get out your trusty planimeter, pencil and calculator and calculate the leads on boats of the type you like. Scale - even units - doesn't matter, because you are looking for unitless percentages, not measurements. Collect the data in a spreadsheet, and using regressional analysis buffered by some fudge factors that you feel appropriate to level the playing field, determine an average lead percentage for a boat of your parameters.

And remember, as you approach the twentieth or thirtieth hour of the data-mining expedition, that this is the glamour of yacht design! tongue.gif :D

[ 08-21-2004, 01:13 AM: Message edited by: mmd ]

[Ian McColgin]

Speaking of fudge factors, well, more like a rule of thumb -- literally.

Capt Pete Culler claimed that a low aspect traditional keel design would come out right if, when you have drawn her on a 1:12 scale, the lead should be the width of your thumb.

In what follows, I can blame no NA for bad training or bad explanations. It's based on observation and my own attempt to explain the observations. I hope a NA will correct where I'm so far wrong as to be dangerous to the naive.

The big problem is that the static measure of lead is not a true measure. The static measure treats the boat like a lever with sail area pushing to one side and keel and underbody resisting to the other. The force vectors are normal to the centerline.

But look at the actual vectors on a beat. We'll not complicate with other points of sail. The force vectors on the sail are not all abeam. Were they, no boat would move. In fact the venturi over the sail gives some forward componant. In otherwords, depending on the sail and this is why lead may be less for some high aspect sails, the net force, which is a somewhat foreward facing vector, must be factored in.

And that vector is from a sail that's not exactly on the centerline pushing the sail effort more forward and the strength of the venturi effect differes on different parts of the sail.

And and and.

All of which net into two places: The pretty much lateral force in the sheets and the locus of the major driving vector in the mast. A saysail is a bit more complex but same theory. Most of the purely lateral force vector of a boomed sail is siphoned off into the sheet, well aft, and this is one reason why the approxomation of needed 'lead' should be forward of the boat's center of lateral resistance.

Static center of lateral resistance is equally ficticious. This can be most readily appreciated on a boat like Gardiner's Gunning Dory and similar boats that have a smallish rudder or in a cat boat where the rudder is large but other factors play. At low to zero speed, like coming out of a very bad tack, the boat may want to get "caught in irons" - just refuse to bear off on either tack. With the wind well ahead of the beam and well ahead of any useful sailing angle the astern and lateral vectors on the sail are high and the boat's hull has no hydrodynamic properties - might as well be sailing a log.

But if you can get moving, the hull and appendages develop some lift. As speed increases there will be lifting venturis for, depending on design, the hull and keel/centerboard and rudder.

As I feel boats, and I hope that a lurking NA will correct this intuition if it's wrong, the boat's effective center of lateral resistance moves forward with the moving net venturi effect. Quite a lot in the first few knots from zero to three or so, and porportionatly less from there up.

This is why some boats need to be way 'overtacked' but then can be brought back up to the wind once they've a little speed up.

Of course there's the further complication of hull trim. Boats as they heel may be putting some more resistance to leeward, making a twist off the wind effect, but this is hugely outweighed by the declining utility of the hull's net venturi. Short fat boats weathercock when heeled hard down.

Edited ot add: Ah - a designer, of what we don't know, and a pilot. That explains the confidence to tinker.

You must know enough about foils to know that the intended use and operating speed makes quite a difference. A bi-plane wing is different from a fighter's wing. The sail that works fabulously with all those cool haarkin blocks on an ice boat is totally stupid on a Chesapeak oyster dredge.

But at least you're hipper than . . . I sail a bit with a local impressionist artist. Trying, vainly, to get him to at least a few points of seamanship and sail trim. What he can do to a jib would make you weep. Anyway, we frequently sail past a nice little Doughdish belonging to a younger member of a large local irish American political family and my artist pal can't resist pointing out all the things Herrishoff did wrong with the design of the boat. Basicly, he'd like her about 2 feet beamier. Nothing to do with actual sailing, but it would make her look more like a boat he remembers from some french painting!

Well C, your desires to fool around at least harmonize with sea and air. Keep on.

[ 08-21-2004, 08:18 AM: Message edited by: Ian McColgin ]

[Corso]

Ian: Industrial designer, but from furniture to bycicle frames things narrowed down to design and modelmaking for the jewellery industry, its the biggest industry in the area i was living in when in Italy and its something i know really well (i dont design things i dont know anything about, it just doesnt work, and usually i prototype what i design, because its required or just for the sake of it). On the side there have have been r/c airplane models.
Now that i moved here in Canada ill probably have to look for something else :(
There now you know what i design.

mmd: started it smile.gif , there should be an online database for things like that, a sort of boat designs collection database, would be handy.

Thanks again for all the explanations.

[mmd]

there should be an online database for things like that - Corso Well ....

A designer of yachts has two things available to him that defines his own little patch of the profession; his artistic talent and his compiled database. The first is non-transferrable and, in my opinion, not learnable. It just is. The other is the results of long, hard hours of unpaid work spent acquiring, calculating, evaluating, and filing. This kind of a database is a large brick in the foundation on which a designer bases his professional work, and he is unlikely to make it public until he sells, retires, or dies.

:D Ya gotta keep some form of advantage over the masses! tongue.gif

[ 08-22-2004, 09:15 PM: Message edited by: mmd ]

[Ian McColgin]

Also, as you can see if my explanation is even partially correct, there are way too many factors to make a practical data base. There are scantlins tables - rather like the engineering tables for any construction - that will tell you how to build to an anticipated strain but the behavior of a boat on that interesting interface of air and water is far more complex than the design issues of wheeled vehicles or flying vehicles. Even in areas where there are well settled data and where the issues of loading and speed are very predictable - tanker design for example - there's still room for some individuality. Get to a sail boat, even if you define general size, use and expected waters, and if you ask any five NA's you'll likely get a dozen options that don't look too much like each other.

[Hwyl]

Originally posted by Ian McColgin:
Also, as you can see if my explanation is even partially correct, there are way too many factors to make a practical data base.


Even in areas where there are well settled data and where the issues of loading and speed are very predictable - tanker design for example - there's still room for some individuality. Get to a sail boat, even if you define general size, use and expected waters, and if you ask any five NA's you'll likely get a dozen options that don't look too much like each other.Ian, I like your explanation above, I teach racing in conjuction with another coach, when we talk about heeling to windward going downwind (especially Lasers and Opti's), I talk about putting the center of effort over the center of bouyancy. He talks about underwater shape when heeled(utilising a banana). We are both equally right --he gets more pottasium than I do.

On the rudder question and related to the five NA opinions you mention, a famous NA of my aquaintance adds half the rudder area (he does design those strange polynesian things though)

[Venchka]

Don't forget the smoke, mirrors, vodoo incantations, dumb luck early on and Rule of Thumb #1: "If it looks right it probably is right."

Folks like LFH and family, the Crosbys, Atkin father & son, Alden, Garden and many others had IT. Thank goodness they shared IT with the rest of us. If the world were waiting for me to find IT we would all be farmers and lubbers.

Wayne
Thankful for talented boat desiners in the Swamp! :D

[John B]

The lead on my boat is between 11 and 12% . gaff cutter, cut away forefoot. The C/E I used to calculate from is a simple geometric centre of sail area. The CLR I used was a combination of LFH 'balance the underwater profile on a pin' method, combined with the (unfair) ' shoving the boat off a dock until I found the centre of the boat' method. They were about the same.

[Corso]

So far looks like all the recent designs fall in between about 12 and 16%, that for what i know fits the normal 10-20%, older designs (example "Solitaire") are well under the 10%, C. Archer seems to give a lot of weather helm to his boats. Is all this the normal evolution through the years or im missing something?
John B: i doubt i could shove that boat anywhere, the sharp edge method (or pin) its easier, calculations are even better (faster for me)
Venchka: got a chicken and a few incense sticks, ill wait for a full moon night, as for the good luck if you tell me where they sell it....

[ 08-24-2004, 08:36 AM: Message edited by: Corso ]