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[George Steele]

I gather that there are various ways to design pond sailboats. I tried a boat design computer program but could not get the hang of it.  I have read about the traditional method of drafting table, splines, ducks, planimeters and have some of that equipment. But a recently acquired book seems most helpful. It is "The Nature of Boats" by Dave Gerr, 1992 ISBN 0-07-024233-X.
   I am interested in designing, building and sailing a square rigged vessel about four feet on the waterline. I want the weight to be under 30 pounds and the draft under 14 inches. Thus I need a displacement of just under half a cubic foot. Since this is to be a cargo carrier I can have the half angle of the waterline forward vary between 30 to 60 degrees. Gerr says the way to go is to draw the sections, calculate the area of each underwater portion using transparent graph paper and multiply by the station spacing.  Then jiggle everything until I get half a cubic foot. Then I will rig and launch and hang as much lead on the fin as needed to get the hull down to the designed lwl. Then I expect it will be necessary to experiment with mast height and sail area  and see how it works out.
    Any suggestions?

[tobyker]

I think there are plans for the tea clipper Ariel at about the size you want in the MB plans service - I used to have a set, but can't find them now.

[Colin Bishop]

You can find all the details of the Harold Underhill sailing ship plans here: http://www.skipper.co.uk/catsmnew.htm

Colin

[malcolmfrary]

The old fashioned way, pre computer, was to use a board and a fairing lath.  Fix the lath at a few points, bending to shape, and you get a faired line.  Work out what you want your sections to be from that, them switch on the computer to start working out volumes and weights.

[George Steele]

Ah the "switch on the computer" bit is a problem. I am a computer KLUTZ. But if you switched on your computer what would you get? I know the underwater volume I want-- half a cubic foot.  So I stretch out the hull so the water line is say 3 feet and the beam one foot and the draft 6 inches. I could check this out by drawing sections and measuring each and multiplying by the station spacing and eventually get the volume I want. But figuring the sail area and the required stability is a bit of a problem  The only method I can think of is trial and error and keep trying. As far as I can tell there are no published data on sail area and the necessary stability.
    Using scaled down plans of actual ships should get me into the scaling problem and an unstable ship. I could assemble the total weight, length on deck, height of mast of lots of racing sloops but few list draft and none list the length of the fin or the weight of the lead bulb.
    So I am back to: build the hull, build the rig,  attach a fin, and vary the weight of lead at the bottom. (Or also vary the length of the fin.)  That is not too difficult, but in this day and age I would think that there would be published data so I could minimize the experimental stuff.

[Boy From the Bay]

    So I am back to: build the hull, build the rig,  attach a fin, and vary the weight of lead at the bottom. (Or also vary the length of the fin.)  That is not too difficult, but in this day and age I would think that there would be published data so I could minimize the experimental stuff.

Hi Mate
No published data will be out there.

Every time you change the hull shape (in any way, not just length and beam, but any part of the profile) the displacement changes.
Change the scale the dispalcement changes, and also the scale effect that effects the ship being koncked over by the wind.

You can calculate the displacement the way you said, or if you know the displacement of the original ship you can calculate it from scale. I think cube root it.
Or put weight in it till she reaches the waterline, and bobs ya unckie.

This weight won't ever change. It is up to you how deep you put it.

The depth of fin can be calculated, and if you are into racing it can be serios math.
But you say you are doing a square rigger, so you won't be worried about friction.
Looking at some of the BIG boats (4ft plus) on the net the external keels they screw on are only about 6-9 inches wide.

[malcolmfrary]

Pre computer, I used the way you mention, using a computer, I used a spreadsheet as a super adding machine with a memory to play "what if".  The one in Open Office does very well, and its a freebie, so you dont shell out for running up a blind alley.
I had to dig out my old maths books to sort out a formula for working out sail areas of triangular/trapeziodal sails knowing just the edge measurements.  To guess centres of effort I cut out a card image and found the balance point by balancing it on a pin.
As the Boy from the Bay says, if you want seriously accurate pre-design, you will have to delve into serious maths, but a play with a spreadsheet can save a lot of work.  And you get a new skill.

[George Steele]

Well, I have a tabulation of about 20 boats that race in the American Model Yachting Assn listing displacement, LOD, height of mast and draft. I could calculate approximate sail areas and height of same above the dwl but no weights for he lead bulbs are listed. (except for one, the Laser, the fin is 16" and the bulb is 4 lbs and the displacement is 9 lbs so I could get the ratio and use that on all the other fin keelers). The max. sail area of the laser is 710 sq. in. so that would be helpful. Steel, Chapman & Hutchinson Ltd. at <modelsailingships> list the displacement of each square rigged ship (and privateer schooner with 3 square sails) but does not list sail area. 
    So this is as far as I have gotten in locating "hard data."

[malcolmfrary]

Thinking back, the rules for "A" class racers, which are really BIG pond yachts, take all the factors that are thought to improve performance, and the formula seeks to even performance by causing a big advantage in one parameter to reduce performance elsewhere.
In one of the model mags back in the late '80s, there was a comparison of the previous several years winners, there being a very wide range of hull weight and sail area, as well as beam and hull and waterline length.  There is a measurement form from which some useful pointers can be gathered on
http://www.modelyacht.org/aclass.html
The example quoted is for a 45lb boat, but it should be possible to scale from this.