If a plan is enlarged 10%, what about sail area and ballast?.....

[Rigadog]

....What percentage does the sail area increase? Does it just scale up 10%

....and ballast?


I'm asking this in regards to Reuel Parker's Sea Bright 23.

[Ian McColgin]

We have a number of discussions, one within the last week, as to why this is a very bad idea. You're really not smarter than Mr. Parker. If you can't find a stock design that meets your needs, give him a call and he'll give you some ideas. Call him with cash and he'll design just what you want.

G'luck

[Ryden]

No, sail area increases by 21%.

But listen to Mr McColgin!

[Rigadog]

I understand what you are saying, but I have purchased 3 sets of plans from him and I really like the 23, just a tad bigger. I had a stroke last Nov around election time and so I have downsized my dreams a bit with this new concept of mortality stuck in my head. I'm a tad embarrassed to bother Reuel again with yet another change of mind, but I guess I probably should. He may give me his blessings with a few suggestions, who knows?

[Ian McColgin]

Reuel has a well deserved rep for open-minded helpfulness. I am sure that he'll work with you and quite the opposite of being judgmental, you'll find him most supportive of getting the choice right.

G'luck

[johngsandusky]

My Wandering Star, 39' ketch, is a successful 10% enlargement of a 36' schooner. The lines were enlarged by the first owner, who contacted the designer for a new sail plan.
Maybe ask R. Parker what he thinks of the idea.

[Peter Belenky]

If all dimensions are increased by the same ratio, the length, beam, and draft are increased by a factor of 1.1, the sail area is increased by a factor of 1.21, the displacement is increased by 1.331, the heeling moment by approximately 1.331 and the righting moment(stability), by 1.464. What you should be considering, however, is the reason for the 10% increase. Are you trying to lengthen the accommodations? Are you trying to carry a greater weight? Are you trying to improve speed? Any such decision involves a multiplicity of adjustments, not just a uniform scaling. Materials have to be strengthened to deal with increased stresses. With greater weight of hull and payload, the ballast to float at the designed waterline may not increase in proportion to the displacement. With greater stability, the sail area might be increased or the ballast or beam reduced....

The Sea Bright 23 is a truly minimalist design, and it may well be that such refined calculations are unnecessary, but Parker would have to tell you whether the placing of bulkheads to support plywood panels or the size of fastenings should be changed. I concur that he should be consulted.

[Rigadog]

Okay I heard from Reuel and here is what he said about increasing the Sea Bright 23 by 10%:

Now I am officially confused...

Yes you can increase all dimensions linearly by 10% (no more). But sail area works opposite—generally it must be decreased proportionately as boat size increases (simple physics—wind pressure on square footage of sail). Increase ballast by 12%.

[MN Dave]

So, what he said was that you can sail a really big ship with a hankie? Perhaps just lengthening it increases the hull speed and decreases righting moment?

The following inexpert ramblings may be entertaining, or even contain useful information, but don't blame me for the consequences. I am just as confused here as for smaller sails on longer boats. I wrote this yesterday, before Mr. Parker replied and my internet provider went haywire again.

I want to disagree with Ian and company, but I have to agree that for a boat such as Reuel Parker's Sea Bright 23, you are looking at a level of risk that requires more expertise than a free online forum can offer. The warnings are well intended attempts to save you from an expensive, and potentially dangerous mistake. Lengthening and shrinking canoes is a trivial thing that is often accomplished by simply altering the space between the forms. There is a limit as to how far you can take it, but I would not be surprised to find that almost any canoe can be built anywhere from 13 to 18 feet by changing the form spacing without ill effect. Unfortunately, this isn't a canoe. In From My old Boat Shop, Weston Farmer widened an Edson Shook design by 25% because he liked the design but thought it was too tender. While Farmer was an accomplished designer well qualified to redesign the boat, he passed it off as a trivial exercise.

You do need to be aware of the math to get the weight to scale properly. Designers do build scale models for testing. There is another thread http://forum.woodenboat.com/showthre...skiff-measures with another link http://www.boatdesign.net/forums/boa...ons-51905.html that links to a spreadsheet credited to H M Barkla, University of St Andrews, Scotland and published in one of the AYRS publications between 1955-1986 and included in a book on yacht design by Larsson & Eliasson in their book if this is the right book.

So as they said on Boatdesign.net, without knowing how these formulae were derived, it is hard to say how reliable they are, but the guy who recommended them is no slouch. I looked at the spreadsheet and reduced with the equations to try and get some idea how it works. All I can say is that the equations are as listed below. I added scale factor to make it simpler. The scale factor is the change in length, so assuming you want to lengthen by 10%, this is the list of equations:
Length x Scale factor
Sailarea x Scale factor^1.85
Beam x Scale factor^0.7
Depth x Scale factor^0.7
Freeboard x Scale factor^0.7
Mast height x Scale factor^1.4
Sectional area x Scale factor^1.4
Wetted area x Scale factor^1.7
Lateral area hull x Scale factor^1.7
Lateral area keel x Scale factor^1.4
Volume hull x Scale factor^2.4
Volume keel x Scale factor^2.1
Displacement tot x Scale factor^2.38
Wetted area tot x Scale factor^1.63
Sail area / wetted area x Scale factor^0.22
Sail area / displacement tot x Scale factor^0.26

This is how it works out for 10% more length:
Scale factor 1.1
Length x 1.1
Sail area x 1.193
Beam x 1.069
Depth x 1.069
Freeboard x 1.069
Mast height x 1.143
Sectional area x 1.43
Wetted area x 1.176
Lateral area hull x 1.176
Lateral area keel x 1.143
Volume hull x 1.257
Volume keel x 1.222
Displacement tot x 1.255
Wetted area tot x 1.168
Sail area / wetted area x 1.021
Sail area / displacement tot x 1.025