Bermudian and Gaff Cutter design questions

[photojoe]

Seeing as smaller is a better way to start, I'm working on a small Gaff Cutter. I've looked at a lot of old designs from various designers who seem to enjoy putting the staysail on a boom, which I hate. I also know that setting up the staysail sheeting angle is a bit tricky and wanted to know if anyone had any tips.

I know that a Gaff Cutter the staysail is sheeted to mimic the jib.

Also what is a good lead for the Bermudian and Gaff cutter? Any suggestions for reefing points and balance? My other question since I can't find information on it is for designing a loose footed gaff main.

And finally does anyone have ratios for pilot berths. Not being short and never having been in one, I thought I'd see if any of the designers had suggest for length width and depth. That would make getting in and out easier for a tall crew member (such as myself at 6'5").

My last question what is formula for taking the sail area of the jib, stay, or main and breaking it down in to Luff, foot, and Leech? I know there is a trigonomic function that can do this (maybe not for a gaff main).

Thanks,

Joe

[Lewisboats]

Originally posted by photojoe:
Seeing as smaller is a better way to start, I'm working on a small Gaff Cutter. I've looked at a lot of old designs from various designers who seem to enjoy putting the staysail on a boom, which I hate. I also know that setting up the staysail sheeting angle is a bit tricky and wanted to know if anyone had any tips.

I know that a Gaff Cutter the staysail is sheeted to mimic the jib.

Also what is a good lead for the Bermudian and Gaff cutter? Any suggestions for reefing points and balance? My other question since I can't find information on it is for designing a loose footed gaff main.

And finally does anyone have ratios for pilot berths. Not being short and never having been in one, I thought I'd see if any of the designers had suggest for length width and depth. That would make getting in and out easier for a tall crew member (such as myself at 6'5").

My last question what is formula for taking the sail area of the jib, stay, or main and breaking it down in to Luff, foot, and Leech? I know there is a trigonomic function that can do this (maybe not for a gaff main).

Thanks,

JoeThe lead is comprised a number of factors. The range for lead is somewhere between 5 and 15% of the waterline length. Narrow beam, high aspect ratio keels and low rigs (gaff, sprit, lug) call for leads in the lower part of the range and beamy hulls, tall rigs and long keels call for higher end of the range. as an example I would say that a Beamy hull with a high aspect ratio (fin) keel (or daggerboard) and a gaff rig would run between 8-10%...adjust with mast rake for balance. Substitute a longish center board and you would want to bump it to 10-12% again adjustable with rake. Bermuda rig on the first hull...again 10-12%, but on the second hull 12-14%

As to the berth dimensions...A 6'1' person is comfy in a 7' x 20-24" space with about 30" above and 4" of padding...add length to taste.
Finally...the area of a triangle is 1/2 height x base. Split the triangle into two right triangles...that is your height then multiply the length by 1/2 and multiply by the length of the base of the whole triangle.To find the area of a square(ish) sail...split it into 2 triangles then do the math. To figure out a length, split into 2 triangles as above, then a2xb2=c2 where c is the hypotinuse (sp?)

Height
/|\
/ | \
/ | \
/ | \
Base

Steve
(the triangle looked good when typing but is looses something in the posting...sorry.

[ 03-09-2006, 01:16 PM: Message edited by: Lewisboats ]

[John B]

Here's a nice 1909 sailplan with a pretty jib/staysail configuration.
http://www.imagestation.com/picture/sraid161/p7198bc552a8f1117bd2dfba52e7f8d44/f4ce8d91.jpg

[Todd Bradshaw]

This PDF has some info and a bunch of drawings that might also help:

http://webpages.charter.net/tbradshaw/C.E..pdf

I don't know of any exact formulas for laying out a loose foot on a gaff sail. Much of it is generally done by eye. A foot round is added below the foot reference line (an imaginary straight line between the tack and clew corners) by drawing a curve with a batten and the shape and depth of the round will vary with the designer and what he thinks looks good. You generally want a gradual entry into this curve, maximum depth about 45% of the way aft of the tack and a fairly flat exit. So far, you have created a flap - and flap is just what it will do unless you transform the sail's bottom portion including this new foot round extension into a modest cup-shaped configuration. This is done by broadseaming - gradually increasing the panel-to-panel seam overlap on certain portions of certain seams as you approach the edge of a sail. In this photo (it's a spritsail, but the concept is the same as it would be for a loose-footed gaff) I have drawn a red, inverted "V" on the bottom of the sail. Seams within this area would be gradually broadened, beginning at the "V" and getting wider as you go down and they approach the edge, with a little bit of a flaring increase, just as you get to the foot edge. You can actually see extra width on some of the broadseams if you look carefully.

http://www.imagestation.com/picture/sraid202/p961a35973ca2a3ba8e5c5110b5b239f7/efe4e138.jpg

As you might guess, this is easier on a vertically cut sail than on a cross-cut where you might only have a couple of seams striking the foot to work with but it's about the only way to have a nice round and end up with a cup, not a flap. It will work on cross-cut sails, but can be a little more subtle and predictable on a vertical cut.

The amount that the seam widths are increased depends mostly on the firmness and stability of the cloth used, as well as how many seams there are to work with, so it's hard to be more specific. Sailmaking texts will usually specify something like "increase seam width 1/2" for every 24" of broadseam for medium cloth - less for soft cloth, more for firm cloth" and they will have charts that show suggested amounts for various fabrics.

Computer programs can visualize the three-dimensional sail and then measure it in 3-D to generate a specific cup shape for the bottom of the sail. Most of us humans aren't that smart, so it's done with a few rough formulas, a good eye and above all, experience and practice.

As for adding reefs, it's generally a lot cheaper to make mistakes on paper than in wood and cloth, so it's worth drawing-up the various configurations to be sure that as you reduce sail area you're not doing something horrible to the combined C.E. affecting the balance and lead. Few reefed plans come out perfectly balanced and sailplans are a tool, rather than an exact prediction of real life, but it's nice to know that you haven't pushed things off the charts somehow when you drop a sail or put in a reef. This is a series I drew for a little boat that I used to own when I was thinking about re-rigging it (thought about changing the cabin, too, as you can see). It uses a standing lug main, but a gaff cutter would be a similar process. You figure out the sail area for what remains of the mainsail and all other exposed chunks of canvas, determine the combined CE using the instructions in the PDF file and sit back and watch what happens to the CE's position and lead amount as you go through the various configurations. If you find one that looks like it may really screw-up the boat's helm balance, you re-draw the reefs and try again or make a mental note that that particular sail/reef combination may not work very well. In the reefed drawings, the original, full-sail CE is the red one and can be compared to the new combined CE of each configuration.

Full Sail
http://www.imagestation.com/picture/sraid168/pfa103c36ff8c8a4c698a6afdc75861c0/f42c3cd7.jpg

One reef in the main
http://www.imagestation.com/picture/sraid168/p0e2b7d4dff08ff3f5752874c770e3dbe/f42c28de.jpg

Jib stowed
http://www.imagestation.com/picture/sraid168/pefadbc490b1b9e333ff7e377f1cbee69/f42c28e2.jpg

Second reef taken in main
http://www.imagestation.com/picture/sraid168/pf591988e394f4534707995a4664367f5/f42c28e0.jpg

This boat pretty obviously needs some sort of sail in front of the mast in all configurations and it could also be worked up using the jib and reefed main and stowing the staysail. Sailing with main alone, even double-reefed really moves the CE aft and the weather helm would likely be awful. In any case, it's nice to know these things or at least have some sort of data to base your performance predictions on before you start cutting up expensive building materials.

[photojoe]

Thanks for the info.

So anyone have tips for leading the staysail sheets? I really don't want a stays'l boom on the fordeck.

Also any suggestions for picture hosting websites so I can post images of the boat in question?

Joe

[Thorne]

Can't help with the staysail stuff, but as for posting pics, you can search this site (LOTS of threads) or try www.picturetrail.com. (http://www.picturetrail.com.) Create albums, upload the pics, then copy and paste the photo's URL to this text area.