converting rig to Gaff cutter - any ideas

[hantsgaffer]

I am considering the conversion of a standard bermudan sloop rig to Gaff cutter. Why ? because I am slightly eccentric! Has anyone done/ thought about this before. I know I should probably go to a Naval architect and get it done properly, but I'd at least like some rule of thumb numbers to start from. Boat details 8m LOA, 6.85 m LWL, existing main 11.80msq, jib 11.5msq, genoa 17msq. I feel its currently a little underpowered! displacement 3,400 Kgs, hull form traditional long keel. Anyone got any magic formulas or hard won experiences on sail area for a gaff cutter? i know i'll need to change all the backstays to running etc, its the sail area i'm struggling with.
thoughts please.

 

[Wansworth]

I ha a sial plan drawn upby a naval architect;was notr expensive if you only ask for basic details of mast position and areas,the problem he will have is determining the centre ofLateral resistance etc.But if your going to invest in a rig for an 8 mtr boat you dont want a dog at the end of all your work!

 

[Mirelle]

Well, I\'ve done it

About 30 years ago, mind you.

You will presumably be adding a bowsprit. Draw the sail plan and establish the CE. You will want to retain the same CE, with the proviso that the gaff rig is rather more tolerant of a variations in lead than bermudian.

Yes, plan on increasing the sail area. By about 20%, I would say.

You may be able to retain the mast, but remember that you cannot stay it below the hounds.

Mast length roughly equal to overall length, boom not more than three feet outboard of transom (to get to reef cringles).

Look at a lot of gaff sail plans before going ahead.

 

[hantsgaffer]

Re: Well, I\'ve done it

many thanks, off to have first discussions on the subject tomorrow, so it will interesting to compare ratios!

 

[Mirelle]

Some magic numbers, handed down from early in the last century

These formulae appear in that excellent (and easy to find secondhand!) book "A Manual for Small Yachts" by RD Graham (he of "Emanuel" and "Rough Passage") and John Tew (naval architect and husband of the late Helen Tew) and are either taken from averaging designs in the Yachting Monthly or are taken from Claud Worth's "Yacht Cruising"

They are in feet and the tons are Thames Tons:

Mast, deck to truck - 4 tonner - Bermudian - 31ft, gaff 28ft
Boom 14ft Bermudian 15ft gaff Gaff 11ft
Mast deck to truck - 7 tonner - Bermudian 39ft, gaff 32ft
Boom 16ft Bermudian, 19ft gaff, gaff 14ft

Dimensions of spars - mast should be 0.3 inches diameter for every foot in length, deck to hounds. It should not taper below the hounds (mine does, though!)

Boom should be .22 inches in diameter for every foot of length, unless loose footed mainsail in which case 0.24 inches per foot

Gaff should be between 0.21 and 0.22 inches diameter per foot of length and parallel

Sail areas

4 tonner - 300 sq ft

7 tonner - 450 sq ft

The combined area of the headsails should be about half the area of the mainsail.

The clew should be 1.4 times the height of the gooseneck above the water (this saves your skull and reduces the risk of the boom end hitting the sea when running and rolling)

Shape of mainsail -

Luff should be between 2/3 and 4/5 of the foot.

Head should be between 3/5 and 2/3 of the foot.

Gaff should make an angle of 35 degrees or so with the vertical - but another way is to say that the gaff should make a right angle with a line from throat to clew.

Foresail clew should be reasonably high and should not come abaft the mast, or only very slightly so. Ballow flresail clew should come 2/5 of the distance from stem to mast abaft the mast and should be cut low.

Hope this helps a bit.

(Thames Tonnage is (L-B) x B x (0.5 x B) divided by 94, where L is length on deck from stem to rudder head and B is beam, all in feet. )

Cheers