Sport Boat 18

Sail Boats 15' and up. Please include the boat type in your question.
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Sport Boat 18

Post by Jeff »

This question arrived over the weekend: "Hello all I have built the Sportboat18 "Lady C" with daggerboard version originally. As I moved to a more exposed area, with stronger winds, I recently converted her to the lift keel version. I note that in strong breeze (20 knts) and upwind, she tends to luff/turn into the wind in the gusts. Looking at the sail plan, I find the sail center to be around 250/300 mm aft of the keel center line. Could this be a reason for this luffing tendency? What would you suggest to correct this? I was thinking to order a new main sail with a shorter foot (500 mm shorter?) or with less roach. Would that be a good option? Any other solution?
Thanks for your help." Jeff

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Re: Sport Boat 18

Post by Jaysen »

weather helm

some folks prefer a "little" bit of weather helm as it tends to move the boat into a reduced power state (toward the chains) if you lose the helm or a super strong gust hits. I believe Jacques designed a slight weather helm into the sb18. All of the "race" boats I have been on have had weather helm by design with factory sails (j24, j30, c27, c&c 31). On all boats we found that when weather helm became excessive, loosening the sheets (main sheet specifically) actually increased speed.

The cause of weather helm is, as you indicated, the COE of the sail being aft of the COR on the daggerboard/centerboard. Remember that COE is actually MIDWAY between the COE of the main AND the COE of the JIB.

Reducing the foot would move the COE of the main closer to mast and UP from boom. This would result in a boat that heels more when the sails are loaded. If you move the MAST forward you would reduce the jib luff a bit but would maintain the COE alignment of both saile much better. Moving the mast is ... not simple. You might consider changing the shape/location/angle of the dagger. Lots of complexities there but moving the COR aft accomplices the same thing as moving the COE forward.

Alternatively, just loose the main sheet a tiny bit until the weather helm is manageable. If you loose the mainsheet too much you get lee helm (bow moves downwind) which is much nastier in my opinion. I always prefer a tad bit of weather helm as it means the boat wont get too far of I go over the side :)
My already completed 'Lil Bit'. A Martens Goosen V12 set up to sail me to the fishing holes.
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Re: Sport Boat 18

Post by jacquesmm »

Excellent reply from Jaysen.
You can reduce the weather helm in different ways but in that kind of wind, weather helm is normal and is safer. In 15 to 20 mph wind (17 knots), you should have a reef in the main.
You can also rake the mast forward but thne you will be disappointed in lighter winds.
Also, check the shape of your sails. Use the Cunningham if you have one, or pull that main sail flat. Excessive heeling will make you luff.
Jacques Mertens - Designer

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