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Old 09-02-2011, 16:43   #1
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Corrrect Sizing of a Heavy Weather Spinnaker

I'm looking to equip my boat with a heavy air spinnaker and look for some help. Here is some backround and design criteria: Corsair 28 Trimaran,19.8 ft beam, 36 ft mast, 130-180 downwind sailing, symmetrical spinnaker, 25kts of wind and would like to keep a comfortable boat speed of around 10 to 12 kts. My present asym would take off like a rocket and produce speeds over 20kts.

Questions:
(1) What dimensions change on a spinnaker sail to reduce its power? Luff/Leech or the foot. I was thinking the luff/leech would shorten and the foot would remain about the same but not sure... hence the question.

(2) Would 1.5oz be necessary for 25kts (true)? A .75oz would be more versatile.

Comments?

Thanks,
John
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Old 09-02-2011, 19:07   #2
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John, I would have thought that for your boat and the target speed of 10 kts in 25 kts true a spinny would not be needed at all... maybe not even a foresail of any sort!

Having said that, I've used a "too-short" kite ( it blew out a couple of middle panels, and I just sewed the remaining bits back together) and it set ok. However, dealing with the pole up that high was awkward, and I was really glad when I could get it properly repaired. Point is that just shortening the luffs may not be the best solution if you really want a smaller kite. I suppose that this means that a combination of reduced girth, especially near the shoulders, coupled with somewhat shorter luffs might do. Seems as though it might be hard to develop a stable shape. Interesting question -- design a slow spinnaker!

Cheers,

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Old 09-02-2011, 19:19   #3
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Have you considered dual head sails? In big winds I will probably move you just fine and be a lot more stable.
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Old 09-02-2011, 19:25   #4
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Quote:
Originally Posted by jkcowboy View Post
Questions:
(1) What dimensions change on a spinnaker sail to reduce its power? Luff/Leech or the foot. I was thinking the luff/leech would shorten and the foot would remain about the same but not sure... hence the question.

(2) Would 1.5oz be necessary for 25kts (true)? A .75oz would be more versatile.
2nd question first, because that's the easy one. The high end for a .75oz chute is 18 knots APPARENT. So, at 25 knots true, if you were making 12 knots downwind, you'd be seeing 13 kts apparent, but if you round up to a beam reach you could blow the sail. For most cruisers, this would not be worth the risk, especially if you already have a chute for lighter air.

As to the first question, I would think that shortening the dimensions of the sail (building a "shy kite") wouldn't be as effective as changing its handling characteristics. You may want to consider a flatter sail. You should understand that a reaching sail is built to a different shape than a running sail, and I would think that once you're moving 12 knots you'll be moving the apparent wind forward enough that you'd rather have a reaching sail. This means that the mid-girth should probably be smaller than the foot girth. In other words, you might want to experiment with a gennaker rather than a true spinnaker.

That said, I have to agree with what was stated above that for your boat, to do 12 knots STW in 25 knots of wind you really shouldn't need a spinnaker.
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Old 09-02-2011, 20:05   #5
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Quote:
Originally Posted by jkcowboy View Post
I'm looking to equip my boat with a heavy air spinnaker and look for some help. Here is some backround and design criteria: Corsair 28 Trimaran,19.8 ft beam, 36 ft mast, 130-180 downwind sailing, symmetrical spinnaker, 25kts of wind and would like to keep a comfortable boat speed of around 10 to 12 kts. My present asym would take off like a rocket and produce speeds over 20kts.

Questions:
(1) What dimensions change on a spinnaker sail to reduce its power? Luff/Leech or the foot. I was thinking the luff/leech would shorten and the foot would remain about the same but not sure... hence the question.

You don't need to design this from scrath. It's is a well known and understood sail. You want an "S4" (Heavy Runner, AWA 120-165, TWS 16-25, Midgirth 100-115% of Foot).

(2) Would 1.5oz be necessary for 25kts (true)? A .75oz would be more versatile.

Yes, you need 1.5 oz if there is any possibility you could get the wind on the beam with this sail
......
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Old 09-02-2011, 20:33   #6
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I'm concerned we'll be reading about you after you stuff a wave with your bows and pitch-pole, or overpower the rudder and round-up. Maybe not right away, but .... Then there will be another outcry claiming multihulls are unsafe, and it'll be that much harder for me to get insurance. "It's operator error" will fall on deaf ears.

You don't need a spinnaker in 25 knots in an F-28. To be safe you've got to size the sails you're flying for maximum apparent wind. Can you safely handle 25 knots apparent with full main and genoa? Not likely at all angles of sail or with the seas that accompany 25-knot apparent winds with any sizable fetch (e.g 20+ miles). You'll be on the edge, at best.

Just my humble opinion based on sailing tris for 10 years.
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Old 10-02-2011, 09:05   #7
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Thanks all. I think the final answer to my question is that changing the dimensions of a spinnaker that will still "fly" is possible but has limitations. On the Corsair the spinnaker could be reduced in size by approx 15% but shaping the girth. This would go from approx 630 ft2 to 535 ft2. This would still be too much for the wind conditions and generate too much boat speed. Another sail combination is the way to go, most likely jib only (175 ft2, 8kts est) or screacher only (300 ft2, 10-12kts est) with no main or 2nd reef main with a target of 10kts boat speed.

For SailFastTri, I appreciate your cautions but the limiting condition of maintaining 10-12 kts downwind boatspeed were enplaced to prevent stuffing, pitchpoling, rudder overpowering, uncontrolled rounding, and any other callamities (I think you named them all). Properly weighted, this boat typically will not stuff the bows at less than 15kts boat speed. At 10-12 kts, you have a pretty good safety factor especially downwind. On this boat having the main up, spinnaker sheet made fast and initial boat speed > 15kts is the common killer. What we are looking at is headsail/spinnaker options to keep a good target boat speed for the conditions. When boat speed goes over 15 kts, the power sheet needs to be in hand.

Thanks all for the discussion,
John
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