Hobie Spinaker - too much power?

Two years ago, I mounted a spin on my SX18.
It was created by Surf City, with Tiger components, and cut by Whirlwind, and we love it.
We'll talk about the learning curve and the 'adventures' later.

Earlier this week, Randy offered me a used F18 spin, so last night, we opened up the old spin to check measurements.
The old one is only 12.65 sq metres, and from what I recall, a 'small' Tiger spin is 19 sq m, and a 'big' one is 21 sq m.
OOOPS ! I treated the spin as a triangle, 1/2 B x H does not work here due to draft.... its probably 19 sq m.

Am I nuts to think of mounting such a large spin? Will it provide too much power?
(We only do Club racing locally).

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2015 H16, with spin,
SOLD 1989 Hobie SX18 Sail # 1947 "In Theory..."
'Only two things are infinite, the universe, and human stupidity. But I'm not sure about the former.'

Hmm, interesting. Would you have to change the mounting point of the tang? Also, is the tang mounted to the comptip?

From a theoretical standpoint, sails are treated as foils, and the resultant force on a foil is directly proportional to projected area and velocity squared. I would expect that with such a bigger spin, you certainly do stand more chance of breaking something. However, I would suggest that you approach it the same way that the America's Cup boats, or any larger competitive sailor might: Use the bigger spin as your "code 0" for lighter air conditions, and stick with the smaller spin for the heavy stuff. I'd figure also that as wind speed increases, the extra lift would go to waste and your focus at that point would be reducing drag, so smaller spin and flatter sails.

The equation btw is:

F=C*.5*rho*A*V^2, where

F = Force (lift/drag)
C = Coefficient of lift/drag
rho = air density
A = projected area
V = apparent wind speed

Of course, I have little actual experience with asymmetrical spinnakers, so take it with a grain of salt. If the F18 spin has a more efficient cut or is more controllable than the one you have, that may change things a bit. I guess the best way to figure it out would be to give it a shot! I'd still shy away from the higher wind speeds with it, as that has a much more prominent effect on force than does sail area.

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Mike

'79 H18 standard 'Rocketman II' sail #14921 RIP
'78 H18 (unnamed) sail #14921
'08 H16 sail #114312
'97 H21SC sail #238

No tang.... I created a set up similar to a Tiger, with a length of Amsteel which is mounted through a hole drilled way up near the top of the mast, and at the lower end it is tied to the halyard pulley. The pulley can move to the port/starboard on an Amsteel bail. The bail is mounted in a hole drilled through the (back of the) mast.

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2015 H16, with spin,
SOLD 1989 Hobie SX18 Sail # 1947 "In Theory..."
'Only two things are infinite, the universe, and human stupidity. But I'm not sure about the former.'

If you're using it downwind, it's sort of out of the regular equation of sail force against rudder,board, etc., force. It really just becomes a matter of how much nerve do you have. What you have then isn't a foil, but a kite. And the beauty of the asymetrical spinnaker is that if you start to get into trouble, quickly steering closer to DDW will collapse it. On the downside, if the wind is up and you get caught with it on a beam reach, things can go bad very quickly...

Tom, while I know that a symmetric spinnaker at ~130+ degrees TWA acts only as a kite to create drag (relative to the wind, but driving force for the boat), my understanding of an asymmetrical spinnaker at ~135 degrees TWA (~90 AWA) is that they're used much like any other sail on the boat, to primarily generate lift, and therefore still acts as a foil, unless the apparent wind shifts further aft to somewhere around 110-120 degrees AWA?

For conventional/monohull & symmetrical spinnakers, they are treated as essentially parachutes with a coefficient of drag of between 1.3 and 1.5. I may have to dust off Principles of Yacht Design (excellent book for the technically inclined, btw) when I get home for a bit more information, but as I recall the formula stays the same, you're just working exclusively with drag rather than a combination of lift and drag.

The cut of the F18 sail will be the key to knowing the answer to the question. Is it better than the H18 sail I have? Bigger will likely be better in lighter air. As wind and your speed pick up, then the cut becomes very important. If it is flat like most of the F18 sails I have seen then you will likely be good there too. I have seen the F18 sail very high with their chutes, meaning they are very flat cuts. This is good. A common mistake in catamaran sails is thinking big is good. Bigger is typically a little better in light air, however as wind picks up it tends to create a lot of drag. A cat is sailed fast down wind by using apparent wind. If the sail is too big to make that work well, then you will actually be going slower but think your are going fast due to all of the forces on the boats. That said, if you can get the wild thing going sooner with the F18 sail then you have some definite possible advantages.

If you decide not to go with the F18 sail, let us know. I do not have a spinnaker and would like to give it a try. One of my H18s has the old all aluminum mast and I think I could make it work on that boat. A least better than no spinnaker. Again you are looking for an answer of, is it better than the H18 spinnaker you do have.

If you do go forward with the F18 sail, let us know what you think.
Thanks

Kaos's post makes a lot of sense to me. I didn't explicitly mention it... and from a theoretical standpoint, Naval Architects generally oversimplify the effects of sail shape, because they are so varied, complex and constantly in a state of change, but the cut of the sail and resulting curvature affects the lift & drag coefficients. It's a significant factor, but one that's difficult to define without getting out there and actually sailing the thing! (Hence why sailmaking is an art, skill and science unto itself!)

That was my point. If you sail into a deep enough window downwind, you can actually pull the boards up and you'll go even faster as underwater drag is reduced and the sail is just pulling the boat along. There is no "lift" to counteract. On the other hand, head up far enough to the wind and those boards will have to be kept down to prevent excessive leeway and heel. The deeper the cut on that sail, the narrower that window is going to be. The flatter the cut, the closer to the wind you can use it and the more it's going to perform like a larger version of your present sails. More like a gennaker for instance.

The tiger spin works just fine! You do need to worry about your bail location proportionately to your pole location. You need to be in the neighborhood of tensioning your luff a bit. You'd run a bigger risk of snapping the comp tip, traveler on mainsail no more than halfway out and mainsheet tight tight tight. I was playing with a couple of F18 spins last year on H18se and H18sx rigs and had a lot of fun.


Tom

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Tom
Fleet 259, Central Coast CA
H18 ('81)
H18 ('85)
H20 ('97)
H18 ('78)

John,
you didn't mention whether the two spins had the same overall height along the luff, so I'll assume they do and the increased area is in the draft of the sail.

I've done an awful lot of asymmetrical spinnaker sailing on big performance racing monohulls, to upwards of 100 SM of spin sail area. In handicap racing, compared to OD, you'd go to a greater spin area because the wind is so light that gybing downwind adds more distance than boatspeed can make up for, so a much fuller spin creates that push downwind. The parachute analogy.

with other wind speeds a flat or full aysm spin acts like a foil, turning the flow around the sail through the slot. when the wind really pipes up you can sail much further down, to an apparent wind angle of 110° or more, because there's so much wind. the full spin gives you the edge, though the horsepower generated can be frightening. conversely you can also sail the flatter spin quite deep to much the same speed on a light cat, because of so much wind.

in moderate air where boatspeed well overcomes gybe distance traveled, it becomes a balancing act, choosing a sail based between speed and downwind wind angle.

none of this answers your question of whether your hardware can handle the larger, fuller asym, only testing on your boat can accomplish that.

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Chris
'88 H18SE Arís