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Do asymmetrics lift the bows on cats?
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stewart smith View Drop Down
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    Posted: 22 Jan 11 at 12:39pm

Do asymmetric spinnakers give lift to the bows on catamarans?  I find that I am more likely to pitch-pole my Topaz 14X when I put the kite up.  I assumed that monohulls experience this lift because the boost in speed raises the hull onto a plane.  This doesn’t happen on small cats like mine because they do not plane. 

I would like to put a bigger kite on my Topaz 14X to sail in light winds against Bandit handicapped mono-hulls in the Wednesday evening races at Grafham.

Does the length of pole have any effect on this mythical lift?  If so how long should I make it?  What other dimensions should I consider?  I intend to increase the size of the kite from 9.9m to 13.7m, the same as the Shadow has done to become the Shadow X.

I hope the response to this post is as good as “masts on cats”. 

Regards

Stewart

 
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Posts: 99 		Post Options Post Options   Quote Teamvmg Quote  Post ReplyReply Direct Link To This Post Posted: 22 Jan 11 at 2:17pm
The lightweight Youth Hobie 16 teams used to struggle downwind in a blow, some of them had to reef the mainsail to get round a course. Once the spinnaker kit was added, the same crews could handle much more wind due to the lift that the spinny gave.
The h16 kite only lifted the bow once Hobie developed the very long pole.
The added umph meant that the crew could also trapeze downwind and get right out the back of the boat
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Post Options Post Options   Quote Guests Quote  Post ReplyReply Direct Link To This Post Posted: 22 Jan 11 at 5:50pm
Originally posted by stewart smith

Do asymmetric spinnakers give lift to the bows on catamarans?  I find that I am more likely to pitch-pole my Topaz 14X when I put the kite up.  I assumed that monohulls experience this lift because the boost in speed raises the hull onto a plane.  This doesn’t happen on small cats like mine because they do not plane. 

I would like to put a bigger kite on my Topaz 14X to sail in light winds against Bandit handicapped mono-hulls in the Wednesday evening races at Grafham.

Does the length of pole have any effect on this mythical lift?  If so how long should I make it?  What other dimensions should I consider?  I intend to increase the size of the kite from 9.9m to 13.7m, the same as the Shadow has done to become the Shadow X.

I hope the response to this post is as good as “masts on cats”. 

Regards

Stewart

 

"Franken-bandit" LOL
How are they gonna work out your results then....it will be like "bandit hell" up there in sunny Grafham.
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JimC View Drop Down
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Posts: 6662 		Post Options Post Options   Quote JimC Quote  Post ReplyReply Direct Link To This Post Posted: 22 Jan 11 at 8:10pm
The spinnaker doesn't lift the bows on any boat... If it did you'd see crews rushing to the front of the boat when a gust hits...
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Posts: 62 		Post Options Post Options   Quote stewart smith Quote  Post ReplyReply Direct Link To This Post Posted: 22 Jan 11 at 8:28pm
Franken-bandit
 
There are two ways to recalculate the handicap on a cat.  The first is to go to the SCHRS website.  Then download the calculator.  Input the measurements of the boat and in this case substitute 13.7m instead of9.9m for the size of the spinnaker in CSPI box.  Click on the Calculate Ratings box.  This will give the SCHRS number for my Topaz 14X with the bigger kite of 1.221, instead of 1.238 with the standard kite.  Now multiply this number by 692.7 to get the PY number.  The result in this case is 846.

 

The mono-hull way seems to be , make up a number and hope to get away with it.  As long you have an uncommon boat , you probably will.

 

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Posts: 514 		Post Options Post Options   Quote Stuart O Quote  Post ReplyReply Direct Link To This Post Posted: 23 Jan 11 at 4:33pm
best way to keep the bows out is get the weight back. It isnt uncommon to see the top crews trapezing off the transom of the windward hull
having said that Scooby will step in here and andd the only way is T-foils....
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Posts: 8956 		Post Options Post Options   Quote Rupert Quote  Post ReplyReply Direct Link To This Post Posted: 23 Jan 11 at 7:53pm
Originally posted by stewart smith

  Now multiply this number by 692.7 to get the PY number.  The result in this case is 846.

 

The mono-hull way seems to be , make up a number and hope to get away with it.  As long you have an uncommon boat , you probably will.

 

Stewart

That is the way to get a trail number. As opposed to a measurment driven system like cats use (there are fewer variables in hull shape, so this stands a chance of working) the PY system is results based.
Unless you are planni ng on racing against monohulls, the PY system probebly won't bother you that much. Can't say if the cat handicap is a nice one or not.
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Posts: 545 		Post Options Post Options   Quote catmandoo Quote  Post ReplyReply Direct Link To This Post Posted: 24 Jan 11 at 12:53pm
all I can say is with experience is that I'd rather go downhill with a kite than without whatever the wind .
its far smoother and easier to control than without .
wether its due to lift or not who cares !
First Lochness proved this emphatically with a 25 mile run in f5/6 gusting 7
 
half of hurricane 5.9 fleet had kites , i sailed with kite gybing at the end (most windiest )and after 4 hrs racing very tired ,proved tricky , but watching the boats with out kites coming in after was quite spectacular , hobby horsing and nose diving every few minutes with spray flying the height of their masts , looked more like bucking broncos , more than prooved the difference to me ! and I was more than glad to have used the kite. Even when you did stick bow in as long as you held on to the kite the bow would be "powered through and back up , or perhaps lifted ?, came up whatever.If you had let sheet go you would definately have gone down and cartwheeled .
 
Second nessy after a slow start ended up pretty similar windwise , but this time solo in a stealth , by the time I turned at fort augustus was soo knackered after beat that i woosed out of kite (hoisting and lowering when solo in high winds can be fickle  , but after two capsizes without thought fek it and put it up anyway , 25 miles later no regrets , though I did have wings on rudders too !
 
Last years stealth scottish at largs (an extremely windy afair) was another great proover for kites , with the one boat who woosed out of flying his kite complaing of many nosedives and near death experiences (though his wings may have saved the day for him  ) whilst the others suffered no dives at all.
 
Theres definately something beneficial going on with a kite , lift or the extra power but all i know is get the rag up whatever .
 
 
Pretty pointless racing a beach cat against keelboats , we did a race once in h5.9 sx against a variety including one off racers up to 40 foot , upwind the bigger keelboats were on par (because they pointed much higher ) off wind not a chance bye bye they were specs in distance behind in mere minutes.
 
we had used the schrs conversion to get  comparible ratings , but still won by obscene amounts .coversion didnt work really with so disimilar boats .
 
Keelboats can hit beach cat speeds however off wind in big winds , but you wouldn't want to be out on a beach cat then :)


Edited by catmandoo - 24 Jan 11 at 1:08pm
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Posts: 113 		Post Options Post Options   Quote Bryn_14 Quote  Post ReplyReply Direct Link To This Post Posted: 25 Jan 11 at 6:48pm
Originally posted by JimC

The spinnaker doesn't lift the bows on any boat... If it did you'd see crews rushing to the front of the boat when a gust hits...

Really? I've got an old International 14 on which the kite can lift the whole bow (past the centerboard) out of the water when we're double wiring. 

I thought the amount of lift a kite produced related to the angle that the luff was at, in relation to the verticle. 

If it was straight up down, all of the pull would be forward, but as they are at an agle some of this is split into a verticle Y component? Correct me if I'm wrong??
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Posts: 514 		Post Options Post Options   Quote Stuart O Quote  Post ReplyReply Direct Link To This Post Posted: 26 Jan 11 at 10:27am
I think this is one of those questions that the theory and the practise never quite add up. There is a long debate on the subject in the dinghy section.
 
Experience does show that asymetric kites do give the bows a lift, but equally get it wrong and they produce a lot of down as well.
 
trying to keep it simple and not go overboard with the theory and maths:-
 
Beach cats tend to use very square top mains, from experience, these tend to give a downward push to the bows, this is due to the force pushing the leeward hull into the water. The kite counteracts this so could be said to give lift
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