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Old 29-09-2007, 08:56   #46
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I don't believe it is wise to rely on any formulas. Sailing the boat should become instinct at a certain point. Reefing is one of these instincts of course. I'm not saying you should try to fly a hull, but you should know from experience where it will start getting "light". Usually hard upwind will give you the most heel.
On another note; It is strange to me that so many cruising catamarans can not fly a hull in 25 knts of wind. I am a strong believer that cruising boats should never try to fly hulls, however it seems that these boats are underpowered, or too heavy. Or both.
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Old 29-09-2007, 14:09   #47
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All cruising cats are underpowered if you compare them with racing and beach type catamarans but who would like to risk it to fly a hull or worse go over and come into its most stable position ?
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Old 30-09-2007, 03:54   #48
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Quote:
Originally Posted by Tnflakbait View Post
... Sailing the boat should become instinct at a certain point. Reefing is one of these instincts of course ...
Itís a good idea to rely (& act) upon your conservative instincts.
The time to take preventative or corrective action, is when you first consider it.
Do it now.
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Old 30-09-2007, 05:43   #49
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I ran the Cat comparison on the Multidynamics website and it shows that my Sspd at 24.39.

This has me scratching my head as I've been under full sail with a 150 genny in 25 knots and experienced no problems (certainly no hull lifting) whatsoever, with a lightly loaded boat. Nor did I expect to as my boat is underpowered, with a short rig, and modest sail area. I don't see how those numbers can be correct.

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All cruising cats are underpowered if you compare them with racing and beach type catamarans but who would like to risk it to fly a hull or worse go over and come into its most stable position ?
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Old 30-09-2007, 09:09   #50
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I ran the Cat comparison on the Multidynamics website and it shows that my Sspd at 24.39.

This has me scratching my head as I've been under full sail with a 150 genny in 25 knots and experienced no problems .
Yeah, I dont think that formula is reliable.


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Old 30-09-2007, 09:42   #51
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Originally Posted by rickm505 View Post
I ran the Cat comparison on the Multidynamics website and it shows that my Sspd at 24.39.

This has me scratching my head as I've been under full sail with a 150 genny in 25 knots and experienced no problems (certainly no hull lifting) whatsoever, with a lightly loaded boat. Nor did I expect to as my boat is underpowered, with a short rig, and modest sail area. I don't see how those numbers can be correct.

I want to repeat myself:

My guess is that number applies if the boat is sitting still in flat water sails sheeted tight and wind abeam.

Other factors then begin to apply ie: movement.

My guess.

I would not try it on a boat of mine but if you guys will please give it a try and post results I would appreciate it.
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Old 30-09-2007, 12:12   #52
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It is a theoretical value. It assumes the sails are absolutely flat, and aligned down the centre of the boat, perpendicular to the wind - so the entire effort of the sails goes into lifting the windward hull, and there is no forward drive generated. In real life with real sails that is impossible to acheive.
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Old 30-09-2007, 12:20   #53
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I think Therapy is correct...the formula works for static conditions. As for me, I 'sail by the numbers'. Hard to admit for a seat of the pants guy but the challenge in cruising cats is that they do not give much warning when they reach their limits. Take a look a some of those catamaran stability curves. Everything is hunky dory until the limit is reached and then things go in the other direction rapidly. You can be just fine but when a gust 5-8 knots higher arrives unexpectedly, then you are in trouble.

Also, since most of us probably learned to sail in monohulls first, we have been subconciously trained to 'feel' those early warning signs of being overpowered... excessive heel, hard helm, etc. Cat generally don't do that. I have a policy to strictly put in my first reef when the wind is consistently gusting over 20 kts. I already know that I could handle 25 knots and maybe even higher but I don't know how much higher. It requires rigorous discipline both because I'd like to know how much she can really take and because it doesn't feel overpowered.

The good news is that the boat performs just fine under my self imposed rules and I have the confidence that short of an unexpected microburst, I probably won't et into too much trouble. I can liv with this.

BTW, I've flown hulls regularly with Hobies and know how to handle this when its both unexpected and when I'm deliberately sailing on the edge. I just choose not to ever take that chance with my loved ones, guests and not inexpensive investment.
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Old 30-09-2007, 14:37   #54
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Also, since most of us probably learned to sail in monohulls first, we have been subconciously trained to 'feel' those early warning signs of being overpowered... excessive heel, hard helm, etc. Cat generally don't do that.




BTW, I've flown hulls regularly with Hobies and know how to handle this

This would be me.
My fear is that when I get a cat and a gust I will forget and turn the wheel the wrong way!
My fear. One of many.
Sometimes they have been known to keep me out of trouble though.
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Old 01-10-2007, 06:47   #55
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SSpd - Not a Real World Number

The Stability Speed formula (SSpd) is used to Compare vessels, not to give a real world figure of when the windward hull will lift. John Shuttleworth on his website, and MDI on its's website clarifies this under the TECHNICAL pages. On the TomCat multhull site they refer to it as a "Figure of Merit". It is not a hard and fast number, but rather a number that allows comparison between vessels. We know that the numbers reported are lower than what a multhull sailboat will tolerate before lifting a windward hull. But accross the board if you keep apples to apples you can compare boats. But on the other hand if you use the formula reported in Multihull Magazine you recieve a number significantly higher than is reasonable and could lull someone into a sense of security that is just not there.

So just to clarify this: The formula is used to compare boats, not to give a "real world windward hull lifting number". The formula does include hull design, dagger or fixed keel, or wave action just to mention a few important variables. We encourage users to read the Technical section of the MDI website to fully understand how the formulas are designed and how they should be intepreted. We developed the Technical section because we find there is a lot of confusion over terms and formulas and how they should be interpreted, what they mean and finally how they can be applied.

I hope this clarifies how MDI uses SSpd.

Kind Regards,

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Old 01-10-2007, 07:04   #56
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Hull Lifting and Thomas Firth Jones

If you have not read Thomas Firth Jones' book "MULTIHULL VOYAGING" then IMHO you have missed a necessary mainstay to understanding, sailing and possibily building multihulls. One thing he suggests is once you have your boat take it out on a high wind day, a bad weahter day, a day you may not normally go out on your boat, stay near shore and bring friends to assist you. Sail your boat hard ,find out where her limits are "and never go there again", but know where they are for your own knowlege. Now this is a paraphrase and I highly recommend his book, read it several times. It is a cheap buy and very valuable book. As you may know he died last year what a terrible loss to the multihull world.

I am from Louisiana but I am reminded of a good friend from Chicago. Who said each year when if first snowed he would take the car he was driving regularly to a large empty parking log on a Sunday, back in the days when shops were closed. He would accelerate and find how how the car handled in the snow. He'd brake, make sharp turns, get a real feel for the car and then go drive safely on the roads with an understanding of how the car would react in those conditions. I think he was trying to give me advice since I had never seen snow and we were stationed in Germany with lots of snow and I was terrified just to drive to the airfield. Essentially that is what Jones is saying. Learn about your boat "hands on", press it hard and then never go there again, but understand her limits.

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Old 01-10-2007, 07:21   #57
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FastCat 435 Green Motion hull flying speed

We at African Cats have calculated the moment nessecary to lift a hull and with what sailing speeds that corresponds.
With Full main and Jib @ 90 degrees apparent the hull will start to fly with a wind speed of 31 knots , the speed of the cat under 50 % load is than 28.9 knots. Fortunately I have not tried that out yet. The maximum speed achieved so far was 26.9 knots in no wave conditions anfd the wind speed was 28 knots .
The windward hull was sitting high , a angle of 8 degrees and the leeward hull started dipping into the water , the bow was about 10 inches above the water surface so we had lots of spray.
I have attached a picture taken when moving at 24.8 knots and a polar diagram at light weight conditions

Greetings

Gideon Goudsmit
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Old 01-10-2007, 11:00   #58
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I would think the point of no return on a cruising cat would have a lot to do with hull shape and bridgedeck design. I've spent a lot of time on a Hobie 16 on one hull, and there is a point of stability to it. But what's happening at that point is that you have essentially turned a cat into a mono, allowing the sail to unload itself through a combination of forward motion (speed) and heeling. The beachcat's hull shape, light weight, and lack of bridgedeck structure allow a predictable transition from the drag of two hulls to the drag of one. Besides the drag, there is also the matter of adjusting fore/aft balance to avoid pitchpole. On a cruiser, I would expect that transition from the drag of two hulls to the drag of one to be unpredictable and perhaps tragic. Having the full weight of the boat expressed through a single hull's draft is surely going to result in a change in drag coefficients, and if some of the bridgedeck structure starts to get into the water, things are going to slow down, perhaps suddenly. When the increased drag prevents the sail from unloading with forward motion, then it can only unload via heeling, and you'd better be quick on the sheets and tiller or she's going over. Given the difficulty of adjusting fore/aft balance quickly on a cruiser, it's a 50/50 bet in my mind as to whether she goes over directly sideways, or digs a bow into a wave and goes over at a 45 degree angle, sort of a half-pitchpole. I've flipped Hobies every way they can be flipped, even backwards, but I can't even imagine what that would be like on a ten-ton boat!
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Old 01-10-2007, 11:23   #59
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SSpd - Formula Clarification

So just to clarify this: The formula is used to compare boats, not to give a "real world windward hull lifting number". The formula does Not include hull design, dagger or fixed keel, or wave action just to mention a few important variables. We encourage users to read the Technical section of the MDI website to fully understand how the formulas are designed and how they should be intepreted. We developed the Technical section because we find there is a lot of confusion over terms and formulas and how they should be interpreted, what they mean and finally how they can be applied.
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Old 01-10-2007, 17:15   #60
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So just to clarify this: The formula is used to compare boats, not to give a "real world windward hull lifting number". The formula does Not include hull design, dagger or fixed keel, or wave action just to mention a few important variables. We encourage users to read the Technical section of the MDI website to fully understand how the formulas are designed and how they should be intepreted. We developed the Technical section because we find there is a lot of confusion over terms and formulas and how they should be interpreted, what they mean and finally how they can be applied.
I knew that...........
Thank you very much sir.
I appreciate your taking the time to make this clear to us.
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