Flying A Hull on a Cruising CAT!!!

[Keegan]

Here is what Phil Berman (broker for Catana, Dolphin and Dean Catamarans said on the matter:

Guys,

It really takes a monumental act of bone-headedness to capsize a modern production cat in any winds under 70 knots. (Over 70, all bets are off, because the wind could catch your underbelly and dump you over. That said, you would be running under bare poles with a drogue, or hove too with a sea anchor, which does reduce this danger.)

Finally, if you think it is time to reef, you should have reefed long ago too. Cats never sail any better if they are pressed too hard, especially upwind.

Finally, if the boat flies a hull, drive off, not up. Most capsizes in the racing cats occur when the skippers head up in a puff, not off. It is typically monohull guys who do not understand how fast the apparent wind swings aft.

Best,

Phillip Berman, President
Racing, Cruising, and Brokering Catamarans Since 1969
BA (University of California), MTS, (Harvard University)

[Keegan]

Originally Posted by fastcat435

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.
Gideon Goudsmit

Nice, Gideon !! That is info an owner can use. That is the kind of info I was hoping to understand about the Dolphin Cat.

In the end , I will be reefing soon enough, but it is nice to know your limits. It is like owning a Porche, everytime you get in it you dont drive it to its full speed but you do like to know what it can do, if needed

Keegan

[Keegan]

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[fastcat435]

Originally Posted by Keegan

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Nice Picture Keegan I have had my St Francis 48 hull out like that doing 23 knots but will not ever try to repeat that.
Please all of you keep in mind that waves combined with strong winds can tip your cat ( Or monohull ) Try never to come even close to the limits , the od thing is: if you do, the speed of your cat will not increase but decrease because the other hull starts digging in and it is one way for me if I ever did that with my wife on board never to see her on my boat again
She is brave but no fool.

Greetings

Gideon

[David M]

Speaking of flying a hull, check out this months cover of Latitude 38 (scroll down a bit).

Latitude 38 - The West's Premier Sailing & Marine Magazine

Edit: Hah...just saw you beat me to it! Isn't that a great photo? One of the best I have ever seen on the cover of Latitude.

[Therapy]

Originally Posted by Keegan

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It appears as though the person in red is rigging their trapeze.

[smm]

FWIW, it appears that Shuttleworth's formula is what Wharram calls "dynamic stability" which is 0.6 * static stability (no explanation for the constant).

Definitionally, I believe that static stability means that the boat would start to lift a hull if sitting on the hard. Obviously, in the water the boat would slip sideways to some degree and convert some of that energy into forward velocity. OTOH, this doesn't take into account the considerable additional force due to the wing-like nature of the sail, apparent wind, different underwater appendages, waves, etc.

Endless Summer (Farrier F41, now sold, alas) has a static stability of 36 knots. As a practical matter, reefing around 15 knots of true wind never seemed to slow us down and gave us a huge safety margin.

When looking at catamaran performance, I find it interesting to compare boat speeds at 50% of the static stability limit: ie. how fast do you go given the same basic level of capsize risk. As a cruiser, that's probably where you're going to spend most of your time.

[Calculator]

If you were to substitute the term "steady state" for "static" and "gusting" for "dynamic", the Kelsall and Shuttleworth/Wharram stability formulas would make more sense. Kelsall gives the formula 13.8578((Disp. X Distance between hull centerlines/2)/(SA X Hce))^0.5 for the hull lifting speed (knots) in steady air. Shuttleworth gives the formula 8.23805((Disp. X Distance between hull centerlines/2)/(SA X Hce))^0.5 for the safe wind speed(knots), indicating that it contains a 40% gust factor. If you divide 8.23805 by 13.8578 , you get 0.6. So they are both using the same formula, one for a reefing wind speed and the other for a hull-lifting (impending-capsize) speed.

With the specifications we have for the Farrier 41, the reefing wind speed is 21.03 knots and the hull-lifting speed is 35.38 knots.

Cal Markwood
Engineering Analyst
Multihull Dynamics, Inc.

[ssullivan]

What is the term "Hce" again? This term I do not know, but would like to calculate these values for my cat.

Originally Posted by Calculator

If you were to substitute the term "steady state" for "static" and "gusting" for "dynamic", the Kelsall and Shuttleworth/Wharram stability formulas would make more sense. Kelsall gives the formula 13.8578((Disp. X Distance between hull centerlines/2)/(SA X Hce))^0.5 for the hull lifting speed (knots) in steady air. Shuttleworth gives the formula 8.23805((Disp. X Distance between hull centerlines/2)/(SA X Hce))^0.5 for the safe wind speed(knots), indicating that it contains a 40% gust factor. If you divide 8.23805 by 13.8578 , you get 0.6. So they are both using the same formula, one for a reefing wind speed and the other for a hull-lifting (impending-capsize) speed.

With the specifications we have for the Farrier 41, the reefing wind speed is 21.03 knots and the hull-lifting speed is 35.38 knots.

Cal Markwood
Engineering Analyst
Multihull Dynamics, Inc.

[Calculator]

Sorry! Hce is the height of the sail center of effort above the center of lateral resistance of the hulls and appendages. On the MDI website, we approximate it with the formula Hce = (2/3)*(SA/LOA)+0.25*LOA. SA is sail area, LOA is length overall.

Cal

[ssullivan]

Cool! My hull lifting wind speed is 34.5 knots.

However, I think given the hull geometry of a Catalac, it wouldn't matter much if you did lift the hull because at that force, you'd be going sideways at 10 knots anyway! ha ha

I have nothing that grips the water. I just go sideways in those kinds of conditions, I think. If the hull lifts out, the boat would heel, spill the sail area term out of the equation, skid sideways through the water and self-correct.

BTW: The equation is misleading. There is no term in it for the ability of a hull to skip sideways. In the extreme case, with completely flat bottoms to the hulls on a cat and no daggerboards, your hull wouldn't lift no matter how much wind you put in those sails... it would just skip over the water sideways.

So why is this equation meaningful? It doesn't seem to take the real world into account.

Interestingly, I have been out in 20kts (approx) on numerous occasions recently. I have been to 7 knots hull speed and was praying for *more* wind! I didn't reef, or even think about it for more than a second or two.

[Therapy]

Originally Posted by ssullivan

Cool! My hull lifting wind speed is 34.5 knots.

However, I think given the hull geometry of a Catalac, it wouldn't matter much if you did lift the hull because at that force, you'd be going sideways at 10 knots anyway! ha ha

I have nothing that grips the water. I just go sideways in those kinds of conditions, I think. If the hull lifts out, the boat would heel, spill the sail area term out of the equation, skid sideways through the water and self-correct.

BTW: The equation is misleading. There is no term in it for the ability of a hull to skip sideways. In the extreme case, with completely flat bottoms to the hulls on a cat and no daggerboards, your hull wouldn't lift no matter how much wind you put in those sails... it would just skip over the water sideways.

So why is this equation meaningful? It doesn't seem to take the real world into account.

Interestingly, I have been out in 20kts (approx) on numerous occasions recently. I have been to 7 knots hull speed and was praying for *more* wind! I didn't reef, or even think about it for more than a second or two.

I think the number is for a static cat. like one said before, like if it is on the hard.

If you slip sideways at 10 kts then your static remains the same - 34.5 but you will lift a hull at 44.5 assuming no other motion.

This just seems logical to me.

And not thinking of reefing in a cat is how I read they get in trouble........but........I know you have a really heavy one with a moderate sail plan and so is one of the safest in the world.

[ssullivan]

Originally Posted by Therapy

I think the number is for a static cat. like one said before, like if it is on the hard.

If you slip sideways at 10 kts then your static remains the same - 34.5 but you will lift a hull at 44.5 assuming no other motion.

This just seems logical to me.

And not thinking of reefing in a cat is how I read they get in trouble........but........I know you have a really heavy one with a moderate sail plan and so is one of the safest in the world.

Oh, right. I missed that post where it mentioned the equation was like a cat on the hard.

One factor though, is that when the hull does lift, the SA starts decreasing immediatly, just like a monohull.

To be clear, I should have said that in 20 knots wind (true wind) I didn't even *consider* reefing because I felt the boat was finally going a respectable speed. It didn't feel "sketchy" at all, and was moving along very nicely. It liked 20 knots a lot.

[Calculator]

I think you are understating the sailing characteristics of the Catalacs. They are deep V hulls, and many catamaran designers around the time of the Catalacs were using them without boards or keels. And they are capable of relatively good windward performance. I guess if you get them heeled over far enough, they would lose their grip and skid sideways, but I doubt it. That would take about 45 degrees.

The nominal SA/D for catamarans in the 25-35 foot Lwl range on the MDI website is about 25. The Catalacs run in the upper teens. So your boat probably has such a high stability speed because it is very conservatively rigged.

Cal

[ssullivan]

Very impressive, Cal.

What you write is the gospel truth about the Catalac.

I was imagining a 45 degree heel angle to skid this boat also, given the angles of the deep V. The boat sails just as well to windward as many cruising monos, so long as you use the 110% and not a huge Genoa.

The boat is 34 feet long (the 10M) and Catalacs do indeed have a conservative sailplan - resulting in the high wind speed I found using your equation.

I do have to break out the spinnaker in lighter winds to get her moving.

Good interpretation of this boat. I'm quite impressed.

Originally Posted by Calculator

I think you are understating the sailing characteristics of the Catalacs. They are deep V hulls, and many catamaran designers around the time of the Catalacs were using them without boards or keels. And they are capable of relatively good windward performance. I guess if you get them heeled over far enough, they would lose their grip and skid sideways, but I doubt it. That would take about 45 degrees.

The nominal SA/D for catamarans in the 25-35 foot Lwl range on the MDI website is about 25. The Catalacs run in the upper teens. So your boat probably has such a high stability speed because it is very conservatively rigged.

Cal