Do cats fly?

whiteoaks7

whiteoaks7

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www.seasolutions.co.uk
Any number of cat owners claim amazing speeds for their boats, far in excess of what a humble monohull of the same length could achieve. Is this exaggeration or do the laws of physics not apply to cats? I always thought the hull speed laws would apply equally to a cat (I'm not talking about the flying boats of Americas Cup fame btw) which might be informative of either owners or their instruments :-)
 
Hull speed figures don't apply to narrow hulls i.e. greater than 8:1 LWL:beam. That's because they don't create significant bow/stern waves. Not that multis like the America's cup machines do not plane but go through the water far in excess of 1.4 x sqrt LWL. To achieve similarly high speeds things like open 60s have to plane.

That said, most of the claims are owners who see a high figure occasionally then delude themselves and others that the figure is typical. Monohull sailors do it too. I have seen 17 knots on the clock but I still plan passages on the basis of 7-7.5 knots. If the claims we hear regularly on this forum were realistic I would expect to see AWBs leaving me in their wake every time I go out.
 
Pretty sure that should be 16:1 beams to the length waterline...otherwise, lots of liners would do 50 or 60 knots...:)
 
dancrane said:
Pretty sure that should be 16:1 beams to the length waterline...otherwise, lots of liners would do 50 or 60 knots...:)
Click to expand...

No, I'll stick with 8:1. Most liners are straight sided with short tapered sections at bow and stern so despite the ratio, they do make waves. In any case with slender vessels you need more than just a fine hull, you need the horse power to push it along. The difference is that with a fine hull you keep applying more power and you get more speed (within reason of course) while a tubby hull's resistance rises exponentially above theoretical hull speed.

Some cats, e.g. the designs of Bill O'Brien and Tom Lack have hull forms that make waves and they are indeed limited to the hull speed formula.
 
snowleopard said:
No, I'll stick with 8:1. Most liners are straight sided with short tapered sections at bow and stern so despite the ratio, they do make waves. In any case with slender vessels you need more than just a fine hull, you need the horse power to push it along. The difference is that with a fine hull you keep applying more power and you get more speed (within reason of course) while a tubby hull's resistance rises exponentially above theoretical hull speed.
Click to expand...

If that's so, does it explain why warships tend to have "undisclosed" top speeds? I assumed they couldn't go as fast as they would have, if they'd only been half as narrow, because they'd be impractical, like giant torpedos...but eight to one is possible...

(Pic in next post, I hope)

This one in Portsmouth Harbour looks pretty damn sleek...but I never heard of an RN ship able to go faster than 35 knots.
 
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We have a 40 foot, newish cruising catamaran. And i reckon that on average, theres not much in it.

On a beam to broad reach I think we sail a bit faster than a similar length monohull but on a close reach (and not very close) we are left behind by a similar size monohull. We also lose ground due to leeway when sailing close to the wind.

But then again, we have a leisurely cruising catamaran. Ask the same of a more racier catamaran owner and you may get a different answer.

Cheers

Garold
 
dancrane said:
If that's so, does it explain why warships tend to have "undisclosed" top speeds? I assumed they couldn't go as fast as they would have, if they'd only been half as narrow, because they'd be impractical, like giant torpedos...but eight to one is possible...

(Pic in next post, I hope)

This one in Portsmouth Harbour looks pretty damn sleek...but I never heard of an RN ship able to go faster than 35 knots.
Click to expand...

The fastest conventional rn warships I think we're the abdeil class minelayers of ww2. They were circa 450 feet long and 3700 tons, and could do 40 knots.... But they had a huge amount of power.. Almost as much as the county class cruisers.

But turbinia was able to do 35 knots... And that was 1894.... And she was only
Just over 100 feet long....
 
I have for years believed (only because I read it - sorry, I forget where) that a displacement hull which is sixteen times longer than it is broad, is finally slender enough not to churn up a wave that will inhibit its pace...

...obviously there are exceptions...but wasn't Turbinia semi-displacement? Looks that way to me:

300px-Turbinia_At_Speed.jpg


As I understood it (right or wrong), if a vessel is not built to plane, it doesn't matter how much power she has, she'll be unable to move faster than her "hull speed", 1.4 x the square root of her LWL...such that the nuclear-powered Nimitz would be limited to 45 knots...

...I presumed the only reasons we don't see nuclear aircraft-carrying fast-cats, 700ft long and able to launch planes by steering into the wind at 120knots, was cost and doubts about safety!

Re the length/beam question: isn't it surprising that the SS United States, built when cold-war competitiveness might easily have justified taking any risk, (particularly as she was intended as an express part-time troop-carrier) wasn't provided with nuclear power?

At more than nine times longer than she was wide, atomic power would have put the Blue Riband in her bag forever, if indeed hull speed is overcome at 8:1. As it was, she didn't go much over 40 knots, ever...which is approximately 1.4 x the square root of her LWL...
 
" Do Cats Fly ? "

Well as we all know cats land on their feet and toast lands butter side down, so it's obvious a cat with buttered toast strapped to its' back will hover while rotating.

Wrap wires around it and Hey Presto, a DIY dynamo !

C. Vizz magazine.
 
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In terms of 'big' ships there are of course the high speed cat ferries and an experimental craft in the USN (according to Wiki which, if I'd thought to read that first, would have saved you all some time educating me). Anyway, I can now appear knowledgeable next time the debate starts in the pub so thank you all.
 
Off the wind my cat which is very much a heavy and comfortable cruising cat is a shade faster than similar waterline length monohulls (25ft) but close to the wind I am slower by a fair bit. I think the confusion is that many sailors do not deduct the rate of tidal flow from the speed figures they are seeing and momentary acceleration down a wave. I have seen 18 knots downwind on the GPS but if I deduct about the 6 knots of tide at that time and a little momentary surge on a wave that was probably about 10 knots true maximum speed. Still a fair bit more than 1.4 x root waterline length which would give me 7 knots hull speed.
 
dancrane said:
This one in Portsmouth Harbour looks pretty damn sleek...but I never heard of an RN ship able to go faster than 35 knots.
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I had a session on Brave Borderer while on a minesweeping course a full fifty years ago. I distinctly remember see the Chernikeef system showing well in excess of 45 knots AND we were making smoke at the time !
 
I too reckon we go about the same speed as a similar sized monhull most of the time, and as you say, maybe a knot more on a broad reach and a knot less close hauled. However...... When the conditions are just right (flat sea and F6 on a broad reach and I have the bottle to fly an asymmetric!) then we can see a much better sustained speed. I have had a twelve knot average on a six hour passage once in Turkey and over ten knots on a few other occasion, does not happen very often though :-(

The close hauled performance of a smilax length cruising monohull is not actually significantly better in terms of VMG, we just need to come a lot further of the wind.

The one area that a monohull does better than us is motoring into wind with a choppy sea. We don't have the weight to carry the motion through waves so our speed will drop to a couple of knots when a wave knocks us back and then accelerate till it happens again...
 
caalac8

catalac08 said:
Off the wind my cat which is very much a heavy and comfortable cruising cat is a shade faster than similar waterline length monohulls (25ft) but close to the wind I am slower by a fair bit. I think the confusion is that many sailors do not deduct the rate of tidal flow from the speed figures they are seeing and momentary acceleration down a wave. I have seen 18 knots downwind on the GPS but if I deduct about the 6 knots of tide at that time and a little momentary surge on a wave that was probably about 10 knots true maximum speed. Still a fair bit more than 1.4 x root waterline length which would give me 7 knots hull speed.
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I would agree with that
On narrow beam cats the inner bow waves of the hulls cross under the bridge deck about 2 thirds back from the bow and in the right conditions lift the stern up , on a reach with the right sails up we often get 9/10 knots and we have seen 12. AFter allowing for tide. edit Gps speed 16 to 18 Knots but with 5/6knots of tide ... upwind we are slow tho !
 
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zzyyxx said:
Planing though, hull speed doesn't apply....
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No, but it's fun though. I spent a week in Brave Swordsman.......lunch in Gosport, tea and refuelling in Falmouth. I seem to remember max speed was about 55, speed at max continuous power just under 50.
 
In Ypres / Leper they hold a 3 yearly festival called Kattenstoat, when they throw cats from the church tower to prove that they aren't supernatural.
Those cats do fly, for a while.
Apparently they don't use real cats anymore, shame.:)
 
Displacement speed is generally approximated to 1.4 x etc.

Planing is generally 3 x etc.

In between is semi-displacement. And very interesting.

Foil-borne is another branch.

A foiler is a displacement hull with added oomph. As are the semi and planing.
 
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