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Old 30-06-2019, 06:48   #1
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Sailing cat speed under engine power

Is a standard sailing cat, uender engine power, subject to the same hull spe limitations as a monohull of similar length? Thanks
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Old 30-06-2019, 06:56   #2
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Re: Sailing cat speed under engine power

Apparently not, but i can't tell you why. If they were, they wouldn't be so much faster than monohulls. I've hesitated to apply the usual 1.34 X square root of WLL formula to them, but I've got a total of three days on cats in 74 years. I hope someone with better knowledge than mine in this area will answer, because I'd also like to understand the issue.
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Old 30-06-2019, 07:03   #3
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Sailing cat speed under engine power

Not usually, if your hull is thin enough you can break the rule to some extent.
Itís my understanding that is why a Naval Destroyer is narrow beamed, for speed.
Big battle wagons can go fast I assume based on their length.

On edit, power required of course still goes up with speed, just the curve isnít as sharp with a fine hull form.
So a very fine mono would be just as fast, but apparently fast monoís go for planning instead
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Old 30-06-2019, 07:33   #4
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pirate Re: Sailing cat speed under engine power

Seeing as the standard cat has around1.3m draft x 2 I would think so to a large extent..
In real life on a Lagoon 45S at top speed with two 45hp engines the fastest was between 8.5 and 9kts.. in pretty calm water
Unless its a shallow V or a planing hull to my mind the same laws apply.
On monos I find if I sail over hull speed my bow starts nosing down.. on one boat 3ft shrank to barely 6inches and I was expecting to pitchpole at any moment.
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Old 30-06-2019, 07:58   #5
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Re: Sailing cat speed under engine power

In displacement mode,


I have read somewhere


that a cat can make 2.0 of sqrt lwl


or there about. Now since sqrt 2 is 1.4241 (and plain banana boats can make about 1.4 of sqrt lwl) then the cat could be faster by roughly 1.5 or so. Now ask if you have this engine hp to attain the upper limi.


In other words, faster (is a cat).


By comparison local mono ferries go at 14 kts and local cat ferries go at 35+ kts. Big heavy displacement boats. Again, the cats are faster.



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Old 30-06-2019, 08:31   #6
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Re: Sailing cat speed under engine power

Yes and no.

The 1.3 at the beginning of the formula is related to wave length formulas. But if you look a the bow and stern wakes, they don't come out perpendicular to the direction of travel. If you look from above, they angle back. So the effective wave length is longer than the boat.

For monohulls with a 3:1 to 4:1 length to beam ratio, that angle pretty close to the same and there isn't a big difference.

For multihulls where 8:1 to 12:1 is pretty common and sportier models can go much higher, the waves tend to angle back at a sharper angle resulting in a longer wave length and thus higher theoretical hull speeds.

Complicating matters is other factors come into play.
- With a traditional monohull running at 6-7kts, wave making resistance is typically the vast majority of drag limiting top speed. With (particularly a sporty cat), skin friction often limits speed before wave making tops out.
- For less sporty cats, wave interference between the hulls can limit speeds as the wakes interfere with each other and can hit the bridge deck.

So for sporty cats, there is a hull speed but it may not be the limiting factor on speed.
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Old 30-06-2019, 08:46   #7
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Re: Sailing cat speed under engine power

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Originally Posted by boatman61 View Post
In real life on a Lagoon 45S at top speed with two 45hp engines the fastest was between 8.5 and 9kts.. in pretty calm water.
If this is the case, a Lagoon being a fairly standard cat, then the same limitations effectively apply.
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Old 30-06-2019, 09:25   #8
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Sailing cat speed under engine power

Look at how fast a racing sculling shell is. Itís way past hull speed, but nearly no wake.
I believe there is even a video of an eight man team pulling a water skier for a short distance. That has to be maybe 15 kts?
Found it
https://youtu.be/_dLFs4cUd2o
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Old 30-06-2019, 09:31   #9
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Re: Sailing cat speed under engine power

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Originally Posted by Srah 1953 View Post
If this is the case, a Lagoon being a fairly standard cat, then the same limitations effectively apply.
With all due respect, Lagoons, while very popular are not the standard cat. They're very much toward the heavier, beamier hulled end of the range.

Tank testing done years ago by (I think) Edmund Bruce established that at LWL:BWL ratios around 8:1 and higher, the hull speed limitation due to the bow wavemaking drag began to disappear.

Most cat's have ratios 10:1 or higher.

Many cruising cats can sail much faster than they can motor, whereas with most cruising mono's there's not so much difference between how fast they can sail or motor, due to the rapid rise of wavemaking drag as hull speed is approached.

With cat's the limitation on motoring speed is essentially the power of the engines fitted, rather than waterline length.
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Old 30-06-2019, 09:37   #10
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Re: Sailing cat speed under engine power

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Originally Posted by valhalla360 View Post
Yes and no.

The 1.3 at the beginning of the formula is related to wave length formulas. But if you look a the bow and stern wakes, they don't come out perpendicular to the direction of travel. If you look from above, they angle back. So the effective wave length is longer than the boat.
Interesting observstion, but.. actually the effective wave length should be SHORTER in proportion to sin of the half of the angle between the left/right waves at the bow.
Moreover, as in this picture the wave progresses at the angle to the boat speed this, too, should have been considered.

I only wonder if the 1.34 coeff takes all the above into account (I think it should)
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Old 30-06-2019, 10:17   #11
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Re: Sailing cat speed under engine power

On monos I find if I sail over hull speed my bow starts nosing down.. on one boat 3ft shrank to barely 6inches and I was expecting to pitchpole at any moment.[/QUOTE]

Wow! That raises some interesting questions about center of thrust versus center of drag, given that most hulls nose up as they try to climb the bow wave after the wave length exceeds the waterline length of the boat, ergo, "plowing."
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Old 30-06-2019, 10:19   #12
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Re: Sailing cat speed under engine power

The 1.34 coefficient takes into account that, plus conversion to knots from LWL in feet and speed in knots. It's an engineer's coefficient.
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Old 30-06-2019, 10:27   #13
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Re: Sailing cat speed under engine power

To significantly exceed theoretical hull speed, beamy hulls must plane. Getting a heavy boat up on plane takes huge amounts of power. Narrow hulls can greatly exceed hull speed without planing.

https://youtu.be/yrCh8NKMXyo

A power cat at more than double theoretical hull speed, still clearly not planing. And generating virtually no bow waves.
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Old 30-06-2019, 10:27   #14
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Sailing cat speed under engine power

Quote:
Originally Posted by tkeithlu View Post
On monos I find if I sail over hull speed my bow starts nosing down.. on one boat 3ft shrank to barely 6inches and I was expecting to pitchpole at any moment.


Quote:
Wow! That raises some interesting questions about center of thrust versus center of drag, given that most hulls nose up as they try to climb the bow wave after the wave length exceeds the waterline length of the boat, ergo, "plowing."


I thought the bow downward was due to the sails center of pressure being way above the hulls center of drag, effectively pushing her bow down.
You donít see that with a power boat of course.
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Old 30-06-2019, 10:36   #15
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Re: Sailing cat speed under engine power

Theoretical hull speed on my FP is about 9 kt. With the fixed props that were on the boat when I bought it, I could do 10.5 kt at 3200 rpm. Feathering props are not as fast.
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