SD Power Cats at Sea

[Gludy]

Please do not read anything into this .... and here is hoping for a peaceful thread:-

I have been discussing how SD power cats behave in heavy seas.

Say you have an SD power cat with say a beam of 24 feet and a length of 60 feet.

Imagine that is a sea with say 15 foot waves - does the cat behave better than a mono SD hull in those circumstances or because it can have each hull in a very different part of the wave does it behave worse?

I am not asking because I am buying one or anything like that. I am asking because I do not know the answer and wondered if anyone else did?

I do not think its possible for this thread to become one of those Gludy style threads ... mind you they all start off innocent ... don't they? /forums/images/graemlins/smile.gif

 

[goochie]

Over to you Bajansailor!

 

[SpiderDiver]

I think Southampton Uni. did some tank tests Paul comparing cat and mono hulls. Conclusion was that cat hulls can take much bigger breaking waves than mono's.

Think this link refers to this;

http://www.powercatamaranyachts.com/no_body_knows.htm

Hope this helps - I must say having been out in some marginal conditions with our power cat I have always felt reassured by this.

 

[DAKA]

I too have pondered on this many a time, there could also be a situation where one is stuck on the crest of a wave, drifting sideways with the wave @ 20 knots /forums/images/graemlins/shocked.gif

 

[Nautical]

If you were going head on then there should be little difference, perhaps a bit more 'slap' between the hulls if its really rock and roll. The problem I see is that beam on in a short sharp sea is the frequency of motion is more violent than on a mono hull as you have two seperate hulls in different parts of the wave perhaps one riding the crest and one half way up the following if that makes sense, mind you with a beam of only 24' they would have to have a high frequency between crests, neverthe less you will have two hulls acting against each other torsionally and therefore the motion will be more violent. Much like the local seacats in the Irish sea even though they drop their speed to low 20's on par with the mono ferries they always appear at least to be more violent in motion and more difficult to walk around the decks.

 

[Bajansailor]

I like Daka's illustration above - it just needs a suitable Peyton type caption now.....!

I think that displacement and semi-displacement power cats generally tend to have very good directional stability - we certainly found this with the cat in my avatar, who is 49' x 16'.
She will track along happily in a straight line with just one engine running - I was surprised, was sure she would want to go round in circles..... and if you point her down the face of a wave she will go down it like a train on rails, with no tendency to broach.
However if you happen to go down a wave at an angle, and then try to correct, she takes a long time to turn, because she has such good directional stability - this can be a bit disconcerting.....

The general consensus seems to be that planing power cats (SD as well?) are brilliant in rough seas, even when punching into them, so long as you keep the throttles down and the speed up, as then they are sort of floating on a slightly pressurised cushion of air between the hulls.

 

[MapisM]

Wave twisting?

Now that's scary! In offroad terms, that's a classical twist situation.
Considering that quite often, once you're stuck up there, you have no other choice but to have someone else towing you out of it, I wonder how to deal with that at sea, unless there's a tug around...!
/forums/images/graemlins/laugh.gif

 

[Divemaster1]

But surely, that is an impossibility .... a wave, unless it is breaking, is virtually stationary ... isn't it ?? surely it is the energy that moves through the water and the surface water does not move.... until the wave frequency changes and it cannot build any higher without breaking ...

 

[Nick_H]

Hmm, i'm scared to say this on a Gludy thread, but I don't think there is a cushion of air between the hulls when they are open at each end, unless you were travelling at very high speed (like F1 powerboat type speeds)

Back to the question, I would expect a stabilised mono to be more comfortable that a cat, but an unstabilised to be less comfortable?

 

[DAKA]

The bigger the wave the faster they travel.

20-25 knots is about as big as I want to be out in.


I once tried to cross a tss by compass course in a heavy beam sea, in 10 miles I was pushed off course by 6 miles.

The troughs were peaceful and the only time we felt able to make headway.










(Two of us both with experienced crew crossed from Aberville to Varne from south to north, missed Varne and Dover ! )

 

[Divemaster1]

Quote MBY Feb 2008, Page 142

"Does that mean the water is Moving?
The energy moves through the water, which remains pretty much in the same place until the wave breaks. Just as well as it would be almost impossible for boats to leave the dock at all if they had to constantly battle against a moving wall of water."

Suggest that you were carried sideways by tide and breaking waves...

 

[DAKA]

The tide was in the opposite direction helping to keep us on course.

I understand your/MBY principal , all the crests were indeed breaking and worse because of the TSS they were breaking on the Port bow which knocked us further off course.


Had we been in a cat my thoughts are each and every wave crest would hit the inside cat hull and surf it sideways, perhaps 50m each wave.

The props would clear the water on a regular basis and also both rudders would be dangling as the crest was surfing us ?????????????????????????????????

I am not certain about this, just my thoughts.

 

[hobiecat]

Can you say all SD monos perform the same in heavy weather? Same goes for SD power catamarans.

Very generally in my experience a SD power cat performs much better going directly into heavy seas compared to a mono - due to a cushioning effect of air between the hulls. A catamaran can be relatively comfortable going quickly into heavy seas. Wider beamed sailing cats cannot compress air so easily and may slam. Downwind in heavy seas both monos and cats are equally hard to control down breaking waves - though they both track well. Beam seas are naturally uncomfortable for both.

Overall, in my opinion a catamaran in heavy seas is relatively more comfortable than a mono. (I stress though - neither is comfortable so it is a relative comparison). I understand tank tests show a power catamaran to be safer.

 

[Gludy]

All good stuff so far - I still do not know enough to form a firm opinion but these are my thoughts at this point.

1. I cannot see how the cushion of air forms - this maybe a myth. At high speeds fair enough there would be a build up of pressure between the hulls but this cannot be the case in hand - slow speed heavy weather.

2. I am confused - I accept high directional stability but how does the square with super easy agility in the marina because the engines are so far from the centre line of the boat? How can you have both?

3. I am still not convinced cats are better in heavy seas - can anyone relate an experience?

 

[Divemaster1]

As a thread drift ... this does not look too bad for a 54 ft Cat..

http://www.yachtworld.com/boats/798684/0/

 

[Gludy]

That is pretty pricey for such a boat but a good example of cat performance.

With a Cat you can swop 2 1000hp engines in the same volume mono hull boat for two 450 hp and go cheaper and faster than with the larger engines.

However I am currently of the thinking that the boat does have to have the hulls wide apart to get all the benefits.

 

[DoubleVision]

fwiw Paul here at Eastbourne all the fishermen run SD cats of 30 to 50 ft. Its not a particularly nice stretch of coast and very exposed to a SW hooly. They seem to swear by them as stable fishing platforms and of course much more working and storage area on the deck. /forums/images/graemlins/smirk.gif

 

[Nautical]

Here's a quite nice looking one, not a cat fan myself but this is one of the better ones i ve seen, somehow a boat with two sharp ends just doesn't do it for me.

 

[Deleted User YDKXO]

I don't believe there's a 'cushion' of air between the hulls either but I guess it might be possible to design the underside of the deck to provide lift at speed and hence increase speed. I wonder also whether the deck can provide wing in ground effect also helping to increase speed. Need an expert here

 

[DAKA]

no expert but IMHO

the narrow hulls are perfect for the least water resistance, two separate hulls running in parallel will not give an increased performance ( mentioned earlier) as they will not be conjoined and offer no stability.


The air cushion is not sufficient to make any difference ( at cruiser level) however when a normal hull hits a wave the whole hull is submerged by the wave and braked, needing power to climb clear and plane free again.

when the narrow hulls of a cat submerge the main flat conjoining hull slaps the wave for support, skims the surface and breaks free will minimal wave breaking effect.

If a normal boat had a flat bottom it would slap around with no steerage, but the cats hulls maintain good steerage.