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Old 18 August 2004, 13:37   #1
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design q's about a cat rib with water jets...

Hi looking to push the evenvelope in rib design.

Thinking about a 10-12 metre rib made out of aluminum,
looking at a cataraman design to aid shallow access.
looking at a pair of yanmar 375's connected to water jets
am i mad what combinations have you tried. whats the view on water jets.
need to seat to 12-16.
any info very welcome.
possibly a semi cabin.....

thanks in advance
James
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Old 18 August 2004, 15:23   #2
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James,

I'm no expert, but I am very interested in powerboat and RIB design. What I'm about to say, you probably know anyway but I'll say it anyway!

Water jets I believe are not as efficient as propellers but do have some very significant advantages. Obviously safety and shallow water capabilities but also turning circle and stopping distances are much lower than with a prop. However, if your engine cuts out at full wack then you loose your steering with a jet, for obvious reasons.

With a catamaran design you will obviously get loads of deck space, but will you see much of an improvement with regards to shallow water access (i assume that is what you mean by "shallow access")?

I also believe acatamaran boats can be faster than monohulls because they tend to act like a wing creating lift and reducing the wetted hull area (therefore less skin friction). A zapcat (and I would assume a larger Cat) works by a principle of air being forced between the deck and the water creating high pressure which lifts the boat out of the water.

Grip in the corners would probably be superior to a monohull too, but again I'm no expert. I think Adam Younger would be the person to talk to on this forum.

I will be watching this thread with interest, as I would like to know what other people think and if what I have written is true!

Good luck with your project.

Cheers

Tim

PS aluminium, light, strong and expensive?
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Old 18 August 2004, 16:11   #3
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"Grip in the corners would probably be superior to a monohull too, but again I'm no expert. I think Adam Younger would be the person to talk to on this forum."

You are right on everything you said bar this one point!!!! Cats do not turn as well as a monohull - nothing under the water!

Would certainly make an interesting RIB BUT would it really be a RIB or a boat with rubber fenders - surface effect etc means it will fly high and the tubes will really be well clear of the water!
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Old 18 August 2004, 19:25   #4
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Codprawn,

I thought I may well have been talking crap when I said that. I just have visions of Zapcats whipping round corners at stupid speeds as if they are on rails. I guess they are a different kettle of fish being so light and small.

Clappers, I would imagine that you would have no problem at all seating 12-16 in a normal 10-12m rib, let alone a Cat. I agree with Codprawn saying about it possibly being a boat with fenders as opposed to a RIB. I also reckon a monohull would give a smoother ride when the conditions get really rough.

Cheers

Tim
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Old 18 August 2004, 19:49   #5
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James, not sure I would want to be first to try this as your not going to get much change out of 150k. Perhaps the people to talk to are Ocean Dynamics in Pembrokeshire, they have a huge experience of Al hulls and jets.


Pete
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Old 19 August 2004, 05:08   #6
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Quote:
Originally Posted by codprawn
You are right on everything you said bar this one point!!!! Cats do not turn as well as a monohull - nothing under the water!
Not true! Cats are capable of turning way quicker than a mono, obviously it has to be of a design intended for fast cornering, but assuming it is, there is no comparison. Circuit racing cats and indeed Zapcats are a perfect example of this, F1 circuit boats which are of course at the extreme end of cornering design, pull many 'G's when turning.
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Old 19 August 2004, 05:38   #7
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Aha! I always said I knew exactly what I was talking about !
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Old 19 August 2004, 06:45   #8
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Quote:
Originally Posted by Jonny Fuller
Not true! Cats are capable of turning way quicker than a mono, obviously it has to be of a design intended for fast cornering, but assuming it is, there is no comarison. Circuit racing cats and indeed Zapcats are a perfect example of this, F1 circuit boats which are of course at the extreme end of cornering design, pull many 'G's when turning.
You obviously haven't watched much class I offshore powerboat racing - the cats are way faster in a straight line in CALM waters but the monohulls ALWAYS made up for it on the turns - you just can't carve in a cat like you can in a mono!
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Old 19 August 2004, 07:07   #9
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Is that so! I guess I should do some homework.

You're obviously a racing expert, what is your backgound Mr Cod?
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Old 19 August 2004, 07:42   #10
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Of the Cat hulled Power boats I've used they corner on rails until they let go and start skipping side ways!!
That can be interesting could also be due to the hull design.
Jelly
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