design q's about a cat rib with water jets...

[clappers]

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

[slimtim]

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?

[codprawn]

"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!

[slimtim]

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

[Pete7]

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

[Jonny Fuller]

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.

[slimtim]

Aha! I always said I knew exactly what I was talking about !

[codprawn]

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!

[Jonny Fuller]

Is that so! I guess I should do some homework.

You're obviously a racing expert, what is your backgound Mr Cod?

[Jelly]

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

[codprawn]

Originally Posted by Jonny Fuller

Is that so! I guess I should do some homework.

You're obviously a racing expert, what is your backgound Mr Cod?

No boat racing yet - only cars - but have followed offshore racing for a long time - obviously now the cats dominate but rough weather still favours monos - always remember the way the monos would make it all up on the turns only to lose out on the straights!

The cats would always turn much wider than the monos - seen it so many times.

[Jonny Fuller]

OK, if you've been 'watching it' for many years, I bow to your greater knowledge.





taxi! (for Mr codprawn)

[codprawn]

Do you race catamarans then?

[Jonny Fuller]

no

[Andy Gee]

I had a commercial Cat Rib for about 6 years and on the plus side they are extremly stable boats when working beam on and generally working in a big swell. They are also excellent when no loaded evenly i.e divers leaving off one side whilst other side is still full.
On the negative side it was a pure cow of a boat when transiting to and from the site, she would slam to the point of nearly stopping however you tried to run either with or against the swell, with no difference to trimming. Only Husycat with the Hydros have overcome this to a degree.
I have looked at a few "Hard" boat cats an the only one that I found not to slam was the Gemini. Most of the others (all about 10m) gave a very hard ride in any swell. One company, I cant remember their name, even tried to legnthen the water line by adding Bulbous bows which helped a bit.
IMHO the only cats that work are either Zap cats (very light and agile) or the big ferries, and even they are restricted in sea states that they can operate.
As for turning Jelly has it right, you can turn them on rails.....so long at theres no swell.
Andy

[Simon A]

This is a response I recieved from Hamilton jets when I was asking their opinion of jets in a 11meter rib. Hope its of interest. Lots of jet ribs in the scillies.

"I suspect that the reason behind your query is simply that there is a completely different technique between*handling boats with jets compared to props.
*
A properly set up jet boat allows a wide variation of power to be applied without necessarily altering the boat speed.* The main reason for broaching is that the steerability of a boat depends on the relative speed of the water flow across rudders, so when that speed drops, there is no effective steering and the boat is just taken by the sea. Underwater appendages ie rudders, "P" brackets etc can help to reduce this effect.
*
With*Hamilton jets, it is easy to maintain a power setting and control the ahead speed and direction simply by controlling the reverse deflector and steering. The thrust from the jets therefore*is independent of boat speed . As an example,*even a high power boat can*have the throttle open to say half power and the boat can still be held*stationary in the water. Small adjustments of the reverse and/or steering can then make the boat move in any required direction. This feature allows far greater control (and almost instantly) when operating in heavy weather.
*
As for losing drive, this can happen if you start to fly but there is an instant pick up as soon as the boat re enters the water. There is No torque reaction and the load on the engine will never go above its normal maximum. There is therefore no need to "work the throttles" just set them to a suitable level and do all the rest of the control using reverse and steering only.
*
As for users, the KNRM - the Dutch equivalent of the RNLI - use almost entirely Hamilton jet powered craft - the largest of which are almost 19M long*full wheelhouse RIBs.
*
The RNLI are currently starting a new build programme with our jets in 14M craft *- they already have*6 smaller boats in service.
*
Jets*are widely used*as rescue craft throughout the North Sea and another new series of 19M wheelhouse RIBs are just getting under way for* North Sea operation.
*
The main reason there is still some worries about jets is more to do with people trying to drive them in the same way as they would props!!

Jets have nothing underneath so it is important to be able to direct the thrust instantly so as to overcome any tendency to broach before it actually happens.
*
Strakes and keels can be used to good effect - within certain limits of course- and if placed carefully, they can actually assist with drying out as well as providing improved directional stability.*
*
There are*one or two*very good training organisations but they tend to be a long way away!* There are also a*lot of individuals who have tremendous experience running big jet RIBs in the worst possible seas and I would be pleased to put you in touch if the project progresses."

Simon

[slimtim]

I reckon Cats can corner fast as if they are on rails when is calmer water but they don't bank over like monohulls do and so being a driver or passenger it might not feel as safe and secure (would feel like you were being thrown out of the boat). I think that if you were to turn a cat very sharply, then you might well loose way too much speed, hence the Cats taking wider circles in racing? I'm not sure the time difference between a cat and a monohull taking a corner would be all that much different, because the cat goes faster, but covers more distance and the monohull goes slower but turns tighter? I could be wrong of course. I think a monohull would be quicker in the rough stuff though?

I just going by what is going on in my head (I have no experience of catamarans) but could a cat dig in too much if cornered too hard at high speed, causing a studden change in direction but dramtic loss of speed? A monohull will always bank and carve a turn whereas a cat will rail. Much like a motorbike taking a corner compared with a quad bike?

I think the thread may be wondering slightly off the topic at the moment, but as always, interesting to read everyones views. Clappers, do you have any other comments? more details of your project?

Tim

[Pete7]

Originally Posted by codprawn

Do you race catamarans then?

Codprawn the question you should have asked JF is:

"Did you ever race mono hulls offshore ?

http://www.boatmad.com/racing.htm


Pete

[Jonny Fuller]

Thank you Peter, I have also raced in 4 litre and 6 litre catamarans, and in a class 1 mono, and I can assure you, cat's go everywhere quicker than a mono, unless it's very rough indeed, there really is no comparison.

But this thread has drifted horribly off course, so lets drop the racing chat.

[Dirk Diggler]

Originally Posted by codprawn

- but have followed offshore racing for a long time - obviously now the cats dominate but rough weather still favours monos -

You need to get out more, I can't remember the last time a mono raced in class one, they're not even allowed anymore!