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Old 22 September 2006, 07:00   #1
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Jet Drive Units?

Jet Drive Units


All.

What do we think of Jet Drive Units for a RIB?

Im making long term plans to build a 7.5 8.5M Inboard Diesel RIB from a bare hull, and am currently weighing up the pros and cons of using a jet drive unit in place of a stern drive and prop.

Bearing in mind I sometimes spend a lot of time in shallow water off the IOW, but also want to cruise down to the channel Islands etc.
I see the situation as below:-

Advantages
- Shallower draft.
- Less to go wrong.
- Slightly safer to anyone in the water.
- Appeals to me in a sad I want one sort of way.


Disadvantages
- Initial expense
- Less efficient than sterndrive/prop (How much?)
- Water inlet can be susceptible to pick up rubbish or sand/mud in shallow water.
- Less adaptable (changing impellor ratio is harder/more expensive than changing prop pitch)


Id be interested in your thoughts on the above, but also the following, maybe stupid, questions.

To make room for the inlet, I presume the engine needs to be mounted a little further forward in the boat?
How does a Jet unit compare weight wise to a Sterndrive and prop?
Although Im convinced Ive been told they are less efficient at turning engine power into forward thrust, I dont know by how much?

Thanks all.


Nasher
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Old 22 September 2006, 07:31   #2
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Nasher I had the same thoughts before I started my build. I'm pleased I didn't go for a jet but with one exception. An outdrive will not lift high enough for you to let the boat dry out and if this is necessary for you the jet may be attractive.

There were various reasons for not going jet, most of which you have mentioned but the big one for me is that after leaving the water you are landing dead stick and that has got to be a hard landing.

Although you can use the jet in shallow water, a jet big enough to power an 8.5mtr is likely to lift large quantities of sand and wee stones so you probably wouldn't want to do that frequently, if at all.

There's also the problem of not having any trim adjustment. Trim tabs work but they can only adjust down.

I don't imagine straight line stability to be great given the lack of skeg although you could get over that with a couple of wee additions to the hull.
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Old 22 September 2006, 10:04   #3
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Quote:
Originally Posted by Nasher
What do we think of Jet Drive Units for a RIB?
The idea of being able to keep all the power turning in a straight line appeals to me. Sadly the only P22 jet that I had the chance to drive was a complete shambles so we never really got to play.

With your new connections a free day at next years Seawork Boat Show could be time well spent, afterall both Ocean Dynamics and Vosper Thornycroft seem to have 7 - 8m jet ribs with respectable performance. A demo trip would be an interesting experience. Do you supply them with electronics ?

Pete
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Old 22 September 2006, 10:14   #4
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JW, Pete.

As usual interesting comments from you both.

Firstly, the sucking up debris would be a big issue for me at places like Priory bay, and I hadn't thought about the total lack of power when re-entering the water after a short flight. Must be like hitting a brick wall.

Mind you, I didn't realise a stern drive couldn't be lifted far enough to beach the boat! I'm sure I've seen a picture of Old Spice beached at Priory Bay.

Pete
At the moment my contract finishes at Raymarine at the end of February, so don't think I will be able to wangle seawork.

More to think about.

Nasher.
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Old 22 September 2006, 10:23   #5
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Quote:
Originally Posted by Nasher
I hadn't thought about the total lack of power when re-entering the water after a short flight. Must be like hitting a brick wall.
is it though ? how do OD and VTs Ribs manage? hence the desire to take one for a test drive, or even a trip in Pressmans Carson
Quote:
Mind you, I didn't realise a stern drive couldn't be lifted far enough to beach the boat! I'm sure I've seen a picture of Old Spice beached at Priory Bay
the second time we saw O/S she was parked in the middle of a tarmaced public road without a trailer, thankfully the stern drive lifts clear enough to park her on dry land.

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Old 22 September 2006, 11:31   #6
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Ocean Dynamics swear by jets - the late Shaun White probably created the finest boat there is for shallow water extreme ribbing.

http://www.mustangmarine.com/ribwork...olio/index.htm

http://www.venturejet.co.uk/index.htm

These people used to work with Shaun and still use one of his boats - they operate in surf all day picking passengers up off beaches.



One of Shaun's great boats in action.

Good luck with the new project Nasher - have a word with Paul Tilley for the tubes - he will be a great help.
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Old 22 September 2006, 12:03   #7
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I think one of the main benefits is that it puts a lot less pressure on the engine. There is an Outward Bound Ocean Dynamics rib in aberdovey and that is a very good boat from what i can see.

James
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Old 22 September 2006, 12:44   #8
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Dancing the boat...

I have an 8.4m Avon with a 340HP Cat Diesel and a Hamilton Jet...
It is a great boat.. however.. if I could go back 3 years... I would have bout a similar sized boat with two outboard engines... Anything goes wrong on a jet and it is a major effort to fix it.. Outboards... you just take off the boat with 4 bolts...
Fuel economy.. we were getting 3 litres to every mile.. at 0.45 a litre sounds good.. but then if RED diesel goes that gets exspensive.
Also when I bought it for a commercial operation I then found out after that you can claim VAT back on petrol.. so that would make outboards a realistic option..
A jet drive is a lot of fun, good turns etc.. but twin outboards do the same.. are lighter and as you say easier to change parts and impellers etc.
Avon do the same boat we have new in outboard and jet drive mode..

http://www.avon-workboats.com/page/sr84d1

http://www.avon-workboats.com/page/sr84od

Jets in shallower water are good, but do suck up things, Twin outboards with powertrim can do shallow..

Having driven the MOD pacific 22 with jet drives on the ships I work on... they are fun too.. but suffered a lot when new with problems, and they seem to go wrong far more than the sterndrive versions..

Have some photos of ours during 'rebuild' if you want to see...
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Old 22 September 2006, 15:13   #9
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Thanks Guys.

The project is at least 18 months off even being started, I'm just doing some preparation, and getting things straight in my mind.
Need to get a permanant job first though!

The plan is to purchase a bare hull and tubes, probably with a small cabin, around 7.5 - 8.5M then build it up myself. Including putting the engine in.

I'm looking to put an inboard diesel in around 300HP and hoping to keep it to around 1.5 to 2l a mile at cruising speed. - Go on, tell me its not possible.

Most of the spec is already on paper, and the jet is a 'nice to have' item, but it looks like I'll forget it.
But I will make sure any sterndrive I opt for lifts to the point of being able to beach the RIB.

Nasher.
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Old 22 September 2006, 15:25   #10
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You could always try a surface drive.....
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