Jet Drive Units?

[Nasher]

Jet Drive Units


All.

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

I’m 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)


I’d 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 I’m convinced I’ve been told they are less efficient at turning engine power into forward thrust, I don’t know by how much?

Thanks all.


Nasher

[jwalker]

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.

[Pete7]

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

[Nasher]

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.

[Pete7]

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

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.

Pete

[codprawn]

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.

http://www.youtube.com/watch?v=CG7uc...latable%20boat

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.

[J S]

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

[badbaws]

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...

[Nasher]

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.

[codprawn]

You could always try a surface drive.....

[jwalker]

Originally Posted by Nasher

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.

Nasher, if you can push it, go for 9.5mtr.

I'm getting a little better than 1 mpl. and that's over a period of 1500lts of fuel. That's 8.5mtr and more than 3 tons with some fuel and crew.

Here are a few pics of drives and transom shields. They may give you something to work on. The Bravo looks to be just about level with the hull bottom but none of the Volvo drives are. There is somthing else I might mention re outdrive v outboards. There is a limited angle of trim above which you can't run the engine. It's determined by the angle the U/J's bend through. For the Volvo, and I'd guess for other drives too, there is also a lesser angle where the engine can be running but only at limited revs for the same reason. That angle isn't particularly high.

[Nos4r2]

Call me ignorant here someone but what hull profile would you need to mount the intake for a jet drive? I've never actually seen a jetboat intact and out of water where I could look that closely.

[Nasher]

JW

Thanks for the images, I've downloaded them to study later.

Nos4r2

Good question, I'd also be interested to see.

Nasher.

[pressman]

Originally Posted by Pete7

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
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.

Pete

Pete you are most welcome, the boat is in the marina at Hythe so whenever really it may dispel some of the negative comments

[pressman]

Originally Posted by Nos4r2

Call me ignorant here someone but what hull profile would you need to mount the intake for a jet drive? I've never actually seen a jetboat intact and out of water where I could look that closely.

These are the best i have!!

[alt]

Originally Posted by pressman

These are the best i have!!

Holy cr4p what is that beast of a thing?! Got any more pictures? I just made a sex wee!

-EDIT- Just did a search for more pictures, double sex wee! Amazing machine my good man, maybe if I win the lotto....

[Nos4r2]

Originally Posted by pressman

These are the best i have!!

That boat is pure sex


So, based on what I can see(and pure conjecture):-

If you want a single jet you'd need a flat on the keel towards the stern (like a planing pad) to mount the intake-or twin jets a la Pressman?

I'm guessing but twin jets like that should suck downwards less than a single jet with the same output due to being mounted at an angle?

Would splitting the intake of a single jet to either side of a deep V hull mean a noticeable decrease in efficiency and would it be feasable to do this and still keep the impeller below the waterline?? Obviously it'd cause some loss of power- everything's more efficient if it only goes in a straight line...

I'm assuming you have to have the impeller below the waterline at rest to at least some degree to get it to pick up water properly?

Am I making sense?