Jet Unit vs Outboard?

gairndeb · 03 November 2002, 12:37

Hi all, and excuse me 'coz I'm new here. I have a few questions and it seems this is the place to find a few answers.

I'm looking at the pros/cons of a Jet unit for a yacht tender? I have found a little from people here already, but I would like some specifics. For example, I can already assess that there is an overall weight penalty, but it is intended that the RIB be kept in a stern garage, so that's not so much of a problem. Dragging 300kilos over a sand flat may be a bit exhausting though

I understand that, even with the back-flush, fouling is a bit of a problem. Anyone with experience of this? Any one with a similar tender on a Yacht? Anyone who has already researched this option?

The main attraction is the ease (not having to cart around outboard motors, the integral fuel tanks, a steering wheel) and the power!! Figure these would make a nice sports car alternative at the end of the day. Really keen to hear comment from anyone.

Now the really silly question - do they all have reverse, and is it effective? Does it work like the aircrat jet engine reverse thrusters?

Cheers

TIM · 03 November 2002, 21:59

Seadoo also make a jet rib similiar to the one in the photo, but its more 'jetski with tubes around it' style than the one in the photo.

As a tender to a yacht they cant be beaten as you dont have to worry about props and landing on beaches etc and if you want to get off a beach with heavy surf, its much easer to do.....

they are a lot of fun too and can carry lots of people/ food/stores and are people safe....

IMHO they are the best boat tender.....

Powerboat · 03 November 2002, 22:34

Looks like a very sexy little boat !

Jets generally soak up about 30% of available power and can also suffer from clogging . You may have no prop to ding , but coarse sand and pebbles can decimate an impeller quicker than you might think. Impellers are more expensive and harder to change than props.

We had a maker of large twin-diesel jet ribs over here to give us a demo a while back.

Against our advice ( we were worried about wash in the harbour ) he opened her up hard in the harbour in a meter of water. he spent the next 20 minutes trying to extract the sea bottom of Hodson Bay out of his innards !

I mention this because with even small jets , you should toodle out until you are in over a meter to avoid sucking up the nasty stuff.

All in all I think the advantages of a jet would out weigh the disadvantages given what you want it for ! ( And it looks like a reallly nice little boat !

Best wishes ,

Stuart
www.powerboat.org

Tiger · 04 November 2002, 14:14

I agree with powerboat, for your situation a jet sounds much more simple to live with.

Power loss is a small problem, but manufacturer has hopefully built in larger engine to give you a decent amount of power.

do small jets have a reverse flush? (helps clean the intakes, if not completely)

Just be carefull applying power astern when you are going fast ahead (emergency situ, full ahead to full astern) as jetboats can really stop and your body might not!!!

Tiger

gairndeb · 05 November 2002, 00:05

That's the sort of info I was after. There are various manufacturers doing these now, and according to them, they are becoming quite popular. No doubt that is why the competition is developing.

Most site 'reverse flushing', or 'back flushing', as an option, and all seem to use an 83hp Yamaha two stroke unit as the power plant. Now, all these RIBS are between 3.4 and 3.5m so the maximum outboard you could safely put on the back is about 25hp, as far as I can tell. Even if you lose 30% for a Jet unit that is still the equivalent of a 60hp prop unit, and I wouldn't fancy lifting that every time we want to go ashore/visiting

The problems with the impellors I felt would be similar to the problems with the impellors in an outbourd unit if you used it in dirty, merky or sandy conditions.

Another consideration is the practice of using the tender as a kind of tug boat for an engine failure or for difficult docking. Can do away with an expensive bow thruster then too.

Does anyone know how reliable these Yamaha units are? How easy is it to get parts around the world?

Tiger · 05 November 2002, 13:34

My simple answer is I don't know, but you are unlikely to find anything better engineered than a Yamaha !

Tiger

Julian · 05 November 2002, 13:44

I beleive that parts for Yams are quite easy to get hold of and like Tiger says they are very well enginered engines so you should'nt have any problems with them. They have been making engines for Jet skies for years and they would probably be the same for the Ribs.

Julian

Scubakid · 07 November 2002, 14:05

Hi

I had a jet powered RIB (6.5M) and you will not find anything more easy to control but I do agree that if you hit the power too soon you will lift the sea bed up to meet you !! The important thing to remember is that whichever way you turn the wheel the bow will go, this is both forward & astern.

John

gairndeb · 07 November 2002, 19:37

So, if you reverse with the wheel to the left, the bow will go to the left? Is that true? Wow!

Does reverse work simply by lowering a reversing scoop over the jet of water on exit and reversing it's flow?

Powerboat · 08 November 2002, 00:10

The impeller damage problem is a bigger issue than you may realise.

The impeller on an outboard is made of nylon and can withstand quite a bit of abuse once water is flowing around it.The intakes are also quite narrow so the only real danger is from mud or sand clogging and blocking water flow leading to cooling problems.

The impeller on a jet unit is normally steel of similar size to a prop ( that is what it is !)and the intakes feeding the impeller are big enough for small pebbles and sand to pass through .

It really is the exact same as driving your prop through a shale or sand bank with consequent damage to the steel impeller blades.

This is why you should be so careful while operating jets in shallows.

gairndeb · 08 November 2002, 04:04

Sounds expensive.

The way I figure it, if you had an outboard, and were sensible, you would go quietly in the shallows anyway, so it doesn't seem to be too much of a disadvantage really. Another option I was investigating was a small hovercraft - but the weight restrictions and inability to handle the surf quickly tossed that idea out the window.

The benefits of a jet would seem to outweigh the negatives at this point, though I really appreciate you drawing my attention to this. I didn't really understand the differences between the impellors - I think a trip down to the local PWC store, for a bit of a squiz at the internals, may now be in order.

Thanks for pointing me in the right direction.

Jelly · 08 November 2002, 09:50

Greetings Gairndeb
Question, which you might have answered before are you intending to use the jet in the surf??
The reason is that the intake must be in the water to keep drive and stearing. Have seen in some jet boats once they leave the water off the wave (obviasly need to throttle back as normal) no water.
On landing it takes a second or so to pump the water through the system where as other drives there is no lag.

Though if your boat is big enough ( can they ever be

) will the intake be out of the water, do want to fly her!

Alan Priddy · 08 November 2002, 20:54

The biggest problem that happens and hids the effectiveness on most water jet boats is the boat builder tries to save money and fit the smallest cheapest unit he can find. Coupled with the fact that most boat builders never go to sea the old saying" there isn't any subsitute for size" is the big issue. If you have a huge jet stuck on the boat which in turn has a massive water capasity you will not have any problems.
If I was building a new boat it would definatly have one (or more!) water jets on the back.
Alan P

thewavehumper · 08 November 2002, 21:19

Alan P

There is a 10 meter Cabin rib in a Rib builders near my house and it has twin diesel lumps powering a pair of jets, It was built as a tender for his yacht originally fitted with yammie 170 Hpthe owner was not happy with performance. To date he has had a pair of Yamaha 170's which were taken out and replaced with a pair of 240's which gave him 5 knots more. So he decided to swap these for Yanmar 300's.

When the Rib was on the land Jasper was talking to the guys from the builders. Who told him the saga he suggested to them that the Jets were too small and therefore restricting the thrust. The comment from the builder was something along the lines of "he might be right" They were tiny little things probably not capabale of properly handlin the original 170's let alone the increased output from the bigger engines.

Alan Priddy · 09 November 2002, 12:34

I rest my case M'Lord