Jet drives on a 60’ boat

longjohnsilver

longjohnsilver

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I know next to nothing about jet drives. There’s a 60’ Aquastar with them fitted. I guess that’s pretty unusual for a boat of that size. 704hp Cats linked to Hamilton fat drives.

What are the pros and cons? My guess is less drag therefore a higher speed and/or more efficient. And possibly more manoeuvrable than shafts, not that shafts aren’t manouverable.

Any other suggestions?
 
Shallow water .
There’s maintenance issues underwater hydraulics for steering and the reversing bucket and impeller wear .
I don’t know what the time interval is for a strip down .

Ultimately they say less drag , no P bracket or rudder so marginally more efficient fuel wise or faster for a given fuel burn compared to a shaft equivalent.

Ropes / weed getting sucked in wrapping around is a haul out job not a diver only job .
 
Portofino said:
Shallow water .
There’s maintenance issues underwater hydraulics for steering and the reversing bucket and impeller wear .
I don’t know what the time interval is for a strip down .

Ultimately they say less drag , no P bracket or rudder so marginally more efficient fuel wise or faster for a given fuel burn compared to a shaft equivalent.

Ropes / weed getting sucked in wrapping around is a haul out job not a diver only job .
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I did think shallow water, but then thought the keel would be about as deep as props.

So just the same as jet ribs, susceptible to rope and weed. There’s lots of it about!
 
longjohnsilver said:
I know next to nothing about jet drives. There’s a 60’ Aquastar with them fitted. I guess that’s pretty unusual for a boat of that size. 704hp Cats linked to Hamilton fat drives.

What are the pros and cons? My guess is less drag therefore a higher speed and/or more efficient. And possibly more manoeuvrable than shafts, not that shafts aren’t manouverable.

Any other suggestions?
Click to expand...

Here is an even bigger boat on Jet drives
https://www.boatinternational.com/yachts-for-sale/force-india--87851
 
I was under the impression that you needed bigger engines and fuel burn to run jets but like aircraft the water jet gives constant thrust whereas a prop will reach a plateau at WOT and top engine /prop revs. I.e jet is not only able to go faster but also to sit in a favourable torque band at a greater range of speeds when accelerating.
 
One of the Brecqhou boats that runs out of Guernsey is an Aquastar commercial hull running Cummins QSC with Hamilton jet drives.

After many years of commercial use, the drives now look very tired. If you are thinking of buying a similar set-up, a bloody good engineer's survey is a must. Spare parts may be hard to come by now too.
 
BruceK said:
I was under the impression that you needed bigger engines and fuel burn to run jets but like aircraft the water jet gives constant thrust whereas a prop will reach a plateau at WOT and top engine /prop revs. I.e jet is not only able to go faster but also to sit in a favourable torque band at a greater range of speeds when accelerating.
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Actually the opposite is true as you can get jets running with as little as 5 HP and again it comes down to application and little more, you are correct about constant thrust of a jet and it doesn't need specifically tailoring to a boat as thrust is proportional to speed and as you state there is no real optimum speed for them to work at.
 
Portofino said:
Shallow water .
There’s maintenance issues underwater hydraulics for steering and the reversing bucket and impeller wear .
I don’t know what the time interval is for a strip down .

Ultimately they say less drag , no P bracket or rudder so marginally more efficient fuel wise or faster for a given fuel burn compared to a shaft equivalent.

Ropes / weed getting sucked in wrapping around is a haul out job not a diver only job .
Click to expand...

Totally incorrect as in a correctly designed unit all the hydraulics and reversing bucket are above the waterline and not below it and can be maintained easily with minimal work and time.

Impellor wear is not really an issue unless you use them for extensive periods in shallow water with a sandy or abrasive bottom which effectively sandblasts them and causes premature wear on the impellor or its housing and as for weed and ropes being sucked in? this is again down to poor design and inlet filtration, where did the idea of rope cutters come from? the jet drive.
 
I was involved in operating fast passenger ferries , 35 knots.

With propeller boats in this speed range the problems were prop cavitation and picking up debris. The hydrofoils used to destroy a set of props in 450 hours through cavitation, so three sets per season per boat.

The waterjet passenger cats and the 74m wavepiercer cat for cars and passengers that I was involved in the waterjet impeller lasted for many years. The advantages were low noise, smooth operation, fantastic manoeuvrability with computerised controls and very low downtime.

Shallow drat was not a great difference because they operated like a huge vacuum cleaner sucking up debris and jamming the intakes. In Hobart I saw a boulder about a metre diameter that had been removed from a waterjet intake.

A water jet works well on a fast boats, the hull has to designed for water jets to optimise performance for efficiency and speed.
 
Assassin said:
Totally incorrect as in a correctly designed unit all the hydraulics and reversing bucket are above the waterline and not below it and can be maintained easily with minimal work and time.

Impellor wear is not really an issue unless you use them for extensive periods in shallow water with a sandy or abrasive bottom which effectively sandblasts them and causes premature wear on the impellor or its housing and as for weed and ropes being sucked in? this is again down to poor design and inlet filtration, where did the idea of rope cutters come from? the jet drive.
Click to expand...

Assumed being a boat forum I automatically had “leisure use “cap on .
Sometimes technology in use with commercial esp if it’s primary design ethos was centred around commercial, when then subsequently transferred to leisure pattern of usage ( infrequent use / long lay ups idle ) reliability gremlins creep in .

There’s a few Med sports boats Mangusta being one that use the Kamewa jets .
Brokerage sites , the better brokerage sites often quote 10 ,s of thousands just spent in service , stuff like bearings , shafts , bushes, seals and indeed impellers .

True the hydraulics are actually out of the water but only at rest .but they and there fittings are exposed to wetting while other then the boats on its berth , again they will have a service life and being outside subject to sea water a shorter life than that type of gear inside a transom .
Secondly “ yard walking “ like outdrives I see a lot of leisure jet drive boats with large holes in the transom and 2/3 weeks on the hard in the yard .

Ok I see a the odd drawn shafts and renewing of cutless bearings and rudder stocks in conventional shaft drives , but that’s a 24/48 hr job in yard time and peanuts compared to stripping down a Kamewa as any yard guy can do shaft work .

It’s a case of dong the re search and making an informed choice .

Perhaps constant use by the commercial guys reduces any static degradation issues that shorten service life in leisure AND the bills when they eventually roll in are more than offset by the fuel savings and timetable efficiency of getting bums on seats by a ferry operator.
Leisure uses have not got that high income stream .

A lot in the leisure sector seem to end up suffering from fouling and barnacle growth and end up having a strip down for a clean .A less than smooth surface of the tube and impeller leads to unforeseen cavitation problems ( that is not there in commercial) hence shorter hrs on the leisure usage impellers .

As said above a engineers report is prudent I would have thought in a leisure boat .
.
A lot of standing around low hrs would ring alarm bells unless as said in the sales blurb they have just undergone a €20 K or what ever refurb .
 
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