Dare Iask a jet drive question?

[burgundyben]

Er, without wanting to get into a never ending discussion....

What is the difference in efficiency between a shaft installation and a jet drive? how much power is sapped by the drag of the shaft and prop? How do Outdrives compare?



<hr width=100% size=1>I didn't knacker TCM's copper it was Trazie

 

[BarryH]

I've always wonder'd this. Mercury do or did a Jet drive outboard. On the cowling it said "Jet 30". But looking at all the gumph about the engine it was actually a bog standard 50 with the gearcase changed.
So looking at it from that point of view.......quite a bit. But no doubt some one will be along with all the techynical stuff and...........need I go on!

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[kimhollamby]

Outdrives remain one of the most efficient installations for most general purposes -- shows through big time on fuel figures / performance when you check boats like Pincess V39 versus V40 -- or look at the Sealine 39 Coupe in its variant forms. Plus they are cheap to install and with forthcoming EC regs on sound where builders can buy them as approved packages from engine bods they are even more attractive. Shame they are so easy to break and can be a bit of a handful in marinas on some types of boat.

General wisdom on jet drives is that get more efficient at faster speeds, above high 20s, although no doubt the jet drive producers have been working on that. Efficiency can also be quite badly effected by fouling and hull shape. You have to say the general principals are fun though and I've always wanted to own one (a jet boat that is).

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[burgundyben]

Ok, I'll refine my question, same boat, same engine, one with a shaft and one with jets, assume max speed of shaft boat is 30 knots, what would be likely max speed of jet boat?

<hr width=100% size=1>I didn't knacker TCM's copper it was Trazie

 

[adarcy]

Re: My guess

FWIW
32-33 w outdrives
33-34 jets as the hull won't be optimised for them

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[wakeup]

Are you going to fit them to your Swordsman, a bit like giving a bald man a bigger comb no???

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[stefan]

A Finnish boatyard Sarins Boats making a boattype very similar to the Targa line, are touring the country presently with three boats of the same model (Minor 27) and with the same motor. The drivetrains are different, one is w. shaftdrive, one legs and the third with jets. They were acatually close to my home last weekend but I didn't have an opportunity to go and test. I'm sure they would be more than happy to send anybody interested some info about the performances. You can find them on the link below.

http://www.minor.fi/index_eng.asp?main=3

I have discussed my new Avon jetrib compared to the old Avon Seasport with an outboard. I find the outboard more seaworthy as the ob leg stays in the water and deliver thrust even if the boat jumps a little. The Jetrib fall as a stone immideately when no water is delivered to the pump and the ride gets very wet and bumpy.

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[kimhollamby]

Fudge answer

I genuinely don't know about jets, think there are many things at play.

One figure immediately comes to mind concerning outdrives. Sealine F36 we ran typically turned in 1.2mpg-1.3mpg on shafts. S36 on outdrives, 60hp less total, typically did 2mpg. Top end performance in realistic terms around 5 knots more. Okay, lower superstructure and hence less weight and air resistance, which would help a bit. But noticeable difference.

Remember MBY did one yonks ago Fairline Corniche 31 new boat test(!) shafts versus outdrives. Big difference on that too! And think some comparison V39/40 figures bandied about.

I'm not sure if there have ever been any jet versus shaft comparisons published. There was that Storebro boat a little while back that had jets and the big Targa at last year's Southampton. Plenty of RIBs with them that also have outdrives but not exactly what we need here I think.

Perhaps I'll go and make trouble on the MBY or MBM shop floor to see if I can get any meaningful stats.

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[aswade]

Specific jet drive info/comparison

I have been dealing with Hinckley quite a lot as my boss was looking at buying one of their new top of the range 55 footers with jet drives. Hinckley arguably have more experience with jet drives than any other production (or semi-production) boat builder. With the same hull (one that has been optimized for jet drives) same engines etc., they quote a 38 knot top speed for the jet drives and "several knots less" for the prop/shaft/rudder set up. At cruising speed in the low 30s, the jet drives are substantially more fuel efficient. This is for a 55 footer displacing 27 tons with twin 1000hp engines.

I don't believe that fouling has any greater effect on performance than on any other planing boats with props/shafts, and if the hull is designed for jet drives from the start, they are potentially more efficient at lower speeds than props/shafts.

There are substantial advantages to jet drives that you cannot really understand until you have seen a Hinckley in action, among them:

Computerized joystick control linked to jet drives and bow thruster allows inch-perfect maneuvering with just a fingertip- makes docking in even the nastiest currents and wind a snip for a total novice. It's like playing a videogame.

Emergency stop from planing speed within two boat lengths. Slam the throttles into reverse, the buckets drop down over the jet outlets and instantly direct 100% of thrust forward. It is quite a sight to see/experience. Surprisingly, the stop is not as abrupt as one might think- akin to heavy braking in an auto, but not a full-on slam on the brakes ABS-juddering stop. As long as you are prepared for it and holding onto your drink, there's no drama. They are also very maneuverable at speed versus a similar sized boat with props/shafts.

No props to injure people in the water (you see quite a few accidents, over this side of the pond at least, where somebody is mangled by props.) This also means no running gear to damage on submerged objects or props to foul in fishing nets etc. There have been Hinckleys that run hard aground on rivers that have freed themselves by slowly engaging reverse thrust with only minor gel coat damage to the hull- no appendages to damage. Most likely you would not have the same result with props/shafts/rudders. Also no alignment problems after a knock with jets as in shaft set-ups.

Some jet installations are set up so the engine runs at a constant RPM, which is much better for engine life than revving up and down often- speed is controlled by adjusting the amount and direction of the water flow and not by engine rpm. This offsets the higher maintenance costs of the drives.

Disadvantages are primarily cost related: cost of installation due to lack of units manufactured/sold, the market has not reached critical mass; also higher maintenance costs versus prop/shaft/rudders, especially in salt water applications. A Hinckley T55 will cost well over 2.25 million dollars by the time it is typically equipped, and this is a low-profile, sleek boat with only two cabins and less interior space than a typical 48 footer. For comparison purpose, over here in the U.S. a brand new Fairline Squadron 58 or Azimut 55 can be had for around 1.4 million dollars. I think in the UK, a Hinckley will cost relatively even more than in the US. Granted, a Hinckley is almost a bespoke boat, so comparing the Hinckley to the Fairline is a bit like comparing a Bentley Continental to a Jag XJ sedan. The Bentley costs more and has less space, but you would expect that.

I would argue that if somebody from another planet came to earth and decided he wanted to design a boat using the best of all available technologies we have available today (and there was no cost advantage for props/shafts due to volume etc. and no built-in advantage due to legacy issues of props/shafts having been invented much earlier) that he would decide that the best solution might be jets. A lot of the denigration of jet drives comes from lack of knowledge or misinformation among the general boating public about today's available options. I was a sceptic until I did all this research for my boss. One ride on a Hinckley and I wager you'd all be converts too! Too bad Gludy's not hanging around anymore because in many ways he was right to ask about jet drives. Then again, like most boaters he had to worry about resale and such, so forgoing jets was probably the right option for him at the time (Then again, Hinckleys hold their value at resale better than most brands, so it is not impossible for boaters to eventually recognize that jet drives should not be summarily discounted at resale.)

In the real world, because of the cost and maintenance issues as well as those cited above, I don't expect to see jet drives in anything but boats from premium builders, but they are far from the bizarre, esoteric, irrational choice that many portray them to be.

Alex





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[tcm]

Re: Specific jet drive info/comparison

excellent post. I agree it's a pity gludy went, tho he never quite hit the nails on their heads as you have done.

I think burgundybens question relates to his own boat - so i wonder how feasible it is to cut about a boat with shafts and retrofit with jets?

Les seriously, what about if the boat is an ole knackered Fairey with fablon all over - won't it fall to bits over 30 knots anyway?

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