Opinions on fitting jet units to a Sunseeker

[Dave2]

I am taking on the restoration of a 1987 Sunseeker XPS34 which has been idle for about 7 years. It is currently fitted with twin Volvo AQAD41DP's (diesels) and these will need extensive work or possibly replacement. I am looking at alternatives to upgrading to Volvos as while the new D4 and D6 look interesting, I believe they would be an overcapitalisation on such an old boat.
There is a highly reputable firm in NZ which produce a quality jet unit and they have advised me that they have matched one of their jet units to a 230HP marinised Lexus (petrol) engine. All this at less than half the cost of the Volvos.
If anyone has any ideas/opinions/advice on the feasibility of fitting such a powerplant/drive to my boat, please post.
PS I would probably be doing less than 200 hours cruising per year.

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[Gludy]

I am a great fan of jets and in particular the Hamilton jets from NZ. However, unless the hull is designed specifically for jets, I would approach the changing over from props to jets with great caution.

Sunseeker themselves made the mistake some time back of offering jets as an option to props without desgning the boats for jets and the option fell flat on its face.


<hr width=100% size=1>Paul

 

[tcm]

ooh. what different hull design features does a hul designed specially for jets need? (note: I have no idea, and this is not a trick question)



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[wakeup]

Well I guess the prop tunnel ain't a lot of use for jets for starters... as the jets usually stick out the back of the transom..

How about a nice set of surface drives...... nice big roosters. You could propapbly use the exisiting engines, or am i talking through my butt, must get thicker pants to muffle the sound!!!!

<hr width=100% size=1>...but then I would say that wouldn't I.....

 

[hlb]

Forget the petrol idea. You've just devalued the boat by maybe twenty grand, probably devalued it even more with the jet drives. Mend the old engines or look round for two second hand replacements.

<hr width=100% size=1> <font color=blue>No one can force me to come here.<font color=red> I'm a volunteer!!.<font color=blue>

Haydn

 

[Gludy]

Props and jets apply the driving force to the boat at different angles - so it maybe that a boat changed from props to jets will maybe be nose up or down more than it should be.

Props tend to be at an angle that would produce an upward lift component as well as a forward component - jets would be about all forward componenet.

I am checking up if there is more to it than that and will post as I get more info.

As regards going to petrol - that would devalue the boat add the risk of jets not working well and im my view the project is too risky - spending a lot of cash and effort to end up with a devalued pup.



<hr width=100% size=1>Paul

 

[BarryH]

Not 100% sure about this, but wasn't the xps34 fitted wiv outdrives. Sure it was or was there an option of shafts.

On the conversion to jets. I'm not a lover of jets. Nil steerage unless in gera with power. Ok outdrives arn't brilliant in that area but at least theres a modicum of steerage when in neautral. Jets need bigger hp as they sap power like no ones business. If jets were that good how come not more builders offer it, even as an option.

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[Gludy]

"Nil steerage unless in gera with power. "

Jets - you set the engione revs and leave them whilst in the marina - you then have the ability to go at any angle in any direction and hold the boat at any angle regardless of wind/current etc - jets in every way beat every other power system for control - that is a simple fact.

"Jets need bigger hp as they sap power like no ones business. "

That is rubbish - the Cara Marine boat is a big 60 footer and yet can travel faster and have better fuel consumption than the prop equivalent per horse power. We are talking here about Hamlton jet systems.

"If jets were that good how come not more builders offer it, even as an option. "

Jets are not really an optional propulsion system - you should design the baot for jets and do it properly or not at all.

All industries become very conservative by their very nature - EMI stated it would never release any of their artists on CDs - IBM totally misunderstood the power of softrware and the PC - hence opened the door for Bill Gates - every industry needs a shake up and one day that will happen in the pathetic boat industry that will be forced to wake up.

More an more jet boats are being offered and once one breaks into mainstream production, the props will start to tumble. You cannot use the interia of any industruy to support the move against change. ...... that is very silly grounds to argue on





<hr width=100% size=1>Paul

 

[Duncan_Hamble]

The hull needs to be designed for jets.

As others have said with a sterndrive the thrust line can be adjusted to alter the trim of the boat. You can't do this with a jet. (Well maybe with a few - but it's not the norm). It's thrust line will be practically horizontal - this is not a problem and is actually quite efficient. However, a jet drive creates "virtual weight" at the rear of the boat. This is caused because as the water is pumped through the jet it is also raised from the level of the bottom of the boat to the level of the nozzle. This could cause the boat to run with a lot of bow up trim - especially on a narrow hull like the XPS34.

I would agree also with what others have said about resale value - petrol engines are not preferred and jet drives are too unusual for most buyers.

My advice would be to put a pair of AD41's in it - you may be surprised at the price you can get these for if you haggle hard.

XPS34 with petrol jet drives worth £25K --- ?????

XPS34 with new AD41's worth £40K +++ ???

Just my 2p

Duncan

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[BarryH]

No I don't agree with all you say. Have a look at Mercury's 50 hp jet. The power head is from the 80hp lump, but they rate it at 50horses. Wheres the other 30 horse gone.
Mmm, yep leave the engine/s in gera etc. Same as outdrives, no different there. I was talking about some form of rudder, jets dont have'em. As far as the Cara go's, can't comment on that as I've never seen or heard on one. We're not talking 60 footers, we're talking 30 odd foot of mono hull sunseeker.

Can you explain the difference between a hull designed for outdrives and one designed for jet drive. I've looked at a few on the web but can't fathom it. All jets seem to come from a transom as do outdrives, dunno maybe I missing something totally obvious. If thats the case and they both come from the transom. Then it should be easy for the builders to offer them as options. If they don't why not. Cost?

As far as I can see, jets are used for specialist applications where the conditions that the boat operates in requires them.

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[Gludy]

We are not talking about fitting 50 hp jets but jets in cruisers be they 30 foot or 60 foot. Hinckley do a whole range of highly succesful jet boats in the USA that includes 30 footers.

"Can you explain the difference between a hull designed for outdrives and one designed for jet drive. I've looked at a few on the web but can't fathom it. All jets seem to come from a transom as do outdrives, dunno maybe I missing something totally obvious. If thats the case and they both come from the transom. Then it should be easy for the builders to offer them as options. If they don't why not. Cost?"

This has already been explained - jets offer a much more horizontal propulsion force and are not able to be trimmed like outdrives.

"As far as I can see, jets are used for specialist applications where the conditions that the boat operates in requires them. "

Just not true - Some nice crusisers from Italy, Hinkleys and Cara are all now jet powered.

Do you accept that such jet powered cruisers offer incredible ease of use in berthing and moving around a marina compared to props?

Have you ever been on, seen or experienced a cruiser fitted with jets?

J
Paul


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[aswade]

A jet drive may lose some hp versus a conventional driveshaft, but it usually makes up for it when you are comparing to inboards with props/rudders because it has far less appendage drag, far less parasitic drag, does not have to deal with the issue of form and frictional drag due to the props being inclined, loss of efficiency due to the shafts/props being inclined, lack of optimal prop size due to clearance limitations (Arnesons do not have so many problems, ergo the same boat with Arnesons will usually be faster than with props or jets) and a host of other things that negate any shortage in power output of the jet drives. In fact, one of the biggest sources of drag with an inboard/prop/rudder boat is the friction of the rotating prop shaft in the water flow- in the words of an engineer friend, "the induced drag associated with the magnus-effect lift caused by shaft rotation."

I have the specs for the new Hinckley T55 in front of me. Offered with both jets and conventional props/rudder. because Hinckley optimize their boats for jet drives, the prop driven version is actually a couple of knots slower all other factors being equal.

Other than people being more comfortable with what they are familiar with, I think it is economies of scale and cost more than anything else. KaMeWa, Hamilton and all do not manufacture jet drives in anywhere near the volumes of conventional drive systems, and in any case the complexity/cost of a jet drive for a 60 foot boat is much greater than for shafts,props/rudders. In a high-end boat like a Hinckley where the added cost is less of a factor, the jet drives offer many advantages.

Many jetskis now come with auxiliary systems to provide complete steering control when power is off, so that issue is a red herring if you care enough to have the same option put onto a jet boat. While the engines ARE running, a jet driven boat with a bow thruster can out-maneuver any other recreational vessel. Go to the Hinckley web site and watch the demonstration video for their JetStick computerized control system. It is as easy as a video game in docking mode and you can squeeze a 26 ton 55 foot T55 into a space with two fingers, your martini in your other hand and not even breaking a sweat that no boat with props could get into.

If you have young kids, the lack of exposed props is another advantage of jets- here in the U.S. there are a huge number of people injured by props after falling overboard. The shallow draft, better protection from flotsam and debris in the water- there are actually a lot of reasons jet drives make sense.

Low end torque and low speed/runningwith the seas directional stability are two areas a jet driven boat typically are inferior to the same boat with props/rudders.




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[Gludy]

I agree with every word!


<hr width=100% size=1>Paul

 

[Dave2]

Thanks everyone for your input. The jet unit that I am considering installing is not the Hamilton unit (a very good unit), but one called the Scottjet unit. If you want to see what I am considering do a search for Scottjet new zealand and you will find their website (I dont know if the forum rules allow me to post a link as it may be considered commercial. I am a potential customer of theirs only and naming the jet manufacturer is only so that the participants in this forum know what I am considering.) These jet units are used on many of the successful jet boats in New Zealand and have been fitted to larger pleasure boats. I am waiting foir feedback from someone who has just had 2 such units fitted to their 30+ footer and will post their response when I can get it.
As I see it, these jet units are steerable and are fitted with a reverse gear (ie thrust is vectorable) so I assume that the same measure of maneuvrability is offered by these units as a sterndrive unit. Granted that with the jet units, there is not such a defined keel as with the sterndrive so a loss of power would probably leave little scope to maneuvre.
As for the power issue, the Volvos that I am considering replacing are 200hp units and the Lexuses will be 230hp (allegedly). For those not familiar, there is no "tunnel" on the XPS34 as it uses sterndrive units. Comments please?
As for resale (this will bring tears to many eyes) I was given the boat by a generous boss who has no further need for it and didnt want to see it rot any more so when I finally choose to part with it, I would be happy to get my money back on it which would be just the cost of restoration.
My interest in the jet unit stems from personal experience riding as a passenger on a number of jet boats in NZ and was impressed by the performance/reliability of this form of propulsion.
Please keep providing the feedback. I appreciate all points of view.

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[jfm]

specialised hull designs

Jet units add virtual weight as duncan said, plus real weight. The virtual weight is the hull being sucked down as water is sucked upwards into the jet drive. The real weight is caused by the fact that the whole drive tunnel is full of water when moving or at rest. You can calc the effect of this by getting the internal volume of the jet unit and multiplying by 10lb/gallon.

A dedicated jet boat should have all this taken into account in the design, but I spect it's a small enuf amount that you can live with it ok in the sseeker. Probly quarter to half tonne I spect, you will notice this (= 4 big blokes sitting cross the transom) but can probly tolerate it.

Also what is the static weight of the gine/jet combo you are looking at, compared with the original sterndrives? You need to factor that in too.


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