Replacing an outdrive with a jet

OK everybody,
I just heard back from the Hamilton guys. The 350 has to go since is not enough power; the replacement can be a 454, 460, 496, Volvo D6 or Yanmar 6LY. Any one of these will be enough for a HJ242 with no gear reduction or a HJ322 with a 1:1.5 reduction. So now that I have all the information time for the homework. I'll keep you posted about the progress.
Thank you everybody for your advices and opinions.
Gabriel

 

I have noticed that you are becoming increasingly vitriolic about jet units and understandably so,you have been the victim of a mismatch of power plant, propulsion system and hull and you're a bit pissed about it This problem alone is the main reason that jets have got such a bad name and it's not really deserved. There is a clue though in something you said "unless you spend real money and have a jet matched to your boat " Well this is the answer ,your jets would work well on a drag boat but are simply not designed to move a heavier hull ,I'm sure you know this ,a much larger diameter jet with a mixed flow impeller is what should have been used
The late Richard Parker was probably the first to realise that instead of using centrifugal impellers or bolting on more and more small diameter axial impellers in stages a large diameter mixed flow pump was needed , he started this trend with the PP90 and went right up to very large commercial jets with this principle and these jets are still amongst the most efficient ever made.
He also pioneered jets made of GRP which would have solved your other problem .
I am following your "tunnel posts "with awe as are many others I am sure

 

You have a point! The expensive and time consuming experiment with the imitation Berkeleys did not enhance my sympathy for jets.
But I also studied the technology, the various concepts and the degree of success, and concluded that jets are not the most efficient devices to propel a vessel.
I live on an island that is in the middle of the marine tourist industry: harbors, marinas and shipyards near every city or village. And I visited them all.
My estimate is, that there are some 10.000 boats and yachts around here, with stern drives, outboards and conventional prop shafts.
I found 5 with jets, from which 2 are catamaran ferries with a capacity of 350+ passengers with custom built MTU water jet propulsion. The other 3 are recreational craft. One has a Castoldi jet with a damaged gearbox, the other two look abandoned and have the remnants of a Berkeley dangling from the transom.
Of course I didn't look at all the boats on this island, just the ones that seemed large enough to accommodate a jet drive. And I skipped the few jet ski vehicles which I consider to be toys.

 

Anthony, CDK - valid points, from both of you.

You've hit on the key problem with jet installations here. Unlike a prop drive, where you can just substitute a different pitch and diameter if things don't work out right, a jet- in addition to being rather expensive to begin with- must be, as a system, matched to the boat.

A year or two ago, I was considering a jet drive for one of my projects (a two-tonne planing hull of about 8 to 10 metres LOA). The guys at HamiltonJet ran my specs through their (proprietary) sizing calculation codes, and ended up suggesting an HJ 274 to handle 150 hp or so. They confirmed that a propulsive coefficient of 0.50 or so would be realistic at just under 20 knots in this application. They also emphasized the important point that, with jets, efficiency (propulsive coefficient) is quoted AFTER all frictional losses related to the drive are considered. Contrast this with a sterndrive, shaft drive or outboard, where your propeller may be nominally 0.70 efficient, but the drag on the strut and/or shaft and/or rudders is considered separately. Once the appendage drag is considered, a properly matched jet tends to come out pretty close to a properly matched propeller, once you're at planing speeds.

A Berkeley-style pump is a very different animal from a Hamilton or KaMeWa. Notably, they seem to be aimed more at light, high-speed hulls. They're good at what they're meant for, but putting one on a heavier boat is like trying to push said heavy boat with a small diameter, high pitch prop. As for our original poster's concerns about control in reverse- again, the large twin-duct deflector of something like a Hamilton is likely to give much better reversing ability than the more compact deflector of a drag-boat pump, which is really only intended for slowing you down as you approach the dock.

Thing is, those speedboat pumps are about a third of the cost of a good continuous-duty mixed flow pump. So it's awfully tempting for builders to specify them in applications that, from an engineering standpoint, really call for a different style of pump. And given comparable efficiency between a properly matched sterndrive and a properly matched mixed-flow jet, I think a lot of people find the jet's advantages (less vulnerable to damage, less sensitive to changes in loading) to not be worth the five-figure premium it commands at installation time.

 

Yes Marshmatt quite correct ,but the biggest problem of all in my experience has been created in the past by the manufacturers or distributors themselves,at least here in the UK. In any range of jets there is always a performance overlap ,so the top end spec of a smaller jet will always merge into the lower end of the next largest model in the range. Now the price difference between different sizes can be considerable. If a manufacturer is asked to quote competatively for a jet and they know that price is a factor,some will quote for the smaller jet, according to the lines on the performance chart it will work , it won't work as well ,but it will be cheaper and they will make the sale. I have seen this happen countless times,and to my mind this is why jets have got such a bad reputation.
I must add here that Hamilton will only quote for the correct size,and have the experience to get it right they have achieved an enviable reputation here because of this ,but it is a double edged sword and it is why they are sometimes unfairly accused of being a bit pricey.
I live in a village on the South coast, we have a natural harbour formed by the esturies of two rivers which more or less dries out at low tide ,on the ebb,on springs the tide can run at seven knots between sand and shingle banks ,all you will get from a chart is "seek local knowledge" jet? no brainer

 

A 34' boat is way out of the league for any of the Berk/AT/Dominator et cetera pumps. And I doubt any of their reverser bucket designs would give you the improved reverse performance you are after.

Like others have already said, the Hamilton and other axial flow pumps would be your only option. But I would be very careful about reverse performance even with the Hamilton's whale tail bucket design.

Find someone in the NW or Alaska that is running a Hamilton on a big river sled and take a ride and see if you like the reverse performance first!

 

Any of you guys worked with Castoldi before? I found a guy in UK that sells recon units with 1 year factory warranty; and the units have their own gear box for pairing with any engine.

Gabriel

 

I've been in contact with Castoldi and Lancing Marine when I started my jet adventure.
The design looks good, the shaft position is high so there are no problems installing an engine with a deep oil pan. And the lever to wiggle the intake grate to remove debris also is a nice feature.

But I live near Italy and am surrounded by marinas. From the many boats I've seen, only one has a Castoldi jet (with a gearbox problem). That makes it very suspicious.

 

We seem to be by and large a friendly and helpful lot, so consider carefully the lack of response to your question .

 

Hi again ,one thing you should understand about jets, in fact several things They are good in shallow water in two modes, on the plane WOT and in displacement mode SLOWLY .The jet reverse is vectored thrust, it does not ,as many assume ,go straight back under the boat, if it did the entrained air in the thrust water would cause serious cavitation ,the thrust lines go down ,to clear the transom, and outwards and forwards on each side of the jet intake from a propulsion point of view this is not very efficient ,compared with a prop .If you open the throttle of a jet boat in reverse in shallow water the jet thrust will excavate the seabed and the jet will pick it all up ,not a good idea. The jet boat party trick of full ahead to full astern is only possible ,because until forward motion ceases there is solid water at the jet intake .this is not indicative of a powerfull reverse although it is often cited as such I don't think a jet will suit your purpose but if you do go for it, ask for bollard pull figures in shallow water because that's what you need.

 

Anthony, CDK,

Thank you for the post. i think I did not make myself clearly understood from the very beginning: I need greater maneuverability and speed going in reverse in deep water and also the possibility to go in shallow water regardless the speed.
The reasons I'm looking at Castoldi right now are:
- the 238 mounts outside of the stern eliminating the necessity of shifting forward the engine;
- because the 238 is design for engines up to 350HP I don't have to swap engines;
- and last of course is price

Thank you,
Gabriel

 

Re read my post of 07-31-09 @10-02 then ask yourself ,is this what you are about to do ,is this the classic mistake you are about to make? How does the spec, not the price ,the spec of the hamilton unit recomended for your boat compare. Impeller size power requirements etc you may think they got their price wrong but trust me they will have got the jet size right I am not neccessarily recomending Hamilton above all others but they will know for sure what size of jet your boat needs .

 

Oh and in case your'e wondering I don't have a Hamilton jet ,I make my own, not commercially just for me.

 

Anthony,
I went by this when I look at Castoldi:
http://www.castoldijet.com.au/PDF/td 337.pdf
Anyway this a project for next spring-beginning of summer; at this point I'm collecting as much info I can.

 

Gabriel
Your link is to a Castoldi 337 a completely different animal to the factory reconditioned 238 you were originally asking about,
Tony