Replacing an outdrive with a jet

[spatialul]

I have a 1986 34' gillnetter up in Alaska that has a Chevy 350 and an Volvo 290 outdrive on. The boat is a bow setter and picker so going full throttle in reverse is a pain; especially in rough weather. Plus i need to be able to go in really shallow waters in order to fish (let's say 18" deep). The boat draws about 4" at the bow and 10"-12" at the stern. Can anybody advise me if is possible to throw a jet in, and how big it has to be.

Thank you,
Gabriel

[CTMD]

what's your top speed at present?

[spatialul]

I think it is around 15 knots at 2800 RPM; the engine will not go over 3000 under load

[Ad Hoc]

Go visit your nearest KaMeWa's FFjet supplier or Hamiltons...find the closest to you..they are best to advise since they can see your boat too. Plus you'll get an idea about cost..which is what it generally boils down to!

[marshmat]

Hi Gabriel,

It's probably possible to replace your sterndrive with a jet. You will probably have to sacrifice a bit of mid-range efficiency and a fair chunk of cash to make the switch.

A Chevy 350 should be going a lot more than 3000 rpm at full throttle. Its normal redline is somewhere in the mid to high fours (I've heard anywhere from 4200 to 5000, depending on how it's set up). So your current setup is almost certainly sub-optimal.

If a jet does interest you, your application would call for one of the beefier industrial-style units (KaMeWa, Hamilton, etc.) as opposed to a jet-ski pump or an Aggressor/Jacuzzi/Berkeley style. I'd suggest dropping HamiltonJet a line ( http://www.hamiltonjet.co.nz/contact_us ). Given the dimensions, weight and power of the boat, their engineers can take a pretty good guess at what pump would work and how it would perform in your boat; they can also refer you to a local dealer who can check out your hull and determine exactly what would need to be modified for the conversion to work.

[jim lee]

I wouldn't be too fast tossing out the Berkeley option. There is no way a Chev 350 can overpower one. We used to run 1200+HP through the one on drag boat I worked on. I think it was plain vanilla berkeley with stainless impeller. The rule of thumb was 650 HP before you needed the stainless impeller. What's the 350 chev put out? 300 - 350 HP?

Now, would it work at the speeds you're running? Don't know.

-jim lee

[spatialul]

Jim,
the Chevy is a 1986 and puts out about 260 HP.

[spatialul]

Also, i just checked Berkeley and is for boats up to 5000#; mine is at least 8000.

[CDK]

Replacing the Volvo with a jet pump will not improve your 8000 lbs boat's performance when reversing. It will probably double your fuel bill unless you spend real money on a Hamilton matched to your boat.

Before mentioning Berkeley again, read this thread:
http://www.boatdesign.net/forums/jet-drives/looking-berkeley-12jf-parts-28320.html

[spatialul]

CDK, thanks for the heads up. I contacted the Hamilton guys yesterday; Gave them all the details and waiting for them to get back to me. I also left a message with the Rolls-Royce guys.

[jim lee]

Before mentioning Berkeley again, read this thread:
http://www.boatdesign.net/forums/jet-drives/looking-berkeley-12jf-parts-28320.html

Well, ok.. Berkeley clone. Legend, Dominator, AM Turbine.. Kinda' like we all say PC, but how many really mean the one built by IBM? (Its their name for them)

-jim lee

[marshmat]

Jim,
I think the problem with a Berkeley, or any of the similar drives, is that- for our questioner's application- speeds are relatively low and the boat is relatively heavy. Aggressor, Jacuzzi, Berkeley, etc. are all meant for relatively light boats that run relatively fast. To get a jet to work efficiently in the sub-30-knot range requires a lower rpm, larger diameter, high flow rate / low exit velocity design.

The fact that our original poster's Chevy 350 is refusing to rev over 3000 seems to me to be a strong indicator that the prop and/or gear ratio are not appropriate for his application. I would bet that considerable improvements in performance could be achieved simply by substituting a larger diameter, lower pitch wheel.

[spatialul]

Matt, you're right about the gear ratio. In the very beginning i had the original sterndrive on the boat and i had it up to 4000 RPM (just me on the boat). A week later that unit broke and I replaced it with a spare i had; with the spare the engine won't rev past 3000 loaded or not; I think there were different gear ratios on the sterndrives. The reason why I want to replace the stern drive with a jet is that:
1. will allow me to fish in shallow water (currently with the sterndrive the boat draws almost 3ft of water).
2. I think it will give me better steering and a little bit more speed reversing; since is a commercial gillneter when you set the net speed is of essence.
I'm still waiting for the Hamilton guys to get back to me.

Gabriel

[CDK]

Well, ok.. Berkeley clone. Legend, Dominator, AM Turbine.. Kinda' like we all say PC, but how many really mean the one built by IBM? (Its their name for them)

-jim lee

We call'em PC, not IBM PC. And it was a good design that conquered the world with over 90% market share.
The Berkeley story is exactly the opposite, lousy design, 0% market share.

[TeddyDiver]

Funny.. I just ripped a jet out and converting to outboard :)

[spatialul]

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

[anthony goodson]

We call'em PC, not IBM PC. And it was a good design that conquered the world with over 90% market share.
The Berkeley story is exactly the opposite, lousy design, 0% market share.
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

[CDK]

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.

[marshmat]

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

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
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.

[anthony goodson]

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

[Doc Nozzle]

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!

[spatialul]

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

[CDK]

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.

[anthony goodson]

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

[anthony goodson]

I have a 1986 34' gillnetter up in Alaska that has a Chevy 350 and an Volvo 290 outdrive on. The boat is a bow setter and picker so going full throttle in reverse is a pain; especially in rough weather. Plus i need to be able to go in really shallow waters in order to fish (let's say 18" deep). The boat draws about 4" at the bow and 10"-12" at the stern. Can anybody advise me if is possible to throw a jet in, and how big it has to be.

Thank you,
Gabriel
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.

[spatialul]

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

[anthony goodson]

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 .

[anthony goodson]

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

[spatialul]

Anthony,
I went by this when I look at Castoldi:
http://www.castoldijet.com.au/PDF/td%20337.pdf
Anyway this a project for next spring-beginning of summer; at this point I'm collecting as much info I can.

[anthony goodson]

Anthony,
I went by this when I look at Castoldi:
http://www.castoldijet.com.au/PDF/td%20337.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

[spatialul]

Tony,

I look at the specs for the 238 and just won't do for the boat, so I found the 337. I'm waiting to hear from the UK guy if he has any coming.
Gabriel

[baeckmo]

Umm, some of the figures mentioned seem odd. If your engine reached 3000 rpm it would probably generate some 190 hp instead of 260 at ~4500. With a weight of 8000 lbs, ie 3600 kg that is ~19 kg/hp. This should result in higher speed than the 15 knots you mentioned, if the hullshape is reasonably OK for planing speeds.

So, either the hull weighs in at about 4 to 4.5 tons, or its shape is incorrect for the speed range, or the propeller/engine matching is wrong, or a combination of all!

Now, Anthony G and others have pointed out the problems to be expected with undersized jet units; this is a never ending theatre. To show the interconnection between the main variables for jet selection, I prepared the nomogram attached. Key: Weight/Power > vertically to curve > horizontally to speed > to actual power > down to required diameter.

There you may see that with a weight/power ratio of 21 to 24 kg/hp you would have some 16 - 18 knots with a decent hull. BUT to have a reasonable efficiency with this combination, you must have a jet inlet diameter of about 300 mm in order to "process" enough water. At this speed, the jet inlet suffers from low "ram pressure", resulting in cavitation. Impeller tip speed would probably be limited to about 30 à 35 m/s, ie ~2000 rpm. A smaller jet than this would need a considerably higher power input for the same thrust.

From this nomogram you can also see that the high pressure/low flow ("Berkeley type") jet with its small diameter inlet (=small throughflow) is fine for for low load-highspeed applications, but will result in disaster for a high load-lowspeed boat!

[spatialul]

Baeckmo,
Thank you for the info and the drawing. The impeller's diameter at the inlet is 337, hence the jet name. The boat weighs in the vicynity of 5 tons with all the gear and crew; I'm attaching some pictures here:
33893

33894

33895

33896

33897

[baeckmo]

OK, those pictures and the weight explain a lot. First: a propeller, just like a jet has to have a certain working area in order to have "authority" over hull forces. Or, in other words: there is a limit to the pressure a set of rotating blades can generate, and thus the resulting force. The Volvo drive uses 16" as a maximum dia, which gives about half the area of what you should have had to that boat!

The immediate medicine would be to substitute the Volvo drive for a Bravo two, where the max dia is 20". Still not quite optimum, but far better. To have reasonable backing performance there ought to be about one propradius of water above the prop, meaning that your max draft becomes ~0.75 m with this arrangement.

For a jet installation, the flat bottom and sloping bow will create a trail of bubbles following the bottom which will disturbe the sensitive jet inlet and result in a loss of performance. Even without this loss, the maximum operating efficiency in the 15 knot range will be less than ~40 %, compared to some 50+ % for a propeller.

As I don't know how important the fuel costs are to your total economy, I cannot tell whether the jet efficiency can be accepted or not. If shoal operation and good manoeuvrability is a must, I would choose double engines with straight shafts and propellers in CORRECTLY designed tunnels instead! The bill for the big jet would probably more than pay for this arrangement! There is an important bonus in terms of redundancy with dual installations in addition to the better fuel economy.

[spatialul]

Baeckmo,
Do to the internal design of the stern area the twin engine solution is not viable. Now, will the jet performance improve at lower speeds or not? The Bravo unit will draw around 18 to 24" depending on the prop size; but I think will not work with my Chevy. I'll look into it; maybe get a small diesel since gas cost is also important. This past season I'd burned 1000 gal in 5 weeks at $5/gal.

Thank you,
Gabriel

[baeckmo]

Nope, the jet efficiency will even reduce with lower speeds! Roughly speaking, the best jet will just barely do what your Volvo drive is doing today! In comparison with a Bravo 2 with correct gearing, the jet will spend over 60% more on fuel and you need a 60% bigger (=more expensive) engine!

In this region of Sweden, we also have some fishing with traps in shoal waters. The propulsion problems are the same; going in ahead is ok, but backing out with crossing wind and sea is a lottery. One problem we have seen is that backing with jets create stronger wash over the bottom, filling the net with algae and debris. And every fisherman knows that dirty nets "don't fish", so I still prefer propellers here!

I think you might save ~100 mm in draft with the B2 drive if you raise the bottom aft with a small tunnel portion. In addition, letting the exhausts go thru' the transom and not through the prop will improve low speed thrust.

Comparing boats of similar size as yours, I think a diesel, 160-200 hp, twisting a B2 would shave your fuel bills considerably! My personal favourites in this span is the Iveco 5.9l six (the 8060 series) and the Cummins 5.9B or BT. These engines come with SAE flanges that fit Mercruiser cardan shaft transmission pieces, making connection to B2 drive a piece of cake.

Guess you need a hydraulic pump for your net hauler as well? The Iveco has a power take-off from the camshaft gearing, with standard hydro-flanging....!

Btw, what is your cost for diesel fuel? Also, while I think of it; arranging cooling water inlet far aft, and far out to the side, plus a good size mudbox is essential to avoid cooling problemsin this application! (Best of course keel- or skincooling!)