Replacing an outdrive with a jet

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

 

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!

 

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:

 

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.

 

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

 

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!)