Just curious why jet drives are not used on the larger offshore boats. I was watching some show on the Navy swift boats and their Jet Drive use...

jets are woefully ineffecient. good for shallow water as long as a gov agency like the military is buying the fuel. otherwise, only good for spectacular roostertails...

woefully inefficent,i raced a blown jet boat years ago that ran 7.70 at 149 mph in the quarter,what is unefficent about that?

I agree!But Roostertails are cool!

A race boat is a lot different than a driver. I have a Berkley Jet and a V-drive. The jet drinks fuel like a fish, the V-drive runs on a small tank of fuel for a long time. The jet is cool in the fact that it feels like a vette, it feels like it rotates instead of turns. But it if you get some sea weed or grass in the pump it loses it's prime. You can work the throttle and blow it out sometimes. But usually you are pulling the cover off & pulling it out. Ice bags suck (no pun intended) and a rope will ruin your day if it wraps around the shaft. Fishing line isn't very good ether. Other than that they rarely break. It's just a glorified farm pump. But the rooster tail is like a fire boat....

If you get into bigger heavier boats, say 34' and weight about 20,000 lbs, jets are great for manuvering and you can parallel park them easier than a car. Dump the buckets and it's like slamming on the brakes. The downside I noticed on our patrol boats was that running hard in open water, the jets gave you a feeling like the ass was sliding around, with outdrives down in the water you had a lot more feeling of a stable ride.

i agree with everything wou said,but i do love the acceleration you get from a good running jet boat,and premium is only 4.27 per gallon in my area lol.

I design lots of boats with water jets. The axial mixed flow units have gotten pretty good as far as efficiency goes. Back in the '90's the crossover point was around 5000 hp where the jet became on par with a prop. Nowadays it's down to about 1000 hp.

Normal water jets like the Hamilton are designed for workboats and commercial vessels. They are great up to about 50 knots, at which time the internal drag and intake drag start to make them less efficient. They are also rather heavy if you are following the recommended power guidelines.

So, they are not really a high-performance item. Sure there are lots of racing jetboats and jet sprint rigs, but they are running small diameter pumps with highly modified impellers. Those don't last long.

Finally, they are quite expensive. Last quote I remember seeing for a Hamilton HJ403, which runs behind something like a 715 hp Cat C18 was about $98,000 a piece. A typical HJ 292 which is used with 450 hp motors is about $35,000 ea.

P.S. Most of the jet setups we do utilize a conventional twin-disc or ZF transmssion to get some gear reduction and also provide the ability to backflush and clear the jet of debris. We've also done ones with two-speed transmissions for fireboats. so that they can be shifted to run in low gear while the firepump is coupled up to the engine.

A smaller percentage of the setups, mostly the one's I've spec'd out for river usage are setup without transmissions. These boats are almost all USCG Subchapter T passenger vessels.

You ask, why use a gear reduction? Well, it turns out that we get better cavitation resistance and better midrange thrust with the higher pitch impellers. A typical gear ratio for these setups is about 1.13:1

Hamilton must have been giving the Navy a discount, when I went for training they said it was about $25,000 to replace them. We were running twin Cummings 6BTA, I think they were about 450hp