Props vs Jet drives

Have they made vast improvements in the efficiency of jet drives in the last few years?

I remember when I had a jet drive ski boat and the prop boats of similar size were fasted and didn't use near the fuel I did.

Why the resugence of this type drive in the picinic boat size and larger yachts? Isn't a prop still more efficient?

Has it got to do with the diesel engines torque curves vs gas engines we used to see in smaller boats?

Dennis

 

Prop vs Jet

The relative efficiency of jets and props is fixed by a few simple aspects of geometry, mainly the size of the jet nozzle vs the diameter of the prop.

High efficiency in marine systems is acheived by accelerating lots of mass through as small a velocity change as possible. Thrust you get is m*v, energy you pay is 1/2 m * v2, so by making v as small as possible and m large, you get more thrust for less energy. One way of getting lots of mass is having a large diameter propulsor, another is to move it through the water fast.

However, a jet, being an enclosed pump, is more efficient than a prop at speeding up the water it encounters and big props tend to have big struts and shafts which cause drag at high speeds. Thus jets tend to have low efficiency at low speeds but increased efficiency as speed gets higher, (mainly due to reduced drag) until props and jets cross, and the jet wins. Underwater props also cavitate, but you can suppress this in a jet with the inlet design. Jets also have shallower draft.

The other alternative for high speeds is a prop without struts or an immersed shaft, i.e. surface drives, which tend to be the winner at higher speeds over both, but may not be applicable because of the exposed gear aft.

Props, either underwater or surface drives, are generally cheaper than jets.

A wise designer looks at all the factors and decides on the relative merits of efficiency, cost and operational issues.

As far as the poularity of jets for some rec boats, it may make sense or may just be hype, even for the same boat model from owner to owner.

 

Prop Vs Jet

First off let me tell you that I have owned both types, prop and jet. I am an avid boater and waterskier. The best performance I have had has always been jet driven. The best handling has always been jet at high speeds and prop at low low speeds. The boats I have owned have ranged from 18ft to 60ft. As far as maintenance costs, the jets win there too as there are less parts to a jet than there are to an outdrive. When you do a complete comparison of both, and I have, there is really no contest as the only downfall to a jet is low low speed handling. Which can easily be overcome by an experienced boater. In my book jets are the best.

 

I'm right at the beginning of the process of converting my 30 ft. twin diesel sedan cruiser from veedrive prop driven to jets. The choice has been based on improvements in waterjet technology, our chosen boating environment, (primarily river travel), unit cost, (new tranny's, shafts, and props vs. jets), simplicity, and serviceability. The path has been a long one, and the choice of manufacturer is still not finalized. There's lots of information out there, it's just a little work to get it.

True, I will undoubtedly experience some increased fuel cost, but that will be well offset by the reliability and serviceability of the waterjets. My old veedrives and props and shafts are worn out, and the replacement costs far exceed the cost of conversion. Not to mention that I will also "lose" all that stuff that hangs down under the boat! No more "propping out" on gravel bars, no more bent rudders or shafts.

I expect to keep a very accurate account of the conversion, possibly in multi-media form, I'll eventually post with details.