Reactive thrust of submerged waterjet

[Frosty]

I lost money on a bet 20 years ago on this. I said that the water from the jet needs the water outside the boat to push against. The guy with me on the boat said so you think that if we park the boat at the bottom of the slip way and fire the water jet up the slipway the boat wont move --I said no the boat wont move, it will just pump water up the slip. Duh!

So we did just that, the boat accelerated away from the slip with as much thrust as always. This was the ski boat from the largest sloop in the world built in Thailand called Miranda (I think) .

[yipster]

well, contradictionary to what i would bet have read on another waterjet site
i now read on twindisk's site: http://www.twindiscpropulsion.com/it/JETFeatures.htm

Some misunderstandings that people commonly have is that the exit jet must strike the water or that the thrust will be greater if the water jet strikes the water. This is most definitely not the case and the essel would be propelled even if the water jet exited into the air.

jets are way more efficient going fast, so would i disagree with twindisk, i would not dare!
did read up on water, jets, scramjets, aurora, x51 and formulas, could not find much on a pulsing waterjet tho
very intriqued but totally confused better round it off for now with these jets http://www.liveleak.com/view?i=1f2_1189625978

[jehardiman]

Originally Posted by anthony goodson

JH is right in that a large slow moving prop is good for bollard pull .However don't disregard the jet, in this respect ,they punch well above their weight in static thrust, The reason for this is that a good waterjet ,when stationary ,can absorb in excess of 90% of engine revs and power,{to the limits of cavitation},whereas a standard propeller needs to advance in order to do this.

Very, very wrong. That sounds like some sales smoke and mirrors using apples and oranges for direct comparisons.

Pump-jets are always less efficient than regular wheels when both are properly designed and free from other constraints, and will deliver less thrust per input hp even when stationary (where efficency is zero) than a conventional wheel. It is just the nature of the beast because they have exactly the same cavitation issues as an open wheel, as well as wall and nozzle losses which an open wheel does not have. It is true that an open wheel optimized for high speed may have cavitation issues at bollard that will not allow it to absorb full high speed power, but conversely at high speed the water jet will have much less thrust than the open wheel. This is why most boats are open wheels, it takes less power at speed than a pump-jet.

What pump-jets DO bring to the table is a draft, disk size, and possibly weight reduction, and you just pay for that in maximum efficiency.

[yipster]

some more advantages and pro-jet info at http://www.hamjet.co.nz/hamiltonjet_...jet_advantages but under history and bringing the jet above the waterline it mentions

1954
A slight modification to expel the jet stream above the waterline proved the turning point in marine jet propulsion,
increasing speed to 17mph and eliminating all underwater appendages.

ok, could be trust remained the same but drag was eliminated but than wonder why -if not held down- that car lifted by fireman dont fly higher but still have to read up more

[jehardiman]

Originally Posted by yipster

wonder why -if not held down- that car lifted by fireman dont fly higher but still have to read up more

The weight of the water in the hoses....the system is self limiting.

[anthony goodson]

Originally Posted by jehardiman

Very, very wrong. That sounds like some sales smoke and mirrors using apples and oranges for direct comparisons.

Pump-jets are always less efficient than regular wheels when both are properly designed and free from other constraints, and will deliver less thrust per input hp even when stationary (where efficency is zero) than a conventional wheel. It is just the nature of the beast because they have exactly the same cavitation issues as an open wheel, as well as wall and nozzle losses which an open wheel does not have. It is true that an open wheel optimized for high speed may have cavitation issues at bollard that will not allow it to absorb full high speed power, but conversely at high speed the water jet will have much less thrust than the open wheel. This is why most boats are open wheels, it takes less power at speed than a pump-jet.

What pump-jets DO bring to the table is a draft, disk size, and possibly weight reduction, and you just pay for that in maximum efficiency.

In the opening line of my post I conceded that a large slow moving propellor was good for bollard pull, fact. You concede in your post that an open wheel optimised for high speed may have cavitation issues at bollard pull that will not allow it to absorb full high speed power,thats true also .But a jet ,which by it's very nature is configured for high speed will regardless of this still absorb 90%ish of its power at bollard pull and there is the difference and my point. It would seem that we agree on this point ,which was the only one I made. So where does the wrong ,very wrong verdict that you made come from ,and where does the greengrocery come into it.

[jehardiman]

Originally Posted by anthony goodson

In the opening line of my post I conceded that a large slow moving propellor was good for bollard pull, fact. You concede in your post that an open wheel optimised for high speed may have cavitation issues at bollard pull that will not allow it to absorb full high speed power,thats true also .But a jet ,which by it's very nature is configured for high speed will regardless of this still absorb 90%ish of its power at bollard pull and there is the difference and my point. It would seem that we agree on this point ,which was the only one I made. So where does the wrong ,very wrong verdict that you made come from ,and where does the greengrocery come into it.

This is the statement that I object to

The reason for this is that a good waterjet ,when stationary ,can absorb in excess of 90% of engine revs and power,{to the limits of cavitation},whereas a standard propeller needs to advance in order to do this.

This statement is disingenious at best, and wrong at worst. A pump-jet will always absorb power to it's cavitation/starvation limit (let us say 90% of delivered shp as a talking point), regardless of inlet speed. Similarily a "standard" prop will absorb 100% (yep it has to...there are no losses...it all goes into the prop) of the delivered shp from bollard to flank as long as it is kept below it's face cavitation limit. The question here is how much useful thrust will be delivered...and the answer to that is " it depends". What you are trying to insinuate by your statement is that an open propeller performs worse than a pump-jet at bollard, and that is not true as a general case. A well designed open propeller/prime mover set will produce more thrust than a pump-jet at bollard just because it has less losses.

Now that said, if you stated that given the limitations of small boat propeller disk size and ICE rpm-torque curves that a pump-jet can produce more bollard than the typical i/o set of the same hp I would agree with that. But that has more to do with the boat design and engine selection than with the capability of the propulsor.