Jet propulsion possible for slower boat?

[mik lenoir]

Hello

Jet drives are popular for small and fast vessels.(lots of hp)
But how about for aux. propulsion in heavier boats?
What would be the limiting factors to apply jet drive to slow going ( let say sailboats) with the typical engines as found in those.
If an external prop with certain rpm can create enough thrust to maintain the 5 to 7 knots at a given design and power, could an internal pump not deliver similar thrust via jet arrangement?
Unlike some jet seadoos taking in water horizontally from the bow area(strainer) removes some of the forward resistance and also creates suction in the desired direction. Or?

Dragging a prop through the water while sailing, or fouling prop with unwanted stuff and other reasons may question the need for alternatives.
Is there some info on this?
Of course high performance is not the idea; just similar to what is found on sailboats.
Please direct my interest to other previous threads here or elsewhere.Have checked; but not found anything.
Thanks for help Mik

[Thunderhead19]

I have two words for you "Tractor Jet"
please see
http://www.ultradynamics.com/section...nal/design.asp
section "6" I think

[mik lenoir]

Thanks Thunderhead19,
very good information on your suggested site.
If I read correctly, application of jet is possible if desired; even so optimal
performance range is a bit higher than 6 kn @ 30 hp or so.
Water intake on regular designs is on the horizontal bottom of hull and more aft meaning not so good for nonplaning hulls moving at hull speed.
By locating intake at the bow in the direction of movement the bow wave can be reduced by the flow amount of jetdrive. That should have some positive effect.
Are there any application on boats around which fall somewhat outside the optimal range of application? any one done some experiments?
please post; 'cause there are more odd things in life.
Cheers mik

[marshmat]

Most production jet drives need the nozzle to be out of water in order to work well. In displacement hulls they don't do so well. Some of the ones for large commercial craft are different but these are probably not relevant to your application. Something innovative and different could be a good solution....

[mik lenoir]

thanks,
but dont understand(lack knowledge) why jet nozzle has to be on or above waterline? Air entrainment maybe but why?
Even if this the way to do it,I don't see reason why nozzles couldnt be located
to meet this criteria. Yes some piping need to be part of it .
appreciate comments thx mik

[marshmat]

If the jet outlet is below water level, water behind the boat creates resistance to the flow and you get high back pressure, reducing flow and outlet velocity. Jets work on Newton's 3rd law- every action has an equal and opposite reaction. The force transferred to the boat is proportional to the mass of water moved and the speed at which it is moved. Blasting the outlet into air gives less resistance to the outlet flow, therefore more water flowing faster, therefore more force, therefore better efficiency.

It's possible to get equivalent thrust to a prop in a displacement hull, but the jet will require more power than the prop to give the same boat speed.