Alternate propulsion system

[outofthenorm]

Hello all, and happy new year! I've been following the threads re: deisel electric and it got me thinking about other solutions. I have no real engineering kowledge, so this question may be naive, but here goes. Would it be feasible/reasonable to replace the shaft and prop with a water jet system? Would it work on a relatively heavy boat (13,000 #)? My engine is 30 HP that I run in the lower part of the rev band, and my prop is offset to stbd (it does not go through the stern post). This means that under power, I've always got 20 degrees of helm dialed in to counteract the turning vector. This creates lots of drag
I'm assuming that there would be a water intake and an output, so there would be some way of getting reverse. . I'm thinking about 3 benefits:
1) eliminating the vector in the thrust by being able to "steer" the output, and
2 ) eliminating the drag of the cutless block, the shaft and the prop.
3) a clean bottom with nothing to snag.
Can anybody tell me if this is a dumb idea, and if so why?

- Norm

[Cuyahoga Chuck]

Not an engineer either.
Jet skis and jet boats work on that principle. The jet is produced by a water pump and that's one of the shortcomings. If the pump digests anything but water damage can be instantaneous. Pumps, even when well cared for, are a high maintainence item.
Pump drives were tried on larger boats but seemed to have faded in popularity. This was a little before my time but I heard that bigger boats seem to suffer because the pump drive was particularly not effective when manuevering the boat in close quarters.
In terms of retrofit, there seems to be school of thought that the after sections of boats must be designed for pump drives to be efficient.
As old hat as propellers are they just keep plugging along.

Charlie

[ssor]

Water jets are most efficient at rather high speeds and most are steerable nozzles. This means that without thrust you are without steering. The best way to move a 500 ton airliner at two miles per hour is with a tractor that has an engine coupled to the wheels through a drive train.
That's not the best way to move a 500 ton airliner at 600 MPH. So fit your system to your purpose.

[Dan McCosh]

A jet drive is basically less efficient at providing static thrust than a propeller of the same diameter as the outlet. This means that for a displacement hull, you are effectively going to be attempting to use a 3 ins. or so diameter prop--exactly the opposite of the best prop to push such a hull through the water. Dunno why an offset prop would be causing help problems. With the same setup, the helm on our boat is neutral. Often the offset neutralizes the natural torque. Maybe prop rotation?

[outofthenorm]

Thanks guys. I figured it was an idea that wouldn't work - but now I know why ;) .

Dan, the helm issue seems to be because the shaft is set at an angle of about 15-20 degrees off centre to stbd so there's a vector that makes her turn to port under power. Sometimes useful when docking (once you're used to it), but irritating when trying to go or stop straight. The prop rotation is CW looking aft - the top of the prop swings towards the hull.

- Norm

[ahp]

I offer tow hazy ideas for propulsion that do not use props. My dad told me that about 1900 someone built a steam yacht that sucked water in thru the bow and then pumped it out through the stern with a direct acting steam pump. It achieved about two knots speed.

Then there was a submarine propulsion system whose details I cannot remember but was part of the technical detail of "The Search For Red October".

[Canoeyawl]

Pulsating Water Engine - The old Pop-Pop boat toy
direct acting steam engine. easy and fun to play with; plans in the old cub scout handbook.
http://www.nmia.com/~vrbass/pop-pop/cubboat2.gif
Read about it here... Pop-Pop Boat (http://www.nmia.com/~vrbass/pop-pop/)

[ahp]

I had a Pop-Pop boat as a kid. I only got it to work about once.

When I wrote in my previous post about a direct acting steam pomp, I meant something quite different. A direct acting steam pump has two cylinders, a steam cylinder and a water(or air) cylinder. They are in line and the single piston rod serves both cylinders. There is no rotating shaft or crank.

They were commonly used as feedwater pumps on steam locomotives, and also for compressing air, for the brake system.

[Gary E]

outofthenorm


Alternate propulsion system One question... WHY?
Seems to me that that you have a small boat with a small motor and that's about as good as it gets... What are you looking for?

[Dan McCosh]

We have the same setup--about a 15 degree offset to the starboard side. From a standing stop, the bow will swing slightly to port, and the stern walks to starboard in reverse. Underway, the effect is not noticable. I use prop- torque to make tight turns to port--which is the only way to make a turn in less than a couple of boatlenghs.

[Ian McColgin]

Back to the jet drive for a moment. I don't know if they are still around or if someone else took up the notion, but in the Pacific Northwest an outfit called Traktor or something such was making high volume low speed jets for fishing boats. Great bollard pull. I heard but can't confirm that they were working towards a higher speed unit. As with props, you can get things that work great at high speed but poorly at low, or visa versa.

[Dan McCosh]

I suppose a Kort (sp?) nozzle, used on tugs, technically is a jet drive. The issue gets back to blade surface area--akin to the disc area supporting a VTOL aircraft. A jet drive is an extreme--just like a jet aircraft engine, that mainly works well with low hull resistance, and high speed.

[outofthenorm]

To Gary E. The 3 answers to Why? are in my original post. Also, I have the whole shebang apart right now and that's the best time to wonder if there isn't a better way to put it back together. Turns out there probably isn't.

- Norm