Get you home engine, would it work.

Davy_S said:
My thoughts are, a generator runs at slightly above tickover speed, dependant on load, so, we convert the output into kw and use this to drive the prop, the kw tend to remain static output at idle speed, I would have thought that a small outboard motor would consume more fuel. as its output is normally rated at max rpm, a generator runs slower, so burns less fuel.
Click to expand...

The losses involved in the generator are considerable. Generator rpm is irrelevant - you can't get something for nothing.
 
Also by fixing the dink alongside at midships the prop stays in the water, whereas if you have it on the stern and there is any wave action it will not unless it is an extra long shaft.

I do agree that without a helper in the dink, close in work will be a challenge but I have done it single handed to get somewhere I could drop the hook.
 
mrplastic said:
I've often wondered if you could get a couple of decent powered Jet washers, on-board water pump fed with sea water, power it all with a genny and strap the spray heads to the side of the boat! Gotta be good for a couple of knots ;-)
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A 'decent' jetwash is around 3kw, but needs around 9kw to get the motor started. So if you fit a couple and try to start them together.... What sort of suitcase genny do you carry? :o) Even staggering the starts is going to need some grunt.

Read about some guy on a small cat who used a petrol strimmer, with a prop mounted on the end, as a mini longtail to get in and out of harbour.
Since I have had two strimmers fail at the angle gearbox, there must be a cheap source of material.
 
mrplastic said:
I've often wondered if you could get a couple of decent powered Jet washers, on-board water pump fed with sea water, power it all with a genny and strap the spray heads to the side of the boat! Gotta be good for a couple of knots ;-)
Click to expand...

jon711 said:
When I worked for a well known pump manufacturer, we did have an inventor approach us with something similar, he had mounted modified pressure washer jets above the waterline, and was able to manouver the boat around the marina using a remote control. He was offering us the marketing rights (He owned the patent). Problem was very weak power (Struggled to overcome a crosswind), and the fact that anybody standing nearby (or on the boat), would get wet, either from a direct hit, or from spray....
It did sound a good idea at first!

Jon
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DownWest said:
A 'decent' jetwash is around 3kw, but needs around 9kw to get the motor started. So if you fit a couple and try to start them together.... What sort of suitcase genny do you carry? :o) Even staggering the starts is going to need some grunt.
.
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The problem with powerwashers is that they work on using a high velocity/low volume water jet.
For moving a boat you need low velocity but high volume.
More like a 2" pump.
 
KellysEye said:
We used our dinghy and out board to move the boat it was tied fore and aft to the quarter. Five horsepower moved our 12 ton boat fine but it was calm, we only did it to see if we could move the boat if the engine failed. It's definitely checking it works for your weight of boat and dinghy HP.
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Tried 2.5 hp with my 30 ton boat.
About as much use as hanging a bar of soap over the stern.
 
Lakesailor said:
The problem with powerwashers is that they work on using a high velocity/low volume water jet.
For moving a boat you need low velocity but high volume.
More like a 2" pump.
Click to expand...

Presumably either works, as long as velocity x volume is high enough in each case.

Like you get the same power with 1a at 240v or 20a at 12v.

Pete
 
No. It's to do with time as well. If you use a high velocity jetting pump at 6 gallons a minute to try and move a large volume of silt in a culvert it stirs it up, but goes nowhere. If you use a gully emptier to dump 200 gallons under no pressure in a minute it all flows away nicley.
High pressure washers are very low flow. The pressure is an indicator of the velocity you will achieve at a given flow through the size of jet you are using. Once the water leaves the nozzle it is at atmospheric pressure, but travelling quickly. Most small power washers are only pushing 3 or 4 gallons a minute. Most domestic cold water taps flow at 6 gallons a minute or less.
 
You'd think a short shaft would work with a bathing platform bracket. In fact ,with my Merry Fisher 805, it needed an extra long shaft.
 
prv said:
I *think* our outboard would clamp onto the lip of the kedge locker in the swim platform, with nothing more than a bit of scrap wood to stop the clamps marking the paint. I should probably get round to trying it some time.

It's only a little 2.2 though, so progress is still going to be pretty limited. And with the throttle out of reach and no neutral or reverse, I'm not sure manoeuvring would be any better than under sail!

Pete
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and when there's no wind ?
 
michaelchapman said:
Tried 2.5 hp with my 30 ton boat.
About as much use as hanging a bar of soap over the stern.
Click to expand...
One horse power used to move 70ft narrow boats at 2 or 3 knots all day, stopping only at canalside boozers...
A full barge must be the thick end of 30 tons?
 
Lakesailor said:
No. It's to do with time as well. If you use a high velocity jetting pump at 6 gallons a minute to try and move a large volume of silt in a culvert it stirs it up, but goes nowhere. If you use a gully emptier to dump 200 gallons under no pressure in a minute it all flows away nicley.
High pressure washers are very low flow.
Click to expand...

I know that pressure washers are low flow - they were invented (or at least popularised) by the tree-hugging Germans as a way of reducing water use. Which is why it's particularly annoyingly silly when some councils ask people not to use them in hot dry summers, as a sort of mini version of a hosepipe ban.

I'm not convinced that your sludge-moving experience is directly applicable to using water jets in air for propulsion. Dredging up my school physics, I think this is (to a first approximation) a simple conservation of momentum situation, so what matters is the momentum of the water being thrown away. Momentum is mass times velocity.

Empirically, when I use my weedy domestic pressure-washer to clean the patio, it has a perceptible recoil force. Using the hosepipe (higher volume/flow, much less pressure) I assume there's a small force, but I can't feel it.

Pete
 
lw395 said:
One horse power used to move 70ft narrow boats at 2 or 3 knots all day, stopping only at canalside boozers...
A full barge must be the thick end of 30 tons?
Click to expand...

That horse power (actually about two horsepower, I believe, for James Watt's marketing purposes!) had a nice firm towpath to push against, though. Michael's outboard probably had a whizzy little eggbeater of a prop for pushing dinghies, which just stirred up the water instead of pushing the boat.

The old Seagull barge-pusher had a great big fine-pitch propellor to develop lots of thrust despite a relatively low-powered motor.

Pete
 
prv said:
That horse power (actually about two horsepower, I believe, for James Watt's marketing purposes!) had a nice firm towpath to push against, though. Michael's outboard probably had a whizzy little eggbeater of a prop for pushing dinghies, which just stirred up the water instead of pushing the boat.

The old Seagull barge-pusher had a great big fine-pitch propellor to develop lots of thrust despite a relatively low-powered motor.

Pete
Click to expand...


Spot on...when it comes to moving heavy weights slowly, torque is your friend :)


Horsepower = How fast you're going when you hit the wall
Torque = How far you take the wall with you when you hit it
 
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