Get you home engine, would it work.

prv said:
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
Click to expand...


....and then try holding a fireman's hose....(oeerrr)
 
Yes. Let's have a race. You can use a Karcher patio cleaner and I'll have a Climax pump on deck. I can dangle the strum box over the side and attach the outlet hose to the rudder.
I don't know where you'll get your water supply from...
 
Lakesailor said:
Yes. Let's have a race. You can use a Karcher patio cleaner and I'll have a Climax pump on deck. I can dangle the strum box over the side and attach the outlet hose to the rudder.
I don't know where you'll get your water supply from...
Click to expand...

Only if the two pumps provide equal momentum to the water they expel.

Pete
 
I guess a reasonable measure of momentum would be to see how many litres per minute each pump delivers into a bucket (or how long it takes to deliver a litre).
 
I just had a look at jet drives (Hamilton etc) It seems some manufacturers rely on lower flow/higher velocity but they are not regarded as being as good as the Hamilton types which are based around higher flow-rates.

It would be an interesting exercise to try the different ideas.
 
We picked up a piece of net just SW of Bardsey at around 9.30 one evening, strapped the dinghy alongside with 2.2hp motor and beached the boat on Pt Dinllean beach at around 8am. It moved our then 34ft boat at up to 2kts in calm neaps conditions but I had to stand in the dinghy to stop the bow rearing up. Holyhead CG wanted to launch PD life boat to tow us but I flatly refused.
 
Here's one:
If the becalmed boat has a bowthruster, then two plastic elbows stuffed into the ports. One facing forward to 'suck' and the other facing aft to 'blow'. Dunno how long you can run the motors for, or ampage use. But for a short distance?

Plenty of people have converted o/bs to use 12v electric motors instead of the petrol unit. (plans on Glen-L site) Run it off your batteries with the genny to try to top them up. Lot of faff though, when a cheapy o/b will do it. But at least 12v is a lot safer than using 230v motors in salt water.
 
DownWest said:
Here's one:
If the becalmed boat has a bowthruster, then two plastic elbows stuffed into the ports. One facing forward to 'suck' and the other facing aft to 'blow'. Dunno how long you can run the motors for
Click to expand...

Most electric bowthrusters have a very short run time - measured in seconds or a small number of minutes - before the motor overheats and automatically cuts out.

Some hydraulic systems (but by no means all) can run indefinitely, but they need the main engine to be running anyway to provide the hydraulic pressure.

Pete
 
prv said:
Most electric bowthrusters have a very short run time - measured in seconds or a small number of minutes - before the motor overheats and automatically cuts out.

Some hydraulic systems (but by no means all) can run indefinitely, but they need the main engine to be running anyway to provide the hydraulic pressure.

Pete
Click to expand...

Thought that might be the case. With the hydraulics, since prop fouling (as above ) seems to be a common lack of drive, then one could still run the engine for the bowthruster.
 
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.

Its a Yamaha Itchfanny special I think its 8 gigawatt but I could be wrong :-)
Click to expand...
 
DownWest said:
Thought that might be the case. With the hydraulics, since prop fouling (as above ) seems to be a common lack of drive, then one could still run the engine for the bowthruster.
Click to expand...

True.

I believe some single-engined mobos that already have a serious hydraulic system (for thrusters, stabilisers, passarelles and tender cranes) sometimes have a hydraulically-driven backup prop. Obviously this is only really worth it if the hydraulics can be driven either by a dedicated engine or the motor of a genset, but in big US-style "trawler yachts" that often is the case.

Pete
 
I once sailed on a diesel/electric tanker, Its the same principal large generators running a drive motor. I guess its just a question of having the right motor, and enough power. Not sure that mains appliances should be cannibalised and dipped in salt water though! it might be better to stick to 12V.
 
robstracey said:
I once sailed on a diesel/electric tanker, Its the same principal large generators running a drive motor. I guess its just a question of having the right motor, and enough power. Not sure that mains appliances should be cannibalised and dipped in salt water though! it might be better to stick to 12V.
Click to expand...

The idea has been around for a long time the theory being that there is a weight and space and cost saving from getting rid of the gearbox, and the engines can be run at their most ecconomical regime without the direct shaft link. My last submarine was proper diesel electric with no connection between the engine and shaft. It can also be used to adopt unconventional machinery installation
 
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