12v water pump boat design

[nwina1]

Found a few 12v water pumps around 20 gpm wonder what kind speed i would get from that?
looking for something that would attain speeds of a trolling motor

http://compare.ebay.com/like/390438034571?var=lv&ltyp=AllFixedPriceItemTypes&var=sbar&_lwgsi=y&cbt=y

I know ..... I know ...... why not a trolling motor

WHERES THE FUN IN THAT ;-)

 

[btrueblood]

The jet force developed at the pump exit will be roughly V*mdot, or the flow velocity times the mass flow rate (getting the units right is the trick, best done with SI units).

Next problem is estimating the hull drag coefficient...there are some online resources that can help, google "hull drag coefficient" and similar terms.

I'd try just putting different size orifices (jets) on the pump exit to maximize the V*mdot. Could be done pretty easily by experiment, or if you can find a pump curve for your pump, use the BEP.

 

[bimr]

If fun is what you are after, you should consider a trolling motor rather than a pumped drive:

Disadvantages of jet pumps include:
[ul]
[li]Can be less efficient than a propeller at low speed[/li]
[li]More expensive[/li]
[li]Higher weight in the boat because of entrained water[/li]
[li]Can suffer more easily from cavitation than a conventional propeller[/li]
[li]Can become clogged with debris; e.g., seaweed[/li]
[/ul]

http://en.wikipedia.org/wiki/Pump-jet

 

[btrueblood]

The same article lists some advantages to pump jets that are contradictory to the list of disadvantages; it really comes down to what the design speed is, and what the expected usage environment (speed, hull depth) is going to be. Cavitation is listed as both dis- and advantage, pointing out that it really comes down to matching the prop or pump to the expected design operating conditions.

I'd say pump jets are less likely to foul with debris, since they are more easily protected by a screen, and placement of the intake, from that debris. The entrained water idea might be true for large systems, for a 12v RV pump I think you can safely ignore it - there's more water entrained in the beer cooler than in the plumbing for the pump .

Finally, for trolling, a jet might be quieter, and probably will be operable in shallower water than the typical trolling motor.

Course, this whole thread might just be a troll...

 

[bimr]

The biggest problem will be with fouling. A 12V motor will have a pump with a discharge line of approximately 1-Inch. nwina1 would spend all afternoon cleaning the pipes instead of fishing. However, when a trolling motor fouls, you simply pull it up, clean it, and put it back in the water.

 

[btrueblood]

I'm left wondering what kind of debris you think will foul the pipes, bimr? Again, put a suction screen on the pump inlet. When the pump starts to make noise, hoist up the inlet pipe (hose really), clear the screen and drop it back in. About as easy and quick as pulling up a troll motor....and a LOT quicker if we are talking about unwinding millfoil or lily pad stalks from a propshaft. Unless we are talking about trolling in a sewage pond, I really can't see how fouling is such a big issue.

 

[bimr]

It appears that you have never been fishing. Stuff happens. The suction screen will also likely foul. Anything is capable of fouling a 1-Inch line; a frog, a leaf, a twig, etc.

http://www.proboat.com/a-fix-for-waterjet-fouling.html

A trolling motor has about 3-5 HP. The water pump will be a fraction of that. Water jets aren't as efficient as props.

It would probably be easier to use a weedwacker:

http://www.youtube.com/watch?v=RgRiirOQdSQ

 

[btrueblood]

Whatever bimr. "The suction screen will also likely foul." Yes, so design it to be easily cleaned.

"It appears that you have never been fishing"

Try cutting ~100' of 50 lb test nylon monofilament off the troll motor prop sometime. A suction screen would have blocked it.

Jet boats are used for fishing here all the time, they have their problems, but fouling is not the biggest one.

"A trolling motor has about 3-5 HP. The water pump will be a fraction of that."

So pick a higher power pump.

"Water jets aren't as efficient as props." Bull. Properly designed, they are as efficient, and sometimes more efficient. I will grant you that a typical off-the-shelf RV pump won't be as efficient as a purpose designed troll motor, but that specific comparison doesn't hold. If you are so sure of that fact, tell us all why large ships have been moving more and more to shrouded props (which is all that an axial pump is)?

 

[bimr]

In this scenario, nwina1 is proposing to use a typical off-the-shelf RV pump and is asking how fast will it go. More than likely, the pump will lack adequate power to move the boat and will suffer from a host of operating problems. So what's the point of off topic discussions about supposed benefits of a shrouded prop, a suction screen, jet boats, etc?

 

[Artisi]

The whole idea of a 12 volt system sounds like pie in the sky to me. Jet pump (actually axial flow pumps)have been round for ages. If you want to do some research start with H D Nelson NZ/ Australian - who built some very sophisticated "jet Propulsion" units some 40 years back.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[bimr]

That's the same point I made. Jury rigging a 12 volt water pump will probably cost more than a trolling motor while providing less performance.

 

[btrueblood]

bimr,

If that was your point, you made it poorly in earlier posts. I agree with your last post, though.