Impeller Design for a Model Boat

why are you attempting to manufacture the impeller? why re-invent the wheel? it seems you will use ready made engines, why not use a ready made pump, complete with housiing, impeller, shaft, bearings and packing all matched and ready to go?

there should be numerous small pumps that could be retrofitted to your boat. i would look at fountain pumps or perhaps pumps for cooling inboard marine engines. both are likely to come in numerous sizes with various characteristics.

benbenben

Don't know if this will help but I looked up what I thought may be an interesting topic when you mentioned 11,000 rpm and it sounded like a hydraulic system. "Hydraulic pumps for use in boat power train" and found some interesting references to the matter. You may want to google it.

GA, but I would have been a little stronger about cavitation. 11,000 RPM is pretty fast for water and, if you don't have a pretty flat prop, I think you might find you're making more bubbles than speed.

Thats a really cool boat and I agree with Andy Germany on the plumbing design very well done. And again I agree with Andy Germany on the two pumps with a motor on each, instead of one pump with two motors. They do the inline thing with tractor pulling and drag racing but they can't ever seem to get the full HP out that they expect.

I think that you might have a problem with the high RPM out of those little engines, 11000 rpm is really high. I know that with model airplanes they have to be very careful with those little motors and propeller size and shape as well as angle. They can get the tips of a propeller going faster then the speed of sound. I do know that you can over rev a prop and at that point all your thrust goes away. You might want to look for some sort of gear reduction system. When you reduce the RPM's your HP will go up, well really your torque will go ups and I won't go into the HP - torque battle. But way I look at it more torque is always good.

I saw someone mention hydraulic pumps. Which reminded me that I have see a real small little hydraulic gear pumps that might work real well for what your doing. I can remember where I've seen them but I will see if I can find them again. If you want to search for hydraulic pumps what you want to look for is a gear pump.

Something like this?

http://www.funrcboats.com/ProductInfo.aspx?productid=GR2349

or this

http://www.hobby-lobby.com/basic_jet_propulsion_boat_drive_118_prd1.htm

Cheers!

The reason why I'm designing drive unit is the size of the boat.

Those little jetdrives would not even move that boat.

I will respond to all other posts after work.

Thnx for now.

sn4il

Sorry to be a spoils sport, but why in the world would you put motors on a deep-v cabin cruser that would propel it 60 actual MPH?

What I see is a 5, maybe 6 foot long model at the most. There will be two 1 or 1.5ci 4 cycle motors there. Evan if you put a scale prop on this boat, it WILL go much faster than it should (scale).

My feelings is that with this fine model of a fishing boat, just put the electric inboards with good props and have a boat that goes 10-20 MPH, shows a realistic wake, and is able to turn on a dime if on motor is reversed (as a real one would).

On a lighter note, I would like to be there on the first launching. It would be quite a sight when 2 ducted fan jetdrives, with motors screaming at 11,000 RPM, gulped their first taste of water, the 3 inch inlets sucking the water out from under the boat and take it straight to the bottom.

I do think a jet drive is an overkill for this fine model, though. No reverse, no manuverability at low speed. Wish you luck.

Someone mentioned "Ducted Fan", got me to thinking ,and it didn't even hurt too bad, about the fans used in model aircraft "jet engines". These are the ducted fan "Jets" not the real turbojet engines. They should be able to handle the RPM's, but in moving water instead of air there will be inefficiency's do to the different liquid density's. Be sure to keep the impeller to housing distance as small as possible. I would try to hone the impeller to the pump housing with the impeller spaced into the housing with shims, then reduce the shim thickness to achieve the optimum clearance. Maybe you could get the impeller, which I believe are of composite construction, ceramic coated to reduce the effects of cavitation, the great destroyer of props and impellers.

Hi,

I'm not trying to re-envent the wheel here. Jetdrives available on the market are simply smaller 40mm dia, mine is 76mm. I searched thousands of pumps from different manufacturer's and could not find a single one that could fit this project.

Tcinc - thnx for the topic.

Andy - thnx for good advices. I think I will start testing with propellers available on the market. The hint with turning propeller down is great. I'm still not giving up on impeller, I think that gap between blades on regular prop is too big to create good pressure inside the tube. Impeller's blades overlaps each other.

Harry - thats a good hint also, i did more research and decided to change power plant and lower rpm's to ~6000 max

Fishpipes - good point with torque

Robin - I'm familiar with these pumps available on the market, these uinits are smaller

Guest (whoever you are) - thnx for great ideas

I made the hull few years ago and it was supposed to be used in mono boat. Few years went by and concept changed :D

I'm simply playing with this boat, adding things as I build. It developed to the point that will be albe to steer by itself through the waypoints sent to the boat via xbee.

CFECO - I did researched ducted fans and impellers. It's not going to work, it's simply not efficient enough due density and blade design.

I decided to change a concept again and swich motor to electric.

I ordered Turnigy Aerodrive 380kv/1820W

Model: SK50-65-380
Kv: 380rpm/v
Turns: 6
Resistance: 24
Idle Current: 1.3A
Shaft: 8mm
Weight: 430g
Rated Power: 1820w
ESC: 80A
Cell count: 4~8 Lipoly

I will power that beast with 6S2P 10000 Mah Lipoly Pack.

It will be much easier to experiment with props/impellers running electric setup. Wattmeter will give me more accurate data to play with.

sn4il

Nice accurate post with a lot of good feedback for us all.

I agree, with an electric motor you are going to have no problem experimenting with various props......it will really help.

An idea just crossed my mind (well its only a short distance!), what if you carefully drilled a prop completely through, and if metal, continued the thread, and put TWO props on the same shaft? It may be a daft idea, but I thought that I would run it past you anyway......turned down so that the both fit with very little clearance in the tube......

By the way, the restrictor on the outlet may not be a good idea.......

Hi sn4il
I'm afraid I have to spoil your plans for revolutionizing the jet propulsion.
Honestly, what you have done with this PVC fittings is a VERY,VERY,VERY BAD design of body jet, please read only a little on jet design and hydrodynamics, you will see that you are building a design filth.
Whith this body design is not necesary to design an impeller to get decent results, you will never get decent results!
The really critical part of the jet is the inlet, inlet losses are about 35% of total looses on a good jet design.

In the real world,the part of the jet unit that really is in need for improvement is the flush inlet and ducting up to the impeller inlet; FORGET about 90 degree inlets. If you are still interested in jet development, go study basic hydrodynamics and pump theory first!

Interesting links.

Radio Control Jet Design:
http://www.rcgroups.com/forums/showthread.php?t=922716&page=7

Boatdesign.net
http://www.boatdesign.net/forums/jet-drives/

Thank You,

I guess I did not write anything about the inlet in the first post because it was not the point of my question.

I have 2 versions, both with modified inlets of course. Single and doube and believe me, its nowhere close to 90 degrees :D

sn4il