Propeller Tunnel / Impeller for child safe motor

[charliep]

I would like to build an electric boat which can be used on the beach and will not have a rotating propeller which can seriously injure people on the stern. I am an electric engineer with generous experience building electric drive trains so this is not my issue. Propulsion will be a 1.5kW DC electric motor.

I want to protect children, swimmers and inexperienced boaters from maiming themselves with a rotating propeller. My thought is to either use an impeller where there is no easy way to get a body part inside or use a propeller inside a tunnel. The impeller / tunnel will be fixed with a rudder at the end for steering. I realize this is not as efficient as rotating the motor but I want to keep the design simple.

The boat will be very light, less than 60 lbs with a payload of around 400lbs with a flat bottom, except for the propulsion system in the middle. The boat will be about 8ft long x 3.5ft wide. Boat speed is not critical (but of course nice). Since the boat will be battery powered efficiency is important for longer range.

The propulsion system will be located beneath the stern. I would prefer NOT to have the motor inline with the propeller to improve efficiency and decrease the diameter of the propeller. The draft of the boat will mostly be the diameter of the propeller beneath the boat. If this motor is not located inline with the propeller it could easily be built into the stern.

Does anybody know if something like this has ever been done before? Product names, pictures, suggestions, websites, ideas would all be appreciated.

[DCockey]

Link to a page on integrated motor/thrusters with the motor around the perimeter:
http://www.tsltechnology.com/marine/thrusters.htm
Don't know anything else about them.

[Tim B]

Welcome to the wonderful world of naval architecture! You have just found out that you can't have it all and have to compromise somewhere. The question is where?

You have a few obvious options:
Propeller with prop-guard. This is well used on outboard motors and is a reasonably good solution, but might not provide the level of protection you require. Smaller meshes will drop the efficiency but you may be able to trade that with propeller size.

Propeller in a tunnel with mesh protection. Half-way to a waterjet, but you may have significant ventilation issues resulting in low performance.

Full waterjet. Very safe, but again smaller inlet mesh will cause greater blockage and lower efficiency. However, it would probably be the easiest package to install. Maneuvering nozzle and reverse buckets would be necessary, but would not affect the draft.

Thrusters are likely to be less efficient than a waterjet of comparable size, but it may suit your setup.

You could also consider a caged air propeller. This is probably the most efficient, but raises stability and control issues.

Don't forget to add the weight of your batteries into the deadweight of your boat. They are not payload (ie. cargo and people). Even lithium-based batteries get heavy quickly in these designs. You are also looking for a hull which is "easily driven" (ie. it has low resistance), so you might look at a racing sailing dinghy hull if you are going to be working in the displacement regime. If planing then a shallow V hull will server you well, but you need to do some preliminary powering calcs to tell you where you are. This requires basic hull designs and weight estimates.

Cheers,

Tim B.

[gonzo]

I don't think a jet is efficient with an electric motor. A tunnel with a grate under is a good compromise. It will allow for a central locatioin of the motor and batteries too.

[charliep]

Batteries and controller will go inside the boat. With a tunnel configuration the motor would probably go inside the tunnel unless I use something like the TSI IntegratedThruster approach. That is pretty close to what I am looking for but it looks expensive.

Any mesh which would go on either end would not be tight enough to significantly constrict water flow. Holes in the mesh would probably be around 2 inches square to keep limbs out. The tunnel would be long enough so that fingers poking through could not reach the propeller. I am more concerned about a wave catching the boat and pushing it into a person than somebody actually sticking their hand into the propeller.

To steer by rudder I think the tunnel approach is better than a caged approach.

[gonzo]

A mesh will significantly constrict water flow; do the turbulence calculations.

[ProtectTheOcean]

Reminds me mostly of what some SCUBA divers use to haul themselves around, no?

This also keeps marine mammals safe, which gets my vote!

[Submarine Tom]

charliep,

You've got lots of power and some good ideas. I don't know of any such designs but am confident you'll do just fine with your intentions.

Do you have any design ideas on paper yet?

-Tom

[rasorinc]

type in Dura jet pump. My computer is acting up so I cannot give you a link. It fits on outboard engines. It is an encased impeller and is very safe. No loss of power they say

[charliep]

The website for DuraJet is:

http://www.actorlando.com/recreational-products.htm

This seems like some fantastic technology. Unfortunately I am looking for an electric solution rather than a gasoline solution. Does anybody know about the underlying technology for this product?

[john zimmerlee]

Since 2004, I've been working on this exact same project!

First, electric power is only for low speed and very efficient hulls.
Calculate 1.34 times the square root of the hull length at waterline.
That's the theoretical max speed in knots. Is that fast enough?

Second: At slow speeds (under 20 knots) large amounts of surface area on the propelling member (prop, oar, paddle, etc) work best. Water jets do not function well at slow speeds. I spent over $100,000 proving that! "WaterJets" include putting a prop or screw in a tube and blasting water out the end.

Third: Props in an inverted "U" shaped tunnel were tried. If the fore and aft ends of the tunnel are swept down to the bottom of the hull, the blades will be hidden and less likely to do damage. I tried this in 2005 and found that too much energy is lost sucking up the water . . . and then forcing it back down. But another manufacturer http://twintrollerboats.com/ is offering a similar product with questionable results.

Fourth: What does work is an inverted tunnel open at both ends. See the earlier version in action at http://www.youtube.com/watch?v=XkknAR1tmLY and the later version at http://www.youtube.com/watch?v=3sZ8cKelDRs

Instead of a prop, I am working on an auger that has a smooth lead-in on both ends. It has thicker flighting and tends to glide over sea weed, hyacinth, coral, manatees, and possibly humans. It ran over a bubble-wrapped sand bag without ripping the bubble-wrap. I haven't volunteered my backside yet! It also has a steeper pitch that the (4" only) standard trolling motor props. This means not only greater forward speed, but equally impressive reverse.

Tom Hoad, who organizes Olympic swimmer competition has expressed interest in the boat to monitor open water swimmers.

john.zimmerlee@gmail.com

[Tim B]

The question with this is compromise. Remove the safety aspect and obviously you'd use the largest submerged propeller you could fit under the hull. It is diameter which most notably improves efficiency, not surface area. The surface area controls the onset of cavitation by changing the blade loading.

Waterjets are well-known to be inefficient at low speed (you can do the maths to prove that!), but it may be a reasonable compromise depending on the operational conditions and through-life repair cost etc.

Changing the head of water is expensive, as you found out, but where noise is not necessarily a consideration, why deflect the water down again (causing extra resistance in the tunnel) ? Propeller tunnels do work and are not an unusual way to reduce draft and/or shaft angle.

We haven't considered the hull-form, of course. I had assumed a monohull, but a displacement catamaran affords a ready-built tunnel between the hulls. All you have to do is fit and protect the propeller suitably. This could even be a combined motor & propeller pod. A trimaran might afford the ability to use a "tanker-type" stern with the propeller well-guarded from the front, giving an easier setup to protect.

There are many solutions, none of which are right, and none are wrong. However, they will have differing levels of suitability. It is the clever designer who can pick suitable bits from an unsuitable design.

Tim B.

[Submarine Tom]

Charliep,

Do you have any drawings yet?

What are your design concepts?

3 - 4 knots?

Tunnel hull?

-Tom