12' Low-Power Electric Planing Skiff – Design Review

[The Wing Guy]

... but a decent concept of the mechanical structure and parts.

Yikes! I could hardly disagree more. It looks like classic AI slop, with the bearings in the wrong positions, a thrust bearing that is in the wrong place and does not look like a thrust bearing, the main shaft seal in the wrong place, a toothed belt that is installed inside out, a piece of the housing called out as a water-lubricated bearing, a fabulously odd looking propeller, etc. etc. No hint re how the nearly 100 mm lower pulley diameter is not carried up the length of the housing, which would then be anything but streamlined, with its width being substantially greater than its length. For the very low thrust than a small outboard can create, ordinary ball bearings can serve for both radial and thrust loads. Three view drawings are much more informative than renderings.

 

[kmcsharry]

Yea, but other than all that it looks pretty good OK no more AI renderings...

 

[alan craig]

Some more AI (Alan's Ideas) for a belt drive solution; belt in water protected partially from weed by two strip uprights in place of a hollow leg. The shaft revolves in bearings in the bullet shaped bit. Also a picture showing a neat solution by the developer of Delftship using a Flipsky motor; this is from the May/June 2026 issue of Watercraft magazine. But I do realise that you already have the Mercury leg, I think the motor you choose will weigh less than that crank alone. I used a Turnigy C80-100 kv130 in mine.

 

[The Wing Guy]

Some more AI (Alan's Ideas) for a belt drive solution; belt in water protected partially from weed by two strip uprights in place of a hollow leg.

AI has come so far since 1985, when I first used it for creating a diagnostic system for industrial mechanics. Then, Pac Man was state of the art for CG graphics. The "drawing" that Alan has provided is very nearly indistinguishable from one created by a human! A really nice touch is the lined "paper" background, which looks so much like the real thing!

On a napkin somewhere, I sketched up a similar thing, but with the belt running up and down in two streamlined housings, which, if I were going to build it, I would fashion from hand shaped extruded styrofoam, subsequently wrapped in carbon fiber in epoxy. Then, in a serious affront to the environment, I'd dissolve away the foam with acetone*. I've thought about a similar idea, but using belt rim drive. Then there is the Yamaha Harmo (which strikes me -- no pun intended -- as an unfortunate name). Systems like the Harmo could reduce the likehood and severity of injury from prop strikes, I'd think. Next-Generation Electric Propulsion System – HARMO | YAMAHA MOTOR CO., LTD. https://global.yamaha-motor.com/business/outboards/products/harmo/

*or gasoline. The resulting mixture could be burned in my gas outboard, I'd reckon. (Or I could pour it into my wife's gas tank -- she still has one of those cars that has in internal combustion engine!) Don't do this.

 

[montero]

OXE OB's have belt transmission.

 

[kmcsharry]

Cool pics and sketch Alan, I really like the belt drive idea. Here is the "Ki" version (draw by me in free version of sketch up) of what I think will be a good design to put atop the lower leg assembly from the outboard. I will probably mock up in plywood first for testing and then build out of aluminum if it works.

Feedback and ideas as always are appreciated...

 

[montero]

What do you need that belt for?

 

[kmcsharry]

I don't really with this outboard propulsion set up but it gives me a couple advantages I think, if I want to try different motors I can set them up with the correct gearing to have my RPMs stable between various motors. I am also really interested in building a belt drive pod that would sit below the waterline and would eliminate the lower leg of the old mercury motor. I have seen this same set up for a electrical inboard design which I was also considering at one point. The short answer is my interest and that this system can be used in at least 3 systems I can think of, outboard, in board and pod drive. The ability to change gears to account for high RPM electrical motors is a big plus.

 

[SolGato]

The older Merc motor might share the same internals as the larger HP, but it’s the housing that matters, specifically the spacing from the prop shaft centerline to the anti-cavitation plate.

As previously mentioned, this is what determines how big of a prop you can run.

Todays family of engines, the 4/5/6 typically share one size housing, with the 8/9/9.9 sharing a more robust casting with larger prop.

With 2:1 reduction your motor was originally reaching 4500-5500 rpm at WOT and was incapable of getting most small boats to plane.

How do you expect it to do any better with an electric motor coupled to it?

 

[montero]

The older Merc motor might share the same internals as the larger HP, but it’s the housing that matters, specifically the spacing from the prop shaft centerline to the anti-cavitation plate.

As previously mentioned, this is what determines how big of a prop you can run.

Todays family of engines, the 4/5/6 typically share one size housing, with the 8/9/9.9 sharing a more robust casting with larger prop.

With 2:1 reduction your motor was originally reaching 4500-5500 rpm at WOT and was incapable of getting most small boats to plane.

How do you expect it to do any better with an electric motor coupled to it?

The 4hp OMC was able to get my boat into semi-planing mode for two people.
The 6hp TOH for one person was a pleasure to plan.
I weighed at least 80 kg back then, plus 20 liters of gas and some beers.

 

[SolGato]

The 4hp OMC was able to get my boat into semi-planing mode for two people.
The 6hp TOH for one person was a pleasure to plan.
I weighed at least 80 kg back then, plus 20 liters of gas and some beers.

Are you talking about a Johnson 2-stroke?

If so, they had 1.65:1 gear reduction which would be better suited for a speed oriented electric conversion.

With that ratio, your prop may have been achieving 3000+ RPM at WOT and 500 rpm can make the difference between getting over the hump or not.

One thing you don’t want to be doing is running your electric motor at semi planing speeds all the time unless you have plenty of battery capacity.

Prop size, material, RPM, gearing, pitch, blade count, etc.. it all matters.

Personally I think the original plan of using a compact brushless motor underwater is the better way to go for high performance.

 

[kmcsharry]

The older Merc motor might share the same internals as the larger HP, but it’s the housing that matters, specifically the spacing from the prop shaft centerline to the anti-cavitation plate.

As previously mentioned, this is what determines how big of a prop you can run.

Todays family of engines, the 4/5/6 typically share one size housing, with the 8/9/9.9 sharing a more robust casting with larger prop.

With 2:1 reduction your motor was originally reaching 4500-5500 rpm at WOT and was incapable of getting most small boats to plane.

How do you expect it to do any better with an electric motor coupled to it?

I don't think it will be "better" for getting the boat moving. It's more of a learning and building exercise and conversion to electric interests me. Hopefully the hull design and motor come together in a useful way. I do understand the prop size is limited by the smaller size motor design but the point I was trying to make is that increasing watts/hp should be ok with the gearing that is in the old motor lower section.

 

[kmcsharry]

Are you talking about a Johnson 2-stroke?

If so, they had 1.65:1 gear reduction which would be better suited for a speed oriented electric conversion.

With that ratio, your prop may have been achieving 3000+ RPM at WOT and 500 rpm can make the difference between getting over the hump or not.

One thing you don’t want to be doing is running your electric motor at semi planing speeds all the time unless you have plenty of battery capacity.

Prop size, material, RPM, gearing, pitch, blade count, etc.. it all matters.

Personally I think the original plan of using a compact brushless motor underwater is the better way to go for high performance.

Its a 1972 Merc 40 (4hp) I have confirmed 2:1 reduction gears. I agree there might be better more modern options but right now I want to start with a known mechanical unit instead of trying to build something up from scratch. The old lower to new electric motor is a proven concept.

Check this out:

 

[SolGato]

I don't think it will be "better" for getting the boat moving. It's more of a learning and building exercise and conversion to electric interests me. Hopefully the hull design and motor come together in a useful way. I do understand the prop size is limited by the smaller size motor design but the point I was trying to make is that increasing watts/hp should be ok with the gearing that is in the old motor lower section.

I’m fully aware that using a leg is “proven concept” and I support it, but you have to start out with the right components if you want to achieve your goal.

You asked for input, and myself and another member specifically said to use a larger motor due to prop size limitation which was in reference to your stated goals.

If it’s just a learning exercise that’s fine, otherwise you seem to have missed the point about the housing size and prop limitation.

Even the housing used in the video is stated to be a larger 9HP which means it has the ability to run a decent diameter prop.

Anyway, I’ll check back in when you’re on V2.

 

[montero]

Are you talking about a Johnson 2-stroke?

If so, they had 1.65:1 gear reduction which would be better suited for a speed oriented electric conversion.

With that ratio, your prop may have been achieving 3000+ RPM at WOT and 500 rpm can make the difference between getting over the hump or not.

One thing you don’t want to be doing is running your electric motor at semi planing speeds all the time unless you have plenty of battery capacity.

Prop size, material, RPM, gearing, pitch, blade count, etc.. it all matters.

Personally I think the original plan of using a compact brushless motor underwater is the better way to go for high performance.

OMC 4hp 2T 2.15 13:28 T .