Prop question for homemade outboard

[Russell Sova]

Yes, I bought the plans to the Homemade Outboard sold on ebay. Well, it definately will work OK and even the techs at the trolling motor company thought it would be fun to try. My question is the prop. The trolling motor prop is a 9x4 and the original electric engine put out 1200rpm max. The gas engine convert will put out 1900 rpm. What prop size would give the maximum speed and make sense cost wise, etc.?

[Garrett Lowell]

I don't have an answer, but I'm highly intrigued that you're making your own outboard. Sounds cool! Can you give us some more info, or do you have a website?

[Russell Sova]

The guy's ad now includes a photo. He's emailed me back a number of times and he is a very nice guy, helpful and seems on the level. Homemade Outboard (http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&rd=2485656681&catagory=50439)

[Russell Sova]

I'll try it again Homemade Outboard (http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&rd=1&item=2485656681&catagory=50439) I screwed up the numbers on the last one.

[Gresham CA]

Keep the trolling motor prop. What goes inside the electric motor housing?

[alteran]

Without betraying the guys secret can you give anymore info?

Looks like a string trimmer motor connected to an electric trolling motor drive.

Am I close?

[Cuyahoga Chuck]

Russell,
The problem with most of these home-brew motors is that it's difficult to marry a small gas motor to a boat prop in a manner that keeps both operating in their optimal rpm bands.
What's required is a gear reduction unit. No matter what prop you use you have to spin it slow enough that it gets a bite on the water. The gas engine, on the other hand, has to be revved up to make enough torque to keep from stalling.
The blurb on the web site is a bit shy on how all this is accomplished.
Electric boat motors operate from O-1200 rpm's. They make max torque at 0 rpm. Their props spend a lot of time below 700 rpm. A 30 cc weed-whacker motor probably needs at least 2500 rpm just to keep from stalling. If the e-bay guy has all this engineering mumbo-jumbo worked out you got your money's worth.
I wouldn't worry about the prop just yet.
Charlie

[Russell Sova]

The motor is geared 4 to 1. All of the gearing info and even where to call and who to talk to are included.

[Tom Robb]

This is a gag, right?

What connects the motor to the prop?
There's no drive shaft in an electric's shaft and no right angle gears in the motor housing, is there?
Besides why would you bother changing the quiet of the electric for a screaming noise maker?

Let us know how it turns out.

[Del Lansing]

Producing 8.8 HP at the prop???? don't think so! I'm not saying the unit doesn't work, but putting reduction gear in the drive line doesn't change the 2.2 HP motor to put out 4x. Something to do with conservation of matter and energy or something like that.

[Russell Sova]

Gotta agree with you Del. I emailed the company that makes the engine and they say it tops out at 1.4 HP. With gear and friction loss I'm guessing 1.2 HP at the prop. That's still much better than what I have with my trolling motor now and in addition the engine will turn a prop that is larger with a longer pitch, i.e., trolling motor prop is 9X4 whereas a 12X6 prop is possible with this motor. Trolling motor 1200 rpm, this motor 1750 after being geared down. It'll be 3 times as powerful and weigh very little, just what I'm looking for.

[Del Lansing]

It'll be 3 times as powerful and weigh very little, just what I'm looking for. OK, just wanted to know what to expect out of it. The reduction gears will give it the added torque to swing a larger prop, but wanted to make sure you understood horsepower doesn't come out of thin-air. Have fun with the project.

[John Blazy]

Very innovative idea this guy has. Did you already have a trolling motor to use, or did you go out and buy one? Weed whacker motor too?

Anyway, I bought a stainless steel Minn Kota E-Drive prop and mounted it on my MK trolling motor powered launch to give better speed, and the higher pitched Edrive prop definitely pushed it harder. The higher torque load on the motor was countered by the kort nozzle I fabbed for it.

I bought the Edrive prop here edrive prop (http://www.iboats.com/mall/index.cgi?prod_id=38867&current_category=trolling%20motor%3Epropeller&cart_id=856876376)
This prop is actually smaller diameter than the weedless wedge props, but the higher pitch and apparently larger blade surface area are also factors. This prop will also convert to the MK shaft ok as long as you rebore the shear pin larger, and mill a bushing to increase the diameter (I made mine from a solid brass door pull, then final turned to the perfect OD right on the actual spinning prop shaft).

[ 08-05-2004, 09:48 AM: Message edited by: John Blazy ]

[Cuyahoga Chuck]

Russell,
Just read your comment about the superiority of the weed-wacker over the electric.
I don't think you've got the range yet. You can't make a direct comparison between e-motor HP and gas motor HP. The differences in their respective torque curves mean that a fudge-factor is required.
If you replace an electric motor with a gas motor the gas motor must be rated at a minimum of 4 times the HP of the electric in order to do the same amount of work.
Standard electric lawn mowers have 12 amp motors which is about 1 HP size. Comparable gas mowers all have @ 5 HP engines.
If I were building this device I would first try to see the power curve for the chosen motor. Trying to run a motor of 25cc-35cc below the lower edge of the power curve is never going to work. Falling below the power curve, even by 500 RPM, could mean a 50%-75% drop in HP if you can keep it from stalling.
I could be wrong, but I have a hunch that the weed-wacker conversion won't outpull the electric.
If you prove me wrong I've got an old Lawnboy 3 HP that runs like a top.....
Charlie

[kra]

Russell Sova
i was intrigued by your post so i ordered plans
i had an old trolling motor laying around and a similar weed eater collecting dust so i figured what the heck
needed a small motor for my row boat anyway
well the plans were very detailed
had to modify things a bit to work with my "junk"
i was really suprised at how well it turned out
pushes my 9' rowboat much faster than electric motor did(same motor used and same prop) lighter than electric and battery for sure.
only ran it for a couple of miles but so far so good
hope yours turns out well also