Weed eater engine conversion

To Roy: glad you and yours are OK after the storms. Thanks for posting the updated plans. I bought a set from you via Ebay a while back, but I've stalled d/t the gearing cost. My boats are light enough taht I can probably do the longtail version.

To All: anyone seriously particilating in this thread should probably go to the library and check out Dave Gerr's Propeller Handbook. It tells you all about how to calculate things like how fast a given weight hull can move with a given hp, and the prop dimensions needed based on shaft rpm. Handy stuff!

Essentially Roy's setup with the reduction gear lets you run a bigger prop, which gains you more efficient operation, and more real-world thrust at low speeds. It does not gain you 8 hp - Roy calculated that wrong, but it should get you low speed thrust more or less like what you'd get with an 8hp motor without gear reduction.

If you do the math in the Gerr book, you can see that a 3" prop will let a weed whacker turn to its full rated rpm. Pitch would be governed by how fast 1 hp can push your boat. But it would have to be pretty flat for sure. This tiny prop would have a lot less thrust and be less efficient. It would have a harder time pushing you against a headwind, for example. It might even be better to throttle the motor back and sacrifice some hp to get lower rpms and run a bigger prop, but this would depend again on what you're pushing.

Here's an extreme example - suppose you used the gearbox from a Mantis tiller. That 240 rpms would let you swing something like a 22" prop! (That's from memory, but I know it was really big.) Still only 1 hp, but using it to make a *wall* of slow moving water rather than a tiny stream of fast moving water.

Seriously, read the book. This stuff is engineering, and that's no place for wimps! The math is simpler than it looks, and once you get it you can do a whole lot.

The short story is to use Roy's plans if you need lots of thrust (e.g. superstructure catching air or a big boat), and use something like ripped off's longtail for a small, light boat with not much weight in it.

Best,
--Rob

 

Here's some math for y'all. Suppose your boat can be driven to 4.75 mph with 1 hp, which I'm assuming a weedeater makes at 7000 rpm. All these are for 2-bladed props. Here's what the calcs tell you:

7000 rpm (1:1) - 3.12" x 1.92" prop (not much prop surface area here)
3500 rpm (2:1) - 4.73 x 3.81" prop (Already WAY better)
2800 rpm (2.5:1) - 5.41 x 4.81" prop
1750 rpm (4:1) - 7.17" x 7.7" prop (this is the Lil' Indian ratio)
1120 rpm (5:1) - 9.37 x 12.03 prop (getting steep, I wonder if it would start well from a standstill)

Notice how the 4:1 ratio gives you close to a "square" wheel - e.g. diameter and pitch about the same? There's nothing magical about this, but it is an indication that you're somewhere in the reasonably efficient range, ASSUMING those numbers are indicated by the formulae. Arbitrarily using a given prop will get you the problems that others have found, like stalling or lack of thrust.

--Rob

--Rob

 

will this setup push a 20' sailboat

I am looking for a small/inexpensive 4 cycle outboard to push a 20'(1350 lb) sailboat just far enough away from the dock on an inland lake so that I can put the sails up.

I have researched this website and others for the last two days for answers but I am still confused.

Do you think the following setup would push my boat approx. 2-3 mph?

Troybilt TBP6160 32cc weed trimmer and some type of prop(I would have NO idea as to which one to use or how to attach it)

http://www.troybilt.com/webapp/wcs/stores/servlet/product_10001_14102_496547_55009_-1

Would there be a noticeable difference in power if I went with the 29cc version vs the 32cc version?

The engine will probably have to be positioned in such a way that would not be the normal cutting position and I was wondering how this would work as far as gas to the carb and oil lube. Also, would I just attach the prop and pack the shaft with grease?

Thanks,
Timmytim

 

Howdy, I,m new on here. Getting ready to build a small longtail moter. After reading all these posts I think I'm going to try a small moter instead of a weedeater conversion. Heres a deal I found
http://www.northerntool.com/webapp/wcs/stores/servlet/product_6970_200333569_200333569

Tecumsuh 3.5 hp for 100bucks.

I know it's kinda pointless to have more hp since my boat wont get on plane but I figure with the right prop I'll be able to load the boat down and still reach max speed around 8mph.
My question is where can I get a simple gearbox simular to RickShaws'? I'm going to take ripped off advice and go with a 2:1.
thanks

 

Beavertail Prop Available

Hello everyone ! I found a small prop in my collection of toys that would work great for a weed trimmer beavertail conversion. The dimensions are :

Main diameter is : 5 3/4"

Shaft diameter is : 3/8"

I don't know what the exact pitch is, but I made about 15 other beavertails for other people and the 31cc weed trimmer engines I used worked very well. The engine did not bog down, and pushed very well. I added a skeg on the bottom of the shaft placed down near the prop. These things are great for 8' - 10' boats. Duck hunters, this is the cats meow !

I will send the new prop free if you need it, just pay the shipping. Contact me at : allaerotech@centurytel.net

I'll be happy to help out any way I can, I have designed, built and successfully tested so many different weedtrimmer conversions you would not believe it, and spent several thousand dollars doing it to get them right. It almost cost me a wife. This knowledge can be yours just for the asking.
Take care everyone.

 

This little motor will get it done without a gear reducer. No bog whatsoever. All I had to make was the bushing/seal on the prop end and bend up the handle out of .750 od .125 wall alum. tubing. I used a trolling motor mount to attach it to the boat. It uses a 52cc Chinese made brush cutter. $179 new. I tested it on my 13/6 " 4 rivers total wgt. 320 lbs. The motor pushes the boat with ease and the conversion was so simple even a caveman could do it. The prop is a Young`s T-12. I had one so I just put it on and hoped . It works great.

 

Looks good John, where can I go to get a power head like that?

 

Heres what I made so far. It works in a tank but I haven't put t in water yet. I used a briggs and stratton 34.5cc moter. It was tough though cause the shaft tapers down to 7/8's and I already welded a 1inch shaft so I grinded alot and made it work.
I finished the throttle cables since these pics were taken. All I have left is the boat mount. I'm doing one like ripped off made since I dont have an old trolling moter.

 

mini-longtail

The unit seen is marketed in the U.S. by Duropower.com it is their 52cc 2 hp brushcutter. It lists for 179.00 but they really lug you for the shipping. The engine is a Chinese built unit that they build by the thousands. They are used a lot on scooters , power boards .Go carts, pocket bikes, and ATV`s. You can find new motors offered on Ebay , Ebay express, and by doing a scooter parts search. There is a lot of aftermarket performance parts available for these motors to improve their power output. I spent about $30 on mine for a velocity stack, hi-performance air filter and a rocket key(wood ruff key) that advances the timing 10 degrees. It really brought the little motor to life and improved the low rpm torque. A very easy modification. I just received a new motor I purchased on Ebay Express to build my hunting buddy a mini-longtail. It came with an electric starter and a 5/1 reduction gearbox. It cost $145.00 delivered. Were going to try to run the 5/1 reduction and pull a 10 1/2" prop. Nice job on your unit. Let us know how it works in the water.

 

why has no one used an edger shaft to get the 90 degree bend? like the ones on an echo...they also can be mounted to bigger echo engines easilly.

 

I would try that if I was making an outboard type moter but here in the Texas hill country I hunt/fish rivers. The Blanco and Guadalupe are limestone rivers and the depth changes alot. You even have to get out sometimes and drag your rig over a dam or shallow area. With a longtail you just hold it where it clears and go.

 

Hey John, got my moter mount done last night. I will be trying it out in the water today. I have a question for you though. Does your moter have a clutch? Mine is really tough to get out so I left it in there. Most people seem to pull them out cause they burn up easily from the water load.

 

clutch issue

The motor has a much larger clutch than a weedeater type, 78mm. This type of clutch has replaceable clutch shoes $15 and locks up at 2500 rpm. It is nice not to have the prop turning when idling. The clutch pictured on the Brushcutter motor is from a 31cc Ryobi. Here are a few additional pics of the motor in its final configuration.

 

Took my boat out and it did great. There moter RPMs were perfect for the prop. No cavitation. I ran with GPS in hand and she did 8mph in about 40 yards. My bracket started slipping from the vibration and the gear oil in the shaft started leaking. I got a few bugs to kill but she works great. At 1/4 throotle she runs real smooth and easy. The moter seems to purr at that rpm. It does 5mph like that.

I'll post pics soon. I need a better bracket. Mine lets the moter sit way to deep when I'm not holding the tiller.

 

Wac m ; Glad to see that you motor worked out so well. The problem you reference regarding your mount allowing the motor to run to deep can be addressed by putting a limiter on the mount . Turn 4 fun has a design, posted earlier in this thread . I have an adjustable limiter on the trolling motor mt. that I used. It is that star shaped handle and threaded rod shown in the pic of the mount. Good job!