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LAWNMOWER ENGINE FOR INBOARD POWER


Roger Peterson

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Hi Everyone,

My brother asked me how to install a horz.shaft 6HP lawnmower engine into a 14' wooden fishing boat as inboard power with F-N-R control.

I'm a sailing guy and don't know anything about power boats. Can anyone point me in the right direction as to "Transmission building" with belts and pulleys?

Do you know of any links to help out a couple of powerboat dummies?

Thanks,

Roger

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A simple transmission is fairly easy, just being a few pulleys, an idler wheel or two, V belts and a lever to engage the engine. Of course you'll also need a stuffing box, rudder and steering gear as well.

One of the biggest hurtles to over come are the limitations of the lawn mower engine. Most small engines of this type are L head designs with suction carbs and oil slinger lubrication. This means it really can't live on much of an angle before it dies a smoky, oil starved death. The angle is necessary for the shaft to stick out he bottom of the boat.

Other issues are: the engine RPM will be quite low for the props typically used, the carb will have a tendency to flood in choppy conditions, at 6 HP you don't have much more then a trolling motor for power on a 14' wooden boat, venting the heat and providing enough cool air for it to suck on.

You could attempt to hijack a transmission out of a garden tractor. Small garden tractor typically use belt driven "gears" and a lever.

You might want to "upgrade" that engine to it's biggest capacity. Most engines are made from a base casting. An example is a set of outboards I want. I'd like to have two electric start 15 HP outboards on this small boat of mine. The 15 HP versions cost almost 3 grand each. I just happen to know the same brand's 8 HP engine is the same engine and costs $1,700 each. I'll change the jets, adjust the timing and install different venturies, plus a few hot rod tricks. They'll produce over 15 HP each for considerably less then the cost of a new 15 HP.

Lawn mower engines are the same deal. You might be able to get 10 HP from your 6. Then again it may already be the biggest end of the model line, with sister engines of 2, 3 and 5 HP available.

As far as specifics, I'd need to know what you have and what you wan to do. My first thoughts are the engine is very small for the boat you have. My next thought is a used outboard isn't very costly, especially after you consider how much a stuffing box, steering and rudder setup and a home made transmission will eat out of your wallet, when everything is said and done.

If you're a reasonably handy "fabricator" then you can slap together this home made drive. If on the other hand, the whole idea of making a set of pulleys spin backwards from the engine rotation is past you, then you'll need some help and would probably be best advised to find an outboard.

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"One of the biggest hurtles to over come are the limitations of the lawn mower engine. Most small engines of this type are L head designs with suction carbs and oil slinger lubrication. This means it really can't live on much of an angle before it dies a smoky, oil starved death. The angle is necessary for the shaft to stick out he bottom of the boat."

We never thought about the mounting angle starving the motor of oil. My brother has a motor on hand and does not want to buy an outboard. If he sets the motor more or less level and runs a belt and pully to the shaft assembly set at an angle, he should be ok.

The boat will be used on the Biloxi back bay and the connecting rivers. Very sheltered waters. No desire to go fast. In fact, he wished for a small diesel pulling a large prop real slow... but he had the lawnmower motor at hand.

We were looking for help "making a set of pulleys spin backward" as you said. Having never seen this arrangment, We thought we would ask for a little help.

Thanks anyway

Roger

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First off you need to "hot rod" the little engine. This way you'll get the most out of it.

Do a "port match" job on it, with the intake to block and the exhaust. Next modify the exhaust to have a straight run away from the engine with no bends if possible. Then a bigger muffler, preferably baffled not the perforated tube most often used. Clean off any casting "flashing" on the carb and intake runner. Smooth out any rough edges or humps in the intake, through the carb and runner.

Next check to see what model you have and if it's at the small end of the casting type. If it is, find out what makes the difference for the larger version. Usually it's just a couple of jets and timing differences. Make these changes.

If the carb is suction type (tank located directly below the carb), connected to it's body. This will be a hard one to over come as you'll not develop enough "head" to suck up fuel through a remotely mounted tank. You could install a fuel pump and float switch to supply fuel from the mounted tank from a remote or just fill from a can.

If you run down to the local lawn mower repair place, they'll have carcasses of spent garden tractors lying about. Have a quick look at the way they handle reverse. It's little more then some pullies an idler wheel or two and some belts. One look and it will explain more then I can describe here. Look at several, as they're all slightly different, but always very simple.

The easiest reverse gear is to have a friction wheel (a solid rubber lawn mower tire will do) on the transmission shaft engage the prop shaft. The transmission shaft would be V belted to the engine output shaft. Neutral is usually just an idler wheel that releases pressure so the belt's slack lets the transmission stop spinning.

Basically you'll need a separate shaft, which will be the transmission. Mount it on some pillow blocks and rig up a pulley at each end. One pulley takes the output from the engine and will have a idler wheel in line so you can disengage. The other end is V belted to the prop shaft. This is forward and neutral. Attach a lever to the idler pulley and you can engage or disengage at will. Reverse is kind of hard to describe easily, but visually easy to understand once you see it. With the transmission in neutral, a friction wheel is pressed against a separate shaft and the prop shaft (or a wheel mount on the prop shaft). It should do so in a way that releases the transmission's forward gear, V belt tension. This is usually accomplished by another idler wheel that lets the V belt get slack, as reverse is engaged. A reassuring little chirp from the rubber wheel(s) will tell you it's engaged. This simple transmission is good to about 20 HP. It'll have maintenance issues of course, but lube it up often and it'll do fine. Keep your fingers away from the spinning parts and you'll be safe.

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Thanks PAR,

I'll send my brother a copy of your post. He has a small engine repair shop close to where he works. I'm sure he will be able to figure out how to make it all work with your advise. Thanks again for your help. I'll keep you posted with his progress as he sends me updates and pictures.

Roger

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  • 2 weeks later...

;D ;D

Sea trout, or as they are known here, Speckled Trout, are one of the BIG three sports fish. Trout, Redfish and Flounder.

There are literally thousands of guys who target sea trout exclusively.

We love them when fresh- hate to freeze 'am though, they tend to get mushy.

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Hi Bruce,

We had the same kind of boats on the back bay in Biloxi, Mississippi. That is what my brother had in mind when he asked me for help with his project. He will be fishing for Speckled Trout and looking for secluded spots to throw his cast net for Shrimp and Mullet. He could make the boat go forward easy enough, it was reverse he was looking for help on.

PS

His boat is actually 12' long, not 14'.

Roger

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My vote for "most creative" design of a small engine to power a boat goes back to my days in Key West when a couple of refugees from Cuba built a raft and powered it with a weed whacker they "converted" by using a beer can to make a propeller for the hub.....In spite of the dubious engineering they made it as far as the reef outside of KW ( 6 miles out) and  were only stopped because they ran out of fuel ( HOW do you guess how much to bring along on a trip like that...). Even the coasties who "rescued" them had to admit it was a pretty gutsy voyage. Last I saw of the raft it was on display down there a few years back...

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  • 8 years later...

Ok im looking to make a lawnmower engine an inboard motor for my 16' jon boat. Would the steering linkage from a vehicle with two collars on each end 1in bolts to the down shaft on the lawn mower motor the other end bolts to the foot would that work with a knuckle in it or if you have any ideas to make a 25 horse lawn mower motor down shaft work as an inboard I like the idea of using the axle from a lawn mower for it to shift gears but couldn't you use it to also Drive the foot as well

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I've done as you're envisioning and it's not as easy as it might initially seem, mostly because the lawnmower parts just aren't very durable, particularly in a marine environment. Garden tractor axles aren't remotely strong enough, nor are the transaxles, particularly with a 25 HP engine. Besides, most transaxles  reduce 3,500 RPM's down to a few 100 RPM, which is way under geared for an inboard which would typically be reduced 2:1 or 3:1.

 

Have a look at "mud motors" for an approuch that uses few parts and works. Of course, you'll have to live with 8' of shaft dangling off her stern. A horizontal shaft air cooled engine can be mated to a go cart trans (FNR) and now you have something you can work with, naturally a beefy shaft and prop, probably a strut and rudder too. In the end, it's way cheaper to buy a worn outboard, repair/rebuild it and have all the power you need, plus vectored steering, FNR shifting and parts availability.

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  • 4 weeks later...

There used to be a guy in St. Pete., Fla. who sold plans for a conversion using a clutch. I don't think there was a reverse. he called himself Capt. _______. He advertised in comic books, Popular Mechanix, etc. I met him once in Gulfport, Fla. where he was running one. Wish I could remember more...

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19 hours ago, PAR said:

I'm not sure who CLP is, but their site hasn't any images of FNR transmissions, no videos, nor any customer reviews, which make one question the company, let alone any product. Can you provide a link to the page?

 

Paul,

On the link I provided above, there is an image of the unit as well as a table for matching gearing with different engines. I can't find much about the FNR transmission on the internet, though. Next time I travel to Bangkok, I will try and scare up some more info. Here is an old promotional video about their reduction gearbox and another one on putting a their longtail setup together.

Rex

 

 

 

 

 

 

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