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LAWNMOWER ENGINE FOR INBOARD POWER


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10 replies to this topic

  #1 Roger Peterson

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Posted 13 February 2009 - 09:41 AM

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

  #2 Craig

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Posted 13 February 2009 - 11:07 AM

Paul can help with that.  He has a friend that made an transmission  much like you need.  Paul???

  #3 PAR

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Posted 13 February 2009 - 12:16 PM

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.

  #4 Roger Peterson

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Posted 16 February 2009 - 02:54 PM

"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

  #5 PAR

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Posted 16 February 2009 - 10:44 PM

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.



  #6 Roger Peterson

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Posted 17 February 2009 - 09:23 AM

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

  #7 Bruce Kershaw

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Posted 01 March 2009 - 03:46 PM

Back in the early 70s, when things were really different here, (It was still a frontier in many ways.) I would always see these commercial fishermen in what looked like oversized, (Im guessing 16-18 ft.) flat bottomed rowboats. They were powered by inboard, air-cooled lawnmower-type gas engines.  They would slow-troll the flats with several lines out on cane poles. I believe they were targeting sea trout. (Not sure why Not a meal I would catch on purpose.) I got close enough to check out one of the boats, just once.  From what I remember, it looked like it was direct drive, forward only. Seems like there was a short piece of rubber hose clamped on each end to a split drive shaft, creating a flexible u-joint/dampener.  The rudder shaft was mounted on the outside of the transom, and controlled by a stick up towards the bow.

For years I have kept my eye out for one with the thoughts of restoring or reproducing it, but no luck. Ive searched the internet, but dont really know what to search under. Few people I talk to remember them, but some do.  Youd think it would be a local classic. There were so many of them at one time. 

  #8 Charlie Jones

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Posted 01 March 2009 - 03:56 PM

;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.

  #9 lewisboats

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Posted 02 March 2009 - 06:29 AM

Don't forget that a LM engine uses its blade as a flywheel...required to keep an engine like that running, especially a 4 stroke. You will need provision for this too in  your setup.

Steve

  #10 Roger Peterson

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Posted 02 March 2009 - 06:38 AM

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

  #11 Docpal

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Posted 02 March 2009 - 12:31 PM

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...