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by Douglas Keith

(Doug mentioned that he had used a Briggs in his boat, so I asked him to share how he did it since others have asked about converting smaller motors.)

Here we go…The tough question you asked is “how would I tell someone how to do this”. Boy, when I thought about it in those terms, I realized how hard this would be. Well, here goes.

First a summary: As for cooling, the Briggs I used was splashed lubricated, so I had to mount it on the horizontal, then come off with a jack shaft to a pillow block with a thrust bearing, then to a universal joint (I used a Spicer), then thru the floor through a 16 degree shaft log. In short, it kept the motor upright and in the relative wind. It is a small boat designed for a special purpose, coon hunting, or you could use it in back waters where wave action wouldn’t be a big deal i.e. sloughs, small lakes, etc. It gets excellent fuel usage and makes about 10 knots full bore. Transfer of power was done thru a centrifugal clutch w/double row chain to the jack shaft. I tried routing the exhaust out the transom, but it was too noisy, so I ended up with a straight up pipe w/muffler.

The details:

1. I used a Briggs model# 130202.

2. Overheating wasn’t a problem, in fact I never considered it. They are cooled with a fan, so it was never a thought.

3. I used a standard centrifugal type go-cart clutch w/double row chain. I found out by trial and error, that a single row works, but durability is not there.

4. After the clutch, down to a shaft with a sprocket, the one I used was held in place by 2 pillow blocks, fore and aft.

5. Then to a u-joint. You need one with strength, so I used a Spicer rated at 40hp.

6. Next a thrust bearing. This is where you get your shaft log angle, so the thrust bearing is mounted at the angle of the shaft log. In my case at 16%. I got the bearing from Kaman Bearings. It was adjustable, so I could mount it flat and then rotate it to match the angle I needed.

7. Then you go through the floor using a standard shaft log and packing gland then to the strut, and prop.

Your Thoughts?


4 Responses to Converting a Briggs and Stratton for Use In A Boat

  1. James King says:

    For universal joints try G and G in Omaha, Nebraska. Their products are in every farm store in the Midwest and I would guess the rest of the United States. You may find centrifugal clutches in the same area of the store. http://www.ggmfg.com/
    Centrifugal clutches with two sprockets for roller chain are available and as the author states they are necessary. I would suggest operating the motor at 1500 rpm and using the chain drive to gear down the rpm at the propeller shaft. I tried routing the exhaust into the water to quiet the engine and found that you can’t go too deep or the back pressure will stall the engine and make it hard starting. This type of project is undertaken by power mechanics, welding and machine shop students and it results in an excellent power transmission education.

  2. scott h. says:

    there are some fellas down in Bradenton, Fl. that adapted a B/S to an outboard drive…..pretty kewl….Steve @ Dave Lucas Boatworks and Happy Hour Club! kewl kats.

  3. david says:

    How about some pictures?

  4. Tim says:

    There’s a couple of ideas in Bueller’s Backyard boatbuilding. If memory serves it was less specific. I would bot worry about a lot of bew fresh air those motors don’t need much to stay cool.

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