35horses

TANKAR's Tech Tips for small plywood boats.
Making the most of your mini-max or doodlebug type hull.
35 Horses on my mini max style hull.

Warning: Do not try this at home unless you have good insurance. I take no responsibility for anyone doing what I have done by putting more horsepower on a boat than the hull is rated for and will in no way be held accountable for anyone else's foolishness in this regard.

When I was building this hull I did not yet have a motor for it. I actually never even put the motor board on it until I got a 35 horsepower Evinrude Big Twin from my wife's step father at a great price. It was a 1959. After I figured out how high my motor board needed to be to get the motor up high enough for performance I cut it out and mounted it up with what I hoped would be a strong enough design with a little extra wood and screws to handle the heavier motor.

The motor seemed to weigh a ton but it's actual weight was probably around 100 lbs. I had a feeling that this was a bit heavy so I set out to reduce it's weight somewhat. I removed the top shroud which got rid of about 8 lbs. Then I removed the recoil starting mechanism which saved about 4 or 5 more lbs. It was also an electric start model so off came the starter and gear from the flywheel. The starter weighed about 6 lbs and the flywheel gear was about 4 more lbs removed. Next to go was the tiller arm and throttle that saved about 2 more lbs and the lower shroud fore and aft on the motor which saved about 2 more lbs. This motor already had a fairly short shaft so it was as stubby as I could get it but I did decide to modify the lower unit some by opening the exhaust up to twice it's normal size. I also trimmed off about three inches of the cavitation plate from the aft side of the lower unit. By opening up the exhaust I eliminated all backpressure on the motor and it revved up pretty quick. It sounded real growly like that too and it got people's attention when I was out running the boat at speed. I also streamlined the leading edge of the cavitation plate to a more bullet shape. These lower unit mods saved about another 1 or 2 lbs. In the end I had lightened up the motor by somewhere between 25 and 30 lbs! Check out what can be removed from this motor to lighten it up in these pictures.
Lighten up the port side.
Lighten up the starboard side.
Lighten up the lower half and another view of the lower unit mod.

I got it to the point where I could actually manhandle it around by myself with out giving myself a hernia. Opening up the exhaust and removing the flywheel starter gear ring allowed noticeably quicker throttle response. There was a fatal flaw that I knew about when I got the motor though. It had been used for decades on the transom of a succession of pontoon and deck boats and was rarely if ever opened up. It had plenty of hard honest miles on it pushing a pontoon full of up to ten people around at a punishing 1/4 to 1/2 throttle all the while wearing away at the needle bearings and detonating in the cylinders and burning through the pistons due to the wrong gas oil ratio for the last twenty years of it's life before I got it. All of my mods made it go faster....in every way! The high revs that it had been denied for decades chewed up what worn out needle bearings were left in the case and one of the pistons was nearly burned completely through. I had the motor long enough to have lots of high speed, fun runs around the local waters and in Michigan. It puked out one day right at the dock after bringing it back from a run. It just tied up as I killed it and never started again. I saw the bearings at home after I took it apart. It was time to find a replacement.

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