@ Kristijan Leitinger, I was already considering some kind of cage around the prop. TBH, mainly to protect the prop from rocks and the likes. (this windsurf board should be able to work in very shallow waters) And with all (small) boats, where there is a risk of you falling overboard, there is this prop milling around. I will also add a “deadman swith” to this engine, because the risk of my falling off at one point, are pretty high. So there won’t be ALL the force applied to the prop. Just inertia of all the moving components. (probably still enough to remove a few toes)
The lower unit (and the prop) came from an “egg” 3.5 hp Yamaha outboard engine. That stock engine would be able to rev up to 5.500 rpm, when pushing a small boat. (for example a tiny rubber boat) Granted, the stock engine was a 2-stroke engine, so each rotation of the engine, would have been powered. So there might be some torque figures. I however do expect that this lawnmower engine shouldn’t be too weak to power this surfboard. It’s probably less heavy, and it’s very flat/sturdy. So it will be faster ontop of the water, and thus needing less torque? (and maybe more HP in the higher rev range/straightpipe/no air filter)
fun fact I’ve had many smaller outboard engines in the past, and I’ve got 4 at this moment. None of them had airfilters in their airboxes. I had always assumed someone removed them at one point. Turns out, they never had an airfilter. The airbox is intended as a kind of muffler, to reduce engine sound. There kinda isn’t any risk of road debris/sand/dust/junk to be removed from the air, because you’re on water.
@ Marcus Norman: thnx for the insights. When I was tinkering with the engine, I had removed the lawnmower blade. And removed the governor and had manual throttle control. It’s idle performance was just perfect. Very smooth. But with zero load on this engine, it would act slightly odd when I would stab the throttle, and go for max RPM. I’ll be honest: it was slightly scary. And the engine would spit brown/black smoke at those max RPM’s. Had the engine at an 90 degree angle, to remove the lawnmower blade, so I had assumed the black smoke came from all the oil in the wrong places. (still can’t explain the brown in said smoke, had it produce brown-ish smoke for a bit, to rule out rust dust from the exhaust…)
Like you and Steve Unruh said: connecting rod is going to fail first. Very scared of such things, especially considering I didn’t plan on adding any kind of plate between my lower back and the engine. While the piston will be “shot” towards me when in operation.
How badly did your engines blow up due to a failing connecting rod? Were there pieces of the block just blown out, or was everything still contained in the engine. Would you recommend some kind of a plate between my back and the engine? If so, what kind? Multiplex/wood or steel? If so, how thick? (not concerned about a big fireball, water will be all around me)