VIntage K&B 3.5cc Engine in SK Daddle Too

[palbietz]

I recently finished building an old Dumas SK Daddle Too that my father had started building in the mid 80s (image attached). At the time he had also bought for it a K&B 3.5cc marine engine, #5464. I don't have the manual for it and while it does run, I have some questions. 1) Is it OK to run fuel blends that use synthetic oils, or should I use the fuels which only use castor oil, 2) I was planning on using 20 to 30% Nitro, is that OK, 3) Does anyone know how well this engine runs, is it a dog or does it compare well with current day engines of a similar size? 4) I had recently tried running the boat in a local lake and every time I launched it, the engine died. If I submerge the prop in water while holding it, it seemed to be OK. I have seen other forum posts that state sometimes it is best to give the boat a little toss into the water while upping the revs. I haven't tried this launch technique, so does anyone know if that is necessary for this engine. Any suggestions would be appreciated.

[palbietz]

Correction, the number for this model is #8384.

[SteveM]

Those K&B 3.5cc marine engines are excellent performers and I ran Byron 25% nitro fuel when I raced them. A nitro content of up to 60% will work, but the higher the nitro the higher the wear and tear on the motor. I always added a few ounces of Sig AA castor oil to my gallon jugs of fuel as extra protection. Marine motors run in adverse conditions and the little extra oil helps. Castor is a better lubricant than pure synthetic but can be messy. If you drop the hull into the water you will like end up with cavitation and subsequent stalling once the prop grabs. I always found it best to toss the hull into the water to get it up to speed faster and avoid this issue. The drive line setup is likely sub-surface, meaning the prop is fully submerged when running and this might also be causing some of your stalling. The latest mono hulls use a surface drive where only the lower half of the prop is in contact with the water. This lets the motors spin much faster resulting is a very fast hull.

[palbietz]

Thanks a lot for the response SteveM, I think it was very helpful.