One thing we have found in several model and full scale projects is that speed is often more about propellers than power. We deliberately try to use sucessful existing propellers. In our electric hydro this meant a quick change gear up drive to match the 5000 rpm brushed DC motor to outboard hydro props that turned around 10,000 rpm.
In internal combustion models we found that propeller rpm was limited by the engine so speeds were limited. Zenoah powered boats were held to speeds around 100 mph for years by their peak rpm. Only by seriously increasing the pitch of stock and custom propellers were today's 107 - 108 mpg speeds reached. The GX1 record of 109 mph is purely a result of the higher rpm of the Quickdraw, even though it's well past its power peak. I think even nitro records are starting to be limited by maximum engine rpm.
Electrics are free from this limitation. The question is now what rpm gives the best efficiency. My feeling is that a pitch to diameter ratio of 2 or less should be best even though we ran much more than this in our latest SAW record boats. There is no question that the propeller and its shaft are a major part of a boat's drag, so gearing two engines together should be better than conventional twins. Gearing up means drive line drag gets magnified by the gear ratio. Our first attempts at model gear up drives was not successful. At this point I think a straight or slightly bent solid shaft suported in ball bearings is needed. If engines can turn 28,000 rpm with good power I think the nitro SAW record can be beaten with a modified 2170 and no gear up. The ultimate model record of over 140 mph will take a gear up drive.
Lohring Miller
