Quote:
Originally Posted by rodscat
Yes currently have the High thrust props on the Honda 20HP, i have been led to believe that the 9.9 perform better
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That depends what exactly do you mean by performing better.
Comparison 1:
Determine maximum speeds for both pair of engines in calm
weather, then measure fuel
consumption at the same speed for both pairs, the speed used for testing being the max speed for the pair that has less maximum speed.
Case a) if all props are selected to achieve maximum speed
case b) if the props of the engine pair giving better max speed are selected for the fuel
consumption test to give the best fuel efficiency while still allowing matching the speed of the other pair.
For 1a) the original 9,9 HT would be the best for fuel efficiency if using the best pitched props, not just what the engine has when purchased, not sure of the newer upgraded model that was initially 8 hp.
for 1b) the winner for fuel efficiency would depend on the speed that the boat in the test would achieve. For faster boats the double 20 is likely to win, but not necessarily for slow boats. You would actually need to do the test to find the answer.
Comparison 2:
Ignore fuel consumption and just compare static pulling force, ie boat tied to land and measure the force of the
rope with max throttle.
If the 20 hp pair has even decent props it will win.
Comparison 3:
Test for max speed and ignore fuel consumption, the 20 pair will make the 9.9 pair look real bad.
As long as the props remain submerged and do not ventilate:
The larger diameter prop combined with larger
gear reduction in 9.9 HT gives significantly more static thrust for hp, and better propulsive efficiency in those conditions, but can not compensate for 50% less engine power if only comparing thrust or speed. The advantage in propulsive efficiency reduces with speed, but will still be there at 10 knots. The larger hub &
gearbox and leg will increase drag, and make the total effect a disadvantage in 13 +-3 knots.
Both engines seems to have props that avoid suffering from cavitation caused by insufficient blade area.
More modern internal combustion engine might also have significantly better fuel efficiency, but that effect is only relevant when not using anywhere near full power. At high power the differences are rather small.
If any prop ventilates due to being too close or above surface, none of the above holds anymore.