Why do we use reduction drives? The Aircraft engine was designed to be a low RPM engine with lots of torque to turn propellers at a speed where the tip speed would not exceed mach 1 and go super sonic. Then we want to use engines with less torque and more HP that turn more RPM's. The answer was a torque multiplier, or what we call a reduction. A V8 engine that produced 350 HP and 400 ft lbs torque at 5000 RPM's with a 2.0 to 1 ratio would produce 800 ft lbs torque at the prop adaptor and would be turning the prop 2500 rpm's. If you ran this same engine on a dyno up to 2900 rpm's and checked the torque, you might develope 325 ft lbs torque and 260 HP. Set up dirrect drive, thats all you would get. The right prop for the direct drive setup would be a 7236 while the right prop for the 2.0 to 1 reduction with the same engine might be a 7848, which might develope twice the amount of trust.
There are three factors in deciding what reduction to use. (1) Engine RPM, Engine Power, and Prop Design. We will start with props that have a warranty limit of 3000 RPM's and list a few known engines used in the airboat industry.
Chevy V8 or 5000 RPM engines 2 to 1 Torque: 350 to 450 ft lbs
Cadilac 472 & 500 cid, 4000 RPM 1.774 to 1 Torque: 500 ft lbs
GM Diesel, 3400 RPM 1.5 to 1 Torque: 750 ft lbs
Now these numbers are in general terms to show how the reduction ratio works. They are very close. The lower RPM engines produces the most torque and uses the highest ratio reduction.
In recent years, some prop designs were developed to work at lower RPM's. A good example of this is the Water Walker Maximus. The maximum rpm for this prop is 2250. The blade is 14.25 inches wide. Turning this prop faster than 2250 rpm can cause trailing edge delamination. With these props designed to develop hugh amounts of thrust at lower RPM, reduction ratio's like 2.37 and 2.68 to 1 are used. The biggest benefit from the slower turning props was less noise.
As for the 084 engine at 3600 rpm, i think a 1.65 to 1 ratio would be best. Then decide what prop length and pitch would work best. Keep in mind that the prop is the engine governor and you can't over-rev any engine with the right length and pitch prop.
Thanks, Chuck
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