CONTINUOUSWAVE

I have just re-powered my 21" Outrage with a Suzuki 225-HP. The original outboard engine was an Evinrude 225. The new outboard engine came with a 19-pitch propeller. I didn't think too much of this because I had run the Evinrude with both 19 and 21-pitch propellers. I am still in the break-in stage with seven hours on the new motor, but have been surprised at the difference in speed at a given engine speed. I did not appreciate the implications of the difference in gear ratios of the two motors until now.

The Suzuki has a gear ratio of 2.29:1 and, as I recall, the Evinrude was about 1.85:1. In my first real test of the Suzuki my speed at 3700-RPM was 22-MPH. On the Evinrude at 3700-RPM speed was in the range of 30 to 32-MPH. The 21-pitch propeller was obviously a little faster, but the only conclusion I can come to for the large speed difference is the gear ratio difference.

I still haven't run the Suzuki WOT yet but will the next time out. The recommended engine speed range is 5000 to 6000-RPM, and I expect with the 19-pitch propeller the engine will rev much higher. I am going to try it with a 21-pitch propeller, but at $800 per each I don't want to invest in a collection of unsuitable props. I am also wondering about after market props. The Suzuki prop is at least $200 more than its after market competitors and I would think Solas or Michigan Wheel would be just as good. Any comments or suggestions are welcome.

My dollar references are Canadian-dollars. The U.S.-dollar equivalent would be about 30% less.

It is common with four-stroke-power-cycle engines that the gear ratio will be numerically higher than an similar horsepower two-stroke-power-cycle engine. By gearing-down the engine crankshaft speed the reduction gear creates more torque.

Suzuki engines tend to have a larger propeller aperture in their lower unit, and this permits propellers of larger diameter to be used.

Two engines of the same horsepower, in your case 225-HP, should push the same boat to the same speed, but, if they have different gear ratios, the propeller pitch will have to be different.

The engine with the higher numerical gear ratio will have its propeller shaft revolving slower, so you have to use a propeller with more pitch. This scales linearly, so you can estimate the pitch as follows:

PITCH-A x [ RATIO-B / RATIO-A ] = PITCH-B

In your case 19 x (2.29/1.85) = 23.52

Now this will have to be also scaled by the maximum engine speed. Often the four-stroke-cycle-engines have to reach a higher engine speed to make their rated horsepower. I don't have the data handy for your two engines, but let's assume they are as follows

RPM-A = 5700
RPM-B = 6300

Now we scale the pitch by 5700/6300 and get 21.3-pitch

You can use the Propeller Calculator to do this math. See

http://www.mercuryracing.com/prop-slip-calculator/

The larger propeller aperture on the Suzuki engines allows larger diameter propellers to be used. This is quite helpful, really. There is a general feeling among propeller designers that a really efficient propeller will have a certain relationship between its pitch and its diameter. This means as the pitch gets higher the diameter should also get higher to keep that pitch-diameter relationship close to the optimum ratio. You might ask, well, what's the optimum ratio for pitch and diameter. That is a good question, and I cannot give you a good answer. I can say that generally a lot of really efficient propellers for outboards have a pitch of about 19 and a diameter of about 15.5. From that we might guess that 1.2:1 is a good ratio.

So in your case, if you are going for a 21-pitch propeller, the diameter might be around 17. That is probably too much diameter for the Suzuki. I guess they might only fit about 16.5-inch; check your specifications for the real data.

In general for the Suzuki the propeller you want to get should take advantage of the bigger propeller aperture and have more diameter. A lot of outboards can only handle about 15.75-inch maximum diameter.

In testing propellers on large yachts, we found that larger diameters were more efficient than higher pitches. A pair of 36"D x 34 pitch props attained the same rpm as a pair of 34"D x 36 pitch, but speed increased from 25 knots to 27 knots with the larger diameter, a nearly 10% increase in efficiency. I think the difference is mainly from the increased blade area.

I would go with the largest diameter prop that the engine will handle and adjust the pitch accordingly. I believe Suzuki has quite a few props available in 16" diameter, and don't rule out a Mercury Enertia Eco, which is also a 16" diameter prop.