21 REVENGE with Mercury 115-HP FOURSTROKE

[GRH]

I traded in the 1999 Mercury 200-HP OptiMax engine for a new Mercury 115-HP FOURSTROKE made in 2015. [The Boston Whaler 21 REVENGE] can make 39-MPH with two people aboard using a 15-pitch four-blade [with the engine] turning 6,000-RPM. I traded a 21-pitch that we had as a spare (for the 200) for a 17-pitch four-blade. I want the WOT RPM to be less than 6,000 with the light loads that we usually run. Can't wait until April to try it out.

[Whalerdog]

That is a lot better performance than my 2007 190 Montauk with a 2007 115-HP and bottom paint. It never hit 40-MPH--38 maybe in neutral water with no tides or wind. Two people [brings] down [the maximum boat speed] to 36-MPH or so. Fuel full then it drops further.

[floater]

You went to a 115 from a 200 on a 21-foot Revenge. How is it getting up on plane? I bet the fuel savings will be quite high.

[jimh]

Boat speed is usually proportional to the power-to-weight ratio to the 0.5 exponent. Assuming the weight stayed about the same, dropping power to 115-HP from 200-HP is a power ratio change of 115/200 = 0.575. Raising that to the 0.5 exponent gives a boat speed ratio of 0.758. This means if your boat used to go about 52-MPH with the 200-HP engine, it would probably go 52 x 0.758 = 39-MPH with the 115-HP engine.

I assume you got the current model Mercury 115-HP FOURSTROKE with 2.1-liter displacement. Its weight is 359-lbs (lightest). The OptiMax 200-HP probably weighed 505-lbs (lightest). You shed 146-lbs of engine weight, so that should also help the 115-HP performance remain good.

I don't think the new engine fuel economy will be drastically better than the OptiMax. The OptiMax was a modern two-stroke-cycle engine and had good fuel economy. Unless you used to run around at full throttle all day with the 200-HP OptiMax, I would expect the 115-HP FOURSTROKE will produce about the same fuel economy.

[Phil T]

Before you do anything, is your motor mounted at the appropriate height? It should be ~ 2 holes up, meaning the bolts in the top bracket are in the 3rd hole, counting down from the top.

-Bolt holes
0 <----- bolt here is "all the way down"
0
0 <--- bolt here is "2 holes "up"
0

If not mounted 2 holes up, raise it and re-test with passengers. You need to run the boat light and empty to see what the WOT rpm's are with the current nondescript 17" 4 bladed prop. Once you know how many rpm's you want to drop, then you will know how many inches to reduce.

Keep in mind prop size and their performance are not universal across materials, different brands and even between models by the same company. If, hypothetically, you were 200-250 rpm's over you would go up 2" in the same make/model of prop you have now.