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Out of curiosity, what sort of output would produce running an unequal engine combo, say 175hp + 25hp? Would it be equal to a 200hp ? Or would the 25hp be useless?
I have no clue about the results? Has it been done before?
With such a difference in hp the 25 hp would probably be exceeding it's rpm limit because it would be dragged along by the much faster 175 hp engine.
If you check my personal page you will see my 1977 custom 21 outrage with triple engines. And yes I used to run all three at the same time. I would run the two 8 hp Tohatsu engines in sync at 90 % power then bring the 25 hp Tohatsu up to 90 % power and all three engines would hum together in sync at the same rpm at about 12 knots on plane. What a sound they made together. Eventually I put a 90 hp Tohatsu on the boat with one of the 8 hp motors for a kicker. You might be able to run the 25 hp as a kicker but you might have interference with a big 175 and also if you turn the big motor too far towards the kicker you might hit props together.
A 175 HP outboard and a 25 HP outboard might not be the same as a single 200 HP outboard, but only because of the added drag of the extra gearcase. The same holds true for twin 100 HP outboards; they won't be the same as a 200 HP single.
There is nothing to stop a 25 HP outboard from running alongside a 175 HP outboard at speed. You just need the right prop to allow the 25 HP outboard to hit its recommend WOT RPM.
This would also mean it would be propped with a very tall propeller and that might make it less suitable for use as a trolling motor, or emergency spare. A 25 HP outboard can't be propped to run alongside a 175 HP outboard *and* be propped to move a heavy load by itself. It's one, or the other.
