Re: Should it go faster
Swapping the (85) 2 to 1 lower unit (If it is indeed 2 to 1) in place of a 1.73 to 1 will normally increase speed by 3-5 MPH ALL OTHER THINGS EQUAL If you increase pitch by 2 inches.
At any rate, with either engine, your speed seems a bit low and you need to investigate why. Weight? Bottom condition? Height of engine on transom? Trim position? Number of people at speeds quoted? etc.
Back to the swap: That means that if your 125 would go, say, 35 on a boat, if you put a 2 to 1 lower unit on it and increase the prop pitch by 2 (17 to 19 for example) then it will now go approximately 38-40 MPH.
UNDERSTAND though that all hulls are unique and respond differently to horsepower. ALSO understand that as a general rule, it takes four times the horsepower to double the speed.
SO 30% more horsepower is not going to make your boat go 30% faster. Since a deep vee rises higher with increased speed and therefore resistance is less, 10-15 % would be a more reasonable guess. SO if you have gained 7 MPH with the 125 you are actually doing a bit better than would be expected.
As another concrete example, I have a 14 foot flat bottom boat set up to go fast. Not a racing hull, just reinforced to take the power and stripped, with only a battery, gas tank, and one seat.
With a Force 90, this hull tops out a 55 MPH. Increasing the horsepower to 125 just as you did increased my top speed to 60 MPH---Just about what I was hoping for. If I install a power jackplate to take advantage of engine height and the stainless cleaver prop I am running I would expect perhaps another 3-4 MPH.
IF top speed is important to you, Spend a bit of money (It always costs more to go fast) and buy a stainless prop. Stainless by itself should give you around 3 MPH more with no other changes. You do need to use the tach or install one, since in order to go fast, you must go fast. You must prop the engine to turn at least 5,000 and preferably right on 5500 RPM or even a tad more.
At 60 MPH, my 125 is turning a stainless 12 X 21 pitch cupped cleaver at 5900 RPM.
You don't have too much choice in props and you did not tell me the year of your engine. You also diid not tell if the lower is dual exhaust or single exhaust--it does make a difference. Again, however, I would expect your 125 on a 17 foot hull to be quite happy with a cupped stainless 13 X 19 Michigan IF it has the older style single exhaust lower unit. Also, if you have a single exhaust unit and are using the black open hub stock prop, I believe you will do better with an after-market prop, OR a stock closed hub Chrysler/early Force closed hub prop made by Michigan.
If you do some research, you will see that while your choice of props is a bit limited, there are options and other manufacturers.
