Mercury Inline 6 Tower of Power 90HP Propeller pitch Need Help!

[frankrizzo]

I just recently tuned up my motor and put a new 17pitch prop on my 21' Center Console VIP. My max speed was 21MPH at 4500 RPM. What can I do to get more speed? Any help would be appreciated.

 

[Chris1956]

You are likely a bit overpropped, depending on the weight and hull type of the boat. However, make sure she is running on all 6 cylinders first, before you change the prop. Take a compression test, spark test and examine the spark plugs. The compression should be within 15%. You should obviously have spark on all cylinders. Also, all the spark plugs should be slightly black or brown. Any that are real clean or white are a problem.

 

[jimmbo]

Something's wrong. A 17 in prop, @ 4500, 2:1 gear ratio and 10% slippage should be 32-33 mph.

 

[frankrizzo]

We changed the plugs before we went out last weekend, previous plugs looked fine. I will have to check compression again but last season it was fine. I have the same motor that is 115HP that I will be rebuilding soon, however everyone i have spoke to said that 90HP motor should be able to push the boat at least 30MPH.

 

[jimmbo]

It is possible you are carrying several hundred pounds of water if the foam floatation is saturated

 

[frankrizzo]

I don't believe I have any foam insulation. Possibly boat is just too heavy? Motor was purring like a kitten last time i went out.