[Fr. Frank]

Two possibilities: Either the motor is not developing full power, or you need a different prop. First make sure your motor is running well.

As to your prop, I can't get you close without knowing your gear ratio. According to my book, the Volvo 280 series single prop outdrive was available with six different gear ratios over the years, 1.5:1, 1.61:1. 1.65:1, 1.73:1, 1.88:1 and 2.08:1.

Here's the formula:

. SPEED X RATIO X C
PITCH = ( -------------------------- )
. RPM X [1 - (SLIP/100)]

If we assume the speed is 36 statute miles per hour, the C in the above equation is equal to 1056. We will also assume an average propeller slip of 15%, and a max RPM of 4800. Now the equation looks like this:

. 36 X RATIO X 1056
PITCH = ( -------------------------- )
. 4800 X [1 - (15/100)]

So give me your drive ratio, and assuming a clean hull bottom, I'll give you a style, diameter and pitch prop to put you in the sweet spot for rpms. Deal?

Of course, knowing the present pitch, speed, rpm, and using 15 as an approximate slip ratio can give you a pretty close estimate on your gear ratio if you can't find it anywhere else.

. RPM X PITCH X [ 1 - (SLIP/100) ]
RATIO = ( ---------------------------------- )
. SPEED x C