Prop recommendations please: 20' Seafari w/ 150 Johnson OB...

[Fr. Frank]

John, you didn't say what your performance was with the old prop.

Were you able to reach the maximum rpm (5500) of the maximum recommended rpm range (4500-5500) with the boat at maximum loading? This is called propping for Max of Max at Max. This is the ideal all-purpose prop performance goal.

Many, myself included, keep more than one prop for different situations. But in no case should you ever use a prop that will not let you reach the Max of Max with a light load. If it's just you and a full load of fuel in the boat, you should always be able to reach that 5500 rpm mark. If you cannot, you are over-propped.

In that scenario, if you take on a little water, put a couple of extra people on board, or find yourself in dangerous seas and now you may no longer have adequate applied torque to perhaps get you and your passengers home safely.

Similarly, an over-propped boat is significantly less maneuverable, is much slower to plane, and puts a great deal more stress on all reciprocating or engaging parts.

On the other hand, a slightly over-propped motor can slightly increase your fuel economy and sometimes (but not often) increase your cruising speed. I never use an over-pitched prop when going offshore just for safety precaution.

I run a BRP 15x17 Viper on my Etec 90 25" on the back of my 18' Seacraft. I can reach Max of Max at Max with this prop.

I also have a BRP Cyclone 14.5x15 that drops my max rpm by about 200 rpm, but I can cruise 4-5 mph faster and get 7.4 mpg at cruise instead of 6.9 mpg. It's a bit slower out of the hole, but I can also trim it significantly higher which is helpful in the shallow water through which I often run inshore.

And I have a plain older 14.8x17 aluminum Michigan prop as a spare/emergency prop that I carry offshore, just in case.