Posts: 3470
Location: Elk River, Minnesota | Hiya!!
If my gut instinct is correct you are about 400 RPM's below your maximum RPM for the motor, and if you are lightly loaded with the numbers you have supplied, a prop change will definitely be in order. Would I also be under the correct assumption you are running an aluminum prop and the motor is mounted flat to the transom or 1 bolt up?
If the two questions are yes (flat on transom or one bolt up, and running aluminum), here is what I feel would be some things to try:
If staying Aluminum, drop to a 17 pitch prop, load the boat only with your equipment, and fill the boat with fuel. The change to a 17 pitch should bring your RPM's UP by about 400 or so, but I have a feeling it will be more than that. If the motor is mounted flat to the transom, raise it up 1 hole. A great prop to try here is to go to SOLAS propeller and order their aluminum. It is a specific squeeze-cast design which allows for a thinner blade for better performance and the blades are progressively cupped to grab really well with higher mounted motor positions. Something like this you could move your motor up to the third hole and probably get away with it.
Many times, when you reach a point where your motor is over-propped, it will drop RPM's significantly. I had a 90hp Johnson 2 stroke that I found worked best with 18 pitch and ran to max RPM (5500 rpm), but tried a 20 pitch just to see what happened....I could not reach 5000 even with trimming... It was just too much prop.
If you were to test stainless props, you could also go to a 17 pitch, BUT...go to a prop with higher rake with tip/trailing edge cupping. With this change, raise your motor to the highest position possible. The reason for this is a highly raked and cupped prop will hold the water much better at higher positions than a standard aluminum, thus the height setting will offset the switch to steel. The material change alone from aluminum to steel will normally drop your rpms by around 200 or so, but each hole up off the transom is roughly going to gain you back about 100 rpms each hole. Good props to try are the older michigan Rapture (it's no longer made), Michigan ballistic, Solas titan (I believe that is the steel..if not, its the amita), I'm unsure of what mercury offers in the midsize gearcase...I don't believe they offer a tempest in that hub size, but if they do, that is a GREAT prop to try.
No matter what you try, there are always trade offs in selecting the prop for your rig, and there is a bunch of testing that goes into it with different diameter, pitch, rake, cup, material, etc. Moving a motor up increases speed and prop slip, but decreases handling. Changing to steel increases handling (to a point-depending on prop design) and many times allows you to move the motor up without losing handling, but gain speed.
Lots of variables here to consider and no doubt I have gotten a touch long-winded.
I will say that holeshot with the 4 stroke will not be as good as a typical 2 stroke, so going to a lower pitch will help your holeshot if you have found it is not the greatest.
Steve
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