Posts: 3508
Location: Elk River, Minnesota | Hi Dan,
Without seeing anything else, I would say you could stay with the same pitch prop, but drop the diameter down to 13-1/4" which should help hole shot a touch, but increase your speed a bit. What physically is going on is you are reducing the prop's surface area, which means less friction. With a stainless prop, you won't lose as much bite on the water either. What the smaller diameter should do for you is increase your overall RPM's by roughly 200 if you went with the same brand and model of prop.
If you are unable to find props with this diameter in your hub size (I'm guessing a 4-1/2" hub so you should be able to find them as it is a pretty standard size for mid-size outboards...) then a drop in pitch to a 13 would be in order...
BUT...before doing any of this, how is your boat normally loaded when you are running the 5100 or so? How are you trimmed...as far up as you can go before either porpoising or losing grip, or are you below that point?. If you are not trimming up much, that will be a huge issue as you want to trim up as far as you can go before porpoising or the prop starts losing grip or doesn't gain any more speed (you'll know because you should be able to both feel and hear how the engine responds. In most cases, when you trim up, you should see a substantial increase in RPM.
If you are heavily loaded with the numbers you have given, I would first start by setting up the boat if you were fishing alone with just your gear....it is at that point you want to maximize the rpms, so when you do get that buddy that brings everything and the kitchen sink along, you don't have to change props and the boat should still perform within the given parameters by Mercury. The heavy load will be reason for the slower hole-shot, thus when lightly loaded, the boat should jump up on plane in little time....a vented prop also aides in this area as well.
Given your numbers are under light load, the last thing to consider is moving the motor up off the transom more. I am not very familiar with Tuffy's models, and from previous discussions like this, I believe they make their transoms a tad taller which can negate how much you can move your motor up. Some of their models already have a set-back built into the hull which also affects what you can do as well. Steve Worrall knows tuffy's well and I would bet he can give some insight on the transom and what is possible.
IF you are able to move the motor up...even one bolt hole, you should see another gain of about 100 or so rpm, which is getting you closer to that max value. If your rpm numbers are under that heavy load, I would bet you will be darn close under light load and trimmed.
Good brands and models of props to try: Michigan ballistic or rapture, mecury has a couple as well in props that are high rake as well. When finding props, ebay is a great source for used props in great condition. And...the nice thing is about the only thing you are out if you try a prop and it doesn't work is the shipping. In most, if not all cases, you can the same money back out of the prop as you put into it.
Steve
Edited by VMS 2/2/2014 4:34 PM
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