Eventually, customers will just plug their boat into a cell phone and it will talk to the servicing technician computer  which will tell the boat how to fix itself. In the mean time, the job of a marine mechanic is pretty secure.   Fortunately, repair and troubleshooting tools are getting better and easier to use. Setting up a boat, motor, and propeller package that performs at it's best is not an easy task (at times) but well worth the effort.

As a reminder, setting proper trim angle and motor height is essential to reducing drag and prop slip. (See HighPerformance Boating)

   The easy part is selecting the boat and motor. (Well, kind of easy) The hard part is selecting a prop that gives the best performance to the package selected. There are many factors that go into the design of a propeller that most of us don't have to deal with. The best place to start is always the OEM props that have already been tested for performance. Their selection of pitch, diameter, and style combinations still gives us numerous options.

Prop selection choices are:

  • Which manufacturer to use.
  • Propeller material: usually aluminum or stainless
  • Prop design: thru hub, over the hub, aerated hub
  • Blade style: un-cupped, cupped, conventional or cleaver blade
  • Number of blades: two, three, four, five, or more

   Since few of us will ever make our own prop, things like pitch/diameter combinations, blade thickness, skew, rake, contour, and construction material are up to the manufacturer. Matching the prop design to the intended application is up to us. Today, often the biggest question is how many blades are needed,  more than what pitch to use.

   This Boat Performance Analyzer (See Special note below, before clicking) takes several combinations of Prop Pitch, Drive Ratio, Engine RPM, MPH, and/or Prop Slip Percentage values then, calculates either MPH or Prop Slip %, depending on which values you have left out.

   For example, if you enter 19" pitch, 1.50 gear ratio, 4600 RPM, and leave the prop slip value at 0%,  it calculates the maximum possible   speed of the boat to be 55.17677 MPH.  Obviously, this actual performance from a boat can't happen because, in reality, 0% prop slip isn't possible.

   But, if you enter 19" pitch, 1.50 gear ratio, 4600 RPM, and your lake test result of 43 MPH, it calculates Prop Slip % to be 22.068651 %.

Since we'd like to get a prop slip of about 15%, which is considered ideal, let's enter 19" pitch, 1.50 gear ratio, 4600 RPM, and 15% slip. The result is 46.900253 MPH.

In our test boat example, the actual speed was almost 4 MPH slower than what would be considered ideal.

Note: Do not trust the speedometer in a boat. Even if it is a specially calibrated unit. Digital speedometers and a digital tachometer are recommended for this purpose. Also, GPS units can work well enough over long runs especially if they have a speed average function. The best bet is include radar gun values. Don't have one? Good time to make friends with a local cop. Ask him if he likes boating? Don't forget to take readings running the boat in several directions, so you can calculate average speed and RPM. That helps reduce calculation errors due to water current and wind.

What does this mean?
I'm glad you asked. First, as you may have noticed, this is called a Boat Performance Analyzer and not a Prop Performance analyzer.

Get under the boat, notice the shrink-wrap tape still on the bottom, left over from the shrink wrap job the dealer did for you last year. And there's that layer of road dirt from last months trip to the lake. I know you were going to wax it, this weekend. And don't forget about that "small" ding in one of the prop blades from that "little miss-hap" at the boat ramp last fall. So before you jump to conclusions, Boat and prop performance go hand in hand and have to be considered as a whole.

Note:  A perfectly calm day, with no breeze, will slow a boat down because of "Surface tension". Barometric pressure and ambient temperature also play a significant role in actual engine horsepower output. I'm not going to cover these items too extensively here, but be aware that it plays a major role in how fast your boat is, at any given moment.

What to do next?
As you can see by our example a little tweak here and there means looking at more than the prop at first. Try to reduce as many of the other influences on performance as possible. That also includes trying to reduce the weight of the boat as well as shifting weight to different parts of the boat if possible. Your objective is to reduce wetted surface of the hull, but at the same time trying to keep the boat safe to operate.

Laying a straight edge to the bottom of the hull might be your next check. Excessive hook or rocker in the hull, either intentional or by defect, must be compensated for. Many boat builders mold some hook into the hull which makes the boat come on plane faster. However, this also has a tendency to reduce top speed. Next, make sure there aren't any obstructions protruding from the bottom of the boat that are directly in front of the engine. These bumps in the bottom can create air bubbles that could be sucked into the prop, reducing it's efficiency. (increasing slip %) Assuming your boat is within spec., working with the prop is your last adjustment.

This next part can be a problem. Obviously if you have access to ten different props, try them all. Looking for changes in your numbers. Since most of us don't, the real answer is to: make sure your engine is running at spec., verify overall performance is satisfactory/safe, and then look at your slip test numbers. In our example, our slip% is only a little higher than we'd like to have.  If you are using a three blade prop, small changes like:  changing to stainless, increasing cup, and/or trying an identical prop from another manufacturer may be all that is needed.  If your slip% is way out of whack, more radical steps are necessary.


======= Work in Progress ============

Special Note: The "Boat Performance Analyzer" link opens in a new window and takes you to's web site.

The Boat Performance Analyzer is an invention of Ed Lojko. A mobile marine mechanic turned multi-media producer. I'll leave it up to him to tell you his story.

"I am a professional developer of multimedia training specializing in Web based training. My previous career as owner/operator of a mobile marine repair service leaves me with a lingering interest in marine related topics. is my way of staying connected with the industry...."

We're always sorry to hear about talented people leaving the marine industry, there just aren't enough good technicians out on the docks. 


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