You’ll need to know:
- Make and length of boat.
- Make of outdrive, motor and horsepower.
- Make and size of current propeller- Diameter & Pitch.
- RPM of motor with current propeller.
What you would like in a new propeller?
- Better Hole Shot
- Fuel Economy
The right PROP makes all the difference
Hey Dennis, I spoke to you a few weeks ago about re-propping the Diesel Duck. Your recommendation was dead on! Just changing the prop resulted in an amazzzzzzzzzzzzzing performance difference. he old prop (15X16) gave us a calm water (in the bay) speed of 12 knots. The only way we could get up on plane was to “surf down a swell”. Now, with the new prop (16X13), our calm water speed is 30 knots!!! Performance out in the choppy swell this morning was wonderful! Plenty of power going into the swells!!! Engine speed was perfect! Max throttle achieved max recommended RPM for our engine (3700 rpm for our diesel). The new prop size is perfect! Thanks for all your help! If any other boat owners are out there, wondering about repropping…….. call Dennis. He’s the man!
Gary aboard the Diesel Duck
The most frequently asked question is about propeller size…
The first and most important thing you need to find is your engine manual. In there you will find a section that will say “this engine has a Maximum HP at some RPM.” You must be able to reach that RPM at full throttle in your boat. If you cannot get all the required RPM your prop is too big; if you go over the required RPM your prop is too small. By knowing the number of RPM you are under or over the required value, you can have your prop adjusted. You can alter your inboard prop three or more inches in pitch up or down as long as it is made of bronze. Diameter is the other factor which is almost always overlooked, if your not sure what your running , ask!
Sizing is an easy process if your boat is put together and running. If you are building or repowering the prop may need to be sized ………… . If you are repowering and the max RPM is not too much different from your old engine and the gear reduction is the same, your old prop will probably work. If the RPM or gear reduction changes your old prop will be less likely to work well for you.
The most important determining factor for propeller sizing is not speed, but RPM!
Most Boats can use different propellers and achieve different speeds and fuel economy. Trial and error is usually the way that most people find their propeller. Lots of people own boats for years and never have the right prop for their boat. It’s the one that came with it and they never even thought about changing it.
Sometimes when you change the layout of the interior, or add fuel or water capacity, repropping can dramatically help your performance.
First step is you need to know that your engine is tuned correctly.
Write down the amount of fuel, Water and any other variable weight on board during testing. (Weight is a big part of performance)
Know that the bottom is clean and propeller is not damaged, Outdrive trim angle (If applicable is same throughout testing) Trim tabs set or used in the same positions.
If boat is equipped with a gallons per hour flow meter, This is the Ultimate Instrumentation needed for the most accurate measurement of performance. If not, by going to the fuel dock and topping the tanks off before each test and keeping a log of the hours and amount of fuel used is the next best measure.
Next— Get a accurate speed and RPM Readings with your tachometer and GPS or Loran C ( or a friends) In different directions and variable conditions. You should see what Rpm’s you see at wide open throttle for bursts up to one minute at a time. Also the speed you would like to cruise at, and the speed at which the boat planes (This is usually minimum of 16-18 mph) .
Write down this information and either take it to your local propeller shop or contact a knowledgeable boat dealer or repair shop (Like AMS).
This procedure could give you a big increase in performance and save you . Remember almost every boat is different from any other!