Props to you... Choosing the right prop

Props to you... Choosing the right prop


First, we would like to say that propping an inboard is not an exact science, the same prop moved from one boat to another may not have the same results. There are many factors to consider, for example, engine HP and tune, hull size/design/weight etc.

Usually, the boat manufacturer has optimized the diameter and pitch to match the engines horsepower/transmission ratio to the maximum allowable RPM (+/- 200) at wide open throttle. It's a safe bet to replace your prop with the same size and pitch as the factory recommendation.

The first number of an Inboard prop designation is the prop diameter the second is the pitch (example 13 X 14, 13 = diameter and 14 = pitch). You also will have prop rotation to consider, LH left hand (counterclockwise) or RH right hand (clockwise). This is determined by standing at the rear of the boat looking forward. Another consideration is shaft diameter, Inboard ski boats will have a 1" or 1-1/8" shaft. They both use the same keyway, prop nut and puller. Prop designation is usually stamped around the prop hub near the nut. Some earlier props were stamped on the hub between the blades. All Johnson (OJ) or Acme props that are sold by Discount Inboard Marine are standardly cupped.


Good = 3 Blade

In the 70's and 80's prop selection was simple compared to today's choices, it was the 3 blade NIBRAL (nickel, bronze, and aluminum). A few stainless props appeared but there is no performance advantage like their would be on an I/O or outboard. The 3 blade standard not CNC (computerized numerical control) is a good choice if you use the boat for cruising and general water sports.


Better = 4 Blade

During the 1990s the 4 blade (non CNC) was seen as an upgrade to the old 3 blade. The fourth blade provided more blade surface for better hole shot and sustained speed as well as a reduced rooster tail with some boats. The fourth blade also added a better balance to the prop for reduced vibration. It is common with PCM 1.23:1 ratio transmissions to reduce diameter from 14" to 13" when switching from a 3 blade to a 4. Early Nautiques 89-96 may over rev at WOT when doing this but they really come out of the hole strong and it is usually not an issue unless you run the boat close to wide open throttle for extended periods.


Best = CNC Machined 3 or 4 Blade

The choice was simple back then - choose the standard 3 blade or upgrade to the 4. Enter the CNC Acme prop that is hailed by some as being revolutionary. CNC is a machining process that mills small amounts of metal from the propeller to ensure repeatable exact tolerances. This machining leaves small tool lines on the face of the propeller and hub where the high spots of metal are removed. In theory, every prop would be perfect out of the box being smooth and vibration free. Acme took this to a new level and designed a 3 blade with more swept blade surface to equal the 4 blade in acceleration and increase top speed. (3 blades are theoretically more efficient than 4). Most reduction gear transmissions still benefit from running the 4 blade CNC prop. If you have a 1:1 ratio then we would consider the 3 blade. CNC props are measured exact and not rounded off like the non CNC, for example, a 13 X 16 non CNC would have the same RPM range as a 12.5 X 15.5 CNC.

Johnson propeller (OJ) countered Acme with their version of the CNC prop the XMP. These props have been around since about 2004 and are getting very good reviews. The 4 blade seems to be a good wakeboard and ski prop but the 3 blade with larger blades and less pitch is supposed to provide even more acceleration for adding ballast water tanks. If you have a late model boat you may want to check some of the online forums for recommendations on the latest and greatest prop for your boat.

If you are not sure call or email us, or browse our store , and we will be happy to advise you on prop selection. To help us determine what's best for you please provide your current prop size and wide open throttle RPM. We can then recommend a prop to utilize your boat's engine RPM to match your application.

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