It All Starts at the Business End of the Boat

Cathouse Marine, LLC
Posted: 6/17/2009

The answer to most basic marine performance questions often boils down to proper propeller selection.

A forum member and I were having a conversation about a trim switch problem, and as usual, just when the conversation was coming to an end he popped the inevitable question "How can I get more power"? Everybody wants more. When I asked him to be more specific with his question so that I could better understand what he was looking for, he simply said that he wanted a little more power to pull up skiers.

The answer I gave him would have nothing to do with adding power, but using the available power he currently had more efficiently! In my opinion, most multi-purpose boats come from the showroom equipped with adequate power and propped for general use. Meaning they have decent acceleration and achieve a compromised top end speed.

From a manufactures standpoint, this is probably the best and most cost effective way to send these boats out to the public. After all, an average bow-rider is perfectly at home wake boarding, tubing or simply carrying the family about the lake. When we leave the "general purpose" arena things need to be changed and looked at a little closer.

In the case above, the suggestion was made that the propeller be changed to accommodate a more "specific use" like pulling skiers. This person currently operates his boat with a standard 3-blade 21" propeller.

For those that don't know, the 21" or 21P as some refer to it, means how many inches the propeller will travel through the water in one revolution of the prop shaft (not accounting for any slippage), I suggested that the person go down one inch in pitch, and add one blade to the propeller, thus he would have a 20" 4-blade propeller.

The added blade certainly makes the propeller more efficient, and the reduction in pitch makes the propeller easier to spin because the blade angles are a little less aggressive on the propeller hub. In addition, most 4-blade propellers tend to climb up to the surface of the water higher than 3-blades thus producing stern lift and less bow lift. The result is a quicker time to plain and less fatigue on both the skier being pulled and the boats engine.

This particular improvement in specific use usually comes at a cost to the other end of the speedometer. Some if not most boats will have a slower top end speed, but general cruise speed may even be better.

Let’s say the same person did not mention anything about pulling skiers, but really wanted the speedometer swinging a little farther to the big number side. The first thing I would want to do is yank the engine out and build a monster that would feed on a healthy diet of 93 octane. But, let’s keep things in perspective and attack it from the business end of the boat like we did above.

Most boats come propped to operate in the optimum engine RPM range. Most stock marine engines are designed to operate between 4,500 and 5,000 RPM when run at wide open throttle. RPM refers to "Revolutions per minute" that the engines crank shaft turns.

If we change the propeller from 21" to 22" and add the fourth blade we will likely see an increase in speed. We would have a more efficient 4-blade propeller. Provided we could still operate in the optimum RPM range, we would have an increase in top end speed. The time to plane or hole-shot would suffer and you might have to drag that skier a lot farther to get him up.
It's hard to have the best at both ends.

If the engine cannot achieve optimum RPM then maybe consider the same 22 pitch on a 3-blade design. Other tricks for pushing the speedometer needle farther to the right are raising the drive height, or X-dimension. There are a few manufactures that offer lower drive units that are "shorter" than stock, thus raising the propeller closer to the surface of the water. This creates less drag, and allows an even more aggressive propeller pitch due to the propeller actually breaking the surface of the water at the top of each blades rotation.

Imagine going 50mph in your boat and holding a 2x4 in the water. The drag would be less at a depth of 4 inches than it would a depth of 7 inches but let’s save drive height for later conversations. They are pretty specific to high horsepower extremely fast applications.

In addition, here is some other general information you should know about your engine/drive combination.

If your engine is turning 5,000 RPM, it does not mean that your propeller shaft is rotating at the same speed. There are several different gear ratios on different makes and models. An example would be a 1.5 out-drive gear ratio. That simply means the engine would turn 1.5 times per 1.0 turns of the propeller shaft.

It is important to know what gear ratio your boat has when speaking with someone about propeller selection.

One last bit of information I would like to share is speed calculation.

Try this formula and you will be able to walk around most (I say most because the go-fast crowd can fool you) boats and take a pretty good shot at how fast it will go.

Here is the formula:
Speed = ( Propeller Pitch x Engine RPM )  /  ( Gear Ratio x 1056 ).
When you get that number take 10-15% off due to prop slippage.

Example using the numbers used in this article
22" prop x 5,000 RPM = 110,000
1.5 Gear ratio x 1056 = 1,584
110,000 divided by 1584 = 69.4 MPH
Take 12% out for slip and the above boat should obtain approximately 61mph

All the above statements, comments and recommendations are not necessarily published facts. They are my opinions and real world experiences and results. Please read this as general information and in this case mostly specific, but not limited to Inboard Outboard applications.

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