How To Achieve Better Inboard Engine Boatspeed

If your thinking about whether your inboard engine boatspeed is up to scratch, this article is written to get to the bottom of that question.
Before giving you the resource to go off and try, it's best to understand what the variables are behind finding the best from your motor.

The basics of velocity are governed by variables where one factor has an impact on another.

Ahead of you going out buying a brand new engine, why not get the most out of the one you've got at the moment? Here are some variable that need to be kept in mind:

Pitch

The theoretical distance the propeller travels in one complete revolution. This is subject to slippage, which is covered a bit later. Slippage reduces the distance traveled by the blades through the water, so lowering efficiency.

Diameter

Is the distance, doubled from the center of the propeller to the outer most point of a blade. As the number of blades varies, this method is best to calculate diameter.

Slip

We looked briefly at slippage early, here is some detail. This is the percentage lose in distance that the propeller looses as it turns through the water. Effectively slippage reduces the ability of the propeller to develop drive, forward or back. The percentage lose is in the area of between ten and thirty percent.

Cavitation and Pitting

Cavitation is the formation of air bubbles on the blades. The bubbles are the product of interference in the aqua dynamics of the boat. Simply put, if barnacles are growing on the hull, in front of the propeller, their profile on the hull causes a disturbance in the water. Bubbles form and float back to the propeller blades. Firstly the bubbles act as a barrier to the blade getting traction in the water. This in effect is slippage.

Secondly, oxidation of the blades increases. This is the beginning of pitting. If the barnacles or the depth sounder is not removed or replaced elsewhere on the hull, the effect of the cavitation becomes profound. The oxygen rich bubbles attack the propeller blades. Causing pits to develop. If there is any previous damage on the blades then pitting will result quicker.

As you can imagine, cavitation and pitting can have a big impact on your propeller's abilty to drive your boat through the water. The degree of slip also increases, which lowers efficiency.

Blade thickness

The design of the propeller and construction material used, determine the thickness of the blades. The thinner the blades, the lower the drag.

Cupping

Is the angle at the propeller edge. Cupping is used as an adjustment to assist pitch. The angle at which the water leaves the surface of the blades is a factor in establishing distance travelled through one revolution.

Really the pitch is the most important aspect of getting the best boat speed out of your inboard engine. If the theoretical distance the blades travel through one revolution is maximized, then optimal speed is reached. If contributory factors are reducing the distance travelled through that revolution then the result is lower speed.

Time to get on the water!

• You will need a tachometer and speedometer

• The engine manual

• Calmish water

First off

From your engine manual, find out the range of RPM that the engine is designed to reach at full throttle

Find a piece of water, which is large and queit, so you can do test runs unhindered, in safety.

Establish a benchmark over a few runs. Note down RPM and boat speed on each run.

If your RPM exceeds the range indicated in your engine manual, then the pitch of your propeller needs to be greater - increase propeller size

If your RPM is lower than the range indicated, then the pitch of the propeller should be lowered - decrease propeller size

You should note that changing altering the size of your propeller by one inch up or down, changes the RPM by a factor of 250. So engine speed will increase by 250 RPM if you decrease the propeller size by one inch. Engine speed will decease by 250 RPM if your increase the sixe by one inch.

Once you have reached maximum RPM when you have completely opened the throttle, then you will be getting the best boat speed you can from your engine.

This method is a simplified one. If you are looking for a more complex formula that includes total boat weight etc, then click on Recreational Boat Building Industry for further information.

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