HydroComp PropExpert is a program for the selection and analysis of propellers for motor yachts and workboats. While it was developed with the inboard propeller in mind, this report validates its use for small high-speed powerboats - both recreational and commercial - and provides guidance on some of the more common pitfalls and concerns.

Accurate operating vessel weight is important. Weight is used to make predictions on potential vessel speed and other vessel data. Unfortunately, all too often the true operational weight of the vessel is not known.

First, it is important to understand what is - and is not - meant by the vessel's operating weight. It is the total weight of the vessel underway, including hull, power plant, fuel and consumables, passengers and any cargo. The following are things that are not weight:

Hull weight or "light ship" weight - This is the weight of the hull alone. It is typically found by weighing the hull on a scale.

Gross or net tonnage - This is particularly confusing to those not working with merchant cargo vessels. This is actually a measurement of the volume of usable cargo space. (The name came from the British "tun", a barrel of 100 cubic feet.) Regulatory agencies typically use gross or net tonnage to determine taxes.

For sterndrives, we recommend the following values:

Evaluating speed is always best using Prior trial data. To determine if the trial data makes sense, you can use the following two relationships. The first is a prediction of expected power based on weight and speed. The second is a "best vessel" minimum power curve. It is unlikely that a vessel would do much better than this best vessel curve.

BHP = Total installed engine power (hp)
W = Total operating vessel weight (pounds)
V = speed (mph)

The following table shows predicted average and best horsepower versus the actual installed power.