If you have a propeller installed (fixed OR feathered), is it working efficiently?

	Does it have the right pitch for your gear ratio/engine rpm and hull speed ?
	Is it the correct diameter for the application?
	What is my hull speed?

If you need to find out the answers to these questions, read on. 
Or jump ahead to the topics with the links below. 
You can use the arrow to get back to the top.

Pitch Factors

Pitch Formula

Prop Diameter

Determine Fixed Prop diameter diagram

Acceptable Clearances

Prop to cutlass-bearing distance

Calculating Hull speed & Pitch

This formula will calculate approximate  hull speed for displacement  hulls only.

hull speed (Vk) = [square root (LWL in feet x 0.3034)] x 2.5

Pitch Factors

Vk = hull speed 

M = Maximum engine rpm (85% of which is usually continuous use)

R = Reduction ratio

S = percentage slip

For diesel engines with R of 1.7 - 2.3,   S will be in the range of 30 - 25%

For diesel engines with R of 2.4 - 2.75, S will be in the range of 25 - 20%

For diesel engines with  R of 3.0, S will be approximately 20%

Pitch Formula

Pitch = (X / Y) / Slip factor

X = (Vk x 6082) x 12 - Distance in inches per hour 

Y = (M/R x 0.85) x 60 - Total shaft revolutions per hour 

Slip factor = (100 - S) / 100 

The above rules of thumb will get us plenty of comments, in the 
meantime use these numbers as a guide. If you get expert-advice 
that differs substantially from the numbers you calculate, 
get a second opinion.

Prop Diameter

Prop size and type are determined by four factors:

1) Engine Horsepower Continuous.

2) Maximum engine rpm.

3) Gear ratio.

4) Space available and clearances needed. click to acceptable clearances

Items 1 - 3 determine shaft rpm and torque available. Using this information 
with the nomogram provided (borrowed from Exalto Holland and Radice Italy), 
you can easily determine fixed prop diameter.

Click the thumbnail for full view of Exalto Holland's nomogram -

The Prop diameters determined on this basis are 3 blade power props. 
Although blade areas differ between sail and power props the result gives 
a fair indication of the 3 blade prop diameter for your sailboat. Why? Because 
power props generally drive less efficient hulls, thus requiring more blade area.

Based on the size determined for a 3 blade prop. A 2 blade prop, according to 
industry standards, should be of at least 1" to 2" larger diameter.

Acceptable Clearances

The fourth factor of space available and clearances needed is where limitations 
come in. Even though the your engine rpm/gear ratio and horsepower may 
require a certain size prop. Your boat may not have the space available for 
it to work efficiently. Causing bad vibrations in the hull or cutting pieces 
out of your boat!

The acceptable clearances are provided as a percentage of the prop diameter 
and in brackets are the absolute minimums:

From tip of blade to hull - 15% (10-12%)

From tip of blade to structure below prop - 6 to 8% (5%)

From aft end of propeller hub to rudder - 50% (30%)

Prop to cutlass-bearing should be at least 1/2" for engines up to 20Hp, 
presuming the engine mounts are in good condition. As power increases 
so should the above mentioned clearance.

This is the absolute minimum dimension for clearance which if incorrect 
will cause the propeller to rub against the cutlass.

It can be rectified by installing a spacer, drivesaver or flexible coupling 
between the shaft and the gear.