At Seahawk we often spend a lot of time checking and confirming data when sizing propellers, to many, our requirement for accurate boat details is painful, as we send then repeatedly ‘head down – backside up’ into the bowels of their boats looking for engine and gearbox ID plates, or making them calculate the ACTUAL loaded displacement.

What follows is our version of ‘Prop Sizing 101’ for displacement hull, sail driven boats, this is not intended to be a technical paper, it is aimed at the typical sailing customer to help them understand propellers and the sizing of them a just a little better.

First lets us make a bold statement, no matter what others tell you, propeller sizing is a “Highly Technical Black Art”, while there are many calculators and software programs available, they all rely on accurate information, then ‘tempering’ with experience to have a successful propeller sizing outcome.

We will work through a fictitious boat example, adjusting the numbers as we go to show you the effects…

Let us start with a 3 ton displacement, 30 foot waterline, 30 hp @ 3000 rpm engine and 2:1 gearbox reduction ratio. Calculating the sizing with just one of the many programs available yields the following.

For a two blade propeller 17” Diameter x 12.5” of pitch
For a three blade propeller 16” Diameter x 12.5” of pitch
For a four blade propeller 15” Diameter x 12.5” of pitch
The boat has a predicted speed of +/- 9.9 knots

It can be seen from the above results that the only difference between the 2, 3 & 4 blade propellers, is the diameter, we shall discuss this aspect later. To keep it simple we will use the three-blade example only.

If we the waterline was longer, say 40 foot, with the rest staying the same, we end up with a faster boat, the propeller sizing changes as we need to increase the pitch of the blades so it screws through the water more with every turn, in effect the propeller needs to be shifted up a gear…

Now we need 16” Diameter x 13.5” of pitch, with a new predicted speed of +/-11.5 knots.

Next we go back to the original specs, but increase the displacement to 6 tons, this is typical, albeit an exaggerated case of the difference between the understated bare, dry ‘brochure weight’ claimed by manufactured, versus the actual loaded ‘ready to sail’ weight’ or maybe you have just cruised the world for a few years and collected all manner of interesting treasures…. as you may figure, the extra weight slows the boat down. As the boat is now slower, we need less pitch, in effect we need the propeller to screw through the water less with every turn…

You now require 16” Diameter x 11” of pitch, with a new predicted speed of +/-7.4 knots.

Having had the boat for some years, you unload all your ‘treasures’ and repower with a bigger 40 hp engine, same rpm, same gearbox, waterline and displacement. As would happen with a car if you added 33% more power, you can now go faster and need more traction. More speed, as we know from the above examples, requires more blade pitch, more traction comes from more blade area, which typically means a larger diameter (or more blades)…

We now require 17” Diameter x 13.5” of pitch, with a new predicted speed of +/-11 knots.

The last of the significant variables used in calculating propellers is the gearbox ratio. In this next example we go back to our original set of data, but due to a ‘catastrophic failure’ you have had to replace the gearbox. The exact same gearbox was $1000 more expensive than a discounted variant with a 2.5:1 ratio, so you opted to save the money. With the 2.5:1 ratio, the propeller turns only 80% as fast as before (down from 1500 rpm down to 1200rpm).

The required propeller is now 18.5” Diameter x 15” of pitch, with the original predicted speed of +/-9.9 knots.

Why?…
Diameter Increase - As the propeller turns slower and in effect has more torque due to the increase gearbox reduction, it needs to be bigger to get ‘traction’ in the water.
Pitch Increase – As the propeller has been slowed down it needs more pitch (higher gear) to make the same speed the 30 hp will allow.

Coming soon...

How many blades are best - more is not always better.

Over or under sized propellers and the symptoms.

Brochure weight versus actual loaded displacement. 

Diesels are not what they used to be, why not to run the revs too low. 

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