Propeller Pitch - what it means and choosing the right one
Posted: Tue Dec 22, 2009 12:11 am
Propeller pitch is quite a simple one really.
Say you had a 26" x 20" propeller. This means it is 26" diameter and 20" pitch.
Just like a thread on a bolt, the pitch tells you how far it moves in 1 revolution. E.g. a thread with a 1mm pitch will move into a hole by 1mm in 1 revolution of the bolt.
So with a propeller, given there is no resistance, as the propeller turns, it will push its self through the water, 1 revolution giving 20" of forward motion in this example.
Prop Slip
But as we know, there is resistance. Its called a boat. This causes propeller slip, where although a propeller does 1 complete turn, instead of travelling 20" it only travels 15".
Working out the propeller slip is very straight forward. You need to know your propeller speed and hull speed.
For example, the hull is doing 6mph and the 26" x 20" propeller is doing 400 rpm.
400 rpm x 20" = 8000" a minute.
6mph = 6336" a minute.
6336 / 8000 x 100 = 79.2% -> This is the % of used thrust.
Thus 100% - 79.2% = 20.8% slip.
In the ideal world, you want 0% slip. In reality, anywhere between 10% and 20% would be ok, but on the smaller steam launches you can often find the slip is as much as 25% or more.
Choosing the correct prop
Again this is really much simpler than just having to guess.
The important thing is to know your engine speed. This will have been designed into the engine, so if it recommends running at 450 rpm, best aim for that. Higher speeds also means more power.
You should also work out your hull speed.
Hull speed (knots) = 1.34 x Square Root of (Length of Water Line in feet)
1 Knot = 1.150779 mph
E.g. for a 15 foot boat, the hull speed is:
Hull speed = 1.34 x SqRt(15 foot)
Hull Speed = 1.34 x 3.742
Hull Speed = 5.19 knots = 5.97 mph.
Say you calculated a running speed of 6mph, and you want to try for 14% prop slip, all you do is calculate the correct pitch.
Adding this into the calculation:
14% is the slip, meaning that there is 86% of useful thrust.
450 rpm x pitch = "pitch distance" a minute.
6mph = 6336" a minute.
6336 / "pitch distance" x 100 = 86%
"pitch distance" = 6336 / 86 x 100
"pitch distance" = 7367" a minute -> This is the distance the propeller would move without slip.
So 450 rpm x pitch = 7367
Meaning the pitch required is 16.4".
You may want to go for an 18" pitch in this case, it is usually better to have the engine slightly slower with the larger pitch than slightly faster with the smaller pitch.
Propeller Diameter
In this case, bigger the better. The larger the propeller the better their efficiency. The difference between a 14" and a 16" will decide who gets the good jetty spot if the boats are otherwise identical.
Steam launch propellers generally have 3 large blades. This also adds to their efficiency as we are not running them at high speeds, surface piercing or anything other than stuck firmly under the water and generally not stressed. There is nothing wrong with 5 blades, its the total blade area which is the key to their efficiency. As long as the pitch is suited for the engine speed and hull speed, you shouldn't go far wrong.
Brass and Bronze are the traditional metals, though a few stainless ones have appeared lately, the odd one being home made.
Say you had a 26" x 20" propeller. This means it is 26" diameter and 20" pitch.
Just like a thread on a bolt, the pitch tells you how far it moves in 1 revolution. E.g. a thread with a 1mm pitch will move into a hole by 1mm in 1 revolution of the bolt.
So with a propeller, given there is no resistance, as the propeller turns, it will push its self through the water, 1 revolution giving 20" of forward motion in this example.
Prop Slip
But as we know, there is resistance. Its called a boat. This causes propeller slip, where although a propeller does 1 complete turn, instead of travelling 20" it only travels 15".
Working out the propeller slip is very straight forward. You need to know your propeller speed and hull speed.
For example, the hull is doing 6mph and the 26" x 20" propeller is doing 400 rpm.
400 rpm x 20" = 8000" a minute.
6mph = 6336" a minute.
6336 / 8000 x 100 = 79.2% -> This is the % of used thrust.
Thus 100% - 79.2% = 20.8% slip.
In the ideal world, you want 0% slip. In reality, anywhere between 10% and 20% would be ok, but on the smaller steam launches you can often find the slip is as much as 25% or more.
Choosing the correct prop
Again this is really much simpler than just having to guess.
The important thing is to know your engine speed. This will have been designed into the engine, so if it recommends running at 450 rpm, best aim for that. Higher speeds also means more power.
You should also work out your hull speed.
Hull speed (knots) = 1.34 x Square Root of (Length of Water Line in feet)
1 Knot = 1.150779 mph
E.g. for a 15 foot boat, the hull speed is:
Hull speed = 1.34 x SqRt(15 foot)
Hull Speed = 1.34 x 3.742
Hull Speed = 5.19 knots = 5.97 mph.
Say you calculated a running speed of 6mph, and you want to try for 14% prop slip, all you do is calculate the correct pitch.
Adding this into the calculation:
14% is the slip, meaning that there is 86% of useful thrust.
450 rpm x pitch = "pitch distance" a minute.
6mph = 6336" a minute.
6336 / "pitch distance" x 100 = 86%
"pitch distance" = 6336 / 86 x 100
"pitch distance" = 7367" a minute -> This is the distance the propeller would move without slip.
So 450 rpm x pitch = 7367
Meaning the pitch required is 16.4".
You may want to go for an 18" pitch in this case, it is usually better to have the engine slightly slower with the larger pitch than slightly faster with the smaller pitch.
Propeller Diameter
In this case, bigger the better. The larger the propeller the better their efficiency. The difference between a 14" and a 16" will decide who gets the good jetty spot if the boats are otherwise identical.
Steam launch propellers generally have 3 large blades. This also adds to their efficiency as we are not running them at high speeds, surface piercing or anything other than stuck firmly under the water and generally not stressed. There is nothing wrong with 5 blades, its the total blade area which is the key to their efficiency. As long as the pitch is suited for the engine speed and hull speed, you shouldn't go far wrong.
Brass and Bronze are the traditional metals, though a few stainless ones have appeared lately, the odd one being home made.