Propellers
Choosing a propeller
We have recently changed the propellor on Allegra from a 2-blade 16x11 to a 3-blade 18x12. Both propellers are from Volvo Penta and it's an upgrade I've wanted to do for some time.
The two-blade prop was slightly underpitched, which meant to achieve a given speed, say 6 Knots we needed to run the engine at a higher RPM, then the engine would also reach its maximum rpm with ease. It is a fine balance between under-pitched and over-pitched because a propeller that is pitched too high will mean that the engine can't reach its maximum rpm, could put extra load on the engine and the boat will travel faster when it's in gear at idle rpm.
Pitch, Size & Rotation
- What does 16x11 or 18x12 mean?
- What is LH or RH rotation?
A propeller that is 16x11, measures 16 inches in diameter of a circle as scribed by the blade tips. So if you spun it around with a pencil on the end of the blade, that circle would be 16 inches in diameter. The 2nd number is the pitch and on each rotation would move 11 inches forward if it was turning through something soft.
So the new propeller is bigger in diameter and also in pitch. This means for each turn the boat travels further forward than it did, the bigger diameter gives it more efficiency and therefore more thrust and we also have an extra blade in the water which helps with the grip. 3 blade props reduce vibration but can add more drag when sailing, luckily a folding prop helps with drag as the blades fold in when its not pushing the boat.
LH / RH are Left Handed Rotation or Right Hand Rotation, this is driven from the gearbox and some output clockwise and some anti-clockwise when you select forward. The S130 sail drive spins anti or counterclockwise when in forward gear, so a LH prop is required.