CaptainBob
Well-Known Member
Interesting stuff all this.
I'm inclined to agree with eyehavit in that the drag is quantifiable by the loss of energy to the rotational momentum left in the wake of the boat - intuition tells me there's less swirl remaining if the prop spins freely. If the prop is locked it's the water that will spin surely. Like an initial stator row in a turbine.
I found someone on another forum who pointed out that if the prop turns at the exact appropriate speed, the water will pass through with almost zero swirl remaining in it... so drag is zero at this speed of prop rotation, but increasing drag as the prop slows, to massively increased drag as it goes into reverse... simple thought experiment - I like it - but it forgets to factor in that props are made up of aerofoil shape blades - so if it's spinning, it's creating lift (all be it horizontal) also. Like in the helicopter example.
A little poking about on google reveals that different people with different boats, props and configurations of the same reveals that in practice, due to a number of hard to computate reasons such as cavitation, shaft friction, hull-prop distance for example, find differing results. Sometimes it's better to lock it, sometimes better to feather it.
I guess the only solution is to try it and see how it works out for you in your boat.
But regardless of which causes the most drag - one is probably better for the gearbox than the other.
I'll just do what it says in my manual.
I'm inclined to agree with eyehavit in that the drag is quantifiable by the loss of energy to the rotational momentum left in the wake of the boat - intuition tells me there's less swirl remaining if the prop spins freely. If the prop is locked it's the water that will spin surely. Like an initial stator row in a turbine.
I found someone on another forum who pointed out that if the prop turns at the exact appropriate speed, the water will pass through with almost zero swirl remaining in it... so drag is zero at this speed of prop rotation, but increasing drag as the prop slows, to massively increased drag as it goes into reverse... simple thought experiment - I like it - but it forgets to factor in that props are made up of aerofoil shape blades - so if it's spinning, it's creating lift (all be it horizontal) also. Like in the helicopter example.
A little poking about on google reveals that different people with different boats, props and configurations of the same reveals that in practice, due to a number of hard to computate reasons such as cavitation, shaft friction, hull-prop distance for example, find differing results. Sometimes it's better to lock it, sometimes better to feather it.
I guess the only solution is to try it and see how it works out for you in your boat.
But regardless of which causes the most drag - one is probably better for the gearbox than the other.
I'll just do what it says in my manual.
