Bukh Propellor

[tdw]

Point of Order.

Isn't the size of your prop determined more by a tradeoff between speed and torque ? Bigger prop allows you to pull up tree stumps, smaller is as week as p..s but gives plenty of revs. I a'int no engineer so could have this completely wrong.

[Alan Wheeler]

Yes that is kinda right. A bigger prop diameter/greater number of blades has greater surface area within contact with the water, thus greater efficiency. But at the same time, the greater surface area also produces greater drag. Or in other words, more effort to turn the thing around. So ultimately a balance has to be reached of power available being able to turn the prop and the prop getting that power into the water and moving the boat forward. It would be of no advantage to have all 10hp taken up just to turn the prop around without getting any of that 10hp into a forward movment.
The number of blades is an even more complex issue and choosing a two blade, three blade aor four blade prop has a lot of science around the choice. Then you get into blade shape and profile. That has so much gobbledegook around the science of it, it is why each prop manufacturer has there own design. what ever the shape and profile is, the main point is that prop design today is very different to what it once was. Decades ago, the design of a prop was thought of as a screw. True in some respect. But the present(last couple of decades or so) day designs are based on a rotary wing. The prop no longer screws through the water, it lifts through the water as if it were a wing. So design is influenced by principles based around wings in a fluid. Or Hydrodynamic/aerodynamic principles.

[seafox]

Whatabout the "wobbles"?

[Alan Wheeler]

They lived underground and over ground wandering free in Wimbleton common I think.

Sorry Darryl, not sure what you mean.

[Marauder]

I think Darryl was talking about the "tooth ache" with my prop. It does have some play but certainally isn't falling off. I am sure that a prop salesperson would sya buy a new one! Was wondering if that has anything to do with lowering the performance or something to keep an eye on so that it doesn't completely flog out?

[Boracay]

Surely any wear or tear that causes free play in a folding propellor will degrade performance?
In particular if one blade opens fully and the other does not then at least one blade will be performing poorly.
Have you noticed any unusual vibration?

[Alan Wheeler]

Chris is right. Vibration is wear somewhere and often on shaft and bearing. Plus poor performance could be an issue due to cavitation, one blade creating turbulance into the path of the other, or one blade just simply not being efficient in the water. My biggest concern however, would be a question of How worn? and how soon will the blade fall off compleately and leave you with the arm strong model of prop. Two oars. At least the Armstrong is a twin engine design;-)

[seafox]

From memory there is at least 5mm of play when you fully open the two blades and give them a vertical twist up and down (not sideways but at 90 deg to the way they open and close).
A local prop guy sucked me into buying a new one with about 2mm of play.
There may be a way to put a bush or washer into the prop to stop the play or wobble of wimbleton.

Hey Andy....dying Southerly on Thursday becoming Northerly 15knots. Mmmmm, I will think of you doing your lawns