Questions on Props

[speedchaser]

A few days ago I bought two Graupner (one's a spare) fixed pitch 42mm 2-blade racing carbon-fibre props (not wobbly edged unknown-branded nylon one's), even though originally I was going to go for a three or four bladed prop, as I thought that more blades would produce more thrust, and so I thought I could go for slightly smaller diameter, say 40mm; but the store told me that the opposite was the case, that less blades will produce more thrust than more blades for the same diameter.
I half believed the member of staff, and so got two 2 bladed ones - but is this right or..? I.e. the more working oars or paddles in a rowing boat the faster it will go, and well, a prop is a kind of rotary paddle :rolleyes: to use an analogy (sorry - posh word)

[Climate]

First off, props are not at all like a paddle. They are like wings. Each blade on a prop produces areas of positive and negative pressure as it moves through a medium.
The blades on a prop do a lot more "pulling" than they do "pushing". Look at a prop blade carefully. It is curved and has a wing shaped planform.
The most efficient design will always be a larger diameter, single bladed prop. When the prop spins it creates turbulence in the medium, as the next blade encounters this turbulence it does not "bite" as much of the medium and slips. For slower moving craft, this is not a problem and a multi bladed prop significantly reduces the overall diameter of the prop, slippage is not an issue.
For high-speed craft, we want to get the most efficiency from the prop we can. The less slip the more speed.
Another thing to consider is "prop walk" this is the sideways force created as the prop blades enter and exit the water on a surface drive application.
The more blades you have, the less prevalent prop walk is. The reason for this is simple. On a 2 bladed prop there is a point where both blades are basically out of the water. On a 3 or 4 bladed prop, there is always at least one blade in the water.

Do a search on the net for "Prop Theory" You will get more information than you know what to do with.

You can then pass this information on to your friend at the hobby shop.

Peter Richards
www.climatemodels.com

[speedchaser]

Many thanks - so from that I understand that a two bladed prop will create more thrust than a three bladed one, and so on...