June 18, 2013

Propeller whys and wherefores

WHY ARE SOME propellers two-bladed and some three-bladed? Why are some fixed, some feathering, and some folding?

It’s all a question of compromise. In the sizes, and at the speeds, usually found in auxiliary sailboats with fixed propellers, a two-bladed prop is often the most efficient.

But sometimes there just isn’t sufficient clearance between the shaft and the hull to accommodate those long thin blades. So the load is spread among three broader blades of smaller diameter. Each blade here is working in water slightly more disturbed by its predecessor than a two-bladed prop would be, so it’s slightly less efficient.

But a three-bladed prop is less prone to vibration than is a two-bladed one, and needs less of a hole cut in the rudder, if that’s the way it’s fitted.

A two-bladed prop, on the other hand, can be nicely lined up vertically behind the rudder post and so avoid much of the drag of a three-bladed prop. And so it goes. In the end, the best advice is probably to fit a two-bladed prop if you can, a three-bladed one if you can’t, and stop worrying about it.

A folding prop causes very little drag, but is prone not to open at all if a fat barnacle grows in the wrong place. So folding props are more suited to racing yachts than to cruisers. Folding props can be tricky in reverse and sometimes need a transmission with a higher than normal astern-gear ratio to make them open properly.

Adjustable pitch props require no gearbox or clutch but are quite rare, presumably because the sophisticated mechanism needed to vary the pitch under way is expensive and more prone to malfunction than are a simple prop and gearbox.

An automatic feathering prop, such as the Max-Prop, reduces drag while sailing, but its fine engineering makes it too expensive for most cruising yachts, which happily drag fixed, reliable three-bladed props around the oceans of the world.

Today’s Thought
As when the devilish iron engine, wrought
In deepest hell, and fram’d by fury’s skill,
With windy nitre and quick sulphur fraught,
And ramm’d with bullet round, ordain’d to kill,
Conceiveth fire, the heavens it doth fill
With thundering noise, and all the air doth choke,
That none can breathe, nor see, not hear at will,
Through smouldry cloud of duskish stinking smoke,
That th’onely breath him daunts, who hath escap’d the stroke.

— Spenser, Faerie Queene

The problem with the person who has an hour to kill is that he inevitably wants to spend it with someone who hasn’t a minute to spare.

(Drop by every Monday, Wednesday, Friday for a new Mainly about Boats column.)


drysuit2 said...

Always a good read.

John Rushworth said...

This puts matters in further perspective. http://strathprints.strath.ac.uk/5670/ and the whole area of the 'right' prop and drag is an area I am researching for my electric sailboat, in which I have now covered my first 330 miles. The nice thing about my electric drive is I can spin the prop with only 20 Watts to overcome friction. Much better than a locked prop about which there is a lot of misinformation. Here is my conversion. http://www.Elektra-Yachts.Co.UK