Propellor pitch

[alanporter]

Can anyone tell me a practical way of determining that a sailboat prop has the correct pitch. The various published formulae only give a rough calculation for a "generic" boat, but each boat has it's own individual characteristics. I have changed the engine on my Westerly Renown from the Volvo Penta with a L.H. 14 x 11 prop, to an Isuzu engine with a R.H. 15 x 9 prop, as recommended by the engine supplier. How do I know if this is correct ? Many thanks in advance.

<hr width=100% size=1>Alan Porter

 

[bedouin]

The only way to tell is by how the boat performs under engine. Does it reach target cruising speed in calm water at cruising revs?, can you reach full revs in most conditions?

In practice there is not necessarily a single "right" propellor, but rather a number of different options, each maybe offering a different compromise.

The manufacturer's recommendation is usually a good place to start.

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[charles_reed]

Unfortunately there is no such thing as a "correct" propellor for all occasions unless it's one of the self-pitching ones like the Hundstedt or Brunton (I exclude the Maxprop because that has to be tuned for the engine/conditions).

The best you can do is to have a reasonable compromise.

If, under calm conditions the boat reaches max power engine revs at about 7/8 throttle, you've got a good compromise.

If the engine won't reach max power revs and produces lots of black exhaust you're overpropped - in any case it will do this heading into a head sea and wind about F6 onwards.

If max power revs come up much under 3/4 throttle you're definitely underpropped.

I'm afraid it's trial and error.

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[max_power]

It is interesting that you have increased your prop from 14" to 15".

I have been given to understand that the width of prop has a much higher bearing with regard to increased power than pitch. Providing you have the room to swing it with decent tip clearances and enough power.

It's a bit like an arm of soldiers doing a wheel on the parade ground . I can guarantee that the ones on the outside will be puffing and blowing a lot harder than the ones on the inside . I think this is why pitch is much stronger near the hub and sails have more depth higher up to compensate.

If you can't swing a big'n you can go along the road of more pitch and larger blade area but , as usual in everything boaty, it's goes downhill after a certain point.

Charles has mentioned the self pitching Brunton which sounds excellent. I assume that the pitch reacts to water pressure ?. Is it like a push bike with high gear for flat water and lower gear for punching choppy water ?

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[charles_reed]

Brunton Autoprop

Asymmetric blades, which have to be tuned to the hull drag/torque curve/power curve of the motor.

Result, when drag is low pitch is increased - ie motorsailing
, when drag/resistance is high pitch is reduced - ie butting headwinds/seas

All however depends on knowing the output curves for the engine (not difficult) and the prismatic coefficient of the hull (very obscure, unless Wolfson tank-test figures are available).

Another variable is the gradual loss of engine power between head overhauls.

Mine was one of the first Autoprops, in use for 11 years.
You'll be hard-pressed to justify the additional cost on fuel saved (it'll take about 15 years) but the greatest benefit is when motorsailing in light airs, greater range under power and getting into Lagos in a F8 on the nose (5 of the 7 boats who rounded Piedade with me retired hurt).

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[Aramas]

Prop matching is one of the great mysteries of the universe, not because it's difficult, but because prop manufacturers will not supply accurate characteristic graphs. Unless you use standard series (mostly for ships, eg wageningen) then you're pretty much out of luck. Generally you need to work out appropriate diameter, BAR (blade area ratio), pitch, skew and number of blades, attempting to optimise for a given speed and rpm range, within the limitations set by the cavitation characteristics of that particular prop.
Which of course is impossible without a set of graphs from the manufacturer.
In the end all you can do is find a supplier that knows what they're doing, or experiment yourself.

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[pheran]

I assume your propellor supplier/manufacturer ran your boat/engine details thro' his computer software first. They all have some form of calculator and some are quite sophisticated these days, allowing you to mess about with all the characteristics eg DAR, rake, skew etc as well as the basics of diameter and pitch.
There are also some simpler programs available on the net as free downloads, always handy to have at home! Try sticking boat propellors into Google and see what you get.

<hr width=100% size=1><font color=blue>A hangover is just the Wrath of Grapes</font color=blue>

 

[chippie]

I agree with Pheran , but am aware that no matter how much information is available re engine; reduction ; room available etc. etc. The characteristics of the hull are the hardest thing to quantify accurately. Most manufacturers would essentially be working from experience of similar type hulls as a starting point.
The finer tuning of selection criteria seems to be the really hard bit.

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[NigeCh]

Your first stop should be to read ...

David Gerr's "Propellor Handbook"

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[max_power]

Re: Brunton Autoprop

Thanks Charles.

It certainly has the potential to be a 'design classic' in sailing along with man made ropes etc. And it's British !

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