Propellors

[Johnjo]

If a yacht was fitted with three bladed propellor say 12 by 8.
Then had fitted instead, A two bladed prop.
The question is, Would the two bladed prop need to be slightly larger
to compensate for the lack of a third blade ?
Or would it already have being taken into consideration when designed ?
To my thinking at the moment, Less drag equates to less drive,
Hence larger prop needed,

Mike

 

[airbubble]

as a rule of thumb - you step up 1" when losing a blade.

 

[kgi]

seriously think about a three blade feathering prop, the two bladers are not that brilliant going astern, and they tend to cause more vibration when motoring

 

[Celena]

I seem to remember Darglow (from whom Celena's Maxprop came) said that it's blade area that's important. We changed from 3-bladed fixed to 2-bladed feathering and we're very pleased (astern as well). Darglow did say performance (over a 3-bladed) would suffer when the going got tough though!

 

[Chris_Stannard]

This depends on what you are trying to achieve. If you are fitting a two bladed propeller to save drag then you should look at a three bladed folding propeller if you can afford it. I have a Flex-o-Fold from CJR of Southampton on my boat and it is very good. It is not as expensive as a feathering propeller and is just as effective as the fixed three bladed it replaced. It was not cheap however, about £850 for a 17 x 13 model. CJRs service was not up to much either but the increase in sailing performance was worth all the effort.

Chris Stannard

 

[johnt]

I'll second that Airbubble...but its one inch in pitch ..or one inch in diameter ..and dont forget the 15% of diameter prop/hull clearance

 

[jimi]

Chris, did it make that much difference? I was humming anf hahing about a folding prop when I had to replace mine after it fell off (do'nt ask or I'll bore you to tears) and was informed that it would'nt make much difference so at the time it did'nt seem worth the extra cost. However I'd be really interested in your before and after story.

Jim

 

[Johnjo]

Thank you all,
I think my question has been answered ,
What has happened is,
Bought new engine from friend who never got around to fitting it,
Subsequently, He sold his boat,
Engine came with 3 bladed prop to suit his boat.I.E motor boat,
Now being fitted in yacht, Hence 2 bladed prop,
Advice on folding props noted and agreed with,
But HWMBO is having palpitations about cost,
To save her from complete breakdown,
I have to take the lower cost option.
Rrgards
Mike

 

[dickh]

Mike27, if you are interested I have a sh folding prop on approx a 5' x 1"Ø shaft which has been sitting in my shed for ages - the prop if I remember is 15 x 8 - if you are interested, PM me.

dickh
I'd rather be sailing...

 

[Johnjo]

Dickh
Thanks very much for the offer,
But as fate should have it, Went out this morning and bought two blader.
Also feel that a 15 inch diameter would be too much to swing.
Regards
Mike

 

[Robin2]

This came up here recently. Search for "prop size" . It gives a link to a site called surfbaud which has a propellor calculation spreadsheet.