How to determine prop pitch and which way 'handed'?

[Babylon]

I need to determine the pitch of our fixed three-blade propeller.

Any original data engraved on the boss has long since disappeared. Is it a black art, or can one work it out oneself?

Secondly, what is the convention for determining which way the prop is 'handed'? IE, observed from standing aft of the stern, the prop rotates clockwise in forward gear - does this mean its a 'right-handed' prop or vice-versa?

Finally, I assume gearboxes reverse the direction of rotation of the prop-shaft relative to that of the engine in forward gear. So, if the engine is rotating clockwise (when facing aft), and the propshaft rotation is reversed by the gearbox, then one would expect the prop to rotate anticlockwise when facing aft (ie clockwise when facing forward).

I need to know all this because, if I re-engine my Bukh with a Beta, I'd like to keep the existing prop if the two engines rotate in the same wise. - and Beta can then fit a bigger reduction gearbox (2.6:1 rather than the standard 2:1) to suit my existing, larger than standard prop.

Thanks

Babs

 

[MapisM]

observed from standing aft of the stern, the prop rotates clockwise in forward gear - does this mean its a 'right-handed' prop or vice-versa?

Yep, that's a right handed prop.

I assume gearboxes reverse the direction of rotation of the prop-shaft relative to that of the engine in forward gear.

Yes and no. That's what the gearbox on stbd engine does, but the gearbox on port engine does the opposite.

 

[earlybird]

/www.ybw.com/forums/showthread.php?t=260236&highlight=measuring+pitch
Not too easy!!
A RH prop. rotates clockwise in ahead, when viewed from the stern.

 

[Tranona]

As usual Google will help with your first question - just enter the term to get a number of methods for calculating pitch. Alternatively take it to a prop specialist who will do it for you in seconds as they have a table set up for the job.

As regards re-using the prop, it needs to be the same "hand". You can check yours by rotating the gearbox in gear. Hand is as viewed from the stern. Going for a 2.6 is good as you can swing a larger prop - I have a 15" with my Nanni 14. However, once you have decided on your engine, do the sums (or get the engine supplier to do them) If the answer means you can use the same prop, that is a bonus. Not worth compromising though - better to sell the old prop and get a new one the right size.

 

[Phideaux]

observed from standing aft of the stern, the prop rotates clockwise in forward gear - does this mean its a 'right-handed' prop? Yes

Finally, I assume gearboxes reverse the direction of rotation of the prop-shaft relative to that of the engine in forward gear. So, if the engine is rotating clockwise (when facing aft - if the point above is correct then it should be rotating clockwise when facing forward), and the propshaft rotation is reversed by the gearbox, then one would expect the prop to rotate anticlockwise when facing aft (ie clockwise when facing forward). Other way round

My thoughts are in red above.

 

[snowleopard]

I assume gearboxes reverse the direction of rotation of the prop-shaft relative to that of the engine in forward gear.

No. For example with Yanmars, a shaft-drive gearbox requires a RH prop but a saildrive gearbox requires LH though the engines always turn the same way. Don't make assumptions without checking.

 

[davidwf]

Put prop on ground with side closest to engine at bottom. Now try to put your foot on the blade whilst keeping heel on ground. Which ever foot it is will tell you if its left or right hand pitch.

 

[scottie]

You are going to spend a lot of money on a new engine .

A prop cost a small part of the total cost ask the supplier for a prop recommendation if you existing old used is anywhere near you have won a watch

The existing prop will absorb x hp at y revs turning z hand

Would you buy a new boat just because it used the same mainsal?
Or a new car just because you have a set of snow tyres that fit