How do yo work out the correct propellor size?

monkfish24

Active member
Joined
24 Apr 2003
Messages
1,597
Location
Cornwall
Visit site
A friend of mine is looking to change a 3 bladed fixed prop to a 2 blade folding prop on a shaft driven Yan 1GM10.

How do yo work out the correct pitch and size?

I've tried google but can't find owt :(
 

sailorman

Well-known member
Joined
21 May 2003
Messages
78,864
Location
Here or thertemp ashore
Visit site
A friend of mine is looking to change a 3 bladed fixed prop to a 2 blade folding prop on a shaft driven Yan 1GM10.

How do yo work out the correct pitch and size?

I've tried google but can't find owt :(

spin the largest Dia you can allowing for hull clearance then size the pitch.
there are web based calc programmes

http://www.castlemarine.co.uk/pitch.htm
 

Norman_E

Well-known member
Joined
15 Mar 2005
Messages
24,609
Location
East Sussex.
Visit site
First test is to find out if the current prop is correct size. Will the engine achieve its rated full power revs with the prop? If not it is too big. If it revs right to the rev limiter the current prop is too small. After doing that test find out the gearbox ratio and give that information plus the rated power and revs of the engine to a propeller supplier, and take advice.
 

BAtoo

New member
Joined
1 Mar 2004
Messages
2,056
Location
East Coast
Visit site
A friend of mine is looking to change a 3 bladed fixed prop to a 2 blade folding prop on a shaft driven Yan 1GM10.

How do yo work out the correct pitch and size?

I've tried google but can't find owt :(

On my experience of buying 2 propellers I took all of the measurements (shaft size, length, taper, clearance to hull and rudder, displacement etc) and spoke to the people I bought the props from and took their advice. Worked well enough for me.
 

bendyone

Well-known member
Joined
19 Jan 2003
Messages
5,384
Location
Oxford
Visit site
On our Hunter 26 with a 1GM10 3:1 gearbox we had a 2 bladed prop 14 x 10 which was the correct size.
 
Last edited:

Tranona

Well-known member
Joined
10 Nov 2007
Messages
41,211
Visit site
A friend of mine is looking to change a 3 bladed fixed prop to a 2 blade folding prop on a shaft driven Yan 1GM10.

How do yo work out the correct pitch and size?

I've tried google but can't find owt :(

As suggested you can get an estimate by using the Propcalc programme from Castlemarine. However the advice to get the supplier to specify the size is the way to go as it is then their responsibility to get it right. There is a lot of variation in the design of folding props so the ideal size may vary between manufactuerers.

As has already been noted you will lose a lot of motoring performance unless the boat is very light as there is little power available from a 1GM. A feathering prop such as a Featherstream is a much better bet, but if course more expensive.
 

BAtoo

New member
Joined
1 Mar 2004
Messages
2,056
Location
East Coast
Visit site
Maybe less motoring performance - only maybe though imho - but a much better sailing performance once the 3-bladed bucket is ditched - in my actual experience....
 

Tranona

Well-known member
Joined
10 Nov 2007
Messages
41,211
Visit site
Maybe less motoring performance - only maybe though imho - but a much better sailing performance once the 3-bladed bucket is ditched - in my actual experience....

Not questioning the potential improvement in sailing performance - just pointing out that when you have so little power, any loss in motoring performance can be very noticable. A feathering prop normally has better motoring performance alongside the reduction in drag.

My 1GM was able to spin a 15" 3 bladed featherer in a heavy boat. It is now in a smaller lighter (rather racy) boat and is quite happy with a 2 bladed folder. So you need to consider the whole package and potential use before choosing the prop.
 

monkfish24

Active member
Joined
24 Apr 2003
Messages
1,597
Location
Cornwall
Visit site
Cheers guys,

The friend is an avid racer who is more interested in sailing performanc than motoring. He's the kind of bloke who will sail on and off a mooring to not bother with the engine. He has a 5hp outboard which he is planning on using as an engine if we can't get the engine going so I'm not really imagining that he will be worried by the performance drop. It is in an Eygthene 24 so not a big boat at all.

I'll speak to Yanmar and see what they recommend.

Thank you everyone.
 

sailorman

Well-known member
Joined
21 May 2003
Messages
78,864
Location
Here or thertemp ashore
Visit site
Cheers guys,

The friend is an avid racer who is more interested in sailing performanc than motoring. He's the kind of bloke who will sail on and off a mooring to not bother with the engine. He has a 5hp outboard which he is planning on using as an engine if we can't get the engine going so I'm not really imagining that he will be worried by the performance drop. It is in an Eygthene 24 so not a big boat at all.

I'll speak to Yanmar and see what they recommend.

Thank you everyone.

you will get a better response from a Prop manufacturer / supplier
 
Top