Right propeller?

[pikeyrm]

I have a Westerly Pageant with a yanmar 1gm10 with 2:1 gearbox in and I am running a 13 x 9 propeller. I was told this was the correct size for the boat and gear box and engine.
After a year of having the engine in the boat I know believe that this is not the right prop as I am only getting about 4.5 knots from the engine. I have also thought that the engine never reved out with this on.
It is a new (2013) engine and the hull is clean, I did think about going for a larger pitch on the prop to gain greater speed under engine but am now thinking that if I lowered it then it might rev out, giving me greater speed.
So the question is, bigger or smaller? (prop lol )

 

[underdog]

Try this site for a prop size calculation: http://www.vicprop.com/displacement_size.php
The figures I put in come out with 12 X 7 for a Westerly Pageant

 

[Tranona]

That prop is far too big, which is why you are not getting the speed. It would work with a 2.6:1 reduction box, but as already suggested a 12*7 is more appropriate.

 

[pikeyrm]

many thanks, looks like another new pro then ouch lol

 

[Minchsailor]

I agree with others, far too big. I have found Norris at Isleworth excellent for all prop related stuff. They may be able to rework your existing propeller - they did for me:

www.tnorrismarine.co.uk

 

[Nigel_Ward]

You need to use the exact gearbox ratio to do a prop calcultion. Yanmar show 2.21 and 2.62 reduction ratios for the current engine.

 

[pikeyrm]

yep many thanks, been down to the boat today and the gear box ratio is 2.21, so with that I believe I will require a 12 x 7 and will be looking into getting mine resized.

 

[ianj99]

yep many thanks, been down to the boat today and the gear box ratio is 2.21, so with that I believe I will require a 12 x 7 and will be looking into getting mine resized.

As long as you are happy with any extra noise from having to run the engine at a higher rpm. Reducing the pitch by 20% from 9 to 7" will mean approx 20% more revs for the same speed. Personally, I'd compromise and settle on a 12 x 8".

 

[pikeyrm]

Please see plate on the gearbox

 

[scottie]

Dia is revs and pitch is speed so decreasing the dia would be better and easier in this case

 

[pikeyrm]

Dia is revs and pitch is speed so decreasing the dia would be better and easier in this case

11 x 9

 

[RichardS]

Dia is revs and pitch is speed so decreasing the dia would be better and easier in this case

I have always thought that pitch and diameter were interchangeable and the overall effect of the prop was the approximately the multiple of both.

I stand corrected.

Richard

 

[ianj99]

I have always thought that pitch and diameter were interchangeable and the overall effect of the prop was the approximately the multiple of both.

I stand corrected.

Richard

Its a black art so you are not entirely incorrect! This where the Kiwi prop scores - the pitch is adjustable in situ with a 4mm hex key.

It would be interesting to see what Norris and Lancing Marine come up with if the OP contacts them.

 

[pikeyrm]

emailed a few prop companies and all coming back with 13 x 7 would this because they prefer to change the pitch rather than diameter?

 

[ianj99]

emailed a few prop companies and all coming back with 13 x 7 would this because they prefer to change the pitch rather than diameter?

Its just what their calculator and years of experience comes up with. The only way you will know if they are right is when you try one and it does what you want.

 

[Tranona]

emailed a few prop companies and all coming back with 13 x 7 would this because they prefer to change the pitch rather than diameter?

Two inches less pitch will bring revs up by 450-500. Is it a 2 blade or 3?

 

[scottie]

I have always thought that pitch and diameter were interchangeable and the overall effect of the prop was the approximately the multiple of both.

I stand corrected.

Richard

One can be played off against the other but I would stand my basic understanding

A question to ask is which would a supplier offer to alter if either to gain performance .

 

[pikeyrm]

£80+ vat to re shape mine in a week, I think is a good price quoted from Hamble Propellers