Propellor choice

[rajjes]

In calculating the prop for my engine/gearbox/boat specs (using the castlemarine program), the results are 12.92 diameter by 11.60 pitch.

Based on this, should I go for a 13X12 or a 13X11 (3 bladed) prop?

Am I right in assuming that with the 13X12 the engine will work harder (is this beneficial on a diesel?) and might not reach max revs, while with the 13X11 there might be some reserve (wasted?) power left at fuil revs?

As props are not cheap I would like to get it as best as possible first time round!

Actually would there be a tangible difference in 1" pitch difference or am I fussing about nothing?

Engine is a 10Hp Kubota, 3000 revs with 2.2 gear ratio on a long keel 26' boat.

Thanks for your help!

 

[Clyde_Wanderer]

Hi I would say go for the 11 pitch as 12 might just put that extra unwanted loading on a 10hp engine, which would cause it to use more fuel and not give any more thrust than the 11 pitch.
Also it will make for worse steering going astern, due to more prop torque.
I fitted a new 13x8" 3 blade as a replacement for an old 2 bladed prop, connected to a 20hp engine, it made a huge difference to the punch through waves, and speed, but cant steer astern now. fin keel. skeg hung rudder.

 

[bendyone]

Talk to these guys T Norris
I have just ordered a new prop from them 14Hp 3500rpm 2:1 gearbox Hunter 26.
They came up with either 13x8 or 12x8 but suggested the larger one as if if proves wrong they can cut it down or even re pitch it.

 

[roly_voya]

Increasing pitch increases the speed the boat tries to go at any particular revs provided the engine has enough power at those revs to turn the prop. Increasing blade area (dia or no & shape of blades) increases the poer put into the water so drives a heavier boat or keeps speed up into wind and waves, again provided the engine can provide the power. If your power/displacement ratio is 3hp/ton or less definatly go for the smaller one, you will get 1/4-1/2kn less on max speed but about 10% more reserve power because you will be reving faster (therfopr developing more power) for a given boat speed so the boat will hold its speed better into wind and waves. If you have plenty of power (4-5hp/ton) the larger one might give you about 5-10% lower revs at cruising speed which will be a little quieter and more ecconomical but will be slightly less efficient flat out because once you reach hull speed the boat wont go faster so you just increase slip if you increase the prop speed. You would probably get a max usable revs of 2800-2900. The safe opton is the smaller one

 

[Gunfleet]

Is 4/5 hp a ton considered normal now? If so my 4 ton boat driven by a Yanmar 1GM10 is half underpowered!

 

[rajjes]

Thanks to all, I've ordered the 13X11, cheers.