Props size - Diameter v pitch

[Stemar]

Castle Marine's prop calculator says I need a 15 by 9 prop. On ebay there's a 14 by 11.

Are they going to be somewhere near equivalent, and would the smaller diameter have less effect on my sailing performance? What would be the advantages/disadvantages?

The boat's a Snapdragon 24 with a Volvo 2003 28hp that can't even feel the current 13 x 8! Yes, I know, but it was affordable when I needed an engine. Anyway, we reckon that a monoski under each keel and we'll be able to take on the fast cats /forums/images/graemlins/grin.gif

 

[steve28]

When i did my prop i tried the online calculators, they proved to be contradicting each other.
I decided in the end get the prop suppliers do the work and gave them the details they in turn told me the diameter and pitch i needed.
This enabled me to source a used prop which gave me the required rpm at full throttle 3600rpm on the yanmar

I think your selection from ebay is a bit hit and miss but you might get lucky

the people i used were bt marine propellers in devon, and im not linked to them in any way

steve

 

[snowleopard]

roughly speaking you can trade an inch from the diameter for an inch on the pitch. it's not exact and a large fine pitch will give you more thrust than a small coarse one.

 

[alec]

Within reason, I would go for the widest prop you can swing with decent tip clearance ( minimum 10%. You do have a mighty engine there fella !

The wider the prop, the more ‘clean’ water the tip blades will come in contact with – hence more bite on the water. Also, area ratio of the prop goes up much quicker with radius .

Unfortunately, us sailors have need of the engine at two extremes. Windless calm water and wild choppy stuff. It might be worthwhile to choose you’re prop on whatever you think is most important.

I have no idea what would be a better set up for calm or wild conditions. Would be most grateful if other forum members might know the answer to this.

Regards,

 

[snowleopard]

just noticed the engine size - my yanmar 27hp runs a 17 x 12 prop. my gear ratio is probably a lot lower than yours but the prop sounds a bit small. do you know the gear ratio?