How to calculate prop size?

[jim99]

This is what I have...

Yanmar 3YM20, 2.62 gear ratio and 2.4 cm prop shaft.

Boat is fibreglass monohull... Vancouver 27, LWL 23', beam 8'8", displacement 9,000 lbs.

Any other information needed?

I want a three-blade prop for long-distance cruising. Diameter and pitch should further that goal.

If anyone knows from that what calculations are necessary I'd much appreciate it.

cheers

 

[NorthUp]

Propcalc, a freeware program, suggests 15 x 12 for a hull speed of 7.2 knots.
Does your displacement include a substantial ammount for stores and fuel? It starts to be a black art fine tuning the prop size for cruising, but allowing for some extra weight v. the engine not being flat out all the time, this is a good starting point.
Stand by for alternative opinions!
Finaly, check how big a diameter of prop you can swing in the hull, you need at least 15% of diameter as a tip clearance

 

[Robin]

That suggests that with that set up it will beat hull speed which on a 23ft LWL is 6.43kts or alternatively that it can achieve a hull speed of 1.5 x sqrt of LWL. I would do the sums again?

 

[saltwater_gypsy]

Here is a Googlebook on Prop. design

Prop.design

 

[NorthUp]

Max hull speed does not occur at an exact point of a graph- its much more vague that that, the formula you quote is an approximation- as is the propcalc program!
Its free, after all- google for it and you can play with the figures changing Hp or revs for comparison. I have had good results with it, and have confirmed it works with existing prop/ engine installations that work.

 

[Ex-SolentBoy]

Whilst Propcalc and other do a reasonable job, you can also do the calculations yourself by reading Nigel Calders Technical book.

Either way I would suggest that the Vancouver Owners Association already know the answer.

 

[alexrunic]

ring t norris they will do it for you