Gear ratio 2.21 or 2.62

[Ken13559]

My friend is repowering his Maxi-95 with Yanmar 3YM30 + KM2P gearbox. Question is, should he go for gear ratio 2.21 or 2.62? What's the benefits of one over the other? Appreciate advise. Thanks.

 

[Ross]

For these low speed boats the larger prop turning at a lower speed will be more efficient. If he has room for the prop the the lower ratio is indicated.

 

[Don12364]

Measure the perpendicular distance between propshaft centre line and the hull.
Deduct 15% and this will give the radius of the largest prop which will fit without causing too much turbulence & vibration from interference between prop tips and hull. i.e. prop walk.
Then do the calculation for the engine/boat/prop combination using the prop calculator on Castle Marine's web site for each gearbox ratio:- http://www.castlemarine.co.uk/pitch.htm.
This should give the basic data for pitch, boat speed and efficiency etc.
If a suitable prop is found for the higher ratio (i.e. 2.62) it should give best propulsion.

BUT a big prop causes much more drag when sailing so this route should only be followed if it is intended to fit a feathering (or folding) prop.

If you are concerned with sailing performance without fitting a featherer or folder then a smaller prop with two narrow blades is required and the 2.21 gearbox will give this - although you will almost certainly be disappointed with both sailing and motoring performance.

 

[Ken13559]

Ross, by lower ratio I gathers you meant 2.62 since he'll need a larger diameter prop with this ratio.

Donalex, he intend to fit a Max prop (feathering). Thanks for the link to Castle Marine Propclac. Very informative.

 

[Ross]

Ross, by lower ratio I gathers you meant 2.62 since he'll need a larger diameter prop with this ratio.

Donalex, he intend to fit a Max prop (feathering). Thanks for the link to Castle Marine Propclac. Very informative.

That is correct. 3:1 is a higher ratio than 4:1. Just as 1:40,000 chart has larger detail than 1:80,000.

 

[Ken13559]

Ross, thanks for the confirmation. Its clearer now. My friend wants a 2.21 for speed but his dealer and I recommends 2.62. He thought he could go fast!! like >hull speed with bigger horsepower.

 

[Ross]

He needs to look at the graphs of speed and horsepower for displacement hulls. It doesn't take much power to motor a 30 foot boat up to about 2/3 hull speed but after that the curve starts to get very steep.