34/2/2012

Reduction Gear and Propeller Guesstimates....

My "theory" on prop sizing is that a prop and a reduction gear need to be chosen for the specific USE of the boat. I did an informal study of salmon trollers for sale to see what, if any, was a common reduction gear. These were both current and former working trollers, all used for cruising distances scouting for fish, some used today as cruising yachts. When they were working they also spent a good deal of time running at a comfortable idle trolling for fish as well as running at a faster RPM LOOKING for fish, which seems similar to our cruising powerboat uses. Their prop and reduction had to be suited, or rather "compromised" is a better description, for both use.
Four of these boats had 3:1 reduction gears (three of these were beamier and heavier than some of the others), one had 2.75:1, two had 2.5:1, and three had 2:1. The 3:1 guys seemed to all have 6 cylinder engines, and the others had 4 bangers and one, a 3-71 GMC. The 3:1 guys all burnt more fuel at cruise, reporting 2 to even 3 gallons an hour compared to 1.2 to 2 for the others. Prop size and clean bottoms effect that but just the same, the 3:1 boys burnt more fuel.
There's two 462 DUCKS out there with slightly different reductions. I dunno about the props. Don's boat, with 3:1, is reporting higher fuel use than Randal, with 2.8:1 I don't know what other variables there might be but if it was me, I'd use no more than 2.5:1 on most cruising boats.
I talked with the Twin Disc sales people at the last commercial boat expo in Seattle and they agreed with my theory. A heavy reduction gear isn't needed on a boat that isn't always driven hard into conditions or that tows loads. For general use it seems to me that it's better to get bogged down once in a while if the other side is cruising efficiently most of the time.
Just the same, I'd run the above by a good prop shop if it was me, before ordering it!

 

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