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Old 01-29-2015, 08:29 PM   #69
Marin
Scraping Paint
 
City: -
Join Date: Oct 2007
Posts: 13,745
Quote:
Originally Posted by manyboats View Post
Who's to say this guy knows what he's doing and this guy dosn't?
The long-term results will say. Done wrong, and engine longevity will suffer. Done right, and the owner/operator will enjoy a benefit.

Grand Banks did it for at least a couple of decades, they did it right, and they sold thousands of boats to buyers who were very satisfied with the speed experienced at cruise with reasonably low noise levels and excellent engine service and longevity.

"Right" in this case, is in the mind of the beholder. You (Eric) think it's wrong from a theoretical standpoint, but as has been shown before with what sells and what doesn't, reality trumps theory every time.

American Marine did is right as far as they were concerned (and I suspect they were a much more successful boat design and manufacturing company than you or the rest of us on TF forum have been with the possible exception of Tad) And American Marine enjoyed the sales of thousands of their Grand Banks boats to very satisfied customers. And those boats continue to be prized on the used market by people who desire this type of cruiser.

So wrong in your eyes, maybe. But in this particular case, which is the only one I feel I know enough about to comment on, what American Marine did with their Grand Banks boats regarding what should more aptly be called FF's "cruise propping", was very, very smart.

We've propped our GB down. The original three-bladed props were cruise propped 24" x 17" on one side and 24" x 18 on the on the other side, and our PO-installed four-bladed props were more or less the same. They are now 23" x 16" on both sides. What we gained for a given rpm was a bit less fuel consumption and engines that work a bit less hard as illustrated on the EGT gauges.

What we lost was some speed. We can get that back by running the engines about 1,000 rpm faster, but now we use more fuel again, the engines are working harder again, and we have a considerably noisier cabin.

When we thought we needed new props, we were going to go back to three blade props (thee blades are more efficient than four) and have them set to the original (coarser) pitches. It turned out our 4-bladed props were still good but they needed considerable working over to rectify the terrible setup job the prop shopt in California had done prior to our buying the boat.

So on the advice of the prop shop we use in Seattle, we had the props set to what they are today.
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