Thread: Backing
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Old 01-16-2012, 01:03 PM   #48
Marin
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Join Date: Oct 2007
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Backing

Quote:
nomadwilly wrote:
*

Marin I do'nt think prop walk on a twin would be any more than a single as the props are usually smaller. Even when they are'nt (like a lot of trawlers on TF) the prop walk is proportional to the amount of power applied so prop walk should be the same (roughly) when backing.*

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Speaking only for Grand Banks boats because they are the only ones I'm this familiar with, the prop used on a single engine GB is the same as the prop used on the port side of a twin-engine GB of the same size and using the same type of engine as the single.* In the case of FL120-powered GB36s, the diameter and pitch is 24 x 18 for their original, three-bladed props.

The only other evidence I have is that we chartered a single engine GB36 before we bought our own twin engine GB36.* To yaw the stern to starboard as a single left-hand prop will do in reverse, our twin with both props rotaing clockwise (port reverse, starboard forward) swings its stern to starboard much more responsively and faster than the single ever did. There are reasons for this other than just propwalk of course, but the combined propwalk of two props turning in the same direction instead of just one does play a signficant role.

One potential factor in our favor is that we have four-bladed props where I believe the single we chartered has a three-bladed prop (but I could be wrong about that).* And a four bladed prop develops more propwalk than a three-bladed prop of the same diameter, one reason why the prop shop folks say "A three backs better than a four."* By which they mean develops less propwalk.* It's also more efficient, but that's a different topic.


-- Edited by Marin on Monday 16th of January 2012 03:12:35 PM
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