Offset Propellers

Hi again. We are in the process of installing a diesel engine in our Mirage. All was going according to plan, until a fellow boater mentioned crabbing, and warned us that placement of the prop is critical. If not offset, the boat will ‘crab’, worse in reverse, than in forward. Next Saturday, the hole is being cut through the hull. I’ve had a look through the Encyclopedia, but can’t find much about proper placement of the shaft/prop, and although many of my boating friends have heard of it, no one can elaborate. I’ve looked at the last few boats in the area that are still out of the water, and find that the shaft is placed dead-centre. Do you have any advice, or do you have a publication that explains the concept? Your help is appreciated. Jim from Canada

Jim: In the olden days, the norm on full keel, attached rudder boats, props was to offset the prop so the builder would not have to cut away the rudder or trailing edge of the keel (and so that the prop shaft and required bearings was an easier installation).

These had to be done so that the offset would be counteracted to some degree by the somewhat asymmetric thrust of the prop blade. However, by the time you got through with the offset thrust line, the fact that the shaft was angled, and the long keel/attached rudder configuration, handling under power in tight quarters became problematical.

With modern boats and the ability to “line bore” for prop shafts, there is really no reason to offset a prop.

Offset in reverse is a totally different issue. This is a function of prop torque due to running the prop backwards, further impacted by hull and fin relationships. You can mitigate this issue by using a feathering and reversing prop, like a Maxi (which looks the same to the water going ahead or astern). Steve

Posted by Steve Dashew  (November 30, 1999)

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