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Author Topic:   OB Setbacks explained Pros Cons
bigz posted 12-26-2000 11:32 AM ET (US)   Profile for bigz  
Might find this interesting
http://www.screamandfly.com/setback.htm

Tom

jimh posted 12-27-2000 08:45 PM ET (US)     Profile for jimh  Send Email to jimh     
That was an interesting article. Clearly the author knows a bit about engine setback.

In general, most of his points had been mentioned in our discussions on the FORUM, but there were two new points that I found interesting and want to repeat here:

1) moving the engine back farther helps to raise the bow just from the static weight distribution. Having the heavy engine(s) a few inches or feet off the transom creates a natural tendency for the bow to rise, even at static trim; and,

2) engine lower unit alignment more nearly vertical causes the hydrodynamic shape of the lower unit to improve. The typical trimmed out attitude of the lower unit tends to spoil its hydrodynamic shape and creates turbulence from the way the lower unit passes through the water.

Another area I found interesting was the mention that the setback may only need to be a matter of a few inches to gain some noticeable advantage. He was talking in terms of 5-6 inches being a large set back. This is in contrast to the Armstrong Bracket or Whaler Drive approach which have setbacks of several feet!

Thanks for the pointer to this link. Good material.

--jimh

Peter posted 12-27-2000 09:04 PM ET (US)     Profile for Peter  Send Email to Peter     
Several years ago a friend of a friend retrofitted a 25 ft Pursuit I/O with an Armstrong bracket and a 225 Evinrude. Interestingly, and consistent with the teachings in that article, he had to put about 100lbs of weight into the anchor locker to counter balance the weight of the 450 lb motor on the bracket. You might have thought that the outboard on a bracket was equal to the old I/O-motor combination but the real difference was that the weight of that old combination was in front of the transom, not behind. That article makes it crystal clear that one needs to take the moment arm created by the offset into consideration.
MikeC posted 12-28-2000 05:23 PM ET (US)     Profile for MikeC  Send Email to MikeC     
I know the guy who wrote that article.You have to remember that they are dealing with very light boats with V6 motors so it takes less setback to get the front end up and most of them are 17-20 ft.My drag boat weighed 700 pounds without motor and thats a lot less than any Whaler so it only needed 2.5 inches to make a big difference.I just picked up a 20 Revenge and am thinking about bolting on a 6in plate that I have laying around to see how it works.Just my 2 cents
jimh posted 12-28-2000 06:08 PM ET (US)     Profile for jimh  Send Email to jimh     
I would be very interested to hear about the results of using a setback with your 20-Revenge. Please keep us posted if you do decide to try it.

--jimh

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