At what point does moving the engine back change the cg

[RUSTYNTABATHA]

would like a little more clearence between the engine and the splash well for access to the bilge hatch.. i have a 6 " jack plate and a 14 inch jack plate.. was thinking of mounting the 14 inch to allow more room to open or close valves in the bilge area when at sea.. as well as open check in there... i like to look with my eyes from time to time even on new vessels... i think the 6 would work fine for now but the 14 would really help out.. this thing is light and when used on bass or racing aplications really change the running attitude of the boat but uncertain at lower speed.. we use soccer balls or the old play ground balls as a cheat for extra floatation on bass boats to keep them from swamping when coming down off plain to fast... the ball floats the stern up on swell coming back into the transom..
any thoughts ???

[Billpotter]

I put 6" set back "step N jacks" on a twin V-6 23 Seavet almost thirty years ago. Boat became noticeably more tender, although a few miles per hour faster. I would not do it again.

[Bushwacker]

Rusty,

ANY movement of the motor aft will shift the CG aft; it's just simple physics! How MUCH it shifts will be directly proportional to how far you move the motor from the current location. If the motor CG is currently 10' from the boat's CG and you move the motor aft by 1', you will have shifted the CG aft by 10%. On a Moesly hull, (19 bowrider, 20, 21, 25 Seafari, or 27 Seamaster) it's probably safe to assume that the CG is approximately at the center of the gas tank, because he designed it so that the boat's trim would remain unchanged as you burned off fuel. Not so sure about the Potter designs (18 & 23) because he didn't have Moesly's extensive aviation and boat design background, but they shouldn't be too much different.

Are you talking about a 20 or 23? The 20 is obviously much more sensitive to a CG shift just because it's smaller. The most noticable effect of an aft CG shift will an increase in min planning speed and a correspondingly rougher ride, so how much of a CG shift you can tolerate depends on where you're starting from, and what your threshold of pain is! An original well balanced 20 with a 300 lb motor on the transom will plane easily at about 12 mph without trim tabs. Same for the 21. Not sure about a 23, as I don't have any experience with them, but I suspect they won't plane as easily as the 20 or 21 because I think they have more deadrise in the outer panels. However I've heard that the later 23 Sceptre's built by SeaCraft Industries seem to be more bow heavy than the Potter models because they used plywood instead of balsa core in the decks, so a later model Sceptre might actually benefit from the addition of a jackplate! Denny

[RUSTYNTABATHA]

Thanks for the help and thoughts... think after replacing the plywood splash board with starboard that may be all the change it gets for a while..