Jackplates setback effect on vessels COG?

[juicegoose]

I currently own a 21' cc bayboat. For the purpose of this question I'll outline the location of major components.
light tackle and trolling motor forward of CC
4 batteries in CC
Driver at CC
fuel tank under CC

500 lb outboard at rear of boat mounted to 10" setback bobs jackplate.

My question is this. Obviously having 10" of setback moves the moment of arm(aviation talk) back more then a 6" or 4" setback would.
What I'm not sure of is by how much. How much effect does moving the engine back off the transom 10" have over moving it back 4-6"?

Anyone smarter then me that can help?

 

[smokeonthewater]

Re: Jackplates setback effect on vessels COG?

Twice the effect....
as for "how much" that would require specific measurements of dimension and weight of the whole package....

To be more practical give us the big picture.... Are you having a stern heavy problem?

moving a couple of your batts to the bow would likely have a greater effect on c/g than moving your motor 5". OTOH moving the motor ALSO changes the amount of leverage it has on the boat I/E effectiveness of trim adjustments.

 

[juicegoose]

Re: Jackplates setback effect on vessels COG?

The problem I am dealing with is porpoising. I've been told by Nautic star that they had issues in the past with 07 2110 tunnels and the 200 optimax package. Essentially there just wasn't enough weight in the front of the boat to offset the heavy optimax and miscellaneous gear that were placed center and rear. Add to that the effect of loosing some planning surface with the pocket tunnel and bam. I know all boats are different and adjusting the trim and motor height to find the best combo is what has to be done. I just figured I'd get more then 3-5 clicks on the trim button before the boat would porpoise.

I've installed trim tabs and they help but the porpoising is still there unless I really bury them.
I did just recently install a Merc rev4 17p prop only to find out that rev4 is a bow lifter.

Edit: No soliciting allowed on iboats

 

[smokeonthewater]

Re: Jackplates setback effect on vessels COG?

Do you NEED a jack plate?..... Seems like you could likely do better without any set back at all....

Also look into moving batts.... All 4 moved to the front might just do the trick.

If you hadn't already installed tabe I would have suggested smart tabs....

 

[juicegoose]

Re: Jackplates setback effect on vessels COG?

I've got no room to move the batteries any further forward then I already have. I do actually utilize the jackplate but just don't need the setback amount.

 

[H20Rat]

Re: Jackplates setback effect on vessels COG?

Keep in mind there is a difference between affecting the COG with a heavy motor, and moving a lighter motor back. Yes, the COG change might be identical, but the leverage moment created by moving the motor further back is also greater. Its entirely possible to have an effective COG that is further forward, by installing a jack plate.

 

[dan02gt]

Re: Jackplates setback effect on vessels COG?

If I were in your shoes I would sell the 10" and put a 6" in place of it.