Does a jack plate make a boat more stern-heavy?

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Author Topic:   Does a jack plate make a boat more stern-heavy?

David Jenkins posted 05-29-2003 09:21 PM ET (US)
I have a 335-pound, 4-stroke, 70 hp engine on my 15' Whaler. The engine is mouted on a jackplate with 4 inches of setback. I am giving some thought to removing the jackplate, thinking that this would make the boat less back-heavy. Does anyone know how much the back of the boat might raise if I move the engine forward 4 inches?
SSCH posted 05-29-2003 10:28 PM ET (US)
Let's see .......... If you move the engine forward 4 inches you should reduce the weight aft of the center of buoyancy because the engine will be closer to that point. I would guess that the change would be roughly the same as the change you would see if you tried standing at the transom and then moving 6 or 7 inches forward in the boat(assuming you're about 200 pounds). Probably not a lot.
You will also reduce the total weight of the boat by the weight of the jackplate. If this is a manual plate, maybe 20 pounds; a power plate, maybe 45 pounds. You'll not see a lot of difference there either.
If you were running larger set back or had a heavier plate, maybe it would make a difference. The plate should allow you to run the engine higher up and reduce the draft at speed over that without the plate.
I'd keep it if you like the performance adder at speed. If it doesn't help when running, I'd take it off, but not expect a major change at rest. If you have a real problem with a heavy transom, its likely the extra 110 pounds of 4 stroke over two stroke that is the problem.
Full disclosure ...... I've never had a chance to measure the changes I've suggested are likely to occur. To this engineer, the analysis sounds logical.
David Jenkins posted 10-08-2003 11:00 AM ET (US)
I can now answer my own question. On the classic 15', the weight of the jackplate and the 4" setback make a huge difference in how the boat sits in the water. I could not use the manual CMC jackplate with the 335-pound Suzuki/Johnson/Evinrude 70, but once I took the jackplate off, the boat was significantly less stern heavy. And by mounting the engine 3 holes up. I was able to have all of the engine mounting bolts terminate in the spalsh well.
Now I just need to figure out what to do with this CMC jackplate....
whalerpoor posted 10-08-2003 11:28 AM ET (US)
I might be interested in buying it.
Jim might get upset with me for talking swaps on this forum though.
kidding you know
Over the LINE posted 10-09-2003 08:23 AM ET (US)
Can't say about the 15' but on my 13' I took the jackplate off for the same reason. The manual with 4" setback (50HP) was too much weight too far back. I think the boat handles better without it.
Good Luck

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Author	Topic: Does a jack plate make a boat more stern-heavy?
David Jenkins	posted 05-29-2003 09:21 PM ET (US) I have a 335-pound, 4-stroke, 70 hp engine on my 15' Whaler. The engine is mouted on a jackplate with 4 inches of setback. I am giving some thought to removing the jackplate, thinking that this would make the boat less back-heavy. Does anyone know how much the back of the boat might raise if I move the engine forward 4 inches?
SSCH	posted 05-29-2003 10:28 PM ET (US) Let's see .......... If you move the engine forward 4 inches you should reduce the weight aft of the center of buoyancy because the engine will be closer to that point. I would guess that the change would be roughly the same as the change you would see if you tried standing at the transom and then moving 6 or 7 inches forward in the boat(assuming you're about 200 pounds). Probably not a lot. You will also reduce the total weight of the boat by the weight of the jackplate. If this is a manual plate, maybe 20 pounds; a power plate, maybe 45 pounds. You'll not see a lot of difference there either. If you were running larger set back or had a heavier plate, maybe it would make a difference. The plate should allow you to run the engine higher up and reduce the draft at speed over that without the plate. I'd keep it if you like the performance adder at speed. If it doesn't help when running, I'd take it off, but not expect a major change at rest. If you have a real problem with a heavy transom, its likely the extra 110 pounds of 4 stroke over two stroke that is the problem. Full disclosure ...... I've never had a chance to measure the changes I've suggested are likely to occur. To this engineer, the analysis sounds logical.
David Jenkins	posted 10-08-2003 11:00 AM ET (US) I can now answer my own question. On the classic 15', the weight of the jackplate and the 4" setback make a huge difference in how the boat sits in the water. I could not use the manual CMC jackplate with the 335-pound Suzuki/Johnson/Evinrude 70, but once I took the jackplate off, the boat was significantly less stern heavy. And by mounting the engine 3 holes up. I was able to have all of the engine mounting bolts terminate in the spalsh well. Now I just need to figure out what to do with this CMC jackplate....
whalerpoor	posted 10-08-2003 11:28 AM ET (US) I might be interested in buying it. Jim might get upset with me for talking swaps on this forum though. kidding you know
Over the LINE	posted 10-09-2003 08:23 AM ET (US) Can't say about the 15' but on my 13' I took the jackplate off for the same reason. The manual with 4" setback (50HP) was too much weight too far back. I think the boat handles better without it. Good Luck