Estimating CG Shift with Heavy Engines

[Capt Terry]

Originally Posted by jorgeinmiami

So on a 20 master angler with the original 70 gallon tank a smaller center console batteries up under the console t top etc.... What would I need to move console up if I put a 140 Suzuki 140 4stroke. Hanging on the transom? Seems like somewhere around 5"????

Jorge- Hey man, I put the CG estimating tool together, you want the answer at the back of the book too? This tool took more time than I will admit; my wife kept asking me if I was still working on it. The math & science part of it was the easiest, then the writing/rewriting to try to get it simple & clear, followed by the time & trouble to get it into CSC without fouling up the formatting of the tables.

SIMPLE ANSWER
Don't know what your engine weighs (check the internet). But let's say it's 200# more than the original & without a bracket assume it's CG is 0.5' aft of the transom, This results in a negative 100 Ft.Lbs. about the reference point of the transom. To compensate for this you want an additional positive 100 Ft.Lbs. added forward of the transom. If the console weighs 100# and that's the only thing you will relocate then you will move it forward 1' If two batteries are moving with the console, let's say that's another 100# (you can weight them), then the total 200# would only to need to move 0.5'. Without a bracket the change is not so severe, probably better off with a cooler of beer between the console & the bow.

[Terry England]

Originally Posted by Bushwacker

Terry - great post that lots of guys should find very useful! Even if the absolute value of the some items isn't real accurate, like CG location of bare hull, it will still be be pretty close at predicting CHANGES in CG for changes in one or two items, like a motor or bracket, provided the numbers for the questionable item remain constant!


Moderators - I think this one is worth making it a "sticky" in the Repairs/Mods section! What say ye? Denny

I second Denny's E(tec)-motion. This is actually a tool members can use, instead of the normal nonsense GFS, Pelican, Spidercrab, DonV, NoBones, Kmoose, EricB, Step Up, Fly4navy, Old's Cool and others post! This should be a serious, strictly technical forum. And wipe that smirk off your face!
("Incoming")
Signed Keyboard Cowboy T-3

[gofastsandman]

Originally Posted by Capt Terry

Jorge- Hey man, I put the CG estimating tool together, you want the answer at the back of the book too? This tool took more time than I will admit; my wife kept asking me if I was still working on it. The math & science part of it was the easiest, then the writing/rewriting to try to get it simple & clear, followed by the time & trouble to get it into CSC without fouling up the formatting of the tables.

SIMPLE ANSWER
Don't know what your engine weighs (check the internet). But let's say it's 200# more than the original & without a bracket assume it's CG is 0.5' aft of the transom, This results in a negative 100 Ft.Lbs. about the reference point of the transom. To compensate for this you want an additional positive 100 Ft.Lbs. added forward of the transom. If the console weighs 100# and that's the only thing you will relocate then you will move it forward 1' If two batteries are moving with the console, let's say that's another 100# (you can weight them), then the total 200# would only to need to move 0.5'. Without a bracket the change is not so severe, probably better off with a cooler of beer between the console & the bow.

Too funny.

Think of people as trim tabs.

[Terry England]

Originally Posted by gofastsandman

Too funny.

Think of people as trim tabs.

Some are even the Lenco "Short Stroke"!

[FLexpat]

Caymanboy has the best selection of trim (tabs) I've seen lately

[Terry England]

Originally Posted by FLexpat

Caymanboy has the best selection of trim (tabs) I've seen lately

The Mormon-Cracker has a point -
Well, maybe two!

[NoBones]

Originally Posted by Terry England

The Mormon-Cracker has a point -
Well, maybe two!

Both can be considered "HIGH" maintence......

Those Lenco's have a high rate of failure with
routine replacement after high useage !

[sidelock]

So how does one figure out or calculate the center of gravity of a hull to do the calculation if a motor is already mounted on the transom. I have an 18SF that at some point in time will ultimately have to replace the 1998 Yamaha C115 @ 330lbs with a heavier four stroke and would like to know how far forward to move the 40 gallon fuel tank and or the center console.

[Bushwacker]

Originally Posted by sidelock

So how does one figure out or calculate the center of gravity of a hull to do the calculation if a motor is already mounted on the transom. I have an 18SF that at some point in time will ultimately have to replace the 1998 Yamaha C115 @ 330lbs with a heavier four stroke and would like to know how far forward to move the 40 gallon fuel tank and or the center console.

I think it's safe to assume that both Moesly and Potter, who were both pilots so they understood the impact of cg location on performance and handling, put the fuel tank on the boats CG so it wouldn't change trim as fuel was burned off. So if you figure that the CG of the complete hull was located at center of fuel tank with a 300 lb motor on the transom, you should be able to back calculate where the CG of the bare hull would have to be.

[Terry England]

Denny, have you ever been able to put enough weight in your cabin to make your Seacraft "plow"? I have loaded so much crap in the front of mine that it sat bow down at the dock where the winch eye was in the water, but when I get going it still runs with the bow up and handles perfectly. I don't have much of a motor back there to counterbalance the weight either. Has anyone on here ever moved their gas tank and batteries too far forward to make the VDH hull plow? Just wondering?