Estimating CG Shift with Heavy Engines

[Capt Terry]

It seems there are questions and discussions about heavy outboard engines, outboard brackets, boat handling, and center of gravity at least a few times a week on CSC. Our performance moderator, Bushwacker, reminds us the engines when Moesly designed the SeaCraft outboard engines were considerably lighter than today’s engines and particularly four strokes. (The weight of my 1976 Merc 150 inline 6 was about 300#; my 1985 Merc 150 V6 about 375#; and my 2012 ETEC 150 at 418#.) Bushwacker has also been very candid sharing what he has learned in using his 20’ Seafari conversion to a full height transom with a Hermco bracket for his ETEC 150 HP, including his pros, cons, and cautions.
As a retired mechanical engineer from the jet engine and then gas turbine world, I decided to try putting a bit of science to assist SeaCraft owners in estimating center of gravity shifts for their engine and bracket decisions. Please realize the attachment is a tool primarily for estimating the change in center of gravity of adding or relocating heavy equipment, not a precise center of gravity calculator.

Estimating CG Shift rev7 pdf.pdf

[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!

After someone uses Terry's method to figure out how much the CG will shift with a given repower option, they can then use this method to figure out how to create the same CG shift on their current rig using ballast (a cooler or trash can filled with water) in the back of the boat, and then go test it to see what effect it has on the boat's ride, handling, min planing speed, etc. and confirm what I've been trying to tell folks for several years in my post on the Pro's & Con's of Brackets!

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

[wattaway2]

Terry thanks what a excellent tool

[jtharmo]

This applies to where your passengers sit also. I recently post in the Verado thread with my wife and son sitting in the transom seats there seem to be nothing I could do to get the bow down. In my mind I equated this to a 550 lbs 4-stroke (350 lbs motor plus 200 lbs of family). Using the calculator that's a cg shift aft by about 6". My wife and son moved to the bow to eat and hang out. On the return trip (i.e. same conditions) with them still up front I was able to cruise in the low 20's. The boat did not get pushed around by waves and barely moved up and down. No one complained. The bow was doing what it was meant to do and spray was coming off mid-ship nicely. In fact I punched it to 30 a few times and no one noticed. Again, putting this through the calculator, this is a cg shift forward of 9". A 15" absolute shift in cg and it felt like two totally different boats.

[Bushwacker]

Originally Posted by SSPBill

. . . A 15" absolute shift in cg and it felt like two totally different boats.

That's what I learned over 10 years ago when I replaced a 300 lb motor on the transom with a 427 lb motor on a bracket, and have been trying to tell folks ever since! The resulting CG shift makes a big difference in the boat's low speed planing performance, and ride when it gets rough! Fortunately, the Seafari's big cabin makes it easy to put a lot of gear up front when you're making a long trip. I even installed pad eyes on the bulkheads so I could keep it all tied down in rough seas!

Do you have trim tabs on your boat? They will help compensate for load/CG shifts. A 4B prop will also give you more stern lift as will an SE Sport hydrofoil on the AV plate.

[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

[workinpr0gress]

Isn't there more to it considering the hull shape and flotation changes from stem to stern ?

[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"????

[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.

[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.