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.

[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"????

[FLexpat]

Originally Posted by workinpr0gress

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

That would be center of buoyancy (CB). CG is independent of CB. When the CG and CB are aligned vertically, the line between the two points is perpendicular to the waterline and the displaced water at the waterline equals the mass - that's how you can mathematically figure out if the scuppers are dry at rest... but Denny's method with weights is much faster and easier.

Edit: Capt Terry's method is perfect for figuring out what the effect of moving stuff on CG should be without actually moving anything - and it does not require cash outlay, drill bits or resin. Denny's method is the easiest way I know of figuring out what the at-rest waterline will be after the CG moves and before everything is final. The combination should seriously reduce the number of holes that need to be redrilled/patched, wasted $, and the @#%^$%^%$hi++ moments!

[Bushwacker]

Originally Posted by workinpr0gress

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

You may be thinking about the impact of a big flotation tub on the bracket. While a large flotation tub on the bracket may largely offset the impact of a bracket & heavy motor on the boat's trim at rest (on my boat the static waterline with the Hermco bracket/new motor is only about an inch higher than it was with 300 lb motor on transom, so it's still self bailing with original deck, but just barely!), the real concern is how the boat performs when it's up on plane and the flotation tub is out of the water, so it's added buoyancy is irrelevant. The original planing surfaces haven't changed, so CG location is what counts. Moesly understood this very well, which is why he had ballast tanks in his race boats, so he could shift CG forward for head seas and aft for following seas.

Mitch, thanks for clarifying the CB issues! But the example and method for calculating CG shifts was Terry's idea, so he deserves credit for it!

[jtharmo]

I think the important thing to remember is the hull shape is not a variable here (assuming no trim tabs), weight distribution fore-aft is. In relation to cg, center of buoyancy will effect a boat's attitude at rest or less then planing speed. On plane, center of lift (which has a smaller buoyancy component) has the biggest effect. Changing the cg trims either situation.

In my example below moving the cg forward 9" was too much. I did need to trim up the engine to lift the bow a bit. All hulls will have a optimum running attitude fore-aft. With the bow up the acceleration of water being displaced laterally is too high due to the shorter hull length in the water and the swallower dead rise of what is left in the water. I think that equates to a harsh ride. Part of the magic of the Moesly hull is the bow can stay down to allow a longer, more gradual acceleration of water laterally. But in doing so there is less resistance in the lateral direction due to the stepped hull shape.