Estimating CG Shift with Heavy Engines

[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")
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