zz28zz

I learned this many many moons ago when working on airplanes.I have forgot most of it, but remember you have to establish a Datum point. It can be the most fwd point or somewhere forward or aft but wherever it is, it remains constant for all the calculations (keeping the Datum ahead of the boat makes the math easier when moving loads around). You weigh the craft at 2 points and multiply the distance from the Datum (called ARM) in inches by the weights to get the "Moments". Add the Moments together and divide it by the total weight to get the CG in inches from the Datum. I think that's right..

A quick example:
Datum= 100" fwd of bow.
Fwd lift point= 50" behind bow or 150" behind Datum
Rear lift point= 350" behind Datum
Front lift point weight= 1000 lbs
Rear lift point weight= 5000 lbs
Front Moment= 150" X 1000 lbs= 150,000 lb/inches
Rear Moment= 350" X 5000 lbs= 1,750,000 lb/inches
150,000 + 1,750,000= 1,900,000
1,900,000 / 6000 lbs (total weight)= 316" behind Datum for the CG.

I started a thread years ago asking abt CG when I was thinking abt trying to lift the boat from the trailer with a 2-post automotive lift. I was told that Perf V's CG is close to the bulkhead near the rear of the bench seat.

Twin O/B Sonic

My bad.

I meant on complete hull.

resurrected

Great topic and informative answers, but most seem to be referring to the center of mass (might be the wrong term, but that's what I use). Getting the center of mass is easy compared to the true CG, zz28zz has a great way to do it, or on smaller lighter stuff I've actually just run a single strap under it and kept picking it up until it balanced. True center of gravity is 3 dimensional, fore aft, up down and port stb. I would love to figure a way to measure that.

Twin O/B Sonic

I understand the theory/concept and it’s effect on performance boats but am interested in the Offshore vee bottom race teams target for this.

Ive rigged/set up a couple of vee bottoms based on the popular “all weight aft theory” that were wrong.

They we’re OB’s, which by design, are able to get to shift the weight bias further aft.

I am on my third twin OB vee bottom and all three fly the bow much higher and the hull much cleaner to the same hull w/similar IO power.

But I am lucky that where I run (Lake Erie) it’s all chop and no long swells like in the ocean so I see no issues w/the weight so far aft.

But Ive had a “come meet Jesus moment” in all three when trying to fly it off a swell like the big boys do!

You only see front deck as the boat comes down w/bow pointing at sky!

And I know a lot of the teams run ballast tanks.

My latest project is a ‘87 Checkmate 25’ Convincor that I converted from single IO to twin OB.

My first two were 24’ Sonic which had 8’ beams.
The Convincor is only 7’6” and runs WAY different!

VERY sensitive to trim, tabs, weight etc.

I built a galley in the cuddy and mounted a 15 gl water bladder in the anchor locker.

Went as big as I could, as far forward as I could to get as much effect from its 125 lb as I could.

HUGE positive improvement on handling!

Also had a new 92 gl fuel tank built so changed C of G on this hull about as much as possible!

Checkmate offered this as twin OB but w/motors mounted to transom.

Mine are on a 20” bracket w/Jack plates which add another 6” of setback!

We have been using the thing as a cabin cruiser this season but am about to head to the local pond w/my 6 sets of wheels to start high speed testing.

So knowing w/adjusting water level, etc, I can effect C of G, what should my target be?

resurrected

I think, as you have said it really depends on the specific conditions your running in. For flat water, as light as possible in the front to reduce the wet surface, but not so much that the angle of attack of the hull increases drag.
On my Thundercat inflatable you can really feel how weight distribution affects the speed. Running into the wind to get as much lift as I can, if I shift my body 6" to 8" it can make as much as a 3 MPH difference in speed. I am still working on props and I need to raise the engine up, but with the four blade if I slide all the way to the back against the transom it's slower than if I move about a foot forward. It's a tunnel hull so totally different animal but hydrodynamics are the same. It's a balancing act between angle of attack and wet surface.

BTW can't wait to see your numbers on that thing!

ICDEDPPL

Hmmm I may need more weight in front.. but them again when you launch of a big roller it`s hard to fly straight .

rvtransport

I always wanted to learn more about cg's but in testing in my 42 fountain I go faster with main tank empty and fuel in only the rear saddle tanks.

Griff

If the CG was forward of the axles, there would not be enough tongue weight. In most cases, CG will be about at 1/3 length of the boat from the stern, but that will still vary

Twin O/B Sonic

I’ve done quite a bit w/rebalancing trailers to get proper tongue weight which is a great example of how much I have shifted weight in the hull.


Although I never checked tongue weight as purchased it did tow perfect at 80 mph on its way home.

Once the power conversion was complete it was scary to tow and swayed terrible!

I weighed the tongue at that point and was only 135 lbs!
Far from the targeted 10% of gross, loaded weight which is 4500 lbs.

Had to move hull as far forward as I could which was almost 2’ and got tongue weight to 450 and now tows great.

As to weight shift in small boats, I designed/built and raced outboard hydroplanes for 9 yrs so I totally get that.
I firmly believe I am as good as I am on trim buttons because I spent 9 yrs w/my ass being the trim buttons 😂

Also spent many yrs playing w/fast single OB hulls.

Brad Christy

Griff,

That is incorrect. If the CG is on the tongue, the ALL the weight would be on the tongue. If the CG were at the transom, there would be less than zero weight on the tongue. We want the CG to just just ahead of the axles. If it is behind, the boat and trailer will swerve, making for a pretty scary ride.

Trailer CG

Thanks. Brad.
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