Center of gravity?
 

I’ve seen this topic on here before but I’ve searched for it and failed miserably.

What is the opinion of you offshore wizards for a fore Vs aft center of gravity for our style of boats?

Second question is how do we measure?

Thanks guys!

I am sure I seen some threads on here also, but didnt find them either. I did find these with google though.

Boating Mag - Article
Boat Builders Article
Microship.com

If the trailer is set up correctly, the center of gravity should be a few feet behind of the center of the axles. I would guess 2-5 feet depending on boat length.
If the axles were exactly at the center, you would have little to no tongue weight.

Griff,

I think you mean "ahead of center of the axles"....?

Thanks. Brad.
(937)545-8991

Twin,

Just to be clear:

Are you asking about the CG of the boat itself, or the boat on the trailer? You didn't specify, but Griff mentioned "CG on trailer".

Thanks. Brad.
(937)545-8991

On a V hull performance boat, I would guess that you would want to achieve 50/50 weight distribution fore and aft. Which with all that iron in the back is hard to achieve but some boats are better balanced than others. This is evident when "flying" the boat over waves/swells. Does the boat fly level or does it come down quickly ass first?

Isn't that why race boats have ballast tanks? As you burn fuel the CG changes and you add/remove ballast to keep it level.

How do you measure this? Not sure....

Pretty sure you will need to lift it and use some sort of measurement on the lift to really find out. I could call my friend that did the LUCUS oil boat if you want.

No, the furthur back the better in a V. Race boats have zero stuff in the cuddy and lightweight layup so they may need weight all trimmed out. Most V hull boats the CG is about a foot in front of the engines. For a fast V hull a little furthur ahead. Stepped can be even slightly further forward. I love CG stuff. I messed with this for months.

Its pretty simple to test. Make sure all the batteries are mounted at the stern. Take all your junk out of the cuddy, run limited fuel, keep moving your weight back or remove it, you will inevitably go faster.

OSO has been around for a long time, there are a bunch of threads. I have to laugh at some of the newer Facebook groups with newer guys in go-fast boats that mount all their batteries under the sink in the cuddy and all their amps under the bed then argue that thats not why their boat is slower. Even saw one that mounted 2 batteries under the bed with plywood and twobyfours! Because nothing could ever go wrong with that, including beaking the mounts and hitting the ceiling from a big wake. But hey, physics are a weird thing...

That is because even though everyone loves to talk about how Bad A$$ rough water boats they have, you give them real 2-3 and they get scared, slow to a craw and talk about the 6 footers they were in.

Techman,

Pretty sure race boats have the fuel tanks right on the CG, so fuel consumption affects CG as little as possible.

Thanks. Brad.
(937)545-8991

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.

My bad.

I meant on complete hull.

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.

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?

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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!

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

https://cimg3.ibsrv.net/gimg/www.off...d6e7cf2dc1.jpg

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.

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

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.


https://cimg2.ibsrv.net/gimg/www.off...4c60fad8e.jpeg
https://cimg3.ibsrv.net/gimg/www.off...467647c74.jpeg
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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.
(937)545-8991

Small outboard boats are a ton of fun.
That shot of the STV is impressive!

Thanks.

That boat was a ton of fun.

Ran a 100 w/a box stock JohnRude.


That’s my favorite pic of that boat.

If you look close, there are no signs of hull to water contact, anywhere.

That was 95 mph and I am flying the boat.
Slower that way but more fun 😂

Was at a run where all my friends were running oneteens so all I could do was show off!

BTW, STVs are the easiest boat to drive fast that there is.
That hull will smack you in the head before it will blow over!

Great shot!

My bow eye would be pointing at the moon if I did that w/my junk!

The only build difference I can see that would affect COG between a factory O/B vs I/O 26' Sutphen is the fuel tank is about 3 feet closer to the transom and a bracket was always used. Pretty sure the 1200 lb heavier model would be a twin 350 Mag/Alpha boat.
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Very cool ad that I’ve never seen!

According to my math, I’ve added 100–150 lbs going from the single big block IO to twin OB’s.

But……, Ive moved the center of that mass, aft by 5-6’!
https://cimg7.ibsrv.net/gimg/www.off...ae3d1fb406.jpg

There use to be a guy on here "Crayola" he was extremely methodical about CG and weight. He used two 45lb gym weights and two 80 LB bags of sand and would move them around based on calculations, photos, setup changes. Yes, you are an outboard guy but for example, he would take pictures of his boat at 100mph and move the bags of sand forward or aft at the point that the water breaks on his hull/keel at 100 mph.

Go get some sand. :)

I think it will always be a compromise depending upon how many people are in the boat and the water conditions that day. For the best ride and control in rougher water I think you need to feel what your keel is up to and a COG under your feet gives you that. Going fast where max hull lift is needed is something else - sit back, tabs up, add some trim and fly the bow.

Yep.
Been there, done that.

Problem is I haven’t been able to light it up yet, so no pixs of it running.

I do have pixs and measurements of water line/freeboard at bow/keel at rest and does sit about 2” higher w/the OBs on it.

I remember a thread on here years ago though where they were talking about that they have a target, for the offshore race guys.

Reminded me of it watching the YouTube video of Bob Scantini (?) and his amazing 41 Apache build.

One of his comments was how well the weight distribution came out on that boat out of the box and maybe for the first time ever.

You can see it in videos of it launching off of monster waves and landing almost level!

Yep.

Goal is to get as fast possible w/full water tank as I need it for the rough.

Then when fuel tank is 1/2 empty, dump the water and see what it does.

I saw 76 GPS when I was just getting the motors to run decent but it was a scary SOB below 50!
(before water tank)

Basically had to drive it like a 100 mile an hour Allison vee bottom.

Get it to about 30, pull the tabs up, set the trim close to where you want it and nail the throttles and let the horse power pick the bow up and carry it until it starts building lift and then dial in from there.

But I thought it was gonna spit me out several times to get to that point.

Heres a shot of my first Sonic.

I dissected that shot a million times!

Took a straight and drew lines for the keel where it was invisible in the spray and another along the transom to estimate wetted area.

That boat had a round keel so not a pad bottom vee.

I calculated the last 3’ of keel and 6” on both sides were the only thing wet.

Perfect attitude (for an OB), perfect trim and the perfect, long flat rooster tail!
Goal is not have tail above top of motor cowlings when WFO.

Was also a handful to drive fast.


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That is such a cool pic you've been granted a pass on the scummy bottom.

Unfortunately the scuzz was in the gel when I bought it it and never got it out.

I painted the second one.
https://cimg3.ibsrv.net/gimg/www.off...f4251609d.jpeg

Clean everything out of the boat literally everything, only run a 1/2 tank and feel the difference For chits and giggles when I did this I pulled all the stuff out of the cuddy, put in construction bags and its was 118 pounds. Later, if you want to get aggressive take out some of the furnishings, bed, couch, fridge, etc.

A boat is a big "lever" 50 pounds of tools under the cuddy bed is way worse than 50 pounds of tools under the helm.
*Dont even get me started on putting batteries for the stereo in the cuddy (anywhere)

Done that too.

The Sonics were totally different boats w/a light fuel load etc.
Acceleration was brutal from a 45 roll!

I normally could run 2” more pitch, light.

As in going somewhere that I can run.

I plan to weigh it again loaded so I know for trailering but am afraid to!

BTW, a porta potti full of water (no waste) is 35 lbs in the bow!

My tongue weight also increased exactly the weight of the water bladder.

And your right on weight placement as well.


https://cimg1.ibsrv.net/gimg/www.off...eb09f1d3f.jpeg
I rigged this boat with all the weight as far aft as I could possibly get it including batteries and gas tanks.
Actually mounted tanks on slides so I could adjust it based on whether I was drag racing, lake running or top end running.

As rigged it would try to blow over at 90 miles an hour with ZERO warning!

After I did that the fourth or fifth time I took the battery from the transom and mounted it under the dashboard and it totally changed how the boat ran!

This is at 101 after the weight move.

For a 100+ mph (543 lb hull) vee bottom it was an amazingly well handling boat once I got it dialed.

Tons of trips to the ramp to get it though!

You guys see the video of The Parvey running at the shootout , that fu cking condo was so far out of the water you`d think it was 25' not 43'.
Very impressive.
and the acceleration at the start .. wow!

so moving weight forward makes the tongue lighter??? please enlighten us

I suspect part of the reason 6s run good is because they are so heavy and so far off the transom that move the CG back

I over thought it when I posted and not sure wtf I was thinking as you are both correct

Well then I`m on the right path!

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Anyone have video?