CG question.

Timdelta

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Any insight into how I can find the Cg for a Q25FSB with a Yamaha 300. I’m getting a lift installed and it is good to know instead of the USMC, SWAG method. (scientific wild ass guess)
Thanks
 

Vikingstaff

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Okay. I guess I’ll go ahead and ask. What does CG/Cg stand for or an abbreviation for - particularly in context to the question/statement above?
 

Timdelta

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Sorry I should’ve said center of gravity. Basically the balance point of a object or in my case a Q25 with a Yamaha 300. It’s important to spread the weight evenly on a boat lift.
 

Vikingstaff

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Thanks guys. I kept thinking I should be able to figure this out in context, but then wasn’t. Don’t know the answer, but hopefully someone can give you a decent answer soon. :)
 

BigKahuna

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Your lift installer should know approximately where the Cg is on your boat. When we had our lift modified to fit our boat all the installer asked was how long the boat was and the approximate weight of the outboard. He installed 12 foot bunk cradles and said that was perfect for our boat and weight distribution. We were leery because we never kept our boat on a lift before. Let's see.... 24 foot tritoon, add 2 feet for the motor, 12 foot bunk cradles. We have 10 to 11 feet of tritoon just hanging there in the front! This isn't going to work! It came out perfect! That was an education for us. What kind of lift are you having installed?
 

Yianni

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Your lift installer should know approximately where the Cg is on your boat. When we had our lift modified to fit our boat all the installer asked was how long the boat was and the approximate weight of the outboard. He installed 12 foot bunk cradles and said that was perfect for our boat and weight distribution. We were leery because we never kept our boat on a lift before. Let's see.... 24 foot tritoon, add 2 feet for the motor, 12 foot bunk cradles. We have 10 to 11 feet of tritoon just hanging there in the front! This isn't going to work! It came out perfect! That was an education for us. What kind of lift are you having installed?
We had the same situation. The installer showed up with 12' bunks for our 25QX. I told him that's not going to work. He assured me it would be fine. Like you said it was a little scary seeing 10' hanging loose up front. After using it for a summer it worked out great.
I guess some of these installers know what they are doing.
 

Vikingstaff

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Ditto the 12’ bunks. We had a bunk style covered lift our first two years with our 2017 24’ SSBXP SPS with a 200HP. Lighter boat than yours, but some of the basics our dealership and the lift company said to us echo in above two experiences.

We were told all our significant weight was in the back third of the boat. Both told us direct bunk support of the front half of the boat was totally unnecessary. Seemed off to us, but they were certainly right based on our two years with the lift before switching to Sea Legs.
 

Timdelta

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I’m having a beam-less lift put in. My issue is my finger pier is short and I can only put boat on side that has the helm. As we all know no entry on that side, so have to enter on front, back, or port side. So if installer has a good idea of Cg he can get boat close to Seawall and then a simple step off to front of boat. I will probably have a walkway on both sides of boat so may end up being to enter in all three doors. Bennington said 16ft triple bunks, so will have that. The slip is about 20 wide and supposed to have a limit of 35 length but neighbor boat is a 43 so I could go quite long. The 4 pilings being put in will be spread out wide and long for bigger boat if ever required. Installer said pontoons are a bit difficult to know Cg due to all the furniture configurations. I did see that pontoons usually have a Cg point somewhere between 60% and 65% point from front of boat. My first boat and lift so it’s all new to me. I have a email into Bennington on Cg question so will let all know if I get a answer.
 

Potomacbassin’

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There are several ways according to the interweb.

Weigh while on a trailer at the axle and at the tongue, then use the available formulas.

Find a marina with a hoist and single strap. Lift the boat at various points until you find the CG.

I would think you could use weights and a level. Place the level on the deck and keep adding weight (human bodies, water, etc) to the bow until the boat is level then use available formulas.

Seems like a fun science project!
 

Timdelta

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I love the input from all. However —-Flaws to the science experiment. 1. Don’t have research equipment, ie no trailer and no boat. Lift is coming before boat. 2. Bennington says to not lift boat with strap, so that is out. However if it was put on bunks and then lifted that would do it. Good idea but again no boat. 3. The level method on water is not the same as Cg. The water method will show the center of buoyancy. Imagine a hollow ball floating on water the ball will mostly be above the water and the center of buoyancy will be very close to perimeter of ball surface. The center of gravity of that same ball will be dead center of the ball. As for a pontoon, if we made the back half of the pontoons bigger than the front, the boat could float level. This would work because the bigger tubes in back would displace more water, but the back end would still be heavier (even heavier with more metal) and the Cg would still be aft of center despite the level float of boat. Make sense?
Sorry for any typos or for the bold but doing this on phone is tough. (For me)
 

BigKahuna

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We had the same situation. The installer showed up with 12' bunks for our 25QX. I told him that's not going to work. He assured me it would be fine. Like you said it was a little scary seeing 10' hanging loose up front. After using it for a summer it worked out great.
I guess some of these installers know what they are doing.
The funny thing is with 10 feet of pontoons just hanging there in the front, the front gate is our main entry/exit point. THAT took alot of getting used to thinking that we were going to tip over every time we used that gate.....
 

Timdelta

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The answer from Bennington:
Hi Tim,
Thank you for contacting Bennington!
There are too many variables for us to determine the center of gravity but a great starting point would be 7-9 feet forward from the back of the outside tubes.
Have a great day!
Team Bennington
 

SEMPERFI8387

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Find a marina with a hoist and single strap. Lift the boat at various points until you find the CG.

I’d never recommend this method. If I recall correctly, Bennington does not recommend the use of straps, let alone 1 strap lifting the entire boat. Your just asking for damaged tubes.
 

Potomacbassin’

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I’d never recommend this method. If I recall correctly, Bennington does not recommend the use of straps, let alone 1 strap lifting the entire boat. Your just asking for damaged tubes.
What I read was that you don't lift the whole boat out of the water, just enough to determine if it's centered or not. But yeah probably better on a fiberglass monohull than a pontoon.
 

Timdelta

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I received a email from a Bennington Director of engineering. His email is below:

A good general number is 35% of the length of the outboard tubes. Measure the outer tube in feet, not marketing boat names. CG is within 5% plus or minus. (25 R, Single example 25'-4" x .35= 8.9 feet forward of the endcap) (Twin 27 models, 27'-0" x .35= 9.5 feet forward of the transom)

For lifts, boat CG may not be enough to base support on. A big live load on the bow of the boat will throw the CG way off.

For splitting hairs.....
10' single is closer to 40%
Twins are closer to 30%
Small boats are found at both extremes.

To know exactly, set up the boat with specific engine(s) fuel batteries and whatever else people and load wise. Weight the four corners with race car scales and crunch the numbers.

Strong points are at the tube section welds.
When not supported well there, check to make sure there is no deflection on the tube when bunks are lifting the boat & boat with live load.

Lifts that use stringers to support the deck (no tube bunks) can be as short as 55% boat length if live load is kept at a minimum.
 

Dsqrd909

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Great info. Will have to check that out next spring when the boat goes back on the lift.
 
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