# Center of Gravity

Discussion in 'Sailboats' started by tom power, Jun 10, 2020.

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### tom powerJunior Member

Good Day,

I one a C&C 30 Mark 1 sloop rigged sailboat. I'm desperately attempting to fabricate a trailer for my vessel. To help with the stability of the trailer I'm researching where the center of mass is on my boat with no luck. I was wondering if anyone has any advice to achieve this.

I realize the keel is the major actor but it would help me a lot if I knew where along the keel, my keel is a sweeping fin type. My boat is restored to original condition.

Thanks for any assistance,

Cheers.

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### gonzoSenior Member

Is the boat on land? You can use scales under the stands to calculate the CG. Otherwise, for an approximate calculation, you can use some measurements from the submerged volume and find the centroid.

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### TANSLSenior Member

Gonzo, could you make a more detailed description, the steps to take, measurements to be taken and how to process that data, in the procedure you propose? Surely the OP will thank you.

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### tom powerJunior Member

Thanks for the speedy advice. My boat is on land. When it is submerged the water line is sitting almost perfect. Would it be possible to measure the submerged distance from bow to stern using a datum line from the bow, them drop a plumb bob from the center point to the a point on the ground. Would that work? or would taking the submerged surface area into calculation be a better option. Thanks.

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### gonzoSenior Member

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### bajansailorMarine Surveyor

Welcome to the Forum Tom.
https://sailboatdata.com/sailboat/cc-30-1-1-506

They mention that her design displacement, I presume to the waterline that you mention, is 8,000 lbs, and the ballast is 3,450 lb.
Are you more interested in the longitudinal or vertical position of the centre of gravity when she is on the trailer - or both equally?

The keel does indeed have a fair amount of rake on it - how is it supported while the boat is currently ashore?
It would have to be supported in a similar fashion when on the trailer. Have you got a prop under the stem to stop her tipping forward?
Would it be feasible to have a sort of 'U' shaped cradle structure forward of the keel on the trailer to support the forward end of the boat?

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### tom powerJunior Member

Oh wow, you guys are awesome, lol talk about a rabbit hole for a newbi. Looking forward with hop now.

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### tom powerJunior Member

Thanks bajansailor, that is indeed my boat, my goal is to find a position a set of tamden axles under the cradle that the boat sits on.

The cradle is 2"x 6" x 1/4" box beam. The keel is currently sitting on a timber on the hard, which is not ideal, I plan to weld in a curved support on the trailer for the keel.

The boat is currently supported by six jack pads, two of which are forward to stop the boat from tipping, there is a lot of weight on the pads. When I attach the tongue to the cradle, I will need to have a stem support to take some pressure off.

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### gonzoSenior Member

If the boat is on a cradle, there is an easy way to find the CG. Get two bottle jacks and lift the cradle slightly from somewhere under the middle of the keel. Preferably run a 4x4 across. The boat should balance when you lift from the CG. If you are building the trailer, don't weld the suspension until the boat is loaded. That will allow for fine tuning.

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### bajansailorMarine Surveyor

It sounds like the cradle is resting on the ground (hard) if the keel is sitting on a timber on the hard (?).
In which case it will be rather difficult to try to jack up the cradle.
Tom, do you have any photos that you can post showing the boat sitting on her cradle please?

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### gonzoSenior Member

Cradles are always set on blocks and they hold the keel too.

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### tom powerJunior Member

Thanks Gonzo for the balancing idea, I'm going to try it, sounds exciting balancing a loaded cradle.

I may have added to some confusion in the earlier post. The boat is currently sitting in it's cradle. The cradle is supported from the ground via timber blocks. The keel is sitting on a six inch timber which is sitting in the cradles bed plate. I plan to replace the timber under the keel with a one inch dense rubber pad. here is a picture to clear up confusion.

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### tom powerJunior Member

The cradle is currently loaded, after I find the ideal CG for the axles. Would there be any chance of having the "trailer" warp when the weight is taken off? The cradle will be level when I weld on the trailer assemblies. I'm able to take the weight off the cradle (raise the boat) if that would help the outcome. Thanks for the help. Cheers

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### Will GilmoreSenior Member

If you can weigh the boat from one end, while supporting it from the other, then move the weigh point in or out and get a second weight. You should be able to get a good idea of how far from center mass your two with points were, relative to each other and the third resting point.

Without taking the time to actually do the math, I haven't that, you set a point at the far aft, just under the transom, for example. This should be braced to prevent lateral rolling so you can then pickup the boat at the bow without her wanting to roll. The keel should help. Just pick her up enough, an inch, pr so, to get your weight. Say, 25 feet from the transom. Let's say your boat weighs 8000 lbs and you measure a weight of 4200 lbs. Good, the center of mass is obviously near half way between the two points. Now, set her down and move your lifting point 5 feet aft. It will be heavier as you move closer to the center. You should be able to calculate from the second point, where you would have to move to pickup the entire weight of the boat. It will be a near linear relationship.

This sounds like a lot of work, though. My personal approach would be to design my trailer with adjustable ways and measure the tongue weight as you position the boat fore and aft on the trailer.

-Will (Dragonfly)

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