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Old 09-06-2013, 22:54   #1
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Center of Gravity - Bow to Stern

I recently acquired a 1985 Sovereign Antares (sloop). Am resetting a trailer for the boat hence need to know the boat's center of gravity (dry and stripped). Is this published anywhere?
Thanks,
Mark
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Old 09-06-2013, 23:17   #2
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Re: Center of Gravity - Bow to Stern

If you can contact the original builder he can probably get you the naval architecture diagrams. All of the measurements I know for a boat are based around metacentric height and buoyancy; obviously the forces will be a lot different out of the water. Not sure if that's on normal architectural diagrams.

Tough one.
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Old 10-06-2013, 09:14   #3
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Re: Center of Gravity - Bow to Stern

Put the boat on the trailer and see how the trailer balance and adjust accordingly.
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Old 10-06-2013, 09:52   #4
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Re: Center of Gravity - Bow to Stern

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Originally Posted by shadetreevet View Post
I recently acquired a 1985 Sovereign Antares (sloop). Am resetting a trailer for the boat hence need to know the boat's center of gravity (dry and stripped). Is this published anywhere?
Thanks,
Mark

You can calculate it yourself

use the formula Xcg = ∑xW/∑W to find the CG along the x-axis and Ycg = ∑yW/∑W to find the CG along the y-axis.
The point at which they intersect is the center of gravity.


X=(∫x*y dx)/(∫y dx)
Y=(∫y2 dx)/(∫y dx)
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Old 10-06-2013, 11:53   #5
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Re: Center of Gravity - Bow to Stern

I am not familiar with the design, but I bet that if it is a fin keel boat the longitudinal COG is close to the center of the fin... at least close enough for getting the trailer balance near enough to be useful. Fine tuning can be accomplished by positioning the boat on the trailer.

Cheers,

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Old 10-06-2013, 20:39   #6
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Re: Center of Gravity - Bow to Stern

I should have given more details. It's a shoal draft boat with over 1200 pounds ballast all placed in the forward half of keel. I haven't actually measured the keel but estimate it to be about 9 feet long and 16 inches deep. Most of the weight of the trailered boat will be on the keel, of course. I'd like to determine the approximate optimum placement on the trailer before bolting the rebuilt truck (dual axle) under the trailer frame. I'll have to do a bit of digging into the history of the builder and see what's available for specs.
Thanks, I appreciate the suggestions.
Mark
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Old 11-06-2013, 05:54   #7
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Re: Center of Gravity - Bow to Stern

If it is a fin keel and depending of the forward slope of the fin it possible that the dry CG can be in the slope just in the front of the base of the fin keel and the forward sections will need supporting. Some time a builder for ease of construction will modify slightly the angle of the slope in such a way that the dry CG is just in the first foot (.3 m) of the front of the fin keel base. To calculate the moment can take sometime. Basically if on the dry the boat requires that the forward section be supported then the dry CG is forward past the front of the keel. Now 16 inches (0.4064m) is not much of a fin. So I guess if the boat is not forward supported then the dry CG would be pretty much where the ballast is.
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Old 11-06-2013, 05:57   #8
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All the boats I've hoisted onto trailers had the CG aft of the fin keel center. Right about at the max beam. At the companionway. Always surprisingly far aft.

On a trailer the ballast center will "always" be well forward of the axles.
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Old 11-06-2013, 18:24   #9
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Re: Center of Gravity - Bow to Stern

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Originally Posted by gindac View Post
X=(∫x*y dx)/(∫y dx)
Y=(∫y2 dx)/(∫y dx)
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Old 11-06-2013, 18:32   #10
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Re: Center of Gravity - Bow to Stern

Hull plans have a vessels center of gravity.

You will want about 10% of the vessels weight on the trailer tongue. You may end up having the shift the boat forward or aft on the trailer to achieve this regardless of where know or estimated the CG is located.
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