Centre of Gravity - fin keel gibsea 282

[nigel261266]

Hi all,
I am in the process of fitting temporary legs and need to locate the centre of gravity. Does anyone know where I start?

Nigel

 

[DJE]

How accurately do you need to know the C of G for temporary legs? Are you looking for longitudinal or vertical position?
You could get some idea of longitudinal position with the boat out of the water by jacking up each end in turn with a calibrated jack and noting the load which just lifts the boat out of her cradle.

 

[sarabande]

I'm just wondering whether locating the legs at the point of balance (fore and aft) is the best idea ?

If the legs are offset toward the bow, it means you can head to the for'd cabin and the boat will be supported towards the bow.

 

[Bajansailor]

If the waterlines were all symmetrical fore and aft of amidships, then the longitudinal centre of buoyancy (LCB) would be pretty much amidships.
But a Gibsea 282 (like most other sailing boats) is broader (and has more immersed volume) aft of amidships, hence the LCB will invariably be aft of midships as well.

Here is a Gibsea 282 for reference - http://www.yachtsnet.co.uk/archives/gibsea-282/gibsea-282.htm

When the vessel is floating in equilibrium, the mass of the vessel acts downwards through the centre of gravity, and this is balanced by the upthrust of the hull buoyancy which acts through the centre of buoyancy.
If the crew move aft, the vessel sinks a bit at the stern, the overall centre of gravity moves aft, and the LCB moves a bit aft as well, until the vessel is in equilibrium again.

Sarabande makes a valid point re locating the temporary legs forward of the centre of gravity.
Following on from this, I am thinking that if you locate your legs amidships (if this is the point of maximum beam, even better) then you can fairly safely assume that they should be forward of the CG (unless you hold a party on the foredeck......), and hence there should be less risk of the vessel falling forward on her nose (assuming that the base of the keel is level, ie not raked aft).

 

[Mrnotming]

centre of gravity

I was told to make a cardboard silhouette of the side view of the vessel.
This then to be balanced upon a pencil held atwarthships.under the fin keel in my case.(Ron Holland Shamrock 30 Club version).This lent credence to the plan to suspend the vessel from a single point hoist,led through the cabin top hatch, blocked off with stout wood cutouts.A welded up inverted T bracket fabricated to be attached to three keelbolts completed the equipment apart from the sized strop with tested shackles.This arrangement worked perfectly and is used by X-Yachts and Dragons amongst others.
The reason for going to this trouble was that the club crane was short in stature and lifting with strops tended to squeeze the vessel.I corresponded with a very helpful design office in Currabinny Co. Cork during this work and was delighted with the personal and helpful advice which was forthcoming.
Designs have changed now to dart shaped vessels which will need owners to correspond with the designers of their particular vessels for hopefully equally helpful advice on how to proceed.fitting legs is the inverse but gravity will nonetheless have it's effect in a similar manner.

 

[Malo37]

Hi all,
I am in the process of fitting temporary legs and need to locate the centre of gravity. Does anyone know where I start?

Nigel

Are you sure you dont mean the centre of bouyancy?

 

[snowleopard]

The accurate way is to attach a plumbline to the centre of the hoist while craning.

To find out while out of the water you could jack under the keel just a couple of millimeters and see whether the boat tips towards the front or back.

If you have a lines drawing you could calculate it from that by working out the areas of each of the station sections. Remember that the CG of the boat is at the same position as the CG of the water displaced.