Low Tide

Yacht Designer Tad Roberts' Web Log

Change in Stability with Dinghy Aboard

I’m often asked about the advisability of adding a heavy dinghy stored on the roof, people wonder how much difference this will make to stability. The answer always involves a lot of questioning about the particulars of the boat and load condition. So today I made a study of a typical 1985 “trawler” yacht, the Californian 43 cockpit MY.

I made a computer model of a hull that’s an approximation of the Californian 43. It’s 43’8″ overall, beam is 13’3″, and displacement is 32,000 lbs in the lightship condition.

Every time we add or take a item off the boat, it’s total displacement changes, draft, trim, and freeboard change, and stability changes. My effort here is show the magnitude of the change instability caused by adding fuel down low or a dinghy on the roof. This is not a precise calculation because I don’t have precise data on any particular Californian 43. To complete these calculations I’ve made a number of assumptions based on experience. As such this post is just my opinion of what happens and anyone is free to disagree.

For arguments sake…..

Lightship is the bare boat, 32,000 lbs
Full Load is the bare boat plus 2856 lbs of fuel and 1166 lbs of fresh water
The dinghy is 700 lbs.

After the hull model I created a simple table of weights and moments, to quantify the effect of adding fuel and the dinghy. All weights are in Long Tons of 2240 lbs. Note that filling the fuel (400 usg) and water (140 usg) lowers the VCG (vertical center of gravity) by approximately 2.5″. Adding the dinghy up on the roof when the tanks are empty moves VCG up 4.75″. Adding the dinghy when the tanks are full moves the VCG up just under 2″.

Californian 43 weight table

The Righting Arm Curve at Lightship shows a max righting arm of almost .7′ and zero stability at 61 degrees heel.

Californian 43 lightship stability curve

With Full Fuel and water RA is about .85′ max and zero stability is at 66 degrees heel.

Californian 43 stability with full fuel and water

Lightship plus the dinghy shows a .6′ RA and zero stability at 58 degrees heel.

Californian 43 with no fuel but with dinghy

And Full Load plus the dinghy shows RA at .75′ and zero stability at 64 degrees heel.

Californian 43 at full load plus dinghy

Once again these numbers are just an indication of what will happen under these circumstances. Real life will be different. Adding a heavy dinghy on the roof will reduce stability on this type of boat. The reduction in stability shown here is not (IMO) enough to cause concern under most typical cruising conditions. In marginal conditions I would find it cause for concern and would take action to improve the vessels stability.

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