Flotation Marks and Weight/Balance


Lindsay Turvey sent us this photo of the first time away from dock for Avatar (years ago Lindsay took part in a Queen’s Birthday storm rescue and sent some amazing sea state photos which are in Surviving the Storm). The FPB 64 is obviously floating high, but is she high enough?

When we work on a weight budget we use spreadsheet that has thousands of cells worth of data. Every item on the boat, from tools to spares, furniture, ground tackle, even art work, has a weight allowance and a position vertically, transversely, and longitudinally. These weights and positions are used to create a center of gravity for the boat and its payload, which determines the center of buoyancy to which the hull is designed.

The weights are based on a mixture of hard data – for example an engine or so much area of 12mm plywood – and “allowances” (fudge factors) based on a gut instinct for how the boat will actually be executed (as compared to theoretical assumptions).

We always want to know what a new boat weighs, and how she trims compared to the spreadsheet data. If we have the dimensions from a couple of spots on the hull to the water, the displacement and trim can be calculated. The FPB 64s have reference beads of weld at the bow, stern, and amidships, for this purpose.

Saturday these were measured at 190mm aft and 240mm forward. This puts the first FPB 64 where she was expected to float. Her fore and aft (longitudinal) trim is also on target (the quarter degree stern down trim in light trim will even out as she is loaded for cruising).

Posted by Steve Dashew  (March 21, 2010)

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