Low Tide

Yacht Designer Tad Roberts' Web Log


Stability information presented to the public is usually simplified to the point of being useless. When I see a published stability curve like that above, my reaction is “That’s nice and it looks good, but it tells me almost nothing.” It’s a nice graphic but key information is missing, mainly data on the conditions under which this curve was arrived at, and how it was ascertained. “Oh no one wants those details.” Well I’m sorry but the curve is only a small part of the story.

This curve happens to belong to the Xc 45 sailing cruiser and was published by Yachting World Magazine . It’s a nice boat and I don’t mean to disparage it or it’s designer in any way. But the presenting of these stability curves has become popular in published work and it’s been bugging me for years. This particular curve is truly bad because they have cut out even the vertical scale, the grid could represent feet, metres, or tenths of metres…..

Information missing on the data is;
1) Name of the boat, or is this a representation of a the series?
2) Load Condition in words (Full load, half-load, lightship, etc.) Somewhere there should also be an explanation of the load condition. For sailing yachts: Are sails hoisted or lowered?
3) Trim and GM or height of VCG at the above Load Condition.
4) Displacement (a number) in this load condition.
5) Downflooding point or points marked by heel angle.

Information missing on how the data was arrived at is:
1) Is this a calculated curve(from a weight study) or is it the result of in water floatation and inclining measurements.
2) As this particular boat has a large cockpit how was that treated to arrive at the published curve.
3) How much of the superstructure was included in this curve.

As a for instance, if the curve is done from a calculated weight study, a floating finished boat, with equipment ready for sea, may have a very different VCG and displacement. This means the curve may be representative, but not accurate for one particular boat. We’ve all seen the additions of huge stern arches loaded with antenna and other items that will change VCG.

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