Thursday, December 4, 2008

Ingrid Performance and Stability

I have been curious about yacht metrics as applied to Ingrids. At 15 tons, an Ingrid is one of the heaviest small boats I have seen.

Pure sailing performance does not compare to today's Beneteaus and JBoats. Still, it is quite easy to reach hull speed in 8 t 10 knots of wind. Going to weather, Maitreya will do 5 knots in only about 5 knots of wind.

There are a group of metrics for stability and comfort that are used by naval architects. These include length to displacement, sail area to displacement, comfort factor, capsize risk and others. Ted Brewer often references these figures when he writes about cruising boats in Good Old Boat Magazine.

I ran the basic equations for an Ingrid and posted the results on Maitreya's technical page. There is a very good discussion of these formula posted by John Holtrop here. John has also published an interesting study of the averages of these formula for many of the major cruising yacht designers. He did not include Atkin or Archer. I will summarize the Ingrid results against John's standards.

Displacement / Length496
Lower number gives quicker response
Sail Area/ Displacement14.8
Racing boats are 18+
Capsize Risk1.43
<1.8Lower is less risk
Comfort Factor54.8
Higher is more comfortable
Length to Beam3.3
3.3 is average
Velocity Ratio.979
General performance metric, Racing boats are 1.8
Roll Period5.9
Less than 4 is stiff, greater than 8 is tender
The Ingrid's displacement skews all of the results to the very heavy side. Owners know that the Ingrid turns pretty slowly as indicated by the L/D. Sail area is on the low side, but a greater area would be too much for a small crew. The capsize risk is the lowest and comfort rating the highest of all of the designers in Holtrop's study.

The question about suitability comes down to the following; Is the maximum comfort and safety too damaging to performance? If you need to exceed 7 to 8 knots on a reach, the boat is too slow and you need to look at a longer water length or a catamaran. The Ingrid performs as well offshore as most boats of it's size. Lighter, faster boats may tend to slow down more in moderate to heavy seas.

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