[Public-List] Incline test on an Alberg 30?

[Robert Kirk]

At 06:39 PM 5/8/2010, JT wrote:
>Baraka was a wreck when we found her in '06. After a hiatus, we're back to 
>putting her to rights (so to speak)! We'll eventually take her offshore 
>and are working with a rigger to ensure her replacement spars and standing 
>rigging are properly sized for that job. While he can work from standard 
>tables, he said the best way was to perform an incline test, as in Brion 
>Toss's book "The Complete Rigger's Apprentice," p. 136: level the boat in 
>the water, then line up several of your buddies along the toerail after 
>noting their weights. Note the heel in degrees. (Hopefully, it's a handy 
>10 degrees.) By measuring the distance they are from the CL of the boat, 
>and multiplying their aggregate weight by that distance, then multiplying 
>again by 3 (if you got 10 degrees), you get the righting moment (RM) for 
>the boat at 30 degrees of heel.


Jeff... I'm not sure about the Righting Moment at 30 degrees, but you can 
get the related Metacentric Height (GM) pretty easily without weighing your 
friends, by sallying ship (timing an induced roll by having a couple of 
people run from one side to the other). The shorter the period, the better 
the stability. GM is approximately (B/(2T))^2 where is B is beam (8.75' for 
an Alberg) and T is the full period of a roll (average a couple of rolls).

Perhaps one of our naval architect Albergers (Hint: Mike) can comment on 
the relationship & usefulness.

Maybe a fun Alberg group project, we could each Sally Ship and report the 
resulting periods, so we can see how stable we are. They should all be 
pretty much the same, but you's expect a boat with a radar aloft and a 
light diesel to have a longer period than a clean topper with a lot of gear 
in the bilges.

If your period is significantly longer that other Albergs, you might worry.

All it takes is two people to induce a nice roll. I've gotten off sandbars 
that way. A full period is from one side to the other, and back again. Best 
done at anchor.

Bob Kirk
Isobar #181






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