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

Tue May 18 05:59:52 PDT 2010

If you do Jeff's experiment you have the righting moment for the heel
angle you measured, but not for any other.
You cannot extrapolate from 10 degrees of heel to 30, because the ship
geometry keeps changing as you heel.
If you can measure heel fairly precisely you can draw the righting arm
curve by sucessivley adding Albergers to the rail.
The Iphone has a nice leveling app that gives rotation to 0.1 degree.
If anyone can do this please post the data!

The GM gives us only the righting moment around zero heel. GM is the
height of the metacenter - the virtual center of heel - above the center
of gravity. We sometimes use GM0 to denote GM at design trim (zero
heel), because it varys with heel.

GM0 is a measure of initial stability - at small heel angles (1 degree
or less) the righting moment, RM=D*GM*tan(phi)
Where phi is heel, D is ship displacment. So GM is the slope of the
righting moment curve (and righting arm curve, which is just moment
divided by displacment).

The beauty of the A30 is her GM is not so high, but her righting arm
keeps building as she heels.

Bob's equation is used to estimate roll period using GM, but it's a
stretch to get GM from roll period. If we do the static experiment for
small roll angle we get a more precise GM.

The roll period / GM relationship is quite fuzzy. Now we are considering
dynamics, and the addition of all those sallyers can really change the
roll period.  It's nice to know the natural roll period independent of
GM. I think a better test would be to heel her by a line from the
masthead, let go and measure roll vs time.  

Mike
Rinn Duin #272

-----Original Message-----
From: public-list-bounces at lists.alberg30.org
[mailto:public-list-bounces at lists.alberg30.org] On Behalf Of Robert Kirk
Sent: Monday, May 17, 2010 3:32 PM
To: Alberg 30 Public List -- open to all
Subject: Re: [Public-List] Incline test on an Alberg 30?

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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