Righting Moment Test vs Selden Calculator

[Tree]

Hello SparTalkers,

I have been noodling on the relationship between displacement changes and RM30. I am hoping some of you more experienced riggers can chime in. Here are the cliff notes:

I recently completed a dockside righting moment test on my Lafitte 44. (We used grilled sausages in lieu of pizza so hopefully that didn't skew our results.)

Anyhow, with 10 friends (many of them holding heavy objects such as anchors, cases of oil, jerry cans, 5 gallon buckets of water, etc) we were only able to muster 1,870 lbs. Unsurprisingly, this didn't dramatically heel us, but we averaged 5.3 degrees.

Working the math, this puts the boat very near the RM30 predicted on p. 142 Fig. 5-24 of Rigger's Apprentice. So despite potential accuracy pitfalls of extrapolating RM30 from only RM5.3 (vs 10+ degrees) I feel that we got reasonably accurate results.

Running the numbers further puts 12mm HSR right at the desired 10% of breaking strength for creep.


Today, I was playing with the Right Moment Calculator on the Selden website.

Because the Selden calculator is a "black box" that takes in data and spits out a number, I have no idea how they are handling the data to get their answers. However, if their calculator is to be trusted, displacement has a fairly large influence on RM.

Using the published displacement of 28,000 lbs vs anecdotal (not personally verified) "cruise loaded" 34,000 lbs is the difference between being at 10% vs 12% of 12mm breaking strength.

I don't like the idea of going up a size based on anecdotal displacement (that could easily be exaggerated in the memories of the reporter) but thought I would seek advice.

Should I be concerned that because my boat is currently somewhat empty that my RM30 calculations could be off?

Cheers,
Tree

[benz]

Just replying by the seat of my pants, I'd say it's OK to stay with 12mm, though I'd really recommend half inch .
It seems all but impossible to me to load 6,000 lbs of cruising gear onto a 44-foot boat. And even if you did, how often would would the rig be loaded up long enough for creep to occur? Unless creep occurs while the mast is at rest (which is what "sizing for creep" is supposed to prevent), it'll be a long time before you strain the rig for long enough that creep will be noticeable.
That's just the opinion of a notoriously unscientific guess-timator. If one of the resident math wizards tells you otherwise, go with them.
Ben

[Tree]

Thanks for the reply, Ben. I also suspect loading 6,000 lbs of cruising gear would be unlikely unless I find a stash of un-cursed but long lost pirate gold.

Also those are good questions for thought experiments. I wonder if there is any published data on, for instance, Creep after 10 hours vs Load %. (I can't find any with a quick search).