Interesting effect on RM of a cockpit.

This is interesting and not what I expected.

The two RM curves show the difference between a central (well aft of centre) flush deck cockpit with substantial side works and a sunken cockpit the seat tops are at deck level, floor 400 below deck level.

There is a significant reduction in inverted volume with the sunken cockpoit and I expected an overall reduction of RM past the flooding angle.

Although vanishing stability decreases as expected the unexpected is an increase in stability well prior to vanishing.

What is happening ............ the cockpit floods and the resulting stern down trim results in a larger GZ as it is a modern vessel fuller aft than fwd.:idea:

 

full context missing

nice picture

but i don not have a clue about the context ??
what are you trying to explain here ??

 

Sorry If tI didn't make things clear.

I was looking at the "rollover" stability effects of removing a sunken cockpit and replacing it with a flush deck with some seating and side bulwarks/ winch mouting walls.
The cockpit is quite large and being sunken is a big hole in the deck, the effect on the RM curve of a flush deck is interesting. Intuitively you would expect an increase in heeled Rm when you get to the cockpit flooding angle. However the RM is actually higher for the sunken cockpit than for the flush deck even though there is more volume enclosed.

The reason has to do with the way the boat trims on the longitudinal axis and the resulting effect on GZ.

In practical terms there's little difference but the result is not what you would intuitively expect, I posted it as a curio only. Probably the best lesson to draw from this is don't trust stability curves from any software where the vessel is not free to trim.

clearer?

 

Interesting, although I'm not sure I follow you when you say "...Intuitively you would expect an increase in heeled Rm when you get to the cockpit flooding angle...." Isn't this a contradiction with what you state in your first post..."I expected an overall reduction of RM past the flooding angle"...?

Also I have a question on this RM apparently contradictory effect: Although the boat with the cockpit shall trim aft more than the flush decked one, for sure, shouldn't the reduced heeled floatation area (because of the cockpit) reduce RM instead of increasing it?

Anyway the real effect of flooded cockpits may vary significantly from what those curves say, as RM (or GZ) curves are 'static' (heeling angle varies at an infinitely slow pace.....) and in real life we'll have to account for speed of rolling, inertias, draining speed, etc. I think these may prove to have more important effect on stability than what the static curves say.
Cheers.

 

At 150 to 155 degrees you are almost completely upside down, and the coaming's bouyance will help turning the boat further.

 

Apparently confusing, my apologies. I'll try this from the beginning once again:

In the computer model I removed the cockpit and flush decked over the cockpit, I then ran the static (free to trim) rollover. As expected the AVS was increased, but when I superimposed the curve with the cockpit extant I saw that its static stability was actually higher between around 90 and 135 degrees (The cockpit version has a higher RM than the flush deck from 90-135 degrees) . Note The vertical scale is RM, in Kg-meters.

Intuitively we would expect the opposite; that changing to a flush deck would give a higher GZ for all angles of heel past the flooding angle due to increased volume.

That’s why I posted the curves, because it caused me to do a double-take and I had to think (always an effort).

This is because max beam and COB are aft of mid-ships, so when the vessel trims down aft due to the cockpits reduced buoyancy it is actually trimming to a more stable state (only within 90-135).

Another way of looking at it would be to consider adding weight on deck aft and finding that your area under the RM/GZ curve to AVS had not changed. Not intuitive.

Absolutely couldn't agree more.