Self righting possible?

Jehardima, you said, "Everyone generally agrees that if knocked knocked over, down flooding will occur" . But I neverheard that term here.....down flooding........what do you mean by that?. I thought people were saying that if the whole boat is a watertight compartment, and its heavier at the bottom, by physics, it tries to right itself.

 

Down flooding is a term associated with the stability and survivability calculations. The vessel begins to down flood when a typically open or non-watertight fitting becomes immersed; whether by damage or roll/list or water on deck. The SV PRIDE of BALTIMORE and SV CONCORDIA were both lost to down flooding before they could rise after being knocked down. A significant number of fishing boats in the Gulf of Alaska are lost to down flooding due to improperly sealed hatches while taking green water on deck.

 

Looking at the blue one it has the same venturi style dry exhaust that many commercial boats have. I'd surmise that causes shockingly fast down flooding.

As to the loose hatches, its kinda the catch all for blame recently. In the case of the destination every fisherman on the water knows it was ice more than a loose hatch on a bait hold aft. I had friends who were closest to them, and when the cold front hit they started building ice at a rate that had the whole crew up and clearing ice. From 13 to 17 we had super mild winters, and 11 and 12 had ice around the season by was fairly ice free during. 17' got brutal and the guys should have known better and rolled folks up early on. All to common some 20 year old with an iPad in gets the late shift and doesn't have enough sense or confidence to roll folks out soon enough.

Consolidated quota and catch shares have dropped the crab fisherman's crew share so low, the toughest and smartest crab crew I know drive tuck and operate heavy machinery somewhere in the midwest. It's why we're seeing more boats go down than we have in 15 years. Problem with ice is it melts and leaves no trace, so we blame a hatch. But hey it's easier than accepting that the management regime adopted had brutal unintended consequences.


Back on topic. I cannot speak to the Norwegian boats as going on one is a bucket list. I'm intimately familiar with their counterparts and someday they may find a wave to roll it, but it's not self righting....

 

Thanks, then thats human error, not the boats fault if people dont batten down everything.

 

I'm pretty sure I read you can get a mechanism that closes them watertight. True?

I though you guys said that if the whole boat is watertight and heavier at the bottom, by physics they have to self right?

 

Down flooding is not so much error as necessity; people and engines need to move and breathe so there has to be some openings. Where they are placed and how they are operated is another matter.

No, do not confuse ultimate stability with initial stability. In most vessels, including the figured fishing vessels, the center of gravity (CG) is above the center of buoyancy (CB). What prevents the vessel from turning over is that as it rolls, it causes two wedges of changes to displacement to form. These are the emerged wedge on the high side and the immersed wedge on the low side. These to wedges have the effect of creating a buoyancy couple that rights the body, i.e. the emerged wedge decreases buoyancy on the high side and the immersed wedge increases buoyancy on the low side. The effect of these two wedges is called the GM of the vessel and the metacenter is the point that the buoyant volume of the vessel appears to, at that instant of time, to rotate about relative to a fixed vertical CG reference axis (i.e. gravity is always "down"). See here for a full discussion of GM (and please ignore the snark in it): Calculation of GM https://www.boatdesign.net/threads/calculation-of-gm.8852/#post-60405
The bad side of these buoyancy wedges is that they are also what cause the vessel to roll. As the wave profile moves across the hull, gravity is always acting "down" but the differential wave profile, port to stbd, is exerting a buoyant roll force on the vessel (n.b. there is also a lot of other factors but for the sake of a simple discussion we'll just stick to hydrostatics). A vessel capsizes when the roll angle, roll force, and wave profile are such that the vessel does not have positive GM at that instant; and will continue to roll until GM is again positive, perhaps with the vessel up side down. To make a self righting vessel, the distribution of buoyancy in relation to the CG must be such that when inverted, the GM is always negative. As has been pointed out previously, this can be difficult in practice. It also means that if GM is positive for a roll of zero, and negative for a roll of 180, there must be a roll angle where GM is identically zero. At this roll angle is where the vessel MUST HAVE a positive righting arm, and where weight "low" in the hull comes in handy.