|* Photo Courtesy of The U.S. Coast Guard.|
Guest blog by Vincent Pica
Chief of Staff, First District, Southern Region (D1SR)
United States Coast Guard Auxiliary
We’ve covered many seamanship topics here over the past several years. Topics such as what to do if you’re sinking (Six Things To Do When Your Boat Is Sinking), how to handle heavy seas (Surviving a Storm at Sea) and what to do if you’ve run her aground (When Your Boat Runs Aground) have been among the literally hundreds of topics we’ve covered. One topic we haven’t covered is the general topic of capsizing. There is a tremendous amount of data on “righting moments”, centers of buoyancy and gravity, thanks to the US Navy and the US Coast Guard, amongst many institutions who literally live and die by these metrics. We’ve also seen a couple of columns here about wind and waves, which are the agents of capsizing (Wave Theory – and Practice.) But there has been very little direct data on what that translates into in terms of my 25’ boat and 8’ seas at the Inlet. This column is about that.
|Diagram A - Courtesy of US Coast Guard|
|Diagram B - Courtesy of US Coast Guard|
Sail boats are designed to operate with a higher degree of heel (greater GM) than motor boats but the principles are exactly the same.
From This to Wave Height?
Yes. You can infer that your motor boat’s center of gravity and center of buoyancy can’t be too far apart when the entire distance from the keel to the floor boards is probably something like 2’ or 3’. Think of her draft. It isn’t a big number, even for a 40’er. No reason to panic but you now realize that M, G and B can’t be that far apart – which means that GM just can’t be that great either. And GM is a surrogate for the righting ability of your boat.
But wait. I’ve been out in some pretty steep seas and I think the boat handled it well. Yes, because studies conducted by the Society of Naval Architects and Marine Engineers (SNAME) determined that 3 things must exist for a capsizing to occur:
1. The boat is broadside to the wave. Yes, a boat can be pitch-poled (tossed end-over-end), but the size of the wave needed to do that greatly exceeds the size of the smaller wave needed to knock a boat down when broadside to a wave.At this point the wave contains enough energy to overcome a boat’s righting moment.
2. The boat is struck by a breaking wave.
3. Wave height must exceed a certain percentage of the boat’s length.
So, what is that “certain percentage?” At only 30% of your boat’s length, (about 6’ from tough to crest for a 20’ boat), things enter directly into the realm of high danger. At 60%, it is nearly certain that one wave will catch you and then you, the crew and the boat may well come to grief.
So, before trying to transit these inlets and bars that control much of our access to the open sea (see Atlantic Maritime Academy, Mastering the Inlet), think about just how much of a righting arm your boat can possibly have…
BTW, if you are interested in being part of USCG Forces, email me at JoinUSCGAux@aol.com or go direct to the D1SR Human Resources department, who are in charge of new members matters, at DSO-HR and we will help you "get in this thing…"