In the book The Perfect Storm, the author makes the comment that a boat is in danger of capsize (or pitch poling) whenever the wave height is more than the length of the boat. Or, in the case of a boat that broaches, whenever the wave height is more than the beam of the boat. My question is, does that also apply to sailboats with keels? Do typical boats such as the Catalina, Beneteau and Hunters in the 34-38 foot range capsize under those conditions, for example? And if they do, do they self-right, or do they turtle? (assuming the cabin is sealed up and doesn't fill with water)
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Yes, it applies.
Search the archives for "capsize ratio" and/or "capsize screen". Lots has been written, check the articles on www.sailnet.com. One of my favorites is the Link below. See "Technical Articles" and "Choosing an Offshore Cruising Boat". A book that I really like is "The Seaworthy Offshore Sailboat" by John Vigor.
Search the archives for "capsize ratio" and/or "capsize screen". Lots has been written, check the articles on www.sailnet.com. One of my favorites is the Link below. See "Technical Articles" and "Choosing an Offshore Cruising Boat". A book that I really like is "The Seaworthy Offshore Sailboat" by John Vigor.
Studies Have Shown....
.....that a "modern style sailboat" caught in the wrong position (broach, more or less beam-on) will likely be rolled by a breaking wave that exceeds ~40% of the LOA. This study also noted that NO boat of ANY design was able to safely and consistently withstand, on the same basis, waves exceeding ~55% of the LOA. (immediate source; Nigel Caldor's new book on cruising; he cites the original sources). What surprised us was what seems like a rather limited range of additional ultimate security offered by "a boat of any design". Kind of puts some of the discussions about "seaworthiness" and "ultimate stability" in a bit of perspective. It's a good reminder that when it gets really nasty, NO boat will "take care of you"; you need to "take care of the boat" and make certain that the wrong wave doesn't catch you in the wrong position. A very "seaworthy boat" may give you a more tolerable ride and make it a bit easier to "take care of it", but beyond that the "seaworthiness" of the skipper and crew would seem to be the real key. Carl and Jule s/v 'Syzygy'
.....that a "modern style sailboat" caught in the wrong position (broach, more or less beam-on) will likely be rolled by a breaking wave that exceeds ~40% of the LOA. This study also noted that NO boat of ANY design was able to safely and consistently withstand, on the same basis, waves exceeding ~55% of the LOA. (immediate source; Nigel Caldor's new book on cruising; he cites the original sources). What surprised us was what seems like a rather limited range of additional ultimate security offered by "a boat of any design". Kind of puts some of the discussions about "seaworthiness" and "ultimate stability" in a bit of perspective. It's a good reminder that when it gets really nasty, NO boat will "take care of you"; you need to "take care of the boat" and make certain that the wrong wave doesn't catch you in the wrong position. A very "seaworthy boat" may give you a more tolerable ride and make it a bit easier to "take care of it", but beyond that the "seaworthiness" of the skipper and crew would seem to be the real key. Carl and Jule s/v 'Syzygy'
Capsize
Tank tests show that it is breaking waves that cause capsizes. It is rare that a wave is breaking all along its length to that one technique is to keep moving and steer away from the breaking areas of the waves. In tank tests a boat towing a drogue was never capsized in the 40% LWL waves. I haven't seen the 55% LWL data. John Vigor's book is excellent and keeps reminding the reader to "think inverted." For the reason why, see chapter ten of Chichester's Gypsy Moth Circles the World. For more on wind waves have a look at Van Dorn's book Oceanography and Seamanship. For more on stability have a look at the section on righting moment in Dave Gerr's book The Nature of Boats.
Tank tests show that it is breaking waves that cause capsizes. It is rare that a wave is breaking all along its length to that one technique is to keep moving and steer away from the breaking areas of the waves. In tank tests a boat towing a drogue was never capsized in the 40% LWL waves. I haven't seen the 55% LWL data. John Vigor's book is excellent and keeps reminding the reader to "think inverted." For the reason why, see chapter ten of Chichester's Gypsy Moth Circles the World. For more on wind waves have a look at Van Dorn's book Oceanography and Seamanship. For more on stability have a look at the section on righting moment in Dave Gerr's book The Nature of Boats.
Beaufort Wind Index
If a boat is in danger of sinking (or at least capsizing) whenever wave heights exceed 55% of the boat's length, then a 36-foot boat should be in danger whenever wave heights are more than 20 feet in height. That corresponds to only about 40 MPH of wind on the Beaufort index. In other words, winds in the high thirties, if sustained, produce wave heights of 20 feet. Many sailors in boats that length report having gone through gales worse than that.
If a boat is in danger of sinking (or at least capsizing) whenever wave heights exceed 55% of the boat's length, then a 36-foot boat should be in danger whenever wave heights are more than 20 feet in height. That corresponds to only about 40 MPH of wind on the Beaufort index. In other words, winds in the high thirties, if sustained, produce wave heights of 20 feet. Many sailors in boats that length report having gone through gales worse than that.
Capsize ratio is important
All boats given the right amount of circumstances can be knocked down and turned turtle. (some obviously take more than others) I'm sure it would have to be one heck of a wave AND it must be a *breaking wave*...20 foot rollers on the open ocean are NOT going to capsize your boat, its when they are really steep 20 footers...thats when yoyu need to watch out. What really matter is *if* you ever do capsize what is the stability of the boat to stay in the inverted position. A Catamaran is *very* stable in an inverted position, but it will take a lot more inertia to get it there than a monohull. And a heavy stable monohull (take for instance a Westsail 32!) has very little stability in the inverted position...Take a look at this link about what happened to the Westsail 32 from the Perfect Storm....(it survived the storm lying ahull with relativly little damage...an attestment to the boat and design...read...heavy-overbuilt and slow) http://world.std.com/~kent/satori/index.html. If you want to get an *idea* of the types of Capzize index for a boat in this range here is a capsize graph chart for a Catalina 36....a little more D/L ratio than some others in the catagory (go here http://albertson.sytes.net/~chris/C36_Polars/gif_clean/C36Polars_as.gif ) Click on the link below it has a great site that explains some basics on stability You can go to one of these 2 sites for an online calculator/comparison http://www.venturaisle.com/weather_links/sailcalc.html http://www.gosail.com/rateboat.html Go here for Plots of average boats http://www.johnsboatstuff.com/Articles/plots.htm
All boats given the right amount of circumstances can be knocked down and turned turtle. (some obviously take more than others) I'm sure it would have to be one heck of a wave AND it must be a *breaking wave*...20 foot rollers on the open ocean are NOT going to capsize your boat, its when they are really steep 20 footers...thats when yoyu need to watch out. What really matter is *if* you ever do capsize what is the stability of the boat to stay in the inverted position. A Catamaran is *very* stable in an inverted position, but it will take a lot more inertia to get it there than a monohull. And a heavy stable monohull (take for instance a Westsail 32!) has very little stability in the inverted position...Take a look at this link about what happened to the Westsail 32 from the Perfect Storm....(it survived the storm lying ahull with relativly little damage...an attestment to the boat and design...read...heavy-overbuilt and slow) http://world.std.com/~kent/satori/index.html. If you want to get an *idea* of the types of Capzize index for a boat in this range here is a capsize graph chart for a Catalina 36....a little more D/L ratio than some others in the catagory (go here http://albertson.sytes.net/~chris/C36_Polars/gif_clean/C36Polars_as.gif ) Click on the link below it has a great site that explains some basics on stability You can go to one of these 2 sites for an online calculator/comparison http://www.venturaisle.com/weather_links/sailcalc.html http://www.gosail.com/rateboat.html Go here for Plots of average boats http://www.johnsboatstuff.com/Articles/plots.htm
Also, Please note there is a difference
Between Capsize Ratio (as per the chart from the other Post) to Capsize risk !(as per the formula from the link johnsboatstuff.com) In that case the CAPSIZE RISK = beam/(disp/(.9*64))^.333 ( derived by the USYRU after an analysis of the 1979 FASTNET Race) determines that a Catalina 36 is somewhere around 1.85 which is well under the value of 2 which is suggested maximum to compete in ocean races.
Between Capsize Ratio (as per the chart from the other Post) to Capsize risk !(as per the formula from the link johnsboatstuff.com) In that case the CAPSIZE RISK = beam/(disp/(.9*64))^.333 ( derived by the USYRU after an analysis of the 1979 FASTNET Race) determines that a Catalina 36 is somewhere around 1.85 which is well under the value of 2 which is suggested maximum to compete in ocean races.
FWIW; here are a couple of thoughts
(1) The CSF (Capsize Screening Factor; if I remember well) -- rather than the term Capsize Risk used by the fuzzy logic author cited by Tom -- is no more than that, i.e. a little crutch for comparing vessels of very similar design. Hoist a 500 lb chunk of concrete to the top of the mast or put it in the bilge; according to the CSF your capsize tendency will remain unchanged..... (2) IMHO Hunter cabin tops and hatches are not built to withstand breaking waves. So, don't even think about lying ahull in heavy seas. Try to heave to while keeping the bow into wind and waves. If that is no longer possible one has to choose between running off with a drogue (to reduce the chance of pitchpoling or broaching) or keeping the bow into the waves with a sea anchor. We have had to run off a few times when suddenly overpowered by wind gusts. After longer periods of time the helmsman is likely to become fatigued and then let the vessel broach (finkeelers broach much more easily than fullkeelers). (3) I expect that most Hunters with open transoms will not take kindly to being slowed down strongly by a big drogue and that the helmsman will get his or her feet washed every few minutes. Maybe no big problem in the tropics but not something I would try at much higher latitudes. In that case My preference would go to a parachute-type sea anchor, even if one may be unable to retrieve it later. (4) If you stay within the Tradewind belts -- and well outside the hurricane/cyclone/typhoon season -- your chance of ever having to deal with breaking seas for more than a few hours is very small indeed. As soon as you venture into the Variables (regardless of whether in the Northern or Southern hemisphere) your chances of running into full gales and worse multiply very fast. In some areas, e.g. North of New Zealand, you may run into "weather bombs" (e.g. the Queen's Birthday Storm) well outside the cyclone season (though rarely inside the Tradewind belt). (5) Heavy-built, slow cruising boats with poor upwind capability have little choice but to make their Easting by leaving the Tradewind belts and venturing deep into the Variables in search of favorable westwinds, mostly associated with depressions. So, yes, they get to see a lot of heavy weather (and thus have to be able to take it; it is essentially a self-fulfiliing prophecy)). Don't let their stories scare you; just make sure you are able to go to windward in the Trades and only enter the Variables when unavoidable; e.g. when returning the US, Canada, Europe, New Zealand, South Africa or Southern Australia. This is why so many passagemakers or circumnavigators say that they encountered the worst weather upon leaving or coming home...... See yah out there! Flying Dutchman
(1) The CSF (Capsize Screening Factor; if I remember well) -- rather than the term Capsize Risk used by the fuzzy logic author cited by Tom -- is no more than that, i.e. a little crutch for comparing vessels of very similar design. Hoist a 500 lb chunk of concrete to the top of the mast or put it in the bilge; according to the CSF your capsize tendency will remain unchanged..... (2) IMHO Hunter cabin tops and hatches are not built to withstand breaking waves. So, don't even think about lying ahull in heavy seas. Try to heave to while keeping the bow into wind and waves. If that is no longer possible one has to choose between running off with a drogue (to reduce the chance of pitchpoling or broaching) or keeping the bow into the waves with a sea anchor. We have had to run off a few times when suddenly overpowered by wind gusts. After longer periods of time the helmsman is likely to become fatigued and then let the vessel broach (finkeelers broach much more easily than fullkeelers). (3) I expect that most Hunters with open transoms will not take kindly to being slowed down strongly by a big drogue and that the helmsman will get his or her feet washed every few minutes. Maybe no big problem in the tropics but not something I would try at much higher latitudes. In that case My preference would go to a parachute-type sea anchor, even if one may be unable to retrieve it later. (4) If you stay within the Tradewind belts -- and well outside the hurricane/cyclone/typhoon season -- your chance of ever having to deal with breaking seas for more than a few hours is very small indeed. As soon as you venture into the Variables (regardless of whether in the Northern or Southern hemisphere) your chances of running into full gales and worse multiply very fast. In some areas, e.g. North of New Zealand, you may run into "weather bombs" (e.g. the Queen's Birthday Storm) well outside the cyclone season (though rarely inside the Tradewind belt). (5) Heavy-built, slow cruising boats with poor upwind capability have little choice but to make their Easting by leaving the Tradewind belts and venturing deep into the Variables in search of favorable westwinds, mostly associated with depressions. So, yes, they get to see a lot of heavy weather (and thus have to be able to take it; it is essentially a self-fulfiliing prophecy)). Don't let their stories scare you; just make sure you are able to go to windward in the Trades and only enter the Variables when unavoidable; e.g. when returning the US, Canada, Europe, New Zealand, South Africa or Southern Australia. This is why so many passagemakers or circumnavigators say that they encountered the worst weather upon leaving or coming home...... See yah out there! Flying Dutchman
seaworthiness statistics
Based on the posts below, I ran the formulas on some common boats to see how they stack up. The two I focused on were the displacement to length ratio and the capsize risk factor. Typically a light coastal cruising boat has a displacement to length ratio of around 200, with higher numbers being better. And a capsize risk factor of 2.0 is considered minimum for offshore work, with anything below 1.7 being very good. Here is what I came up with, using fin keel statistics: Catalina 36: 218, 1.92 Catalina 380: 236, 1.79 Catalina 34: 201, 1.99 Hunter 356: 217, 1.74 Hunter 386: 212, 1.95 Beneteau 331: 209, 1.82 Beneteau 361: 190, 2.07 Crealock 34: 333, 1.63 Crealock 37: 338, 1.65
Based on the posts below, I ran the formulas on some common boats to see how they stack up. The two I focused on were the displacement to length ratio and the capsize risk factor. Typically a light coastal cruising boat has a displacement to length ratio of around 200, with higher numbers being better. And a capsize risk factor of 2.0 is considered minimum for offshore work, with anything below 1.7 being very good. Here is what I came up with, using fin keel statistics: Catalina 36: 218, 1.92 Catalina 380: 236, 1.79 Catalina 34: 201, 1.99 Hunter 356: 217, 1.74 Hunter 386: 212, 1.95 Beneteau 331: 209, 1.82 Beneteau 361: 190, 2.07 Crealock 34: 333, 1.63 Crealock 37: 338, 1.65
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