How small of a wave can capsize my boat?

D

Doryman

New member
Found this article on Three Sheets Northwest. The takeaway concept is that, for powerboats, a broadside wave with height 30% of the boat's length gets you into capsizing territory. This translates into a 6' wave, trough to crest, to capsize a 20' boat.

I'm looking forward to Bob's take on this.

Warren
 
I'm not sure this applies to C-Dories so much as to taller boats. As I have sat in many marinas and have yet to see a boat that you can sleep on that sits any where near as low in the water than a C-Dory 22,23 or a 25. Not being top heavy, combined with having a relatively flat bottom and the low center of gravity which lets them slide off of waves. Being light and responsive to control inputs does not hurt.
D.D.
 
I am sure there are factors other than those mentioned in the article that can affect the question, such as speed of wave development and travel, as well as the actions of the operator. We have a little stretch of river down here called Clatsop Spit that can pick up a boat and toss it like a pancake with waves having just a couple feet of surface height. On the WA side outside the north jetty is Peacock Spit which can do the same trick to 60' crab boats with waves nowhere near 30% of the vessel's length. I think that if a wave is breaking as a result of its own power and the sea floor beneath it then all bets are off and the math won't keep you safe.
 
Wil-C and Tyboo both give excellent information. If you want to peruse the subject further, I recommend tow of my favorite books: "The nature of Boats" by David Gerr, and Oceanography and Seamanship by William Van Dorn.

The steepness of the wave's face, if it is breaking, the period (distance between waves) all enter into the factor. How the boat operator takes the wave also is extremely important. A 6 foot steep, breaking wave is very significant. Under the right circumstances it could roll a C Dory. Also the depth of the water is important--shoal water, the wave is more likely to crest and break.

The weight aloft--and center of flotation/center of gravity issues are also important. (This is when I get a bit concerned when I see big dinghies with other gear on a cabin top of a C Dory 22). I have seen 25 foot boats with a full tower which will capsize fairly easily with a couple of people up there.

I have been on large sport fishers where it gets frightening, with several people on the flying bridge in heavy seas.

The chances of an actual capsize of a C Dory are very small--and a lot has to do as earlier suggested with the ability of the boat to slide sideways, and not trip and roll.
 
I read the article and it specifies breaking waves of one third the length. If I were being hit by 7 foot breaking waves on the beam it would mean my motor was dead as there would be no way that would be an allowable situation. I guess it could happen in a following sea causing a broach. I would be very afraid and calling the coast guard with my location.

Further the article says at 60% of the length of the boat in breaking waves you are probably going over...duh. A 12 foot breaking wave broadside is may day territory for sure. Probably issued from your portable vhf right after you set off your plb or eprib.

In one way this seems rather obvious. Waves steep enough to be breaking over three feet would be very scary. On the other hand 8 foot waves with a 20 second period even broadside are kind of fun if you are not subject to mal de mer.

Ron
 
If that kind of wave height was in the weather forecast my boat could not capsize. It would already be on the trailer and either be in the driveway or headed for there. I don't mind big ground swells but breaking six to eight footers or more I'm so out there. Once the ocean starts breaking white I pay close attention. Running inlets where wind and tide are opposite can cause some interesting scenery. While up in the Apostle Island area I had read to keep a close eye to the south as that is where most fast approaching storms come from. One day we were at Little Sand Bay and I saw some dark clouds coming up from the south, I headed for the dock at warp speed. The ole gitch a goome is not getting me like it did the Edmond Fritzgerald.
D.D.
 
Yikes who is going to get sideways to breaking waves. Not good at all. I most always run into or with. At all costs avoid going sideways in breaking swells. Larger ships can take some sideways as I saw our cutter take a pretty good one on the side while entering our harbor while towing a fishing vessel back. I would not want to set myself up for one on the side.

A good thing about small boats is that when they lose power, the wind and swell usually pushes them with the swell. At least in our area wind and swells usually run the same direction but not always. I don't usually go out in confused sea state. To find out if your boat will go with the flow kill your motors and wait a few minutes to see your position. A larger vessel is going to turn sideways. Once flowing you should be fine until you approach obstacles. Have plenty of anchor line, a good anchor, get on the horn and let those who listen know that you need help.

One of the first things I tell a new surfer is to not get the surfboard sideways to a breaking wave. It can hurt and these things only weigh 20 lbs.
 
How a vessel will lie/ride in heavy seas depends on both her underwater hull configuration, and the superstructure. The C Dory has little under the water, except the outboards aft, so it will tend to slide bow off, with some drag from the engines. There is a fair amount of windage from the C Dory cabin and hull form. One of the features of a semi dory hull is its flat bottom and the ability to slide sideways in breaking seas--and that is what saved the Banks Dory's in heavy weather (granting there is some difference in a Bank's Dory and a C Dory.

Most offshore small vessels will currently carry a Jordan type Series Drogue. This is the opposite of a sea anchoring that it is steamed from the stern, and is multiple small drogues. The sea anchor might be more appropriate for the C Dory--because of the large cockpit, which is not self bailing and relitatively low freeboard aft with the motor well-which would hold the boat head to wind/seas.

We sailed for 6 days in hurricane force winds on one Atlantic Crossing where the breaking seas were documented at close to our Length Waterline--or 45 feet. The boat had a small storm sail forward, to keep the bow off the wind, and the engine ticking over, to give water flow (even in turbulent breaking wave crests) across the rudder so that the auto pilot would keep the boat on course. We did take a roll to 90 degrees, and if we had not been in a sailboat, with a heavy keel, we probably would have gone all of the way over. There are tactics which one can use to prevent catastrophic results.
 
I've been over to 45+ degrees in a destroyer in the North Atlantic. No ice up high or it would have been a lot worse. We did lose both boats on the starboard side (motor whaleboat and personnel boat), the accommodation ladder, life lines and deck capstan on the fantail. Also broke out bridge windows and did other misc damage. It was not a good night.

We had ballasted empty fuel tanks which was an extreme measure but necessary. Very close to flooding low ammunition magazines.

The waves were nowhere near as high as the length of the ship or they'd have been 500+ feet high!:disgust

Charlie
 
I recently had an experience with a freighter wake that really woke me up to the variation in wake forms. Funny, physics being what it is, one would think that water only stacks in certain ways and waves behave in predictable patterns. Yes, even the washing machine irregularly irregular, and wave on top of and across wave.

Crossing Juan de Fuca Strait on a glass flat day, ran across a small freighter, moving slow that created a very unusual wake. A very low front rise, followed by a very deep hole, with both the front and back walls of that hole very steep, and the cap of the following wall was breaking into the trough. That breaking top was again followed by a very deep, (again about 6 feet), steep hole, with a following steep wall, and then back out on top it was nearly flat and to glassy in about 3 boat lengths.

I was very surprised by the depth of the "holes" and the lack of height on the front wave. These were not like a sinusoidal pattern generally produced by the ships I see and the wakes I cross. When the freighter crossed my bow it was a few feet over 1 mile (per the radar rings) and when I crossed the wake I was nearly a mile and a half from it.

Had I not seen this coming, and been watching for it, I would have crossed that wake at a 45 degree angle and it would have been a very rough pitching. As it was, bow on and dead slow, we bobbed through it OK, OB props in the air for a while but all in all, OK.

Had I not seen this and caught it on the beam, I would have, at best had a terrible mess to clean up, IF I came out right side up. It would have been a very violent pitching for sure. Sure would like to think the C-Dory would have sloughed it off, and gone on home fine.

You can read the more graphic story on this thread:

http://www.c-brats.com/viewtopic.php?t=18694&highlight=

Harvey
SleepyC :moon
 
Years ago "captd" Darrel, posted pictures of the 22' C-Dory that he owned at that time. The pictures are still in captd's photo album, although, I'm too computer challenged to transfer them on to this thread.
The pictures show his 22' being tossed around in the surf (ie: Big Waves) on the coast of Mexico after a weld broke on his anchor. The C-Dory did not roll and if I remember correctly only very minor damaged occurred to the vessel.

In viewing the photos he posted, it would appear that the wave pictured is well in excess of 6'.

Although, I do remember that the current vessel "Scout" that is owned by one of our C-Brats was a victim of rolling in the surf off the Oregon coast. This vessel has been lovingly resurrected by its' current owner.
 
Thanks for posting those pix Roger. On second viewing, perhaps they are not 6 footers, but at least a viewer gets an idea of what the C-Dory was able to endure.
 
I read an article in this morning's newspaper that a "22 Foot Cabin Cruiser" rolled in the surf at Ocean Shores on Monday 9/2/12. It stated that the 4 occupants tossed overboard were safe and that it was Captained by a 64 year old man. Apparently the vessel lost engine power.

Just "google" Boat Capsize in Ocean Shores, Washington.

I hope it wasn't one of our C-Brats!
 
I found pix of the boat that capsized in Ocean Shores on the "North Coast News" website., fortunately all aboard were safe. And it was an "Olympic", not a C-Dory.
 
I saw a photo of the boat in this article. Here's the image
boat-capsize-1024x768.jpg
.

It's not a C-Dory.
 
That boat looks pretty good, from here, to be a roll over. The bow rail looks OK and looking closely, the one antenna is still sticking up, and so is the anchor light, although there may be an antenna down on the port side. It is right side up. All good ways to be after a roll. Although the articles said they say it roll, I wonder if they saw it pitch, and then the ensuing splashing hid the righting. Could have been a lot worse..... and everyone walked away.

Harvey
SleepyC :moon
 
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