Wavesize vs danger on a sailboat

[Reefmagnet]

Another thing to keep in mind with any seas is the phenomenon of "synchronization". This refers to a vessel's relatively excessive rolling or pitching when it starts to resonate to the frequency of the waves. I'm sure most people at some point have stood on the deck of a (mono-hull) boat and introduced significant rocking simply by shifting weight slightly from side to side and seen just how much even a heavy boat can be made to roll. This demonstrates synchronization. Due to this effect, it is possible for even small seas to really get a boat rockin' and rollin' to the point of being dangerous. The fix for this is simply to change course or speed.

[GILow]

This all reminded me of our delivery trip home. A new crew member joined us at Robe in South Australia. At his first spell behind the wheel he was faced with a lovely 5 to 6 meter swell coming from abeam on the port side. We had all become used to them by that part of the journey and we took a terrible delight in watching his face as the first one approached towering over us and looking like it was going to swallow us up. Of course he reacted, put the helm hard to port and took it bow on, white as a sheet. Of course we all looked at him, shook our heads and asked "why'd ya do that?!"

The point being, as others have said, wave height is only a small part of the equation. With a wavelength of, I dunno, maybe 40 or 50 meters, that swell made for some of the most superb sailing I have ever had before or since. Took the new guy a good hour on the helm before he relaxed and took them on the beam without flinching.


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[Don C L]

Originally Posted by FamilyVan

I've been pondering this much of the afternoon (in between working hard of course). I've been considering the hydrodynamics and is a full keel better in big beam seas? Or is lateral resistance actually a disadvantage in a beam sea? Could it cause the boat to trip like a buried gunwhale or leeward dagger board on a cat? Is this a point of sail where a fin might outperform?

I don't know the answer, this is a question, not an answer.

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A fin keel boat, being faster, has a much better shot at surfing out ahead of a breaking wave as long as it is not right on the beam of course. I don't think it is catching the rail that is the issue, it is that the hull is slammed on the windward side just as it loses all its flotation (water) on the leeward side and the boat falls into the trough. How far the boat will heel then is more a matter of center of gravity, ballast weight, beam, thus righting moment, regardless of keel design, I THINK.
oops, I see, you didn't mean catching the gunwale, you meant the length of keel and the great lateral resistance of the keel being the catch. I am not positive but I think we are not talking about a boat being pulled from the beam but falling to the upper part of the hull, i.e. a greater angle. I suppose all other things equal, same hull shape, same center of gravity, same righting moment, just different keel shape, the fin would heel more quickly and right itself more quickly. Naval architects out there???

[FamilyVan]

Originally Posted by Don C L

If the wave was standing, he must have been moving, with the current. In that case not only does the wave lift the downstream rail, it will catch the upstream rail, push it, and continue the boat's roll all the way over.

Ya, I guess he might have been falling off to port and hit a curler on the port side which flipped him, I wish they had a diagram in the report.

One thing that caught my eye was the coxuns 363hours of sea time, in other words GREEN, probably shouldn't have been out alone, never mind training other people in a 12 knot current and 12c air temp.

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[DumnMad]

Here's a quote from a US Coastguard report on drogues.

"The incidence of breaking wave capsizing decreases sharply with an increase in boat size or displacement. Many 30 to 40-foot boats have been capsized but very few boats over 60 feet have been capsized by a breaking wave. It is apparent that there are few breaking waves with enough momentum in the crest to drive a 60-foot boat up to wave speed."

[Don C L]

Familyvan, Could be his inexperience but unless you have experience in river travel you may (somewhat) forgivably underestimate the ability of a standing wave to capsize a boat. BUT I will check the link to see...

[korrigan]

Originally Posted by Don C L

Naval architects out there???

This is the sort of conversation where Jeff H would have been very helpful.

I wonder where he's gone. Perhaps chased off the web because he couldn't say nice things about every boat.

In any case this subject has been well studied but it seems as if it's like global warming and everyone is an expert.

I debated countless hours with Jeff and I read and re-read Marchaj's book until I decided it was mostly mistaken and had a bias against fin keel boats. I believe the flaws in his arguments were well considered over the years in various places and it was shown conclusively that fin keel or full didn't really make a difference on resisting a capsize and that the over all factor for increasing rollover resistance was the size of the boat.

The fact is that Marchaj's book was written in response to the 1979 Fastnet and was an attempt to pin the blame on the fin keel rather than poor construction, or poor seamanship. He set out with an agenda and didn't do an exhaustive study that held up over time.

The basic argument is that a boat with a high roll moment of inertia won't tip over because it's too slow to respond to the wave surface while the fin keel boat with a low rolling inertia would. The reader is asked to believe that boat that is less stable is safer at sea.

My personal experiences as a boat owner are with two 10,000lb boats. One a full keel 32 footer that was strong as an ox and sailed fairly well off the wind. The other is Korrigan a 36 foot fin keel boat. I would say based on sailing both boats in the ocean and in sheltered waters that it was far more likely that the full keel boat was going to roll over than Korrigan. From my personal experience there's no comparison in seaworthiness between the two boats.

So all that rolling inertia etc sounds good but it doesn't represent my experience. I'll take the fin keel and high ballast displacement ratio every time. That other stuff sounds rather hypothetical.

In any case there are quite a few discussions around this topic that can be found on various forums.

I also think it's interesting to consider Dudley Dix's thoughts and experience. He should know after all. He wrote an interesting article about this for Professional Boat builder a while back but I'm not seeing a link right off.

[FamilyVan]

Originally Posted by Don C L

Familyvan, Could be his inexperience but unless you have experience in river travel you may (somewhat) forgivably underestimate the ability of a standing wave to capsize a boat. BUT I will check the link to see...

Don, I do have a little bit of experience with river travel and standing waves, as well as with the make and model of the boat involved in the incident.

I thought the incident might add something to a discussion on waves and their tendency to sometimes capsize vessels in certain conditions and not capsize them under certain other conditions.

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[korrigan]

Here's the discussion by Dudley Dix on water ballast and capsize. I think it's different than the one in PBB but discusses the same topic.

Dudley Dix Yacht Design: February 2014

I've personally put a lot of thought into this. Which is the best I can do since I'm not an architect.

I believe that something heavy with most of the boat below the water line and a decent hull shape is probably the best for surviving the ultimate storm.

The idea is to sit on a lower layer of the water and let the surface wash over you as much as possible.

But as I mentioned elsewhere just being heavy and old fashioned looking isn't enough. The list of ultimate survival production boats I'm aware of is rather short. The Westsail 32, Ingrid 38, and Alajuela 38 come to mind as authentically bullet proof survival boats that actually are what they are made to look like.

[Don C L]

Originally Posted by FamilyVan

Don, I do have a little bit of experience with river travel and standing waves, as well as with the make and model of the boat involved in the incident.

I thought the incident might add something to a discussion on waves and their tendency to sometimes capsize vessels in certain conditions and not capsize them under certain other conditions.

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oops, hope you didn't think I was implying YOU didn't have river experience, only the skipper mentioned! One thing about that boat, if I understand the description of the design, is as its center of gravity is pretty high and its ballast (people in this case?) being movable probably fell out offering little to the righting force. All speculation 'cause I still haven't seen the link yet.

[Nicholson58]

We added the Chicago - Mac in 81 and encountered waves in excess of 30 feet with sustained wind above 70 knots. Heritage One Ton. We kept strict attention with severely reduced sail. Wind was on the nose so we were sliding in our own slick. Several near disasters in the fleet but no loss. Just rudders and masts. None breaking - just very long period monster rollers.

[FamilyVan]

Originally Posted by Don C L

oops, hope you didn't think I was implying YOU didn't have river experience, only the skipper mentioned! One thing about that boat, if I understand the description of the design, is as its center of gravity is pretty high and its ballast (people in this case?) being movable probably fell out offering little to the righting force. All speculation 'cause I still haven't seen the link yet.

I didn't do a link, just a Google search parameter in post 27

Centre of gravity is quite low in a Hurricane (500 Liters or so of gasoline below deck), investigators, definitely cited inexperience but not design as a contributing factor.

Its a good read, it made me think any way.

http://www.TSB.GC.can/eng/rapports-r...2/m12w0070.asp

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[Steadman Uhlich]

Originally Posted by Nicholson58

We added the Chicago - Mac in 81 and encountered waves in excess of 30 feet with sustained wind above 70 knots. Heritage One Ton. We kept strict attention with severely reduced sail. Wind was on the nose so we were sliding in our own slick. Several near disasters in the fleet but no loss. Just rudders and masts. None breaking - just very long period monster rollers.

That would be something to see and would make a lasting impression.

[Don C L]

Originally Posted by korrigan

Here's the discussion by Dudley Dix on water ballast and capsize. I think it's different than the one in PBB but discusses the same topic.

Dudley Dix Yacht Design: February 2014

I've personally put a lot of thought into this. Which is the best I can do since I'm not an architect.

I believe that something heavy with most of the boat below the water line and a decent hull shape is probably the best for surviving the ultimate storm.

The idea is to sit on a lower layer of the water and let the surface wash over you as much as possible.

But as I mentioned elsewhere just being heavy and old fashioned looking isn't enough. The list of ultimate survival production boats I'm aware of is rather short. The Westsail 32, Ingrid 38, and Alajuela 38 come to mind as authentically bullet proof survival boats that actually are what they are made to look like.

If I am reading the graph correctly, the righting moment for the Didi 950 shows that at by 130 degrees of heel angle the boat has a negative righting moment. Yikes, that is not very encouraging! I remember reading Olin Stephens saying something about the importance of a boat having a positive righting moment through 150. In other words, aside from being upside down 180 degrees, the boat should always be eagerly trying to right itself. This discussion came up in talking about his narrow beam designs and whether their tippiness meant they might be more likely to capsize, which they aren't. I know I keep coming back to this boat, and I have never been on one (would love to) but I'd like to add the Pearson Rhodes 41 to your list of boats with a low center of gravity and not a lot of freeboard, and a narrow beam, as a good survival boat. Any Rhodes 41, Reliant or Bounty II owners out there? How do they do?

[Sailor SAN]

It's a super dynamic situation and one that is complicated to model, calculate or predict- A vessels "dynamical" stability can be referenced to areas under the GZ curve however synchronized and other dangerous motion (breakers) can develop. Some of this even as a result of length and not beam. And will be affected by sail plan and trim. Who here has been rolling rail in and out while the only sea is running is directly on your head? "Parabolic Rolling" is a phase , , perhaps wrongly, I've heard to describe this and is looked upon as a function of a vessels design and condition and radius of gyration about the motion axis is question. In a rolling situation (wx on beam) if your vessel is shipping lots of water it's bad. If it's not shipping water and it's deck is aligning parallel to wave faces it's now synchronized and worse (very dangerous). If you feel unsafe, you can always detune synchronicity with a course alternation. It's harder however to deal with breakers.... Always best to stay clear of translatory choppers. This being said I always find myself in the "wrong" length of wave for by boat and desired heading. I think it's Karma coming back to get me.