Seaworthiness

Discussion in 'Stability' started by Guillermo, Nov 26, 2006.

  1. Guillermo
    Joined: Mar 2005
    Posts: 3,644
    Likes: 185, Points: 63, Legacy Rep: 2247
    Location: Pontevedra, Spain

    Guillermo Ingeniero Naval

    Let's discuss here what we understand by 'seaworthiness', as it seems everyone has his/her personal point of view about this.

    To begin with, on my side I propose to discuss seaworthiness of said cruisers derived from racing classes, as the Class 40. Let's discuss the Pogo 40, i.e.

    One of the problems I see with this kind of boats when brought from the racing circuits to the cruising grounds is that most cruising crews are not trained to withstand the punishment of countless not sleeping and miserable hours (or watchs scheme) as to ride (and steer) seas in a gale or storm at high speeds, the only way this kind of boats are able to defend themselves, as they are not conceived to hove to.

    Very deep and narrow keels stall easily at slow speeds, so you need to keep the boat moving at an extenuant pace, as you have to keep sailing at an speed and course adequate to avoid resonant rolling. And as speed is high, reaction time is less, this calling for a demanding steering.

    A 100 ft long wave (30,48 m) travels at 13,40 knots so you have to keep the boat planning all the time. And this 100 ft sea's waves beging to break when they reach 4,35 m (14,29 ft). Realize that this sea state is reachable with only force 9 winds, maybe even as low as 8 depending on the particular conditions at the place.

    On the other hand, the Pogo 40 has a max beam of 4,40 m (and well extended aft), pretty much the same as that wave height. So it would be posed in a very dangerous situation if by any reason gets caught beam to seas. With those forms and a deep and narrow keel, probably she will tend to get like that if stopped. And we have to remember that with that wide beam going all well aft and her small over deck volumes, inverted stability will be not negligible, indeed.


    The period of encounter with waves is given by formula:
    Te = Lw / (c-Vs*cos (beta))
    being Lw, the wave length, c the wave speed, Vs the boat speed and beta the angle of encounter measured from the stern.

    So for beta = 90º (beam seas), the period of encounter is the same as the waves period, as expectable. In this situation the 30.5 m long - 4.3 m height waves we are considering, have a period of 4.4 seconds. For beta = 60º, the period will be Lw/(0,5 * c), so the period of encounter will be 8.8 sec, twice the wave period. For a boat with a natural period of 4.4 sec this wave period and its armonics may induce resonant rolling....

    ....Ooops! Enough for today! Time for some obligations. I'll work deeper on Pogo's 'rolling' numbers and other matters next week.

    Cheers.
     

    Attached Files:

    3 people like this.
  2. marshmat
    Joined: Apr 2005
    Posts: 4,127
    Likes: 146, Points: 0, Legacy Rep: 2043
    Location: Ontario

    marshmat Senior Member

    Good points Guillermo.

    Firstly, as to the Class 40. I'm really starting to like these boats, and I'd seriously consider getting one- as a cruiser, not a racer- if I had $250k to toss around on such things.

    Then again, here on the Great Lakes, speed- and instrumentation- really can give some safety. Knowing bad weather is on its way, a quick boat can get to safe harbour here in short order. It's common in the ocean to go a few hundred miles out of your way to avoid a storm; here on Lake Ontario, going ten miles off course will often put you in a corn field. Better to throttle up and get to shelter, if possible.

    In an ocean cruiser, I'm not so sure if speed is such a valuable tool anymore. When the seas get big and the winds get bad, I kind of like the idea of dropping the drogues and curling up in the saloon with a good book, sound in the knowledge that my boat can take whatever the waves will do to it. Given all the crap (40' isocontainers, etc) that floats around in those storms, I'm not sure blazing ahead at 20 knots is the best course of action for a cruiser. For a racer, yes, but they're in it for the adrenaline; I go boating for fun.

    So I've got a few points I'd consider when trying to see how seaworthy a boat is. Among them:
    - How much damage does it take when hit broadside by a 20' wave? How much when knocked down? How much when capsized? There must be no structural damage or losses of watertight integrity in any of these cases.
    - If it capsizes, does it right itself? Or does it remain stable and afloat? Or does it simply sink? The first two are good; if the third option is dominant, I'm not interested.
    - Is the instrumentation adequate to avoid the crappy weather in the first place? This is always my preferred option.
    - If caught in the bad stuff, how will it behave? Will it lie bow to the breaking waves, or beam-to, or quarter-to? A comfortable motion when hove to is definitely a good thing. Does it buck and kick and bounce, or does it roll gently in the breakers?
    - Say there's a complete failure of the propulsion machinery, no fuel, and the battery's burnt out. What changes?
    - Say we hit something anyway. An isocontainer, perhaps. How big an impact can she take, before something gives? What gives- the stem? the keel? the hullsides? How long does she stay afloat afterwards?

    Any boat can go out in the ocean. An 8-foot Pelican pedal boat can go out in the ocean. The question of seaworthiness, IMHO, has nothing to do with whether or not a boat CAN go out there, or what it can do while it is out there. To me, it's a question of what it does when something goes wrong out there.
     
  3. RHough
    Joined: Nov 2005
    Posts: 1,792
    Likes: 61, Points: 0, Legacy Rep: 793
    Location: BC Summers / Nayarit Winters

    RHough Retro Dude

    I think Guillermo has put up a moving target. :)

    Trying to define "Seaworthy" in broad terms will be very difficult.

    For me, a boat is seaworthy if it has the ability to survive any conditions for which it was designed.

    A cruising design intended for being sailed across oceans by two people (the basic cruising couple) will have a different set of characteristics that make it seaworthy than an ocean racing boat. The storm survival strategy must be matched to the number and experience of the crew. Thus, a crewed boat like a VO70 does not need the ability for passive survival. A Sundeer 65 absolutely must have that ability.

    I don't think you can condemn a design just because it's primary survival response is active. I think you can condemn a designer or builder or salesman that does not make it clear that the boat requires active survival response when the boat is marketed/sold.

    Then again, can the response of the Pogo 40 while laying to a series drogue be predicted? Just because it doesn't heave to shouldn't mark it as un-seaworthy.
     
  4. marshmat
    Joined: Apr 2005
    Posts: 4,127
    Likes: 146, Points: 0, Legacy Rep: 2043
    Location: Ontario

    marshmat Senior Member

    I'd agree with that.... I might add, 'able to survive any conditions that may arise in the type of use for it was designed'. A boat designed for calm lakes may still be caught in bad weather on one. Lake Simcoe is a prime example; it can be calm enough for 14-footers at a mile out, but within an hour it can churn up a storm that'll give those boats a run for their money.
    Seaworthiness has a lot to do with knowledge... and if a boat designed for inshore use, or for offshore use with a big and competent crew, is marketed as being seaworthy in offshore conditions with only a few crew... that's very irresponsible on the part of the sales team.
     
  5. rayk
    Joined: Nov 2006
    Posts: 297
    Likes: 15, Points: 0, Legacy Rep: 146
    Location: Queenstown, NewZealand.

    rayk Senior Member

    Re seaworthiness I expect a vessel and crew to weather a gale with no damage or injuries. Up to storm force may cause damage but shouldnt prevent the vessel from making port unassisted, unless the crew has a medical emergency. Conditions exceeding storm force I think you get what youre given. Who can draw/build/endorse any cruising yacht gauranteed to survive F11, F12...

    A cruising yacht should be expected to float after a storm.
     
  6. rayk
    Joined: Nov 2006
    Posts: 297
    Likes: 15, Points: 0, Legacy Rep: 146
    Location: Queenstown, NewZealand.

    rayk Senior Member

    When the crew is incapacitated that is the true measure of seaworthiness. Seasick or wont leave the bunk? Broken arm? These survivors will praise the boat if they survive.
     
  7. Crag Cay
    Joined: May 2006
    Posts: 643
    Likes: 49, Points: 0, Legacy Rep: 607
    Location: UK

    Crag Cay Senior Member

    A seaworthy vessel is one that does everything the crew ask of it, and not only brings them back to port safely but also 'happy'.

    This definition combines the technical necessity for the boat to have certain qualities and the fact that different sailors have extremely different ideas of what they want a boat to do in bad weather. Some, for example, want to speed away while other prefer to be cosseted below while hove to. But as long as the boat and the expectations of the crew are in harmony, then everyone's happy.
     
  8. Guillermo
    Joined: Mar 2005
    Posts: 3,644
    Likes: 185, Points: 63, Legacy Rep: 2247
    Location: Pontevedra, Spain

    Guillermo Ingeniero Naval

    Interesting thoughts from all of you.
    Just a question before going to bed: Shall a dismasted cruising sailboat (ocean crossing intended and defined as seaworthy) in gale/storm conditions still be able to come back up again (in a reasonable short time) from capsizes...? Or just keep afloat, even upside down...?
    Cheers.
     
  9. rayk
    Joined: Nov 2006
    Posts: 297
    Likes: 15, Points: 0, Legacy Rep: 146
    Location: Queenstown, NewZealand.

    rayk Senior Member

    I think that a seaworthy boat is the crews last line of defence. Running is a 'tactic' only. When the crew runs out of options the time has come for the crew to experience formulas and ratios that the designer agonised over for weeks at a time.
    A crew that has taken a hiding in a stout yacht that can handle it is unlikely to be making design revisions for taller rig, lighter scantlings etc...

    Right side up for a keel boat, absoloutely! Skimming dish monohulls that float upside down are slower than a cat with the same design characteristic. Get a cat.
     
    1 person likes this.
  10. MikeJohns
    Joined: Aug 2004
    Posts: 3,176
    Likes: 198, Points: 63, Legacy Rep: 2054
    Location: Australia

    MikeJohns Senior Member

    A seaworthy vessel remains stable watertight and controllable in all expected conditions.
    To this we need to add a level of sea-kindliness and habitability depending on the expected use, the experience and tolerance of the crew.

    In severe conditions even a hardy crew can be considerably incapacitated by the high accelerations generated in an un-sea-kindly boat.

    People have mentioned heaving-to, a family oriented offshore cruising boat should be able to heave-to or even lie ahull in some comfort. The modern racing boat has a more limited number of options since it is inherently less seaworthy (by the definition above).

    Sea-kindliness is hard to achieve without weight, limited beam, and inherent damping . Ted Brewers “comfort formula” is a pretty good indicator of the sea-kindliness of a vessel. I think your Pogo 40 looks to be a hull-form that works for racing 60 footers but is a poor offshore design for a 40 foot offshore boat racing or otherwise ( we have yet to see the stability curve so this is conjecture).

    For ocean work a cruising person buying a lightweight racing form gets inadequate stability, compromised strength and rigidity and poor durability and an abysmal level of comfort in bad weather.

    Heavier boats represent a good platform to design a strong durable and forgiving hull with a good interior volume and load carrying capability, that also perform well. Forgiving means being able to sail away after you have been pulled off the reef you hit at 7 knots, it means not exhausting the crew in heavy weather, it means coping with tons of gear poorly stowed without becoming dangerously unstable or slow. It means being able to heave-to for periods to affect repairs at sea or await daylight.

    I asked a fast talking salesman at the Brisbane boat show what he thought the chances were of being able to heave-to in a fin/bulb beamy hull, he replied “ No one heaves to any more” This represents to me the willful ignorance of the whole industry, the lack of real seagoing experience once again, at all levels from the designer to the purchaser.

    Too much gets sacrificed to the great speed god. How many designers today would happily sacrifice speed for comfort and a higher level of offshore safety? How many actually design with comfort and safety priorities?

    As for inverted mono-hulls. Where has seaworthiness gone when designers consider escape hatches for upturned mono-hulls as a safety feature!
     
    1 person likes this.
  11. M&M Ovenden
    Joined: Jan 2006
    Posts: 346
    Likes: 69, Points: 38, Legacy Rep: 527
    Location: Ottawa

    M&M Ovenden Senior Member

    Mike
    I couldn't agree more with your post. When things go bad they usually also goes fast and it is a nice feature to be able to slow everything down. It's only a few years ago I found out how nice it was to have a good “heaver”, and it's my boat that showed it to me. A short story (it all happened very fast) made longer. We were sailing our small schooner in a nice breeze all canvas up when my husband called me up on deck ( on a really hurry sort of tone). Some not so good looking squall appeared, it was coming very fast on our side. We had no time to reduce canvas and had to take it as is. As we were both stunned, shame on us, the wall of weather hit us (this happened within seconds) . We hadn't had time take any decision, barely to think. The gust caught the main sail, threw the *** away from the wind and everything stopped, the boat was heaved to. My husband and I just looked at each other stupid, we were double stunned, our sweet little vessel had taking care of it all. Buckets were poring, wind howling but on board everything was stable and amazingly peaceful, nothing more for us to do, we went down in the boat and waited for it to pass.

    On the definition of seaworthiness I believe the traditional sailing work boats of the late 1800-1900 have lots to show. Most of those boats had to go in every weather and there was no rescue to expect, the last resource was always the boat itself. They matched amazingly well there purpose of there job, without giving to much safety away. Just think of pilot vessel, longliners, high sea or coastal fishing boats, dredging boats, all the different cargo boats depending where they were meant to go ( down the coast or half way around the world) and there characteristics.
    Of course not many of them could roll right around and back but why would've they if they couldn't expect rescue and if with the construction methods of the time the vessel would unlikely withstand it. There best bet was to stay up right, no hatch under the hull.
    Anyway, to obtain a fast but safe yacht it's interesting to look at pilot boats. For a maneuverable but roomy cruiser the smaller offshore fishing boats are good examples. And so on.
    Not saying we should all go back to old rigs and wood hatches or stop evolution, but all the boats we tend to agree are the most seaworthy have shapes and characteristics that are very comparable with what our grand fathers came up with from there grand fathers experience. All our numbers and curves are very interesting but the true seaworthiness was defined before modern engineering.

    Murielle
     
  12. Guillermo
    Joined: Mar 2005
    Posts: 3,644
    Likes: 185, Points: 63, Legacy Rep: 2247
    Location: Pontevedra, Spain

    Guillermo Ingeniero Naval

    Nice post Murielle,
    let me quote for you Marchaj at his book 'Seaworthiness the forgotten factor':

    "Admittedly, yachtsmen interested in small cruisers liable to be caught occasionally in what is, for the size of the boat, very rough weather have to things in common with fishermen: the need for reasonable comfort (seakindliness) and the hope of expectation of getting there and back in safety (seaworthiness)."

    Mike and rayk:
    I fully agree.

    Cheers
     
  13. Stephen Ditmore
    Joined: Jun 2001
    Posts: 1,388
    Likes: 44, Points: 58, Legacy Rep: 699
    Location: Smithtown, New York, USA

    Stephen Ditmore Senior Member

    I had a personal experience 20 years ago that makes me skeptical of heavy displacement boats' claim to be more comfortable in waves. I was visiting Hawaii and sailed as a guest on board a thirty-something cruiser.... perhaps a Pearson, in a regatta. We were not expecting to be competitive, but in the steep Hawaiian swell not far from prime surfing areas our boat pitched horribly, and couldn't seem to make headway at all. By contrast, the two Wylie Wabbits competing zipped around the course like there was nothing to it, upwind and down, handling the waves with aplomb. We finished the regatta last, nauseous, and impressed with the small ULDBs. http://www.wyliewabbit.org/specs.htm

    I don't have enough experience with the extreme beam light displacement types to know how they handle waves, but I join Steve & Linda Dashew (experienced cruisers) in thinking the narrow ULDBs do very nicely. http://www.setsail.com/dashew/dashoff.html
     

    Attached Files:

  14. rayk
    Joined: Nov 2006
    Posts: 297
    Likes: 15, Points: 0, Legacy Rep: 146
    Location: Queenstown, NewZealand.

    rayk Senior Member

    After an experience like that it would be time to take stock of the value of some of the cruising trade offs that have crept onto a boat.
    Sail furling conversions, accessory radar arch+solar panel array+wind generator, raising the water line, stowage in the ends, fuel and water containers lashed on deck, outboards on pushpits...

    Putting the boat on its lines, and restoring the CG will improve performance and seaworthiness. Makes sailing fun :)
     

  15. longliner45
    Joined: Dec 2005
    Posts: 1,629
    Likes: 73, Points: 0, Legacy Rep: 505
    Location: Ohio

    longliner45 Senior Member

    seaworthyness: does the boat do what it is intended to do in all applicable conditions<<<<example can you make 40 ,,5day fishing trips per year in the gulf of mexico is that what the boat is for ? in all weather? payload?exctra,,,
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.