Seaworthiness

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

  1. MikeJohns
    Joined: Aug 2004
    Posts: 3,192
    Likes: 204, Points: 63, Legacy Rep: 2054
    Location: Australia

    MikeJohns Senior Member

    From my own observations a long sojourn where folks actually carry on happily wandering is more likely in a heavy sailing performance "disaster":) like a Roberts Spray than a light performance-cruiser racer.

    It is not from marketing hype and racing performance that designers like Roberts have become the number one cruising boat designer worldwide. I would also observe that Women tend to have some very firm and often much more sensible views on what constitutes an attractive home than possibly her performance oriented partner.

    IMHO

    Cheers
     
  2. MikeJohns
    Joined: Aug 2004
    Posts: 3,192
    Likes: 204, Points: 63, Legacy Rep: 2054
    Location: Australia

    MikeJohns Senior Member

    Is this a rhetorical statement :confused:

    Read post 61, I am aware of several other boats that also did this. I would be happy to dig up some more data for you. But for a start I would suggest that you look up the Benetau 390 accident reports. Also Spanish and Portugese accident reports would cover their rescue areas and would be a better source of data since that's often where boats get in trouble sailing from europe south.

    Where are the Mini accident and rescue reports?

    You cannot draw conclusions from limited sources.
     
  3. Stephen Ditmore
    Joined: Jun 2001
    Posts: 1,388
    Likes: 44, Points: 58, Legacy Rep: 699
    Location: Smithtown, New York, USA

    Stephen Ditmore Senior Member

    Wouldn't a masthead float also "significantly aid recovery" (i.e. increase LPS dramatically) on a monohull? Perhaps your idea of requiring LPS>135 is achievable, even for existing racers, if masthead floats are allowed.

    You make some good points, and I take the other side & debate them with you partially because it's fun. One thing I agree with you and others about: this business of keels breaking off is inexcusable. A little bit more beef at and a little more thought concerning keel attachment need not dramatically slow anyone down, and the rules, and standards of responsible naval architecture, should require it.

    Though I may not be one of the true believers, I bristle a little where implicit criticism of the designs of Mr. Finot are concerned. His work, taken as a whole, shows him to be a responsible designer of seaworthy and capable boats. In many cases, they've won because they didn't break or get in trouble. So while I don't think his extreme beam racers should be immune from criticism, I think they've been well engineered, and have proven both more successful and safer than some might have predicted.
     
    1 person likes this.
  4. RHough
    Joined: Nov 2005
    Posts: 1,792
    Likes: 61, Points: 0, Legacy Rep: 793
    Location: BC Summers / Nayarit Winters

    RHough Retro Dude

    Guillermo my friend,

    Of course it is not fair to compare a Pogo 40 to a Catalina 30. My point is that "seaworthiness" is a "compared to what" question.

    A leaky bathtub that makes it across a bay is more seaworthy than a paper and glue punt that does not make it.

    While I make no claim that my Befana was designed to be an all weather any ocean sailing boat. I also know that several people (perhaps many) have sailed these boats around the world. I conclude that in the right hands the boat is seaworthy.

    I also think that comparing boats of near equal displacement is a valid idea. I feel that somewhere between 10,000 and 20,000 pounds (~5,000 - 10,000 kg) is a comfortable size for a cruising couple.

    Do we have a base line of what the numbers look like for boats that are the current choice for cruisers? Looking at the list of boats in the latest Atlantic cruising rally would be a good start.

    If we take the average numbers for the current cruising fleet, we have an idea of what numbers are "good enough". I wonder how many qualify as "A" class boats?
     
  5. fcfc
    Joined: Feb 2005
    Posts: 779
    Likes: 29, Points: 28, Legacy Rep: 399
    Location: france,europe

    fcfc Senior Member

    More on this.

    STIX formula is :

    STIX = (7 + 2.25Lbs) * sqrt(FDS*FIR*FKR*FDL*FBD*FWM*FDF) + delta

    Lbs is the main factor.
    Lbs = (2 Lwl + Lhull)/3 It is basically the weigthed average hull length and waterline length.

    All other factors are corrective factors which have a limited range:
    FDS = Dynamic Stability Factor. 0.5< FDS <1.5
    FIR = Inversion Recovery Factor 0.4 < FDS < 1.5
    FKR = Knokdown Recovery Factor 0.5 < FKR < 1.5
    FDL = Displacement Length Factor 0.75 < FDL < 1.25
    FBD = Beam Displacement Factor 0.75 < FBD < 1.25
    FWM = Wind Moment Factor 0.5 < FWM < 1
    FDF = DownFlooding Factor 0.5 < FDF < 1.25

    Delta = 5 if insinkable per the norme, 0 otherwise.

    That's all for sailboats between 20 and 80 ft.

    So the bigger the boat (Lbs), the more seaworthy. You will have trouble to make a 20 ft sailboat as seaworthy as a 40 ft, and same between 40ft and 80 ft. After all, the wind and waves are the same for all.

    But sadly, you will see in this interesting thread from E. Sponberg : http://www.boatdesign.net/forums/showthread.php?t=13657 that the length is also the determinant cost factor. (this for initial cost, not maintenance).

    So, buy the biggest boat your budget allows, it is likely to be the most seaworthy.Do not forget to include in your budget maintenance costs. And do put some liveable items inside.
    That's all.

    MikeJohns spoke of Roberts designs vs performance cruiser-racer. This obey to this simple basic rule. For the same budget, you can have a bigger boat by building yourself a steel hull than with buying a high maintenance cost cruiser-racer.
     
  6. rayk
    Joined: Nov 2006
    Posts: 297
    Likes: 15, Points: 0, Legacy Rep: 146
    Location: Queenstown, NewZealand.

    rayk Senior Member

    Comparisons between boats...

    A hull, deck, and keel built with endurance in mind.
    Fatigue and 'cycles to faliure' are designed out and built out permanently.

    With their finely honed sense of self preservation women understand the difference between
    'unlikely that it will ever encounter those conditions' and 'if the boat encounters those conditions...'.
    Wives also note Roberts marketing is restrained in the use of thin women in bikinis.

    These these two quotes compliment each other.
    Wether its a cruiser or an ULDB a heavier boat stands up to heavier weather.
    On a smaller boat it may be comforting to have self righting demonstrated so ably, but it is also a little wearying.
    Two boats of equal displacement.The long and lean one is the same weight as the short fatty.
    The most obvious difference to my eyes is scantlings.
    • Short good design is even distribution of redundant strength.
    • Long good design is an even reduction of redundant strength.
    Equal displacement is equal amount of materials.
     
    Last edited: Dec 3, 2006
  7. RHough
    Joined: Nov 2005
    Posts: 1,792
    Likes: 61, Points: 0, Legacy Rep: 793
    Location: BC Summers / Nayarit Winters

    RHough Retro Dude

    This is why they market "safe" vehicles with dozens of airbags to women and driving schools to men. :) These are also the crew members that squeal in panic when the boat heels 10 degrees, large multihulls are also marketed to women.

    It's the difference between passive reactions and active reactions. At some point we are allowed to enjoy activities that our mothers consider "too dangerous". :D
     
  8. RHough
    Joined: Nov 2005
    Posts: 1,792
    Likes: 61, Points: 0, Legacy Rep: 793
    Location: BC Summers / Nayarit Winters

    RHough Retro Dude

    Wow.

    I just looked at the entry list for the ACR. The boats in the fleet are getting larger.

    Of over 160 boats planning to make the 2700 passage, the average length is 45+ feet (13.8 metres) If I assume a 38 foot LWL, and a D/L of about 250, that gives an average displacement of over 30,000 pounds.

    For a decent SA/D the average boat needs a 900+ sq ft sail plan.

    Seems a bit large for a normal crew of 2?

    The smallest division in the ACR is Class H, the average length is almost 37 feet. At D/L ~250 the displacement is ~20,000 pounds. Only 700 sq ft of sail are required.

    The Class A boats are above 7 knots average, the Class H are around 5 knots. That is an extra week at sea for the slower yachts. The samll boats will be at sea for 22-23 days, the large ones only 16.

    Interesting to note that the multihulls are sailing almost exactly the same speed as the mono's of equal length.

    Not sure waht all this says about seaworthiness, take a look at the list and see if there are designs that you think are not seaworthy. I note the lack of Pogo40's ... :D


    Entry List
     
  9. fcfc
    Joined: Feb 2005
    Posts: 779
    Likes: 29, Points: 28, Legacy Rep: 399
    Location: france,europe

    fcfc Senior Member

    First, in my post Lbs is the length for stability per ISO rule 12217, not the abbreviation of pounds (lbs).

    And, second, compare thess 2 boats : I agree they are barely comparable. But they just happen to have the same displacement:
    http://andrewsyacht.com/andrws65.htm
    http://www.bruceroberts.com/public/HTML/S40.htm

    I suspect the 65 ft is more seaworthy : It can still go upwind when the 40 has no other choice but go downwind.

    You comparison is only valid for same material, with same engeenring, and same building technolgy.

    The most frequent boat is the Beneteau First 47.7. 41 ft at lwl, 25000 lbs, and 13000 sqft sail.

    The two most brands are Beneteau and Bavaria. People do not cross Atlantic in what they SHOULD own, but in what they CAN own. And they adapt their navigation program for what they own. ie, they do not cross alone, they cross within the ARC. Except the swan 100, a bit lost there...
     
  10. rayk
    Joined: Nov 2006
    Posts: 297
    Likes: 15, Points: 0, Legacy Rep: 146
    Location: Queenstown, NewZealand.

    rayk Senior Member

    Comparing boats pound for pound.

    Sorry I used your quote out of context, I thought it was spelling.



    The Roberts Spray has greater redundant strength in its scantlings.
    The Andrews 65 has lighter scantlings with no extra weight.
    As for heavy weather sailing, a 15000lb boat will be reefing well before either the Andrews or Roberts.
     
  11. Guillermo
    Joined: Mar 2005
    Posts: 3,644
    Likes: 185, Points: 63, Legacy Rep: 2247
    Location: Pontevedra, Spain

    Guillermo Ingeniero Naval

    Very nice posts, Mike.

    Stephen and others:
    As we know transversal moment of inertia is one of the important factors to seaworthiness and it is also related to seakindliness through the roll period.
    Usual formula for whatever moment of inertia is I= mass* radius^2 = m*k^2

    In the STIX, could we consider moment of inertia is somehow reflected in the Beam Displacement Factor (FBD) as this factor is partially intended to indicate the resistance to capsize in breaking waves? Maybe, but I think this is not enough. It should be more specifically considered, as Eliasson asked for.

    In what I call the the 'Old' ratios and parameters system, moment of inertia is estimated as (imperial units):
    I = (disp^1.744 )/35.5
    and roll period as:
    T = 6.28*( I /(82.43*LWL*(.82*Beam)^3))^.5
    Roll period, on its side, is related with roll acceleration for a 10º roll angle at a position located 1.5 ft inwards from the maximum beam:
    Roll Acceleration = (6.28/T)^2*radius*(roll angle*3.14/180)/32.2
    The Stability Index is
    SI = T/Bmax (normal flared sides)
    Curves for SI and its effect on crews can be found in Marchaj's book, page 75.

    Usually a roll period in seconds in the range of the beam in metres (maybe even slightly bigger) is desirable from the point of view of seakindliness (comfort), but also from the point of view of seaworthiness, as this means the boat has a reasonable transversal inertia to improve behaviour in steep or breaking waves conditions, not tending to follow the wave's slope, but remaining more close to the transverse resting position when moving (mast tendency to tilt towards the wave instead of outwards). Very long rolling periods would be, on the other hand, not only also uncomfortable, but even dangerous. It is also usually considered that T should be between 4 and 8 seconds, depending on the vessel.

    On the other hand, a beamy and light boat with it's short rolling period, may enter resonant rolling with shorter transversal componet of waves than a heavier one. As period of encounter is related to waves' speed, this to their length and this to their height, a heavier boat is comparably able to resist bigger seas, once again generally speaking.

    So seakindliness (comfort in these threads has to be understood as seakindliness) and seaworthiness are closely related. But seaworthiness is more than seakindliness, because it's also related to watertightness, structural strength, etc, etc.

    (By the way: Ted Brewer's Motion Comfort Ratio, MCR = DISP / (.65*BEAM^4/3*(.7*LWL+.3*LOA)) is also related with the inertia (mass) of the boat, but in this case relative to heave motions. A heavier boat will also have more gentle heave motions than a beamier and lighter one. Maybe the name 'comfort' confuses some people, making them think in carpets, sofas, washing machines and the like. Probably 'heave ratio' or the like would have been a more adequate name).

    Longitudinal inertia is not analized in the 'old' ratios, neither in STIX (Displacement Length Factor, FDL, is not intended to that end, but related with the general responsiveness of the boat, penalizing it if excessively high. Remember STIX is a Mom&Dad's intended index, not a racing one). And maybe it's a pity, but as the longitudinal gyradius is much bigger than the transversal one, longitudinal movements use to be more comfortable than transversal ones. But resonant pitching has also to be avoided, as it can be also very dangerous (I think, Stephen, your 'bad' experience when sailing that heavy boat you mentioned, was related to this. Probably a rhumb change would have helped, what you'd probably had done if cruising. But as you were racing....)

    Of course Seakindliness and Seaworthiness are not the only legs in the table. We need a third one to keep it stable: Seamanship. So behaviour of a boat at sea is always a 'three Ss' matter. We should always talk about the triple S combination, or SSS as has been suggested by several posters, absolutely. Could we think about an SSS index to be given to boat-crew combinations....? ;)

    Altering rhumb and speed of the boat as to avoid resonant rolling or pitching is an skipper's decission, not a boat's (Excepting Murielle's one? :) ). And this brings an interesting question to me: Is there any autopilot/reefing/trimming system able to decide and execute a change in rhumb and/or speed if things become dangerous? It would be pretty nice for solo or reduced skippering.

    More in this line of thoughts...could we get an authomated sailboat with a high SSS Index, not needing any crew....? (Definitive triumph of machines over humankind... :D )

    Cheers.
     
  12. Pericles
    Joined: Sep 2006
    Posts: 1,948
    Likes: 106, Points: 63, Legacy Rep: 1307
    Location: The heights of High Wycombe, not too far from Rive

    Pericles Senior Member

    The RNLI designed Tamar http://www.rnli.org.uk/who_we_are/boats_and_stations/lifeboats/tamar slideshow/page_1
    is a fine example of seaworthiness. The crews (men & women) demonstrate the most magnificent and highest levels of seamanship in some of the least sea kindly conditions you can find. I mention the Penlee station in Cornwall.

    1981 On 19 December hurricane force winds had blown the cargo ship Union Star off course after it suffered engine failure. The lifeboat Solomon Browne launched into very difficult waters, so rough that the crew of the Royal Navy Sea King helicopter from RNAS Culdrose were unable to lift any of the eight crew from Union Star.
    Coxswain William Richards made several attempts to get alongside and managed to rescue four people who jumped from the Union Star's wheelhouse onto the lifeboat. The lifeboat made a further attempt to rescue the remaining four when radio contact was lost. Her last message was: ‘We’ve got four off at the moment’. Ten minutes later her lights disappeared. The lifeboat had been completely wrecked with the loss of her crew of eight. The coaster was also lost. There were no survivors.
    Coxswain William Trevelyan Richards was posthumously awarded the RNLI’s Gold Medal. Bronze Medals were awarded posthumously to the remainder of the crew: Second Coxswain/Mechanic James Stephen Madron, Assistant Mechanic Nigel Brockman, Emergency Mechanic John Robert Blewett, Crew Members Charles Thomas Greenhaugh, Kevin Smith, Barrie Robertson Torrie and Gary Lee Wallis. http://www.rnli.org.uk/rnli_near_you/southwest/stations/PenleeCornwall/fleet

    Buying an RNLI Arun class that has been withdrawn from service would be an excellent purchase in my opinion.

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

    Guillermo Ingeniero Naval

    Admirable people and admirable boats.
    Cheers.
     
  14. fcfc
    Joined: Feb 2005
    Posts: 779
    Likes: 29, Points: 28, Legacy Rep: 399
    Location: france,europe

    fcfc Senior Member

    Do not confuse looks stronger and is stronger. To be really stronger, you need engeenering data.

    BTW, steel may not be the best material for deck and superstructure. I am aware of several steel hulls with aluminum, composite or wood superstructures or deck. I am not aware of any aluminum, composite or wood hull with steel superstructures.
     

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

    Guillermo Ingeniero Naval

    May I extend this line of thought and say most of people usually do not own what they should from an SSS point of view, but what gives them more home-like comforts and size (volume) for their money?
    Cheers.
     
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.