Full Displacement of semi

Discussion in 'Boat Design' started by tdamico, Jul 31, 2003.

  1. tdamico

    tdamico Guest

    Is it true that only full displacement offers comparative safety in offshore cruising during rough seas? All opinions welcome. In looking for a Trawler Yacht to be used as a live aboard, 95% of boat builders offer semi hulls, only a very few offer full displacement.
     
  2. Portager
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    Portager Senior Member

    Quoting Michael Kasten, "For seaworthiness in ultimate conditions there is little question that a true displacement vessel is the most suitable platform, having a more sea kindly behavior, a deeper hull, and ordinarily a larger range of positive stability." http://www.kastenmarine.com/idealMS.htm

    That being said, there are many acceptably safe semi-displacement boats especially if you are cautious relative to the weather conditions. It all depends on your intended application and intended cruising area. For coastal cruising where you would be less than a few hours from a safe port, semi-displacement hulls are generally preferred for their higher speed capability and better fuel efficiency in the speed to length ratio (S/L) range of 1 to 1.3. For live aboard applications semi-displacement boats tend to provide more spacious and open living accommodations per foot of boat and more boat for the dollar than true Passagemakers. Many trawler owners wish they could afford a Passagemaker but few would choose to live in a Passagemaker for coastal cruising conditions. OTOH, for true ocean voyaging and circumnavigation, a full displacement boat would be the best.

    Regards;
    Mike Schooley
     
  3. gonzo
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    gonzo Senior Member

    I have sailed ultralights, single and multihull, in very rough weather. They behaved better than many heavy displacement boats in the same conditions. A good design can be made to different specifications. The heavier the boat, the harder the hull will pound.
     
  4. tdamico

    tdamico Guest

    Michael:

    Great information, thanks. I am talking about two, relatively inexperienced boat handlers (only experienced with small boats in the 20' range on inland seas) We wish to purchase a live aboard trawler. Our goal is not to sail across, but we will also not tie up to a marina and live like an RV. We will sail extensivly up and down the eastern seaboard. The great circle route, and extensively in the islands, perhaps anchoring months at a time. Given this criteria, should I be concerned (as I am now) with purchasing a semi-displacement boat?
     
  5. tdamico
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    tdamico Junior Member

    Gonzo:

    I respect your experience, but I simply do not have the level of experience you do. For a generally inexperienced boater using their boat in the conditions I described above, would not the full displacement be a safer boat? Sorry for the stupid question, but explain what you mean by "pounding"
    Thanks
     
  6. Portager
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    Portager Senior Member

    Tdamico;

    For your intended application there are many semi-displacement boats that will do just fine. The great loop is sheltered most of the way with only a few stretches of the ICW and Great Lakes that are exposed. Some (foolhardy) people have even done the great loop is a houseboat. Choose a boat that meets your budget and has the accommodations that you want/need. Your boat selection will determine if you need to wait a couple of days or a few days for good weather to cross the exposed stretches.

    Full displacement trawlers are for people who want to cross oceans or cruise the West Coast of North America. The route from Seattle to Cabo is very exposed and rough. There are semi-displacement boats that could do it, but it would be more comfortable in a full-displacement boat.

    With all due respect, I disagree with Gonzo’s statement. Comfort level is in heave, i.e. vertical acceleration due to wave impact is generally considered to be a function of waterline area to weight ratio with the heavier boat providing a more gentile ride than a lighter one. David Gerr has an excellent discussion of this in his book, “The Nature of Boats (Insights and Esoterica for the Nautically Obsessed)” starting on page 106 subtitle “Heave and Waterplane Loading”. Pounding generally refers to relatively flat bottom boats which tend to belly flop on the water after getting airborne. Pounding is a common problem on planning boats and racing boats in rough seas when they do not want to slow to displacement speeds. These conditions do not occur on full-displacement boats and only occur on semi-displacement boats when going too fast for the conditions. Roll stability is a more typical problem on trawlers except it doesn’t apply to most of the great loop.

    If you are new to trawlers and planning such a big step, I would recommend the trawler world list and/or the great loop list. They can both be accessed at http://www.trawlerworld.com. They also have extensive archives which can be read and searched. You will find that nearly every question you have has been asked and answered many times and in many ways.

    Good luck and keep us informed of your progress.

    Regards;
    Mike Schooley
     
  7. tdamico
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    tdamico Junior Member

    Ahh...but eventually we do plan on making the passage to the West coast. Whether or not we ship the boat or sail the boat, in time, once experience and confidence builds. A West coast sojorn is not out of the question. I had narrowed my list down to full displacement boats like: Mirage Harbor 37 or 47, Selene 46, Krogen 39, or Nordhavn 42. I will still continue to learn, but still don't want to limit options by going for a semi for a few more creature comforts only to wish in 5 years, I could visit Alaska!
     
  8. Portager
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    Portager Senior Member

    I wouldn't put the Great Harbor 37 and 47 http://www.mirage-mfg.com/html/nav37b.html in the same class as the Selene 47 http://www.seleneyachts.com/47.htm, Krogen 39 http://www.kadeykrogen.com/krogen39.htm and Nordhavn 40, 43 or 46, http://www.nordhavn.com/home.htm. As a crude measure of stability, look at the above/below waterline area ratio (if you can find it) for a passagemaker it should be below 3. I can't find the A/B ratio for the Great Harbor 37 but it looks like it would be >3 to me, the Selene 47 is 2.5 full load to 2.7 light http://www.fridayharboryachts.com/selene/s47/s47-specs.html the Krogen 39 is 2.6 http://www.kadeykrogen.com/krogen39specifications.htm and the Nordhavn 40 and 46 are 2.3 http://www.nordhavn.com/40/specs.htm , http://www.nordhavn.com/46/specs.htm . A better measure of stability is the IMO offshore stability criteria, but nobody wants to provide that information. Also you will rarely find shoal draft monohull that qualify as Passagemaker.

    The Selene 47 is actually a semi-displacement hull, note that maximum speed is a S/L of 1.65 and cruising speed is S/L of 1.34, but it is far more sea worthy that some full-displacement boats.

    You might want to buy a boat that is adequate for your East Coast cruising and then, when you are ready to switch to the West Coast, sell it and buy a boat on the West Coast. That would avoid the time and expense of transporting the boat across country or going through the Panama Canal.

    Regards;
    Mike Schooley
     
  9. tdamico
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    tdamico Junior Member

    Michael:

    To me this is a wonderful discussion. Obviously I need to act like a sponge and absorb everything I can. You certainly are very knowledgeable. Can I get your reaction to the following?"

    "The A/B Ratio Is Largely Meaningless

    By Lou Codega
    Naval Architect

    Sooner or later potential trawler buyers will come across a term called the A/B ratio, as a measure of a yacht’s seaworthiness. Forget about it. The A/B ratio is, for all practical purposes, baloney.

    Since its first publication in 1975, Robert Beebe’s Voyaging Under Power has been almost universally accepted as the authoritative source for information on long-distance cruising under power. This was undoubtedly a pioneering effort and, although dated, remains a wealth of practical considerations on many aspects of cruising boat mechanics. But it is not, and was never intended to be, a text on naval architecture as it applies to power craft. It is extremely weak in its treatment of many technical subjects in that it relies far too heavily on overly simplistic rules of thumb and comparisons made between a limited number of mostly similar boats.

    I have tremendous respect for much of Beebe’s work, but I have a large ax to grind with his treatment of stability. There has probably never been an overused and yet so fundamentally useless a term as the A/B ratio. But it’s concept and calculation is easy for the layman to grasp, and its use has been perpetuated to some extent by those builders whose designs appear “good” by its measure. That’s too bad, as in this case a little knowledge is a terrible thing; I want to do my part to help it along a path to extinction.

    Beebe proposed the A/B ratio as a measure of transverse stability, calculated simply as follows:

    Projected profile area above the waterline

    Projected profile area below the waterline

    He then goes on to discuss ranges of this ratio that are found in then current trawler yachts, and those that he considers acceptable in boats that are, in his opinion, “qualified as ocean-crossers.” That’s it in a nutshell, and the discussion in the original text is only about twice as long as my summary. And there is no other mention of stability in the rest of the book, except to say how individual designs stack up to the A/B criteria.

    This analysis is simplistic beyond credulity. The ratio is not now, and so far as I’m aware, has never been used by naval architects. It has a bit of utility when used to compare similarly proportioned and sized boats. But it neglects everything that is important to stability. The height of the boat’s center of gravity, its beam, weight, shape of the watertight boundaries, hull form, tankage and watertight integrity are all vital to an intelligent discussion of stability, but all are completely ignored. Take any boat that you want as an example. It has the same A/B ratio whether it is two feet wide or 25, whether hard chine or round bilge, if it has engine room air inlets on the side of the hull or amidships, or if it weights 100,000 pounds or 10,000.

    The direct calculation of static stability has been done for 80 or 90 years, and, with the widespread use of computers, there is just no reason to simplify something so fundamentally important to the safety of the boat and its passengers into a rule of thumb ratio, and a half baked one at that.

    A designer routinely models a boat’s form, calculates the weight and center of gravity, and in a matter of seconds mathematically rolls it while calculating it’s tendency to either roll upright or continue rolling to a greater angle. (In the “old days” the same calculation was done by hand, using an instrument of torture known as an integrating planimeter while seconds turned into days, but that’s another story for another time.) The complete watertight boundary, location of the center of gravity, weight, in short, all of the factors that affect intact stability are modeled exactly. The result is the so-called righting moment curve that you’ve all seen.

    The example below, by the way, is not for the Navigator/Great Harbour boats and shows righting and heeling arms rather than moments. The moment is simply the arm multiplied by the boat’s displacement. The roll angle, usually zero through 180, is plotted on the horizontal axis. The vertical axis depicts the righting moment, which is the tendency of the boat to roll back upright or roll further. These typically depict a sine-like curve that starts a zero moment and zero degrees, increases to a maximum at say 50 degrees and then returns to zero at say 120 degrees. Up to 120 degrees, the so-called range of positive stability, the craft tends to return upright. Beyond that, the craft will tend to increase in roll, usually to 180 degrees or completely inverted.

    I guess that I wouldn’t be so upset if the A/B ratio was backed up by some science. I could live with it if, for example, Beebe had done some true stability calculations and came to the conclusion that the A/B ratio was a good indicator of the stability characteristics proven by the more involved calculations. But there is no evidence that he, or anyone else, ever did such a thing. I haven’t either, and frankly I’d be shocked if it were the case.

    By the way, an outstanding and very accessible discussion on stability, as well as most every other aspect of boat design can be found in Preliminary Design of Ships and Boats by Cyrus Hamlin. I can heartily recommend this book to anyone wanting to further his knowledge of boat design, intended as it is for the engaged amateur or beginning professional. It is remarkably well written and goes beyond simplistic ratios and rules of thumb to give a true understanding of underlying design principles.

    One more point while we’re at it. Stability calculations and criteria are intended to be comparative, not predictive. This seems perhaps to be a minor point, but it is crucial to your understanding. The righting moment curve that results from the calculations I just described result from an artificial condition that the boat will never encounter. Remember, the designer mathematically rolls the boat in still water. This same calculation, by the way, is made for most any ship around, including warships and cruise liners. This is clearly not an accurate representation of what happens when the boat is at sea.

    What most folks don’t realize is that this curve is only the first step in the process. Overlaid on the righting moment curve is a curve of wind heeling moment, (as shown in the example above) or the effects of off center loading, or high-speed turns, or the firing of guns broadside, as is appropriate to the service of the ship. The two graphs are then compared against each other as well as accepted, and in many cases mandated, standards on a pass or fail basis. These standards are based on past experience for similar types of craft. What is implied by this approach is that other craft having similar stability characteristics have proven sufficiently safe in service and that by extension, so should the new craft. There is no expectation, much less a guarantee, that a boat that has a positive righting arm up a stated angle will survive a roll to that angle in a fully arisen sea. It may, it may not. But stability calculations and criteria do not claim to offer an answer.

    Which returns us to the A/B ratio. It’s a rule of thumb, and like all rules of thumb it is easy to grasp and understand. But it is a gross simplification of a vitally important issue, and to my mind one that does far more harm than good. Its time, if it ever existed, has long since past. Any builder or designer worthy of your consideration will be more than happy to discuss their boat’s stability characteristics and how they relate to your anticipated cruises. But please, leave A/B ratios out of the discussion. "
     
  10. Portager
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    Portager Senior Member

    A/B Ratio and righting moment curve

    I agree with Lou Codega, the righting moment curve is a superior measure of stability and it should be the preferred method. Unfortunately most manufacturers will not provide that data. Since a more useful measure of stability is not available, A/B ratio is generally the best information available. I would not buy a boat without the righting moment curve and if the boat were used, I would insist on an incline test to confirm that the boat stability had not been compromised by the previous owners. Boats have a way of gaining weight over the years and this weight rarely improved stability.

    What the A/B ratio is somewhat useful for is measuring the sensitivity of the boat to cross winds, although the measure should be based on the sum of each segment of area times its distance from the waterline, which would make it proportional to the disturbing moment. The righting moment curve is a measure of static stability with no wave induced disturbing moments or wind disturbance moments. A boat with a high A/B ratio and marginal stability could be in serious trouble in high winds. (I also find the trend to enclose the frybridge with eisenglass very disturbing in this regard. If you want a boat with a pilothouse you should buy a boat with a pilothouse, not enclose the flybridge.)

    The righting moment curve is also used in the IMO offshore stability criteria, which is the measure of stability that I have choosen for Portager.

    Here is a list of manufacturers of Ocean Voyaging Power Boats that I copied from a post by Georgs Kolesnikovs on the Trawler List http://lists.samurai.com/pipermail/trawler-world-list/1998-August/004194.html .

    Branson Boats
    Cape North
    Cheoy Lee
    Grand Banks
    DeFever
    FairIsle
    Fleming
    Kadey-Krogen
    Kanter Yachts
    Linssen
    Nordhavn
    Northern Marine
    Park Isle Marine
    Real Ships
    Willard

    Regards;
    Mike Schooley
     
  11. gonzo
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    gonzo Senior Member

    I have worked in trawlers in the North Atlantic. In rough weather it is impossible to lay in a bunk. You are airborne half the time. This is not an exageration, we use to tie ourselves. In the same conditions, thirty foot seas, a James Wharram catamaran was much more comfortable. A heavier boat has a higher bottom loading regardless of speed. Slow trawlers pound hard. Herreshoff, among others, studied the influence of hull design on roll, pitch and yaw. He found it affects the movement more than displacement or metacenter. Heavy displacement boats are slow, which keeps you in the rough weather longer. Also, they end up with all kinds of appendages and gizmos like outriggers to keep them half way livable.
     
  12. tdamico
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    tdamico Junior Member

    Is a Power catamaran a "live aboard" boat? Is is sea worthy for extensive cruising in weather?
     
  13. Portager
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    Portager Senior Member

    This will probably initiate controversy, but I’ll don by flame retardant underwear and take a shot.

    The short answer is yes, provided they are large enough.

    Catamarans will pound in choppy seas if the height of the bridge connecting the hulls isn’t sufficient for the conditions.

    Catamarans have very high initial stable and, in comparable sea states, they will have smaller roll angles than monohulls, however too much stability can be just as uncomfortable as too little stability. When a Cat is disturbed by a wake or beam seas the roll affects are lower in magnitude but higher rate, which is generally more disturbing. The motion of monohulls tends to a more gentile rolling action.

    Some monohulls supporters emphasize monohulls advantage in stability range and point out that multi-hulls are stable inverted, however the higher initial stability should compensate for the reduce stability range and in a power catamaran (without the vertical close line) capsizing conditions are extremely improbable.

    The practical downside of Catamarans; they are more difficult to find slips for and you are always racing to the next marina to beat others to the last end tie, almost all Cats are dual engine which increases maintenance cost and effort, they are very hard to control with one engine out or off, they are very difficult to transport over land, I have seen very few moderate sized Cats with a decent pilothouse.

    The advantages of Catamarans; they can have shallower drafts than monohulls (although too shallow and they will pound), for comparable displacement they have lower specific fuel consumption and longer range at the mono-hull cruising speed and significantly higher maximum speed, Catamaran pricing is very competitive, for a comparable displacement, Cats provide more living space and are more open (although in the smaller range the narrow hulls can be a bit confining).

    I’m sure this list could be expanded given more time.

    Regards;
    Mike Schooley
     
  14. tdamico
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    tdamico Junior Member

    Actually a little controversy isn't always a bad thing, I enjoy different opinions.

    Would you say that a Catamaran in the 45' range would qualify as being large enough?

    I have seen several Catamarans in this size that have flying bridges and a "raised" look to them, sort of like a "Trawler cat" Would this design tend to reduce the pounding you mention in choppy seas?

    Regarding the difficulty in finding slips, as a retired live aboard we could tend to arrange our visits to the marina during week days or times when activity is less. Storms aside, is this a solution?

    Also, it was my thinking that I would go with a two screw trawler for peace of mind and better handlig, so the increased maintenance in having two engines would be a wash. I had heard that handling was much easier with two screws, than one, especially in wind. Do you have an opinion on the validity of this? Would controlling a monohull with one engine out be as difficult as handling a catamaran?

    The advantages of Catamarans do seem obvious. More room for the money. With enough weight, more general stability. 45' boats can have the room of a 55' monohull. I am seriously beginning to change my thinking from a monohull to a catamaran. A month ago, I would have said a full displacement hull trawler was the only way I would go. Now...I'm not too sure.

    TomD
     

  15. Portager
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    Portager Senior Member

    IMHO, no. TrawlerCat Marine ( http://www.trawlercatmarine.com/index.html ) claims to have the highest waterline to forward deck clearance on the market at 7’, yet they list the 43’ model as an Island Hopper. They do not use the Passagemaker label until they get to 54’. By the way, on Yachtworld.com the new 43’ Island Hopper starts at $445K and the 53” Passagemaker starts at $689K.

    I’m not sure what you mean by a “raised” look, but the only way I know of to avoid pounding is to increase the forward deck to waterline clearance and then avoid conditions that exceed the boats capacity. Increasing the clearance requires a wider beam, which requires longer length for directional stability. If you can’t avoid sea states where you pound, then your only alternative is heading into it because the aft waterline clearance is usually lower than forward and the swim steps don’t take crashing waves for very long.

    I think you’ll have good luck in the off season, but in peak season mid week only helps a little. You will have problems in the great loop. The inland marinas are not equipped to accommodate big cats. Maneuvering will be a problem and haul outs will be very few and far in between.

    A review of boat assist records show that for diesel boats, 85% of engine out problems are fuel related and propeller/rudder damage is far more common than engine problems. In addition, electronic engine controls have made modern engines far more reliable than they were a decade ago. Most modern diesel boats are going to single main engine and single prop/rudder protected by a full length keel to reduce propeller damage. The net result is far fewer breakdowns that a twin engine boat.

    On the handling issue, it is true that twin screws are more maneuverable than a single, but a single with a properly sized bow thruster is better than two props. I think if you need help maneuvering, get a bow thruster, it is far cheaper, more reliable and more effective than twin screws.

    I would argue that the disadvantages of Catamarans are more subtle. Maybe there is a reason why monohulls have such a large market share and Catamarans have so little resale value. I think this decision requires a great deal more research and ultimately I would find a way to spend several days and nights on one before buying. I can’t say I ever slept on a Catamaran, although I spend a night trying.

    Regards;
    Mike Schooley
     
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