Hull Water Loads

Discussion in 'Boat Design' started by Karsten, Jun 25, 2004.

  1. SeaDrive
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    SeaDrive Senior Member

    Eric, did you happen to notice if the waves were 'statistically normal'?

    (That's a joke, son.)

    Sailmakers do give a wind range for many sails. I don't know that you could say it comes from an 'engineering analysis' but it's something. I have also heard of multi-hull designers giving advice on when/how to reduce sail to avoid capsize. The European A/B/C system is also of value to consumers. There is a lot of info out there for the end user, if he cares to pay attention.

    Designers are in a delicate spot if asked if a design is suitable for crossing an ocean. They can certainlyl disqualify a boat based on design, but an affirmative really should only be for a specific boat in it's actual condition and actual equipment (and actual crew).
  2. SailDesign
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    SailDesign Old Phart! Stay upwind..

    I think we may be guilty of over-analysis in this thread. :)

    Bottom line is that even the EU A/B/C stuff does you no good when the weather changes suddenly and unexpectedly, and you are in a "C" boat in "A" weather and far from home.

    The other factor is the driver. How many others here have witnessedthe Tilley-hatted and life-jacketed family with 2 reefs capsize while the kids in an identical boat wearing nothing but shorts and grins zoom about all over the place having a whale of a time with no reef at all. (Sorry, there should have been a question mark, but it didn't look right.. :)) The worst sailor in the world will always be able to break a boat in calm weather, noi matter how well it has been engineered. Contrari-wise, the best skipper in the world usually has a chance of surviving the worst storms in a pretty useless boat.

    But... The best skipper in the world is not going to be able to save any boat under some circumstances or conditions.

    The problem, in Karsten's terms so to speak, is being able to design the perfect, lightweight aeroplane, but it has to be capable of surviving if all of its wings fall off a mile up. Not going to happen.

    The fact that Around Alone and Globe skippers have been rescued from such remote places, on boats that were barely afloat (in some cases not afloat, see below) is a reflection on the skill and tenacity of their skippers, not the cleverness of their designers (sad, but true)


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  3. Thunderhead19
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    Thunderhead19 Senior Member

    I have a 30 X 20 print of RMS Titanic hanging over my desk. The 23 that I posted in the gallery is built like a battleship.

    Aircraft design today, as Karsten implies, relies on massive computing power and wide ranging statistical data. I once met a retired AVRO Canada engineer who worked on the CF-100 "Canuck" fighter interceptor back in the late forties and early fifties. He told me about calculating wing loads on a hand-cranked mechanical calculator. They nicknamed that jet the "Clunk" and the "Aluminum Crow". It was the only non-swept wing fighter to regularly break the sound barrier. It was a tad over-built. The aviation industry has benefitted from advancing technology quite a bit in the last half-century, and so has Naval Architecture when the cost has been warranted. The Queen Mary was the greyhound of her day, but she'd break her owners today if the same hull design was used on the QM2.

    We are designing "Canuck"s. We are not designing QM2s or "Raptor"s. We have no major technology race to win here friends (except when it comes to building them). People use these craft for pleasure. I've worked on 80' Power Cruisers that I know damned well will never leave the marina for more than an hour a month. I've also worked on 30' charter boats that I know will get thrashed. I go to my charts and tables and make my little drawings without fear. A "Raptor" once in a while would be nice though.

    Have any of you ever had one?
  4. CDBarry
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    CDBarry Senior Member

    In the 80's Sid Sircar, Ryan Young and I were in the oil industry and had access to what was then some pretty heavy computing resources and software for motions and loads, and statistical weather and wave data for a North Pacific slow voyage.

    We used this to check the design of a 40 foot cruising sailboat and found that the then new ABS Rules for Offshore Racing Yachts, and the DnV Boat rules both resulted in reasonable probabalistic factors of safety for such a voyage for a paper that was published in the Ancient Interface Symposium in 1982.

    Note also that all the classification societies allow first principles analyses based on structral reliability approaches, whole vessel FEA (MAESTRO, for example) and so on.
  5. Guest

    Guest Guest

    Student, naval arch.

    Hi I read all of your posts about an interesting subject! I am working on with my thesis around design loads on sailing yachts. One thing you all might be interested of is that one finnish research center called VTT did an experiment about yachtloads. They put ca. 60 force sensors all around a halftonner, and tested it in all kinds of weathers. Go look at the link page below!
  6. Dutch Peter
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    Dutch Peter Senior Member

    Just to add some info to this discussion:

    LRS Yacht & Small Craft Rules are indeed no longer available, and the last update is from 1994. The same year the EU came up with the Recreational Craft Directive no. 94/25/EC.
    This Directive is to eliminate trading barriers within the EU.
    It is mandatory to comply with this Directive, for boats of 2,5 - 24 mtrs, when they are placed on the market. So also imported boats have to comply.
    With this directive comes the obligation to provide a "Owner's Manual", that goes for the whole range, not above 6 mtrs.

    The different Design Categories, A/B/C/D, were also discussed. And I have to say that Eric was not completely right. LRS SSC Rules has a similar way of craft into groups, with the Service Area Notation, G1(sheltered waters) to G6(unrestricted), but this is a geographical restriction ( xx nautical miles from a refuge).
    One might argue it has the same effect. The idea is the likelyhood one encounters certain weatherconditions in a certain area and use that as a design parameter.

    Might be nice to know that the weather conditions (wave and wind!!) for the lightest design category for CE, namely D (sheltered waters), can be found on the Atlantic and the Pacific for to or three months! (on average that is)

    The Special Service Craft Rules of LR are mainly for craft above 24 mtrs, but can be used for the smaller/CE sizes. It's true the construction will be a bit on the heavy side than. But when you use the calculation program, you're able to adjust the design pressures used in the program.


    I'd like to know, how are you doing with the design? What set of Rules did you use?
  7. Karsten
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    Karsten Senior Member

    My little design project is going slowly. Just not enough time. It started because I wanted to learn ProEngineer and draw something useful. Probably not the best program for boat designing but with a few tricks it works.
    I got “Principles of Yacht Design” and also the German Lloyd rules. I’m just a bit confused about the panel reduction factor. That the slamming loads are “spread out” over bigger panels and therefore the bottom design pressure is reduced seams to be very strange. I understand that pb2 is a static load and that very local and short term slamming loads might actually be greater. But I don’t understand why you would reduce the design pressure just because the panel is bigger? Is it because bigger panels flex more and therefore the dynamic slamming loads are reduced?

    My plan is to define possible load cases and then to apply the loads to the FE model to see what the stresses are and limit them to about 0.5 times the “yield” strength for good fatigue performance. Some load cases like “hitting a submerged container at 12kn” will be allowed to generate higher loads or even failures. To deal with such situations it is probably better to install a few watertight bulkheads or a good bilge pump.

    My design is optimised for lots of speed, fun and enough room for the occasional overnight trip. I went for something long and skinny because I like the look of classic yachts. To make it easily to handle with 2 persons the sail area had to be small and therefore the weight low for good performance. I hope there is an image attached to this post.


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  8. ErikG
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    ErikG Senior Member

    It looks as if the keel is a quite a bit forward or is it just a matter of eye fooling the mind?
  9. Karsten
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    Karsten Senior Member

    No, your eye is right. Where to put the keel seams to be the second gray area of yacht design. I fond some computer model research on the rig of a 470 and they concluded that the jib produces about 1.5 times the side force of the main per area. If you use that model the keel ends up where it is on the photo. After reading "Principles of Yacht Design" I put the keel further back but the photo is still of the old version. I also have to check the C of G. The model above is actually only a surface model. The model where everything has a thickness and therefore a weight is under construction. I might have to move the keel boulb in relation to the fin to get the C of G in the correct position. It's only a draft.

  10. redcoopers
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    redcoopers Member

    It's correct what you say about the jib producing a lot more sideforce per area, but this is only when sailing close to the wind.

    However, most keels are a little further back so that the rudder will need just a little weather helm closehauled. In some tank testing, I've found that in about 3-5 degrees of yaw angle, an "average" boat will sail faster with about 2-3 degrees of rudder angle. Don't ask for a good explanation - this is mostly from tank testing, and it's a little hard to see what vortices are doing while tank testing.

    I've got some sail coefficients for a masthead sloop on a page here. I think the 14th CSYS has more info on fractional sloops, but overall, there's not too much of a difference.

    Personally, I agree that many keels could be placed a little more forward (they don't seem to have moved much as sail technology has increased). However, I think a bit of testing would be in order to verify my assumption.

    Oh, by the way, that's a really nice looking drawing!
  11. Eric Sponberg
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    Eric Sponberg Senior Member


    You are right in your guess about slamming loads and the pressure being reduced as the panel gets bigger. Back about 1978, two engineers named Alan and Jones wrote a technical paper on slamming loads of high speed power craft whereby they proposed the "reference area" of hull panel structures, which was intended to be a minimum area which had the highest loads. Any other panel larger than this reference panel would have lower loads because the magnitude of the load is dependent upon the elastic reaction of the panel under the load. It is a very dynamic and immensely fast response, and counterintuitive, but it has been proven by further analysis that as the panel gets bigger, the load goes down.

  12. jehardiman
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    jehardiman Senior Member

    Karsten and Erik,

    I think I can answer you question as to why larger panels have lower loads. Slamming loads (or other hydrodynamic impact or shock loads) are dependent on how much and how fast energy is transferred to the panel, understanding that the energy can only be transferred at the speed of sound in the fluid. A larger panel even though it has more area also has more deflection for a given load.

    Lets compare two panels of similar width and thickness (modeled as point loaded simply supported beams for ease) with different lengths, one 1 unit and one 2 units. The load to cause a deflection of 1 is P=(48EI)/(L^3) or to rewrite for deflection delY=(PL^3)/(48EI). Now 48EI is a constant for both panels so the load to deflect the 2 unit length panes is 1/8 the load to deflect the 1 unit length panel.

    Now we have to look at how the load is applied to the panel. Of course P= 0.5*rho*(differential velocity of the fluid to the panel)^2. This "differential velocity of the fluid to the panel" is the important part. It is a function of ship velocity, panel geometry, panel deflection, wave velocity, wave orbital, and speed of sound in the fluid. As slamming load is applied to the panel, the panel begins to deflect inward at a rate over time that satisfies the four equations of P=panel mass*DelY/t^2, P= 0.5*rho*(differential velocity of the fluid to the panel)^2, delY=(PL^3)/(48EI), and delY/t<speed of sound in the fluid (this last may not true for all cases but is true for most slamming speeds).

    In solving this, we can see that because it takes less load to deflect the longer panel and the rate at which the panel can be deflected is limited, then the longer panel has less load. Conversely, the longer panel must deflect more to dissipate the same amount of energy as the shorter panel.

    There are other things to consider of course, such as primary and secondary structural loads and how they are transferred, but the above generally sums up the reasons that slamming loads are reduced in larger panels.
  13. SeaDrive
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    SeaDrive Senior Member

    I am reminded of an article (by Kurt Hughs, I think) in Multihulls Mag (I think) describing attempts to get the USCG to sign off on constuction plans for wood charter catamarans. He pointed out the Rudy Choy had designed his cats with much less in the way of framing (and therefore larger panel size) than the USCG would contemplate without any failures. I wonder if Choy had done some advanced arithmetic.
  14. Karsten
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    Karsten Senior Member

    The explanation makes sens. Having flexible panels is an advantage and also putting heavy items on the inside of the hull would help because you would only end up with compression in the thickness direction.

    To the rudder question. (1.) Basically the rudder is in the vortex field of the keel. To get the same cl on the rudder and keel you have to have a greater angle of attack on the rudder to overcome the negative effect of the keel voritices. This unfortunately also creates an additional drag. (2.) If the rudder has a greater aspect ratio than the keel (which most rudders do) the rudder produces less inducted drag due to the lift. Therefore it can produce lift more effficently than the keel. Big negative effect is probably the gap between the rudder and the hull. (3.) Draging the rudder through the water without any or little side force doesn't make sense.

    The whole problem is basically equal to the jib and main sail problem and the wing and horizontal stabilizer interaction on an aircraft.

  15. SeaDrive
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    SeaDrive Senior Member

    Are we saying that oil-canning is good?
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