Personal Offshore Catamaran Design

Discussion in 'Multihulls' started by JCD, Jul 4, 2006.

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How should the price of the plans be established?

Poll closed Aug 5, 2006.
  1. Retain study on comparable designs adjusted for quality.

    1 vote(s)
    16.7%
  2. Driven by the market. Ebay bidding...etc.

    1 vote(s)
    16.7%
  3. Forum member opinions.

    0 vote(s)
    0.0%
  4. Design value based on NA or NE opinion.

    1 vote(s)
    16.7%
  5. Create one class and establish maximum sail numbers.

    1 vote(s)
    16.7%
  6. Establish minimum cost for number of deposited orders and set reserve.

    0 vote(s)
    0.0%
  7. Depends if full Cad drawings, spreadsheets and all other tools are included in cost.

    2 vote(s)
    33.3%
  8. As a percentage of cost to build.

    3 vote(s)
    50.0%
  9. Other...posted in thread for future poll consideration.

    0 vote(s)
    0.0%
Multiple votes are allowed.
  1. JCD
    Joined: Jul 2006
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    JCD Follow the Bubbles!

    Hello Chris...
    Thank's for the response.

    My expectations are high that I will be very pleased with the performance and excellent turn of speed for the design for two reasons. The first is that all calculations are accurate and take aerodynamic and hydrodynamic forces into account. The second and most important is because even if the performance is illusory and I were to achieve only 1.4(Lwl^1/2), then I exceeded the waterline limit and sailed in comfort and in an upright position with the ability to go offshore or crawl into a little nook or crannie somewhere to wait out the storm of the century while I walk onto the beach.;)

    As far as pointing ability...I think she will do well and I don't forsee myself in a position where I have to claw my way off a lee shore when I have two perfectly sized engines and enough insight to know to head offshore when the going gets rough or stay offshore when it is already rough.

    Volume...hmm. Tried to design for the least volume so that one of my height (6') does not have to step into inclement weather or a mosquito carnival in order to take the crook out of my back. Also, the centerline-midship queen size berths have sitting room on them, but you can't hardly tell can you? The loft on the bridgedeck profile is a naca 15 for aerodynamics. I could lower the height, but at the expense of a crooked back or more draft, the latter preferred. Also...don't forget that the design is for blue water, so you definitely want to have some room to live in although most experts say that you live "on" a boat and not "in" a boat. Windage was approached a little differently by me...and yes, I know what all the experts say about it. I'll explain.

    Consider a box 2x6. The area of that box is 12.
    The dynamic load on a flat plate in a stream is expressed as q = dv^2A/2g.
    Where d is the density of the fluid in cubic foot,
    v is the velocity in feet per second (multiply by 1.69 for speed in knots),
    A is the area of the plate, Frontal area = Area(Sin of Deg.)
    g is gravity, 32 ft./sec.2.

    No..this is not a math lesson as I'm sure everyone here knows this math all too well...I want to present my approach and belief about windage since there is an overly concern with it.

    Lets exclude everything except the frontal area portion of the calculation. Perpendicular to the box area of 12 you would be faced with maximum loads or for this case windage. In this case and only in this case will you have both maximum leeway and maximum heel...pound for pound. Any deviation from perpendicular will automatically reduce this maximum because the frontal area is reduced by an amount equal to the sign of its angle to perpendicular. For example: 12(Sin90)=12 but, 12(Sin45)=8.49.

    First Leeway. If you remember your navigation, you know that you can calculate your course in a manner that gets you from point a to point b by calculating your drift, set, speed over ground etc. I can explain this further but it doesn't pertain to the thread. But simply stated, you point to the left or right of a target before you fire, the amount of left or right offset depending on how much "windage" will carry your round over to hit the bullseye.

    Now heel. Bottom line is that I would be more concerned with reefing at perpendicular than I would at any other angle...but lets address it. A box would be hard pressed to reduce any kind of directional heel against a force. But, if that force is allowed to attach itself and flow around the object, then it stands a better chance of heeling less or actually staying put. Streamlining is the key to reductions in the order of great magnitude. Whether it is hydrodynamic or aerodynamic, it will allow the fluids to attach and flow around the object. Take a good look at her lines and notice that very little is actually without stream.

    Maneuvering in a tight anchorage...well, I guess that might present a challenge to anyone...but I readily admit that it could be a challenge for this design. I don't believe that it is a challenge that cannot be overcome with some caution on approach...appropriate use of the motors...or maybe even anchoring out and rowing in until it gets a little more comfortable. For me, the spectacular events that may arise are from skipper negligence and the never ending rush to get nowhere really fast. Is windage high? I think that compared to a mono or a spartan race multi...yes. Compared to similar multis that range 25% windage with huge square houses...no.

    Submerged volume to displacement...I'm not sure if you mean displacement, shape, draft, area etc? Are you referring to: S/(V^2/3) or the wetted area to displacement ratio? I need clarity on your question. However, I'm not certain that it would make a difference if it were because the least wetted area equals the least drag and looking at the profile we see one hull, but there exists on the other side, another hull. Even mono's that have insane amounts of draft can be designed to "plane", which then reduces the ratio but does not change how much more weight could or can't be added. In any event, the underwater lateral area is 16.5% of the windage lateral area. Not sure what that it means...but since the density of water is 800 times greater than air, you better believe I want as little area down there as possible. Increased area = increased frictional penalties. All of the above is of course my own opinion and thinking.

    There may be a young whippersnapper that may wish to push her to the limit...in which case, the windward hull is shallow and much easier to fly with reduced loads, while the leeward hull has flare to keep her on the rise. It is beyond me why anyone would want to have a hull unstuck and on the brink of flight instead of just designing for less frictional area or longer waterline.

    The construction lines are nice and clean and as you have already surmised...they started as plywood construction. The reason I decided on chines is because I have devised a way to attach them so that the tape does not have to be faired (it is countersunk) and the layup works as longitudinals I beams. This may not be a new idea, but once you look at it you will see that it is definitely improved upon and superior to the old if you compare design and labor. I tried to post it as part of the attachment, but didn't get a chance. I can do it later.

    Well...please post again and thank you for allowing me the opportunity to discuss my opinion on the issues that you brought forward. By the way...don't be dishearted about the windage and draft...the hulls were designed with chines so that draft and windage could always be subtly modified if needed...very, very easily. It would take me no more than 2 hours to scale accurately and maintain all other hull performance characteristics. I did it already to 34'LOA and 13% windage with the same headroom!:D

    Thank's
     
  2. JCD
    Joined: Jul 2006
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    JCD Follow the Bubbles!

    Russel...let me know if you can't get it...it was hell trying to get it on here...had to cut out like 20 detailed drawings to meet the 2mb limit.:eek:
     
  3. fhrussell
    Joined: Dec 2004
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    fhrussell Boatbuilder

    Still not able to see the drawings....:(

    If it's possible, you are welcome to email them direct. I'm really interested in seeing what you have here.
    Thanks!
     
  4. JCD
    Joined: Jul 2006
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    JCD Follow the Bubbles!

    Hello Nero:

    Glad to hear that everyone has managed to get the drawings.

    I like it when opinions are so absolute. I will try to convince you as painlessly as possible that absolute opinions, although valid, might be inaccurate.

    Fore and aft clearances are given for the designed bridgedeck as a percentage of Lwl. Mathematical calculations...( no I won't get into it, just believe me), will show that the clearances are ample for the designed Lwl and fore and aft distances for waves offshore. If it is really critical, you can always loft the bridgedeck to a naca 12 or 10 profile and have enough clearance for a tsunami, but you lose sitting headroom in the bunks. The loft can be either flush with the deck of the amas (that is how their arc is designed) for less windage and clearance or they can remain at the designed height, for the same windage but even greater clearance, but scantlings cannot change. How did you conclude that the clearance is shallow?

    The beam overall, the waterline beam for the ama, the overall waterline beam and the center to center ama beam is given in the design. Other widths are also given. Which beam or width is too narrow or "looks" too narrow and how did you conclude that it is too narrow? Different calculations are used for different beams in relation to Lwl and all calculations produce specific results...some of which I have denoted on the original post such as Bm/Lwl = 65% etc. Please be more specific. Do you mean the 5'5" width of the hull is too narrow?:confused:

    I'm glad to hear that you like the design, thank you and what exactly draws your attention to it? Please elaborate as I am interested in establishing the most significant nautical aesthetic of the design.

    The rig...ahhh, the rig. I designed several although only one is shown on the plans. The others are in "frozen layers". The current sail plan and a high aspect sail plan and several mast sections and material.

    The first is a foil shaped wood mast...rotating on a high-load tapered cylinder thrust, roller bearing with obviously all the associated rigging. The same bearing would/could be used on the other masts.

    The second mast designed was a full carbon foil but that thing was a b***ch to design because of the different directional schedules and computations. Even with the spreadsheet it was nerve wracking.

    The third mast designed was easy...find the inertias...calculate for compression, safety factors etc and then look up existing extrusions which were comparable. The spreadsheet for that made that job so easy I thought it had to be wrong.

    The final mast designed...is a tapered round section and as above...it was so easy. The problem with this one is that it is custom and no one that I have engaged has it in stock or is willing to make it so I'm looking at a way to have a cylinder cut and welded in case if someone is interested.

    Here is the funny part since you mention free standing. The rigs I designed are designed to the point where they are free standing without any safety factor and addittion of shrouds and stays just adds to the safety. Column buckling is calculated at K=2 right from the start and safety factors for the mast are rounded up as the calculation is solved so that the you will see a stick standing there when father Neptune is done playing with you in the middle of the Atlantic. You do not need all the shrouds and diamonds that you see on the design because I calculated for 408 inches but the effective length of the longest panel is only 151" (mast head to top spreader). I have them there because I was trying to find one set up that would work for all masts in case if someone wanted to interchange their masts. One spreader and diamond with one symetrical shroud is all anyone could ever need with any mast on this design.

    So you like my dual helms huh? Got some special design into those too. The wheel is a 30" diameter on a 4" "winch" hub sprocket which gives a power ratio of 30:4 or 7.5:1, so 1 pound of force applied to the wheel rim delivers 7.5 pounds of force to the steering cable which is continuous from centerline to helm, with 4 or 5 turns on the winch, out to the next helm and winch with the same amount of turns on the cable and then back to the centerline with heavy duty springs to absorb shock loads. Each rudder has a load of 2469#'s at maximum hard over of 63 degrees at 20Knots. With the different windings and the radius there is a total power ratio of about 70:1. Therefore, one finger applying a force of 1 pound to the wheel rim delivers 70 pounds of force to the rudder when the boat is broaching at 20 knots full hard over. This translates into 2469# x 2 rudders /70 = 70.5#'s worst case scenario. Divide that by two arms for 1 helmsman or four arms for 1 helmsman on each side and you are looking at pie in the sky while all hell is deterioating around you because you manage to evade exhaustion.:D

    I will be showing the simplicity of this system without all the chains, screws, and this and that that makes up a distributors "total system" with 500 parts and capable of breaking down in 500 different locations at the most inopportune time...like, when motoring into a crowded marina on a windy day.

    Didn't design for strip plank because that is HARD, HARD work to glue,sand and fair, but I'm sure it can be done. Nope...not too many chines...I needed the chines to get the appropriate stream so that liquid can flow around the topsides and below the waterline. Also, easier to handle and wanted to incorporate the I beams into their connections to eliminate the longitudinals. Tried to make the design amateur friendly (because I am an amateur) and stupid simple (because it should be).

    Well...I hope that all of this writing doesn't put someone into a spin.

    Thank's for your "opinion" and know that I welcome them. Let me know if there is anything else I can clarify on the design.

    Thank's
     
  5. JCD
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    JCD Follow the Bubbles!

    Hello Russell:

    You know were boys from the same road...:D
    I'll send it to you as soon as I get home...what I'll do is send you the one I was going to originally send and had to cut up to meet the 2mb limit. You can then e-mail it to the other members so they have the whole file also until I can get everyone's e-mail and am able to do it myself.

    Hey...I put lots of info on the thread...read it to give you an idea before you view the files...also...got a poll up. Please vote and encourage others...sorry, I'm a time freak manager and like to see things in constant motion.:cool:
    Thank's
     
  6. rayaldridge
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    rayaldridge Senior Member

    I have to agree with Chris that there doesn't appear to be enough displacement to support those enormous hulls.

    In my opinion, standing headroom in a 30' cat is likely to lead to more disadvantages than advantages. Is there some reason you are restricted to 30'? I think if I had the cash in hand to build something as complex as your design at 30', I might choose to go longer, but simpler.

    Ray
     
  7. Chris Ostlind

    Chris Ostlind Previous Member

    About the windage issue as you responded...

    I love that you are spending a great amount of time to take a look at all the technical issues surrounding the potential performance of this new boat. I would suggest that it's not just the maths of how much windage you are designing into the above water shapes, but equally so, the style and placement of those shapes that makes for a well behaving craft underway as well as on the hook.

    Having lots of raised and exposed surface area well forward of the mid point will contribute to the boat wanting to fall-off in a serious way. It's my understanding that a prudent design would work to minimize the form well forward, so the hull will want to point into the wind as a natural consequence. Otherwise, you will be putting a sizeable amount of windage/rotational moment into the underwater appendages; mostly falling on the rudder while sailing.

    I'm sure this has been addressed in the design study, so I'd be interested to hear how you arrived at a solution.

    I'd like to thank you for your candor and willingness to share your large body of work by putting this study in front of us. It shows not just an openness, but also a very solid sense of confidence.

    I'm looking forward to the comments of Brian Eiland and Frank Russell when they get an opportunity to respond.

    Chris
     
  8. JCD
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    JCD Follow the Bubbles!

    Hello there Ray:

    I also agree that standing headroom in a 30' creates disadvantages...one of which is windage...especially for someone tall. But if you ask anyone about the amount of windage that is ideal...everyone will give a different answer for different designs and everyone agrees that the least amount the better. Some will quote 6% LOA but they fail to tell you that to get the 6% and have headroom you now need to have a 100 footer for a 6 foot person. Ridiculous use of space when 12% in that length would get you 2 levels above the waterline.

    I went with 30' because I needed to see the effect between a high windage, streamlined cat and cats that I have seen that look really boxy and square and then it took off from there. Why did I continue to design her from there? Well, because the design has specific parameters that can be easily modified to reduce all disadvatanges with relative ease. I'm glad you brought it up because everyone should know that the design can be very easily modified, whether in LOA or Draft so that windage is minimized.

    Twice it has been mentioned that the underwater wetted area may not support the displacement of the hulls without any qualification and I will address it differently. Take 2 boxes, any 2 boxes, measure them, weigh them and float them. Measure the draft and the wetted area. Now ask and answer the following. Is there enough wetted area displacement to support the box? The question is moot because if they float, then there is enough displacement.

    The ratio is non dimensional and used to establish by comparison which will carry more than the other assuming the same wetted area without suffering the friction or drag penalty associated with the extra weight. The lower the ratio on hull (x) the more it can carry than hull (y) for the same wetted area...or conversely, equal drag/friction for more displacement at the same wetted area.

    If you do the same with a ball...you will find it has the lowest ratio of all shapes, but that doesn't mean it will make a good vessel...unless you put paddles along one axis on the outside and steps on the inside and get inside and walk it across the ocean.:D Just a little joke.

    I am at this point interested in hearing (reading) the different ratios believed to be adequate to carry specific displacement for a catamaran. Please advise. I will calculate the ratio for my design and see how it compares, although this sounds like its going to be difficult.

    Don't forget that with enough qualified convincing, I can and will change it but it must prove unequivocally that an increase in wetted area will create a greater advantage structural or otherwise, in comparison to the increased drag and friction.

    Since we are on wetted area ratios, the Wetted Surface to S/A Ratio (Sa/Wet Area) is significant in light airs...less than 10 knots. The lower the ratio, the slower the design. I guess I can solve for this also while I'm at it.

    Thank's again for your comments and keep them rolling...even if they send me back to the calculating board;)

    Thank's
     
  9. JCD
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    JCD Follow the Bubbles!

    Hello Chris...

    I can see that you are taking the time to look at all the angles and I want you to know that an observation that leads to thought processes is formidable and powerful just by implication. Continue to do so.:)

    There is no need to thank me for my candor or for sharing my research and design. You will find that you might not like or agree with what I say but it won't be bull. As far as sharing...well, I'm not greedy like that and in a way, it helps me to release and flow some of this information overload in my grey matter.

    I'm glad to hear that you love that I'm spending so much time calculating the performance of the design...rest assured that all the calculating is not something I love, but I have fun and it has purpose, so I do it. I agree that style and placement of the different shapes is necessary, but I cannot agree with your statement that I have spent any time whatsoever into designing windage into the form. Instead...I have taken steps to reduce windage by modifying the form through streamlining. Relax...I know what you meant.:cool:

    I can't say that I see lots of raised and exposed surfaces forward of the mid point as you have pointed out...did you receive the same file I sent? In the absence of understanding, I will assume 2 subjects. Namely, the bridgedeck, which in my mind is 6" from being flush, and the bows, which are forward of midship and are a significant exposed lateral surface. I will comment on both.

    The bridgedeck viewed from the bow (section) looks like a flat plate but viewed from the profile, it is a naca 15 section. Wind from the bow or slightly off the quarter will attach and flow with minor "assumed" turbulence at connective panels or plates which is expected at all junctions, but minimized with generous fillets.

    The bows, these bows, are imfamously winded. Notice how vertical they are? I guess it's my way of ensuring that there is sufficient bouyancy to counter the fear that comes from an attempted submarine dive. Viewed from the profile, you see a flat plate but viewed from the bow (section), it is well rounded. Not fillet rounded, but a good solid radius that allows good attachment and flow both upwards and across the bridgedeck and downwards towards the waterline. The profile is very proportional fore and aft, therefore, I expect forces acting upon them, (assuming zero attachment and flow) to be almost equal fore and aft. I envision the design evenly leewaying oppossed to the bow or stern kicking over and swinging around.

    If left to their own devices, most if not all designs will want to point to weather as a natural consequence... and I would think the longer they are compared to their beam the better. But it would be a simple solution to keep any boat pointed to weather at the moor by attaching a small sail to keep her headed up. Sailing is quite a different story altogether. You already know about weather and lee helm and that those manifest due to the balanced distance between the center of effort and the center of lateral area. That distance being named the leed. Weather helm is preferred because she heads up and can pretty much take care of herself. The leed is usually expressed as a percent of the waterline, usually 15%, but I don't consider it accurate for a cat. it is however, a good starting point until you calculate all your loads and how they translate into "your energy" to counter them. The Center of effort must be, and should be aft, for weather helm.

    You're right that form forward should be minimized, as on all objects on nautical designs...but how do you qualify the statement? Sail...Multi or Mono? Power...Bass or Trawler? Since we are discussing a cat...almost all cats designed have bows that are higher than the stern with a straight sheer. Great for forward bouyancy, but contrary to forward form minimization.

    Rotational moments on the rudder, or loads, were calculated by using the worst possible scenario. The rudder hard over to 63 degrees dropping into a trough on a broach at 20 knots, or 6.25 times the (Lwl^1/2). Aft and forward is not relevant because the rudders kick-up.

    Frank Russell I have already contacted...informally. Brian Eiland sounds familiar...I might have read something by him. I also look forward to their response as I believe it will be most enlightening.:)

    Lets us not forget that I am not an expert...I'm just very familiar with my design and all things that were required to bring it to fruition.

    Thank's
     
  10. Chris Ostlind

    Chris Ostlind Previous Member

    Displacement???

    Perhaps you could share with me how you arrived at the 4000 pound displacement figure you show in your specs? What materials are you planning to use, what sort of stores you may be including, etc. I ask about the materials because you list two types of very differnt construction types.

    This looks to be an amazingly lightweight cat for a 30 footer. All the other cats I reference have a displacment number more along the lines of the mid to high 6000 lb. area. That would be a whopping difference and absolutely critical to overall structural loading, draft, sail performance, etc.

    Malcolm Tennant's Red Shift design comes in at an estimated 3,300 lbs. and it is a pretty stripped racing cat with hull nacelles and minimal accomodations and yet, the displacement difference is only 700 lbs. overall. While that's a lot for a racing boat, it's truly a reasonable safety factor, load difference in a 30' cruiser.
     
  11. fburton
    Joined: Sep 2005
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    fburton Junior Member

    Yes it’s a nice boat

    Yes it’s a nice boat. Height to Length is a designer’s toughest challenge. I think you pulled it off with some art.

    Alas Ilan Voyager no longer posts but your parameters hit all his ‘ratiocinations’ (one of Ilan’s words) for a consummate cat.

    With many designs particularly the smaller boats it seems the interiors have first influence over the water shape. So which side of the plywood are you designing for? My real question here is Trim. You have large flare particularly forward but why carry it all the way to the stern? Some would say a flared stern would agitate the rocker. A flared midship (even if slightly convex to mitigate this) prescribes the angle of freeboard with the underwater section. This gives poor girth to area ratios upping both forms of resistance.

    Trim commonly demands LCB around 0.54/0.55 which seems a popular choice but one pays a big price for moving the volume backwards; adding up to10% resistance. None the less it would be nice to hear a designer justifying it.

    Where did you put the LCF? How significant is it on a cat?

    I like your profile, a delayed rocker and a shallow stern, though quite a classic shape these days. You have a little bit of a forefoot so I was wondering what your thoughts are there?

    Highest respect for taking it on the chin.
    Fred
     
  12. rayaldridge
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    rayaldridge Senior Member


    Well, the question is not whether the box will float. The question is whether the box will float on its lines. I'm guessing that the thing that struck Chris about the drawings is what struck me: how small and shallow your hulls look, below the waterline, compared to any other design I've seen. Presumably your hulls are designed to be extraordinarily light for their enclosed volumes, particularly considering the amount of structure in your bridgedeck, but I have to wonder why other designers have not discovered how to do this. Can you tell us the weight of your boat, unloaded?


    Ray
     
  13. JCD
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    JCD Follow the Bubbles!


    Hello Chris and good morning to all...

    Excellent observation. In this case, things are a little more complicated and equal the sum of many things but to make an extended explanation short to the point that it gives clarity, here it goes.

    Understand first that the design is for 4K to 6K pounds, so naturally all scantlings, regardless of specific material modulus, or better yet, specific material properties, must and have been designed with safety factors for cycling, loads, etc., at the higher displacement so that the design's structural components are strong enough at the higher displacement...note also that the mast and every other component of the vessel is designed at the higher displacement right from the start, with all "accepted" safety factors (+ a little more because I understand very well that the only thing that sits between me and Davy's locker at 1000 fathoms is the design and how it is constructed.)

    Secondly, understand that in my mind, 6K#'s brings the waterline to kiss the transom and any further rise beyond that point would just create drag...and dragging the transom looks pretty bad when all other designs have their tails up in the air. Not that it can't be submerged, it just lowers the bridgedeck clearance, etc., etc.

    As mentioned earlier in a post, I have retained Stuart Bloomfield, former chief designer at Crowther to check things out...(been working with him for quite a while now and I might be braking a rule here...but I have to put in a plug for him because he is a very intelligent and considerate, understanding professional. I not only recommend him but would house him if he ever came to NY. He has integrity that I believed was extinct like the dinosaurs). So far so good, but he is still working out the response for the scantlings. I mention this because I want to see 4K#'s as displacement but it could change a little, so the design can remain unchanged up to 4.8K to 5K#'s before i have to revisit it.

    Now comes the kicker. I arrived at the displacement through carefull calculation of different areas, cubic displacements etc. What does it include? Hulls, all components and 2 motors of less than 100#'s each. I don't just utilize mass for strength, but geometry and specific properties for maximizing design strength. This is not easy and it didn't come easy and that is why I take my sails down for those that do this for a living.

    The hulls and components have "oppossing load" geometries which handle a substantial load before they begin to reach zero, so in essence there is a reserve load incorporated into all plates. This is not new...just not practiced that often because flat plates are "quicker and easier" than rounded. An example would be a flat plate and a plate with a small arc, both of similar material, weight and dimensions...place a load on the flat plate and measure your displacement beyond the plane, now load the arc until you reach that same displacement and you will see the difference can be huge.

    The interior weight savings comes from the fact that the design is a solid monocoque geometry without all the associated, and unnecessary displacements such as intermediary frames, longitudinals, etc. Does it save weight...you betcha. Also, the bridgedeck platform are the bunks, so, no associated framing or plates there. The accommodations are not...I repeat, they are not solid anything. They are aesthetic tube frames encircling a strong mesh for seats and tables and lockers are nicely designed as well...huge savings in weight there.

    The comparison with the racer is a good one although it may not be a fair one as it compares only displacement. I'm not being defensive or combatative. I have given performance variables and have carefully enumerated all other displacements whether cubic, linear or non-dimensional for the design up to the 6K# heavy limit. The entire design should be compared so that a whole picture is achieved. For example, my rig is designed to be offshore capable and must therefore meet specific accepted criteria...in this case, and the manner I chose to calculate, the stability factor based on bruce numbers. (I will publish these here later after I look them up), not to mention the fact that the sail area is moderate as oppossed to excessive. Why are sail areas excessive? I think it may have to do with insanity...but, good sail area, good underwater form and whala...you can only go so fast anyway.

    Yes, the design you compare has more displacement, and I don't disagree that most other designs that can be compared may have or do have larger displacements, but what about their sail areas? I'm willing to bet that their sail areas are huge by comparison right along with their SDR's, but are they considered offshore capable and are their rigs safe for offshore use?

    Well, I hope that I was able to give some clarity...continue your persistence...it is worthy of praise.

    Thank's
     
  14. fhrussell
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    fhrussell Boatbuilder

    Finally got them opened! Thanks Chris! ( ...had to go to the darkside and use a pc;) )

    Nice looking design and with interesting and well thought out features.
    I like the main traveler arch, but wonder if it can be extended further across the beam of the boat, ...and is it high enough so as not to be a nuisance when moving to your aft bench (nice!) Speaking of the aft bench, I am concerned that your displacement aft may be a bit tender for more than one person in that position in addition to a helmsman. It "appears" she may 'squat'. Perhaps fuller sterns? Also, how will she powered? Unless you drop an outboard through the bridge, you will likely be adding more weight in the aft sections.

    Initially, I feel she will likely draw more water than designed as is common for us optimistic designers. Also, it would be helpful to know what your building materials and/or laminating schedule will look like. I think the drawings are decieving though. According to your spec's, you have a waterline beam of 2.31. This is certainly adequate. The question will be "How light (heavy) is your bridgedeck and topsides/ crew/ supplies/ rigging?" It just appears to be a lot of topside structure for the amount of wetted displacement you have spec'd

    I won't comment on the rigging as it has been addressed earlier.

    Again, your bridge clearance may be a bit low at the stern. For a cat with this DWL, I like to see a minimum of 2', although so many cats these days get away with much less and on larger waterlines, so who knows? ...my preferance ... there is certainly nothing wrong with getting that bridge as high as possible. I do believe that anything less than 2' will yield some discomfort when beating. You did mention a concern for having sitting headroom in the berth.

    Take a look at this design:
    http://www.ikarus342000.com/P86cat.htm

    The design parameters are similar in that they have similar DWL's and similar accommodations, and hull & bridgedeck cross-sections (windage). I think you'll find that the P86 does have quite a bit more boat in the water than your design.

    The daggerboards and rudders are very efficient shapes for most speeds, particularly the envelope in which she'll likely spend her sailing life (5-15 knots). My only concern is when you are powering though a tight anchorage at 3 knots; will the rudder area be adequate?

    I'll admit that my comments and opinions are not mathematically backed-up. But, as a builder and sailor, I can give 'real-world' advice and opinion. We built a 44' catamaran sometime ago with a surprisingly similar hull crossection from the shear down to the keel. She has nice flair forward, relatively full stern sections, plum bow, and a slim waterline. When I looked at her lines, my gut feeling was that she'll sit lower than designed and possibly squat, which all were the case when said and done. She sat 3 inches too low and her transoms were a good 4 inches under too. It was the designers first catamaran design and he is a VERY well known NA, with hundreds of boats of all shapes and sizes on the water and he knows his stuff.

    I'll take a look at the new set you sent! Thanks! Looking to further the discussion......

    Frank
     

  15. JCD
    Joined: Jul 2006
    Posts: 359
    Likes: 3, Points: 0, Legacy Rep: 36
    Location: Coney

    JCD Follow the Bubbles!

    Hello Fred:

    It seems that this forum is requiring a substantial amount of my "response time" and I'm willing, but everyone please be a little patient if I don't respond right away and no offense will be taken if you remind me that I may have skipped you during my responses. I will go to your post and respond immediately.


    Thank you for the compliment Fred...it took a lot to get art and science (since I am neither) to fuse with the design difficulties that I set for myself and subsequently encountered...as duly noted, by everyone, windage and now more often, wetted area.

    Ilan Voyager rationizations for a consummate cat? I'll let that one go so I don't kick myself, but it's too bad he no longer posts...sounds like it could be helpful.

    I am designing for water form below the waterline outside, and the inside is a product of that design. Aerodynamic form above the waterline outside and then, the inside interior is a product of that design, always remembering that I will have to live in the thing and want to do so in happiness without having to feel like I'm in a dungeon.

    I love my flares! They are above the waterline, so no reason to be worried about the rocker or freeboard as it relates to below the waterline. Careful observation will show that the chine we are discussing just barely forms an inside corner threby negating the effect once it cuts the waterplane. Small little observation, but significant, because that turn of the chine would be more suceptible to "negative load reserve"...which is countered by the way I designed the connections.

    Thre are a couple of more things on the flares...they are aesthetically nautical oppossed to a straight up plate, they fend off waves associated with crabbing and forces them down for a drier ride and flater wake, they allow better attachment so that flow goes up and over the hull, they add upper room in the interior without added weight, they also provide an insane amount of bouyancy for the day that you're coming off a storm crest while a wave breaks of your windward beam and lifts your design to the point where:
    1. You need just enough reserve bouyancy there to keep her upright while she slides off the crest and,
    2. You managed to reef, took control of your faculties and hove her to long enough to change your soiled underwears.:D

    Trim. The following cut/paste is from a post I already submitted.
    CB X- 15.84' Aft (53.85% Aft)
    CB Y- .75'
    CG X- 15.66' (53.23% Aft)
    CG Y- 3.24'
    CE X- 16.05' ( Lead = 6.3% of Lwl @ 51.5% Aft @ Combined CE)
    CE Y- 20.35' (@ Combined CE)
    CP .57

    Notice that the CB for the hull is at 53.85% aft. Notice also that the CG for the hull is 53.23% aft. Both terms relate to the Lwl and not LOA. The combined CE is at 51.5% aft. There is a small moment there because they are not in line with each other. I have made every possible attempt to maintain that distance aft and that moment while incorporating all other components into the design. The LCF is positioned as far aft as I could possibly put it...around 57.1% aft. I know this sounds like it will increase friction, but let me explain what I remember reading and it made an insane amount of sense.

    There is a relation that exists to the rigs center of effort. If the LCF is positioned as far aft as possible the pitching motion is reduced if the LCB moves forward quickly as the bows are pressed. In most cases the CE is placed aft or near the CG...which in my mind will slow the movement of the LCB foreward when the hulls are pressed. (This is open for debate because I have been unable to find data or research to disprove it.) The design's CE is forward of the LCG and the LCB. While they work against each other trying to reach equlibrium, the LCB will move forward more quickly under the greater influence of the CE. (This also is open to debate as I have been unsuccessful in disproving it.) Another reason I went a little further aft is because the designed hulls don't have the wide hulls typical of modern cats in trying to eliminate or control hobbyhorsing.

    Thank you for yet another compliment, but delayed rocker just stretches out into a nice long run for ease to plane and the shallow stern keeps her from dragging...both pretty sensible and typical design considerations...but thank's for noticing.:)

    Small forefoot is small because it goes with the plumb bows and makes a good connection for the panels. I tried to increase it a little in radius but I kept losing depth, bouyancy and Lwl so I gave it up. The geometry for the panels didn't quite work too well either because they created a bulbous effect and killed the fine entry.

    Thank's
     
    Last edited: Jul 10, 2006
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