Hat Section Bulkheads? These I DO plan to install.

Discussion in 'Boat Design' started by CatBuilder, Dec 25, 2011.

  1. upchurchmr
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    upchurchmr Senior Member

    Catbuilder,

    Bulkheads and ring frames have two different purposes.

    A bulkhead will take major loads spread them out and keep the shape of the boat stable locally. It has a significantly bigger capability to take loads and hold shape than a ring frame. It can be full of holes and still work for that purpose if you design it right.

    The ring frame is basically a stiffener. It does not have the overall strength to take as much load, nor as heavy of a point load (like the mast load). The ring frame (made in any number of different shapes) also redistributes load, just with not as much capability, and will distort with the hull since it does not have as much stiffness or strength as the bulkhead.

    Hat stiffeners work great, actually can be more structural than other open section shapes (C frames, Z frames, blades - either thick like a 2x4 or thin like a solid fiberglass shape). You can use anything to make the shape, if you use styrofoam just make the laminate thicker, cause the styrofoam is useless - it bends too easily.

    You can do the same thing as a bulkhead with a hat stiffener, but the hat will be so heavily built that it will weight more than the bulkhead, and it will not be the dainty little thing you showed.

    We use them in aerospace but do not replace bulkheads with them. They are used for local stiffening, not major structure.:D

    Marc
     
  2. Richard Woods
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    Richard Woods Woods Designs

    I agree with these comments and also with what Kurt says.

    Basically the top hat stiffeners are a heavy and inefficient way of stiffening a panel and don't replace a bulkhead. Use sparingly and locally. To be honest, they are usually added afterwards when sailors find the hull skin flexes in certain areas.

    Better to make a stiffer skin in the first place.

    One reason why I don't like open plan boats - there isn't enough furniture to act as stiffeners. Also of course in a seaway - even on a catamaran - you want to be able to move across the boat safely, and that means being able to grab a handrail, brace against a locker front etc.

    Houses are empty boxes, but are immediately filled with furniture. Empty floor space is wasted space as its only useful to stand on. Are you sure you cannot alter your interior layout to avid the use of these tophat stringers/frames?

    Richard Woods of Woods Designs

    www.sailingcatamarans.com
     
  3. CatBuilder

    CatBuilder Previous Member

    I think I only need one of these in the galley and one in the other hull opposite. Just in place of 2 bulkheads. Will put a picture up after work today.


     
  4. upchurchmr
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    upchurchmr Senior Member

    Catbuilder,

    Careful, they really are not equal to a bulkhead, certainly not two. There is no magic in structure. It has to be stiff (no noticable flexure) and strong.

    Of course the original two bulkheads might have been overkill. Excess is very common in design. It takes a good design/ analyst to get it right and that depends upon what they plan for loads (someone will always find a bigger wave or storm than planned).

    I have to disagree with Richard on the inefficiency. Properly designed hat stiffeners are more efficient than any others in the right situation - but every design has a "best" geometry, and you have to be real good to first reduce all of the structure to minimum weight. Then you can get caught in paragraph two above.

    One of the Forumites has a saying "do what you want, you will anyway". You can make it work, it just might be heavy. That works for either bulkheads or hat stiffeners

    Marc
     
  5. Richard Woods
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    Richard Woods Woods Designs

    There are a number of designs that have had to have extra top hat stiffeners added to help solve hull flexing problems. Team Phillips and Formula Tag for two.

    But I say they are inefficient for two reasons. First, if the panel is stiff enough in the first place you don't need the stiffener, so it just adds weight.

    Second, a stiffener added later has to be bonded to the hull skin. Say a 2in x 2in stiffener needs say 2in bonding onto the hull. Total width is 10in, yet really only 2in is doing any work, so only 20%, pretty inefficient. And roughly a stiffener only helps stiffen a panel for about 10 times its depth. So a 2in stiffener is needed every 10 inches. Just like a stringer frame plywood boat.

    In the 1950's racing cars were built as a birdcage structure with a light skin. Racing boats in the 1970's were built that way as well. But designers soon realised it was more efficient to design cars and boats with a cored shell.

    But unlike racing boats, cruising boats have an interior and that should be used to help stiffen the structure. So adding a top hat stiffener has to be a last resort and by implication means it is inefficient

    As we all know, boats are usually strong enough, its their stiffness that causes problems.

    Just my thoughts late on Boxing Day and BTW I'm talking generally, not specifically about one particular boat

    Richard Woods of Woods Designs

    www.sailingcatamarans.com
     
  6. CatBuilder

    CatBuilder Previous Member

    Ok, I said I'd upload a picture of what I'm doing. Here it is.

    Sorry for the large size picture, but it's low bandwidth. I know it might wreck some formatting, but the details need to be seen.

    So what do you think of this modification? I might add that there are hat section bulkheads in one of the many interior plans I have for an interior that utilizes bunk beds (the original design of this boat). In that design, Kurt knocks out the bulkhead you see touching the aft bunk (not the main beams, but the bulkhead near "closet"). He turns this into a hat section to fit more berths in at 90 degrees to the berth pictured.

    Also, this hull is stiff to start. The plans call for 3/4" 5lb/6lb (18mm M80/M100). I got a good deal on 1" (25mm) 6lb (M100) foam, so my hulls are made entirely of that. WAY stiffer than spec already.

    The idea of this hat section is just to get that bulkhead out of the galley counter area.

    [​IMG]
     

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  7. eyschulman
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    eyschulman Senior Member

    Is that a KH design?
     
  8. CatBuilder

    CatBuilder Previous Member

    Sure is. He's a neighbor of yours, I see. :)
     
  9. Richard Woods
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    Richard Woods Woods Designs

    So your hull skin is over 2.5 times stiffer than designed. It's got to be a major top hat to do anything useful. Why not just let the galley worktops shelves and lockers do the work??

    But where is your main chainplate - somewhere near I'd guess.

    Richard Woods of Woods Designs

    www.sailingcatamarans.com
     
  10. CatBuilder

    CatBuilder Previous Member

    Wow, Richard.

    I'm impressed. I can't believe you picked out the chainplate location that accurately without any type of mast or sail plan.

    The chain plate is anchored to the hull (not to a bulkhead) just aft of that bulkhead I'd like to change.

    It's exactly where that round sink is pictured in the layout above.

    You are picking up on my hint of the "entire area filled with galley worktop/counter, cabinets, etc..." I had figured I could use those to some extent, to support the hull.

    However, I am left wondering: The lower cabinets could support some of the hull section, then there is a blank space above the worktop, then upper cabinetry.

    That blank space would be unsupported. So, I figured I need the hat section back there, behind the cabinetry.

    Am I building this too much like a monohull? :D

     
  11. Richard Woods
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    Richard Woods Woods Designs

    The dissipation of the rig loads is a huge problem compared to the hull flexing between lockers. I assume you have lots of glass running down the hull side from the chainplates

    Personally I wouldn't bother with a ring frame in the situation you describe. Usually panels 2000mm x 500mm can support themselves.

    But of course I am not your boats designer so don't have all the facts.

    Richard Woods of Woods Designs

    www.sailingcatamarans.com
     
  12. upchurchmr
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    upchurchmr Senior Member

    Richard

    "... if the panel is stiff enough in the first place you don't need the stiffener"

    I have to agree completely, however the reason to add stiffeners is to thin out the panel (skin) to the point where it is not adequate by itself, then add the stiffener/s to make the total structure adequate. If you are willing to make a thicker/ heavier panel it certainly is simpler.

    "Second, a stiffener added later has to be bonded to the hull skin. Say a 2in x 2in stiffener needs say 2in bonding onto the hull. Total width is 10in, yet really only 2in is doing any work, so only 20%, pretty inefficient."

    Simplistic baloney. The value to any stiffener is determined by the Moment of inertia which increases as the CUBE of the height. So the 2" width of the cap is 8 times the value of the same area bonded to the skin. Of course, the cap is not the only thing adding stiffness, there is the 4 inches of "flange" bonding the stiffener to the skin. Additionally the upstanding legs add to stiffness, just not anywhere near what the cap does. So for a local area we have 8x+ the stiffness added.

    "And roughly a stiffener only helps stiffen a panel for about 10 times its depth. So a 2in stiffener is needed every 10 inches. Just like a stringer frame plywood boat."

    Simplistic but basically true. The actual spacing depends upon how minimal you want to make the weight and how good your analytical skills are. It also depends upon the width of the stiffener. A 2" stiffener on 10 inch centers actually only leaves an 8" unstiffened panel, one of the real benefits of a hat vs a blade.

    ".... designers soon realized it was more efficient to design cars and boats with a cored shell."

    They also soon after realized that typical problems with core bit a lots of people is the a**. Come look at the old Tornado balsa cored fiberglass catamaran in my back yard. You will wonder where the balsa core dissappeared to - rot is typical. Foam is typically very low strength and suffers local shear failures which leaves you with the stiffness of one skin, not even the 2 skins worth. Core can be upsized to make it work, at the cost of weight but it can still be much better than solid skin. Just don't put a bolt thru it. What cars do you know of that have core? None except for pure race cars.

    "But unlike racing boats, cruising boats have an interior and that should be used to help stiffen the structure. So adding a top hat stiffener has to be a last resort and by implication means it is inefficient"

    Anything you are going to have should be used for multiple purposes - I agree completely with using the interior to stiffen the boat.

    "As we all know, boats are usually strong enough, its their stiffness that causes problems."

    Richard in spite of my great respect for you and your boats, you just missed the point and pointed it out yourself. A hat stiffener is to solve a "stiffness" problem. Not a strength problem - which is why I previously said if you need a bulkhead you should not use a hat "stiffener".

    You could have brought up the "typical" homebuilders cored structure - fiberglass with a plywood core (or cedar core for kayaks and canoes). The properties and stiffness of glass is so much higher than wood that it really makes a difference, unless you use so much core the glass doesn't matter. One of the real reasons it works is the wood holds the the thin fiberglass straight - without the wood the thin glass would buckle out of the way and not be valuable at all.

    Hat stiffeners make a very structurally efficient panel, approaching that of a cored panel, without the issues of a cored hull. Of course they have their own issues involving peel off, especially in a collision.

    Simplistic comments do not educate the person who wants useful knowledge. Of course catching up to a professional yacht designer will be real tough to actually do. For what it's worth.

    Respectfully,

    Marc
     
  13. upchurchmr
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    upchurchmr Senior Member

    Catbuilder,

    Hughes knows what he is doing, ask him.

    You just saw what the total knowledge of a pro can do with Richards assumptions about the chainplates.

    Don't screw around with such a large investment.

    Good luck,

    Marc
     
  14. CatBuilder

    CatBuilder Previous Member

    Just ask him... :D If there are any KH builders here, they will tell you that while he is available once a month or so for a question, he does not take kindly to little questions like this. He is also re-certifying all his inspected vessels to meet the new weight of the average American passenger, so he is up to his eyeballs in paperwork right now.

    If I could ask all my questions to Kurt, I would. You can't ask him little stuff like this. I already asked him about this some time ago. My responses:

    "You only need a bulkhead every 8 feet"
    "Do a hat section"

    Me: "What's your definition of a hat section? Scantlings?"

    Him: "I'll make a blog post"

    http://multihullblog.com/2011/04/properties-of-hat-sections/

    That's all the info I have from him.

    So, you can see why I need to ask things here.
     

  15. Richard Woods
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    Richard Woods Woods Designs

    To upchurchmr

    Some people may think we are disagreeing. I don't. Its just the medium we are communicating in is so difficult for a discussion.

    For example I agree about the 8 times stiffness on the top of the hat. That's why I said that was the 20% part doing the most work

    And of course I'm writing as I think, so maybe too quickly. So I could have made it clearer that I was only ever talking about racing cars and boats. But then no car has a real chassis these days, it's all held together by the floor pan.

    I write simply because I know many people reading this forum don't know as much as you do and "every little helps"

    Richard Woods of Woods Designs

    www.sailingcatamarans.com
     
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