Reducing floating cockpit deflection.

Discussion in 'Fiberglass and Composite Boat Building' started by fallguy, Nov 2, 2018.

  1. fallguy
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    fallguy Senior Member

    The angle irons were only deep enough to fit the floor. So a 24mm deck would get say a 3/4" angle iron. A one inch iron even starts to get rather heavy with little stiffnes gained.

    Did you have autopilot G?

    Where did you home the pump?
     
  2. groper
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    groper Senior Member

    yes i had autopilot. I mounted it directly under the helm with short lines T-d into the steering lines. The outboards were linked with a hydraulic link between each steering cylinder and a bypass valve which could be opened in case the engines needed to be aligned. In reality - they never went out of aligment.

    I thought as much about your floor using small angles to join them. You have a big problem in the center approx 1/3 the way forward from the rear edge - which im assuming is also supported? if the floor is supported along the rear edge and entire perimeter, the maximum deflection is going to be located here, x marks the spot;
    upload_2018-11-5_21-16-5.png

    If the rear edge is unsupported the max deflection will right on the rear edge at center.
    You wont gain much stiffness from the angle irons- they just dont have the depth - so you can basically ignore their contribution to stiffness.
    The beam i made to support the center line of my floor was nearly 10inches deep - so it was very stiff.

    I think you could do something similar and incorportate the joining process at the same time. Ie - use a beam to join the floor panels down the center and solve the stiffness problem at the same time. You also retain your conduit run for cables etc inside the beam :)
    Or you can further split the panels by using 2 beams which means the unsupported floor spans are now 1/3 of the athwarthship span. This also assumes you can get good connections to a foreward and aft athwartship main beams from which to connect these said fore/aft floor support beams to.
     

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    Last edited: Nov 5, 2018
  3. fallguy
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    fallguy Senior Member

    The reason for the post was my concern at the x essentially. I don't plan for a fighting chair, but it would be right about there.

    The rear is supported with angle iron held to the aft athwartships beam, but the beam is up pretty high for any connections like another for n aft beam. Not saying it wouldn't be possible, but the beams are ally sailboat mast. I won't disclose the size for copyright purposes. I suppose some connection could be bolted to them. The angle irons were going to be 316ss. Once I put it together and don't like the deflection, I could probably attempt an ally center beam or 1/3rd beams or even 1/4 beams. Something like ally vee shaped and bolted. Maybe like 4" deep each. The weight wouldn't be too bad.

    Another person told me honeycomb core would deflect less. And if I run the glass strands the right way on the bottom; that would help.

    Does anyone know how to calculate the deflection? Especially to compare say honeycomb to corecell?
     
  4. groper
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    groper Senior Member

    the core material type has almost zero effect on the stiffness when considering all of your typical lightweight core material like foam, honeycomb etc.
    Please see the attached PDF which explains the material and physical parameters which effect sandwich panel stiffness and other structural engineering of same in full mathematical accuracy. Typical material mechancial properties are located in the appendix at the end.
    Calculating the deflection and strength equations are all in there too. I put all these equations into an excel spreadsheet so that i could easily manipulate the parameters to satisfy design criteria - however the spreadheets are in my old computer so i cannot share with you as i dont have access to it at this time...
     

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  5. JamesG123
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    JamesG123 Senior Member

    With all the metal and non compositey bits you could just as well weld the thing up from SS or Al, tack down some perforated tread plate, and call it a day.
     
  6. fallguy
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    fallguy Senior Member

    I have to be able to move the panels and store them in the cabin for transport.
     
  7. JamesG123
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    JamesG123 Senior Member

    Tabs? Slots? Hinges?
     
  8. fallguy
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    fallguy Senior Member

    They are going to screw in from above with a flat head socket head cap screw that will have a dollop of loctite low.

    The angles underneath will have the nut welded in place.
     
  9. groper
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    groper Senior Member

    How did you go fallguy, did you find what you were looking for? As you can see below on bending stiffness;

    upload_2018-11-6_8-27-49.png

    See the quote above "the most efficient way to reduce deflection is to increase core thickness and thus increase the skin seperation and the value of h"
    The value of h is the total depth (thickness) of your panel and you can see from the equation stiffness is proportional to h squared. Therefore if all else is constant, a 24mm cored panel will be 4x as stiff as a 12mm cored panel. (12x12=144 vs 24x24=576) in the multiplier. Stiffness is simply a number which you can use to compare other materials or stuctures with.

    If you want to calculate deflection then you have another step to complete;


    upload_2018-11-6_8-20-18.png


    From this you can also see that the deflection is proportional to the l cubed. l is the distance between supports. So if identical panels and you double the distance between supports you will have 8x the deflection in a simply supported scenario.

    Do you need anymore clarification with any of this? I hope i have been able to give you something to think about.
    1 more thing thats not covered in the above document - design deflection for floors. In std construction for buildings and homes etc the std deflection limit is usually 300:1. Which means max deflection for the live load is usally defined as 1/300 of the span between floor joists. This is a bit excessively heavy and difficult to design for on a recreational boat where we are trying to keep things light. So a good number to use is about 1/200. You will feel a slight springyness but not too much and as is what i designed for in my boat which felt acceptable and confident underfoot as opposed to cheap and flimsy. The live load i used was the weight of a 100kg man or 1000N.
    So for my boat with 1200mm max spacing between supports, a 100kg man standing in the center, farthest from any panel support would not produce a max deflection exceeding 6mm. I needed a 20mm foam core to achieve this minimum.
    I hope this has helped.
    All the best,
    Nick
     
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  10. fallguy
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    fallguy Senior Member

    We worked on the boat all day.

    No time to review the data, but thanks a bunch.

    I can tell you the aft beam is above the floating cockpit.

    Any midship beams would attach to the bottom of it.

    Conversely, the middle beam of the boat is just under the floating cockpit and any attachments to it would be onto its side.

    I will work on the data in excel. Thanks.
     

  11. Ad Hoc
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    Ad Hoc Naval Architect

    Hi Nick,

    Life, same a ever, just bimbling along.
    I can see you've had one hell of an adventure, insanely jealous!! But you deserve it, worked hard on the boat - kudos matey.
    Mind you, I did do much of my global travelling after Uni and in and around different jobs years ago. My travels now are paid for by clients, but usually just short trips, not endless vacations like yours :(

    You're giving Fallguy good advice, no need for me to chip in.
     
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