Reinforcement of panel cut-outs

Discussion in 'Boat Design' started by Midday Gun, Apr 20, 2021.

  1. Midday Gun
    Joined: Mar 2019
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    Midday Gun Junior Member

    On most boats you'll see a lot of cut outs in the deck / cabin sides / cockpit / coamings etc. For windows / instruments / hatches / hardware etc. Presumably these all reduce the strength of the panel.

    I'm just wondering if there are any rules of thumb as to how much is safe to remove over a certain area, distances between cut out etc.
    I have an area to starboard of the helm on my 27'er that's a little cluttered, cut out for bilge pump, autopilot electrics, someone put a access spin off panel for a speaker, a pad eye to clip on etc.
    It may also need to receive the tiller pilot mount soon which will put some force on it as well.

    I was thinking of laying up some glass or maybe bonding in some G10 plate in way of the larger cut outs at least to try and add some strength back into the area, is there any quick & easy to way to estimate how much is needed, or am I worrying over nothing?
     
  2. wet feet
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    wet feet Senior Member

    Other than wanting to be assured that the pad eye is sufficiently strong for the job,I would be inclined to ask myself how a large load could be applied to the area.If it took a lot of imagination to come up with a good answer,it may be that there is no more than a very remote likelihood of it happening.If we were discussing a traditional wooden boat that had a lot of holes in a solid piece of wood,there may be a possibility of the short grain between holes breaking,but as the stated preference is to add glass then it seems likely the boat is GRP.If you have just bought the boat I would have thought a surveyor would have highlighted the issue.If you have had it for a while then perhaps you should ask why you need to fix something that isn't broken.
     
  3. Midday Gun
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    Midday Gun Junior Member

    I'm talking in generalities really. Its something I'm curious about, how much is 'too much' when it comes to cut outs. I've seen boats with coamings that look like swiss cheese from old instrument holes & other cut outs.

    With regards to my example, I'm sure its fine now, but I'd like to move my tiller pilot mount there which will add back and forth load onto it, the GRP isn't mega thick & is uncored in the area so can flex a bit. I was simply wondering if there are good rules of thumb that are followed.
     
  4. TANSL
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    TANSL Senior Member

    The correct thing is to surround the opening with reinforcements, separated from the edges enough so that they can be correctly attached to the panel.
    You should never go "too much", the designer should try to get "enough" to support the existing loads on each element. And "enough" is obtained by calculating structures, applying the safety factors established by the standards.
    The best "rule of thumb" is calculation.
     
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  5. Midday Gun
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    Midday Gun Junior Member

    Point taken.
    That said, most people aren't going to learn structural calculations just to install some speakers in their 30 year old boat or commission a naval architect to do the same.
     
  6. fallguy
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    fallguy Senior Member

    I think some pictures best. Start from farther away and then closer in from same angles.

    tansl makes a very good comment that overgeneralizing in boats is not best

    on the other hand, cutting out a spot for a switch creating a problem means the panel was underbuilt to start..many things in boats are designed with safety margins and a small cutout is not an issue, but to reply that here is incorrect ftmp because your post is jist too vague

    further, g10 is not likely needed, but a layer of glass may be needed

    with your picture; also point out the planned mounting
     
  7. TANSL
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    TANSL Senior Member

    It seems to me that you are overestimating, "too much", my comment. It is "enough" to place some reinforcements similar to those that the ship already has in the hole area. It's that simple, without learning anything nor commisioning any technician, just common sense.
     
  8. Midday Gun
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    Midday Gun Junior Member

    Pictures aren't great as I've not took any specific in this area.
    20170620_145247.jpg
    IMG_20180725_122305.jpg
    You can see already two large cutouts, one for bilge pump & the other has been made for speakers at some point.
    The other holes are much smaller and just fasteners in most cases.

    Autopilot sits as it is on a drop in panel at the aft of the cockpit, this is less than ideal as the thing isn't a mega tight fit, it moves around as the autopilot works, introducing play into the system. Additionally when racing we take it out anyway (the drop in panel) as it weighs a fair bit and takes up room in the cockpit. Its normally only useful for covering the liferaft.

    I want to mount an autopilot cantilever arm probably just below the existing stuff.
    Layup in that area is (at least according to the drawings):
    layup.PNG

    Its a non cored area, so it would seem to imply its all CSM.

    I mean its not going to cause the boat to break up, but I don't want to start causing stress cracks or whatever through lack of suitable reinforcement.
     
  9. Midday Gun
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    Midday Gun Junior Member

    My plan was just to add a layer or two of biax on backside, and normally I would have just gone ahead, but I was curious so I wondered about possible rules of thumb. Hence the post. Now I feel like its blown out of all proportion simply from my curiosity!
     
  10. TANSL
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    TANSL Senior Member

    As I was saying, in this and in everything, it is good to apply common sense. No matter how many layers you add to a panel that is poorly supported on its perimeter, you will not achieve anything practical because the panel, although very resistant, will bend.
    In the picture you will see the similar or equivalent perimeter reinforcements that I was referring to.
    Snap2.jpg
     
  11. gonzo
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    gonzo Senior Member

    You don't have a structural problem. Those boats are not ultralight and highly stressed structures. Boats will develop gelcoat cracks with age; it is inevitable. However, that is largely caused by UV degradation of the gelcoat.
     
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  12. TANSL
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    TANSL Senior Member

    It is dangerous, and unprofessional, to claim that. The ship as a beam will not suffer, the structure as a whole will not suffer, but it can be said, generalizing, that there may be (and will be) local problems if the openings are not properly reinforced. The cracks (not only gelcoat craks but laminate cracks) will not take long to appear, although, of course, the ship will not sink.
     
  13. fallguy
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    fallguy Senior Member

    It is not blown out of proportion. When you ask others a very vague question; it is bound to result in more questions and confusion. The other forum members have a history of some back n forth. But honestly, they both have a degree of correct in their postings here, imo.

    If you can calculate the forces on the lever arm you plan to add; you would be able to properly determine the required laminate and/or backing plate.

    If it is a cantilever force, for example, it can quickly become a lot and yes, it can stress, a chopped strand mat layup. If it is at the bottom, the sole and that 90 also provide some rigidity.

    I do not profess expertise in providing the laminate schedule. But the rule of thumb you speak of does not exist.

    The force on the skin from an active load is calculated and then the skin is specified.

    And you are correct, that it does not require an NA for this application, but it does take some prudence.

    Based on the crude pictures, your knowledge of forces on the tiller, you come up with the calc.

    For example, if the maximum force on the tiller is 80 pounds; it appears to be applied to say 6 square inches, if the base is backed. Thus the force on the skin you cited is roughly 13 psi (using my guesses). You then establish some margin of safety; say here 2-3. And then you say my force is 35 psi with a good safety margin on 6 square inches.

    And then someone with a wee bit of engineering knowledge can answer whether the skin supports that. Or you stick a finger in the air and say I like 5 psi.

    I am not sure Tansl has considered you can open the deck plate and use a considerable backing plate here and bond it to the inwhale. But that is what needs to be done if you ask me. I cannot answer the size real well, but somewhere in the neighborhood to 5 psi seems decent. So, in order to reduce 35 psi to 5 psi; you need 7 times the back or 42 square inches. Roughly 6.5" backing plate. It could be jist about anything, but bonded glass of 1/4" thickness would be really good.

    Anyhow, this is how it is done crudely.
     
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  14. wet feet
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    wet feet Senior Member

    looking at the autopilot installation, I would say that is the weak link in the system and it seems that is in the process of being modified. We can discuss theoretical situations until we are blue in the face but the cutouts for the bilge pump,shore power and inspection hatch all have round corners.If you had engine instrument panels with square corners that raised stress then you would be more likely to have stress cracks in the corners.In essence,the builders of the boat had an idea of what they were doing and built accordingly.The fellow that did the autopilot installation wanted it to work but didn't do a great job and I would imagine that the revisions under consideration will lead to an improvement.Getting behind any of those cutouts to add glass won't be a particularly pleasant job and if the surface has been flowcoated, it doesn't get nicer as you have to thoroughly key the surfaces.
     
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  15. missinginaction
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    missinginaction Senior Member

    Gonzo has it correct. Look at your gelcoat. It is your early indicator of an over stressed structure. I'm seeing no cracks in the gelcoat AND the two ports that the OP mentions are circular and oval shaped. These shapes are going to dissipate loads. I don't see the need to fix something that isn't broken.
     
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