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Dinghy attachment to boat deck

Discussion in 'Technical Discussion' started by dan1000, Nov 1, 2010.

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  1. Capt J

    Capt J Senior Member

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    I too like the silicone in the screw heads, a very nice touch.

    I have run a boat with removable chaulks like that. I would highly recommend making a wooden dowel that goes between the two of them (bolts) through the eye hole to keep them from rotating loose in a rough sea and coming out. I have had that happen before with that same design, from the same place.
  2. dan1000

    dan1000 New Member

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    Hmm. UMT finally sent me a picture (attached) of the underside of the part that goes into the deck. I'm not sure whether I should be impressed or not. Certainly it looks pretty ordinary on the underside (ie: not "heavy duty").

    They way they are approaching it, each of those holes carries a screw that supports about 42 lbs of weight (assuming a 1000lb dinghy/engine/fuel/chocks).

    IMG_0031.jpg
  3. Bamboo

    Bamboo Senior Member

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    First off- resize the photo's so they fit in the screen. Many liferafts and dingys are attached with screws not thru bolted. I had the liferaft on the bow for many many years with screws and no issues. UMT is a AFAIK a reputable company- do you think they are cutting corners with the screws? Ask around and I think you'll find it's standard/acceptable to do what they are doing and you should have no problems if installed properly. BTW- I've had tef-gel on outside dis-similar metals with zero corrosion/galling after 6 years, and I'm not the only one who knows about this "magic snake oil". You make the call on the gel- listen to someone who doesn't use it and then dismisses it as snake oil, or someone who does and the results are proven.
  4. K1W1

    K1W1 Senior Member

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    Hi,

    Tef Gel works ok and so does natures very own Lanolin, the latter is even a reasonable anti fouling according to a couple of Trawler operators I once talked to.

    As for having your liferaft on the foredeck, I guess that is an easy way to get into it when it is washed onto the sundeck.

    You must be very lucky with your weather to be able to carry those sort of things on the foredeck.
  5. Garry Hartshorn

    Garry Hartshorn Senior Member

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    The fitting looks fine, but over the years I have witnessed many fittings that were screwed down being ripped out. I highly recommend that it is through bolted with a large backing plate and some thought as to how to deal with core material compression. Several options there, delron blocks, dig out as much core material as possible and flood with epoxy resin the list goes on.
  6. dan1000

    dan1000 New Member

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    There's one thing I haven't heard from anyone: Nobody has so far said "I mounted the dinghy this way or that way, and it was bad for my boat, or had to be redone. If you have experiences like that, I would appreciate hearing them. I know that can be a touchy thing to post publicly. Please PM me if you'd prefer.

    and in response to Bamboo:

    Slap on wrist appreciated, and accepted. I did shrink the filesize, but not the viewing area on a couple of the pics. Mea culpa.

    I agree about their good reputation amongst S. Floridians I've asked. Actually, the thought about cutting corners came from them. They said words to the effect of "Yes, we can do it the way you suggested (scrape and fill a larger area of core), but of the hundreds we do each year, only 1 or 2 are done in any manner other than just straight screw down with 5200. If there were problems with that technique, we'd have known by now. Our way involves much less labor, is much cheaper, and it works."

    But, this advice contradicts information I've read in Steve D'Antonio's and David Pascoe's articles about attaching things to cored surfaces, and in various articles (example: here and here), and clearly contradicts the advice of several posters here.

    TO EVERYONE: Thanks very much for sharing your opinions and experiences. I was hoping for a gem of wisdom that would make the decision easy, but it appears that a diversity of opinion is held on this subject. I asked a similar question on the Trawlers and Trawlering list, and received a similar diversity of opinion.

    Dan
  7. Marmot

    Marmot Senior Member

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    "But, this advice contradicts information I've read in Steve D'Antonio's and David Pascoe's articles ..."

    Pascoe? Oh, yeah the guy who tells his clients that the fuel in a diesel is compressed in the cylinder until it is hot enough to explode. :rolleyes:
  8. Bill106

    Bill106 Senior Member

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    After looking at the photos of the screw mounts you posted, I thought virtually all of the advice posted was in agreement with both Steve D'Antonio and Dave Pascal's publishings? In a nutshell, f the deck is cored and you are trying to support a heavy weight on localized points, you need to consider the crushing issue. Whether it be a metal or composite fill-in, tapped or not, something needs to transfer the loads between skins, period.

    The two big factors in any composite laminate are the skin composition and core thickness. Both directly affect the decks properties and can allow you to "get by" with less than optimum mountings. If, for example, you have a deck that has about 1/4" thick glass on both sides and a core of 3/4" (typical for polyester based decks), your dinghy would probably sit just fine on those mounts without any backing, filling or anything but screws into the top skin and would likely stay there unless you were in some really bad conditions.

    If it was a "higher tech" deck built to optomize the structure such as an epoxy based, vacuum-bagged laminate, the skins would likely be much thinner and the core thicker. Those thinner skins wouldn't hole the screws and the deck would probably dimple in short order unless you had some type of insert.

    Properly engineered, both methods will produce a strong deck, but the bagged, lighter weight deck will need extra attention because it's skins will likely be much thinner and can't spread the loads as easily as the thicker ones would of the heavier deck.

    I didn't intend to get too technical but I wanted to show how it isn't quite so simple a solution to just mount something on a composite structure without knowing all the details of her construction beforehand. If I'm not mistaken, didn't you host a herd of talent from these very pages during the boat show? The expense of a cold beverege or two sure would have been cheap compared to a naval architects fee for a design analysis, even if it had to be ran through a universal translator in the morning to make sense of it:D
  9. dan1000

    dan1000 New Member

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    In this case, the installer says that the screws themselves will grip both the upper and lower skins. I can see this being true if the threads don't strip. Clearly not as thorough as having a spacer to prevent crushing, or having solid resin to prevent crushing.

    Part of my problem is that while I'm hearing a lot of opinions about how this approach might cause problems, neither here nor on T&T am I hearing about people who have actually had a screw down approach for dinghy cradles over nidacore, and who have suffered as a result. My hesitation is really caused by the super-good reputation of the installer (UMT). I mean, heck, if you can't trust someone with a good reputation, who can you trust!

    Also, and in response to a prior comment, I believe (and am checking) that my dinghy's tie downs will (if I go with this installer) attach to the cradles themselves, not to the boat deck (as you can see in the pics, there are large rings on the cradles for this purpose). Therefore, there will not be a large tensile load on the boat deck. The boat deck loading will be caused by weight and inertia as the boat moves, but not by the pull of tie-downs.

    I did indeed, and beer was provided courtesy of JW Yachts. If only I had thought of these issues before the party, I would have had the benefit of a variety of equal and opposite opinions in person, rather than a variety of equal and opposite opinions over the Internet.:rolleyes: :)


    Dan
  10. lovinlifenc

    lovinlifenc Member

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    Well said!
  11. hamall9

    hamall9 New Member

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    hello dan1000,

    I would never through-bolt a core without reinforcement ... you will finish uo flexing the skins and crushing the core.
    Nor would I ever embed metallic sleeves.

    Perhaps you might like to consider this approach

    1 .. drill your bolt holes directly through both skins and core

    2 .. bend the tip of a small nail (several mm larger than the bolt) to act as a router, attach it to a small battery drill, angle it into each bolt hole and clean out the core... then carefully vacuum out the core holes ... make certain you get all the loose material out of the holes

    3 .. masking off the under skin holes with masking tape

    4 .. mix your epoxy and very carefully work it into the holes from the top side avoiding any air pockets.

    5 .. after the epoxy has set hard, sand off any expansion excess

    6 .. redrill the core and through bolt the fitting using whatever 'sticky stuff' you favour for waterproofing

    Have a beer (or two) because now you have joined the high-tec composite team

    hope this helps
  12. Marmot

    Marmot Senior Member

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    Yeah, and his mailbox probably needs cleaning out too! ;)
  13. dan1000

    dan1000 New Member

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    Just to report back to the group that the approach ultimately taken was essentially that reported above by hamall9. Holes were drilled, Nidacore was removed using an angled tool, the hole was filled, left to harden, and then drilled as a solid.

    Thanks to all for their input.

    Dan

  14. hamall9

    hamall9 New Member

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    Glad it all worked out for you.
  15. tumbleweed tim

    tumbleweed tim New Member

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    hamall9 is the man! I have a similar project and I will try you method. Thanks
    Tim
    Last edited: Jan 11, 2011
  16. hamall9

    hamall9 New Member

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    It is very important to preserve the core at all times and resist the urge to take a shortcut and screw a fitting thru it in an unprepared manner.

    Even screwing a hose bracket into the inner skin is a big no no ... no matter how short the screw ... in this case you should epoxy a pad [plywood] to the inner skin and then screw into the pad without touching the skin.

    goodluck
    h9