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Old 05-12-2020, 07:59   #1
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One for the engineers

Installing my cantilever davits, Iím running into a quandary.


The davits are made from 3Ē square structural tubing.

They are fastened to a roof and overhang by 3ft which provides the davits.

They are being through bolted to the roof using 1/2Ē stainless steel nuts and bolts.

Already drilled 1/2Ē holes through the top face, the bottom face and on through the roof. The bolts are bought to mate the lower face in contact with the roof to the roof. They were not sized to go from the upper face of the tubing through the 3Ē tubing and then through the roof.

Drilling this all 1/2Ē was actually a mistake because if Iím using a socket to tighten, I needed to use a 3/4 or 1Ē hole saw first on the top face THEN the half inch from the lower face through the roof.

So how to proceed??

I was thinking itís better to leave the 1/2Ē holes in the top face of the beam and use a hole saw to make holes in the side face of the beam in order to fit a box wrench in to hold the bolt while tightening from below. My reasoning is that in all box beans I see in life, people make holes in the web or side face. Never in the top or bottom. Thinking of car frames, the beams that hold my boat together, etc.

My other choice is to increase the size of the holes in the top face to use a socket.

Which will result in a stronger beam?

Thanks!
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Old 05-12-2020, 08:29   #2
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Re: One for the engineers

i'm very partial to using counter sunk hex bolts on external plates with the ends nicely polished up if there is sufficient wall thickness
https://www.westfieldfasteners.co.uk...RoC6qoQAvD_BwE
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Old 05-12-2020, 09:13   #3
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Re: One for the engineers

How about filling the 1/2" hole a wood plug in to guide the hole saw mandrel bit/drill for your 3/4" or 1" hole saw, then just re-drill the holes larger?
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Old 05-12-2020, 09:36   #4
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Re: One for the engineers

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Originally Posted by Bill O View Post
How about filling the 1/2" hole a wood plug in to guide the hole saw mandrel bit/drill for your 3/4" or 1" hole saw, then just re-drill the holes larger?
I can do that, but making holes in that part of a beam weakens it more than making them in the side. Thatís what Iím getting at. Which one would be better to do? Structurally
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Old 05-12-2020, 09:54   #5
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Re: One for the engineers

C-
Is the diagram close? My first thought was to buy longer bolts......the larger top hole might be the easiest from not having to feed bolts or nuts into the tube through the ends or side slots.

I would check the bolts and nuts to see if they are 'normal' or 'heavy' for 1/2". My 3/4" impact socket is 1.095" in diameter and my 7/8" is 1.255"

Some way to spread the load of the nuts on the underside of the roof would be good.
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Old 05-12-2020, 10:16   #6
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Re: One for the engineers

excellent question. I'm eager for a structural engineer to answer it. I don't think all readers have understood the question so far.
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Old 05-12-2020, 10:23   #7
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Re: One for the engineers

My opinion is the strongest solution would be to buy longer bolts, and put a spacer inside the beam so you can't crush the beam by tightening the bolts. That way you get the benefit of having both sides of the beam held down by the bolt. This is the same concept as putting core between two pieces of fiberglass for your hull, lots of rigidity.

I am an electrical engineer with lots of experience making mechanical things.

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Old 05-12-2020, 10:30   #8
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Re: One for the engineers

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Originally Posted by Spot View Post
C-
Is the diagram close? My first thought was to buy longer bolts......the larger top hole might be the easiest from not having to feed bolts or nuts into the tube through the ends or side slots.

I would check the bolts and nuts to see if they are 'normal' or 'heavy' for 1/2". My 3/4" impact socket is 1.095" in diameter and my 7/8" is 1.255"

Some way to spread the load of the nuts on the underside of the roof would be good.
This diagram is pretty much perfect. Except the davits of course go much further across the roof. I already have stuff to spread the load on the roof. Just looking for advice on whether or not to feed it from the side, or to enlarge the holes on the top. Structurally which is better? Thatís the question.
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Old 05-12-2020, 10:36   #9
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Re: One for the engineers

How long is the portion of the davit tubing that is on the roof? What is your bolt spacing? How far is each bolt from the edge of the roof? I assume two bolts per davit arm? What is your expected dinghy load?
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Old 05-12-2020, 10:44   #10
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Re: One for the engineers

I would not worry about a round hole weakening yr structure as long as it is perfectly round with no pointy bits, the stresses go around the hole so nothing to worry about. So i would simply make the hole larger so you can get a socket in there but make sure you add the largest washer possible. Another solution might be to get a longer bolt, pass it into the hole from the bottom then add one large washer and two nuts on the bolt to hold the lower section on to yr roof, assuming you can get an open ended spanner from the open end of the SS strut such that you tighten the first one then tighten the second against the first as a lock-nut, then you will still be left with some of yr bolt protruding out of the top where you could add a large washer. Better than having to use two nuts you could use a pair of locking washers made by guys like Nord-Lock. Then do the same again, alternatively a serrated type locking washer such as from Nordlock and one nut. If there is still a bit of bolt left you can cut this off to make a nice finish and avoid anyone injuring themselves. Alternatively if you decide the make the hole large enough for a socket to tighten the inside nut you could have a short length of thick steel plate made to span the larger hole and then to add locking washers and a nut.
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PS: I use a pair of the Nord-Lock washers under the nut that holds my feathering prop. The boatyard where i had the prop fitted had never seen anything like it in their life and insisted that I signed a waiver letter. Prop still doing fine. Never dropped off after another 3,000 nm and the nut remains tightened to the correct torque setting.
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Old 05-12-2020, 10:48   #11
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Re: One for the engineers

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Originally Posted by SaltyMetals View Post
I would not worry about a round hole weakening yr structure as long as it is perfectly round with no pointy bits, the stresses go around the hole so nothing to worry about. So i would simply make the hole larger so you can get a socket in there but make sure you add the largest washer possible. Another solution might be to get a longer bolt, pass it into the hole from the bottom then add one large washer and two nuts on the bolt to hold the lower section on to yr roof, assuming you can get an open ended spanner from the open end of the SS strut such that you tighten the first one then tighten the second against the first as a lock-nut, then you will still be left with some of yr bolt protruding out of the top where you could add a large washer. Better than having to use two nuts you could use a pair of locking washers made by guys like Nord-Lock. Then do the same again, alternatively a serrated type locking washer such as from Nordlock and one nut. If there is still a bit of bolt left you can cut this off to make a nice finish and avoid anyone injuring themselves. Alternatively if you decide the make the hole large enough for a socket to tighten the inside nut you could have a short length of thick steel plate made to span the larger hole and then to add locking washers and a nut.
Andrew
PS: I use a pair of the Nord-Lock washers under the nut that holds my feathering prop. The boatyard where i had the prop fitted had never seen anything like it in their life and insisted that I signed a waiver letter. Prop still doing fine. Never dropped off after another 3,000 nm and the nut remains tightened to the correct torque setting.
OK! Great. We are talking about holes along the upper face that are 1 inch. Just enough to get that socket in there.


I am using massive washers. Huge. Huge fender washers. And I use Nylok nuts on all things that I put together these days. So they won’t be going anywhere. LOL
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Old 05-12-2020, 10:49   #12
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Re: One for the engineers

if you're concerned about the square tubing collapsing when you tighten down on the bolt.....why not add a piece of similar material to the top...and bottom of the square tubing...in essence a square washer ??
Another thought....if you can reach in there, is to put a supporting tube (compression tube) inside the box....big enuff to allow the bolt to pass through....doesn't have to a tube....can be some square aluminum with a hole drilled thru' it...even with the bolt location furthest in, you can get a tube in there....just attach to a long piece of wood, with some bubblegum or something to stick the tube to the wood.........extract the wood, once bolt is in place...
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Old 05-12-2020, 10:51   #13
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Re: One for the engineers

I got a degree in 'engineering technology' specialized in fluid power (hydraulics) but did have some structural courses... from about the time the pyramids were built.

But, I wouldn't go in from the sides, stick to the top. Longer bolts would be better. If you already made large holes, could they be covered by larger washers?

Having said that, the sides are what support the load (bending monment) and keep the tube from buckling. If the side openings are far enough forward on the roof (if I understand Spot's layout correctly) toward the end of the tube opposite the overhang, then you can probably get away with it.
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Old 05-12-2020, 10:52   #14
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Re: One for the engineers

If I fully understand, you forgot to provide room for a socket to hold the bolt head when tightening?
-Buy a star washer to put under the head when you install it.
or
-It may tighten fine unheld with nothing actually.
or
-Put a common lock washer under the head bolt.
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Old 05-12-2020, 10:52   #15
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Re: One for the engineers

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Originally Posted by Muaddib1116 View Post
How long is the portion of the davit tubing that is on the roof? What is your bolt spacing? How far is each bolt from the edge of the roof? I assume two bolts per davit arm? What is your expected dinghy load?
Answering your questions. The davit tube continues 7 feet into the roof from the cantilever point. Bolt spacing is 21 inches, with an extra one at the far end. The end furthest from the dinghy. The bolts are nowhere near the edge of the roof. That is sort of irrelevant. The davits are each located about 4 feet from the edge of the roof. So they are in the middle of the roof in a lot of ways.

I donít recall all of the math that I did for the dinghy load. But I assumed a 14 foot rib, filled with rain water, filled with gasoline, with the outboard on it, and then doubled that. LOL

I have an 8 foot very light rib with a 9.9hp. And I donít let water sit in the dinghy.

My only question here is what is the appropriate way to make holes in a square tubing section to make it the strongest. I have two choices. Go in through the side, or large in the holes in the top. All of what I see in life shows holes through the web on the side. As I had mentioned in that first post. So I am trying to decide between enlarging the holes on the top, to about 1 inch, or going in through the side and making them maybe inch and a half.
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