One for the engineers

[Chotu]

Oh boy. I guess I’m not being clear. Or no one is understanding this. I’m just looking for advice on what makes a beam more strong. Putting holes in the side? Or putting holes in the top. That’s the question.

[Cheechako]

Originally Posted by Chotu

Oh boy. I guess I’m not being clear. Or no one is understanding this. I’m just looking for advice on what makes a beam more strong. Putting holes in the side? Or putting holes in the top. That’s the question.

Generally I would say putting the hole in the top is stronger. The vertical sides of the tubing hold the load much more than the flat top metal. In reality, even using a channel shape, open top, in lieu of tubing would be nearly as strong.

The high stress part of the loaded "beam" will be right where it exits the roof. So keep holes away from there for best situation.

[Rapanui]

The force on the aft bolts will be insignificant, they just locate the davit laterally. The force on the forward bolts will be ~= 1/2 W x ratio( beam overhang/bolt to bolt distance) where W is the total davit load.

You don't mention what material and thickness the davit beam is and the overlap of the beam on the roof but unless the davit beam is very thin or its material is weak, the load on the forward bolt is pretty small, which depending on grade can take upwards of 5 Tonnes. So unless the bolt to bolt distance is really short. there should be no issue bolting through just the bottom of the box section.
Ideally you should a large washer under the bolt head to spread the load. If you can insert the bolts and washers from the end of the section into the holes, you can cut a spanner in half and extend the end so you can lock the bolt while you tighten the nut under the roof. Alternatively use socket headed bolts and drill small holes in top of davit beam to insert an Allen key to lock the bolt. If you have insert the bolt through the top section of the beam, use socket headed bolts and drill the top section just large enough diameter for clearance which will smaller than using a hex bolt.
Best would be longer bolts through right through davit beam with washer.

[garyfdl]

Originally Posted by Chotu

Oh boy. I guess I’m not being clear. Or no one is understanding this. I’m just looking for advice on what makes a beam more strong. Putting holes in the side? Or putting holes in the top. That’s the question.

In your application? Putting holes in the top.

Where's the rainwater going to go?

[Muaddib1116]

Originally Posted by Chotu

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 don’t 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 don’t 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.

These details inform us of what the load on the bolts and the area of the beam where the bolts will be, it is not irrelevant to your initial question. That being said, with the way you've described it and how much you've over built it, having a 1 inch hole in the top of a 3 inch box beam shouldn't be a problem.

[MicHughV]

what about the pulleys and cleat attachment points ??
have you allowed for lateral movement ?? Dinks in davits sway quite bit.
Have you allowed for a safety factor...someone hanging from the davit, etc...

To avoid confusion, a sketch or diagram would be most helpful here ??

[canyonbat]

Originally Posted by Cheechako

Generally I would say putting the hole in the top is stronger. The vertical sides of the tubing hold the load much more than the flat top metal. In reality, even using a channel shape, open top, in lieu of tubing would be nearly as strong.

Cheechako has it right. The sidewalls provide vertical strength and the top and bottom provide lateral strength which would seem to be less critical in this application. I am not a structural engineer but I did manage a steel fabrication company that specialized in very large steel bridges.

[Echtagon]

A larger hole in the top just big enough for a box wrench will not drastically reduce the strength from the one you have already cut.
However I would suggest putting some epoxy glue under the head of the bolt and pushing it home hard. Tighten from below when epoxy is set. It doesn't take much torque to stop a bolt turning if the threads are clean.
If that fails you always have the choice afterwards of drilling a larger hole for the box wrench.
Or you could cut a small slot in the bolt head and hold it with a screw driver with some one else tightening nuts from below.
I am structural engineer but also an old boat and old car enthusiast and am always having to find cunning ways to reach awkward nuts and bolts.

[MicHughV]

you'd definitely want to be thinking about some x-bracing here....I'd like to know the wall thickness of that 3x3 square tube.....

like some others here....I'm a retired structural marine engineer....many ways to slice this pie....but need some more input from you...

[Chotu]

Ok thanks!

My observations of the world seem to be amiss here.

All the other structural square beams I see have holes in the web only for some reason. Frames on trucks, the box beam that holds my aft end of my catamaran together, etc.

Based on observation with zero research, I figured cutting holes only in the side was the way to go.

Thanks to those of you who contributed to the thread. I was actually put on errands during the thread and was deciding if I should pick up a box wrench to go in from the sides. Happy I’ll be able to use a socket from the top now. Very happy. It’ll be a lot easier and will look better too.

[wrwakefield]

Have you considered using socket head cap screws so you can use an allen wrench to hold the bolt head through the hole already drilled for the fastener?





Or you could cut a slot in a hex bolt head to accommodate a screwdriver... finish tightening nut with an impact wrench so not much holding torque is needed at the head— especially if a star washer is inserted under bolt head...

In case this would work for you...

Cheers! Bill

[Thumbs Up]

Originally Posted by Echtagon

A larger hole in the top just big enough for a box wrench will not drastically reduce the strength from the one you have already cut.
However I would suggest putting some epoxy glue under the head of the bolt and pushing it home hard. Tighten from below when epoxy is set. It doesn't take much torque to stop a bolt turning if the threads are clean.
If that fails you always have the choice afterwards of drilling a larger hole for the box wrench.
Or you could cut a small slot in the bolt head and hold it with a screw driver with some one else tightening nuts from below.
I am structural engineer but also an old boat and old car enthusiast and am always having to find cunning ways to reach awkward nuts and bolts.

Don't mention epoxy to Chotu! You can't use a box wrench from the top. You need a socket. Neither method allows you to insert a large washer. You can use tape on a nut or bolt head to create a tight fit into a wrench or socket for difficult to reach positioning.
I suggest using carriage bolts through the top face of the beam. The rounded heads will neatly cap off the top holes and eliminate the need for top washers. Put the nuts on the bottom. I am curious to what type of fender washers you are using. They need to be very thick to be effective. Stacking them doesn't help much. The bolts, nuts, and washers should all be 316 stainless, if not they will bleed rust stains in short order. You may need to use compression sleeves. Pushing them in from the end of the beam to the correct position will be a challenge. I thought of a way to position the sleeves, let me know if you are interested.

[Chotu]

Originally Posted by wrwakefield

Have you considered using socket head cap screws so you can use an allen wrench to hold the bolt head through the hole already drilled for the fastener?





Or you could cut a slot in a hex bolt head to accommodate a screwdriver... finish tightening nut with an impact wrench so not much holding torque is needed at the head— especially if a star washer is inserted under bolt head...

In case this would work for you...

Cheers! Bill

A brilliant work around!

I’d do this in a second, but I have all the hardware already and it takes weeks to come in.

I’ll go at it by slightly enlarging the holes on the top face to allow a socket to fit through.

Based on all the responses, this won’t compromise the “upward and downward” strength of the beam.

Easiest solution too. Very happy it’s also structurally a good one.

[Chotu]

Originally Posted by MicHughV

you'd definitely want to be thinking about some x-bracing here....I'd like to know the wall thickness of that 3x3 square tube.....

like some others here....I'm a retired structural marine engineer....many ways to slice this pie....but need some more input from you...

1/4” wall. Frp. I did all the calculations and they’re up here on the forum actually.

Basically at the install stage here, although I hope you pop in on other similar threads I will post at some point.


Thank you

[Howard888]

The best way to spread the load is with a compression sleeve that fits around the bolt between the two inner surfaces of the box beam. That way the bolt goes through both top and lower sides of the beam and the force is distributed evenly.
An issue I see with things like star washers and the like is that you will be setting the structure up for dissimilar metal corrosion in a salt filled environment.
I am an aircraft engineer.