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Old 31-10-2011, 17:28   #1
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Drilling Holes in Structural Bulkhead

Is there a rule of thumb for drilling through structural bulkheads? I would like to install a hot air heating system on my cat, with the burner in the starboard engine room. This would call for holes about 3" in dia through several transverse structural bulkheads in order to route ducting. The builder was clearly careful passing through these bulkheads because there is one reinforced conduit through which hoses and wires all lead, but it is not big enough.
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Old 31-10-2011, 20:46   #2
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Re: Drilling Holes in Structural Bulkhead

As long as you're not thinking of sailing to the Moon at Mach II,
in the words of Nike, "Just Do It".

If I had a dollar for every hole I've bored in my boat, I could buy a whole new boat.
I'm perfectly happy with the one I've got.

Getting ready to bore another 1 1/2" below the waterline,
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Old 31-10-2011, 21:20   #3
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Re: Drilling Holes in Structural Bulkhead

Big difference between a tri and a cat when it comes to structural bulkheads and the sizes of holes they can accommodate. Your Dragonfly essentially has NO bulkheads that are vulnerable to penetrations.

SVNeko, it would be helpful if you told us what type of cat and describe the bulkheads in more detail, particularly where you wish to cut the holes. Generally, you want to limit the size and number of penetrations through main beams, but smaller bulkheads like those in floors, etc tolerate much more.

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Old 31-10-2011, 22:40   #4
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Re: Drilling Holes in Structural Bulkhead

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Originally Posted by senormechanico View Post
As long as you're not thinking of sailing to the Moon at Mach II,
in the words of Nike, "Just Do It".

Remind me not to let you near my boat.

Through the ply on mine where there is no bracing, maybe, but I would rather not

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Old 31-10-2011, 23:42   #5
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Re: Drilling Holes in Structural Bulkhead

As I am sure you know some of the bulkheads in a Cat are watertight. Make sure it is not one of those.
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Old 01-11-2011, 00:44   #6
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Re: Drilling Holes in Structural Bulkhead

I spent 33 years as a structural engineer for Boeing. Here are two rules of thumb which are valid for both metal and composite.

1) Make all holes at least 1 diameter from an edge; i.e., at least 3'' from the edge for a 3" hole. Example: If a 7" wide bulkhead is supported (bonded to the hull, for example) on several sides with the remaining sides unsupported (a door opening, etc), you should favor the supported side to ensure a 3" edge distance from the unsupported edge (leaving 1" clearance from the supported edge in this example).

2) Make the new 3" hole as close to the existing hole as possible. The logic here is that (a) the bulkhead structure is already violated by the existing hole, and (b) the designer placed his hole in an area of light loads. The two closely placed holes will form a figure "8". The figure "8" should be oriented the same way as the longest dimension of the bulkhead; i.e., a 2'x5' bulkhead should have the "8" parallel to the 5' edge. This leaves the 2' dimension unaffected by the new hole.

If it makes you feel any better, bulkheads are usually sized to prevent buckling, and taking material from the middle of a panel does little to weaken its buckling strength.

Hope this helps.
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Old 01-11-2011, 06:27   #7
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Re: Drilling Holes in Structural Bulkhead

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If it makes you feel any better, bulkheads are usually sized to prevent buckling, and taking material from the middle of a panel does little to weaken its buckling strength.
How about if he cuts a 3 inch hole through the structural beams or uni's ?

Don't you think the strength would be considerably weakened then?
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Old 01-11-2011, 08:10   #8
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Re: Drilling Holes in Structural Bulkhead

Quote:
Originally Posted by Captain Bruce M View Post
I spent 33 years as a structural engineer for Boeing. Here are two rules of thumb which are valid for both metal and composite.

1) Make all holes at least 1 diameter from an edge; i.e., at least 3'' from the edge for a 3" hole. Example: If a 7" wide bulkhead is supported (bonded to the hull, for example) on several sides with the remaining sides unsupported (a door opening, etc), you should favor the supported side to ensure a 3" edge distance from the unsupported edge (leaving 1" clearance from the supported edge in this example).

2) Make the new 3" hole as close to the existing hole as possible. The logic here is that (a) the bulkhead structure is already violated by the existing hole, and (b) the designer placed his hole in an area of light loads. The two closely placed holes will form a figure "8". The figure "8" should be oriented the same way as the longest dimension of the bulkhead; i.e., a 2'x5' bulkhead should have the "8" parallel to the 5' edge. This leaves the 2' dimension unaffected by the new hole.

If it makes you feel any better, bulkheads are usually sized to prevent buckling, and taking material from the middle of a panel does little to weaken its buckling strength.

Hope this helps.
This is the first really useful and informed advice I've ever seen posted on this subject. Thanks and well done, mate!

Cheers,

Jim
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Old 01-11-2011, 08:22   #9
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Re: Drilling Holes in Structural Bulkhead

Since the subject is structural bulkheads, as in transverse bulkheads, besides the good information by Captain Bruce M, you only need to consider how the "structural bulkhead" is being used. You can augment or increase the remaining bulkhead's structural properties by doubling up its thickness around where you want to drill the hole. Or, add additional structural attachment tabbing to the hull.
- - Loosely analogous, an "I-beam" is a structural member but contains a lot of metal whereas a "Truss-beam" made of elements is just as structural but contains a lot less material.
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Old 01-11-2011, 11:31   #10
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Re: Drilling Holes in Structural Bulkhead

If one is cutting through a main beam on a catamaran, which has simultaneous compression, tension and torsion forces on it, doubling the thickness around the hole and/or adding additional tabbing is not going to help much.

Again, it all depends on which bulkhead(s) and where the OP wants to cut.

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Old 01-11-2011, 11:40   #11
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Re: Drilling Holes in Structural Bulkhead

A hole placed well away from edges in a non watertight bulkhead is not going to harm anything. JMHO.
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Old 01-11-2011, 13:19   #12
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Re: Drilling Holes in Structural Bulkhead

Be sure to cut a ROUND hole or a square hole with rounded corners, and have no sharp corners from where stress cracks would be prone to radiate. I would agree with thickening the surround of the hole to strengthen up that vulnerable area.
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Old 01-11-2011, 15:06   #13
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Re: Drilling Holes in Structural Bulkhead

Thanks for all the replies. Very thoughtful. The boat is a Switch 51. I don't know how to post pictures but there are many on the net. The bulkhead in question is just forward of the engine room. It is just aft of the helm. It is not a cross beam, nor connected to a cross beam. It is a watertight bulkhead, but I don't think that matters so much. The existing hole and this hole will be high enough such that if the hull is sunk that low, there is water forward of the engine room anyway. I think I will cut a round hole and glass in a piece of pipe, building up the layup around it for strength. Maybe overkill, but why not.
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Old 01-11-2011, 15:27   #14
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Re: Drilling Holes in Structural Bulkhead

Here is an idea - use a 3" bronze exhaust fitting bolted to the bulkhead and then attach the hose to each side.
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Old 01-11-2011, 15:45   #15
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Re: Drilling Holes in Structural Bulkhead

Osiris, good idea. The kit comes with a plug to attach the ducts to, but this looks much stronger.
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