Engineers? Opinions on this please...

[Snowpetrel]

I just ran a few numbers, got something like 2.37 mpa max fibre stress vs whatever figure you want to use for wood. Ive found wide variations online, anywhere from 8 mpa through to 100 mpa? So if these figures are somewhere near correct it seems like a pretty decent safety margin.

The aluminium beam on its own was not so flash. 597mpa max fibre stress vs 276 yeild. Though the fact that it is imbedded inside the beam it should mean it sees much less load due to the neutral axis being much further from the edge. However the real neutral axis would be hard to figure out when you allow for flex.

I've only got a diploma in mech eng, and I am a bit rusty so it would be good to get these figure checked.

[patprice]

Originally Posted by wyb2

One note after re-reading - in a simple rectangular beam increasing the width increases it's strength linearly, make it 2x as wide and it will be 2x as strong.

Increasing it's height (what you are calling thicker) increases it's strength to 3rd power - 2x as tall will be 8x as strong.

I = base * height^3 over 12

I think I am correct in saying that the downward force exerted by the mast is equal to the combined tension forces in the windward shrouds. Upper and lower etc. Riggers have ways of measuring these forces by measuring the strain, or stretch, in the rigging members.

The above may be simplistic?

[jimbunyard]

Any 'engineer' that says that this



or this


is stronger than this




has no reason to be taken seriously at all...

That's not to say that there aren't other considerations to be taken into account, but the substantial increase in strength of the modification over the original is not in question...

[Snowpetrel]

Originally Posted by patprice

I think I am correct in saying that the downward force exerted by the mast is equal to the combined tension forces in the windward shrouds. Upper and lower etc. Riggers have ways of measuring these forces by measuring the strain, or stretch, in the rigging members.

The above may be simplistic?

Probably the smartest way to size the compression post or beam is to use the buckling load for the lower panel of the mast plus a decent safety allowance since it is better that the mast breaks than the compression post. To get this we need the Ixx for the mast, the modulus of elasticity, and the panel length from deck to the lowest spreader.

The total conpression loads are pretty hard to accurately estimate. We can pretty easily calculate windward shroud loads but leeward shroud preloads, backstay and forestay tensions are harder to work out, as are halyard loads and dynamic loadings.

So using the mast breaking strength as a start point makes some sense, at least to me.

[Snowpetrel]

Originally Posted by Snowpetrel

I just ran a few numbers, got something like 2.37 mpa max fibre stress vs whatever figure you want to use for wood. Ive found wide variations online, anywhere from 8 mpa through to 100 mpa? So if these figures are somewhere near correct it seems like a pretty decent safety margin.

The aluminium beam on its own was not so flash. 597mpa max fibre stress vs 276 yeild. Though the fact that it is imbedded inside the beam it should mean it sees much less load due to the neutral axis being much further from the edge. However the real neutral axis would be hard to figure out when you allow for flex.

I've only got a diploma in mech eng, and I am a bit rusty so it would be good to get these figure checked.

Attachment 149663Attachment 149664

The discrepancy between the smaller aluminium beam and the larger wood beam was bugging me. So Ive rechecked and found the error (at least this one, there may well be more) anyway I had forgotten to divide the bh^3 by 12. So that makes the max fibre stress 28.44 MPa rather than 2.37 Mpa. Vs whatever you decide to use for the wood. This is at 50Kn or near enough to 5 tonnes.

[Snowpetrel]

Sitka spruce
Douglas fir
Hoop pine

Static bending
fibre stress at proportional limit
[pounds/sq. inch and MPa]
6100 psi 42 MPa
6700 psi 46 MPa
8160 psi 56 MPa

Static bending
modulus of rupture
[pounds/sq. inch and MPa]
10 400 psi 72 MPa
12 300 psi 85 MPa
13 100 psi 90 MPa

This is interesting, comes from some aircraft design infomation here

https://www.recreationalflying.com/t...th_values.html

So pick a number from this table and compare it to 28.44 mpa (if my figures are correct) and you have a reasonable margin of safety, especially since the solid core wood is higher density and stronger, and I haven't used any of the wood in the aluminium section.

A lot of the figures for plywood show about half the MPa of solid wood. Probably due to the fibre orientation? In my calcs I just ignored half the cross sectional area from the plywood parts that have the grain running the wrong way. It would be worth checking the percentage of cross grain and the dimensions I used.

[RaymondR]

You have done excellent work in fabricating the beam however it is not a very good design.


Laminating solid timber together, shaping it and then bolting and bonding an alloy or SS flat bar flange onto the top and bottom of the laminated and shaped timber beam would much better emulate the I beam structural beam and provide for a far greater bending moment.

[roverhi]

Look at the original beam and the one the PO laminated up. Which do you believe is stronger. Yes a beam laminated from solid timbers to the depth of the plywood would be stronger. Either would be many multiples stronger than the one that lasted 50 years with part of rotted away. Think many of the posters have lost touch with the reality of this fix.

[J Clark H356]

Based on a depth of 6 inches and a 6 inch width, a plywood beam should conservatively carry 14,200 lbs of load in the center.

The vertical load on the beam is the weight of the combined mast, boom and sail. The stays are to keep the mast positioned and stable. They don't add to the vertical load. I am guessing your mast load is under 1000 lbs including the sail. If the mast were pumping due to heavy seas and wind loads, it might be effectively 4 times that or say 4,000 lbs.

I think your design is adequate, but I am making no representation that it is. You should hire a marine architect to analyze it if you are the least bit concerned.

[sailorchic34]

So the original beam looks to be about 8" wide and perhaps 3-4" deep, made of solid wood/glass, Looks like the bulkhead supports it.

On the aluminum, I should have said fatigue limit of aluminum is rather low, which is not the same as yeild, though I sometimes think of it as the same.. Why we don't see bridges built of aluminum. Aluminum will fail after some time from flexing, It may be many years, but it's there.

To my eye the aluminum looks to be about 1/8" thick maybe only 1/16" thick. One must remember that Aluminum yield point at ~22,000 PSI is per square inch. A aluminum beam 1/8" wide will NOT have a 22,000 Pound yield.

Then there is the steel bolts touching the aluminum thingy. That will cause galvanic corrosion and will weaken the aluminum even more over time.

[sailorchic34]

Originally Posted by J Clark H356

The vertical load on the beam is the weight of the combined mast, boom and sail. .

Ah, you have to add the compression load from the stays too The shrouds are generally tensioned to say 20 percent of breaking strength. This means there is a downward force far greater then just the weight. Probably in the range of 5000-7000 pounds of downward compress force for the typical size boat. Then you add the weight of the mast, sail. So depending on the diameter of the stay you could have 4 tons of downward force or quite a bit higher.

[wyb2]

Originally Posted by jimbunyard

Any 'engineer' that says that this

...

or this
...

is stronger than this
...

...

has no reason to be taken seriously at all...

That's not to say that there aren't other considerations to be taken into account, but the substantial increase in strength of the modification over the original is not in question...

Originally Posted by roverhi

Look at the original beam and the one the PO laminated up. Which do you believe is stronger. Yes a beam laminated from solid timbers to the depth of the plywood would be stronger. Either would be many multiples stronger than the one that lasted 50 years with part of rotted away. Think many of the posters have lost touch with the reality of this fix.

This is what the OP should be focused on. Seeing more and more people saying it's "not strong," but don't think anyone has claimed it's weaker than what was there before, which is what actually matters.

[Cheechako]

Originally Posted by wyb2

This is what the OP should be focused on. Seeing more and more people saying it's "not strong," but don't think anyone has claimed it's weaker than what was there before, which is what actually matters.

Yeah, as usual CF has taken this way outside the box. The reality is many boats that size and bigger have deck stepped masts with simply the main bulkhead under and that's all. My guess is the predicted loads are estimated way higher than reality.

[Cadence]

I would bet all of the through bolting is actually weakening your laminated truss. It appears you have built some beautiful trusses. Forget the aluminum orient the ply layers at 90 degrees and glue.
Although I think you have all the strength you need

[AndyEss]

Your assumption that the aluminum will carry most of the bending load is probably correct. The plywood will probably carry mostly shear loads - which it is good for. It's also easy to work with for fitting, so good choices.
I suspect you will not develop very high loads from your standing rigging because the hull will probably be pretty flexible/low stiffness, so it will deform before you could develop very high loads in the mast.
As another poster mentioned, the PO stainless steel straps did nothing to stiffen the original damaged arch. Increasing the depth of a beam is a far, far better way to increase its strength/stiffness than increasing its width.
What are your posts sitting on? They look like they will distribute a lot of loading to the bulkheads, but all of this load has to go someplace on the hull. Again, the original system worked for decades, so this probably isn't an issue - unless there is damage here too.