Engineers? Opinions on this please...

[NevisDog]

Originally Posted by Don C L

... I think I am starting to feel some of my sailing summer slip away... and seeing the tools coming out again...

Wooden yachts have traditionally been built by eye - if it looks right, it probably is right. Your wooden/composite mast beam looks over-built (assuming quality plywood used). Every yacht ever built has shortcomings and my thoughts would be that your newly constructed mast support will outperform and outlast many other parts of the hull and rig. Maybe epoxy the whole thing together, seal the entire beam with dynel/epoxy, add a tiny amount of unidirectional along the underside of the beam if you really wish to, then go sailing!

[J Clark H356]

Based on a depth of 3.5 inches, I think the safe load is now about 13,000 lbs. still not sure of the load on it based on the Dyna article. This load is based on a simple beam supported at both ends with section modulus of 22.

[NevisDog]

One more thought - the simplest way to calculate the maximum possible downward load on the centre of the beam is to find out the breaking load of the shroud wires holding up one side of the mast, i.e. take the breaking strain of the 1x19 stainless wire (or whatever you have), add up the number of wires on ONE side (the windward side carries all the wind loading), say an upper and two lower shrouds, and that (plus the small weight of mast and sails) is the maximum downward load your beam can ever experience (or just a fraction less, as the lowers are not vertical, so a small part of their loading is taken horizontally by the deck being squeezed inwards).

That gives the maximum possible theoretical loading - but the designer has allowed a safe working load for those mast wires, probably around one quarter of their breaking load, and that safe working load should hopefully be somewhere close to the safe load you can calculate for your beam, which has a very short span and can be rigidly fixed at both ends (which will significantly increase its load-bearing capacity). Also, attaching it firmly to the deck underside will create a sort of I-beam (or T-section at least - some unidirectional solidly attached to the underside completes the I-beam) which will massively increase the load-bearing capacity.

[Cadence]

Originally Posted by NevisDog

Wooden yachts have traditionally been built by eye - if it looks right, it probably is right. Your wooden/composite mast beam looks over-built (assuming quality plywood used). Every yacht ever built has shortcomings and my thoughts would be that your newly constructed mast support will outperform and outlast many other parts of the hull and rig. Maybe epoxy the whole thing together, seal the entire beam with dynel/epoxy, add a tiny amount of unidirectional along the underside of the beam if you really wish to, then go sailing!

He has over built in all probability.

[hamburking]

Originally Posted by NevisDog

One more thought - the simplest way to calculate the maximum possible downward load on the centre of the beam is to find out the breaking load of the shroud wires holding up one side of the mast, i.e. take the breaking strain of the 1x19 stainless wire (or whatever you have), add up the number of wires on ONE side (the windward side carries all the wind loading), say an upper and two lower shrouds, and that (plus the small weight of mast and sails) is the maximum downward load your beam can ever experience (or just a fraction less, as the lowers are not vertical, so a small part of their loading is taken horizontally by the deck being squeezed inwards).

Don't forget the forestay and backstay are also pulling down.

[barnakiel]

Originally Posted by NevisDog

One more thought - the simplest way to calculate the maximum possible downward load on the centre of the beam is to find out the breaking load of the shroud wires holding up one side of the mast, (...)

Nevis,

This is a very elegant way to present the idea. Thank you!

Can I ask a side question here?

If the mast is a non-swept single spreader stick. With one top shroud and two column wires holding the mast at just below the spreaders (about 50% of mast height).

Can I in the above model assume the lowers are there primarily to keep the mast in column (rather than carrying any significant portion of the wind load)? So then I would assume the maximum load would be the breaking load of the top shroud alone, and the safe load would be about 1/5 to 1/3 of this wire's BL.

I ask this as by adding up all the breaking loads of all the wires on one side (one top, two lowers) I would end up with the figure of 9000 kilograms, which is 3x times the displacement of our boat. This at first sounds somewhat extreme in a boat with barely 30 sq m of SA. I mean we are not too likely to take a 140 mph squall with all our white sail up. (By the above I mean I am not certain if the total load can be assumed ever bigger than the total wind load).

I am also rebuilding our beam/box here. We are leaving the old beam in situ and doubling up the whole beam/pillars design on the opposite side of the mast bulkhead.

Best regards,
barnakiel

[Snowpetrel]

^^ Nevis Dog, you also have to add in forestay and backstay loads, and vertical sheet loads. Halyard and vang loads may be a factor, but in general on most rigs they are kept within the mast itself.

But even at max load all the rigging won't be fully loaded. Eg falling off a wave will likely load up the backstay, cap shroud and the aft lowers, but the forward lowers and forestay will slacken.

Rigging is often sized to deal with deflection or stretch as much as breaking strain. I've never seen rigging that has yeilded, I am sure it happens on highly loaded race boats but its not common on normal boats, the rigging normally fails due to a combination of CSCC, fatigue, crevice corrosion and stress risers due to swaging at a far lower load than it's UTS or yeild strength. I doubt any wind or wave conditions could take properly sized rigging near its yeild. Probably the only things that could take rigging past UTS would be a violent rollover.

It seems to me it would be better to estimate the compression strength of the mast and use this as a basis for the maximum beam load.

[Snowpetrel]

Originally Posted by barnakiel

Can I ask a side question here?

If the mast is a non-swept single spreader stick. With one top shroud and two column wires holding the mast at just below the spreaders (about 50% of mast height).

Can I in the above model assume the lowers are there primarily to keep the mast in column (rather than carrying any significant portion of the wind load)? So then I would assume the maximum load would be the breaking load of the top shroud alone, and the safe load would be about 1/5 to 1/3 of this wire's BL.

The spreaders act to take a significant portion of the load into the lower shrouds as tge cap is loaded. The exact amount depends on the spreader length. A very short spreader that barely deflects the wire will transfer virtually no load. A very long spreader will put a lot of load into the spreader and the wire under the spreader.

Then we have the different load cases eg reefed main, or just headsail. Running vs beating etc so each wire is sized to deal with each worst case scenario. But to answer your question in general you can't just consider the capshroud and ignore the lowers. They do much more than stabilise the middle of the rig. They also deal with the considerable spreader thrust from the caps shrouds.

[Jim Cate]

All this talk of humongous loads in the rig are interesting, but do they represent the real working loads that this poor beam will encounter? Seems that the dynamic loads (excluding inertial loading from wave crashes, etc) are limited by the righting moment of the yacht. The slack bilge, narrow WL beam of these older designs aren't very stiff overall, and thus the immense loads being bandied about are pretty unlikely, especially in a cruising situation, with no rail meat upping the ante! To assume loads similar to the breaking strength of the wire, or even that of its MWL do not (IMO) represent reality. The historical evidence of non-failure even with a rotted beam suggests strongly that even this non-optimal replacement will be more than adequate.

Give poor Don a break and stop the hyperbole and extreme examples!!

Jim

[Snowpetrel]

^^ +1 Jim, it isn't going to break, at least not before far worse stuff starts happening.

I do get carried away with theoretical stuff, but I think Don is pretty safe going sailing this season. If he wants to make it stronger sometime possibly adding some uni glass to the bottom of the beam would be easy enough. But I dont think it is necessary.

[Uncle Bob]

Don,it looks far stronger than the original, finish making it look good and go sailing. To put your mind at ease measure and monitor closely at first then occasionally

[Snowpetrel]

Just ran some more figures to remove the cabintop and just load the beam itself, this puts extra load at the point where the beam steps down to its smallest dimension around 1/4 span. So this is a worst case scenario ignoring the aluminium, the canterlever effect of the end of the beam and the curved or chevron shape. It also assumes a point load of the mast rather than a distributed load over the maststep. All these should make it much stronger in reality.

The upshot was with 80kN of load (8.16t or near 18000lbs) of load the beam is near rupture at yhe 1/4 span, assuming wood of equal strength to spruce of 72MPa. In reality if the beam is fastened to the cabintop the stress here will be much much less.

In this load case the midspan point is around 55m Pa vs 72 mPa.

This is not using any of the cross grain of the plywood in the calcs. Or considering the middle part is much heavier and stronger wood.

I would be pretty happy with this.

Oh and any real engineers, I would be honored if you would check or find any fault in my workings, or suggest improved methodology. I am always up for learning more about the subject, hence playing with real examples like this.

[Snowpetrel]

With true canterlevers at each end, as your design seems to have done reasonably well, (though the strong central core of solid wood is only simply supported) the max fibre stress mid span is halved and the 1/4 span points see no bending stress. There is a reversed bending moment at the ends of the beam equal to the max BM. So real loads should be somewhere near half given it is canterlevered to a large extent this gives an even better margin of safety.

[Don C L]

Originally Posted by Jim Cate

All this talk of humongous loads in the rig are interesting, but do they represent the real working loads that this poor beam will encounter? Seems that the dynamic loads (excluding inertial loading from wave crashes, etc) are limited by the righting moment of the yacht. The slack bilge, narrow WL beam of these older designs aren't very stiff overall, and thus the immense loads being bandied about are pretty unlikely, especially in a cruising situation, with no rail meat upping the ante! To assume loads similar to the breaking strength of the wire, or even that of its MWL do not (IMO) represent reality. The historical evidence of non-failure even with a rotted beam suggests strongly that even this non-optimal replacement will be more than adequate.

Give poor Don a break and stop the hyperbole and extreme examples!!

Jim

except for that "non-optimal replacement part!"

[Don C L]

Originally Posted by Snowpetrel

Just ran some more figures to remove the cabintop and just load the beam itself, this puts extra load at the point where the beam steps down to its smallest dimension around 1/4 span. So this is a worst case scenario ignoring the aluminium, the canterlever effect of the end of the beam and the curved or chevron shape. It also assumes a point load of the mast rather than a distributed load over the maststep. All these should make it much stronger in reality.

The upshot was with 80kN of load (8.16t or near 18000lbs) of load the beam is near rupture at yhe 1/4 span, assuming wood of equal strength to spruce of 72MPa. In reality if the beam is fastened to the cabintop the stress here will be much much less.

In this load case the midspan point is around 55m Pa vs 72 mPa.

This is not using any of the cross grain of the plywood in the calcs. Or considering the middle part is much heavier and stronger wood.

I would be pretty happy with this.

Oh and any real engineers, I would be honored if you would check or find any fault in my workings, or suggest improved methodology. I am always up for learning more about the subject, hence playing with real examples like this.

Attachment 149744

That is fantastic, thank you. The fly in the ointment is that aluminum is in there taking up some of the space and is bolted through.
As I was saying, I thought the majority of the strength would come from the 3 aluminum chevrons, the fact that they are angled and the bolts that hold it all. In any event I will take some solace in the calculations. I'd sure like to take a class in this stuff too. I will say the thing sure FEELS solid!
Thanks to all of you. I should have run it all by you all before putting the mast back up! But given your inputs I think I will tighten up the rigging and push it hard before I do my summer sailing and see if it makes any noises. Today would have been a good day, we have gale warnings up.
And then next fall, I may go back to the drawing board... The perfectionist in me can't quite find peace with a sub-optimal creation!

BTW my boat has single lowers, not that it makes much difference. And it is quite tender as Jim was pointing out, 3120# lead 4 feet down on a 22.5' waterline and 8' beam. That is probably why the squishy strapped plywood arch has lasted so long.

Thanks again all, great stuff!