STRONGBACK DISCUSSION etc.

[Tony G]

Jan

Nice arc! That will look 'shippy' in place. Heck, looks nice just sitting there. Do you have any other plans fro changes aboard?

[Commander 147]

As I have mentioned before I plan to remove my commander compression post and replace it with a strongback. So today I started reading some of the prior posts to determine the demensions I should aim for on the strongback. Then I started measuring my commander and it seems that there are some differances between the the two boats in the amount of curvature of the deck unless I'm missing something else.

The drawing on page 23 of the manual shows the bottom of the strongback as a straight line from one side of the cabin to the other (or so I assume since the full width is not shown).

ebb said that his original strongback was 4-1/4" tall in the middle and tapering to 2-1/2" tall at the ends and 2-3/4" thick. If ebb's original was straight accross the bottom, that would indicate there was only 1-3/4" of camber from the edge of the cabin to the center.

ephifany said his original was only 3-3/4" tall in the middle but from the picture his does not look like it was flat on the bottom. So I may be making an assumption I should not.

In any case, I have 4" of camber in my deck. I placed a straight edge accross the cabin touching the bottom of the deck on each end and in the middle I had a rise of 4". So I will not be making my strongback straight across the bottom. If I want to end up someplace in the 2" thick area on the ends that means it would extend down 6" in the center. That is a bit much IMHO.

Below is a sketch of the approx. layout I'm looking to do. I plan to make it from 3ea. 4/4 boards of white oak laminated together vertically with resorcinol which will give me approx 2-3/4" thickness. I will butter the top of the strongback prior to putting it in place with thickened epoxy and then tab it to the deck and the bulkhead supporting it which will also be tabbed to the hull and deck. And of course I will have white oak bracing on the v-birth side which will be covered with 1/4' mahogany plywood as will the aft side of the bulkhead.

Any suggestions or comments about the plan?

[ebb]

Jerry, Just took another look at the old strongback that came out of A338.
It isn't straight across on the bottom as you might think because of the doorway to the forward stateroom. The original door was cased.
In fact the top liner of the opening is often the first indication that something is going on under the mast.

So the strong back definitely looks curved top and bottom - until I sighted along the bottom and saw that the mid part had been flattened (looks like with an AX) obviouisly to square the doorway.


I laminated my 'upgrade' and later decided that it would have been BETTER and SIMPLER to bandsaw the beam out of white oak. And so easy to make the slight chamfer along the top to snug the cabin top.

Commander's have a COMPRESSION POST.
My guess is that the fiberglass of the cabintop can be kept honest with a plain curved beam top and bottom. In other words I think you can skinny the beam depth as long as you still support the center with a post. I might make the beam a little WIDER - to make up for the cheat.

The beam on the Ariel is 4' long.
Not much to get bent even with a point load in the center.
In fact, bet there never has been an actual bent beam in any of our boats.
Yesterday's Alberg Assoc breakfast there was talk of the Triton's beam getting pressed down and the cabin sides getting pushed out!!! Could see the bulging! That just wouldn't happen with our cabins.

Making sure our shortie is married to the cabin top will imco cancel the remotest possibility of that happening in a Commander.

I'd make certain the center post cannot move.
Actually you could scroll into the beam a PAD in the center over the post, to fatten the beam there if you want to skinny it down. You'd do this all at the bandsaw, right? no gluing in anything extra.
That would remove any tendency to bend. In other words, leave the beam deep in the center but scroll out the sides to remove head bangers. Really round the edges, someone's going to head the beam anyway!

In a modern 'engineered' mast compression POST set up they'd probably just have a disk of some material under the mast - with no beam at all!!!!

[Commander 147]

ebb thanks for the feedback. I'm always interested to hear what you have to say.

So the bottoms were curved also. That explains a lot about the dimensions.

ebb, you may have missed that my whole reason for doing the mast beam is to eliminate the compression post. The unsupported portion of the beam in the center will actually be less than 2 foot wide when my mod is complete.

BTW the Precison line of sailboats made here in Florida are current production models with mast support beams that are made with plywood laminated together and then glassed to the bottom side of the deck. They have only a small portion of the beams ends supported by a plywood bulkhead that is just screwed to the beam. You can see it a little bit in the picture of my old Precision 18.

[ebb]

Sorry Jerry,
assume is the mother of all foul ups.
The key still is we can have nothing move under the mast.
And each of us has our own unique solution it seems when we stray from the box.

I love the sweet lines of the Commander.
The curve of the cabin roof is particularly pleasing.

Maybe, maybe it's possible to design a kind of 'break water across the cabin under the mast.
Have the beam outside.
If it wasn't a breakwater but a rounded slope sided and gel-coated mound it might just disappear and not mess with the lines.

[ebb]

Craig, Looks right on, the AL angle, great idea.

Don't forget the cabin laminate has a balsa core on top.
Ended up cutting something like a 1'X1' square of A338's cabin top out and replacing it with solid layup of glass.
The original mast electrics hole had invited rain water in which caused balsa rot. Balsa did not rot in the '60s.
Whose idea was it to put balsa under the mast?

Discovered this with the mast removed and springing a batton over the top. Could see the top was flattened.
But also, later discovered that the damaged composit had sprung back almost to its original curve.
The cabin top was crushed mostly just under the maststep where the core had rotted.
Inside, could see that the strongback had a space over its top - with the mast removed - and the composite returning to its original curve.

Found that all Pearson support framing on the bulkhead was only screwed together.
My theory was, and still is, that, along with the shrouds pulling the mast into the boat over time,
the factory support framing (which was NOT glued in) also had settled, SHRUNK, if you will, over time.
The white oak beam I took out of A338 was in original condition....NO ROT....STILL CURVED....NOT FLATTENED in any way.
And I still have it somewhere in the garage to prove it.

Pearson used the strongback also to squeeze and hold the forward part of the cabin liner in place.
So on the V-berth side of the beam there was wider space between it and the cabin laminate because no liner inside.
And Pearson, had slipped in some sort of funky shim pieces to make up the difference. Not wedging, just filler.
But, like I say, the beam had air along the top. The 4' beam is also bandsawed square, while the cabin top is angled.
They had also, in A338, misalined the liner when assembling, and the interface was a really wierd mess to look at.
Not really knowing what was going on when trying to figure it out the first time - too funky to believe.
Have to conclude the assembly was never tight to begin with.
But over time Ariel has proven to be forgiving, anyway!

Think everything in the beginning was loose as a goose. But cinched up bent out of shape by the shrouds over decades.
Cabin top is thick laminate and a pronounced curve. Not easy to flatten.
Most A/C's are only sailed seasonally and on weekends, sit unused in marinas, they rest alot, and don't see much bluewater.
I found nothing positive with the B-grade plywood bulkhead*-oak beam-and fir framing stuff to keep the mast from flattening the cabin.
If the core is not rotten, then, perhaps all that is needed are well shaped shimes or wedges tapped in along the top of the strongback.
*the plywood 'bulkheading' across the doorway was a separate piece, a space filler doing no work.

CONSIDER THIS....If I remember, the vertical 2X framing at the V-berth doorway - SEEMS to support the strongback beam - but stops on top of the V-berths.
Just the mahogany trim goes down to the step......
Could say the V-berths themselves (some cleats & plywood tabbed on one side to the hull) and a few wood screws are really what is holding the mast up.
In fact, that's what Pearson did in A338.
So again. in my opinion, quick-n-dirty mechanical construction...... just settling over time.
[Admit I've gotten bent out of shape with Pearson's half-way measures.... on what they sent out from the factory as a coastal cruiser.]


Imco, the aluminum angle will help stabilize the inward pressure of the mast.
But you still may find the shrouds loosening with passage sailing. Hopefully not!
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Sorry for the repetition of 'my thing' - found in many earlier posts. For me,
MORC (Pearson designated: Midget Ocean Racer Cruiser) sounds more like a goblin... than a pretty Alberg sloop.

[ebb]

9/10/2021
A couple of observations from the future. Please excuse repetitions!

In short, my laminated beam across the cabin has no other support
except for the overbuilt gussets at the chainplates at the center shroud.
I laid in a substantial floor (Cross piece timber) at the step up into the
front cabin. Which in Litlgull no longer exists because the bulkhead
is removed (forward as a 'crash bulkhead').

I recorded the height from the floor timber to the bottom of the
laminated beam. But then I read that the tanins in white oak may react
with the resorcinol.. So I thru-bolted the whole thing to the cabin top
with bronze carriage bolts. Measured the height with mast and rig and
found barely 1/8" less. Seems OK. The unitized laminate, bolted and
glued up-graded deck (balsa replaced with solid frp), immovably supports
the mast with all its active downward pressure. We'll see! And, just in
case, a compression post, like the Commander's, could easily slip in
between the cross-floor and the cross-beam.
Someone here pointed out a pole in the middle of the accommodation
isn't a bad thing -- you use it to swing around or just as a steady-hold in
an often very active little ship..

That's A338. Our discussion here finds that the problem with the strong
back is not rot but the settling of the dry fitted pieces under rig pressure
over time.

For those skippers restoring their Ariels, I would suggest removing the
main bulkhead. Put in a substantial floor across the bottom at and as the
step up. Block the crossbeam into its final position if it's in good shape.*
Might GLUE it to the cabin top with rubber cement (4200). Then reframe
the passage way with substantial posts to support the venerable beam.
Thus we have the hull and cross-floor supporting the strongback. Finish
the bulkhead in an interesting plywood of your choice. Doorway can be
kept open with a privacy curtain for air circulation.
*May have to return the mast-DECK to its original curve with a jack -- and
repair any balsa-rot. If I remember the balsa is 3/8". Remove the deck
laminate carefully. Replace rot with glass mat and glue the deck back on.

Hope this still holds true: You cannot find a better marine plywood to
sheathe the bulkhead than BS1088 Meranti. It is made with phillipine-like
mahoganies and bonded under high pressure with phenolic glue. The
veneers are knotless thruout, panel is flawless and deadflat. Could get
by with the 6mm but 9mm will allow left overs for shelves and such. Easy
to varnish or paint.

This will provide all necessary support for the beam and rig overhead
forever. IMCO