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Thread: STRONGBACK DISCUSSION etc.

  1. #31
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    Red face

    Kurt

    just a voice of caution: I recommend you take a look at Brian Toss' book on rigging or a couple of web sites...

    http://www.sponbergyachtdesign.com/A...ngineering.htm

    or

    http://www.classicmarine.co.uk/Artic...ging_loads.htm

    (there are others)

    from the sponberg site a rule of thumb is:

    (mast_compression) = ((weight of boat) * (righting moment from CM to CB)) / (distance mast to chain plates)

    rough numbers (eye-balling the boat at 30 degrees of heel)....

    mast_compression = ((5200 lbs) * (1.0 to 1.5 feet)) / (4 feet) = (1300 to 1950) lbs

    This number is higher than some other more detailed models. doubtlessly, Moderator Bill has some better numbers for weights & moments, but the point is there will be a large point load under the mast and you are looking at a large span.

    before you commit to a radical change, you may want to build a full size model and load it with a jack or hydraulic press.

    Maybe I have misread the article.. I'm just a software guy.

    cheers,
    bill
    Last edited by bill@ariel231; 08-29-2006 at 09:48 AM.

  2. #32
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    flexation

    Kurt,
    the end result of your mast beam project will be zero flex of any sort.

    As to the deck flexing, I believe it has to be rotton core. Right? Like 338 you may have a composit whose inner layer is damn thin. In the past I questioned whether a recore could be done from the top because the inside had little or no integrity. I found a single layer each (on the inside of the composite) of cloth and mat under the mast. Easily moved with the fingers. I've just assumed that later number boats were skimped on, but it may be individual ones.

    I believe now when doing a radical coresectomy that the foredeck area should be carefully supported from inside with boards and battens or even an inflated bag or two. Cutting the deck out in small sections at a time so as not to loose the camber of the deck.

    No bulkhead or knee was built-in to the underside of the deck/cabin roof in 338. Seems to be the way it was done back then. I would make sure they are well tabbed in this time so that the deck becomes part of the system, not a symptom of something wrong. How this can best be done is obviously open for discussion. The forward knees for the lowers ought to be able to support the deck. Wide tabbing and healthy core would take care of any flex in this area. No?
    {The widest part of the foredeck and the rounded nose of the cabin is unsupported from inside - except by its monocque shape and the sides of the hull. Flex in this area would be strange - unless it was contributed by weakened deck.
    The mast is supported by the whole top of the cabin and indeed by the whole top of the molded deck. Imco it's possible that some of the downward force of the mast could be translated into that cabin-nose/wide foredeck area. Loss of camber would be an indicator. Long straight-edge would show it.}

    I managed to get glue under the liner port and starboard sides and carjack the sags and openings 338 had there up flat to the roof - I felt very uncomfortable messing with the liner so I opted for monolithic knees off the hull. I may go back and tab them in to the top. (But I might also remove them because my chainplates are going on the hull.) The liner as you know does not extend all the way out to the hull under the side decks so there is actually some prime attachment for part of the top of the aft knees (aft lowers) and the main bulkhead (or what remains of it under the decks.) Would not tab onto the liner. The knees tabbed in solid must contribute a good deal of stiff to the hull and deck in way of the shrouds what ever method of chainplate attachment.

    IMHO, NOTHING should be able to move in the shroud/mast/deck/cabin/beam area. That is, nothing of the boat itself (the plastic) should move at all. The rig depends on it. The shrouds in big water can't suddenly get loose because of 'oilcanning' of hull or deck and not support the mast, both sides. Sudden loose shrouds could be a cause of the mast breaking in a knock down, doncherthink?

    It does seem far fetched: but if the weight of the boat is supported by the mast (ala Sponberg) then two tons of concentrated force could do some strange distorting of plastic laminate! Would have to see it to believe it - like in frame by frame time lapse. Monocque structures can be crushed, I'd think they can be twisted on an axis, and relative flat thin planes can be oil-canned. Generally '60s plastic lams were substantial and made from excellent ingredients. That's what we gather, anyway. But composits with cores must have been fairly new and simple back then - now we know that the opposing skins must be equal in strength/thickness. David Pascoe, our favorite online marine surveyor talks about this on
    http://www.yachtsurvey.com/HiTech.htm

    I feel that the mast bulkhead along the hull from the deck down to the keel has to be very strong WITH WIDE TABBING. - and ideally the bulkhead should be connected across the keel, so that mast forces are shared on both sides of the boat. As above, so below. The circle must remain unbroken...in the bote, load, in the bote ....
    Last edited by ebb; 09-02-2006 at 07:24 AM.

  3. #33
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    Bill - Thx for the suggestion, I have Toss' tome on the subject, will have to reread the pertinent chapters. The websites you posted are most informative, and give much food for thought. Once I decide on *something*, I will indeed make a test piece first, and do what I can to see that it will be up to the job!

    Ebb - I don't see any obvious signs of core rot in the area, but do plan on making danged sure of it before wrapping it up. Eventually I'll be going with external plates, and the areas around the bulkhead and chainplate attachment points will recieve much strengthening and stiffening in the process. Like you, I don't want it to move at all, oilcanning is for powerboat engine rooms only.

    The circle shall remain unbroken.
    Kurt - Ariel #422 Katie Marie
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  4. #34
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    I took a good look at an Alberg-designed Cape Dory 26 yesterday, checking out how the strongback was engineered in a very similar boat to ours, 20 some years after our boats were in production.

    The CD26 has a center-pole support for the mast, with port-offset access to the v-berth. The mast support is apparently a solid wood, 2.5-3" square pole. Interestingly, the pole is offset from centerline *to starboard*, about 1.5". There is some sort of arched structure up against the overhead which the half-bulkhead attaches to (mechanically, no tabbing that I could see), but it is small, and hidden by the cabin liner. It is an upside-down triangle in shape, approx 3" wide against the overhead, and 3" to the apex of that triangle. The corners of the triangle are radiused about 1/4".

    The wide piece of wood on the bulkhead against the overhead you can see in this pic is simply trim, it didn't appear to serve any structural function. Also note the seeming lack of cabin trunk side support on the port side at the bulkhead station.

    From tapping against the overhead, it was evident that the deck construction in that area - forward of the bulkhead at least as far as the forward edge of the mast base - was more solid than in other areas of the overhead.

    Last, and somewhat unrelated, the size of the standing rigging was obviously smaller than that which we have on our boats.

    Interesting.
    Attached Images  
    Last edited by epiphany; 09-03-2006 at 05:48 PM.
    Kurt - Ariel #422 Katie Marie
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  5. #35
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    Quote Originally Posted by epiphany
    Last, and somewhat unrelated, the size of the standing rigging was obviously smaller than that which we have on our boats. Interesting.
    The Ariel used the same rigging and mast (cross section) as the Triton. Sort of explains why these are such a tough little yacht.

  6. #36
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    beamless CD25D

    Very shallow beam, (don't really see it!) - no side supports at the cabin sides.

    Lots of similarity to the Ariel of 20 years earlier - looks like heavier buttocks, to keep her from squatting. But the beamless interior is very interesting. Did you knock on the sides? They'd have to be solid, wouldn't you say? Otherwise typical Alberg. I have to wonder how it is done? maybe a single post really can act like a keel stepped mast. Do the shroud-plates come thru the deck?

    Numbers say only 78 of these were built.

    Hard to imagine any boat near the size of an Ariel with less than 3/16" wire.
    Wonder how the general construction quality compares between the two?

  7. #37
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    Bill - Right on, that was one of the attractions of the Ariel, for me. The guy with the CD26 said he is going to be staying at our marina, so I'll have to take a turnbuckle or something over, take a pic of the two for a comparison to post here on the site, it's so evident.

    As an aside, I look at every boat in this size range which comes through the marina, none have the strong rigs we have... In fact, most late vintage boats (Ben-ter-lina's, primarily) have even smaller scantlings on boats which are 3-4' longer than ours. Not that I'd expect different there...

    Ebb - I looked all over for an interior shot of the CD25D which illustrates how the mast/bulkhead/strongB is set up on that model, couldn't find one... wah. Looking at the line drawings I could find, it is obvious that the mast sits well *aft* of the forward bulkhead location, so it would be interesting to see if there is any kind of evident internal reinforcement on those boats...

    I didn't knock on the trunk sides of the 26, but shall. It would seem that the sides must primarily be under compression from the mast, if they need no reinforcement from other structure... Still, I will build mine up a bit. Tapping on them just now, they seem to have more of a solid sound than the deck. I don't think there is coring in there, is there? Have you seen any during your (de)construction?

    The sidedecks on the 26 are obviously strengthened against compressive forces from the mast/rig by some sort of side-strongback, you can see it in the pics. That fits in with my plan to have my eventualbeam wrap down around and under the sidedecks...

    The chainplates on the 26 are through-deck, connected to knees on the hull, just like the Ariel.

    CD's have a reputation as generally well-found boats. Judging from what I could see, it seemed put together with at least as much care as an Ariel, or perhaps a bit more.
    Last edited by epiphany; 09-04-2006 at 05:52 AM.
    Kurt - Ariel #422 Katie Marie
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  8. #38
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    Looking at the CD25 Sadler, the beam build-down is also slight. What is obviously the compression AREA is bracketed by wooden structures creating alcoves where lockers and heads are usually put. Haven't researched this at all, But the s/v Allia (with a good inside shot) has a #738 on her sail. That is a surprising difference in numbers of boats between the two designers if that is indicative.

    If there has never been a problem with the CD25D mast, it is very worthy of study. And inspiration. We know that Alberg was a tyrant when it came to messing with his designs - the 'engineering' inside under the mast must be well thought out.

    It's fantastic you have two live models to draw from.

    I would like to see a cross-section and a diagram of this CD25D mastbeam puzzle. Maybe not, because I would be hard put not to change what 338 has now!

    A bunch of luck with the project!
    Last edited by ebb; 09-04-2006 at 08:22 AM.

  9. #39
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    Strongback success??

    I was admiring the work that you did in reconstructing your strongback (view below) and that gave me ideas for mine. I would like to open up the inside much as you have done, but being very new to this and rather ignorant besides, I was wondering if you left that space open or if you reinforced and closed the bulkhead back to its original format? Would it be possible to leave it open like that? Could you include pictures of your finished product? I know I would enjoy seeing that.
    Attached Images  
    Peter

  10. #40
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    strongback

    Peter,
    We are outa luck here.
    TonyG and Kurt are also opening up their interiors simularly.
    Can't remember right now if any Ariels are sailing with this idea put to the test.

    The span is less than 4'. Even less with the legs holding it up. My replacement for the original is laminated white oak of slightly larger section.
    With a select piece of airdry, I'm certain a similar arch could be bandsawed out of timber just like the original with NO loss in strength. In 338 the strongback is epoxied to the bulkhead across the top (which supplies zero strength) and the white oak legs are lapped with the beam and epoxied to the bulkhead going down to the V-berth top (which supplies some strength and a lot of stability. imco)

    AND no deflection. It is impossible to imagine any load that would deflect the arch. Any push down by the mast and rig to straighten the arch would have to push the sides out. Can't see that happening. The legs coming down from the ends of the strongback, imco, have to distribute the load to the hull, another strong curved structure. The V'berth structure at the remaining bulkhead is considerably beefed up in 338. I think I stacked at least two more 'ends' of 3/4" plywood inside the V'berth spaces under the struts to support them AND to distribute the load to the hull. So it's about 3" thick there now. The struts are well supported. Theory.

    What the Factory has there to support the mast these 40 year has been discussed here. I have my theorys about the problems we have and what a solution is.

    On pg16 #234, I asked whether a compass arch would be even stronger. I was looking for more side room for bunking. And decided that the arch showed there would intrude into our limited space. But noone could have an arguement against the shape for strength. A smaller compass radius strongback that would fit up into the trunk and bear on a point directly under the mast would be the strongest possible. Kurt has a definite arguement with that. I did not make a doorply model for that shape but it would be interesting to see how it relates to that opening into the V-berth. It's been done with other small boats and sometimes makes a pleasing keyhole shape.

    I've continued the bulkhead across the bottom of what used to be that doorway, creating a 'floor' or load bearing area across the keel. Lot of stuff going on here in 338 -the cabin sole ends, super-gooped pvc pipe scuppers go thru on way to bilge, the step up into the V-berth stateroom, and the bilge tank ends there. It's pretty solid by extending the V-berth sole 3 or 4 inches onto the cabin sole with solidwood floors under the mast above.
    and JUST IN CASE, a center pole support can now be installed to take the rigging load directly to the keel from the strongback.

    Sorry about no action or photos. Wish I was CRUISING!!!
    Last edited by ebb; 02-13-2007 at 08:29 AM.

  11. #41
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    Ian Nicolson describes a great and very affordable way to do a compression post in his book titled "Build Your Own Sailboat". The post would be easily removable, if you wanted, so you could keep that lovely span, Ebb.

    Take two pieces of angle iron, same length, make holes a few inches apart in them as if you were going to bolt them together on one side. These will be set onto the floors under the mast running fore and aft, wide enough apart to accomodate the post. With the post in place, bolts are slipped in ahead of and behind the post, and wooden chocks are used to take up any space between the bolts and the post (My thoughts: one could easily make the holes just far enough apart to obviate the need for the wooden chock blocks, and or have a piece made custom to fit the bottom of the post that wouldn't allow it any movement).

    On the upper end of the post are threads, and a cap which screws down onto the post. The cap has 3 holes drilled into it; 2 directly across from each other near the end, and one slightly below the other two which is threaded to take a bolt.

    Put the post into place, stick a rod or screwdriver into the two cap holes and use it as a lever with which to screw the cap up to the underside of the mast foot/pad/deck. Once it is adjusted properly, put a bolt into the threaded hole and tighten it to prevent the cap from screwing down accidentally (loosening the post).

    Simple, effective, affordable, and easily doable with common pipe and fittings, I believe. The pipe could be blasted and painted w/a tough epoxy paint, or a sleeve could be made out of the same cloth as the interior finishings, and zipped around the pipe to make it esthetically pleasing.

    I've been trying to think of drawbacks, but none have come to mind as yet.

    One other thing I can see in the above picture which I would not have noticed if I hadn't seen the same on Craig's "Faith" just a few days ago - the longitudinal stringers on both your boat and Craigs are mounted much lower than the ones on my Ariel. From the pic, yours are about midway up the opening in the forward bulkhead, where mine are approx 2/3 the way up. I will put a measurer on mine and see where they lie exactly in relation to the deck, but a ballpark guess would put them 3-4" higher up the hull than are yours and Craigs. Since my boat was made near the end of production (#422), and information about these boats is mostly lost in the mists of time, I'll always wonder if this was a Carl-specified change, or just something which came about due to the decision of some laborer in the factory... Remember that mine was also missing diagonal supports on the main bulkhead, much to the chagrin of my compression beam.

    And another edit: Your stringers run forward of the fore bulkhead all the way to the bow it looks like, where mine start/stop just aft of it.
    Last edited by epiphany; 05-04-2007 at 05:24 AM.
    Kurt - Ariel #422 Katie Marie
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  12. #42
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    Ian's compression post

    Kurt, Ian's tip is a good one, too.

    Critique. Caps and pipe threading, not many turns. A coupling might provide a bit more length. Maybe threads could be deepened with more turns of thread doing the work of holding the roof up. The opposing thread of the coupling could be cut off. IE the coupling cut in half.
    The coupling has a 'flat' top. You'd have to have a plate on top to take the rim when you are tightening against the wooden beam. And this plate could have a stud or some washers welded on it to hold the coupling and the pipe exactly in place when it's screwed up in compression.

    Imagine the torque that post would take with a human swinging around on it.

    The bottom of the post could be handled in the same way. The plate on the Ariel might turn out to be a convenient piece of larger angle supported over the keel and cut to fit the angle of the hull sides like a floor. I would hold it back from the hull a little. The horizontal flange would have the post connect ie the pipe inserted over some welded on washers. The vertical face of the angle could be for the bolts holding it in place. No bolt heads or angle iron on top. The post would appear to be floating.

    I would probably cheat the compression post in place by jacking the beam with a bottle jack. You know, just put everything eggzactly where you want it while the jack has the pressure. Do again to get it purrfec. I'd see if you even need a coupling extension with the bottlejack. Tremendous pressure can be achieved with a jack. Might just cut all the parts to fit and slip em in place.

    Galvanizing is the best coating you can put on steel. Galvanizing is cheap and sometimes they will take pity on the poor sailor bringing in his precious handmade stuff and put it in a bucket with another job. You'd also be galvanizing your new compression post INSIDE as well.
    __________________________________________________ _____________________________________________
    Interesting observation on the stringers. Location may be a trick of camera angles. The stringers in 338 were what the shelves rested on. Guess that their place on the Ariel hull is pretty much similar. Couple inches up or down?
    338's stringers appeared to be jammed into place and slapped on with matt. They are for the most part crooked and not even parallel with the sheer as they could be.
    I just decided to let them be. But added more stringer completely to the bow and all the way to the end of the cockpit. Added to make them appear like they came factory. Think the stringer going toward the bow sloped up some because it looked better.
    The stringers, of course, are not level with the waterline.

    My feeling at the time was to even the loads on the hull and I didn't feel that the stringers should just stop where it was convenient for Everett. What you see are stringers going clear to the bow. While they are stiffening members they are also stress points, so I have also radiused the corners with fillets. Argueable if any of this is really necessary, but I feel better that the boat is more symetrical, if that is the word.

    I see settees and berths and other tabbed-in furniture providing longitudinal strength like the stringers. More so, because they are web frames providing immense stiffning.
    I feel that these lines should be extended wherever possible in a remodel to elimnate hard points.
    Last edited by ebb; 05-04-2007 at 09:34 PM.

  13. #43
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    compression post

    About this compression post thing.
    I think the post should be designed as a single (welded) piece that is bolted thru well fitted flanges to the beam and the floor. There should be no way that it could come apart.

    There could a scenario where the boat is thrown on its side and the compression post be popped out of its seats. Pipe over flat washers as in the example previously. Possible. Could think that the compression post is holding the cabin up as well as holding it together.

    Maybe this is a simpler solution: If the overhead beam is wide enough for an actual pipe flange (so named) which is a common pipe threaded fitting usually with chamfered holes for screws already drilled, one of those at each end of the pipe to widen the point load and hold it in place. They come threaded for pipe or unthreaded like tube fittings. Various collar heights and amount of thread. You use the cheap threaded ones, using that bottle jack to positively hold open the distance between beam and floor, the pipe would be micro fitted by tightening or loosening the flanges. Then welded to set the length of the post and close the seams and lock-in the flanges. Making it a single piece. Then galvanized, have to zinc the threads and fill the crevises.

    Have to decided for yerself if the screws you put thru the flanges into wood are enough. The post is going to get yanked on, screws in sheer, so it's probably good. imco

    This structure is simple - allowing positive fastening - and easy removal.
    Careful cranking on that jack. Don't want to distort anything. It's an extra hand.
    Last edited by ebb; 05-05-2007 at 06:00 AM.

  14. #44
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    Ahh - I see what you mean about using a pice of angle as a floor - good idea. Maybe of Titanium, so no corrosion worries. ;D Seriously tho - galvanized, bedded in and coated with epoxy - the galvy finish wouldn't ever need replacing, right? So sealed it wouldn't corrode, couldn't wear off, and would be good basically forever - or am I missing something?

    Whaddabout you put the coupling in the *middle* of the post, and lengthen/adjust the post just like you do a turnbuckle?

    Ian's book mentions having a socket for the top of the pole to sit in - like you, I would want that plenty deep enough to know it isn't going to pop out of place if everything was going crazy. Especially if everything was going crazy...

    Question for you, since you've done it already: Regarding evening up the gap twixt the house and the liner @ portlights - I was looking thru McMasterCarr website, wondering if some of their pre-made FRP slab could be put into place with epoxy to make it a uniform thickness. Would doing so maybe be easier/quicker than the method you used with thick paste? If so, what thickness would be needed, IYO? Thx.
    Kurt - Ariel #422 Katie Marie
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  15. #45
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    Slow wireless... my apologies...
    Last edited by epiphany; 05-05-2007 at 06:00 AM.
    Kurt - Ariel #422 Katie Marie
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