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: Bimini sag  ( 1715 )
forsailbyowner
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« : March 17, 2014, 06:34:31 AM »

 Im working on a fairly large bimini at the moment. 8'x10'. The prior bimini had sagged and stretched very badly. I got to thinking, "why do they stretch so excessively"?  Could it be from the weight of the center bows with their never ending pull with no stop?  Ive done my last few with adjustable straps holding the center bows in place. Im wondering side by side the difference would be in the long term in a  Straps -vs- Pockets comparison. Ive also noticed the thing usually stopping the biminis from staying tight throughout its life is the reinforcing strip along the lower edges. They stay tight and the rest of the bimini sags between the bows, It would seem that the adjustable center straps could extend the useful life by raising the bows to compensate as it stretches. Another benefit is less exposed structural stitching and needle holes overhead to leak. At this point Im thinking sewing the center pockets in a bad practice compared to support straps. Thoughts?
Mike
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« #1 : March 17, 2014, 06:46:50 AM »

Well at first i used to make top with straps for the center bow as they all sere but then i saw more and  more center pockets   I always thought it was to hold the can as down so it didnt lift up as the boat was moving.  I found this sag to be the problem when there wasnt enough crown to the boaw and once a puddle of water stands on the canvas it straches and grts bigger.
Have you tried using recacril that claims to not streach or shrink ? Ive used it with people who have had bad shrinking from sunbrella. No complaint yet.

Mike
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« #2 : March 17, 2014, 06:51:21 AM »

Perhaps the proplem is the edge reinforcemennt ?
Ive seen cockpit cover where the edge shrunk like a rummer band around the edge to tight a d the center lose   Maybe the edge has tightened more then the center could?  Making the sag   I dont add all the edge pieces

JuneC
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« #3 : March 17, 2014, 06:21:03 PM »

I think the design of the frame can accelerate the sag.  If the center bow(s) are cut too short where the attachment to the fore or aft main bow is very high up on the frame, the weight of the bow will drag down the center.  If those bows have long enough legs where the legs actually support the weight of the bow, the sag will be less dramatic.   Also, the span between the bows needs to be kept fairly short.  If you go leaving more than 3 feet of canvas between bows, expect sags.

June

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bobslost
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« #4 : March 18, 2014, 05:34:08 PM »

June is right the design of the bows have a lot to do with stretching . Standing water and the wind are the culprit . Bow pockets seam to work best to hold the bimini in place, better than straps.
Make sure the bows have a nice crown and the center one is slightly higher, so the water drains .
The span between the bows should be about 36 to 40 inches . I like to use straps on the front of the bimini , because it gives you some adjustment (you can tighten it up as needed). However all canvas stretches  especially polyester . Polyester can be challenging  to work with , it gathers when you sew and most binders do not work the same on polyester binding. And over time it stretches more than canvas , the only bonus is the price and the weight.
bobbin
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« #5 : March 18, 2014, 06:21:23 PM »

I haven't made a bimini or dodger in nearly 14 yrs. (Boss never assigned them to me).  But in the first shop I worked for biminis were constructed in multiple pcs. (first to second bow, second to third) and there was a zippered pocket to encircle the frame.  All the pcs. were cut down the selvedge of the goods (the stretchiest part of the goods) which meant that the crossgrain (least stretch) was the grain that ran fore and aft.  Reinforcements were cut 4" wide, on grain, folded in half and applied to the required edges.  The edge was then bound with 3/4" bias binding.  Straps were used routinely to add additional tension.  I believe that positioning the most stable of the grains fore/aft goes a long way to adding stability.

Boss always cut a bimini down the selvedge, joining widths with a fore/aft centre seam to get the necessary width and darting the areas that needed to conform to the arcs on the frame.  I saw at least 2 biminis constructed that way collapse under the weight of pooled rainwater... in spite of tensioning straps fore and aft.  Why? because the selvedge grain of the fabric has considerably more stretch than the crossgrain!  (that's why sail covers cut down the selvedge "shrink" when you stitch them!)

But, I haven't made one in years (don't care if I ever make one again!).  So what do I know?
Peppy
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« #6 : March 18, 2014, 08:04:57 PM »

I agree with everybody!! ;) Warp-faced fabrics stretch most in the direction of the warp ie up the roll. I see lots of tops like you're talking about Bobbin. Lots have big saggyness and the seams going front to back are tight. But bad design and water pooling are more to blame IMO.

We use pockets and straps on middle bars. The pockets keep the top from lofting and the straps provide adjustment. We always use bars front and back so that's the only real adjustment in our Biminis. We cut a little slit in the main bars pocket to let the strap through. It's a little chessie TBH but it works and its been done that way for 30 years and nobody cares. Well the ones that do care have more problems than a little slit anyway.

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Mike
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« #7 : March 18, 2014, 10:21:00 PM »

when i started i used too have a lot of little 18 20 boats so it was cheaper less material to run a center seam also and with a 2 bow convertible top peppy i had straps as did the old top to hold the forward bow i didn't have a slit though what i did was i added binding to the forward edge of the rear bow pocket and where my strap were to be i didn't sew a 5" or so section for each strap to pass though it looked better, but i have made mistakes and to get by did the slit thing peppy,.
normaly on a large top i will do as you say bobbin running side to side it s more econical  yardagewise usually also but for a simple small shade bimini 6 feet long by 8 feet wide ill have a center seam fore to aft less yardage for top 5 yards rather then 6 and more wayste

bobbin
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« #8 : March 20, 2014, 07:22:30 AM »

I was at a marina yesterday and this thread came to mind.  I mentioned the method that cuts individual pcs. and seams them over the bows so the cross grain runs fore and aft and stated that that method was superior, IMO.  Here are a few more reasons:

When biminis are fitted to frames with multiple pcs. (instead of a flat blank) you have the opportunity to add more crown to the framing/accommodate more crown because you can cut the shape into the individual sections.  It's harder to shape the flat blank method, if there is greater crown in the frame. 

Zipper pockets:  in the first shop they were cut to coincide with the shape of the pcs. they were to secure.  They were split down the center and those raw edges were bound.  The zipper was then inserted.  The pockets were stitched into the seams that joined one pc. to the next and that seam was topstitched.  The seam line rode along the top of the bow! so there were two lines of stitching to keep the pocket secure as the topstitched seam degraded in sunlight (pre Goretex).  The pocket was secured to the adjoining pc. (used seam sealing tape) and the ends were given the "crow's foot" anchor.  Any zipper pocket requiring straps had little half moons cut in it and those were bound as well.  I NEVER saw a bimini constructed that way fail!

And while employed by Boss, I restitched several biminis I'd made in the first shop... but they were NEVER bagged out of shape, loose or saggy. 

More work? certainly! more expensive construction? yes.  Probably not the best method for small boats with aluminum frames, but for large boats, stainless framing? definitely the way to go, IMO.
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« #9 : March 20, 2014, 08:48:50 AM »

I do the multiple pieces & pockets cut to shape, generally works ok for me.
I did have trouble with one that had a huge single aft panel, I needed to join three pieces & put windows in it, yep sagged on one window, the gap between frames was just to big, but it was a recovering an old frame & the owner didn't want to add any more poles. Replaced the panel with a blank one, no windows & it works fine.
I did wonder if zipped window covers would have kept the tension in the panel, but decided to cut my losses & just replace it.

Suzi
« : March 20, 2014, 08:51:20 AM Grebo »

Mike
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« #10 : March 20, 2014, 09:14:38 AM »

Bobbin you make them as i do on big stiff little top peoe want cheap.
Even te cheap aluminum biminis with 4 straps to support it i mKe the bound hacl moons on the leadi g edge for the strAps to  ome ou o the pocket and pull the top tight core to aft and also side to side so the canvas will not creep op the cormers. I see some thT just attach the strap to the frame one say or another. Not as good imo

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