The Folding Strut
By John LeMole
Streamline your collapsible frames with folding strut.
Dogleg bends in the forward frame can create neat stowage for folding struts. In two articles last summer, I introduced marine fabricators to the Drop Top dodger, a new design utilizing the Gemini System hinge and folding strut (Gemlock) and featuring a permanent windshield with a removable top. Now I'd like to broaden the discussion into the use of folding struts in collapsible frames for both dodgers and biminis. The frame basics are the same for both, whether in two- three- or four frame construction.
Let's take a standard three-frame bimini as our point of departure. Tension in the cloth top is usually achieved by pulling on it in opposite directions, using either straps or solid struts. It has become very common to combine the two, using struts aft to serve as storage legs for the folded top, with straps forward for tensioning. Once solid aft legs have been employed, the forward straps can be eliminated and replaced by two horizontal struts, which push the frames apart. This form of compression strut is commonly called a spreader bar. The aft legs are not involved in tensioning the top anymore; they serve only as posts to hold the frame in position.
Often called "strapless biminis" for obvious reasons, these structures provide a very solid frame, usually permanently deployed. The lack of a forward strap frees space for passage to and from cockpits or afterdecks. On small boats (and most boats really are small), this decreased clutter/obstruction is a great improvement.
Our next step is to take those spreader bars and add a hinge to them. This gives the struts-and the entire bimini-the ability to quickly and very easily fold onto its support/storage legs.
In this sample three-frame bimini, we don't need to change the frame proportions from the norm. As usual, we'll use a long mounting frame, a slightly shorter forward frame attached near the base of the aft frame, and a short middle frame that is likewise attached to the mounting frame. All are secured with jaw slides and eye ends, as are the aft support legs (attached just below the corners on the mounting/base legs). The spreader bars use the same hardware and are positioned just below the jaw slide of the middle frame.
Since a spreader bar pushes the two end frames apart, it must in some way avoid the middle frame(s). In this model, it will remain below the middle frame, but I'll introduce another method later. I have not found that this relatively low position affects the spreader bar's ability to tension the cloth top, even on four-frame Biminis. It has the added advantage of placing the folded and stored spreader bar neatly in line with and between the other frames.
If this bimini were made with rigid spreader bars, the frame would now be ready for patterning. Since we're using collapsible spreaders, a few modifications are necessary.
Notice in the photographs a small dogleg bend at the base of the forward frames. This bend allows the end frames to lie parallel when folded. It also provides a perfect storage space for the struts, now nested neatly under the middle leg(s). Without it, the forward frame would stand apart from the others, due to the interference of the spreader bars and their hardware.
My shop uses a four-inch-radius bender to create the dog-leg (three-inch is also available, both for 7/8-inch or one inch tubing). Specifically, we choose a Holsclaw "Handy Bender," available through Manart-Hirsch and such industrial supply catalogs as McMaster-Carr and Manhattan Supply Company (MSC). These portable benders have gradations and are reversible so that you can make mirroring bends in opposing frame legs. They can also be used to make bends on-site on already installed frames.
In the Biminis shown so far, the bend angle was about 25 degrees. When using split-type jaw slides, which stand out farther from the frame, the angle must be increased to about 35 degrees to create greater clearance between the frames. A passing note: On Drop Top dodgers with heavily sloped or toed-in legs, such as are needed on bow spray dodgers, the dog-leg bend is also deceptively toed-in, more than you would think. Just remember to bend in the same plane as the set screw on the eye end.
Once a dog-leg bend has been used in conjunction with a spreader bar, it is absolutely imperative that you remove the eye end's set screw on the dog-leg frame and replace it with a rivet or machine screw. If you don't, the tensioned cloth on top will pull the frame Out of its socket, using the spreader bar as a very effective lever. If you happen to forget, the frame will remind you soon enough, probably while you're patterning. Likewise, it's standard to replace the set screws in the aft support struts, as well as those in the mounting frame. The set screws at the ends of the spreader bars are relatively secure because they face compressional loads, but since the struts often store hanging downward, it can only help if you replace those set screws as well.
These days, almost any powerboat can go 20 knots or better. Such speeds mean considerable lifting loads on the front edge of a bimini, and the strapless bimini style lacks front support for these loads. All the stress is passed down to the four remaining mounting bases. The aft legs are compressed, but the main mounts are seriously levered. Whatever bases are used, whether deck hinges or side mounts, should be through-bolted and provided with backing plates to spread the load on the fiberglass.
Actually, there's a better solution, but because it bucks current design standards, it may encounter resistance. The answer is to turn the bimini completely around so that the support legs are on the front and the unsupported overhang faces aft. Except on sailboats where the legs really need to be aft, a reversed bimini makes the most practical sense because it clears the frame out of the working area of the boat, the rear.
This strapless bimini makes for a clear, uncluttered boat.
Think about fishing or water-skiing, and how lines need to pass from one side of the boat to the other through the stem area. There is no need to take the bimini down to do either (well, less need, anyway). What you and your customers will need to get used to is seeing the bimini stored facing forward. Styles change, as reverse radar arches have proven, and the boating public seems receptive to changes in form if the change offers functional advantages. But it's best to check with the customer first anyway.
One final note before leaving the subject. If the boat's layout allows, hinges can be installed in the aft struts also. The entire assembly can be laid down on the after deck without disassembling it-folded down just like an automotive convertible top. It's a very tidy package.
If you're wondering whether folded struts fold either up or down, the answer is yes, depending on circumstances. I prefer to see them hang, since the space below the middle frame seems a perfect place for them. But the legs on dodgers are fairly short, especially if the corner radius is large (10-14 inches, for example). In this case, the struts must fold upward. Happily, they only project up past the frame a few inches, and aren't an eyesore.
Thus far, the method for installing struts has been to attach them to the frames using jaw slides placed just below the middle frame(s). This works for three- or four-frame tops and has the virtue of keeping the stored struts in line with the rest of the frames. But some jobs may require attaching the struts higher up on the frame, closer to the cloth.
For these occasions, Tom Barber of Spenard Upholstery, Anchorage, Alaska, has suggested another way to mount the struts. King Marine has a concave nylon mount adapter which allows the use of flat mounting hard on cylindrical tubing faces. Yes, we all know about concave rail bases, but their forks face the wrong way for our purposes. The mount adapters give us the ability to attach two-hole side mounts to frames.
In the current case using horizontal struts, we would mount the spreader bats to the inboard faces of the fore and aft frames via side mounts and adapters. They will stick out like inboard grab rails, thus bypassing the intermediate frames. The storage will not be as neat, but this system places the struts higher up, providing greater stiffness and decreasing somewhat the leverage required to stretch the cloth. Although these mounts clearly do not have the load bearing capacity of the jaw slides (to combat shear and twisting forces on the connecting screws or rivets), they're a useful answer to a technical problem, and will surely find other applications well. File it where you can find it.
Regardless of how the struts are attached to the frames, the geometry of the folding struts remains the same. We have a triangle made up of the strut and the frame leg sections below it, and their proportions determine where along the length of the strut the hinge must be placed, Usually it is off-center, which leaves us with the problem of how to find the exact right location on every frame.
We create this problem, of course, by our desire to see a horizontal strut. It looks proper when it's horizontal, The frames below it, however, are likely to be at a strange angle, depending on relative frame lengths, mounting position relative to other boat structures, overhangs needed, etc.
But anyway, we make the strut horizontal because it looks good. When the frame is folded up, all this lack of symmetry can mean that one of the struts' jaw slides sits higher than the other, This means the legs under the strut are different lengths, which in turn means that the two halves of the hinged strut cannot he equal. Of course, if the hinge is placed wrong by as little as half an inch, the strut won't fold down all the way. And the proportions are infinitely varied.
Failing to be adequately armed in either mathematics or computer software, I came up with a no-brainer approach that requires only a tape measure and one measurement. Here it is.
First, set up the frame to desired proportions and install the jaw slides and the eye ends of the struts. I put the eyes into the jaws because it keeps my strut measurements in real terms and gives me a more complete view of how the hardware is fitting into the storage space. Measure the length of the proposed strut from the inside base of each eye end-that is, where the tubing rests in its socket. Measure both the port and the starboard sides. They may be differ, but if the frame looks right, it is right This does mean, however, that you'll have to customize each strut.
Memorize that number and fold the frame into its storage position, with the eye ends facing either up or down. It doesn't matter which direction they face, as long as they're going the same way. Now you need a special tool, a metal tape measure narrow enough to fit in an eye end all the way to the bottom (3/4-inch wide will do). For the sake of clarity, let's pretend that the length of the strut is 12 inches, Take the tape measure and place the zero end into one eye all the way to the bottom Allow it to fold, and align the 17-inch mark along the outside of the other eye at the place where you think the bottom of the socket is, hold these in place, and with your third hand, pinch the fold in the tape The number in the crease of the fold is what you're looking for.
You have just created a facsimile of the folding strut, and the crease is exactly where the center of the hinge must be. Assuming you have a strut with some extra length already at hand, simply cut it to the proportions shown on the tape measure and stick it in the frame. Sure, be cautious and double check everything before you cut. It can't hurt. But the method is reliable.
These are the frame basics for using collapsible struts in dodgers or Biminis. Once you're comfortable working with hinges, you'll find that your customers will come up with the applications. I know a few shops that put hinges in the solid aft pole that's used in zip-attached dodger awnings. The pole can be folded inside the cloth, and the awning stores in half the usual space. Hinges also can he used to make to make gates in solid lifeline rails, and they have successfully been incorporated into the crowned sections of frames. The possibilities are many and varied, I do hope you will give it a try.
John LeMole, MFC, is owner of Gemini Marine Canvas, Rockland, Maine. For folding-strut product support, contact us.
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