This page was printed from https://marinefabricatormag.com

Constructing snow flies for the Alaskan climate

January 1st, 2017 / By: , / How-To Articles, In the Shop, Projects, Resources

Lady Marie is cocooned for the winter. The installation of the snow fly, from arrival to departure, took 29 minutes.
Lady Marie is cocooned for the winter. The installation of the snow fly, from arrival to departure, took 29 minutes.

Our winters in Fairbanks, Alaska, can be cold or very cold with lots of snow or little snow; it all depends. One condition we don’t see often is wind. In my 26 years of living here, I can remember only one time when the wind blew and the snow flew. Most of our snow falls gently from the sky and piles up on everything. The tree boughs can look like something from a Disney movie, and overhead powerlines become dressed with garlands of snow draping side to side.

That is why if we put a sharp pitch on a flat surface, the snow will build up some, then slide off, until the piled snow gets so high that there is nowhere for it to go. Usually, just stomping down the snow does the trick, though on occasion we have to shovel it. It all depends.

To pattern a caternary curve, position a chain from two brads until it drops to a depth point, then dot each link with marker. A plastic pattern is then made and used to trace the curve onto the fabric.
To pattern a caternary curve, position a chain from two brads until it drops to a depth point, then dot each link with marker. A plastic pattern is then made and used to trace the curve onto the fabric.

A two-component system

We build our snow flies as a two-component system, which is different from a typical camp dining fly where the fly, poles and tension ropes all work as one. Our snow flies have a separate pole setup and then a fly covering. This approach is similar to our portable structures or ridged frame awnings where there is a framework and there is a cover.

The main advantage to this process is that as the cover degrades over time, the pole system remains intact. This greatly reduces the advent of falling poles. Naturally, the webbing straps need to be inspected for any wear before being put back into service.

Our fastening point at the bow is a 2-inch web strap with “D” rings that goes around the tongue of the trailer. There are bolts where the strap is pressing against so we made a sleeve out of 28-ounce PVC to protect the webbing. We did cut out the bottom outside center of the sleeve to permit water to drain. The stern is anchored to the trailer eye bolts for strength.

Photos 3-4. The fastening point at the bow is a 2 inch web strap with D rings that go around the tongue of the trailer. There are bolts where the strap is so a sleeve of 28 ounce PVC is used to protect the webbing. The stern is anchored to the trailer eye bolts for strength. Photos 5-6. The webbing straps connect to the one and half inch schedule 40 aluminum poles via SS eye bolts and SS carabiners. The eye bolt with its attachments can simply be dropped into the top of the pole, and the bottom of the pole can be placed over the block base.
Photos 3-4. The fastening point at the bow is a 2 inch web strap with D rings that go around the tongue of the trailer. There are bolts where the strap is so a sleeve of 28 ounce PVC is used to protect the webbing. The stern is anchored to the trailer eye bolts for strength.
Photos 5-6. The webbing straps connect to the one and half inch schedule 40 aluminum poles via
SS eye bolts and SS carabiners. The eye bolt with its attachments can simply be dropped into the
top of the pole, and the bottom of the pole can be placed over the block base.

The webbing straps connect to the 1½-inch schedule 40 aluminum poles via SS eye bolts and SS carabiners. The eye bolt with its attachments can simply be dropped into the top of the pole, and the bottom of the pole can be placed over the block base. The block base protects the deck and helps distribute the load. Using different colored webbing helps identify which strap goes where. It can get confusing when you’re under the fly trying to figure this out during set-up.

The poles are held in place with a minimum of three web straps. This makes positioning the poles vertically easy to do. It also makes the poles very stable.

Catenary curves

Generally we use a catenary curve for the ridge line and a shallower catenary curve for the corners (one at the bow and two at the stern). To get a pattern for the curve, we measure the distance between the two end points and guess as to how deep the curve will be. Fortunately, we’re not building a cathedral or the Golden Gate Bridge; fabric is very forgiving.

Photo 7. The poles and webbing straps were staged. Photo 8. The fly was positioned over the boat. Photo 9. The bow pole was held in place using three webbing straps. The two blue straps were anchored to the stern.
Photo 7. The poles and webbing straps were staged. Photo 8. The fly was positioned over the boat. Photo 9. The bow pole was held in place using three webbing straps. The two blue straps were anchored to the stern.

Patterning a catenary curve

On the cutting table, we’ll lay out some patterning plastic; in this case we used 12-ounce high-density polyethylene (HDPE). We mark a line horizontally, mark our two end points, find the center and draw a perpendicular line.
We measure down that line and mark the depth. That will be the center bottom of our catenary curve.

From here we tack the patterning material to a wall, ensuring the horizontal line is level, which is very important. Now by putting in two brads, one at each of our end points, we can hang a chain. We hook the first link to one of the brads then go to the other brad and adjust the chain until is drops to the depth point. Using a felt-tipped marker, we dot each link. We end up with a catenary curve built for our project. By measuring the chain from link to the link we used, we also have the length of this curve, which might be helpful if a pocket needs to be added to the curve.

As I mentioned, because the fabric is forgiving, an alternative to a catenary curve is a simple arc.

Eric Walton, MFC, is a co-owner of Custom Canvas Alaska in Fairbanks, Alaska.

Print
Many years ago, I worked for Phil Mickelson, then owner of New Haven (Conn.) Awning Co. He gave me a formula that has come in handy over the years. This formula determines the diameter (D) of an arc when having a known measurement for the chord (C) and a known measurement for the height (H).

We also use this formula to determine the radius of welded frame waterfall awnings. It takes the guesswork out when we draw the frame layout onto the floor. See top diagram.

To find the length of the arc, you’ll need a protractor; since you know the diameter you can figure out the circumference. You then divide the circumference by 360 degrees; that will tell you how long 1 degree of this arc is. Using the protractor, you can determine the number of degrees, multiply that by how long 1 degree is and you’ve got your measurement. See bottom diagram.

Of course, you can also use a tape measure and in small increments measure the arc. This will give you a close enough measurement. Calculators can also be found online. What is most important about the curve is that it is smooth.

Leave a Reply

Your email address will not be published. Required fields are marked *

Comments are moderated and will show up after being approved.