Cruiser A/C: Design an Awning to keep your boat cool

Adding an awning to your boat is a fabulous way to add shady, rain-proof living space to your boat while you cool the interior. An awning cools your boat by reducing solar gain and can be considered essential equipment in Mexico and similar climates. Even in relatively temperate Hawaii they are a godsend. Luckily an awning for a monohull is relatively easy to design and only the large size makes it a little awkward to sew. Foredeck awnings are also helpful, especially if you sleep in the forepeak, but are not the subject of this article. However once you’ve sewn your main awing the foredeck awning will be a simple smaller project.

So what are the characteristics needed for a great aft-of-the-mast awning?

• Durability – the material is not inexpensive!
• Structural stability – stays up for weeks at a time, maybe months for some cruisers so must be able to withstand normal wind storms likely in an anchorage without problems
• Secure attachment to boat – probably at the mast, over the boom, the backstay and along the edges
• Easy installation – needs to come down quickly in emergencies and nice if it goes up easily as well
• Roomy – should enhance, not further limit your outside space – need to be able to walk under it, fill the water tanks, sit comfortably with some view, etc.
• Storable – must be stored while underway and is large enough to need some forethought on storage
• Light colored for optimum coolness, although a dark underside can enhance the coolness as long as the top is light to reflect light and heat
• Water Resistant – should help protect the cockpit from rain storms

I looked at many awnings and decided I liked what they were doing at Shade Tree Fabric Shelters. These awnings use fiberglass tent poles which are light, strong, and break down into 2 foot lengths for easy storage. The dome shape and the flexible mounting system adds stability in winds. They make theirs from coated Dacron, but I decided that Sunbrella would be easier to work with, slightly less expensive, and probably more sun-resistant. I decided I could make my own. I looked for the tent poles at many places but ShadeTree sells these so I bought them from them. They were very helpful and got the parts to me quickly. You can also of course order the whole thing from them – from the comments I’ve read in several sailing forums they have an excellent reputation and their awnings are very robust.

But if your budget doesn’t extend to a custom awning and you are excited about sewing your own, read on. These instructions will be most useful for other monohull owners, although the method would be similar for multihulls.

Measuring Your Boat for your New Awning

Length

Measure the longitudinal by placing one end of a measuring tape at the aft side of your mast ABOVE the sail stack. Stretch the measuring tape back to the backstay, keeping the tape approximately level with the top of the highest point of the sail stack. That is your maximum longitudinal distance. Your awning can be no longer than this mast-to-backstay distance. As you cross the topping lift while making this measurement, also make a note of the distance from the topping lift to the backstay at the height of the sail stack. You will need a slit in the awning to match this distance. Measure from the deck to the top of the sail stack. Make sure you measure to the deck, not the cabin top. You only need to be accurate to an inch or two for this measurement. Also measure the height to the top of the life line from the deck.

Make a Profile (or Elevation) drawing using graph paper and it should end up looking something like the one at right.

If, like me, you are insane enough to start this project while your boat is out of the water and the mast is in the mast yard, you will have to use some nice messy trig to compute these distances. Contact me if you really want to do this and don’t know how.

If, as you walk along your deck, the sail stack is NOT at least as tall as the tallest crew member, you may want to take your awning even higher (which will also make it shorter, longitudinally, as the backstay will hit it closer to the mast). Make sure you measure the length high enough to be convenient for walking under and yet long enough to cover the part of the cockpit or rear deck that you want covered. I suggest sketching this out both in “deck” plan and in profile as well as a cross-section. Draw in the mast, boom and sail stack height, boom length, an “x” for the backstay where it crosses the line at the top of the sail stack, and, for the profile and cross-section, a line for the height of the top lifeline. See example drawings below.

Boat Width

Measure the width of the boat from lifeline to lifeline every 4 or 5 feet from just abaft the mast to the end of your awning (where it hits the backstay). This distance is NOT the width of your awning material, but is the distance between the legs of the awning. Your tent poles will be every 4 or 5 feet, spread equally down the length of the awning. The variation in these widths will give you an idea about whether a rectangular awning will be sufficient, or whether you need to do some variation on rectangular. If you’re creating a foredeck awning (on a monohull) it will be a tapering triangular shape, but many abaft-the-mast awnings can be rectangular. Create a scaled Deck Plan Drawing like that shown here.

Cross-Section Drawing

Using the numbers measured above, create a third drawing at the same scale that shows a cross section of your awning design as if you were standing on the transom looking forward. Include top of sail stack, widths at the 4 or 5 locations along the longitudinal (narrowest and widest places as a minimum), and height of lifelines from deck as in the drawing at right.

Design the Awning and Test the Design

With your reference drawings in hand, go to Shadetree, Available Models . Start by looking up your proposed awning length from the “length” section above. Since my 41’ boat is narrower than many at that length (11’3” at widest), I wanted to make sure the standard model would work for me. My measured awning length on our 41’ sloop was about 230” and ShadeTree recommends 5 wands of 10 sections for this length, so that was a good starting place. I cut out a piece of graph paper that was 10 x 26” long (260”) using the same scale as my cross-section drawing. You can see this model drawn on the bottom of the Cross-Section drawing above. This represents one of the tent poles that would support the awning. The pole is going to run from about 6″-8″ below the top lifeline on one side of the boat to about 6″-8″ below the top life line on the other. When you place the sliver of paper on the cross-section drawing in this manner it will naturally curve over the width of the boat. The paper curve and the awning curve won’t be the same, but it’s a reasonable approximation. If it clears the “x” where the top of the sail stack is, then it is long enough. If it clears the sail stack by a lot, and you don’t need the extra height to walk up the side decks, bend up 26” and try the next shortest configuration. By doing this I discovered I really only needed poles of 9 sections, and indeed my last pole would only need to be 8 sections to be roughly the same height as the other poles.

Determine Finished Width

Once you know the length of the awning poles you are going to buy, you can determine the finished width of the awning. The pole will go about 6″ below the top of the life line on each side, held there by a webbing strap that traps the life line and the end of the pole. The edge of the awning should be no more than two to three feet above the life line. This gives you adequate sun protection but allows those in the cockpit visibility out and allows breeze in. So total width of the awning will be about 6′ or 7′ (3′  to 3.5′ on either side) shorter than the pole. Write down the actual difference between your awning width and your pole length. It will be used when you are calculating Adjustable Strapping length!

Order the Awning Materials

Tent Poles for the Awning

Now that you know how many and what size tent poles you are going to use for your awning, you can order those from ShadeTree using the prices and specification from their Replacement Parts list. You might also want to order some snubbers from them (additional bungie cords to secure the front and rear edges of the awning to shrouds or other parts of the boat). Notice that you cannot exactly order these online, but if you tell them what you want using the Contact Us form, they will get back to you quickly by email or phone and complete your order.

How Much Sunbrella for the Awning?

Now that you know the designed size of your awning you are ready to compute the amount of fabric you need. I decided to run my seams the same direction as my poles so that the seams did not interfere with inserting the poles. I thought the extra bulge of the seam might interfere. And for the same reason I did not want my poles to run along the seam – I wanted the seams offset from where the poles would run. So I suggest drawing out the basic shape of the awning on graph paper, using the desired finished sizes. Sunbrella is 60″ wide.

The first and last panel will be 57.5″ wide (allowing for one edge hem and one interior seam.

Each interior panel can be 58″ wide (allowing for two interior seams). Draw this out on paper.

Now lay in your poles at equal intervals over the whole. The first and last pole can start right after the edge hem, or even a few inches inside. Draw them in over your first layout, as shown. If your webbing straps interfere with your seams you can decide it doesn’t matter, or change the length of your awning to create more offset, or move the first and last straps a little inboard and create larger hems on the fore and aft edges and move the interior poles to account for the change. When you are happy with your layout you are ready to buy your materials.

Sunbrella can be bought at most canvas stores or at Sailrite. Choose a light color for maximum reduction of solar gain (I used Silver, which is much lighter color than it looks in Sailrite’s photos).

Sunbrella = the length of any panel times the number of panels.

How much Webbing for the Awning?

I used 2″ webbing straps in two ways on the awning, departing slightly from the Shade Tree design. First, just like ShadeTree, you will need to create two 3′-4′ long Adjustable Straps for each pole in your awning. These straps have a male buckle on one end and are folded over on the other end to create a pocket to tuck the end of the pole into. An additional 4″ piece of webbing, the retainer strap, is sewed about a foot from the end pocket to make another open-ended pocket that the pole is inserted into on its way to the pocket. These straps are connected to the awning by female buckles on the awning itself. This allows the straps to be synched down to tighten the awning. I made mine 5′ long and figured if they were much too long I could shorten them once installed.

I also used webbing all the way across the outside of the awning at each pole. Shade Tree appears to just apply a female buckle to the awning, and not run webbing all the way across to the buckle on the other side. This would save you some money and work. However I wanted the webbing to strengthen the awning where the pole channel attaches to the awning. It seems like this area would have the strain of the pole channel pulling on the sunbrella material as well as the strain of the adjustable strap pulling on the Sunbrella. By placing a webbing strap on the outside of the awning the pole channel is tied to the awning with the additional strength of the webbing and also the Adjustable straps will pull against the webbing, not the Sunbrella. It can also make nice stripes across your awning if you choose a contrasting color. I call these the Pole Strapping.

Now you can calculate the amount of 2″ webbing that you will need. Add the Pole Strapping (if you are using it) to the Adjustable Strapping totals to get the complete amount. Then I urge you to buy 2″ polyester webbing, which has great strength, low stretching, and good resistance to UV. It can be a little hard to find. My local canvas and webbing store only carries polypropylene or nylon. Polyester is better. Sailrite has white, or you can order webbing in 11 colors from Strapworks.com.

Pole strapping = (number of poles) x (width of awning + 8″)

The extra 8″ gives you 4″ on each side to sew the female buckle on. The female buckle at the edge of the awning is what the Adjustable strap clips into when you are erecting the awning.

Adjustable Webbing Strap = (number of poles) x (5.5′) x (2)

The .5′ is to add the 4″ retainer strap above the pocket + the foldover for the pocket. If you know that the length of your poles is not more than six feet longer than your awning width (3′ on each side) you can reduce the five feet closer to four feet.

This meant created a “pole channel” on the underside of the awning where the pole would be inserted. I made these out of Surlast, an abrasion resistant material. You could also use Top Gun or similar products. I cut 4″ wide strips of the Surlast, sewed the strips together to create the channel length I needed, then sewed the channel edges together to make a long tube, and made one for each pole. Note that Surlast has an inside and outside so be careful when you go to sew the tube.

Surlast = (finished width of awning in inches) x (number of poles) / 12  = feet of channel needed.

1 yd of 60″ surlast makes 45′ of pole channel

What Else do I need?

You will also need lots of thread (v-69 or V-92 thread at SailRite is UV resistant), lots of seamstick, and 2″ YKK Side-Release buckles. You may want to add leach line or 1″ webbing inside your hems to give something for bungie snubbers to hook onto.

Buckles = (# of poles) x 2

What’s Next?

Your design is complete and you have ordered all the materials. The next step is of course to put these together. You can probably figure it out from what we’ve gone through so far, but someday I will put together details on that and write a post.  But a few hints now. The main thing is that the cover is large and your machine (like my SailRite LSZ-1) may not have a large throat – so sewing space is tight! You will want to plan out each panel, sew as much on each panel as you can, then second to last step, connect the panels together, always keeping the panel you’re adding to the right (under the throat) and the rest of the awning outside to the left. Then the last step will be to hem the long sides, once again leaving the bulk of the awning to your left. Another thing to keep in mind on this project is that there are layers to keep from sewing together too early. The top webbing over the channels must be secured first (I basted it on right down the middle of the strap). Then the surlast channels for the poles are added to the underside. You want to sew these only at the very edges to leave unobstructed room for the poles. By using 2″ webbing straps your channels should fit neatly underneath and as you attach the channels from the underside you will also be sewing the webbing edges down on the top side. You also want to make sure not to sew the webbing straps down too close to the hem. You need to leave room for the hem itself, and you also need room to attach the female end of the buckle , tuck the end of the strap under and sew the strap together to secure the buckle permanently (see image below). It’s easier to add the buckles AFTER the side hem is done, but be sure not to sew the straps down as you do the hem.

For now wait for your materials to arrive and imagine sitting in your cockpit in the shade while in the tropics…