This project could have been placed in several areas, but I chose the canvas project section as it has to do with modifying the flybridge's canvas enclosure. One issue that many flybridge enclosures have is that with all that isinglass, it is like a hot-house; hot enough to raise chickens! This is not only the case with a flybridge, but many express cruisers as well, so this project can apply to many boats.
I am going to put a vent into the side of the enclosure. The vent is going to have a fan in it to aid in circulating the air. Ideally, the fan should shut off during the evening so that you are not taxing the DC systems too much. I prefer DC power as you could potentially use this system while you are underway if you prefer.
The fan of choice is the small "boxer" type fans you often see on electronic equipment and computers. Many of these fans are brushless, so that electrical noise generated by brush contact won't interfere with communication and navigation equipment.
Since many of these fans are low power; my first attempt was to use a solar panel. A solar panel was attractive to me as it would only run during the daytime, and would not drain the boat's battery. I found a solar battery charger for about $25 that provided about 1.8Watts of power @ 12VDC; or about 150mA of current. There are many fans that fall within that range - at least on paper. After trying several fans, I found that every one of them took more than the specified power requirement. I am not referring to startup current, but just keeping them running, the solar panel had difficulty. As well, the solar panel would not produce enough juice unless it was in the direct sun. Add the mounting problems, and I came to realize this was not the ideal solution (unless I spent well over $100 for the solar panels).
An electronic kit manufacturer I like to use has a thermostat control circuit that runs off 12VDC, has an independant relay, and is low cost - Perfect! The Velleman MK138 is priced below $10, has an adjustable turn on/off range between 41 Deg F and 86 Deg F, and can be assembled in a few minutes. You can mount the temperature probe at the circuit board, or remote it if you wish. I believe you can buy this item pre-assembled as well if you are not liking to building one.
The temperature module has a SPDT relay, so you can make something turn on or off at a particular point. The module is designed to energize the relay when the temperature is below the set point threshold. Therefore, you need to wire the fan to the normally-closed contact, so that it will be on when the relay is off. The LED will come on when the relay is closed, so it will be off when the fan operates.
A bit goofy, but on the other hand, the LED will be on when the fan is off, so that you know the unit is in standby. When the fan comes on, the LED will be off, but you know the unit is operating sans the fan running.
Since the thermostat will be located at the flybridge, I decided it should be mounted in a waterproof enclosure with a bit of double-sided tape. I used one of my favorite Hammond 1554 series enclosures, and a Heyco cordgrip. I decided that leaving the temperature sensor inside the enclosure would work well enough. See Constructing waterproof enclosures for more information.
Mounting the enclosure is simple. We made this pouch out of Sunbrella and a bit of Velcro that attaches to a rail on the flybridge. There are a couple of velcro loops on the backside that will loop around the rail.
The fan attaches to the flybridge canvas by use of this mounting panel, made of Sunbrella. When you account for the hems, the machine had to sew through as much as 10 layers of Sunbrella. The fan is 3.25" square, and the panel itself is 8.5" square.
The completed vent system. It is completely portable so that it can be easily removed from the flybridge when the canvas is stowed, or if it is not needed. The system is simply powered by a 12V accessory socket at the helm. We also made a couple of "cable ties" from Velcro and canvas strap to secure the power wire.
Perhaps the most difficult part of the project is to sew in the vent in the flybridge enclosure. The vent should be able to be closed when needed, as this one does. The triangle side pieces consist of a piece of 1/16" thick plastic sandwiched between two pieces of Sunbrella. This gives the sides some rigidity. While I did not do this for the front flap, it could also benefit from the plastic stiffener.
From the backside, you can pretty much see the vent detail. To close the vent, the side triangle pieces fold in, and the front flap closes. Again Velcro is used to stick everything together (except for the top hinge of the flap - which is permanently sewed into place).
From the exterior of the enclosure, you can see how the vent tucks in nicely. Care must be taken to align the vent with all of the various radar arch up-tubes and other geometric shapes so that it does not look cockeyed.
All that is left is to velcro the fan into place, attach the temperature module to the rail, and plug it in. The temperature adjustment of the Velleman kit, is a bit goofy, so you may want to adjust the set point during the evening at the temperature you desire the fan to turn off.