Mast camera (2)

Home Design Build Race Links Reports Other Topics


I've set up my mast camera to take single snaps.  A micro-servo activates the shutter release using the third channel of my Tx.  The camera is caged in some aluminium channel that I've milled to suit.  There was a choice to run a very long cable from the boat Rx up the mast, or to mount my spare Rx and a small battery with the servo, and use the same frequency crystal.  This set up illustrates the second option (175 g total).

Here are the close-ups.  The micro-servo (10 g) arm can be seen close to the shutter release button on the camera.  The USB port to the computer can be seen on the side of the camera through one of the milled gaps in the cage.  The angle of the whole cage to the mast attachment tube (15 g) can be adjusted via the two screws on the far left, and some support is provided by a wire from the cage to the mast attachment tube.

The small battery pack (60 g) is the yellow lump, the Rx is the black lump (20 g).  The cage (35 g) gives the camera (40 g) a little wriggle room, so its aim can be positioned.

Here is the sort of result achieved.  This snap shows the blown-out head of my main, wrinkled and without proper camber.  What I hadn't seen before, but the snap makes clear, is that the shape at the foot of the main is a little fuller than the shape of the middle of the main.  I think I need to move the outhaul a little more to flatten the foot some and match it to the rest of the sail better.  Similarly, the snap shows that the shape at the foot of the jib does not have nearly the same pronounced draft-forward shape as that at the lower draft stripe.  So -- the camera might tell me something useful after all.

The sample photo analysis of the jib (the point of all this palaver, after all!) shows that draft has moved forward considerably in the region of the lower draft stripe (called the "middle section"), to about 27% of chord, while the shape at the foot ("bottom section") has max draft at about 51% of chord, and at the upper draft stripe ("top section") it has moved back to about 38%.  We can probably conclude that my jibstay sag is a little excessive.  Camber at both draft stripes is around 9.5% or 10%, while at the foot it is just under 7%.  That's OK (I think!), the flatter foot gives a much finer entry angle, just what is needed to accommodate the wind gradient at deck level.  According to the sail measurement spreadsheet, entry angle at the upper stripe is around 27 degrees, rising to 41 degrees at the lower stripe, and a slim 18 degrees at the foot.  Relative to the boom, twist at the lower stripe is about 7 degrees, and at the upper stripe is about 14 degrees.

Unless the winds are pretty light, it turns out that 175 g at the mast head is quite a counterweight.  To get sail shape snaps when the wind is above 'middle of "A" rig', I think it'll have to be a long cable up the mast, and a little more lightening of the cage and mast attachment tube.  Alternatively, since we are really only interested in sail shape when we have this contraption at the top of the mast, and not with boat speed or racing, let's take a Marblehead fin, some 250 mm longer, and pop that into the hull instead.  That should redress the lever arm problem more than some.

The other detail is that the whole assembly puts some torque into the mast, and rotates the spreaders some when the boat heels.  Ideally, the battery pack should counter-balance the camera, and be positioned on the other side, hanging down.  So that'll be version three then!

A couple of sailors have asked why the camera isn't on the deck -- this would certainly keep the weight in the right place.  It would also give a better perspective of the sail.  Besides the problem of waterproofing, the major difficulties are that the camera would not be able to get the entire foot of the sail in shot, would not get much of the boom, and would probably not be able to get the whole of the bottom draft stripe within shot either.  As you can see, the snap from the mast head just gets the top draft stripe, a length of about 100 or 120 mm, in the frame.  It wouldn't be able to do this if the stripe was 350 mm long.  (The boom is important to establish sheeting angle and the "base" twist.  The sail foot is important to judge the change of camber distribution vertically.)  Also, the mast head position gets the jib into shot as well.  A deck mounted camera would have to be moved to take separate jib shots.

Finally, a couple of wrinkles from experience.

One is that the camera position is quite good to windward of the mast (about 100 mm, I don't think it needs to hang out any further).  However, it is aft of the mast by around 40 mm as the photo shows.  This means it doesn't get good sight of the jib leech, so it needs to be re-positioned forward of the mast.

Two is that this camera has a built-in auto-off function.  Thirty seconds from last use, it powers down.  Hmmm...  So you have to take a snap every 25 seconds even if you don't want to.  I've come back in from more than one photo session to discover it must have taken me 31 seconds to launch the darn boat and get it to an interesting position with the wind on the right side at the right angle and at the right strength after switch-on.  I'm thinking that a buddy with a second Tx would be useful;  he'd concentrate on sailing the boat to my instruction while I take the photos and keep track of the necessary 25 second intervals as needed.

Three is that this power-down is there for a reason!  The camera consumes AAA alkalines like there is no tomorrow.  (Non-alkalines can't be used in this model -- not enough volts or amperes.)  Take a six-pack if you plan on a lot of shooting, particularly if you use its "video" mode.  I've flattened a new battery after 65 seconds of video, and then of course lost the whole sequence 'cos its internal RAM wasn't being kept powered.  Alright, so it is a truly cheap 40 camera.

Four, is to think about the contrast of the stuff that needs to show up in the photo.  Not only could the upper surface of the booms do with being painted a dark colour, but so could the spreaders.  Alternatively, if the deck patches were a dark colour, then the booms and spreaders need to be white.

2005-12-18


2011 Lester Gilbert