How do I make and test a radar-reflecting hat?
At significant risk of tin foil hat jokes, I’ve read compelling evidence that a homemade hat incorporating angled and crumpled aluminum foil may slightly increase a kayaker’s visibility on radar — not enough to provide real safety by itself, but as a small addition to other best practices. Thanks to this answer to What can I do to be more visible to other ships’ radar when traveling in a sea kayak? for the link to Radar, Reflectors and Sea Kayaks: A Visibility Study:
How do I replicate or improve the performance of this hat while making it something I’ll actually wear on a long ocean paddle? Most importantly how do I make sure the design is doing its job as intended especially after use and wear?
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Here is a suggestion, though somewhat wacky is perhaps more realistic than the "hat" idea.
The page Sailing Safety Secrets - Are You 'Radar Visible' to Ships at Sea? says that the reflector should be as high as possible. For a boat with no masts, that can only be done by towing a kite, or a helium balloon.
What should the reflector be? A tinfoil hat or other random affair might not reflect radar back in the same direction. This diagram shows that a pair of mirrors with a 90° angle will reflect back in the incoming direction.
Of course, a single pair of mirrors might be tipped at any angle, and not work at all. But this page: Really Lightweight Radar Reflector Built With CDs suggests using a 3D version of the idea, which will reflect back incoming radar from any direction, to where it came. In this case it is made of three orthogonal reflectors.
It is small, but light, and perhaps several of these can be suspended from a kite or balloon to provide a radar image. It should not matter how they spin or wobble: rays from any direction are always reflected back.
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It's a very good question.
I found an article that has some suggestions:
Finally, it is important to consider your visibility on radar and at night. The best way to ensure that you are seen on radar is to use retro-reflective tape on your kayak, paddle blades, and PFD. I recommend putting 6inch long strips of Solas ® retro-reflective tape on both sides of your kayak near the bow and stern and on the back and power faces of your paddle (especially if it is carbon fiber). Many PFDs have retro-reflective tape on the shoulders or back of the vest as do some paddling jackets.
Kayak Dave
I added the bolding to help focus on the key parts.
PFD = Personal Floatation Device
The rest of the article talks about selecting colors for visibility which also might be useful.
BTW I am not nor do I know Kayak Dave
Ships, boats, emergency gear
Retroflective tape is recognized and recommended by the International Convention for the Safety of Life at Sea (SOLAS) because of its high reflectivity of both light and radar signals.
Product info - as an example, not an endorsement
Whatever you develop - part of your question is how to test it.
The best thing would be to find a ship with radar that you could test with.
I have no idea how open your local Coast Guard would be to helping you but it seems like a place to start.
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Your question implies that having a radar reflecting hat is somehow going to prevent you getting hit by a large ship that is using radar. This is at best wishful thinking.
In most cases human powered boats have the right of way, except in narrow channels.
A vessel of less than 20 meters in length or a sailing vessel shall not impede the passage of a vessel which can safely navigate only within a narrow channel or fairway.
A vessel shall not cross a narrow passage or fairway if such crossing impedes the passage of a vessel which can safely navigate only within such channel or fairway. The latter vessel may use the sound signal prescribed in Rule 34(d) if in doubt as to the intention of the crossing vessel Source
If the large/fast vehicle is not able to visually see you, the radar hat is not likely to make a difference. While not real, this YouTube video Navy Ship vs. Lighthouse provides an insight into the mind of the operators of large vessels. The ships/boats using radar are using it to protect themselves from running into something harmful to them.
If you are in a narrow channel or shipping lane, they have the right of way; you need to keep out of their way. If in the open ocean, they are not expecting anything tiny that needs to be avoided; you need to practice defensive boating. Kayak like you are invisible, because... well, you are.
The graph in the question is about being visible to a ship at ⅛ to ½ mile; the person in the kayak can see the ship at 2+ miles away.
Sitting in a kayak: The distance to the horizon for someone sitting in a kayak is approximately 2.1 miles. When you’re sitting in a kayak your eyes are about 2.5 feet above the surface of the sea–this is why waves appears so big when you’re in your kayak. 7 x 2.5 = 17.5 and 17.5 / 4 = 4.375 and sqrt of 4.375 = 2.09165. Source
The kayak is significantly more maneuverable than a large ship, the kayak can see the ship sooner and get out of its way, and is the only vessel at any risk in a collision. The kayak is not allowed to cross a shipping channel if they are not able get out of the way ("shall not impede the passage"). In the open ocean, a kayak theoretically has the right of way, but realistically there is no point in being "dead right".
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