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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:

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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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    I don't know if that hat will reflect radar, but it is likely to deflect girl-friends. – ab2 Jul 20 '19 at 20:03
  • I'd look at something besides headwear... something like a feather flag pole or a long cane pole in a rod holder, with lots of long mylar strips. Get the tip 9+ feet up in the air, use strips 6+ feet long (just be sure they aren't in your face when no wind, etc) and you should have both a nice visual indication but also a decent radar return signal. – ivanivan Jul 21 '19 at 0:57
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    @ab2 unfortunately the study did not include a graph establishing any positive or inverse correlation between radar-reflecting hat use and finding true love. Images from the report suggest that a good mustache may be an important precaution to offset any potential negative effects of the hat. – mmcc Jul 21 '19 at 2:18
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    You don't need this. If you are going to paddle in areas where you think other ships are a risk for you you have already taken the wrong decision about your location. – user15958 Jul 23 '19 at 7:20
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    You better use high-visibility clothes, as, contrary to radar, visual watch on a ship is mandatory at all time. You may additionally use a PLB for, hopefully not happening, SAR operation involving you. VHF is a great tool too, as this is mandatory too, and may allow you to speak to boat and MRCC at a range more or less 10 miles with a 6W power (maximum allowed power for a portable station) – Cailloumax Jul 31 '19 at 8:17
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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

From Wikipedia

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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  • Thanks—is there SOLAS retro reflective tape which is reflective to radar? Any product info? I always thought that meant retroreflective to light (I know you’re quoting the article but thought I’d see if you knew more). – mmcc Jul 22 '19 at 21:31
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    @mmcc I updated my answer with more information and an example product link - not an endorsement. – David D Jul 23 '19 at 0:50
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    I think it may be important to note that although retroreflective tape may act as a reflector for radar, it's unlikely to act as a retroreflector for radar. The properties required for retroreflection of light (with a wavelength around 500 nm) are very different from those required for retroreflection of radar signals (with a wavelength around 20 cm, which is 5 or 6 orders of magnitude larger). As far as I know, a retroreflector for radar pretty much has to consist of three flat electrically conductive plates mounted at right angles to each other. – Tanner Swett Jul 23 '19 at 16:27
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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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    I don’t think the question implies that and if anyone believes their tin foil hat makes them impervious to container ships that’s on them. If however something like wrapping 3M tape around your surf helmet significantly increased radar visibility that would be good to know. Doesn’t seem to be the case unfortunately. – mmcc Jul 23 '19 at 13:15
  • The main reasons for increasing your radar signature is when you're adrift or at anchor and hence causing a hazard to navigation. – Separatrix Jul 24 '19 at 7:13
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    @Separatrix a kayak with a person is not a hazard to navigation, anything big enough to have radar is not going to sustain any damage running right over the top of a kayak and person. Chances are they will never know they ran over you. If you want to live, stay in the shallow water where they can't travel or stay alert and out of their way. Most big ships can neither stop nor turn easily. Wearing a reflective hat in an ocean kayak, is like carrying a bright light while walking on train tracks, at best the operator is going to know they just killed you. – James Jenkins Jul 24 '19 at 10:08
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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.

enter image description here

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.

enter image description here

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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  • The study says that the hat works better than the commercial products, so I don't see why your idea would work any better – Charlie Brumbaugh Jul 22 '19 at 16:34
  • @CharlieBrumbaugh I don't want to get into an argument but the study does not say that: it's the (incorrect) interpretation of the study in the linked previous answer. The study itself says "a radar-reflective hat provides better visibility on radar than a deck bag made of radar reflective materials". And "this homemade hat reflector generated among the clearest returns." And "the hat yielded decent results." And "More research is needed on each of these possible designs." – Weather Vane Jul 22 '19 at 16:50
  • ... I simply posted an answer which is a wacky as wearing that foil hat, which is clearly impractical. – Weather Vane Jul 22 '19 at 16:54
  • @WeatherVane look at the legend on the graph I cut and pasted, at least one of the hats did ok! Also why is it clearly impractical? Non-rhetorical question I’m curious whether this concept could be developed into something useful (or maybe I’m just a standup paddleboarder in Ray-Bans and a comfortable pair of shorts who would love it if dedicated sea kayakers looked even more ridiculous) – mmcc Jul 22 '19 at 21:38
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    Using a kite or balloon was actually a serious suggestion, and not one that was mentioned (that I saw) in the material, because the report does say "Commercial and homemade radar reflectors provide increasingly better visibility as height above waterline is maximized." And "We can also correlate reflector height above the waterline with performance." And "Affixing a mast of any height, even a couple of feet, with a radar reflector at its apex increases windage and thus decreases stability." – Weather Vane Jul 22 '19 at 22:27

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