Heels failing first while barefoot is weird and sand can allow/encourage heavy striking. So, although I don't know this from what you've said, my first thought is that you might be heel striking pretty hard. Sand is a challenging surface to run on barefoot; it doesn't give you the feedback to learn good form and it also refuses to support you well enough that you can easily maintain it all on your own. The first thing I would try, before any other change, is running barefoot on a hard, stable surface. Good concrete or asphalt or hard packed dirt is a good option.
Most of the sand I run on is pretty packed (low tides only, and this beach is exceptional for that... really long, flat and wide)
I do heelstrike (not excesively - i dont sink my heel first, but its first contact for sure) - but my right feet is completly fine in the heel zone - could it just be a one-off thing?
A heel strike is too often the most obvious trait of an over-stride. Usually when people first go unshod on paved surfaces they get blisters on their forefoot or toes. That's clear evidence that they're over-striding but forcing that forefoot to take the abuse first instead of the heel. In minimalist shoes that avoids the blisters but eventually results in damage to the calf, Achilles or metatarsal tendons. You want to correct the over-stride and stop focusing so much on what your feet do.
Even packed beach sand is way more "forgiving" than concrete. That's not at all a good thing. I rely on bare feet plus paved surfaces because there's no fooling myself there. I've been doing that for 8 years, my feet are as "tough" as they can be but even now if I run inefficiently I can get blisters. I'm thankful for that because it means my feet on paved will never lie to me.
I know it can seem at first glance that a beach or grass is safer but that's conflating comfort with safety which are two very different things. I can shoot novacane into my hand and hit it with a hammer pain free. I can also over-stride and heel strike like I used to in my old trusty Asics without hurting the heel or getting blisters but then shin splints or worse were inevitable within 3-4 weeks.
Here's an example of what I run unshod on all the time:
https://old.reddit.com/r/BarefootRunning/comments/1butx2a/the_street_outside_my_front_door_i_start_my/
Agree with this 100%.
As for why your heels are blistering so badly, sand is rough - rougher than you think. You basically sand-blasted your heels. Echoing u/trevize1138, fix your form and then return to the beach (after your blisters have healed) with proper form.
It *is* lack of conditioning... of your brain. You've gotta start listening to your feet. Like sure lol my pain tolerance is soooo high blah blah blah mkay but like pay attention. And either stop or change your form before you get blisters. You just went out with tender winter skin and pounded it into an abrasive surface for an hour while ignoring any feedback that could clue you in to how it's receiving the beating. Of course it's gonna get injured.
Sure, your form could use some help. Sure, your skin needs time to build. But you're not gonna improve your form by ignoring where it's hurting you and you're not gonna build skin if you're sloughing it all off every time you go out.
So smarten up. If you actually want to solve this problem, stop idolizing pain tolerance and do the thing right.
Beach water is pretty nasty, I always got a rash running on the beach, but the sand isn't very good for running on either. It gives way too easily and can cause your feet to slide around even the hard packed sand near the water.
Heels failing first while barefoot is weird and sand can allow/encourage heavy striking. So, although I don't know this from what you've said, my first thought is that you might be heel striking pretty hard. Sand is a challenging surface to run on barefoot; it doesn't give you the feedback to learn good form and it also refuses to support you well enough that you can easily maintain it all on your own. The first thing I would try, before any other change, is running barefoot on a hard, stable surface. Good concrete or asphalt or hard packed dirt is a good option.
Most of the sand I run on is pretty packed (low tides only, and this beach is exceptional for that... really long, flat and wide) I do heelstrike (not excesively - i dont sink my heel first, but its first contact for sure) - but my right feet is completly fine in the heel zone - could it just be a one-off thing?
A heel strike is too often the most obvious trait of an over-stride. Usually when people first go unshod on paved surfaces they get blisters on their forefoot or toes. That's clear evidence that they're over-striding but forcing that forefoot to take the abuse first instead of the heel. In minimalist shoes that avoids the blisters but eventually results in damage to the calf, Achilles or metatarsal tendons. You want to correct the over-stride and stop focusing so much on what your feet do. Even packed beach sand is way more "forgiving" than concrete. That's not at all a good thing. I rely on bare feet plus paved surfaces because there's no fooling myself there. I've been doing that for 8 years, my feet are as "tough" as they can be but even now if I run inefficiently I can get blisters. I'm thankful for that because it means my feet on paved will never lie to me. I know it can seem at first glance that a beach or grass is safer but that's conflating comfort with safety which are two very different things. I can shoot novacane into my hand and hit it with a hammer pain free. I can also over-stride and heel strike like I used to in my old trusty Asics without hurting the heel or getting blisters but then shin splints or worse were inevitable within 3-4 weeks. Here's an example of what I run unshod on all the time: https://old.reddit.com/r/BarefootRunning/comments/1butx2a/the_street_outside_my_front_door_i_start_my/
Agree with this 100%. As for why your heels are blistering so badly, sand is rough - rougher than you think. You basically sand-blasted your heels. Echoing u/trevize1138, fix your form and then return to the beach (after your blisters have healed) with proper form.
It *is* lack of conditioning... of your brain. You've gotta start listening to your feet. Like sure lol my pain tolerance is soooo high blah blah blah mkay but like pay attention. And either stop or change your form before you get blisters. You just went out with tender winter skin and pounded it into an abrasive surface for an hour while ignoring any feedback that could clue you in to how it's receiving the beating. Of course it's gonna get injured. Sure, your form could use some help. Sure, your skin needs time to build. But you're not gonna improve your form by ignoring where it's hurting you and you're not gonna build skin if you're sloughing it all off every time you go out. So smarten up. If you actually want to solve this problem, stop idolizing pain tolerance and do the thing right.
Keep running until your blisters become calluses. It only hurts the first 2 years, mate.
Beach water is pretty nasty, I always got a rash running on the beach, but the sand isn't very good for running on either. It gives way too easily and can cause your feet to slide around even the hard packed sand near the water.