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AKU SL Pro II GTX Mountaineering Boots

AKU SL PRO II GTX Mountaineering Boots
AKU SL PRO II GTX Mountaineering Boots

One of my goals for this winter has been to switch to a more comfortable, lightweight mountaineering boot without a removable liner, at least for day hikes. While my other pair of plastic boots, Scarpa Omegas  (5 pounds 2 ounces/pair), are probably the lightest weight and most comfortable boots with a liner (no longer made), they still feel like you’re wearing cinder blocks on long approach hikes – a not-to-uncommon sensation with plastic boots.

While boots with removable liners are often required for overnight trips where you can’t risk having your boots freeze from the previous day’s foot perspiration (up to 8 ounces per day per foot), they sacrifice fine motor control and climbing feel on more technical routes because they’re so much larger and wider than mountaineering boots without liners. Since most of my winter hikes are day trips and also require very long approach hikes or snowshoeing, I am ready to lighten up and try a smaller volume boot.

At 4 pounds 6.5 ounces/ pair, the new AKU SL Pro II GTX Mountaineering Boots looked like a good candidate to try. AKU is an Italian boot maker best known for their Spider Kevlar GTX boot, a bomber ice climbing boot that is known for its comfort on long approach hikes. Comfort on approach hikes is a usually a function of rocker, or a slight lengthwise curvature of the sole ,which makes walking more natural.

Though not as warm as the Spider which is rated for 6000 meter alpine summits, the SL Pro II GTX is rated for 4000 meter ascents and ice climbing. It is insulated using  Gore-tex Duratherm, a hollow fiber insulation laminated to a waterproof liner which ensures warmth and breathability, and is used by many other mountaineering boot manufacturers. In addition, the SL Pro II GTX comes with an aluminum coated footbed designed to reflect your foot’s body heat back into the boot and which prevents it from being radiated into the ground.

Although it’s still early in the winter season, I have already used the SL Pro II GTX boots for long approach hikes in snow and steep climbs over mixed rock and ice, which is typical for the White Mountains of New Hampshire where I spend many of my weekends in winter.

Structurally, the AKU SL Pro GTX II’s are very rigid, which is desirable for a boot rated for step-in crampons. The rigidity comes from an exoskeleton adhered to the outside of the boot, high rubber front and heel rands for protection, and a carbon fiber last which is lightweight but extremely stiff. This translates into excellent control, particularly for climbing on steep, dry rock, something I don’t get with my much higher volume plastic mountaineering boots.

Warm Feet while Melting Snow in Camp
Warm Feet while Melting Snow in Camp

Fit-wise, the boot is much lower volume than a boot with a separate liner, but it also has fantastic arch support eliminating the need for the superfeet insoles which many of us turn to provide extra support or to use up extra volume in mountaineering boots. The fit around the toes is noticeably narrow, while wider around the heel, which may require some creative sock shimming depending on your foot shape.

For myself, I found the best combination is to layer a liner sock over a medium weight wool sock that I’ve cut the toes off of to keep my toes mobile, but take up the extra width around my heel and ankle. This lets me also add a vapor barrier liner as a base layer so I can use the AKU SL Pro II GTX for overnight trips, but that requires a separate article at a future date.

I am pleased with the performance of the AKU SL Pro II GTX boots so far and look forward to many big hikes with them this winter and beyond.

Disclosure: Philip Werner (SectionHiker.com) received a complementary pair of AKU SL Pro II GTX Mountaineering Boots for this review. 

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  1. My Aku Spiders are on their 5th year now, and have been thoroughly abused. My wife has the SL Pros & loves them. I really think Aku make some of the best single-skin boots on the market.

  2. I’ve been using synthetic winter climbing boots for many years and they are my preferred footwear for winter climbs. For my difficult to please feet the Millet Alpinist boot has worked wonderfully. Millet no longer produces the Alpinist so I’m looking for an alternative. At this point my boots are still in excellent shape. Dave Page has confirmed that he can resole them but I know at some point they will need to be replaced. I’m going to look into these boots.

    • You know the drill, size up a bit and sock fit. These are far more rigid in the mid boot than my omegas and more comfortable around the ankle. Definitely an eye opener after years of plastic boots.

  3. My question is a bit off topic, but I figured you would have some good insight on the issue. Under what conditions do you use a mountaineering boot as opposed to a regular winter boot.

    The reason I am asking is that some time ago I got overly excited about the whole mountaineering thing, and bought a pair of plastic double boots. Since that time I have tried a few pairs of single/leather mountaineering boots as well. I wore my plastic boots once, and just didn’t see the point. My ice climbing skills are not advanced enough to require German technique or any serious front pointing, so the stiffness of the boot gave me no advantage, while it made the rest of the trip feel like I am wearing cinder blocks on my feet (as you said).

    I ended up switching fairly quickly to a regular winter double boot with a semi rigid sole. They can take on most semi rigid crampons, stay warm, and are comfortable for a wide range of winter trips, from walking on moderate terrain, to snowshoeing, to moderate climbing.

    So for your mountaineering purposes, under what conditions do you pull out the plastic mountaineering boots as opposed to regular winter boots? Thanks.

  4. Good question. I’d use the double plastics (provided the temperatures are 10-20 degrees or less) for the following situations

    1) overnight trips where sweat from my feet will freeze overnight unless I take the liners out and sleep with them in my sleeping Bag.
    2) above treeline hikes where the consequences of an unintended overnight are very severe (frostbite or death) For example, when I climb Mt Washington or any of the other above treeline peaks in New Hampshire, I’ll wear double plastics.
    3) Hikes/climbs we undertake in very cold weather. For example, when I climbed two peaks last winter in -20 below zero weather.

    Those are pretty much the primary scenarios. New Hampshire’s mountains can be pretty extreme.

    • Thank for the answer, but why not just use regular winter double boots? I currently use the Merrell Norsehund Alpha, which have proven to be comfortable and still have the removable insulation. Baffin, Sorel and other manufacturers also make winter double boots that are much easier to walk in, and are used in cold places all over the world, including Antarctica.

      Double boots certainly have an advantage for the reasons you mentioned, but there is a fair number of non-plastic double boots on the market. Other than for technical ice climbing (i.e. using ice tools, etc), is there any advantage to the plastic boots over the other ones, especially considering the walking discomfort that comes along with mountaineering boots?

      • I think this ultimately comes down to personal preferences, but for higher angle hiking, which is what I do, I like a mountaineering boot that is step-in crampon compatible and hard enough to kick steps. I also use aluminum step-in instead of steel because its about a third of the weight.

        Let me add to that. The amount of extra control and precision provided by a step-in crampon is much better than a strap-on because the boot and crampon become “one”. Makes a huge difference for me in tackling sketchy routes and boosts my self confidence.

      • Makes sense. Thank you.

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