TL;DR – Packrafts can hold a tremendous amount of weight without sinking, but it’s up to you to determine what is a safe load.
First, what is Capacity?
“Capacity” is the maximum payload a boat is designed to carry. It’s usually expressed in pounds, kilograms, or a number of persons. Another term for capacity is “useful load”.
The capacity or useful load of a packraft is only a fraction of the amount of weight it would take to submerge it – you might be surprised to learn that you could carry 800 pounds of lead in a medium-sized Skeena DIY Packraft and it would still float, even if a big wave came and filled the cockpit with water. With that much weight on board it wouldn’t be stable or easily controllable even in calm water, however, so that’s not a very useful number to know.
Two friends and I (about 215 kg or 475 lbs total) crammed into the smallest single-person Skeena to see what would happen and it floated alright, but because of the high center of gravity it was pretty unstable, and because all of our weight was centered in the cockpit, the bow and stern were high above water but there wasn’t much freeboard along the side tubes.
Over the years people have come up with different ways of calculating a boat’s capacity, and we’ll look at three of them below. The capacities listed on the DIY Packraft product pages are based as much on my personal experiences as on any of these formulas. They should be considered ballpark estimates of what’s safe in the conditions the packraft was designed to be used in, and not any sort of guarantee or advice.
Comparing Capacity Between Packraft Brands:
Comparing capacity between brands is not easy because none of them tell you how they arrived at the numbers, and only Alpacka indicates what their numbers are intended to mean:
- Alpacka says: “Max Capacity is the maximum total weight (paddlers and cargo) that the boat can reasonably carry in flatwater under ideal conditions.” It’s unclear how they arrived at their numbers.
- Kokopelli doesn’t say how they determined their numbers or what the numbers are intended to mean.
- Anfibio gives a “buoyancy” number in kilograms, but doesn’t explain what it means or how it was calculated.
- Aire lists “load capacity” for only some of their models and doesn’t indicate what the numbers mean or how they are calculated.
- MRS doesn’t say how they determined their numbers or what the numbers are intended to mean.
Given the above, if capacity is a major factor in your purchasing decision, instead of relying on the published numbers, I think you’ll be better off comparing the external length, external width, tube diameter, and hull shape to get a sense of the relative capacity between brands.
Calculating a Boat’s Capacity:
Different jurisdictions use different formulas to calculate a boat’s maximum legal capacity. If your jurisdiction uses another method, please post it in a comment below.
Method 1:
This is a rule of thumb for boats less than 20′ (6 m) in length. This method is usually used for powerboats, but it gives similar results to the other methods and it’s easy to calculate so I’ll include it here.
Capacity = Length (in feet) x Width (in feet) x 10 pounds (per square foot)
Example: The 130 cm / 51″ size Skeena DIY Packraft is 9.2′ long and 3′ wide
Packraft capacity = 9.2 x 3 x 10 = 276 lbs = 125 kg
Method 2:
US Federal Law 183.37 – Maximum Weight Capacity: Boats Rated for Manual Propulsion and Boats Rated for Outboard Motors of 2 Horsepower or Less
Rated capacity must not exceed 3/10 of the difference between the boat’s maximum displacement and the boat’s weight. “Maximum displacement” is the weight of the volume of water displaced by the boat at its maximum level immersion in calm water without water coming aboard.
This is more difficult to calculate than Method 1 because you have to know the boat’s maximum displacement. For the DIY Packrafts, I can calculate that using CAD models, but it’s extremely tedious. Once the maximum displacement has been calculated, the rest of the calculation is easy.
Example: The Skeena DIY Packraft from Method 1 has a maximum displacement of about 400 liters (0.4 m³), so
Packraft capacity = 3/10 x 400 liters x 1 kg/liter = 120 kg = 264 lbs.
Method 3:
This method of calculating capacity for inflatable boats is mandated by Transport Canada, and in my opinion it’s better than the others because it takes into account the number of air chambers in the vessel and their actual volume.
GL = (Vt × b × 0.75) – WV
Where:
- GL = recommended maximum gross load in kilograms
- Vt = the total volume of the inflated tubes in cubic metres
- b = constant buoyancy factor = 1000 kg/m³
- WV = the weight of the vessel in kilograms
- The 0.75 is a safety factor, reducing the capacity by 25%
A further load reduction is applied based on the number of air chambers:
- 1 air chamber = 50% load reduction
- 2 air chambers = 33% load reduction
- 3 air chambers = 25% load reduction
- 4 air chambers = No load reduction
Example: If a packraft has a tube+seat volume of 0.375 m³ and a mass of 3 kg, then plugging those numbers into the formula above we get
GL = (0.375 m³ × 1000 kg/m³ × 0.75) – 3 kg = 278.25 kg
Now we apply the load reduction factor based on the number of air chambers:
If you prefer to think of the packraft as having a single air chamber, we apply the 50% reduction and get a capacity of
Packraft capacity = 278.25 kg – 50% = 278.25 kg x 1/2 = 138 kg = 303 lbs
If you prefer to count the inflatable seat as a separate air chamber, we apply the 33% reduction and get a capacity of
Packraft capacity = 278.25 kg – 33% = 278.25 kg x 2/3 = 185.5 kg = 408 lbs
If we add an airtight zipper to the packraft and place gear in a dry bag inside the tubes, we have three air chambers so we can apply the 25% reduction:
Packraft capacity = 278.25 kg – 25% = 278.25 kg x 3/4 = 209 kg = 460 lbs
If we place two or more dry bags inside the tubes then we have four separate air chambers and we can apply no load reduction, so
Packraft capacity = 278.25 kg – 0% = 278 kg = 612 lbs
That means that even if with a 612 pound load on board, if the packraft got swamped by a big wave, over 25% of the tube volume would still be above water.
OK, cut the BS. I didn’t come here to do math. How much weight can I carry in my packraft?
As with everything else DIY Packraft-related, it’s up to you to determine what’s safe in the given conditions. You can certainly load up a DIY Packraft with more weight than I’ve indicated in the product descriptions – we can see from Method 3 above that it would take a tremendous amount of weight to actually sink a packraft (over 800 pounds to submerge the 130 cm size Skeena). But what’s a safe load?
I’ve gone out lake paddling with about 135 kg (300 lbs) total payload in my smallest V1 packraft and there was plenty of freeboard and it handled just fine, but that amount of weight in that little packraft is probably a bit excessive for going over waterfalls. As mentioned above, I’ve also had three people in a packraft designed for one person, but again, that was in a calm lake.
In practice, the safety of a packraft’s load depends as much on the water conditions and load distribution as it depends on the gross weight. In general, for a given load,
- The calmer the water, the safer it will be
- The lower the load is placed in the packraft, the safer it will be
- The more evenly distributed the load is, the safer it will be
- The more secure the load is, the safer it will be
If you’re attaching your backpack over the bow tubes, make sure it’s secure, and if you expect the water to get rough, consider securing it on the floor at the front of the cockpit to lower the center of gravity.
If it’s not securely attached, a heavy backpack on the bow will swing from side to side and it could pull you off balance as you roll with the waves. The higher and heavier it is, the more you’ll feel this effect.
Given all of the above, I can’t really tell you what’s a safe number in pounds or kilograms for a given packraft, but what I will say is that if you want to maximize your weight carrying capacity, add an airtight zipper to your packraft and install attachment points inside the tubes so you can secure gear there (i.e. where the weight is low, secure, and evenly distributed).
If you have any other comments or advice, please post below.
-Matt
1 Comment
Alan Bol · February 1, 2021 at 2:29 pm
Matt, The “N” number for buoyancy is Newtons. I know it’s used for the buoyance of European PFDs. At sea Ievel, one kg exerts about 9.8N of force (call it 10). One kg is equivalent to the weight (or buoyancy) of 1 liter of water. So if you see 50N (like some PFDs) that PFD has (essentially) 5 liters of volume. I suspect that Anfibio just posted the buoyancy of their fully inflated boat (volume in liters times 10).
Bottom line: N might be a nice way to compare boats, but it’s not a useful number for the useful capacity.