I bought my son a Prodigy XS last year which has a 150 pound max weight capacity. He had a growth spurt and now weighs about 135 pounds. I am hoping to get one more season out of this kayak and was wondering if adding stern float bags, which I intended to do anyways, would increase the weight limit at all?

physics

Says no.

Float bag

A float bag will not make it sit any higher in the water. It will keep it from sinking.

How about helium in float bag?

interesting

I’d like to see the math.

No one likes to pay attention to physics

and physics says it all

http://www.snorkelingonline.com/how-water-affects-your-buoyancy/

He

If float bags are filled with Helium (He) instead of air, there will be an additional buoyant force on the kayak.

Numerically, the increase will be equal to the difference between the weight of the air in the bags and the weight of the He replacing it. Using approximate values, the specific weight of air is 0.075 lbf/ft3, for He its 0.011 lbf/ft3. The difference is 0.064 lbf/ft3 (lbf/ft3 is a pound-force per cubic foot).

The kayak in question has a stated maximum capacity of 150 lbf. This is equal to the weight of the water it displaces (Archimedes Principle). At 62.4 lbf/ft3 for water, that's a total displacement of 2.4 ft3 below the waterline.

Let's make a wild assumption that the full volume of the kayak is ten times the displaced volume. If filled completely with He instead of air, that would give an additional buoyant force of 1.5 lbf. I don't think the volume of the boat is quite that high, and float bags don't come close to filling a boat, so the actual increase in buoyancy would likely be measured in fractions of a pound.

Weight limits are somewhat subjective

The nature of how a boat floats means that the weight limit isn’t absolute. There will be a range weights that are ideal, a range of greater weights that are less than ideal, and range of weights that are too heavy but manageable. Further, some boats are rated more conservatively and others more liberally.

I know a guy who’s body weight so overloads his favorite kayak that it looks a lot like the Monitor (a Union warship from the Civil War. Look for photos of it online if you don’t already know what that ship looked like). He has fun anyway.

Of greater importance than the actual weight limit of your son’s boat will be how well the boat performs for the conditions in which he paddles. In general, it’s a good bet that the rougher the water conditions, the more strict you would want to be about staying under the weight limit. In calm water, especially for near-shore paddling, all that matters is that he still enjoys the boat.

kyak capacity

Adding floatation inside the boat will add nothing to the kayak’s buoyancy. It will help with flotation only if the boat fills with water.

Let the kid’s experience tell you

If he’s comfortable in it at 135 lbs and continues to be, just add the bow and stern bags because you should already have done that anyway. Has nothing to do with increasing the boat’s payload; they just reduce the amount of water taken on if capsized.

As guideboatguy said, some boats are rated more conservatively than others, and a lot depends on the paddler’s sense of balance even without the rating variations.

Before I bought a WW kayak, I rented the only two kid’s versions that seemed likely to fit me decently (I’m a small adult). They were rated similarly for weight, but their shapes were noticeably different, and this made one feel much more tender than the other. Because I was near the top of both kayaks’ weight ranges, and because I only tested them in flat water, I went with the more forgiving design. It worked out well when I did take it in WW and I have no regrets. Your son’s butt reaction can tell you a lot, so don’t get hung up on numbers alone. If he starts feeling “tippy” AFTER being used to the boat, it’s time to get a bigger one.

The way I look at it is

that the weight of the bags is part of the payload so you are just adding weight to the boat. Not enough to worry about - but it does add weight. I doubt it will be a problem.

As they say…

adding float bags to a compartment already filled with air only adds the weight of the float bag to the total displacement. Float bags will only aid floatation if the hull is compromised and keep compartments from filling with water or if the bags are attached externally to the hull (which I doubt is practicable). In truth, I doubt one could make any reasonable bouyancy modification to the hull and produce a positive result.

Still, 135 lbs. in a boat rated for 150 may well be optimal, as long as one does not add a great deal of gear to the mix. Not that I’d recommend it, but one could possibly get by with being slightly overweight (a few pounds at most) with this hull as long as one realized that a hull rated for 150 lbs. will likely be more performance compromised by a few extra pounds than a hull rated for 200.

Note that with most engineering specs, there is a margin of safety at the edges. This applies to kayaks as well, but the margins in this boat are probably quite low.

Rick

my Dirago 120

has aweight load of 300-350#.

I weigh in at 170# so 300/350-170 = 130/180 pounds of gear.

Now that 130 to 180 pounds INCLUDES my PFD, clothing, granola bars in my pocket, paddle, water, tent, sleeping bag, stove, etc.

Figure water @ 8 1/2#/gallon per day (I carry a filter but expect it to fail to take at least a gallon at all times. my tent is 5#, you see where this is going.

PLUS I am a gear-junkie and keep REI in business.

I’ve done a lot of weekend trips but my first multi-day trip down the Colorado I hauled 172# of gear + my 170 = 350# total… pushing the limit here.

My deck was STILL 6" above the water and the boat handled! Sluggishly, but I was able to keep up with the group.

So that 300-350# is IDEAL! If you remain below that 350#, your boat will ride hige and paddle safe. If you exceed that limit, the boat rides low and is sluggish to turn and a pain to paddle.

I think it is fear of lawsuit as the makers know that people like me exist who will exceed the recommended limits, the nsue them when the boat sinks because we pout 500# in a 300#-rated baot, so they under-list the load limits.

So you CAN exceed that limit, it may even be safe… it just won’t be as much fun!