Just out of curiousity, how heavy can a kayak be before it’s too heavy? What I mean is that is there an upper limit of weight beyond which the boat cannot be controlled on the water? Supposedly the equations show that a boat’s overall speed is not hugely affected by a change in weight, but what is the upper limit on water? On land it’s a different story, I know that more than 50-60lbs a single person cannot handle, and 85-95 for a lot of tandems is well, a lot to carry but that’s not what I am curious to know. Indeed the plastic tandems are just fine once you get them on the water, especially with another person paddling with you at that weight.
If I could build any paddle-craft I wanted from scratch, what would be the weight beyond which it would become very difficult if not impossible to control? From my own experience the nicest paddle-crafts I have ever handled are the lightest and stiffest so weight must count for at least something. Is weight that big a deal and where does it become one?
I’ve owned boats from 40# to 65# and frankly, the weight of the boat has very little to do with being able to control it. The design of the hull, the wind and the water conditions are the biggest issues. The rougher the water conditons, the less weight matters. A lighter boat will accelerate somewhat faster, but the difference is minor and a heavier boat actually maintains its speed better. Lighter boats are obviously easier to handle off the water, but that’s really the only place that weight matters significantly.
Given the choice between lighter and heavier versions of a boat I like, I would prefer the lighter boat, but I wouldn’t choose an inferior or less suitable boat simply because it was lighter.
Weight comes with inertia, which affects how easily you can change speed and direction. If you only ever paddle on flat water and don’t have to maneuver quickly, weight won’t be an issue. If you want to navigate tricky river features, you want the boat to be as light as it can be, so you can maneuver it quickly.
Got it. Subjectively of all the boats I have tried the lightest and stiffest ones seemed the nicest to paddle, but with stiffness (and less weight) comes fragility. Plastic is the least fun and most durable. I tried a high pressure inflatable that felt stiffer and was easier to paddle than plastic but not quite composite (Grabner), but it weighed less than similar boats made of even carbon fiber! Fiberglass felt nicer, and kevlar/fiberglass a little nicer still. They seem to get going more easily, require less effort to paddle and maneuver better.
Is 180lbs for a completely unloaded kayak (no person, no gear) too much?
While I realize this thread asks about weight in the context of kayaks, I believe the same principles apply as with open canoes. A lighter canoe of the same exact design will perform better than a heavier version. It will be more acceleratable, which is crucial in whitewater. The lighter swing weight of the ends allow it to be turned more snappily and to rise more buoyantly over waves. And, of course, the lighter the weight, the more easy on Sherpa portages, or even to carry from the Rolls Royce to the water.
I also have three kayaks, and prefer the lightest one (in fairness, partially for design reasons): my 36 lb. Surge, the first kayak to use infusion technology.
I know someone with a 180lb boat they built as a sort of hybrid boat/kayak and it doesn’t feel nice at all to paddle. His point is that weight doesn’t matter but over the years I’ve tried more than a few boats and while I am partial to plastic due to the rocky area where I live I know the other materials are a lot nicer. That said 180lbs feels like a beast, it’s very hard to control be it on or off the water in a way that is more or less logical with how boats in the 24 to 95lb weight range I’ve tried (less always seems to feel the best).
Total weight for a boat to handle OK on the water is a function of hull design. Going to 300 pounds would be too heavy for my Vela to handle right, assuming you could find a way to get that much into it. A similar weight is nearing upper limits but not out of bounds for the NDK Explorer.
Not quite sure why the boat itself matters extant from the design limits of the hull.
@CA139 said:
Got it. Subjectively of all the boats I have tried the lightest and stiffest ones seemed the nicest to paddle, but with stiffness (and less weight) comes fragility. Plastic is the least fun and most durable. I tried a high pressure inflatable that felt stiffer and was easier to paddle than plastic but not quite composite (Grabner), but it weighed less than similar boats made of even carbon fiber! Fiberglass felt nicer, and kevlar/fiberglass a little nicer still. They seem to get going more easily, require less effort to paddle and maneuver better.
You are bundling together stiffness and weight in your comparison. What if all the good properties came from the high stiffness of those boats, and not from the low weight?
I am not saying that it is so. I am just saying that you need to separate these two factors in your comparison.
I can add that in a sea kayak, which is often paddled empty ( 20-25 kg), and sometimes paddled loaded for camping (50-60 kg), weight doesn’t really matter for maneuverability in my experience. They respond more or less equally to edging, which I use a lot for direction control, and sometimes the loaded kayak even reacts better to this,
I will assume that the loaded sea kayak does turn slower, but it is not really something I think about, since the problem with a sea kayak usually isn’t the speed of the direction change, but rather that the sea kayak wants to go in another direction than the paddler does. So when I consider (lack of) maneuverability, I mostly think of the forces which are trying to prevent me from turning in the direction I want, rather than the inertia.
180 pounds unloaded?! Is that a typo? When I said that the weight of a boat doesn’t matter much, I was referring to the typical ~25# weight range for composite and plastic kayaks, not something that weighs 3 times as much. Of course that ridiculous amount of weight is going to make a difference in your ability to accelerate and turn. What kind of “kayak” weighs that much?
Two topics then:
First, am I bundling weight and stiffness together? Yes and no. I have found that lighter and stiffer boats tend to be nicer to paddle. I think they can be more or less bundled together because stiffer materials don’t have to be as thick, thus they don’t weight as much so it’s not a 1:1 or 100% correlation, but lighter tends to usually mean stiffer and vice versa. Of course, lighter and stiffer always means more expensive to buy, more fragile and needing more expensive repairs!
Second I am not talking about a loaded boat in terms of weight. I am talking unloaded or weight as built without paddler or gear.
@bnystrom said: 180 pounds unloaded?! Is that a typo? When I said that the weight of a boat doesn’t matter much, I was referring to the typical ~25# weight range for composite and plastic kayaks, not something that weighs 3 times as much. Of course that ridiculous amount of weight is going to make a difference in your ability to accelerate and turn. What kind of “kayak” weighs that much?
Someone I know who had some training in building larger boats ended up trying to design a kayak that blurred the lines.
It was explained and shown to me the equation where extra weight doesn’t affect speed much but nowhere did I see anything about moment of inertia. It’s almost as if his point of view was that it would be almost impossible for the boat to be too heavy to control but I felt otherwise.
A 180 pound kayak will be more sluggish to turn than an identical normal weight kayak because of the higher moment of inertia as you suspect. If you add about 3 inches of water to a canoe it handles more sluggishly.
@CA139 said:
Second I am not talking about a loaded boat in terms of weight. I am talking unloaded or weight as built without paddler or gear.
No, I am - to prove a point, which apparently was lost:
If the weight of unloaded vs. loaded doesn’t change the handling much, why should the weight of the boat itself.
So I am pretty sure that the difference is in the stiffness rather than in the weight.
It’s true that the difference in straight-line acceleration of a 40# vs 80# kayak is not going to be huge. Assuming a paddler weight of 180#, then the total weight goes from 220# to 260#, an increase of 18%. Meanwhile, the skin friction and wave-making resistance of the hull will not change much assuming a minimal increase in draft. So the overall increase in paddling effort to accelerate the hull might change by 10% or so, and the effort to paddle at constant speed will be the same. The increase in inertia, meanwhile, will give the hull increased glide so in some respects the hull may feel faster once it’s up to speed. Much of this is mentioned above.
EDITED TO REFLECT CORRECTIONS (in italics) MENTIONED BY ALLAN IN POST BELOW:
With respect to maneuverability, the rotational inertia of a long thin body is dominated by weight at the ends (see mass moment of inertia, proportional to mass times distance from center of rotation squared). The weight of the paddler contributes almost nothing since it is located near the center of rotation. Doubling the weight of the boat, then, effectively doubles the mass moment of inertia making the boat more resistant to turning. This is why heavy boats are more difficult to handle on land. In water, the effect is mitigated somewhat because the hydrodynamic resistance to turning (due to the shape of the hull and skin friction) remains the same. But I have to guess that the difference may be large, not sure by how much.
A 180# paddler in a (ridiculous) 180# kayak, would have 1.6 times the linear inertia and 4 times the rotational inertia vs a 45# kayak. Again, the aggregate increase in paddling effort would be reduced by the unchanging magnitude of fluid resistance in each case, but still…
None of this says anything about stiffness, which is undoubtedly a factor, but is not really part of the weight question that was asked.
@carldelo said:
Doubling the weight of the boat, then, effectively quadruples the mass moment of inertia
Mass moment of inertia has a linear relationship with mass, and a quadratic relationship with distance from C.O.G. But we aren’t changing any distances here.
If the mass distribution of the boat along the length of the boat is equal, and the boats are equally long, the boat with the double mass will have double the mass moment of inertia.
Anyway, I will still claim that when turning a kayak, the effort used to overcome inertia means next to nothing compared to the effort used to overcome opposing forces from wind and water, which try to either prevent you from turning or even in a lot of cases try to turn you in the opposite direction of what you want. If overcoming inertia was such a huge part of turning, then we would have to use a lot of effort to stop the turn when we have reached the direction we want.
I will throw another aspect in: super-light skin on frame boats like those that I usually paddle (both the “rigid” type and foldable craft) tend to be faster and more stable in rough confused waters than similar sized stiffer boats of any weight because they flex somewhat to absorb and ride over the force of waves rather than being buffeted and knocked around by them.
@carldelo said:
Doubling the weight of the boat, then, effectively quadruples the mass moment of inertia
Mass moment of inertia has a linear relationship with mass, and a quadratic relationship with distance from C.O.G. But we aren’t changing any distances here.
If the mass distribution of the boat along the length of the boat is equal, and the boats are equally long, the boat with the double mass will have double the mass moment of inertia.
Anyway, I will still claim that when turning a kayak, the effort used to overcome inertia means next to nothing compared to the effort used to overcome opposing forces from wind and water, which try to either prevent you from turning or even in a lot of cases try to turn you in the opposite direction of what you want. If overcoming inertia was such a huge part of turning, then we would have to use a lot of effort to stop the turn when we have reached the direction we want.
Allan - you’re right, you caught my careless mistake. My internet access has been down but I will edit the original post now if I can.
You make a good point about the relative magnitude of inertial vs fluid effects while turning. I don’t have a good estimate of that. The estimate of a 50/50 split between inertial vs fluid effects in straight line acceleration seems about right.
@CA139 said:
I have found that lighter and stiffer boats tend to be nicer to paddle. I think they can be more or less bundled together because stiffer materials don’t have to be as thick, thus they don’t weight as much so it’s not a 1:1 or 100% correlation, but lighter tends to usually mean stiffer and vice versa.
A little confused by what you are saying here.
If two Sairy Gamp’s are made of the same material – say S glass – and one is made of two layers while the second is made of three layers, the second will be both heavier and stiffer.
On the other hand, if you are talking about two Sairy Gamps being made of completely different materials – say one made of polyplastic while the second is a lamination of S glass and Kevlar – then the second could be both lighter and stiffer. Most sophisticated paddlers would probably prefer the on-water performance of the SK Sairy Gamp. People who are less concerned about subtle on-water performance differences, but are more concerned about durability for bashing on rocks and dragging along parking lots, may prefer the poly Sairy Gamp.
As to Willow’s claim that skin-on-frame boats are faster in rough waters, I doubt that even though I have never paddled a SOF boat. No racer in any paddling discipline I’m aware of – marathon canoe racing, whitewater slalom and downriver canoe and kayak racing, outrigger and surf ski ocean racing, or Olympic sprint racing – uses a SOF craft.