Influence of paddler weight on ski characteristics/stability

3 months 2 weeks ago #30534 by AGA
Apologies for the size of post.

Does paddler weight affect the characteristics of a surf ski, such as stability? With paddler weight varying dramatically, and the potential impact large, this must surely be a highly important issue for surf ski paddlers, yet one which seems to be frequently overlooked, despite being fundamental to ski performance.

We can put the issue of speed to one side. The impact of weight on speed is well documented. Added weight increases the volume of the hull in contact with the water, and the drag reduces speed at around 1/6th of the percentage increase in weight.

But what happens to stability? The generally accepted concept is that a larger paddler weight increases stability. But is this really correct?
There are several factors that would appear to decrease stability as paddler weight increases:
• Added paddler weight may raise the centre of gravity
• Added paddler weight could increase the momentum accompanying ski rotation
• Paddler weight also changes the position of the waterline relative to the hull. For heavier paddlers, the waterline may sit where the sides of the ski are more vertical – which would decrease stability.
• For lighter paddlers the waterline would sit where the hull has greater curvature which would increase stability. (The greater hull curvature results in more water displacement and resistance on the side of the ski dipping into the water, than on the side rising out of the water).

With these questions in mind, I thought it may be worth doing some practical testing, and I’ve described the results below. The tests I conducted are highly subjective, brief, limited in scope, largely unscientific and most definitely prone to error. Nevertheless the tests did start to give me some practical insights.

The most obvious limitations were that I was adding dead weight, not live weight, and could not replicate the way increases in body weight would be distributed over a whole body. I’ve tried to take this into account when interpreting the results. I was also limited to testing this in a deep water location while carrying a fair amount of extra weight. If I had access to a shallower location I would have liked to run the tests more aggressively.

For safety I ran preliminary-tests in a pool. This gave me a rough idea of what was going to happen and enabled me to confirm that remounts were feasible. I made sure that any weights attached to my body were dive-style weights which could be quickly removed if needed, and I was wearing a life jacket sufficient to provide floatation to the total weight. There is a fine line between being buoyant and sinking. When close to neutral buoyancy, the addition or subtraction of a single dive weight was enough to swing the pendulum from buoyant to sinking or vice versa, and I wanted to be on the right side of that equation.

The next step was to run some tests out on the water proper. The bare paddler weight was 68kg, ski was a Stellar SES Excel 11kg, conditions were 5-7knots, small wind chop and mild boat wakes. Apart from an initial, longer run, the tests involved paddling the ski (with differing weight configurations) for around 5 minutes each over a short circuit involving paddling into wind, wind behind, wind across, a sharp turn and some manoeuvring into a landing zone. As noted, in a shallower environment it would have been good to extend this.

Test 1: Base Case No Weight Added
Took the ski for a 45 minute paddle to get a good feel for the normal set-up. Wind was stronger than the later tests (around 12 knots) and a greater number of boat wakes. Boat performed well in all directions, riding satisfactorily over 2 foot wakes taken broadside, some bounciness could be felt broadside to wind-chop.

Test 2: Added 10kg to Paddler Torso (dive weights added to life jacket - approximately waist level)
Very large increase in sensation of instability. Needed to carefully control body movements and paddling stroke to avoid taking a dive. I believe the dominant effect here was that the added weight raised the centre of gravity and that this overwhelmed any other benefit that may have accrued from the added weight. This was surprising as the added weight at waist level was only fractionally higher than hip level where I believe the centre of gravity for a paddler normally sits.

Test 3: Added 10kg near Paddler Thighs (weights added directly under lower thighs)
Moderate increase in the resistance of the ski to outside factors. The sensation of the ski bouncing over a broadside wind-chop disappeared. There were no problems in paddling aggressively. Felt a little bottom heavy requiring the paddler to manhandle the ski. I would interpret these outcomes as (a) the weight slightly lowering the centre of gravity and (b) the added weight increasing the inertia of the ski relative to outside factors such as the force of a wave.

Test 4: Added 5kg to Paddler Thighs and 5kg to Paddler Torso (waist height)
Moderate increase in the resistance of the ski to outside factors. The sensation of the ski bouncing over a broadside wind-chop disappeared. There were no problems in paddling aggressively. Felt less bottom heavy than Test 3. I suspect this test did not change the centre of gravity from the base case (with weight added both higher and lower) and accordingly this may most closely represent a heavier paddler of similar weight distribution to a lighter paddler. As the results were similar to Test 3, it suggests that the dominant factor in Test 3 may have been increasing the inertia of the ski, rather than lowering the centre of gravity.

Test 5: Added 10kg to Paddler Thighs and 10kg to Paddler Torso (waist height)
Moderate increase in the resistance of the ski to outside factors, but NOT noticeably more than Test 3 or 4. The added weight at waist height increased the sensation of instability, despite this being balanced by added weight at the thighs. A doubling of the added weight should have improved the inertia benefit, however overall stability declined. One possible explanation is that the 20kg of extra weight was sufficient to sink the curved area of the hull below the waterline leaving the less stable vertical sides of the hull at water level. A second explanation is that the greater dead weight around my waist restricted my ability to manoeuvre my body. This test may represent the situation for a heavier paddler who struggles to manoeuvre their body effectively. While the added weight may improve inertia, it was more difficult to keep the weight balanced.

Test 6: No Weight Added
This was a re-check of the base case. The general stability was no different to test 3 and 4. The ski was easier to manoeuvre and accelerate. However the ski was definitely more bouncy across a broadside chop than in the tests carrying extra weight.

Concluding Thoughts
Ski stability appears to be a product of (a) hull shape near the waterline (b) centre of gravity and (c) the ability of the ski to resist external forces through (c.1) inertia (dead weight) and (c.2) effective body positioning.

Assuming that the ski is sufficiently sized relative to the paddler, the hull shape (a) should be a non-issue for paddlers of different weight, but would be an issue if the paddler was too heavy for the ski.

Centre of gravity (b) potentially has a large impact, and does vary for paddlers of differing builds. It is possible an individual with more relative weight in the legs may have greater inherent stability in a ski than an individual with more relative upper body weight. However an increase in paddler weight that is proportionate across the paddler’s body may not change the centre of gravity or stability.

The paddler’s ability to successfully manoeuvre their body weight to counteract the influence of the sea (c.2) is clearly a significant influence on stability. I include in this category the paddler’s ability to generate speed and acceleration. To the extent that this ability is attributable to experience, technique and flexibility, then paddler weight is irrelevant (unless it unduly affects technique or flexibility). Speed and acceleration may be influenced by paddler weight and strength, but this is a two way street. Increased weight slows the ski, but this may be offset by increased power.

The inertia or dead weight (c.1) of the ski and paddler clearly has some impact on stability, with the influence relating to how fast, how slow, and how much the ski will be pushed around by the sea. The ability to resist the sea conditions will depend on the weight of paddler and ski relative to the surface area of the ski impacted by the force of the sea (ie not all of a ski’s surface area is affected by a wave). For example, the force of an approaching wave may be spread over only a few centimetres of exposed hull above the waterline, but for some or all of the length of the ski. This force will be resisted by the weight of the paddler and ski on the other side of that hull.

While this theory makes sense it is worth considering that in the tests this inertia factor was only reflected in the presence or lack of bounciness, it did not tip me out, and it was much less of a factor than body position and centre of gravity. So while the inertia (bounciness) factor has some impact, it is difficult to say how material that influence is. At the least it is probably fair to assume that in significantly heavier conditions or at differing levels of skill, the inertia (bounciness) factor is one contributor to staying upright.

Assuming the inertia factor has some influence on stability, it follows that – for a fixed ski size – a heavier paddler would provide greater resistance per metre of ski surface area affected by a wave than a lighter paddler. Or flipping this around for ski designers – to have the same resistance/inertia characteristics – ski size (predominantly length) would need to be proportioned to the combined weight of paddler and ski. IF the relationship was straight line ( I’m not sure that is the case), this would suggest that if 6.4m is an ideal ski length for a 90kg paddler and an 11kg ski, you could achieve the same stability in a proportioned ski at around 5.7m for an 80 kg paddler, 5.1m for 70kg, and 4.5m for 60 kg.

However this would also need to be considered against any impact that changed length may have on speed. I note that Ralph Baker’s work on optimal length for speed shows 6.0m +/- 0.5m as optimal for a 90kg paddler and 5.3m +/- 0.5m as optimal at 70kg (assuming a ski weight similar to my example), with a greater penalty for being too short, and only a small penalty for being too long.

I also note Ralph Baker’s conclusions that optimal performance from a ski would be obtained by tailoring ski size to the individual athlete’s physical attributes – particularly weight.

The current suite of elite skis on the market vary in size from 6.1m to 6.4m, which appropriately appears to err on the side of greater length, but which is clearly a far more limited range than Ralph Baker’s work on speed and my very rough testing on paddler weight suggests. The recent introduction of the Epic V11 at 5.8m opens this range a little wider.

It is/will be great to see designers with the requisite facilities researching and testing the benefits and disadvantages of ski sizes. All paddlers are different sizes and weights and I’d guess that pretty much everyone is interested to know whether proportionately sized skis will make a material difference or not to overall performance.

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3 months 2 weeks ago - 3 months 2 weeks ago #30535 by LakeMan
I need a nap after reading that. You must be an engineer.

All I can say is that if I eat a donut I can't fit in my ski.

"Success is not final, failure is not fatal: it is the courage to continue that counts." - Winston Churchill

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3 months 2 weeks ago #30537 by AGA
I like the donut concept. The test would be more accurate and the weight spread better if I'd bulked up 10kg on donuts. Better fun factor too.

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3 months 2 weeks ago #30540 by mike k

Thanks for taking the time to post your experiment thoughts. I am interested in your opinion on the experiment with weight under your thighs and its effect on the ski. I am still on a learning curve with my new ski in moderate sea conditions and the most challenging being a side on wind and or waves. From your testing do you think there is enough difference in how the ski felt in those conditions with low down weight to the stability. I was wondering if I could do it adding say 5kg of removal weight under my thighs/calves on the floor of the ski to add a little more stability. Being fairly tall I seem to have enough room after the hump and before my heels to maybe add a moulded shape into the gap that can be removed easily and carried separately to make moderate sea conditions more pleasurable (other than carrying it to the launch site that is.)

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3 months 2 weeks ago #30545 by AGA
Hi Mike,

Adding the weight was good for the test, but definitely don't add weights for normal paddling.

It was very quickly clear that adding dead weights made it much harder to move my body and/or the ski. From a safety perspective this means it's harder to remount and there are more things that can hit you in the head. From a paddling perspective I believe the ability to move freely greatly outweighs the weight benefit.

The only way to get the same effect safely would be the "donut" suggestion above - bulk up - or wait for manufacturers to trial some proportionate skis.

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3 months 2 weeks ago #30546 by Wombat661
Here is a good article on kayak stability.

Looks like you may help or hurt stability depending on the exact shape of the hull. Will need computer analysis to figure that one out.

If you are going thru the trouble of adding weight, why not add flotation instead. Double side tape a strip of foam on each side of the boat like a pool noodle just above the waterline. Kind of like a training wheel for the boat. Just a crazy thought :)

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3 months 3 weeks ago #30547 by wesley
That is a lot of reading including Nick Schade article on stability which I admit-tingly skipped to the Interpretation and Conclusion sections. I get this question more these days. Chris my friend of is 6ft 3, 212 and I am 5ft 9 180 so we often wonder how much our weight influences the stability of the same boat. We assumed the heavier paddler in this case Chris would be more stable but Nicks article suggest this may not be the case.
I happen to see Nick this weekend since he lives in CT and often paddles where I paddle in Newport/Jamestown. Nick will be designing a surfski in the future as we talked on the water last week. He said the hardest part is designing the bucket for his customers that buy a kayak or surfski plan from him and build it themselves from plywood. So shaping to bucket is key. I told him to come on over and sit in the the skis I have currently. Nick is an expert paddler and designer. Below is the video he took this past weekend. I got in the video at 1:05 in my Nelo 550 just before taking it into some of the rough water.

Wesley Echols
Former Stellar Kayaks and Surfskis, Performance Director, USA ,, #1 in Surfski Reviews.
The following user(s) said Thank You: Watto

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3 months 2 weeks ago #30548 by Wombat661
That is some incredibly beautiful wooden boats.

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3 months 2 weeks ago #30550 by mike k
Hi Aga,

Thanks for your thoughts. What I was mulling over and I will try is I have a 5 litre water bag that I can strap down on the ski floor using the unused leash anchor point. This would give me potentially 5kg of very low down weight to maybe make the ski a little more friendly in side waves by lowering my overall centre of gravity. It did seem strange to add up to 5kg to a 14kg ski but when considered with body weight as well its only about a 5 percent increase in total weight but concentrating the extra as low as possible. If people are noticing a difference in stability from lengthening foot pedal length and lowering their legs a little and therefore centre of gravity in my mind its worth trying without much effort. I may well be wrong but think its worth a try and don't believe that set up would compromise safety. If the water bag is filled to the brim there should not be any slop and movement of the water to upset the balance.

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3 months 2 weeks ago #30552 by AGA
With a few unresolved questions in my mind, I thought I should head back out on the water to re check some items.

Firstly I checked how the extra weight impacted on the waterline relative to the hull curve. A difference of 20kg raised the waterline by less than 1.5 cm - not much, although this was at a point where the shape of the curve was steepening. My conclusion on this remains: Probably not much impact on stability unless paddler weight moves to the limits of boat capacity.

I then retested the extra 10kg (5 under thighs and 5 at waist) against a base case of no extra weight but over a longer duration (20 minutes each) and in heavier conditions.

The trial with the added weight again produced the sensation of less bounciness, so that small waves and water movements would not be felt, but only in the lighter part of the test. However as the wind chop got larger the water movements were again noticeable. Conclusion: the added weight ratio provided some better resistance briefly but it didn't take a much stronger wave force to offset the 10kg.

Importantly in this trial I considered the lighter (not the heavier) configuration to be more stable in the choppier and slightly trickier parts of the trial, such as coming over the crest of boat wakes, taking larger wakes and chop broadside or turning across the face of wind and swell. My conclusion here is that once conditions outweigh the weight benefit then the ability to position and manoeuvre the body effectively is vastly more important for resisting the wave forces and maintaining stability. Carrying dead weight certainly detracted from this.

Nevertheless, I wouldn't rule out the possibility that greater live weight, which can be manoeuvred successfully, could conceivably be better at resisting stronger forces and tougher conditions.

And of course it took noticeably more effort paddling with the extra weight. My heart rate was elevated compared to the trial without weight.

So in response to the natural question of whether it may be worth paddling with extra weight, particularly if placed down low in the ski: I think the answer is no. You'll be punished by the extra effort and once conditions are stronger than the weight benefit I suspect you may feel less stable (think about the feeling of instability when the ski has 5 litres of water in the footwell). I reiterate that weight, water and skis are a dangerous mix for paddler and ski alike. I have a few new dings in my ski to prove the latter.

Adding weight in these tests is really a backwards way to test the effect of improving the ratio between weight and the surface area of the ski that is affected by a wave force. If there is a benefit of improving this ratio, then the way to harness that benefit is to shrink ski length to suit paddler weight, while retaining the inherent characteristics of the ski design, but probably only if this can be done with no penalty to speed.

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3 months 2 weeks ago #30563 by Ranga
Why O why do people persist on skis that are way to unstable for their ability. Clearly you are not having much fun and I don`t see the point.
Never heard of such a thing to add weight to a ski? The main reason skis get more expensive is to get them lighter so you can go faster and accelerate quicker?
Get a stable ski and have fun.

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3 months 2 weeks ago #30570 by AGA
As noted above, the objective is not to add weight to the ski. However playing with the weight is one of the few ways to test differences in the weight to ski length ratio.

I like the ski characteristics of a light-weight, narrow ski with a curved hull. I like the SES I currently paddle. Its a great ski. It is one of the few skis partially optimised for paddler weight. I certainly wouldn't swap that for a heavier, wider, more flat bottomed ski.

However I suspect that whatever a person's paddling level they won't be getting optimal speed or stability out of a V12, a Glide , a Max, a Swordfish or a V10S or any other ski unless there are different versions to suit paddler weights. A V12 is surely going to perform differently for a 70kg paddler, a 90kg paddler and a 110kg paddler.

An optimised V12 for a 70kg paddler might conceivably be shorter, possibly narrower, have the same hull curve and may be 10% or 20% (?) lighter than the same ski for a 110kg paddler.

Sounds attractive to me, if its right. But I'm just making an educated guess. I recognise that more rigorous testing could still prove otherwise.

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3 months 2 weeks ago - 3 months 2 weeks ago #30607 by nell
Nice article. Very well thought out. A couple of thoughts:

1. A bit off topic, but - my problem with data from Ralph Baker's article is that he computed "optimal" lengths for paddlers of varying weight at 4.2 m/sec - which is 9.4 mph. This is a far greater speed than what would be applicable to nearly all flat water paddlers and therefore his results would only apply to short course sprinters and world class downwind paddlers - though it might not even apply to the latter. The reason this is significant is that frictional and wavemaking resistance comprise a continually variable percentage of total resistance across a large swath of our usual paddling and racing speed range because one is fairly linear and the other is more or less logarithmic. I suspect the results (of optimal ski length) would be different if he plugged in 7.5 mph or 8.0 mph - though I could be wrong about all this as I haven't the software to run the numbers.

2. I suspect that center of gravity only affects balance/stability if it can be categorized as "dead weight" like you'd have with a barbell weight, fluid bladder, or even the "pot belly" of an person. Paddler seated height shouldn't be a factor, and might possibly be an advantage if taller, due to what you termed "effective body positioning" - which is a great term for this. Similarly, taller people don't have more balance difficulty with walking, standing, snow skiing than short people do. You can almost test this in the ski by leaning backwards onto the rear deck or well forwards. While these maneuvers will lower your center of gravity, they will worsen your ability to effectively position your body to keep upright, and your dynamic balance in the ski will be much worse.

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