Paddler Weight & Ski Weight Ratio and Speed

The ratio between the body weight of a paddler and the combined ski weight and body weight of the paddler shows how much body has to drive how much ski.

For eg. an 80kg body on an 18kg ski has a ratio of 80 / (80+18) = 82%.

The ratios for different body and ski weights are shown below:

Ratio of Body / [Body + Ski]
Body kg 18kg Ski 10 Kg Ski
100 85% 91%
90 83% 90%
80 82% 89%
70 80% 88%

How significant are the differences? Should a 91% ratio deliver 14% better speed than an 80% ratio, all things being equal?(ie 91%/80% = 114%) OR is the difference in speed more or less significant than this suggests?

The difference in speed is less than 14% for between a 100 kg and 70 kg paddler. I'm not even sure the heavier guy would go faster for a specific hull, since the boat is riding lower in the water with more wetted surface. You need to account for what speed you're talking about, i.e. average crusing speed vs. top speed? Because form drag and friction drag will have more or less effect depending on how fast you're going.

As a rough rule, a 10kg reduction in weight (boat or body) should increase speed by 2-4%.

This is based on an assumption that there is no change to power output and paddling in flat conditions.

So, in theory 1-2 mins could be shaved off a 1 hr time trial if 10kg can be dropped.

Thanks for the thoughts guys. I'm thinking more about average speed, than maximum.

Patrick's point about the wetted area must count as well. That set me thinking that a better Muscle to Load Factor may also increase the ease of getting the ski to a more efficient travelling speed (planing?).

Sounds like the equation for the impact on speed may work along these lines:

A. A higher Muscle to Load factor (ie body weight / [body + ski weight])should improve speed proportionately
B. Higher total weight will increase the wetted surface area which will subtract from speed
C. A higher muscle / load factor may increase the ease of getting the ski to plane reducing the wetted surface area and increasing speed.

For a person going to a lighter ski, each of A, B and C should create additions to speed.

However Thames' comment that in practice the benefit is only 2-4% on a 10kg drop in weight would suggest I'm missing something in the equation, as the numbers suggest the benefit could be above 10%.

On the "all other things equal" basis I'm excluding ability on the ski at higher and lower ski weights from my thoughts.

I recognise that rowers scale the boat size up and down, presumably to optimise this muscle/load factor.

I can also see that paddlers are rapidly aligning their ski choice to the brands and models that do the same, which again suggests that the benefits of optimising the muscle / load factor feel significant when paddlers try these different skis.

Just to add some more variables.
Paddling a Surfski has a different power to recovery ratio for the stroke.

I have bookmarked an investigation into rowing scull weights that suggests that for some configurations, as in how many paddlers versus boat weight, that the lightest boats available today may actually be too light.
This is because of the long recover time during a rowing stroke and the hull losing too much speed.
It is a lot different to Surfski though, because the movement of the rowers has an effect on momentum.

When I get to my computer I will try and dig it up, it's interesting reading.

The topic of weight and speed is covered briefly in this article:

www.surfski.info/images/stories/2009/06/...ing%2C%20Pt.%201.pdf

That's a valid point on recovery RAB. It suggests to me that the benefit of momentum is the missing factor in the efficiency equation.

A paddler uses energy to push the weight of the ski with each stroke, but the momentum of the moving ski also contributes some of the force to propel the ski on subsequent strokes.

Momentum is the benefit of weight x speed overcoming the resistance of wind and water. So as the total weight decreases, the momentum benefit diminishes, partly offsetting the efficiency benefit of the lower weight.

The layman's formula for assessing the speed benefit from a change in the weight of a ski would become:

A. Improvement in Body / Load
B. Less Momentum Benefit Sacrificed
C. Plus benefit from lower wetted surface

For an 80kg paddler moving from an 18kg ski to a 10kg ski, this would suggest:

A. Body on load ratio improves from 80/(80+18)= 82% to 80/(80+10) = 89% giving a benefit of 89/82 = 8.5% improvement in speed or efficiency

B. Momentum benefit lost, assuming momentum contributes 70% (guesstimate) of the force in each stroke, would be 70% of 8kg/98kg = 5.7% decrease in speed or efficiency.

The Net Improvement in speed or efficiency would be 8.5% benefit less 5.7% loss = 2.8% improvement in speed + any impact from a lower wetted surface area.

Those numbers then end up exactly in the 2%-4% range suggested by Thames.

Apologies to any genuine physicists for the homespun maths above.

Some other snippets of research I've come across, including a 2010 topic on this site are:
Erik Borgnes commenting that the speed improvement from weight is tangible but small;
Others suggesting it has most impact in the ability to accelerate (makes sense as the momentum factor is less important when accelerating);
And another noting that rowers get around a speed benefit of 1/6th of the weight change, compared to around 1/3rd as the calcs above suggest for a surf ski. (As RAB notes the momentum contribution in rowing may differ significantly from paddling.)

After much number crunching a couple of years ago, I come up with my rule of thumb.

The percentage loss of speed is one sixth the
percentage increase in weight.

(Assuming an unpaddleable, ideal fully semicircular hull which then has straight sides at the waterline.)


But the real question, is in our real world of accelerations to catch waves, how important is this really?

In addition to the paddler's intrinsic geek factor and possible paddling benefit of weight loss, what is the purpose of this discussion? Are you trying to determine an ideal boat size for a specific paddler weight? Or is there something else?

Semdoug, as I am planning on buying a new ski in January, I am definitely following this thread to find ideal ski for weight.

If someone could post a table showing all currently available skis and paddling power required to maintain say 15-17km/h with a body weight of 85kg that would be great! Thanks in advance!

Below is a list of all surf ski's that will allow this 85kg paddler to maintain 15km/h on flat calm water with no wind or current assist:











Easy wasn't it.......

Ahh, RAB, quick on the keyboard as always. But I was after paddling power required, must have been inadvertently left out of your post. I am holding my breath in anticipation!

What I find amazing is that ICF kayaks come in a range of sizes optimised to the paddler weight, yet with surf ski, we are lucky to get two.

I'm only guessing, but I think surf ski sales are currently running at more than K1 style boats.

So where are the weight optimised surf skis?

Realistically, in training over a distance like 10km on flat water, in an out and back situation where tidal currents etc tend to cancel, I get about 11.9 to 12.5 kmh.
Mainly depending on how rested I am.

In my V10, I can do 15kmh over 500m, but that's flatout and I'm getting slower towards the end with maybe a peak of low 16's in there near the start.

Due to the nature of water and it's increase in drag as a cube, I'm guessing that 15kmh is simply out of the range of all paddlers to maintain in a surf ski in flat water.

In the ocean where average speeds kick up a bit, the hull is never really fully in the water anyways, so flat water resistance is probably a poor predictor of performance anyways.
It is common for 3 feet either end to be out of the water at any given time.

I think that really, there is no better test than to paddle with a buddy who is close to you in speed and swap boats regularly.

It's the only way to get a real world take on the speed of the thing.

And I'm betting it would be more accurate than any laboratory based test.

So a little bit of reasoning.

Currently in my aging lazy state of fitness, I get about 330watts at 168 beats on the bike at my anaerobic threshold, or critical speed for those a little younger than me.

Using that as a benchmark and knowing my paddling threshold is currently 162, then a little bit of math will spit out a figure close to 320 watts for a V10 at around 12kmh for a 85kg paddler in flat water.

There is your first data point.

So after some more thinking and examination of my results examining paddle size and length, I've come to the conclusion that having the right paddle is more important than the difference in speed between the current crop of skinny hulls.

Who would have thunk it, it all comes down to the engine after all.....

So RAB's number one tips to get faster without being stronger/fitter/training more than you have time for.

1 Have the right paddle
2 loose the blubber belly
3 Don't fall off/be comfy in the ski you are in
4 Know how to pace yourself


Now somewhere down the list will come:


Have the latest greatest lightest ski around.



And don't forget, learn how to read the ocean like Oscar!

My motivation to think about this issue is that I've been paddling a fibreglass layup V10, and as I clock over into a new year I'm wondering if I should bite the bullet and upgrade the ski.

I'm at the lighter end of the spectrum and so the question of whether I would get a more energy efficient - and faster ski - out of a lighter model is a key question.

One choice is to go for a V10 or V10L in a light weight layup (and I've heard the decals work fabulously). But how light? There's an 8kg drop from the heaviest to the lightest layup, and shades between.

Or go for a ski like the Carbonology Vault or Stellar SES which sound like they are skis trimmed down to suit lighter weight paddlers.

A 2-3% difference in speed/efficiency from an 8kg change hasn't been enough to get me away from happily paddling the fibreglass layup to date.

But as I think about the way in which the weight change impacts speed, and as I look at speed charts across different conditions its giving me a few ideas.

The better power/load benefit of a lighter ski has got to be more efficient in flat water, even if there is a slight loss of momentum which partially offsets the full benefit.

In messy conditions I can see that average speed of a paddler drops because the speed becomes inconsistent, moving rapidly from highs to lows due to wind and wave, meaning the paddler needs to keep accelerating the ski to get the speed back up. Whenever acceleration is needed a lighter ski is again more efficient.

These thoughts are telling me it probably is worth pushing a fair way down into a lighter layup, but probably not the lightest.

Whether to go for the skis that are slimmed down in size as well like the SES or Vault over V10? We all know that's a well documented 50 page discussion in its own right, with no firm conclusions either way apart from the practical suggestion - test them out.

I always get higher average speeds in the ocean.

A lighter boat always accelerates better, I'll see if I can post up some graphs of a 15kg versus 9kg boat over a series of 500m flat water sprints.

It is quite easy to see the faster initial acceleration, but at the end of 500m there is only a couple of seconds in it, which is well within the noise figure for the setting that the tests were performed in.

You can though, quite easily see a steeper curve in the graph at take off and a higher top speed in the first 30 seconds.

I believe there would be a much more pronounced difference in the ocean, as getting all the small holes that pop up (down?) will give you a boost if you can get them.

If you are light, I suggest that then V10l will suit you much better and I would definitely have a go at the SES.

Having both a Performance V10l 15kg and a Stella SES 11kg and no data to prove it, it feels to me that the SES is a lot easier to paddle upwind, quite noticeable for me, downwind I think the Epic is a bit better catching waves, I put this down to the extra rocker the Epic has and a slightly flatter broader hull. I haven't paddled the Epic since Feb, but I seem to loose ground on the downwind legs in the Stella, where as I seem to hold my own in upwind against the same paddlers. If I was to paddle the Epic again it would give me a fresher impression but I'm planning on selling the Epic cause the height of the leg hump just does not work for me.

I appreciate the comments guys. Your thoughts are aligning closely with mine.

My hesitancy in changing ski has been:

A. Because I've thought it better to improve the paddler before I get too finicky about the ski.

B. Also because I've been extremely happy with the V10, which although its a big ski has been exceptionally comfortable, hasn't had a fault in many years of use, and has been like a good friend in carrying me some way up the paddling curve, and

C. I keep expecting Epic to launch the 6m, 42cm, 8kg V10LC, being the supercharged, superlight, super stability compact model for 70kg paddlers

I'm going to head out and do the practical testing, so will see what comes out of the actual stats, and feel certain that once I've bitten the bullet and shelled-out for a new ski from the current range of models and manufacturers, it should only be another month or two before Epic surprise us all with the release of the V10LC.