Speed versus Length

In my endless searching for the next 'perfect' boat I've been reading everything I can about intermediate surfskis.

One of the things I keep seeing mentioned is that unless you are paddling consistently well over 10km/hr on flat water a longer boat is actually slower because of the increased surface area.

Open ocean, light wind I seem to average about 9k over a 2 hour paddle. More touring than racing! When I'm down-winding in 15 to 20 knots I often see sub 6 minute kilometres but I've never consistently done under 5 minute k's. So in other words pretty slow, but I've only been paddling for about 6 months and I'm on a Club spec V10S. Self coaching, all via youtube etc.

Does that then imply a shorter boat would not only be faster, probably more fun in down wind and a better craft to hone technique. The logical ones - V8, Think Eze come to mind. Was thinking of a Vault as I mentioned in another post.

Here is a little video from a month ago - feel free to rip into the technique!



Sorry just realised I posted in the wrong section

Hey Jonathon, took a look at the video, a bit hard to be critical because of the rear angle and what you were doing on the video, really need to see what your technique is like when consistently paddling on flat water and see what your legs are doing as well as how much rotation you are getting.

The Eze and V8 will be slower than the V10 sport.

I wouldn't change ski's for speed at this stage of your paddling, I would invest in more technique training.

I was self taught for the first couple of years also and managed to climb from 9km/hr to around 10.5 in 12 months on the same ski you have now.

Paddling with others helps, as they see things you cant see from the side or front.

I then went and got a V10L, got my speed up to around 10.9 after 9 months in it but didn't really like the feel of the ski downwind.

Here I am about 4 years after getting my V10 sport and I could race over 12km/hr after 12 months in a Vault.

I am about to jump into an elite ski (yet to choose one) after 16 months in the Vault, it does take time and practise, enjoy the journey.

Hi Jonathan,
I was about where you are a year or so back and also paddle a V10S.
As you can see from AR's post it takes alot of time on the water to improve technique and speed. I paddle on average 4 times a week and have been getting coaching as well which has been hugely helpful. Although it is good to be always challenging yourself I wouldn't rush into getting a so called faster boat too soon. You will just end up going slower until you get the balance issues sorted. there is plenty of potential left in the boat you have. I recently averaged 11.5 km/hr over a 19 km flat water harbour race on the V10S. Next year I want to hit 12 then I'll upgrade.
I also have a multisport kayak which is theoretically faster but over this sort of distance I am always quicker in the V10S as I can always get 100% effort into it.
Looks like you are comfortable in the v10 so I wouldn't go back to a V8.

Thanks guys, I realise there aren't any shortcuts. More concerned that I'd actually gone for something more difficult than I should have with the V10S, but I am quite comfortable in it so will get some coaching for sure.

The question about length versus speed was partially academic. I've found the same situation with Stand Up Paddle boards. Often a 14 foot board will be faster than a 17. Just like surfski's so much more involved than just longer is faster, rocker, width, stability etc etc. FreGquently smaller paddles will be faster on shorter boards. Seems like there is a certain amount of brute strength required to tip the weight/length/speed balance.

I'll give you my 4 cents worth. (inflation

You are getting sweet FA out of the forward part of your stroke.
You then severely drop your top hand to keep the blade in the water to try and get some propulsion out of the rear part.

The only rotation is back twist and only then because your stroke starts so late that you have to reach backwards to complete it.

Reach forward to start your catch, move your top hand closer to your paddle side to get your paddle more vertical and allow a closer catch and more verticality throughout the whole stroke.

Keep your top hand high in front of your face and don't let it drop until it reaches forward to start you next stroke.
This will start your stroke earlier and just as importantly end your stroke earlier.
Once you get that starting to happen you will feel a natural body rotating happening and you just need to work on optimising that leg drive more.

If you were paddling with me I'd constantly keep telling you to reach for the catch and keep the top hand high until you got sick of me talking.

You look like you have come from a wide kayak background, get out of the forced bad habits, you are in a missile now, make use of the bloody thing.

The boat you have will take you far and you look like dropping to a more stable one would be a waste of time and money.

Thanks RAB, appreciate the time and effort. I'll keep playing a little mind recording of you yelling at me tomorrow when I'm out there!
Never paddled anything like a kayak before getting the ski, lots of stand-up but nothing with a blade on each end of the shaft. So it's all new.
I know about reaching for the catch in stand up paddling but somehow had this whacko idea that on the ski the rotation was just about all after the catch, which probably explains why I'm doing the back rotation. Gotta get some coaching......

9kph, sounds familiar, be good to see some video taken from the front, shows much more detail. I've been trying to improve my technique for the past year, little to show for it in the ski although I can go pretty quick in my kayak. I would go better in a slower/more stable ski, its just finding one thats not too much of a step back and one thats available.

I think your point about the shorter boat being "faster" got lost in the replies so far... Even at the speeds you are paddling, I think you are over the threshold where wave making resistance is more important than drag from friction. So, for 2 hours or so paddles, I'd say you are in the right boat or you may even go faster on the average in a longer boat, as long as you do not have stability issues.

If you go for a 17 footer, then what you are likely to find is that you will be a bit slower when trying to paddle all out on shorter distances. But if you paddle slowly (under and about 5mph) or over very long distances (again at 5mph or less) you will be less tired in the 17 footer than in a 21 footer.

I'm guestimating here based on some charts I've seen that measure the resistance from friction and dynamic/wave against speed for a number of different boats. And also recalling that an 18 foot boat has usually about 10-20% more wet area than a 16 footer (sorry, don't have numbers comparing a 21 footer to a 17 footer)...

I have the following personal experience: paddling a very efficient 13 footer kayak (narrow, good swede form hull) was really very easy at speeds under 4.5 mph. Paddling a longer kayak 19-20' takes more effort at these same speeds. However, I pretty much maxed out at about 5 mph over 5 miles in the 13 footer where I could do about 5.5 mph in the 18 footer and about 6mph in the 20 footer for a similar effort over 5 miles.

So, simple: if you are paddling really slow, at under 5mph, you will be better off with a shorter boat. Otherwise, if you paddle faster, a longer one will be easier to paddle at higher speeds.

Of course there's a lot more to paddling fast than just the length of a surfski, but a UK university student just completed an 8 month long project looking at this precise question. Here's the article:
What's the optimum length of a surfski

Great work Ralph!

Recognition of the theory that Hein and the guys at Carbonology put into thier designs.

Hopefully Ralphs body of work will inspire other marine researcher/designers to continue this area of study.

RAB will be going off like a frog in a sock sifting through this research

Rab wishes he had access to software so he could do the job properly.
The only free stuff I can find won't work with our boats.

In my opinion he screwed up big time by keeping a fixed beam of 45cm.
None of our boats have a waterline that wide.
A waterline that wide will greatly influence a shorter boat more than a longer one.(and I think this shows in his results and outcomes but is simply missed.)

One of the biggest influences in boat speed with paddler weight will be the thinner waterline of a smaller paddler.

The table of results presented for the mid sized paddler just doesn't make sense compared to the other two and doesn't fit with the conclusions arrived at.
There must be a big mistake in the data of the first 10 lines or so of that table.

And I really think that what we all want to know is,'how will the shorter hull interact with the swell length/height/wavespeed etc' that we typically see.

It also ignores that a shorter hull has the potential to be lighter, have less windage and possibly require less rocker than a longer hull and will probably be a bit narrower as in an Stellar SES.

The speed chosen doesn't take into account the higher speeds seen on a wave or the aerated nature of the water under a hull that is planning.

I hope that somebody like Epic have far more sophisticated tools for modeling than this one.

I might have missed the 45cm constant width assumption with varying length burried somewhere in the 90 pages, which IF TRUE of course makes the whole thing not that useful.

However, I thought I saw a very different assumption: constant displacement. To achieve a constant displacement with varying lengths, one has to vary the width, right? Perhaps the max width stays the same in his model, but the underwater volume has to stay the same in order to have a constant displacement, so width has to vary somewhere...

Displacement is shown as a variable in the modeling of the ski with the three different weights. It's simply done by taking the weight of the boat and paddler into account.

Very easy to be critical of a project like that, I think it was excellent for what it was and is full of statements pointing out the shortcomings. The single width for all boats is a real issue in making the research meaningful, but just take it for what it is - a contribution to the science, with limitations. It was never going to tell you which boat to buy.

I'd be very surprised if Epic or any of the other companies had super sophisticated software to work out optimal hull length and width. My guess is that the industry just couldn't support the costs of developing software that could take all the variable you want RAB. I'm sure software is used to smooth things out and give feedback on volumes etc but as far as a computer program coming up with a better boat from scratch, fantasy IMO. I work with a high end 3d cad solid modelling program on a day to day basis, nothing to do with naval architecture, but I do have some clues about the limitations of software in terms of optimizing complex designs.

First off, it`s awesome that Ralph has taken the time to do this for everyone`s benefit, and to have the balls to publish his findings in front of such a hard-arse jury (let`s not mention any names!). Good effort Ralph.
That said though, I`ve got to agree with RAB on one point - the top half of results for the medium weight paddler do seem to be out of whack with the rest - some kind of typo perhaps?
As for the other criticisms of the work, I think Ralph comprehensively outlined the shortcomings that could be levelled at the research and properly stated that to fix this would be way outside the scope of this study, so to knock him for it is not really good form.
What I take from it though I find very stimulating. Things such as the optimum length for a big guy - think Oscar on one of his faster boats - the guy`s a genius, influencing most of the world`s best paddlers to ride skis that were too long for them for years!
What I think this really shows is that manufacturers, if they truly were to cover all bases, would have to not just build three boats, one for each category of beginner, intermediate and advanced; but should produce each length of boat in skinny, medium and wide, depending on the weight and ability of the paddler. In other words, beginner boats are nice and wide, but why not lengthen them to 6.5M if the paddler is a big bloke - it will help him/her to ride higher in the water and make paddling easier.
At the other end of the spectrum, we`ve seen some of the top elite women using shorter narrow boats. What I believe would be interesting would be if we could get someone like Dawid, who doesn`t have quite the bulk of the Big O, to paddle a 42 or 43cm skinny version of the shorter Swordfish (or a boat that comes close to those specs) and to compare his results with his normal, longer skis. Could make for another great article?

Hey RAB, getting back to my technique shortcomings, did about 35k or more of 20 to 30 knot down winding over the weekend. You were right there telling me to reach and keep my top hand higher. Thanks mate, made a huge difference, just wish you'd been telling me not to fall in so many times!

JonathanC wrote: Displacement is shown as a variable in the modeling of the ski with the three different weights. It's simply done by taking the weight of the boat and paddler into account.

Very easy to be critical of a project like that...

I'm pretty sure I know what a displacement is and that's why I mentioned it - because if the displacement is constant and the length varies, guess what - to maintain the same volume of water displaced for the same weight paddler (!) you got to vary the width. Very simple...

As for being critical, hopefully you are not addressing my post as it was not critical, on the contrary - pointed out that constant displacement is a good assumption. And understand that "constant" applies for the same paddler/ski weight combo but across different lengths. If you change the paddler weight you no longer have a constant displacement, obviously...

Hey Kocho,

Wasn't trying to tell you what displacement was. Meant exactly what I said "displacement is shown as a variable in the modeling of the ski", why don't you read the document, there really isn't much point discussing it unless you take the time to read it. The displacement for the three different weights is cleary discussed as is the width of the ski being constant at 45cm.

Was referring to the other comments about being critical, thought it was pretty obvious who came out with guns blazing! Sorry to mix that in with a response to your question.

Cheers, Jonathan

To be more specific about the fixed beam, what he did was fix the boat beam but to maintain displacement the waterline width must vary.
What happened is that the fixed beam decided the parabolic shape of the hull so as to meet the fixed beam and the displacement requirements.

This leads to 3 different boats with 3 differently shaped 'in water' hulls, which puts differing constraints on the wetted area due and to the different geometry.

I would have much preffered to fix the parabolic, and allow the beam to vary.
That way the wetted area could be optimised for the differing length hulls and a true comparison made.

There is no mention of rocker and it's effect on in water hull shape and wetted area in the whole article.

From what I could see, the program he was using was nothing better than an excell spreadsheet.

After many years of loudspeaker design using pen and paper and filter theory, then a small programable calculator, then a old slow laptop with crude software at about the level it seems he used, and now comparing that to modern software where most variables can be altered in real time and even darwin type evolution used to find optimum alignments, I'm sure that any manufacturer worth their salt has better software than this.

The guys that were asked to study the effect of the maximum boat length rule for K1 boats had much better tools than this and their study is fairly old now.
They studied an almost endless amount of iterations, only constrained by the then rules of the day of minimum beam width and no concave surfaces.
Much more comprehensive than this.

All I can see here is a paper to demonstrate that he is capable of doing this type of work to meet his study requirements.

But overall, nothing revealing and pretty mundane and still leaves the question no closer to being solved or even pointed in the right direction for further study.



And JC, if you didn't fall in, you either know it all already, or you are not prepared to push the limits to find what these boats are truely capable of.
Push the limits, fall in and learn, works for me.

If we could assume that the manufacturers actually model the hulls of the different models, it would have been very interesting to see the speed/power graph for the different skis available. Such graphs could be a guide as to focus on technique, stability or to change ski.

Eg, if you are paddling an intermediate ski with an avg flatwater speed of 12 kph, is it then time to move on to a faster hull, or does the hull have more speed potential before the speed/power curve reach Olympic power levels

Such tables would also be interesting documentation for trading down to more stable surfskis to gain speed

Until prove otherwise , it seems to me that surfski design is more based on practical experience and “art”, rather than “science”.

Darn. my post just disappeared, so sorry if it double-posts. Here it is again:

The paper uses fixed beam, takes 3 displacement values, and varies the draft and length for each displacement (e.g., paddler + ski weight).

The paper calculates the "optimal" length for speed of 4.2 m/s, which is over 8 knots (over 15km/h, 9mph). This gives a speed to length ratio of about 1.8. For ski length of say 21 feet, this happens to be way over the "hull speed" speed to length ratio of about 1.33.

So, at speeds way over hull speeds, can one ignore "lift"? I don't know for sure but that's not the main issue I got with that paper.

All the author did, it seems, is to use an already existing software and optimize 1 variable (length) by fixing 2 variables (beam and displacement) and varying the 4th (draft).

---> This paper gives me no useful information whatsoever in practical terms: I still don't kinow which ski is best for my weight...

As I think Rightarmbad mentioned, the paper can very easily become much more useful with the same available input information and using the same software:

*** Find out the *optimum* hull specs for use at a realistic target cruising speed that a paddler of certain weight can achieve under his/her own power. Say for a 180lb person wishing to paddle at 7 mph how long, wide, and drafty craft would give the minimum total resistance?

To do that simply optimize the *combination* of all 4 variables: hull length, beam, displacement, and draft.

A few hours of scripting should be able to automate a bunch of runs on the Mitchlet software (or something similar) to get enough data so one can get an approximation of the best hull as I mention above... It will still not be the "real" answer, because the finer porints of hull shape will not have been accounted for (such as swede form, realistic cross section that varies across the hull, ect.), but will be more informative compared to what the paper is as published...