Have you skin in the game? Wave swimming in English Channel swims

There has been some discussion, questions and indeed argument arising from Trent Grimsey’s record-breaking English Channel swim, within the marathon swimming community. This centres around whether Trent swimming on or behind Gallivant’s bow wave makes it an assisted swim. Just in case you don’t know what I’m speaking about, riding a bow wave is where a swimmer of sufficient speed and ability can swim in the wave coming off the bow at the front of the boat to gain speed.

From that simple definition, it is of course an assisted swim. But is it really that simple? No. And I’m going to explain why.

Let me first say, because I was there, I almost feel like I’m cast as one of the chief defenders of bow-wave swimming, or whatever we’ll call it, though before Trent’s swim, I’d never heard nor been party to a discussion on the subject. In fact I’d never thought of nor heard the subject raised. So it’s worthwhile trying to look at this subject more deeply.


People should first consider that there’s not much point getting up out the pitchforks after the fact, or without an understanding of what happened. There’s a universal rule in law and sport: You operate within the law or rule in effect NOW. For example, if you cut down your neighbour’s overhanging tree ten years ago and there was no law against it, but then a new law was brought in five years ago, you can’t be retroactively punished. The same for any sports rule. Trent operated within Channel rules.

The Pilot

Trent’s record Channel swim was an extraordinary experience to be part of, and Owen and I were lucky to be there. And as I have said before, recognising that luck, I wanted to share it with the rest of the marathon swimming world, and just as importantly, for any of you who are simply interested. So much of our sport happens by necessity with only a few direct observers that most great swims pass instantly into mythology. We are the first generation of marathon swimmers, regardless of our own ages, who can use the internet to share information and connect with other, and make our stories public and our sport better understood.

When Owen and I sat in on Trent and team’s briefing with Mike Oram, Mike’s statement that riding the bow wave as essential to reaching the record, let alone breaking it, was new to me. I’d never considered it. I checked with Owen and he’d felt the same.

Let me clear up a serious misconception here. Some people seem to think Trent showed up at Dover, the FINA Number 1, professional swimmer, ready to bend all rules and slots to his desires and surf that bow-wave and that somehow things were different for him. But that’s just not true. Trent is a respectful, calm and pleasant young man without a huge ego. He booked his slot three years ago, like any regular Channel swimmer, and had, like many, dreamed of the Channel since he was young.

And he’d never heard of nor swum a bow-wave either.

His plan was to be three to five metres (his words, on video) off the bow. As about 12 people to whom I shared the briefing video would agree, the bow wave was explained to Trent, and us … by pilot Mike Oram.

Trent followed one of the cardinal rules of Channel swimming…he did what his pilot told him.

My first contention therefore, is that the concern is arising now, only because for the first time, many swimmers are now aware of this possibility, and that’s precisely because of my detailed posts on Trent. You’ve seen it with your own eyes from my videos and report, and it’s making you think. People need to think. And then think more about their initial reaction. Why?

Is this an assisted swim? I am not trying to split hairs, my record on this blog and elsewhere like the forum, if that I’ll happily support anything I understand. I have no problem with other assisted swims, my problem has been where rules are deliberately flouted or where people mislead others about their intentions or the nature of their swims, or mislead people into believing they are undertaking a marathon swim, but in practice, they aren’t.

No-one should compare Trent Grimsey to people who have misled others about their swims. No-one should compare his swim to being slip-streamed behind a boat, or equate it to wearing a wetsuit or being towed in a shark-cage.

The Community

But Donal, you say, you are obfuscating. There is the letter of the rule and the spirit of the rule.

I accept that. But I still say Trent’s swim is valid and legitimate and within the spirit. Before I go further with this, the question of legitimacy of his swim has only been raised by a few marathon swimmers. As they are entitled to so do. What is important is that the swim is recognised by the validating organisation. It has been. Trent Grimsey is the English Channel record holder. If you don’t accept this, you fail to accept basic precepts of our sport, those of observation and validation.

This year has seen some behind-the-scenes discussion of asking whether it is possible to reach a rapprochement of marathon swimming rules across the various worldwide associations. The discussions haven’t really led anywhere that I’m aware of, and I don’t for now expect that to change. But they were another part of the genesis of the marathonswimmers.org forum. This means that each association generally agrees that other associations have the right to their own rules, their own wrinkles of marathon swimming, and we’ve discussed those previously … NYCSwim’s MIMS in-water start and lightning evacuation clause and Cook Strait’s Swimming Association’s shark evacuation procedure as the two best known examples. The only real exception continues to be the CSA who refuse to recognise any CS&PF Channel swims. So like many CS&PF swimmers, I don’t take well to criticism from the CSA, though I have many friends who are CSA soloists. But the day after Trent’s swim a CSA swimmer in Dover described the CS&PF as the Dark Side, and not as a joke.

Each organisation determines its own rules (and only the CSA feels free to judge others). So as a proud (no-longer-paying) CS&PF swimmer, I don’t see how non-CS&PF or non-Channel swimmers can judge Trent. Lest this be seen as defensive, when I have commented on other swims or swimmers breaking rules, it’s where we were misled into believing which rules were being used.

Sciencey stuff

Let’s talk about waves themselves, because it feeds into answering that spirit of the rules question. This post took, as you can imagine, some time to write, as I had a day-long opportunity to write and this is the fourth revisions as I read more of the science, which was far from clear.

There are really two types of waves generated by a boat relevant to our discussion, which we’ll call bow waves and displacement waves.

A bow wave is generated by a boat and different aspects influence it:

  • Bow-shape
  • Boat speed
  • Hull/keel shape (depth and beam)
  • Weather and water conditions

A wide bow and hull will throw a bigger bow wave. A narrow hull or shallow keel throw a lesser wave. A boat throws less bow waves when going slower. A bigger bow wave will be generated in flatter water with less chop to impede it. (But choppy water can lead to random waves and spurts being thrown off the boat also, as anyone who spent time on small boats can tell you).

What do you think of a bow-wave as? Is it this doodle on the right?

That’s a reasonable assumption. And you imagine Trent body-surfing and swimming that wave. But it’s not correct.

A bow-wave from our point of view is actually more complex. On one hand there is a shock wave that disperses through the water from the forward, (bow), part of the water, dictated as I said above by speed, bow shape and width . Hulls are narrowed and elongated to reduce this shock wave. Ships don’t have square bows (except some very slow barges optimised for packing). Bow-waves slow boats down. You’ve all seen cargo ships with a torpedo-like bulbous protrusion at the front, which are actually designed to break the water in front of the bow, reduce the bow wave, and therefore increase speed and efficiency. On the other hand the entire boat displaces a volume of water behind the bow wave, which is pulled forward, the displacement wave.

Many Channel swimmers have experienced the bow wave off one of the big cargo ships. (I’ve never heard anyone complain about the effects those, except obviously the sudden swamping and disruption. Does anyone want to ask if big ships give a speed-boost to a swimmer, in light of this discussion?)

We don’t swim in the slipstream behind any boat in Channel swimming, because we can quantify it as an artificial help, regardless of boat size, speed or shape. Trent, as you can seen in the video, was swimming along the side, right outside Mike’s pilot door, about one metres out. Did he go up toward the front of the boat? Yes, mostly during the difficult fifth hour. During that fifth hour he also slipped toward the back of the boat more. Because of the difficulty in staying aligned. A difficulty that every Channel swimmer I know has experienced, as the boat adjusts speed, slips in and out of gear, and tries to match the generally low speeds of swimmers. Because Trent can swim fast enough, the boat could mostly stay in gear. For many swimmers, boats often have to do the opposite, and throw out drag-chutes, to slow them down sufficiently to stay with the swimmer without straining the engine by constant low speed.

Hulls displace water all along their length in what’s called the boundary layer starting from In Front Of the bow. Trent was swimming in the boundary layer (of laminar and turbulent flow) caused by the drag of the interface of the water and the hull.  Laminar flow operates in very thin layers (millimetres) of different speeds very close to the hull (less than probably 30 centimetres maximum and is not relevant).

As long as the boat is moving steadily forward, there is that displacement flow or displacement wave. Any displacement flow Trent experienced was higher because Trent can swim faster. Because Trent can swim faster, the boat can move faster. He brought that advantage with him to the Channel because of talent and training and his own speed! Any advantage he gained was by virtue of the fact that he is already so fast.

And that’s if there was any advantage. With all this talk, I’d assumed there was some. But there might not be at all…

The whole process is non-linear and chaotic, meaning tiny changes in initial state lead to huge non-predictable changes in effect. Turbulence increases in a boundary layer to balance the equation of the water that is being forward. (The overall system must balance, where water is pulled forward water must flow back). Both flows exist congruent to each other and in an ill-defined region. In fact, the slower the boat the further out the boundary layer of turbulent flow begins.

But more importantly as I said initially, a slower moving boat with a deeper hull, and wider beam, has proportionally greater bow and displacement waves.

The greatest  speed is where the bow-wave flows off the maximum curvature of the hull, from the bow to the sides. The maximum area of speed is the leading edge of the wave. The trailing end of the wave is characterised by turbulent flow, some of which spirals in opposing directions.

I’d like to show two of the most compelling pieces of evidence that where Trent was swimming was far from the most beneficial, and in fact may even have been counter-productive.

A short video that illustrates the point:

Yes, dolphins. Engineered by evolution to maximise efficiency and speed versus power in water.

And another different case:

Yes, surfing. This time a longboard surfer on a small wave, so you are not confusing my point because I used a big wave.

What do they have in common that illustrates the Trent case so perfectly?

They are both IN FRONT OF THE WAVE, where the speed is, as surfers say, where the water is green. Humans as surfers think consciously about this, dolphins don’t, but have been selected to do so.  The end result is the same.

The displacement of a boat pushes water forward in front of the bow. Speed is attained on the leading edge. As any surfer will tell you, the turbulence behind the face (front) of a wave is chaotic and the difference between being pushing forward and sitting in place is a state change line along the top of the wave. Pass that line even on a big wave, and you are not getting pushed forward.

Trent was behind the bow wave and only experiencing turbulent flow from the trailing edge. At the only times he inadvertently went past the forward section of bow, which happened maybe twice, and for seconds only, it was a mistake and he returned to his previous position. Most of us have inadvertently swum off the bow of a boat.

Our apparent “common-sense”, the experience of pilots, and our understanding of what is happening, what we all took for granted, as with so much of science, may in fact not be correct.

Everyone decrying his advantage? Is wrong. As were we in misunderstanding it.

Our terminology has been wrong: We need to differentiate between the bow wave and the displacement wave. Trent was in the displacement wave, as the pictures and video show.


Let’s look at a picture of two Channel boats. First we have CS&PF pilot Mike Oram’s Gallivant.

Then CSA pilot Reg Brickell’s Viking Princess. Gallivant is an ex-pleasure cruiser with a shallow steel hull. I don’t know exactly what she displaces but it can’t be more than 20 tons, the general specifications and descriptions for the entire CS&PF fleet can be found here. I wrote 15 tons first but I’m adding another 25% to be extra conservative, something of which I’m not often accused. 🙂

Viking Princess is a steel trawler. She is the biggest boat in the Channel swimming fleet, twice the weight at least of anything else. She is 50 tons with a concrete-weighted deep rounded hull and designed to hold lots of densely packed fish in rough seas, so she has to be bottom-heavy, deep and with a wide beam to be stable (and stable is relative term, as she is the rockiest boat I’ve ever been on). She is 5 metres broad whereas Gallivant is 3.5 metres.


What the hell are you on about Donal?

Hang on, I’m getting there.

Given its shape, depth, beam and displacement, the English Channel fleet boat that incontrovertibly provides both the biggest bow-wave and the biggest displacement wave, regardless of conditions or swimmer or speed, is the CSA’s Viking Princess.

On two consecutive days, in different conditions with different swimmers, I was on both these boats. The day before Trent’s swim Owen, Jim Boucher and I crewed for Alan and we could easily see, in the prevailing Force Two Southerlies and south-westerlies, the shelter that Alan could avail of on Viking Princess’s leeward port side.

Since Alan’s swim, I’ve been incontrovertible that Viking Princess is the best boat in the combined Channel fleet to swim off (though not for crew).

These are different advantages provided by boats. These advantages have been provided by boats since Channel swimming started.

Alan availed of shelter during his Channel swim. So did I. With the wind hitting rising past Force Three, and reaching Force Five, I was also on the port side of Seafarer II, a small but high-sided ex-pleasure cruiser. Wind shelter isn’t complete, anymore than displacement wave assistance is complete.

I’ve stood on Shakespeare and Samphire Hoe on different years and different tides, and seen different swimmers go out with different pilots, and the one thing almost all had in common in choppy conditions,  was they all swam to the port side to avail of wind shelter from the south-westerly prevailing wind blowing on the starboard side.

One last area before I start pulling these threads together.

Amongst the defining aspects of English Channel swimming are the traffic and lack of kayak support. One is a consequence of the other. It’s just too dangerous to put kayaks out there in such rough changeable water with high traffic. This means all swimmers must swim by the boat. Swimmers have tragically been lost in the Channel after being separated from the pilot-boat.  Some like to swim away from the boat, some closer in. Some people changes sides regardless of wind, due to references or breathing requirements. These all add to the challenges. And the constraints.

Reductio ad absurdum

Can you eliminate the possibility of wave assistance?  Of course you can. Given the safety constraints of the English Channel, kayaks will remain out of the question, as will swimming away from the boat. The only way therefore will be to scrap the existing fleet and require that the new fleet be of a single standard hull design, designed with a team of marine engineers and hydro-dynamacists to reduce displacement flow off the port side, where we’ll have to insist all swimmers take up position.

Of course all swimmers will have to able to breathe to the right. I have no idea how to stop swimmers getting the benefit of a wind break, I look forward to your suggestions. maybe we can make them swim in front of the boat. Of course now that we know that’s actually the place where you’d actually get bow wave assistance…

Maybe some type of open-rigged catamaran, where the foils are open to the wind, and the swimmer is placed in the centre under the raised hull, and the foils are far enough apart to counter any flow? Or maybe a hull design where the maximum hull curvature would be at the stern, like a reversed teardrop, opposite to the way boats are normally designed. Of course, cat-s and tri-marans are less stable in rough water and I haven’t seen that many backwards-designed boat…

Or move Channel swimming from the current situation to similar to Gibraltar with a pilot-boat and punt where the swimmer swims off the punt. Of course, then you have safety issues again…

If it’s not the English Channel, Cook, Tsugaru, North or any other location where only pilot boats are used, then simply use kayaks or a rig as the primary swim support craft. Should the kayaker need to be evacuated because of conditions and swimmers need to move to main boat, then the swim is called off.

A simpler suggestion would be to eliminate Channel swimming in greater than Force Two winds. Either way of course, we’ll have to tear up the records and start again. Who will the first Channel Swimmer? It’ll be exciting, just like the 1870s, the whole world wondering if anyone will ever swim the Channel.

Yes, I’ve engaged in reductio ad absurdum, extrapolating a specious argument to display its  flaws.


Part of the problem is our imprecise terminology, like I used in the opening paragraph, but deliberately this time, and some people leaping to condemnation. The language carries meaning that may not at all relate to the reality. “Riding the bow wave“. I’ve said it, and not questioned it closely myself previously. I shall not be using it again, because it is incorrect. Mike Oram was incorrect, those seeking to deny Trent because of it are incorrect.

But this article has set out to do investigate those misconceptions and the evidence is very, very strong.

Every swimmer who has ever swum the English Channel, every single one, has been assisted. They may or may not have been assisted by availing of wind shelter behind the boat. But without a doubt, in the absence of kayak support: Every. Single. Swimmer. Has been affected by flow, aka that displacement wave, that suddenly is of concern now, its strength related to many factors including the swimmer’s speed, and the hull characteristics.

What I and they can’t identify, is to what extent if at all assisting flow exists for swimmers. If they stayed ten metres off on the windward side, even so, every time they came near the boat they entered that wave. If the day was a once-a-year, flat-calm glass sea and they stayed 20 metres off, every time they came in to feed, they entered that wave. Every regular swimmer who found themselves at some point going from stern to aft alongside the boat, was in a displacement wave. Any swimmer who swam beyond their boat, a couple of metres off, entered that wave and stayed in it for a long time.

I swam that wave, I got the wind-protection assistance. Find me a Channel swimmer who says they did not gain assistance from their pilot-boat, and I will show you someone who doesn’t understand what happened during their swim.

Any assistance is defined and accepted within the rules of English Channel swimming. It has not changed. To assert Trent did something new or somehow violated the spirit of the rules, is to misunderstand those rules, to misunderstand what Channel Swimmers do and to deny we all do the same by the same methods. And I have no choice but to ask if there is an ulterior motive in some of the questions.

The very spirit of Channel swimming is in teamwork. A swimmer, a pilot, a boat. And in the observation and validation of those swims. Trent Grimsey swam a 100% legal swim, and was validated by the Channel Swimming & Piloting Federation and most importantly did nothing that others haven’t done, including some of those now, unjustifiably, critical. He is the English Channel world record holder, like the swimmers before him. There is not even a hint of implication that his swim will be challenged. Because there’s nothing to base a challenge on.

If you swim Catalina or Manhattan or elsewhere where you are supported by a kayak and the traffic is lower and the conditions are not as bad, you can define your different rules.

I’m not going to run a poll on this. I’ve spent quite some time researching and writing this, including talking to a fluids physicist. A handful of people even saw the previous third draft of this article, and can therefore vouch for significant changes from that to this as the understanding evolved.

If you are a marathon swimmer you can have your own opinion and we can disagree.

But you’ll now need to prove to me that Trent or anyone else got undue assistance that every other Channel swimmer did not have. And if you are a CSA swimmer condemning Trent, you better explain how you can judge when a boat in your fleet generates the greatest displacement wave.

If you are not a marathon swimmer but have an opinion, you are, as we say in Ireland, a hurler-on-the-ditch, an expert without having much expertise. As the good people in America say, you must have some skin in the game. I’ve put my skin (literally) in there, so has Trent. And so has every English Channel swimmer.


Thanks to the people who reviewed earlier drafts of this, Owen O’Keefe, Zoe Sadler, Nick Adams and Niek Kloots. Thanks most especially to my friend Justin, the sailing physicist, who helped me cut through some of the dead-end research areas I was lost in (I spent a lot of unnecessary time on laminar flow!) and clarifying the issue and explanations.


Fair winds, calm seas.

Related links:

The physics behind dolphin bow-wave swimming.


One thought on “Have you skin in the game? Wave swimming in English Channel swims

  1. Pingback: Channel and Marathon Swimming Articles Index & adding a Donate to LoneSwimmer.com, the world’s most popular open water swimming blog option | LoneSwimmer

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