In last month's newsletter, I wrote a short bit on the advantages of various hull cross-sections: semi-circular having the least drag, and those that have a flattened curve providing more roll stability. (You can find the article 'Flat-bottom Boats' in the sykesna.com Resource Centre.) These are pretty safe generalisations to make. The longitudinal section of a boat, however, is a much more nuanced matter.

Almost all boat builders will agree that the semi-i-circular cross-section has less drag, but believe that the reduced stability can create too many difficulties for the rower that will thus offset the reduction in drag. In many ways the choice on cross-section shape comes down to assumptions about the technical abilities of the crew.

Conversely, those builders who do not like a lot of rocker in the keel line of a boat -- the longitudinal profile of the boat -- feel that the troubles are more inherent in the design itself and not so much in the abilities of the crew. They'll argue that the benefits that I'm about to outline below cannot be sustained without the potential downsides reducing overall performance. In this article, I talk about those downsides as well and how Sykes addresses them in our designs so as to optimise those boats of ours with pronounced rocker. And what's more, I'll explain why I believe the deciding factor in the end for most builders is more a matter of market perception rather than actual boat performance.

Incidentally, some of the points in this article are going to sound similar to points I've made previously in an article regarding boat trim (also available in the online Resource Centre). That's because the interest in properly trimming a boat is an effort to find the boat's most effective waterline. Toward the end of this article, I'll tie in my comments on design into how you can properly trim your boat for various race speeds.

But before I continue: What exactly do I mean by 'rocker'? If you look at a boat's keel line from bow to stern, a boat that has a lot of rocker will look much like a rocking chair rail. It will have a long curving line that lifts up at the ends. In other words, the ends of the boat will be very shallow compared to mid-ship. This is something you'll see in our m20 or in a traditional Van Dusen 1x, or that you'll see in our m1 and m40 eights, or even in the newer Vespoli and Hudson eights. An example of a line of boats that has almost no rocker are the Hudson small boats. If you look from bow to stern, they are quite nearly flat.

Having rocker in the keel line achieves several goals in reducing drag:

In general, a minimum-drag displacement hull will have a semi-circular cross-section with a smooth parabolic keel line that is closely mirrored at the water line on each side. The important matter of length-to-beam ratio is highly dependent on other factors, particularly boat speed. As I've talked about in the discussion of the development of the m40 eight in previous articles (also available in the Resource Centre), the speed of the boat does affect several components of drag rather significantly. It's not necessarily important to the discussion here, though.

So why don't all rowing boats look like this if this is the ideal shape? In the case of eights and fours, the problem immediately arises that those lines won't afford room enough for the coxswains and rowers in the ends of the boat. In the case of doubles and singles, though, the question as to why more shells don't have more rocker is more interesting. The portion of the hull dedicated to seating the crew is small enough that it really doesn't affect applying the ideal shape.

To me, the answer to this question seems to come down to some of the classic trade-offs in boat design as well as interests that aren't matters of performance so much as matters of perception.

Most 'traditional' shapes in small boats (I'm going to focus on small boats given they're easier boats to optimise) don't have much rocker in the boat. The reason given typically is that, by having more volume in the stern and bow, the boat will trim more evenly. And the concern about a boat coming out of trim – where its actual profile through the water diverges considerably for its designed optimum – is three-fold:

So the thinking among those that build boats who use a flatter keel line and keep a higher percentage of the buoyancy in the ends of the hull is that the boat will keep a more consistent waterline. This waterline will be closer to that particular boat's optimal line no matter where the crew's mass is during the stroke cycle. From this perspective, the fact that the boat was designed with a sub--optimal longitudinal shape is arguably trumped by the fact that its profile through the water is more consistent and predictable. Explained another way: The boat never experiences minimal drag (due to increased wetted surface) but also never experiences moments of dreadful drag.

I'd counter this argument by saying that it really isn't necessary that a boat with a good deal of rocker will experience moments of notably higher drag. In fact, the very shape of the rocker can be made so that even as the boat pitches, the length of the waterline remains nearly constant. Some pitching is unavoidable by shifting the crew's mass for and aft, but the rocker can take that into consideration by the very nature of its shape just as the rails on a rocking chair maintain steady contact with the floor. That is, for every length of bow that rises out of the water, a similar length of stern enters the water. We're not talking about a matter of metres here, but rather of centimetres.

Beyond that argument, the Sykes design team tries to reduce pitching in the boats with rocker by holding a good deal of volume (and thus buoyancy) in the mid-section of the hull. This is easier to do with bigger boats given the existing constraints of fitting the crew in the boat. (It's also more important to reduce pitching in the big boats so that the rowers at the end of the boat don't go through considerable height variation, making a horizontal stroke more difficult to maintain.)

The newer moulds – m33 and m40 – have taken into consideration other factors that will affect optimal trim, most notably squat from sustained high speeds. This is where these boats show substantial performance gains over our competitors. Boat squat should also be a consideration of how you rig/seat any boat given its expected boat speeds. You really want to be mindful of properly trimming any boat you're racing beyond just how it was designed.

As I said at the outset, having a boat trim as it was designed to run through the water will get you the best results in reduced drag. So I offer the following suggestion to assist you in optimising trim, and think it's of particular usefulness to folks rowing in boats with considerable rocker.

Observe (as coach or video recorder) the behaviour of the bow at the finish of the stroke cycle. It's very important that this is done at racing speeds to take into consideration squat. What you want is to see is the boat pitched slightly downward at the release, with the tip of the stern slightly higher than the tip of the bow. Having the boat completely level at the finish/release is fine, but I prefer to see a little nose down since the boat is about to pick up speed on the recovery and that's where I want the boat level and with the least amount of wetted surface. From the finish, the boat will come back to even pitch and then to slight bow-up pitch as the rowers' mass moves into the catch. But focus on where it's pitched at the release.

Having the bow pitched up at the finish is what you really don't want. If it is, or you like my suggestion on getting it slightly nose down, move your footstretcher into the bow a good one or two centimetres and re-examine the pitch at race speed. If you can move your rigger into the bow as well, go ahead, but don't worry about that now if it's not convenient. All we want to do first is get your weight properly distributed to trim the boat the best we can. Keep making this adjustment until you find the optimum.

It's important to emphasise that you want to do this at full race speed for whatever kind of race you're about to do – be it your race pace for a 1000-metre sprint or a 6km time trial. Boats do trim differently at different speeds.

If after you find the optimal position for your crew weight in the hull, you may need to move the rigger. To do so you may need to add additional holes in the rigger flange of your boat, particularly for those of you with quick-release riggers. I can lend you a template for doing this or help you make your own.

In pairs and doubles you may want to go through this same exercise with different folks in stroke seat. In fours and eights, you may want to do this exercise not by moving the foostretchers or riggers, but rather by moving your crew members around to different seats.

This approach to hull design and boat trim will mean that those lighty women rowing our m20 singles will see a good deal of their stern out of the water at the release/finish. This is truly inconsequential to what's going on below the waterline, where the real business takes place. Some folks just won't like how that looks, and that may affect what people end up buying. I'm willing to bet, though, that the current environment and sophistication of modeling drag will create a greater openness in the market to non-traditional shapes, especially when people feel how quick a boat with rocker can be. If I'm right, this would mark a change in the marketplace, at least as far as Sykes is concerned over our 40+year history.

“All of Jeff Sykes early designs had a lot of rocker and fine ends,” Stuart Wilson explains. Stuart is the head of production and development at Sykes in Geelong, and started working with Jeff Sykes back in the 1970s. “If we would have kept going with that in the 80s, we would have been out of business.” The European designs were the dominant shapes of the day and they involved lot of length and volume in the ends and made the boats trim very evenly through the stroke cycle. If you modeled them today on the sophisticated computer programmes, though, they would not perform very well.

The take-away point here is largely to inform: knowing why some boats have rocker and others do not is important. You'll have to decide for you and your team which boats you think are best. Perhaps for some of you, reading this article will give you a new persepctive by which to examine hull shapes; that from below the water line where, like I say, the real business of hull design takes place.