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Ask the Boatbuilder
What is Hydrodynamic Lift, and how does it affect my rowing?
March 31, 2016
Steve Killing, Designer for Hudson Boat Works

A Hudson shell in the towing tank

The term hydrodynamic lift is not something I hear in casual conversation around the boathouse, and consequently I thought we should rephrase the question before tackling the answer. Hydro is water, Dynamic refers to motion, and Lift is a force, perpendicular to the direction of motion (in the case of rowing boats, vertical). So the question could be further interpreted as "When I am rowing at race pace, what forces are acting on the hull, and how can that knowledge be used to maximize speed?"

When a boat is moving through water two things will change - the trim (bow up or down), and the sink (an unfortunate term, but it refers to whether the boat's center of gravity rises or sinks vertically). These changes to the boat's attitude in the water are a result of the varying pressure on the underwater surface of the hull, combined with the wave form developed along the side of the boat (you can see the waveform in the towing tank photo above). The faster you go, the more the bow will rise and the stern drop while overall the boat "sinks" in the water.

If you want to relate that directly to the term "hydrodynamic lift", the boat is subject to an upward force (positive lift) near the bow and a downward force (negative lift) near the stern due to the motion of the water past the hull.

There are various computer programs that calculate how trim varies with speed, and we have made use of both the simple and the complex in our assessment of rowing shells. The simpler programs use empirical formulae or wave and drag theories to calculate both drag and trim changes, while the more complex use unsteady CFD (computational fluid dynamics) that calculate from basic principles the motion of the shell as it varies speed and trim with each stroke. Each has its merits, cost, and time requirements and we choose the software to best meet the task at hand.

In order to gain faith in the computer software, we have confirmed the sink and trim of some or our shells in the towing tank. The plot shown here is the towing tank output on a full size shell and has a few interesting points. Whether you are racing at 4.5 or 5.5 metres per second, the trim for this particular shell is fairly constant at about 12 mm bow up (and the stern would be about 12 mm stern down). And that value is something you, the rower, and we, the designers, can make use of.

Although the boat changes its pitch due to your cyclical stroke, on average, for maximum speed, we want the boat to run level. So before you leave the dock, the boat should be 12 mm bow down, so that at speed the hydrodynamic effects will trim the boat level.

We, and we presume other manufacturers, have already located the cockpit to give you that bow down trim at the dock and you can influence it further with crew placement. The crew's location can easily be varied by moving the riggers, and foot stops fore or aft depending on the desired trim outcome.

The sink which shows on this plot to be about 5 mm for the shell analyzed, is something you won't be able to discern on the water and it is also something you can't influence. There are two separate effects that cause the shell to sink lower as it is rowed. The first is the waveform along the side of the boat which has a trough near the mid length of the hull. With the surface of the water lowered where the shell has the majority of its beam and buoyancy, the boat sinks correspondingly. The second reason comes back to the discussion of lift. If water passes with some speed over a curved surface there will be a force created in the direction of positive curvature. In our case the accelerated water flow under the hull creates a downward force (Bernoulli's Principle) and the faster the shell goes the larger the downward force.

Hydrodynamic forces, due to the motion of the boat through the water will lift the bow and drop the stern and since the goal is to have the boat nominally level at race pace, you will want some bow down trim at the dock for maximum performance. Your foot stretchers and riggers should be adjusted so that the boat is slightly bow down at the dock and at training pace, but level at race pace.

Steve Killing
Designer for Hudson Boat Works

Hydrodynamic lift effect at the bow of an 8+ at race pace


Comments

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SDslacker
04/18/2016  12:29:07 AM
Does this suggest that a lightweight masters sculler whose speed may be at the low end of the speeds shown in your graph should position the rigger more toward the stern to achieve a more level trim "at the dock"?

muesli
04/15/2016  3:34:19 AM
sir, since you mentioned to be glad to have another "ask the boatbuilder" session on how stability varies with crew weight, i would like to put "my problem" to you: is there a "optimal" stroke-angle? when discussing "stroke length" and angles "forward and aft" i believe there should be an optimal angle...? regards arthur

BioRow
04/03/2016  10:51:44 AM
What boat type this data belongs to? Single, double, four, or an eight?

Steve Killing
04/06/2016  10:41:48 AM
Even within a given category (single, 2, 4, or 8), the trim and sink will vary quite a bit depending on your particular hull shape, so I don’t want you to take these values for this particular shell and assume they are the correct numbers for your boat. My goal was to have you take notice of the trim at speed and be comfortable with the fact that your boat will be bow down at the dock.


N3673699
04/02/2016  10:15:14 PM
Steve. When at the dock and 12mm stern down, where is the rower positioned. At the finished? Just so I understand, on the power stroke, the bow sinks as the rowers weight moves forward.

Steve Killing
04/06/2016  10:42:23 AM
In the example quoted, the shell would be 12 mm bow down at the dock with the rowers at mid slide. Then, on average, the boat will be level at speed. Both the center of gravity of the rowers and the inertia of their body swing will put the bow down at the finish and the stern down at the catch.


ScullerX
03/31/2016  1:31:08 PM
This is a great way to simply explain a complex topic - thanks very much. It also gives a rational explanation of one reason why you might mess with rigger positioning, etc. It would be interesting to hear your opinion on how or if the chart would vary with lighter vs. heavier body weight in the same shell class. For example, if the hull is the same, does an extra 10 lbs. of average body weight cause the Speeds of Interest band to narrow, widen, shift, or does body weight not really affect this discussion?

Steve Killing
04/06/2016  10:43:37 AM
The speed of interest is simply the normal training and race speed for the class of boat, so that won’t change much with a slightly heavier crew. The dynamic trim action that you see on the water from catch to finish will increase if the crew weight increases - simply more weight moving back and forth in the boat. You do want to make sure you are rowing in a boat that is the right weight category for you. I have also done a lot of work on how the stability of a shell changes as it is loaded with various weights of crew. Maybe we should have another “Ask the Boatbuilder” session which discusses how stability varies with crew weight. I'd be glad to participate.



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