# Rowing Physics: Why choose a dynamic ergometer?

This is an extract from the excellent Physics of Ergometers website.  Section 12. Dynamic v. Static Ergs.

A fundamental difference between the linear mechanics of a ‘static’ ergometer (such as a Concept) and a boat can be illustrated by the following test:

• If you sit at front-stops on an erg and then push your legs down you move backwards relative to room by an amount equal to your leg length
• If you sit at front-stops in a boat and then push your legs down (oars out of the water) you only move backwards relative to the bank by an amount ~20% of your leg length – the rest of the motion is taken by the boat moving away from you.

This is a result of the action-reaction principle (Newton’s 3rd Law). The force applied by your legs to the stretcher acts equally on you and the stretcher. In the static case, the stretcher is effectively attached to the whole planet so doesn’t move – you do all the moving.  In the dynamic case, the mass of the boat is much lighter (typically 10-20%) than you, so it moves further than you do.

This is not just a matter of the frame of reference: in the static (ergometer) you are actually performing more work in accelerating your body weight than in the dynamic (floating) case where the work is split between accelerating the boat and your body in opposite directions.

A ‘dynamic’ ergometer, such as the Rowperfect, attempts to simulate the reaction effect by having the stretcher/flywheel (together weighing approximately the same as a sculling boat) also mounted on a rail so that they also absorb most of the motion.  Putting the Concept on ‘slides’ also simulates this effect, although since the Concept erg is much heavier than a sculling boat, it’s not as realistic as the Rowperfect.

OK, those are the principles. Here’s the maths …go to the original site to read the theory.

That is, on a static erg, the rower is required to put in six times as much energy accelerating/decelerating just their bodyweight, compared to a boat or a dynamic erg where the energy is split between the bodyweight and the boat/erg.

Cas Rekers (designer of the Rowperfect) has performed tests comparing the ‘indicated’ power output with and without the flywheel fixed – the subject gained about 10-20% power output in the second case, representing the additional power that could be applied to the flywheel instead of accelerating the bodyweight.

More energy is used up by accelerating just the body backwards and forwards than by accelerating the body + (lighter) boat/erg in opposite directions.

This is also one reason why the ‘catch’ on a static erg feels relatively ‘slack’ compared to a boat: the initial pressure on the feet is actually being used to decelerate/accelerate the body so the acceleration of the handle (as sensed by pressure in the hands) can only begin once the body has changed direction.  The catch in the handle feels ‘late’ compared to the catch on the stretcher.

Read Carlos Dinares’ recent post “The Dynamic Erg is changing the game

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### This Post Has One Comment

1. Alastair Moir

A pretty shaky grasp of the physics, I fear. The main point is that the centre of gravity of the system, crew and boat, stays in the same place unless an external force is applied. In the example given, the rower makes up about 80% of the total mass, a quick calculation shows that the boat has to move 4x as far as the rower, in opposite directions, to keep the CoG in the same place. The only external forces are the friction on the boat shell from the water and the air resistance on the body of the rower, which can be ignored.

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