Stop the Shaking

Well actually, as Perseverance has not yet seen any water, there are no shakes to stop, so this blog post is about avoiding the causes of shaking. The shaking I am thinking about is propeller induced vibration. Remember, Perseverance is all about silent, smooth almost stealthy cruising so we need to minimize vibration wherever we can.

Propeller Vibration

A perfectly symmetrical propeller turning in a uniform stream does not cause any vibration. This is why aircraft have the propeller at the front (usually). Of course, there is always the exception that proves the rule, and as the Piaggio P180 Avanti is a beautiful looking aeroplane it’s an excuse for a glamour photo…

Let’s get back to narrowboats. What happens to the propeller which sits behind the swim? The flow of water coming from the port and starboard sides of the boat reach the propeller at different angles, so as each blade passes the swim it experiences a change in direction of the water. This causes a change in the forces on the blade and leads to a momentary reduction in force then, as the blade passes the vertical, an increase in force. This in turn leads to acceleration and deceleration of the propeller which is transmitted into the boat as torsional vibration of the propeller shaft. It’s an inevitable consequence of putting the propeller behind the boat.

Is it a big thing? Well, not really, and narrowboats are going to be less affected than broad beam boats which tend to have less elegant swims. On the other hand, I have only ever cruised on diesel boats where the engine, and in particular its flywheel, have high inertia and will tend to absorb such forces and the vibration coming from the engine will tend to mask propeller vibration. I really don’t know whether propeller vibration is noticeable or not. The snag is, if we set off in Perseverance and feel propeller vibration it’s too late to do much about it. I therefore decided to include some simple features in the design as a precaution.

Swim Shape

The first step is to reduce the change of angle of the flow coming into the propeller. This is achieved by making the swim “S” shaped, so that the aft end of the swim comes to a sharper point than a normal stern. Here is a view of the swim on Perseverance.

In this photo you can see my cardboard motor sitting on its mountings, of which more anon. The key thing is the concave sides to the swim as they meet at the stern (left of photo)

Propeller Location

This is the simplest thing to do. Just positioning the propeller away from the end of the swim gives the water more time to achieve steady flow, and hence reduces variation in flow speed.

Torsional Absorption

Normal driveshafts are usually designed to be torsionally compliant. That is, they are made stiff to transmit the engine torque to the propeller. I decided to include a coupling which has relatively low torsional stiffness so that torsional vibration could be absorbed by the coupling. The nature of an electric motor is that the armature will tend to dampen variations in rotation as well, so I hope that the combination of these factors will achieve the goal of gliding smoothly down the cut.

As it turned out, elastomeric couplings are in common use in industry for all sorts of drives, and so they are a lot cheaper than dedicated marine components. I can hear shouts of "what happens if the coupling shears" and I reply, we'll drift to the edge of the canal and have a cup of tea. OK, this is not suitable for offshore use, but it's a nicely made component and can accommodate significant amounts of lateral, angular and axial misalignment. The only snag we found was that, being an American part you can get metric bores but the grubscrews are Imperial!

Will it Work?

Curiously, we may never know if all this was worthwhile. I dare not suggest to the Boss that we should build a Perseverance Mk2, with a normal swim and conventional couplings, just to see if it makes a difference!