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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!

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7 Kommentare


Ian Dedic
Ian Dedic
08. Dez. 2020

Starting/stopping "power" is basically bollard pull, the rule of thumb for this (from various marine sites) is 180N (about 18kgf) per kW for a normal-sized prop. A big slow prop (like the 18" x 14" 4-blade) might do 10% better than this, a small fast one (like the 12" x 7" 3-blade) might do 10% worse than this.


So the small prop (12kW@1800rpm) might give about 195kg pull, the big prop (4kW@600rpm) might give about 80kg pull. Yes the big prop is 20% more efficient per kW at pulling, but this in no way makes up for it turning a third as fast and having a third the power from the motor. If it was correctly matched with a gearbox…


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Ian Dedic
Ian Dedic
08. Dez. 2020

Hi Dave


Having been round this process myself I'm well aware that a bigger prop turning more slowly gives better "grip" on the water (aka bollard pull) and better starting/stopping power, and that PMAC motors (constant torque up to field transition point) like going faster to generate more power, but the only way to do this is with a gearbox or belt drive which I don't want for the same reasons you don't.


If you take your 16bhp/70nm/1800rpm motor and connect it directly to your 4-blade 18" x 14" prop it'll be massively overpropped and has got got no chance of ever getting anywhere near 1800rpm, being a PMAC motor it'll be constant torque (70nm) below this and the fastest…


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Dave Jesse
Dave Jesse
08. Dez. 2020

Ian,

Thanks for your comment. In fact I selected a 4-blade prop, as these are reputedly smoother than the 3-blade version. Now, onto the real meat of the matter. The Tema motor (like most electric motors) is happier turning faster than the propeller design data implies. If you look at Lynch motor installations, or Mothership Marine, they both use reduction belts to reduce speed from the motor to the prop and use conventional prop dimenstions. The alternative could be to fit a smaller, faster turning prop as Vicprop indicates. I had the same recommendation from Michigan Marine, who supplied my prop.

So why did I pick a prop that looks too big? Design programs like Vicprop are aiming to achieve…

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Ian Dedic
Ian Dedic
08. Dez. 2020

Hi Dave, I've been following your blog with interest because I've been looking at doing something very similar, and even reached some of the same conclusions (e.g. BetaGen 10). But I'm very confused about your choice of motor and propeller, these don't seem to work...


I assume from your comments and videos that you're using the TEMA SPM132-1 (73kg, 12kW/16bhp/70nm@1800rpm), but you're also using a 3-blade 18" x 14" prop and direct drive -- am I correct?


If so, have you checked propeller sizing using the Vicprop tool which is regarded as reliable?


https://vicprop.com/displacement_size_new.php


This gives the optimum narrowboat 3-blade prop size for 16bhp@1800rpm as 12.3" x 6.5". To swing an 18" x 14" 3-blade prop it needs to turn…


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Kevin Wheatley
Kevin Wheatley
02. Okt. 2020

We had the privilege of seeing Perseverance today at Ortomarine while paying the deposit for our build. It will be interesting to see the development being made so that we can get the benefit for our build in August 2022.

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