My three key requirements for Slapdash Lilly, a 57’ Semi Trad narrowboat were as follows:
1. To be able to cruise silently at normal canal speeds (4-6 km/h)
2. To be able to cruise efficiently at normal canal speeds (1-1.5kW)
3. To have sufficient power to be able to handle tidal or fast rivers (a max speed of 9-10km/h – which is I believe the minimum specified for Ribble Link)
Unfortunately, I could not get the original motor and prop to deliver on these points. I tried various different prop sizes 13×7, 13×9 and 15×9 (the numbers I quote below are from the 15×9). However, in all configurations I was unable to meet the requirements to my satisfaction. In terms of silent cruising the small prop spinning fast created significant cavitation noise. For efficiency, I was cruising between 2 and 3.5kW power draw (a combination of motor inefficiency and prop inefficiency – motor inefficiency evidenced by rising motor temperature (i.e. heat loss) and prop inefficiency by cavitation). And, the maximum speed I could attain was around 7-8km/h and that was drawing upwards of 9kW with lots of cavitation.
This is why I reluctantly decided to swap the motor out.
Why Mothership Marine’s Tidal-15?
When making my choice for an alternative the key factor for me came down to efficiency of the system. The cavitation noise was a manifestation of prop inefficiency. The lack of sufficient power was again largely an issue of the system not being able to efficiently convert the power to forward thrust. In my view, 10kW should be ample to propel a narrowboat providing the motor and the prop are combining to allow that power to be converted into forward thrust efficiently. There are of course other factors affecting efficiency: depth of water, depth of prop in the water, hull design and probably others too. But there was nothing out of the ordinary with my set-up and the results I’m getting with the new system would seem to confirm that neither hull design nor prop placement were a factor.
Propellor efficiency at low speeds
To get prop efficiency I wanted an 18/19” prop. There is a reason why this is the typical prop fitted on a diesel powered boat and that is because it is efficient. If a smaller prop were more efficient, then diesel boat manufacturers would have converged on that and geared the motor accordingly. So, given that choice of prop size, I needed to find a motor that could efficiently turn such a prop. I wanted a direct drive motor so that I did not have any drivetrain noise and also to reduce moving parts. I studied motor efficiency charts from numerous motor manufacturers and found a few contenders. In the end I decided that MM’s was the best one for me.
I’m very happy with the results. The new motor and prop are significantly quieter, draw between 1-1.5kW at cruising speed (approx 50% reduction in power) and generate significant additional thrust when I need it.
Motor Power
Lastly, there is a lot of discussion about motor power 10 vs 15 vs 20kW. However, in my opinion, for the motor, the ability to efficiently generate a high torque at low RPM is the key thing. A higher torque allows for a larger prop, which is more efficient. As the power increases, the efficiency of the system (motor and prop) decreases and you reach a point where to increase speed even by a small amount requires a large increment of power. Because of this, I can’t see me personally ever putting more than 10Kw through the motor as the gains in speed at that point would be marginal if anything at all. And, as I said earlier, cruising requires just 1-1.5kW.
This is an evolving space. At the end of the day, it comes down to identifying what is important to you as an individual and looking for a motor and prop combination that delivers that. Before buying any motor I’d demand to see an efficiency chart from the supplier and look at the efficiency you will get when the motor is operating in the RPM/torque range that you expect. All manufacturers should be willing to provide you with this. If they’re not, then ask the question why.
Slapdash Lilly Specification
- Name: Slapdash Lilly
- Boatbuilder: Bath Canal Craft / Paul Widdowson Shell, 57ft’
- Motor: 15kW Mothership Marine water cooled direct drive with 18×13″ prop
- Solar: 2.5kW flexi roof mounted from Verditek
- Batteries:LifePo4 – 6 x Pylontech US3000 + 2 x Pylontech US5000 giving 30.9kWh @ 48V
- Power Electronics: Victron with 8kW Quattro
- Genset: Mase 610S (5.6kW nominal).