You have gone electric. You have picked your motor and sized your batteries. Now comes the part most people rush.

The propeller.

It looks like a simple lump of bronze. But it decides how far you cruise on a charge. It decides how well you stop at a lock.

At Torkmar, we see one mistake more than any other. Owners spend weeks choosing a motor and minutes choosing a prop. Yet the motor does not move the boat. The propeller does.

Our advice is simple. Choose your prop first. Then choose a motor that can turn it.

Why electric changes the rules

A narrowboat diesel idles at around 1,200 rpm. It finds its power between about 1,800 and 2,500 rpm. That is too fast for a propeller, so it needs a gearbox.

An electric motor delivers full torque from a standstill. The Torkmar 15kW motor gives 245Nm of instant torque at a nominal 1,000 rpm.

That changes everything:

• Torque from zero. The motor turns a big prop straight away.
• No waiting in reverse. You get instant thrust when you need to stop.
• High efficiency. Electric motors run at over 90% efficiency. A diesel manages about 30%
.• Every watt counts. A wasteful prop costs you miles of range.

So do not simply keep the old diesel prop. It was chosen for a different engine.

Diameter and pitch in plain English

A “19 x 14” prop has a 19-inch diameter and a 14-inch pitch.

• Diameter is the width of the circle the blade tips sweep. Bigger moves more water with each turn.
• Pitch is the blade angle. It is how far the prop would screw forward in one turnthrough solid water.
• Blades are usually three or four. Four grip harder but can be noisier. Three runquieter but stop less well.

Go big and slow.

Here is the golden rule. A large prop turning slowly beats a small prop spinning fast.

Think of rowing. Long, gentle strokes move you further than short, frantic ones. A small, fast prop churns the water. It creates slip and cavitation. You hear it, and your batteries pay for it.

Owners who upsize prove the point. One 58ft boat moved from an 18 x 8 to an 18 x 14. Power at 2 mph fell from 1.4kW to 1.1kW. It stopped better too. That is around a fifth less energy for the same speed.

What Torkmar recommends

For most narrowboats with a direct-drive Torkmar motor, we fit a 19 x 14 Vetus propeller. It suits the motor’s high torque and low revs. It moves plenty of water gently and stops the boat well.

Some owners go bigger still. David and Sandy on Stelvio chose a 20-inch prop. As they put it, “A big propeller gently moving large amounts of water means efficiency."

We also favour direct drive over belts or gears. It is quieter, simpler and needs less maintenance.

Proof on the water

The narrowboat Solmate shows what the right prop can do. She runs a 15kW Torkmar motor with a 20 x 14 Hyperform propeller. Its blades work like an aircraft wing, creating lift as they turn.

• On the canal: the motor turns at just 320 to 400 rpm and draws 0.8kW to 1.2kW.
• On the tidal Thames: she cruised from Hampton Court to Limehouse in under four and a half hours.
• Over three weeks: she travelled 263km from Pyrford to Bristol with no generator or shore power. As the case study puts it, on the canal “it is about torque, not speed.”

Check these before you buy

• Space in the swim. Leave at least 10% of the prop’s diameter as tip clearance.
• Your motor’s torque. A bigger prop needs more torque. Too big, and the motor struggles.
• Gearing. Some motors use reduction gearing. That changes the prop you need.
• Shaft fit. Make sure the new prop fits your shaft taper.

Mistakes to avoid

• Keeping the diesel prop. It was sized for a fast engine and a gearbox.
• Going too small. A small prop wastes power and stops poorly.
• Going too big. One owner fitted a 23 x 24. The motor could only turn it at about 200 rpm.
• Ignoring noise. A prop that “sings” is wasting energy.

The bottom line

The right propeller makes an electric narrowboat quieter, smoother and more efficient. It stretches your range. It helps you stop cleanly at locks.

So go big and go slow. Check your clearances. Match the prop to your motor.

And if in doubt, talk to us. At Torkmar, we are always happy to help you find the right prop for your boat.

Choosing the right prop for your electric boat is very important. Having an electric motor is not merely about changing the fuel source; it is a fundamental shift in the physics of how your vessel moves through the water.  For more information or advice, get in touch with Torkmar.

https://torkmar.com
07311 390840
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