Propulsion

Installing Bruntons Autoprops

Our previous catamaran was fitted with two-bladed fixed propellers supplied as standard by Yanmar when the boat was new. They worked just fine, but I always knew things could be better. On our current cat, the original units were similar two-bladed models, but for some unknown reason, a previous owner had switched these out for three-bladed fixed props. Ignoring the increased drag from an extra blade, ever since owning this boat, I’ve been convinced they weren’t the right pitch or diameter. Running with one motor, the boat was just trying to go round in circles, and so much opposite helm had to be applied that it was counterproductive. With both motors, it was simply slow. So, this time around, we’ve dug deep into the cruising kitty and treated ourselves to a pair of Bruntons Autoprops and… and decided to fit them ourselves.

Propulsion

Published 02/10/2026

By Graham Shaw

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Issue: 210

Published: dec. / jan. 2027

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Right from the outset, a sailboat propeller is destined to be a compromise: it’s that trade-off between maximizing propulsion under power and minimizing drag when under sail. Fixed props are ok if you’re coast-hopping and not too bothered by a knot of speed here or there, but if you’re ocean passage-making, that difference can translate into days. Fixed-bladed propellers - and folding or feathering models that can only have their pitch adjusted on the bench in the workshop - are designed for use at maximum speed. A good analogy is that of an automobile: my car has a five-speed manual gearbox, and fifth gear is great for going along at the maximum (permitted!) speed. But imagine that being the only gear you have… It wouldn’t be all that efficient. And on a sailboat, nobody motors at full engine speed all the time, I hope. What appealed to us most about the Bruntons Autoprop is the clever design and shape of the variable-pitch blades. Each blade rotates on its own bearing, and the prop will always optimize the pitch to the correct speed and power the boat is using at any given time. At lower revs, it’s always going to give more – and this translates either as greater boatspeed, or, using less power (i.e. less fuel) for the same engine speed. Selecting the right propellers for a boat is somewhat of a dark art but armed with all the crucial information (waterline length, displacement, engine size, transmission ratio, etc. etc., the clever folk at Bruntons will work their magic to establish the precise characteristics of the props your multihull needs. Having met them at last year’s La Rochelle Boat Show, along with several other prop manufacturers, we were convinced the Autoprop was the way to go. Now, I’ve got a confession to make… I motorsail. Probably far too much, but I doubt I’m the only one. It always seems to be a question of getting into somewhere unfamiliar before it gets dark, or arriving before Customs closes, or making it through a tidal gate. And I’m sure I could come up with plenty of other excuses if needed. But for me, motorsailing is where the Bruntons Autoprop really stands out from the competition. The variable-pitch propellers sense the force of the sails pushing the boat through the water. A fixed or non-adjustable folding prop can’t do this and when motorsailing, will effectively be holding the boat back until it is rotating at a speed faster than the boat is already going under sail. The Autoprop, however, is always finding the right angle of attack for what is being demanded, meaning it is both more effective and more efficient than any other type of sailboat propeller. Let’s take a look at the installation!

Installing Bruntons Autoprops

Step 01

The existing three-bladed propellers were totally unsuitable. If left in gear when under sail, they had the same effect as towing a bucket behind the boat. If left to spin, they sounded like they were going to wear out the bearings in the saildrive leg.