Propellers 101

A propeller converts the rotary output of a boat’s engine into an accelerated stream of water. This acceleration creates thrust that pushes the boat through the water. To work efficiently, propellers must be properly matched to their engines and transmissions. Other important factors include the boat’s displacement and waterline length and the clearance between the propeller shaft and the bottom
Author:
Publish date:
kiwiprop_flexofold

A propeller converts the rotary output of a boat’s engine into an accelerated stream of water. This acceleration creates thrust that pushes the boat through the water. To work efficiently, propellers must be properly matched to their engines and transmissions. Other important factors include the boat’s displacement and waterline length and the clearance between the propeller shaft and the bottom of the hull. The terms and measurements used to describe a propeller are explained in the figure below. Specifying propellers for particular boats and engines is a job for experts, but owners can still choose between various types and brands.

Fixed Blades

Fixed-blade propellers are strong and cheap. They are most efficient when operating at the rpm and boatspeed they were specified for, and are less efficient under conditions such as motorsailing or powering into headwinds. Fixed blades also create a lot of drag under sail, whether they are locked in place or freewheeling. Sailors are generally better off with folding or feathering propellers that reduce drag and present a smaller, more streamlined shape to the water when sailing. Most folding and feathering props will fit standard propeller shafts and are available in two-, three-, and (in some cases) four-bladed versions.

Folding

Folding propellers (Gori, Martec, Varifold, and Flexofold are a few established brands) have long been popular with racers and are also favored by some cruisers. Some folding props have gears that ensure the blades all open together, but the simplest have blades that pivot independently at the hub. In both cases the blades are pressed backwards by the passing water and nest neatly against one another. When the engine is engaged, the blades are opened by centrifugal force, and in forward gear are held strongly against their stops by thrust pressure. In reverse, the water flow instead tries to fold the blades, so relatively high engine revolutions are needed to hold them open. The performance of folding propellers in reverse is notoriously poor. A rubber band is sometimes fitted to prevent blades opening due to gravity.

Two-bladed folding props have the least drag compared to all other types and are least likely to catch passing weed or debris. Three-bladed versions generally fold less compactly and create a little more drag.

Related

01-Opener-P1090199-copy

The Great Schooner Race

I arrived in Rockland, Maine, on an early afternoon in July with a backpack, my camera and little else. Though traveling to an unfamiliar place to race with a crew of strangers has been a common theme in my sailing career, I had no idea what to expect from the massive wood and ...read more

westonsquallpic

Cruising: Camaraderie in the Squall

Darkness had fallen and the howling wind almost drowned out the voices coming over the VHF radio. “Lights on! Blow horns!” “Make noise to alert the crew!” “Shine the spotlight on the cat by the shore, it’s dragging dangerously near our bow.” These were calls to action, not ...read more

Lagoon-50a

Boat Review: Lagoon 50

Anyone under the impression that change in today’s production catamarans is about little more than cosmetics needs to check out the Lagoon 50—an all-new design that went on to become the winner in the 40 to 50ft cruising multihull category in SAIL’s 2019 Best Boats awards. ...read more

shutterstock_543237994

The Slow Route to Cabo

Each November, cruising boats start leaving California for “a winter of fun in the sun down Mexico way.” And having spent the summer and autumn on a leisurely passage down the West Coast on board Distant Drummer, our Liberty 458 sloop, my husband, Neil, and I were now in San ...read more

MHS-GMR_3549

New Multihulls 2018

Farrier F-22 New Zealander Ian Farrier ushered in a new genre of sailing with his folding-ama trailerable trimarans, the best-known of which are the Corsair designs. Farrier’s last project before he passed away last year was this sweet little tri. Available in three versions, ...read more

shutterstock_373701682

Cruising: Island Comeback

The U.S. Virgins Islands have surged back from the devastation of the 2017 hurricanes, with new infrastructure plans that will benefit charterers and cruisers alike. After hurricanes Irma and Maria roared through the Leeward Islands in September 2017, it was impossible to ...read more