Propellers 101 Page 2

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:

Self feathering

variprop_autoprop

Feathering props (Max-Prop, Autostream, Kiwiprop, and Variprop are a few examples) usually have relatively flat blades that pivot at their bases in a hollow hub. When the shaft is stationary the blades turn edge-on to the passing water. When driven in forward or reverse, the blades are held open by stops at pre-set pitch angles. In forward gear flat blades are not as efficient as those with more curved surfaces, but running astern the performance of feathering props is often superior to that of fixed and folding props, as the blades can pivot around to put their leading edge forward in either direction.

On most feathering props the blades are linked by gears or cranks within the hub so they open simultaneously. Most also have adjustable pitch stops, and that allows owners to experiment when matching the pitch of the prop to the boat and its engine. In some cases, pitch can be adjusted by a diver with the boat in the water. Owners can also transfer their expensive propeller to another boat or engine. The mechanisms must be lubricated periodically.

Self Pitching

The only self-pitching propeller is the Autoprop, which has independently pivoting blades, with the pivot offset by a dog-leg. When sailing the blades point nearly straight ahead, but under power, centrifugal force and water flow pivot them to an appropriate pitch angle for the shaft rpm and boatspeed. This can mean extra thrust and improved fuel economy, especially when a boat is motorsailing or powering against a head wind or sea. Because the prop’s diameter increases in reverse, extra space is needed to accommodate the blades.

Controllable pitch

These are propellers whose blade pitch angles can be controlled from within the boat. The ability to tweak the propeller’s pitch to match conditions over a wide range of boat and engine speeds improves engine loading, fuel efficiency, and thrust, particularly when motorsailing. Mechanical details vary considerably, but controllable-pitch props are very expensive, and require a special hollow propeller shaft. They’re seldom found on recreational boats.

Related

mcarthy-and-mouse

Experience: McCarthy and the Mouse

Sitting at the helm in a light breeze, my arms crusted with a fine rime of salt, my skin so dry I’d lost my fingerprints, I heard a clatter and a curse from below. There were only three of us a thousand miles from shore and only one on watch at a time. Usually, the off watch lay ...read more

2018-giftGuide

2018 Holiday Gift Guide

Brass Yacht Lamp Does someone on your gift list spend the whole winter missing the warm days on the water? Let them bring a little bit of nautical atmosphere home with this new lamp from Weems & Plath. The glass enclosure means the flame cannot be blown out even by ...read more

image001

Opinion: On Not Giving Up Sailing

E.B. White was 64 when he wrote his now-famous essay “The Sea and the Wind That Blows,” which begins as a romantic paean to sailing and then drifts, as if spun around by a pessimistic eddy of thought, into a reflection on selling his boat. Does an aging sailor quit while he’s ...read more

TOTW_PromoSite

SAIL's Tip of the Week

Presented by Vetus-Maxwell. Got a tip? Send it to sailmail@sailmagazine.com A Helping Hand  This is a real-world solution, and I expect correction by my betters. However, anyone whose seacocks are modern ball valves rather than the grand old tapered cone variety may care to ...read more

1812-JeanneaueNewsVideo

Jeanneau Sun Odyssey 410

Designed by Marc Lombard, the Sun Odyssey 410 shares much in common with her older siblings including of course, the walk-around deck. Other features that set the 410 apart from other models being introduced this year include the 410’s “negative bow” shape allowing for a longer ...read more

shutterstock_698968441

Cruising: The Bahamas

“The ‘Explorer’ chartbooks. All three.” “An unlocked phone. But good luck with BTC.” “Spam. It’s ‘spensive there!” These were just a few suggestions we received from fellow sailors who had cruised the Bahamas when we asked how to best prepare for the trip. In fact, several ...read more

windsensor

Gear: B&G Wind Sensors

Sense the Wind B&G has launched a new line of wind sensors, including the WS320, a wireless system that is suitable for masts up to 80ft. Wireless wind sensor technology has been hit-and-miss, with some users reporting intermittent signal failure on tall rigs, but B&G, citing ...read more