Jan 14, 2021

Parts of a PropellerStyles, shapes, pitch and diameter of props are widely discussed on online boating forums, YouTube videos and in boatyard scrimmages. The pros and cons aren’t always debated well – so lets take a look at some of the terms, options and choices to allow you to make some educated decisions.

Props differ widely between sailboats (referring to those with inboard engines) and power vessels (which may have inboard or stern-drive/outboard arrangements). There are many options for powerboats, so we will cover sail auxiliary first.

Sailboats

On sailboats, the engine is referred to as the auxiliary – meaning that the engine is meant as a secondary means of propulsion. The prop is significantly smaller than those found on powerboats. Engines/props are not designed for long-distance cruises, and props are meant to balance two opposing needs: It must be large enough to provide thrust (especially when engaging in reverse), but must be small enough to prevent the slow down and drag that will prevent a cruiser or racer from sailing well.

Sailboat props can be dived in two: Fixed and folding. A fixed prop has blades that remains in position at all times, while a folding prop has blades that fold towards the stern (allowing it to have less drag while sailing, but expand when the shaft begins to turn. Folding props are considered one of the best upgrades available to a sailboater looking to maximize the drag/thrust issue.

In recent years, the saildrive has become popular: An inboard engine, with a leg that runs through the hull, just aft of the keel, resembling the leg of an outboard engine (and functioning much like the leg of an outboard). They are quite small and deliver a great amount of thrust considering that size. They don’t seem to create quite as much drag as a traditional shaft/prop arrangement, and there are few options available to changing out the prop to gain any greater efficiency

Powerboats

Options abound in powerboats – because the nature of the prop in the powerboat is to push a load (the boat) with speed, duration and direction. It requires some careful design to achieve these ends simultaneously – and engine manufacturers work hard to find the best balance. That said there are many ways to upgrade or to change the prop to achieve your own specific wants or needs.

Props for inboards stern-drives and outboards are available in aluminum, bronze, and stainless steel. They may be 2, 3, or 4 bladed (and some workboats have 5 and 6 bladed props).

Props are designed with two dimensions: A diameter and a pitch. These are typically engraved on the body of the prop for reference. The diameter refers to the measurement from the tip of one blade, across to the arc of the opposing blade. The pitch refers to the distance forward that the boat is pushed in one revolution (a 21” pitch would move a boat 21” forward with every revolution).

These variables are probably the single most important elements to consider when looking to increase performance. At the same time, two user variables need to be considered: The speed that the boat is moving (Miles per Hour – MPH), and the engine speed (RPM – the engine revolutions per minute). You would think that the as the engine turns faster, the prop turns faster and therefore the boat moves faster – which is theoretically true – but these can be tuned by adjusting changing the prop to one with a different diameter and pitch.

Propeller PitchHere’s how it works:

Diameter usually increases for propeller used on slower boats and decreases for faster boats. Generally, this is pre-determined by engine manufacturers, based on the use of the boat, keeping in mind the engine horsepower that is being used.

The pitch allows you to play with the variables of acceleration and top speed. A higher pitch will give a higher top speed, but slower acceleration. A lower pitch gives quicker acceleration, but offers a lower top speed.

Each inch of pitch is worth about 200 engine RPM’s – so moving from a 21” pitch to a 23” pitch will decrease the engine RPM’s by about 400.

The caveat, here, is that the engine is designed to run at a certain maximum (wide open throttle) RPM: If the prop pitch is too low, the engine may run beyond the desired RPM’s and damage can be caused. If the prop pitch is too high, the motor may run below the ideal wide-open throttle RPM’s, making it feel bogged or sluggish, and potentially causing engine damage

The next piece of this puzzle is the determine the number of blades and the material:

3 vs 4 blades - in general, 3 blades allow a boat to go faster than 4 blades. 4 blades were introduced to allow for better handling at low speeds, and have since evolved to allow boats better fuel economy and low-speed planing ability. 4 blades are usually seen on larger boats and power cruisers, where 3 blades are found on go-fast runabouts. These can be interchanged to meet your performance goals.

Aluminum vs Steel: Aluminum is lighter and more cost effective. Steel is stronger but more expensive. In shallow waters, or in areas where there are underwater hazards, stainless steel props have been known to hold up better – but are more costly to replace if damaged.

Putting it all together:

When making changes by selecting a new prop, you’ll need a number of pieces of information: The boat type and typical use (including the hull shape and engine size), The current diameter and pitch of the propeller, the engine RPM’s at wide-open throttle (when loaded and trimmed appropriately).

Next – what are your performance goals: Acceleration or top-end speed?

Choose a prop (perhaps with the help of a your local marine chandlery) that will best suit your engine and performance goals (handling, fuel economy, resistance to damage, acceleration and top-end speed). Once you’ve determined the ideal material, number of blades, diameter and pitch requirements, you’re read to buy.

Modern props are purchased using 2-piece propellers, allowing them to be customizable. There are many permutations of diameter and pitch, and there are many engine types with different shaft sizes and spline configurations. So, to make it simple, props are available in each diameter/pitch combination, and a hub (the inner core of the propeller) is purchased separately, to correspond with the engine type and spline configuration on your engine. Prop dealers typically have catalogues available to assist with the correct choice of prop and hub combination – when in doubt, ask!

 

I hope this helps with winter dreaming and allows you to reach your performance goals: racing, cruising, handling or go-fast. Enjoy the journey!

Andrew McDonaldAndrew McDonald is the owner of Lakeside Marine Services – a boat repair/maintenance firm based in Toronto. Andrew has worked in the marine industry for 12 years and is a graduate of the Georgian College ‘Mechanical Techniques - Marine Engine Mechanic’ program.

Questions or comments for Andrew? Email him directly via: This email address is being protected from spambots. You need JavaScript enabled to view it.

Related Articles

Wednesday, 09 August 2017 02:52

The moment we all dread. It’s a warm sunny day and you’re out for a cruise. Suddenly the boat shudders beneath you, pings, dings and clangs are heard and you realize that you’ve hit something...

Wednesday, 11 October 2017 05:00

If you walk the aisles at a boat show, visit a marine store, or stop in at a repair shop, you’ll likely be inundated with digital displays, vivid touch-screens, NMEA compatible devices, and...

Wednesday, 08 November 2017 04:38

Water has a funny way of making its way into a boat: through through-hulls, stuffing boxes, leaks, hatches, windows and portholes 

Tuesday, 12 September 2017 06:10

The moment we all dread. It’s a warm sunny day and you’re out for a cruise. Suddenly the boat shudders beneath you, pings, dings and clangs are heard and you realize that you’ve hit something...

Wednesday, 10 May 2017 00:43

Question: Can I buy generic automotive parts or products for my boat, or should they specify ‘marine’?

Tuesday, 11 July 2017 07:16

For most of us – this is the time to make the most of the boating season – launch and set up are done. Systems have been recommissioned. Let the fun begin!

Boat Reviews

  • Prev
Once again, Cruisers Yachts is leading the market for day boats with their new 42 GLS model that ...
Optimized sailing performance and comfortable living – a sweet ride. The expression that came to ...
This is such an exciting time in boating! While we feel very sorry for people whose health and ...
For many, the 2020 sailing year will be one to go down into the books as “different”. With delayed ...
What perfect timing! Beneteau is has just announced their new Antares 11 model for North America ...
Commodore’s Boats is a full-service shipyard with over 50 years of generational history and ...
The Oceanis Yacht 54, younger sister of the Oceanis 62, embodies the innovation that has always ...
Beneteau announces the launch of the latest addition to the Antares range! With a length overall of ...
I had been looking forward to a sea trial aboard the Greenline 33 because I was hoping it would ...
New at the end of 2019, the 58 Salon Express design features large windows to flood the living ...

CY Virtual Video Boat Tours

Virtual Boat ToursWe all love boats and nothing can break us up! So, what better way to spend our time than looking at interesting boats and going aboard in a virtual ride or tour. We have asked our friends at various dealers and manufacturers to help us assemble a one-stop online resource to experience some of the most interesting boats on the market today. Where the CY Team has done a review, we connect you to that expert viewpoint. If you can’t go boating, you can almost experience the thrill via your screen. Not quite the same, but we hope you enjoy our fine tour collection.

 

Read more about the CY Virtual Boat Tours....................

 

Cruisers Yachts 42 GLSBy Andy Adams

Once again, Cruisers Yachts is leading the market for day boats with their new 42 GLS model that premiered at the Fort Lauderdale Boat Show at the end of October. The concept of a large day boat is now a very well-established trend made possible by the amazing new power and efficiency of the latest four stroke outboards.

Buyers are looking for a different boating experience and we think that the 42 GLS nails it. Fast, handsome and versatile, the 42 GLS is designed for fun and adventure.

Read More

Destinations

  • Prev
In Part I, Sheryl Shard ended the story at June and the start of Hurricane Season when they were ...
You likely aren’t quite ready to travel yet, but we have our fingers crossed that we can all fly ...
Ontario’s best-kept secret, the Trent-Severn Waterway National Historic site holds the key to ...
Located on the sunny south shore of the harbour, the Marina is on pilings over the water, offering ...
The approach to the Chemainus Municipal Dock from Stuart Channel is straightforward and is ...
I leaned my head back into the water and floated easily. Having spent my childhood playing in ...
History: right after gym and just before chemistry class. Fifty minutes of naming the prime ...
On May 19, the New York State Canal Corporation today announced an updated opening schedule for the ...
If you have four hours to enjoy a fine tour of one of Canada’s most interesting waterways (let’s ...
Boom & Batten Restaurant is suspended over the water adjacent to the Songhees Walkway and ...

 

Bahamas - There and Back Again IIIn Part I, Sheryl Shard ended the story at June and the start of Hurricane Season when they were once again joined by friends.

This time it was Noel and Tracey Dinan, whose new shallow-draft Allures 49.5 was in build at the time, we headed north from the Exumas across the expanse of the Great Bahama Bank, dodging coral patches as we sailed to Eleuthera then Marsh Harbour, Great Abaco. Another commercial centre in the islands, we cleared out of the Bahamas here after provisioning for our offshore passage up to the Chesapeake Bay on the US mainland and out of the Hurricane Zone until mid-November...

Read More

Lifestyle

  • Prev
OK, stop the presses. This photo just came in from Beacon Bay. Clearly those folks know how to get ...
Back in the day, the publisher of a magazine would receive a bound copy of the year’s monthly ...
Boaters on BC’s West Coast have heard the story of the garbage pickers of the Marine debris removal ...
Skipper John “Drew” Plominski is hoping that lightning doesn’t strike twice. Plominski, whose boat ...
The Association provides a forum for exchanging information, tips and access an advocate on behalf ...
Kristin Cummings, Operations Manager at Beacon Bay Marina took this shot after the skies broke ...
Our Photo of the Week (two, in fact) comes from New Zealand where the second America’s Cup AC 75 ...
The Marine Debris Recovery Initiative (MDRI), a collaboration with the Clean Coast, Clean Waters ...
The International Joint Commission (IJC) is reviewing Plan 2014 and could use your help. The plan ...
The Council of the Great Lakes Region (CGLR), thanks to funding from Environment and Climate Change ...

Marine Products

  • Prev
On Monday, Volvo Penta announced the availability of their fully integrated assisted docking system ...
Perhaps the ultimate audio solution for boat owners, the JBL by Harman BassPro Go from Prospec ...
It only takes one foggy, disorienting day on the water to make a boat owner understand the value of ...
It’s a voyage everyone wants to undertake, but few get to make. The Whales of Lake Erie is the ...
Over the years I have had a real soft spot for the Jeep Wrangler line of models. Recently I had the ...
Wait no longer, the 2021 Rideau Canal & Lower Ottawa River PORTS Guide has returned! Purchase ...
The Tundra 65 is Yeti's most versatile cooler, just as adept at keeping catches cold as it is ...
Fireball self Extinguisher. It's a revolutionary self-detonating device designed to extinguish a ...
The problem with driving any full-size Pickup Truck or Sport Utility Vehicle is that when you are ...
Wait no longer - the 2021 Rideau Canal & Lower Ottawa River PORTS Guide will be available for ...