How do propellers work and how do you pick the right one? We explain the basics.
February 15, 2022
With the possible exception of golf, fishing and football, boat propellers seem to attract more ill informed chitchat than anything else in the world. And yet these little chunks of metal are precisely the things that enable us to pursue the pastime we love, so we owe it to ourselves to pick the right one.
First-off, to get an idea of how propellers work, watch the video below.
Understanding the numbers
The size of a propeller is expressed as the diameter multiplied by the pitch. This means that a 14x21 prop has a 14-inch diameter and a 21-inch pitch but what do these figures actually denote? Well the diameter is simply the size of the prop, measured as the width (in inches) of the circle created by a blade in one complete rotation. The pitch meanwhile is the theoretical distance (without any slip) that a propeller would travel in a single rotation. In this case, therefore, our 14x21 prop is designed to ‘screw’ 21 inches forward in one perfect rotation.
Horses for courses
The issue of blade number is beset by compromise. In theory, a one-bladed prop would be the most efficient option because it would cause the least drag - but of course the vibration would be impossibly aggressive, so in practice, props tend to use between two and six blades. Deciding where you position yourself within this range is all about what you want from your boat...
While fewer blades mean less drag and in theory more speed, the extra drag of additional blades is compensated for by means of a smoother ride, because the period between each load is shorter. In terms of real world applications, you can see this in action on watersports boats, where you need a good low-speed plane with plenty of grip and powerful acceleration to lift skiers from the water. For this kind of work, a five-bladed stainless steel prop is an ideal choice. It is only toward the top end of the rev range that the extra drag begins to peg back your performance (for other tips on improving your boat's performance see 10 ways to upgrade and improve your powerboat).
By contrast, those in need of outright speed often use fast-spinning three-bladed props. Again, stainless steel (rather than aluminium) is the favoured material, as it performs with far greater responsiveness and efficiency, courtesy of its inherent stiffness. However, the compromise with these fast three-bladers can often be a bit of slip when the throttle is applied hard at low revs, so for something between these two extremes (with a bit of bottom-end grunt, a bit of top-end pace and plenty of mid-range cruising efficiency) a moderately pitched prop that enables the engine to spin close to its top end at wide open throttle, is usually the way to go.
A bit of fine-tuning
Once you know what kind of boating entertainment you most enjoy, you should be able to have a pretty good stab at picking a prop with a suitable blade number, diameter and pitch. And it’s at this point that the finer points come into play - namely cupping (the lip on the trailing edge of the blade) and rake (the amount the blade is angled back).
Increased cupping is designed to maximise grip and acceleration out of the hole, while minimising cavitation - and because it can add up to an inch of pitch to a prop, it can sometimes be used to transform the performance of an over-revving prop. Increased rake, meanwhile, is designed to give more bow lift, shifting more of the hull clear of the water and generating greater top speed on rapid planing boats. It can be great for an economical fast cruise but you need to be careful, as excessive rake can actually make a boat rather skittish at the top end.
Opt for a test day
It is far too easy to get confused by the almost endless variables that can be thrown into the prop choice equation. So rather than get bogged down, take the foolproof route and organise an on-water test day with a batch of props and a small fleet of boating friends.
Based on what your boat already achieves and how you want its performance to alter, you should gather as many propellers of an appropriate size, shape and pitch as you can. Then equip yourself with the necessary gear to accurately measure and record the engine revs and the speed of the boat. Find out what the nominal rev limit of your engine is supposed to be and then aim to get as close to that as you can - ideally within 500 rpm for a decent top end and some relatively efficient cruising.
If you are stopping well short of the top rpm, chances are you have an over-pitched prop, which will be reducing top speed, increasing fuel consumption and putting an increased load on the engine and gearbox. You should try reducing the pitch and going again.
If on the other hand, you are repeatedly hitting the rev limiter, chances are you are under-pitched. In this instance, acceleration is likely to be excellent, but you will struggle to see any other tangible benefits, so you should try increasing the pitch. As a general guide, a one-inch change of pitch might be expected to make around 400 rpm difference to your engine revs.
Your propeller might not be dead!
If money is tight and you are struggling on with an old propeller that seems bent, warped, pitted and abused beyond the point of repair, you might be pleasantly surprised by your options. The fact of the matter is that these days, almost any prop can be repaired. And if you spend perhaps £50 to £100 on reworking your old stainless prop, you could easily save that much over the period of a season, merely by virtue of the extra efficiency it will buy you - and that’s to say nothing of the potential damage an unrepaired and unbalanced set-up could otherwise do to your gearbox. For some top advice on what can be done, take a look at the following websites:
(1) Propeller Solutions
(2) Streamlines Propellers
(3) Steel Developments
Propellers: a summary
It certainly takes time, effort and patience to find the right prop - and despite all the theory, the behaviour of a boat on the water can sometimes confound your expectations. But when you do manage to find the perfect propeller for your boat, engine and general usage, you will be left in no doubt at all. Everything will snap into focus and you will finally see just how good your boat can be.
If you decide to change the propeller on your boat yourself, read our guide here. The ‘Propeller Handbook’ by Dave Gerr is a top resource, with 176 pages dedicated to explaining (in relatively understandable terms) how to choose, install and understand your boat’s prop. At around £15, it’s a very worthwhile reference.