Finding the right prop is a very dark art, particularly for planing boats. But if you can get to grips with the basics, some light should begin to shine. John Cooke explains . . .

Nothing in the world attracts so many myths and old wives’ tales as the propeller. And yet it deserves so much better because this humble chunk of metal is the very thing that enables us to do what we love. Without it, an engine is nothing but an inanimate chunk of combustion - and that makes your boat no more than a floating lounge. But get yourself the perfect set-up and the first time you drive it, your boat will feel brand new. Everything will snap into focus and finally, you will realise what you have been missing . . .

The basics
Props are defined by two numbers, the diameter and the pitch. A 14x21 prop has a diameter of 14 inches and a pitch of 21 inches. In theory (without any slip) this means that the boat will travel 21 inches for each rotation of the prop. The make and model of the prop will determine its other characteristics such as rake (the amount the blade is angled back), cupping (the lip on the trailing edge of the blade), and the number of blades.

In theory, a propeller with one blade would be the fastest, most efficient prop because the drag caused by the amount of metal in the water would be at its minimum. In practice of course, it is not feasible to use one blade because the vibration would be too severe. As a result, props tend to use between two and six blades. The more blades you have, the more drag you get but the smoother the prop feels as it ‘grabs’ more water with shorter periods of time between loads. The fewer blades you have, the less drag you get and the quicker (in theory) it will be.

The test process
If you know what kind of boating you want to do and you still think you can get more out of your boat, it is time to ask questions of other people with similar boat and engine packages. You can then take your craft out with a selection of props and get a far better perspective.

When you go out, make sure you record the engine revs and the speed of the boat. If you’re looking for top end, go out in flat conditions. If you’re a skier or a wakeboarder, make a note of whether the boat is struggling to pull a skier out, or if the boat is struggling to stay on the plane at wakeboarding speeds.

When you think you have all the data you need, find out what the rev limit of your engine is. You should be close to that (within 500 rpm) for good maximum speed and for useful cruising applications. If you are either hitting the rev limiter or need to increase the revs, then both the diameter and the pitch will alter that. To make it simple, an increase in both pitch and diameter will decrease the rpm, but it is normally just the pitch that you would want to change.

An inch of pitch will make around 400 rpm of difference to your engine. There is never a reason to over-pitch a prop, as it will reduce top speed, increase fuel consumption and put increased load on the engine and gearbox. Likewise, if you under-pitch a prop, there is a danger of damaging the engine by hitting the rev limiter too regularly. Although acceleration will be improved with an under-pitched prop, it will not help in any other area at all.

The side effect of increasing the diameter of a prop is more grip but, depending on the type and size of boat and the size of engine, it may adversely alter the handling. With a right-hand rotating prop, the extra diameter and, to a certain extent, the pitch will try to push the boat over onto its left side more than is desirable and this will only be exacerbated when the engine is trimmed down.

Heavy metal?
Another big question is whether or not to bother with a stainless steel prop (instead of an aluminium one). Well certainly, changing to a stainless prop can give quite noticeable differences. They are more efficient, with better acceleration and up to ten per cent more speed at the top end. It is largely the extra stiffness of the stainless blades that makes the big difference here.

The three boating types
So how do you achieve that perfect set-up and unlock the hidden potential in your boat? Well the first thing you need to do is decide what you need it for. There are essentially three basic groups of boaters - those who want cruising, those who want sport and those who want speed. With cruising you are looking for an all round package giving you economy and a prop that gives the boat easy handling. With a sport type prop you might want better acceleration for skiing and wakeboarding or for towing inflatables. In fact there are even props, like the High Five, that will make the back end of the boat hunker down in the water to generate a bigger wake for wakeboarding. And then there are the speed props, which should give you great economy when cruising and a long legged top end at Wide Open Throttle (WOT), but may sacrifice quite a lot in terms of low speed acceleration and be much harder work to drive.

Other prop characteristics
Once the prop diameter and pitch are sorted out, you can then start to look at other characteristics to achieve your goal. An increase in rake angle (with a prop such as a Mercury Raker) will give more bow lift, which (for planing boats) will get more of the boat out of the water at wide open throttle, in turn reducing drag and increasing top end speed. This is all very well, but in a light boat with a lot of power it will take some skill to keep the boat under control. But then again, it should also mean that, for cruising at three quarter throttle it will be quite economical. More blades (such as the five-bladed High Five) will be best suited to skiing and especially wakeboarding, as the extra blade area without any increase in diameter will give the best acceleration, while keeping the boat on the plane at lower speeds and be an easy prop to use, with great grip in the corners. Obviously, it will have a slightly slower top speed and the extra blade area won’t help the economy either.

Cupping is the other main variable in the design of a prop, and an increase in cupping will decrease cavitation, be faster onto the plane and give better cornering. The act of adding cupping to a prop can also add up to an inch of pitch to a prop, potentially transforming an over-revving, poor handling prop into something that is perfectly matched to your boat.

Anodes
Anodes should be replaced regularly as they protect your prop and leg from corrosion. Having said that, even if your prop has been neglected and looks beyond rescue (see below), almost any damage can be repaired. Just £50 to £60 to fix a damaged stainless prop could save you that much in fuel in a season. And that’s to say nothing of the potential damage to your gearbox bearings over time from an unbalanced set up.  

Bush
The bush in a prop has a number of functions. It acts as a cushion for the gearbox, and also as a kind of ‘fuse’. If you are unlucky enough to hit something in the water that tries to stop the prop, the bush should give way before destroying the gearbox or prop shaft. The modern ones are easily removable and it is always a good idea to take a spare on the boat as well as a prop spanner, block of wood  (yes it’s okay to use that), and some pliers to deal with the lock tab. With this lot, plus a spare aluminium prop, you will still be able to get home should the worst happen - and that’s especially reassuring in a single engine application.

Removing the prop at the end of the season means that it won’t seize on the shaft, it won’t get pinched and it will give you time to inspect the blades and the bush in the relaxed luxury of your living room. The wife will love it.

Best for speed
Speed props can take a bit more effort to get going at the bottom end and can lack a little grip if you really enjoy throwing your boat around and playing hard with the throttle at low speeds. A fast spinning relatively low-pitched three-blader is a good place to start looking. Again, if your boat deserves it, the stiffer blades of a stainless prop will see you get that bit closer to realising the outright potential of your boat.

Best for cruising
With a cruising prop on a planing boat, you are looking for an all-round package, incorporating mid-range economy and easy boat handling. Make sure it’s not over-pitched, as you need the revs to be spinning close to the top end at wide open throttle. High speed props like Lasers and Rakers can be very effective for this job, providing tremendous efficiency at a solid and dependable three-quarter throttle cruising speed.

Best for sport
What you need for sport is hard acceleration for pulling people out of the water or towing inflatables and a good low-speed plane with plenty of grip. Look for a good stainless five-bladed prop and then play until you get the pitch right for your boat. The High Five will do a great job across the board for tow toys and watersports and only start to get a bit woolly when you get right up into the higher regions of your rev range.

FURTHER INFO

Steel Developments
02088 747059
www.steeldevelopments.co.uk