Propeller
By Perry Mack
To some extent it’s also important to know what your main use for the boat is. Do you need low end pulling power to pop skiers and wakeboarders out of the water or are you looking for top end speed to get home before the sun sets or to the hot fishing spot to win the derby. I know you wouldn’t be rushing to make last call at the yacht club… ?
Let’s get some basics out of the way. Prop size is typically defined by diameter and pitch. You get the diameter by measuring from the centre of the hub to any blade tip – times 2. Too small and your boat is underpowered, too big and the increased drag slows your boat down (because you ain’t got the horsepower to turn the bigger prop) Pitch is more difficult to calculate – it’s the forward movement of the propeller through one revolution.
Typically, smaller boat – smaller prop, bigger boat – bigger prop. Lower pitch gives you more acceleration and pulling power (think low gear) while higher pitch gives more speed. Now before you go out and buy the highest pitch prop you can, remember that you have to keep to engine rpm within the recommended range at Wide Open Throttle (WOT) or you could see pistons popping like fireworks and an engine that looks like a bowl of metal granola inside.
Props also typically come with either 3 or 4 blades. Without getting into the hydrodynamics (yawn) – a three bladed prop gives you a little bit higher end speed, it’s slower to plane, there are usually more pitch and style options and they are typically cheaper than 4 bladed props. Conversely the four blades will give you a faster hole shot and reportedly better performance and fuel economy at mid-range rpms, better holding power in rough water and better low speed handling.
Choosing between an aluminum or a stainless props isn’t too difficult as long as you understand yourself and your budget. Aluminum is cheaper – cheaper to buy & cheaper to repair (providing you don’t damage it too often) and in a classic case of ‘you get what you pay for’ – worse performance.
My suggestion is you buy both. Use the stainless prop as your main prop - pick the pitch, diameter and number of blades for the way you boat most of the time. Invest in an aluminum prop for your other style of boating and keep it as a back-up on the boat. Think about it. If you damage your prop and it ends your day – sunshine, boat full friends and family on vacation – add in the cost of the repair and you will be truly miserable. If you can salvage the day with a back-up prop it will be a lot less painful.
Modular Composite Props
The concept is simple. Have removable blades attached to the hub so that in the event of a prop strike you simply replace the damaged blade from your spare set. You can also purchase blades with a different pitch – swap out the blades instead of the whole prop. The blades are typically composite which means that they can’t be repaired only replaced. Performance is slightly below the best stainless steel props but the cost to purchase and repair are lower. Essentially they make a good performance upgrade from aluminum props without the expensive of going to stainless.
Replacing Your Prop
If you’re happy with your boat’s performance, get a prop with a similar pitch and diameter. Maybe upgrade to stainless or go with 4 blades. If the old blade is salvageable – get it repaired and keep it as a spare.
If you’re not happy with the performance, change it. Rpm and pitch have an inverse relationship (sounds like a marriage). If you increase pitch, you decrease rpm and conversely if you decrease pitch you increase the rpm.
Let’s try this out in an example. Imagine you have an outboard and your redline is 5300 rpm. At WOT, your engine revs at 4900 rpm. Since a change of 1” pitch usually changes your engine speed by 200 rpm, you buy a prop with 2” less pitch to bring your rpm at WOT to just 100 rpm shy of redline. In this case, you should get more acceleration and odds are your top end speed will stay the same (maybe get better) since your engine kicks out more horsepower closer to redline.
If you have specific questions about your particular boat or prop, visit your local qualified boat dealer or marine repair centre and get the best props for you and your boat.