When I think back to the first year that I raced boats, I am amazed at my naivety. While I spent considerable time fussing over minor details on the boat, I clearly missed the three most critical ingredients to a fast boat - - run with the least weight, the highest horsepower and the most appropriate propeller. As a result, my first boat racing efforts succeeded in doing little more than hardening my determination to win.
While the technology and design found in today’s boats and motors have improved dramatically, the three key elements to performance have not changed. But today’s boat buyer is not likely to rip things out of his new boat to lighten the load or buy a costly racing engine just to improve the performance of the family runabout. That leaves the propeller as the most important performance item on a pleasure boat. Lets look at how easy it is for every boat to run with a propeller that maximizes its potential.
Prop Talk:
Learning basic propeller language is a prerequisite to locating the ideal prop for your boat. Don’t tell those prop folks that I told you this, but understanding propellers and their terminology is really very simple and it’s great fun to impress your friends by dropping a few terms like “rake” and “diameter area ratio”.
Propeller sizes are usually identified by two numbers with a ‘times’ sign between them. The first number is the “diameter” in inches of the circle cut by the blade tips. If your prop is two or four bladed, just measure the distance from one blade tip to the opposite blade tip. If your prop only has three or five blades, the easiest way to calculate the diameter, is to measure the distance from the center of the hub to the tip of a blade to get the radius, then multiply that number by two. The second number is the theoretical distance in inches that a propeller moves forward in one full revolution. (As if it was screwing itself through a solid.) Applying these numbers, a “14 x 21” prop has a diameter of 14” and a pitch of 21”.
The center part of the propeller is called the “hub”. This is the part that slides onto your “drive shaft”. On propellers where the engine exhaust flows through the propeller, as is the case with most of today’s outboards and sterndrives, there is a “barrel” around the hub onto which the blades are attached.
How do props work?
Propeller blades push water in one direction and the boat moves in the opposite direction. (‘For every action there is an equal and opposite reaction.’) Interestingly, as the blades spin and push water back, they also create a vacuum on the forward side of each blade. This vacuum pulls on the blade helping to move the boat forward. So strong is this vacuum that the extreme low pressure area can explode surface air bubbles with such a force as to chip the paint on your propeller. This phenomenon can occur when air is introduced through “cavitation” or during “ventilation”. Here is the difference between the two terms. “Ventilation” occurs when air bubbles running down the bottom of the boat or off the transom in a tight turn, or when air is pulled in by surfacing blade tips, finds its way into the propeller and “vents” it. “Cavitation” occurs when the propeller spins on its own accord, like a car tire on a slippery road, producing air on the forward side of the blade. The unmistakable symptom of ventilation or cavitation is a rapid over-revving of the motor and the correction required is to reduce throttle until the prop reconnects with the water.
What a difference a shape makes:
In prop lingo, a “cup” has nothing to do with hockey equipment, but rather a lip that is shaped into the trailing edge of a propeller. This lip both helps flick the water off the blade making its thrust more effective and improves the propellers grip in the water, thereby reducing slip. “Slip” is the amount of ineffectual spinning that occurs with a propeller and is usually measured in a percent. For example, if a propeller with a 25” pitch, rotated four complete times it should in theory move forward 100”. In practice it may only move 90” which would mean the prop has a 10% slip factor.
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Blades themselves can be shaped in several different ways. The most common shape is the standard “round ear” or elliptical blade. This shape delivers a good balance of thrust and speed. Other blades are “tapered”. These have less drag and are more high speed oriented. Some blades called “cleavers” or “semi-cleavers” are chopped flat and thicker at the trailing edge. This shape allows the blade tips to run closer to the surface without ventilating and have extra strength where the water is thrown off the blade.
If a blade sticks straight out of a hub, in other words, is perpendicular to it, that prop has “zero rake”. Blades can be designed for props with zero rake that produce “stern lift” for boats that squat too much when running. If the blade leans back towards the trailing edge of the prop, it is said to have “rake” and if it leans way back, it is a “high rake prop”. Such rake can be measured in degrees and generally, the higher the degree of rake the greater the “bow lift”.
A blade whose shape sweeps in a curve that follows its rotation is said to have a “skew”. This shape is ideal for props running where there is underwater growth, as they are less likely to tangle in the weeds.
Tough – tougher – toughest.
Early propellers were made of bronze. Bronze propellers are still used today and hundreds of different sizes are necessary for the wide variety of inboard applications. Nickel has become a popular strength additive and where used, these props are called Nibral. Inboard props are very specialized and not easy to find in the middle of a busy boating season.
As aluminum became more plentiful manufacturers found they could build aluminum props cheaper and faster using this material with the added benefit of weight reduction. Recent modifications and upgrades to aluminum designs and manufacturing techniques have resulted in some excellent results when comparing performance to costs. As a result, most small boats still come standard with aluminum props.
Marine grade stainless has become the preferred material choice where strength and performance are mandatory. With marine stainless steel being seven times stronger than aluminum, manufacturers are able to make their steel props thinner without sacrificing stiffness and still have props that are much stronger than most applications demand. Incidentally, unless you are running a solid hub, racing prop, stainless is not as big a danger to your drive as some folks will tell you. The reason is they have rubber hubs that spin or break if you hit something, just like their aluminum counterparts. When boaters buy stainless props, they generally want the metal polished to a shiny finish. Hence we have “high polished” props and the not so polished “satin” finish props. You may be surprised to know that there is no performance gain in polishing your prop. Stainless props are still the top end choice and generally cost a little more than twice that of an aluminum propeller.
The most recent material to be used to make propellers is plastic, or more correctly, high tech composites. With today’s advanced resins, nylon and carbon fibers, we see extensive use of composites in high-end boats and aircraft as well as in the propellers that they use. The better composite propellers have a strength factor greater than aluminum and offer other worthwhile benefits such as lifetime hub warranties, quick-change replaceable blades and of course they will not corrode or blister. This takes all the worry out of being close. The best news is that they are priced very close to aluminum.
How many blades do I need?
One of the most frequent questions I am asked about props is whether to go with three or four blades. There is no hard fast rule, but this explanation will help. As you fill in the area of the circle cut by your blade tips with either fatter blades or additional blades, you increase what is called the “diameter area ratio”. While more blade area adds to the surface area that is pushing your boat, more blade area also creates drag. Think of wider tires on a car and you have a good analogy. Since blades create drag, your starting point should be that the fewer blades the better, with the minimum of course being two. Over the past few years the average horsepower used in boats has increased dramatically, creating cavitation problems for two blade props. Accordingly, most boats today come with three blade props.
More recent hull designs have focused on reducing the wetted surface drag generated by their hulls. This has been accomplished by reducing weight through the use of lighter materials and composites and through hull design improvements such as adding “steps” to the bottom of the hull. This opened the door to the use of four bladed propellers.
If your boat and horsepower can handle a four blade prop, you will notice several benefits. Unlike three blades, a four blade will always have the same number of blades pointing up as it does down. This translates to a smoother operation, which is easier on both the boat and its passengers. You will definitely notice that a four blade will accelerate faster from an idle and the fourth blade seems to permit slower minimum on-plane speeds. As a rule, top speed will not increase and can even decrease slightly. But the biggest benefit to a four blade is a measurable fuel saving while traveling at cruising rpm levels. Lots of boaters switch to four blades for this feature alone.
So from my experience, the bottom line on the optimum number of blades is this: Boats over 23 feet in length such as light cruisers, generally will performance best with a four blade prop. The added drag is offset by the added thrust. Smaller boats that are heavy, or are used mostly for pulling can also perform better with a four blade. Aside from these situations, stay with a three blade and save your money.
In the second installment of ‘Propellers 101’, we’ll take a look at the most comprehensive propeller test ever conducted by a magazine and tell you how each manufacturer faired. Then we’ll provide a step by step guide as to how you can calculate the best size propeller for your boat.
Tricks
You want to go fast but can’t go up to the proper pitch because you need a lower pitch to get on plane? Go with a pitch that is as high as your motor can push and vent it with a tube that you run between the top of the leg and the tip of the propeller. Once running the air will not get into the prop blades and you will achieve the higher speed of the higher pitched prop.
All boats are designed to run their peak performance at a specific “attitude”, or angle between the hull and the water. Set the trim in your boat and then have someone else drive it past you at that setting. Ask yourself if it is operating at the attitude that it should. Play around with this a little and soon you will know if you need to change anything. Props can be purchased that will give you the bow lift or stern lift that you need to hit optimum attitude.
Trim
It is not possible to overestimate the importance of boat trim - -but that’s another story.
Also, I chose not to delve into the effects of various gear ratios, because while they play an important role, the ratios applied were a constant in our prop testing.
However, when it comes to props, for every rule there seems to be an exception. The only sure way to make your boat run at its best is to follow the theoretical guidelines that outlined in this article and then start experimenting.
Visit Ace Propeller for more information.
Bill Jennings also owns Canvas2Cover.com. Pick up a boat cover from the largest on line cover sales company.
