How Props Work
Have you ever wondered why there are so many different sizes and blade shape of propeller? Does it make any difference to the average boat user which propeller is used? Well the answer to both questions is a definite yes. The propeller converts precious power into forward motion. If it is mismatched or damaged in any way its ability to do this will be severely affected and this will reduce your boat's performance. There is no point in having an engine in tiptop condition if you run it with the wrong propeller. So, with so many different types and sizes of propeller available how do you choose the correct one? Unfortunately there isn't a simple answer. The following is only intended to be a rough guide to propellers, as it is beyond the scope of this article to cover every possibility, but you should begin to appreciate the different types and their applications. However at the end of the day it is always wise to seek professional advice.
Firstly a propeller will have two sizes, diameter and pitch, and are usually measured in inches. For example 13" x 19". The first dimension, in our example 13" refers to diameter and the second 19" is the pitch. Diameter is simply the distance across a circle swept by the extreme tips of the propeller blades. Pitch is the theoretical distance travelled for one revolution of the propeller, in this case 19". However as water is not a solid substance this figure is less, due to slip.
Propellers are either left hand or right hand. A right hand propeller will rotate to the right clockwise when viewed from the stern.
A propeller blade has a leading and trailing edge. The leading edge cuts into the water first. The trailing edge is the edge from which the water flows away. Many propellers have a slight upturn on the trailing edge, this is known as cupping. Cupping has the effect of reducing cavitation. Cavitation is commonly confused with bush failure, as the effects can be very similar, IE. Sudden rises in engine rev's and loss of speed. Cavitation is caused when the surface of the blades become covered in tiny vacuum bubbles. A smaller degree of cavitation will cause vibration and if left in this condition for long periods will cause "cavitation burns" which is where the vacuum bubbles implode with enough force to start sucking metal off the blade surface, obviously not a good idea. Severe cavitation can cause the propeller to "break away" completely thus causing the effect mentioned earlier of engine rev's suddenly rising. The reason for cavitation is using either damaged or heavily fouled blades or overloaded blades due to incorrect propeller type/size. E.g. Too high a pitch for the engine power / boat speed.
So knowing these basic facts how do you determine what size propeller you need? You may think that a propeller with a very high pitch will give the fastest speed, and to some degree this is true. However pitch is directly connected to engine rpm. The higher the pitch the less the engine will rev. So if your engine should rev to say 5,000 rpm but you can only achieve 3,000 rpm then the pitch is too high. If you lower the pitch so you can get 5,000 rpm you will actually achieve a faster top end speed than with a higher pitched propeller. The equation is simple, you must achieve maximum rpm. This equals maximum horsepower development and therefore optimum performance. Under certain circumstances you may want to fit a propeller that will enable you to gain more rev's. If you are using your boat for water sking or wakeboarding for example, a propeller with a lower pitch will give increased acceleration and bite at lower speeds, ideal for pulling up waterskiers etc. However you must be careful not to exceed the recommended maximum rpm of your engine. Generally when you use a boat for skiing you do not run flat out so this is not a problem. As a rule of thumb the higher the pitch, the faster the top end speed but the lower the rev's. The lower the pitch the faster the acceleration and the higher the rev's.
The next consideration is the number of blades. Surprisingly the most efficient number of blades is one. This is because the single blade has nothing to disturb the water flow ahead of it, but in practice this is not realistic as a single blade would be impossible to balance. The next most efficient is a two blade, but this can be impractical as generally you would require a propeller with a very large diameter to get enough blade area to generate sufficient thrust. For this reason most propellers have three blades. Some propellers have four or fiveblades (or even more in some cases). These propellers are useful for two reasons. Firstly they reduce vibration will give a much smoother ride. Secondly they increase acceleration for the same given pitch. This is why boat owners who want increased acceleration, turning abilities and to maintain planning at lower speeds for skiing will often choose a 4 or 5 bladed propeller. Similarly 4 or 5 bladed propellers are very good for cruisers who want relaxed and smoother cruising. All propellers are a compromise and the choice of a 4 or 5 bladed propeller will reduce top end speed as the drag caused by the propeller increases. The drag factor is one of the reasons why surface piercing propellers are very efficient. They are designed to run with the water line at hub level. This keeps the majority of the blade area clear of the water and this reduces drag to a minimum and propulsion at maximum. However they are not suited to many applications as they can take longer to get the boat on the plane and they can cause manoeuvring difficulties, as surface propellers are notorious for poor astern characteristics.
A propellers blade shape will affect its performance. Most outboard and sterndrive propellers are constant pitched across the entire high-pressure face. Some are progressively pitched. A progressive pitched propeller is less prone to cavitation and will give increased acceleration and general performance. This is because the leading edge will be a lower pitch so the water is picked up under less pressure. The water then flows along the blade surface increasing in pitch to the trailing edge where it is forced into a tighter thrust funnel and this gives higher efficiency. Some propeller have blade rake, rake is when the blades lean or slop either forward or aft as viewed from the side. Blades that slop aft have positive rake and this will give the effect of increased top end speed, but less acceleration and is generally designed to increase bow lift. Blade area has an effect on performance, propellers with relatively high blade areas will have reduced blade loading and therefore suffer less from cavitation. Blade shape also has and effect. Generally pointed blades, such as cleaver types, will give stern lift. Rounded high rake blade shapes generally give bow lift. Some propellers have vent holes or slots at the root of the back face of the blade. This introduces air from the exhaust to the propeller, which reduces cavitation. Since cavitation is air bubbles in vacuum the aeration from the exhaust gases helps prevent cavitation from occurring. This is called ventilation i.e. air introduced to the propeller blade. These propellers are generally constant pitched. The latest and most efficient propellers are progressive pitched. These are less prone to cavitation and therefore do not generally require vent holes or slots as the blade loading is significantly reduced.
Surprisingly the material the propeller is made from also affects performance. Most outboard and sterndrive propellers are made from aluminium. Fitting a stainless steel propeller will increase performance. The reason for this is simple. Stainless steel is a much stronger material than aluminium which enables the propeller to be manufactured with a thinner blade section. If the leading edge is thinner less air bubbles will be formed on impact and thus cavitation is reduced. A thinner blade is much more efficient that a thicker one. For racing applications the blade section can be thinned to such an extent that you could literally use the propeller to have a shave. For most applications such a thin blade section is not desirable as it is more prone to failure under stress. For the enthusiastic boat owner, simply changing from an aluminium propeller to a stainless steel one will give improved performance. For significant improvements try a stainless steel progressive pitched propeller with cupping.
For outboard and sterndrive propellers, diameter is related to pitch, generally the lower the pitch the larger the diameter and vice versa. The reason for this is generally boats that require low pitches are slower and heavier and they need propellers with a larger diameter to give improved thrust. Faster lighter boats need less thrust to achieve planning speeds and therefore less diameter but larger pitches to give optimum performance.
In conclusion, when you decide on a propeller size you need to carefully consider how you intent to use your boat. Are you looking to tow skiers, or do you want out and out speed. The same propeller cannot deliver high speed and maximum acceleration. You can either go for a compromise, using and in between propeller. or have a selection, so you have the ability to change propeller according to boat loading and conditions. As it is always advisable to carry a spare propeller the latter is the best option. Once you have a propeller it can be "fine tuned" to your boat set up. It is possible to alter the pitch on your existing propeller up to a maximum of one inch or any variation in between, to gain or loose those extra revs. You must keep your propeller in good condition. If it suffers any damage, however minimal it may seem, it will severely affect performance. You should have your propeller professionally repaired and serviced. Trying to do a DIY job will probably cost you more in the long run and you may even render the propeller beyond repair. Any repair costs will easily be saved by reducing fuel consumption and keeping the propeller as efficient as possible. You can increase your boats performance significantly if you have the best propeller suited to your application. If you are unsure, or want advice on any propeller related questions do not hesitate to contact a propeller specialist.