It is possible, with empirical tests, to determine if a certain propeller fit a certain boat, applying simple but reliable rules of thumb, which can be useful for the serious yachtsman wishing to improve performances when steering under power
THE PERFECT PITCHMost of sail boaters, use their boat in the summer time when, nobody knows why, every course is always against the wind. I have noticed, thanks to my 20 years experience sailing across the Mediterranean covering more than 20.000 miles, that exist an average ratio of 1 to 2 relating under sail and under power mileage: one who cover 1.000 miles, has surely steamed 500 miles. All this highlight how important it is, on sailboats to have, a reliable and powerful engine, and to choose the correct propeller in order to optimize enginès working condition (which will then surely last longer).
Sails-men, unfortunately, seldom thinks about "pitch" and "diameter", and very few know the maximum allowable rpm for their engine, both in neutral and under way. On the other hand, the related technical literature is not an easy-to-use reference, at least for the average boater, and, however, it does not always answer to all questions. In fact a universal theoretical rule to apply to any floating body does not exist, due to the incredible amount of factors influencing propeller selection, such as enginès power, shaft speed, boat's dimensions and weight, hull's geometry...
This article is not for those seeking the latest high-tech engineering on propeller design, rather is for the self-made yachtsman wishing to find the right propeller for his boat.
There are several different types of marine propeller, and numerous technical terms indicate their behavior, such as: blade section (ogival or airfoil), positive, negative or no rake, trailing and leading edge shape and so on, but lets let them to propeller's manufactures.
For our purpose, however, only two factors are really important: diameter and pitch.
Diameter: it is the diameter of the circle swept across the extreme tips of the propeller blades. Shaft speed (usually engine rpm divided by the reduction gear ratio) and SHP are the factors influencing the diameter. SHP (Shaft Horse Power) is the power actually delivered from the engine to the shaft thus to the propeller, about equal to the BHP (Brake Horse Power, meaning the maximum engine horse power as tested at the factory) minus about 3% of power loss at the gearbox and 1.5% per bearing. Generally the larger the diameter the greater the propeller efficiency.
Pitch: it is the distance a propeller drives forward for each complete revolution, assuming it is moving trough a solid element, just like a wood screw does. For instance, if the propeller cover 100 millimeters per turn through a solid, then its pitch is 100 millimeters.
There are three main propellers' families:
Constant pitch propellers: this type of propellers blades are welded to the hub, and their pitch, as suggested by the name, is fixed. Their structure is surely the stronger, because they are manufactured from a single casting, usually through CAM (Computer Aided Manufacture) assisted machinery and they have no moving parts. Such propellers, usually have a 50 % efficiency loss in astern motion, and are not suggested for sailboats, due to their excessive drag under sail.
Folding propellers: they have folding blades; under sail the hydrodynamic pressure keeps them closed, thus considerably reducing drag. Their astern maneuverability is poor.
Controllable pitch propellers: in this type of propellers, the user can modify the pitch, while underway, by mean of a hydraulic mechanism or a direct mechanical linkage. Feathering propellers, in particular, are a special controllable pitch propeller type, ensuring low drag, because of their characteristic blade design.
Controllable pitch propellers are very practical because by modifying the pitch they allow for thrust optimization under different load conditions. Most modern sailboats are fitted with this type of propeller. Lets discover together how to use it.
For the majority of engine and propeller manufacturers the ideal propeller will cause a loss of 5 to 10% in engine maximum revolution per minute; if, for instance, the engine rated maximum rpm are 3600, the loss will approximately be 200 rpm, in calm sea, with no wind, with no overload on board and with a clean hull bottom, while it will be about 360 rpm in rough sea, strong wind etc...
If the total actual loss is bigger, then the propeller is "overloaded" and so is the engine, while if the propeller is turning too fast it is "under-loaded" and is not using all the engine power. On the other hand someone believes that one should keep the pitch as long as possible in order to achieve the cruse speed at lower as possible rpm.
For example, lets suppose that a 6 knots cruise speed is reached at 2800 rpm. Increasing the pitch (and of course keeping the diameter constant) the same speed could be registered at 2000 rpm. In this case, advantages are: lower engine speed, less shaft vibration, less noise thus longer engine life.
The question is: which is the right choice?The "high pitch and low rpm" solution , although appearing interesting, is not the correct one. The engine is actually running slowly, but it is overloaded thus lasting shorter, much shorter than an engine running faster but with less "job" to do. This is due to the higher stress concentration on the engine pistons, crankshaft and bearings, which can lead to some serious damage such as engine seizing. Having an "overloaded" engine and propeller is just like someone driving on a steep mountain road on the fifth gear instead of the third: the engine is overheated, the speed does not increase and fuel consumption is higher.
On the other hand the "5 to 10% loss on top rpm" rule will surely not overload the engine, while it will generate noise and the transmission gear will be in danger. The propeller will turn faster, thus increasing shaft and bearings vibrations.
In my experience, the ideal solution is an average of the two and can be obtained with practical tests.
The first thing to do is to find in the owner's manual at which rpm the engine reaches its maximum power (BHP). Lets say, for example, that the maximum power is obtained at 3600 rpm.
Then we have to check which is the actual rpm reached by the engine, accelerating in neutral. If a 3700/3750 rpm are achieved, everything is fine, if not you have to adjust your revolution counter to that value (in fact, and normally, an engine should increase, in neutral, 3 to 4% its maximum rated rpm, because, usually, the manufacturer takes into account the loss due to the reduction gear). All this is applicable to all well maintained engines, and in particular to those with clean fuel filters and perfectly working injection system. This means, for instance, that an engine which has lost compression will not achieve its top rated rpm. Once the revolution counter has been verified, we can start the trial which will allow us to know if and at what rpm our engine is overloaded.
The sea state must be calm, and no sail should be up. Keeping a constant route, we have to increase engine speed with a 200 rpm step. We will plot, for each rpm range, the boat's speed, observed at the LOG (GPS could be too inaccurate for this purpose). Speed should increase constantly for each rpm range. Meantime, we should check exhaust water and fumes color, which must not change. If speed does not increase constantly or does not increase at all, then the engine is overloaded (be sure that you have not reached the hull speed); exhaust fumes quantity and water color will proof the overloaded engine condition. In fact, increasing engine load, quantity, density and color of both exhaust fumes and water will become darker and darker, till they rich a black color, meaning pitch is too long. In this situation, increasing rpm will not increase speed, some of the fuel will not be burned and fuel consumption will increase without benefits.
The same test should be carried out with rough sea and wind and the results plotted; these will indicate if your propeller's pitch is correct or if it should be increased or decreased.
Then lets check again the enginès owner manual, where we will find the maximum horsepower output and the hp/rpm ratio. Lets, now, find the best hp/rpm ratio.
We will assume our engine will deliver the maximum horsepower output at 3600 rpm, and that a 2 hp power increase is attained for every 500 rpm till 2800 rpm, then 1.5 hp till 3200 rpm and then 1 hp till 3600 rpm. The best hp/rpm ratio is at 2800 rpm.
We know that cruise engine speed is 20% less than its maximum speed (3600 rpm): the closest we go to this value the better is our propeller pitch.
For instance, if our engine has its maximum efficiency at 2800 rpm and its maximum full ahead rpm are respectively 3500 in calm sea and 3300 in rough sea, than our pitch is correct (3500 rpm minus 20% equals to 2800 rpm). This is true if our test result confirm that the engine has not been overloaded in the 0 to 2800 rpm range, otherwise the pitch has to be reduced.
I have carried out some tests with my Panda 31, a sailboat 9.60 meters LOA, fitted with a 38 centimeters diameter Max Prop, and a Buk engine with 24 hp at 3600 rpm with its best hp/rpm ratio in the 2400-2600 rpm range. The reduction gear ratio is 2:1, and the hull was just dry-docked. Tests were carried out on a distance base; mechanic Gianni Magurno from Buk assisted me. Later I performed long distance tests, covering more than 2000 miles and evaluating both plusses and minuses of each solution.
Here are the resultsFirst trial: blade angle 26°, for a 350 millimeters pitch. Engine speeds are 2500 rpm in calm sea and 2400 in rough sea. Top speed is 6.5 knots. Cruise speed in calm sea is 5.5 knots at 2000 rpm. The engine is noisy and clearly overloaded. At 2200 rpm, exhaust fumes and water are dark.
Results: pitch is too large. Engine power is not completely used and, in fact, speed do not increase in the 2200-2500 rpm range. The boat is too fast at slower rpm. A counterblow can be easily felt when inserting the gear.
Second trial: blade angle 20°, for a 260 millimeters pitch. Engine speeds are respectively 3450 rpm and 3300 rpm in calm sea and rough sea. Top speed is 7 knots. Cruise speed is 6 knots at 2800 rpm, in calm sea. A good power reserve is available in rough sea. At cruise speed the engine is noisy. Exhaust fumes and water are dark at 3400 rpm.
Results: the pitch is in accordance with the famous "5 to 10% loss on top rpm" rule. In fact the rpm loss is respectively 5% in calm sea and 10% in rough sea. Now the engine is less loaded, cruise speed has gained half a knot, but it is achieved at 2800 rpm, thus increasing noise and vibrations. The enginès best hp/rpm ratio is not achieved.
Third trial: blade angle 22°, for a 290 millimeters pitch. Engine speed is 3200 rpm in calm sea and 3000 in rough sea. Cruise speed, in calm sea, is 6 knots at 2600 rpm. Speed increase constantly till 2800 rpm, for a maximum speed of 6.5 knots. Dark exhaust fumes and water appear above 3000 rpm.
Results: among the three sea trial, the best one was the third, which feature the higher speeds, especially in calm sea. Optimum sound and vibration limits were achieved. In rough sea I would have appreciate more power reserve; however even on most severe conditions I never had to run above 2700 rpm, and just in case I got some help from the jib.
In closing, the purpose of this exercise is to give suggestions to those yachtsmen wishing to identify their boat's most appropriate pitch, to suit their own requirements.
Now all you need to do is try, without forgetting attention and patience. Have fun!