Propellers

PROPELLERS AND PROPULSIVE EFFICIENCY.

The Powerboat Achilles Heel.

In the past virtually all people intent on cruising the worlds oceans or even making long coastal voyages did so in sail boats. In a purely economic sense voyages were made cheaply albeit with the attendant discomforts and slow passage times. About the only running costs were on sail and rigging maintenance.

Increasingly powerboats are now cruising the same oceans. Their means of locomotion is mechanical, burning fossil fuel, resulting in a sharp rise in comparative voyage cost. Marketing literature generally focuses on range and miles per gallon of fuel consumption. These statistics, although important, tend to ignore the actual specific efficiency of their propulsion installations.

The purpose of this article is to give trawler yacht owners and those considering purchasing a vessel for long range cruising a reasonable understanding of the factors affecting the propulsive efficiency of propeller driven craft and the means of making a comparison between different designs. In particular screw propeller design and the means of power transmission are covered in some detail. These factors have the biggest influence on the propulsive efficiency of power craft and are about the only area where improvements are possible. Of necessity we have had to resort to what many readers may consider an overly technical approach, hopefully we may be forgiven for this.

The overall thermal balance of a powerboat is in reality something of a horror story. Even in a well engineered vessel, of every 100 horsepower worth of fuel poured into the fuel tank barely 16 horsepower is available as power to overcome the resistance of the vessel. Fully 84% of the power is lost along the way.

These loses can be divided broadly into two sources:

(1)               Losses in power generation.

(2)               Losses in power transmission.

Losses in power generation are the total losses in a diesel engine in generating its flywheel Brake Horsepower. Of our 100 HP worth of fuel supplied to the engine loses occurring amount to:

29        HP in cooling the engine.

3          HP in the after cooler.

4          HP in the oil cooler.

22        HP in the exhaust gases.

4          HP in heat radiation from the engine.

Leaving 38 BHP available at the flywheel.

Virtually none of these power generation losses are practically recoverable in a small trawler installation. Attempts have been made in large ship engines to mitigate some of these loses, notably in the Still engines of the 1920s. A brief description of this unusual machine is given at the end of this article.

Losses in power transmission are the total losses between the engine flywheel and the effective propulsion of the vessel.

In a well engineered conventional shaft drive, of the 38 BHP available at the flywheel after generation loses, the following loses occur:

3% 1.13 HP In the reduction gearbox
2% 0.75 HP In rotating the propeller shaft
44.5% At the propeller
8.5% 3.22 HP Through propeller/hull interaction

Leaving 42% or 16 HP available to overcome the resistance of the Hull.

This is termed the Effective Horsepower (E.H.P)

The ratio            is called the Propulsive Coefficient (P.C). In this case P.C. = 0.42.

For a trawler yacht a P.C. of 0.42 is considered above average, a P.C. of 0.33 poor and a P.C. of 0.55 rarely achievable.

Fortunately, unlike the generation losses, it is open to the powerboat designer to minimize some of these loses, notably the huge loss at the propeller and the others to a lesser degree.

Sadly in many trawler designs insufficient emphasis is given to maximizing their propulsive coefficients. These vessels have to labour under this handicap for the rest of their lives.

We shall now discuss in detail each of these factors making up the propulsive coefficients.

Propeller Design.

Except perhaps for paddle wheels the fixed pitch marine screw propeller is by far the most efficient form of propulsion. Screw propellers can have a peak efficiency of 75-80%

In most Trawler Yacht installations however even 50% efficiency is rarely achieved. This state of affairs is caused for four main reasons: -

(1)               Restricted draft

(2)               Restricted propeller diameter

(3)               High propeller RPM

(4)               Poor hull form

The propeller efficiency for any particular installation can be quickly and simply determined by calculating the vessels Power Coefficient (Bp). This coefficient is a good basis for comparison between different designs. Using an ordinary pocket scientific calculator which has all the mathematical functions required, a reader may calculate the Bp for their own or any vessel they may be considering.

Bp = DHP x N

Va 2.5

DHP    is the horsepower at the propeller and for conventional shaft drives with a reduction gear can be taken as 95% of B.H.P i.e.: a 3% loss through the gearbox and a 2% loss in the propeller shaft as shown in the transmission loss table.

N         is the propeller R.P.M which is the engine r.p.m. divided by the gearbox reduction ratio.

Va        is the speed of advance in knots, and is typically 80-90% of vessel speed for a single screw vessel. This phenomenon is caused by the passage of the hull through the water imparting a forward velocity to the water in the vicinity of the propeller. In other words the screw in effect operates at a lower ahead speed than the actual boat speed. Va is mainly dependent on the vessels block coefficient (Cb). The higher Cb is the lower Va is and vice versa. For the purposes of this calculation Va can be taken as 0.90 x boat speed.

For a well made four-blade screw propeller the Bp-efficiency relationship is shown in table 1:

Bp Propeller Efficiency
5 79%
10 72%
15 66%
20 62%
25 59%
30 56%
35 54%
40 53%
45 50%
50 47%
55 44%
60 42%
65 40%
70 38%

Table 1.

It can be seen from the table that the lower Bp is the higher the propeller efficiency.

For a Trawler Yacht as a rule if a Bp of 45 is not attainable the whole propulsion system should be reinvestigated.

In the equation Bp = DHP x N

Va 2.5

It can be seen that to minimize Bp either the numerator ÖDHP x N must be reduced or the divisor Va must be increased. As the B.H.P of the engine is fixed for the particular vessel this leaves us with the two variables N and Va.

As previously mentioned Va is determined by the hull form and can not be increased short of a complete hull redesign. This leaves us with N the propeller r.p.m as the only variable available to improve the propeller efficiency.

Where Bp is found to be less than 45 (ie propeller efficiency less than 50%) N should be decreased to achieve at least this figure. Readers may well ask why not simply reduce N until Bp corresponds to a propeller efficiency of say 72% for example. The reason this is not possible is that the propeller diameter increases with a reduction in Bp and eventually reaches an impractical diameter for the vessel.

To illustrate this we shall take an actual example. Our W54 Trawler Yacht has the following propulsive characteristics:

Main Engine                            250 B.H.P @ 2100rpm

D.H.P                                      237 B.H.P

Reduction Gear Ratio             4.5 : 1

“N” propeller rpm                     467rpm

Propeller Dia.                          48”

Bp power coefficient               39

Propeller efficiency                 53%

P.C.                                         0.42

V                                              8.90 knots

Va                                            8.01 knots

Draft                                        7ft

For this particular vessel the Bp, propeller efficiency, rpm and screw diameter relationships have been calculated and are shown in table 2.:

Bp Propeller efficiency N (rpm) Propeller diameter
5 79% 60 149”
10 72% 120 100”
15 66 180 81”
20 62% 240 70”
25 59% 300 62”
30 56% 350 57”
35 54% 420 53”
40 53% 467 48”
45 50% 529 46”
50 47% 590 44”
55 44% 645 42”
60 42 700 40”
65 40 763 38”
70 38 825 37”

Table 2.

From table 2 it can be seen 72% propeller efficiency can be achieved by reducing propeller rpm from 467 to 120 rpm corresponding to a new Bp of 10. However the propeller diameter has increased to 100". This is obviously quite impractical because it would involve an increase in draft from 7ft to over 12ft and a 17.5: 1 ratio reduction gear would need to be fitted. Gear ratio’s available are generally limited to about 6:1.

Although this is an extreme example the table does illustrate dramatically the price to be paid for fitting small diameter, high rpm propellers in vessels of the trawler yacht class.

As the above example shows there is a limit to the improvements that can be made by adjusting the variables in the power coefficient. Is there any other means available to improve the situation?

For vessels with very high Power Coefficients (Bp > 50) it may be worth considering fitting a propeller nozzle to the vessel. These are essentially a ring aerofoil shroud fitted around the propeller. They have the effect of accelerating the flow into the propeller disc and thereby increasing Va. Because Va is raised to the power of 2.5 in the Bp equation we can see that it can result in a marked reduction in Bp.

Interestingly, the nozzle itself generates the increased thrust and not the propeller so there is no nett drag associated with the nozzle even at free running speeds.

The beneficial effects of propeller nozzles were discovered almost by accident in the 1930s. A German Engineer, Kort, when investigating ways of reducing propeller wash erosion of canal banks decided to try fitting a shroud around a propeller. Instead of a simple steel cylinder he had the shroud fabricated with an aerofoil shape to reduce its resistance. Immediately it was noticed this devise improved the propulsive performance of the vessel.

Tugs and Trawlers are now almost universally fitted with nozzles because of the marked increase in propeller efficiency over an open screw. Other benefits obtained are a reduction in propeller noise and vibration along with the protection they give to the propeller itself. Nozzles need to be fitted with a special square tipped “Kaplan” propeller (which would be more properly described as an “impeller”). Figure 1 illustrates a twin screw nozzle installation on a 50ft Tug of our design.

image2

Note the added lower strut and guards. These were fitted as protection for the nozzle and propellers as this vessel routinely works over coral reefs.

We have now established the base parameters needed to design the propeller. Propeller design is a huge study in itself and is beyond the scope of this discussion to go into the theory of their hydrodynamics. Trawler designers must base their propeller designs on a range of Standard Series charts that have been published by various research institutes over the years. It is not really possible or indeed wise to attempt to design a Trawler Yacht propeller from first principals. In the case of large ship propellers the screws are often individually designed after exhaustive model testing of the hull to establish the flow conditions in way of the propeller. Such testing is not justified for Trawler Yacht propellers.

Each Standard Series of design charts are derived from the results of tank tests of a family of model propellers having the same familial geometric properties. These model propellers were typically 10-20” diameter. They were run in a cavitation tunnel where the R.P.M, input torque, thrust was measured for each variation of the pitch, blade area and input velocity (Va). These results were cross plotted, corrected for various scale effects and published as charts based on Bp versus Pitch Ratio (P.R) for each blade area ratio in the series. Figure 2 illustrates a typical example of the design charts described:

image4

The principal charts of interest for trawler yacht propeller design are those published by the following research establishments:

(1)               Taylor Series – Washington, U.S.A

(2)               Gawn-Burrill Series – University of Newcastle on Tyne

(3)               Troost-“B” Series – Netherlands Ship Model Basin

The main variables in any particular series are:

(1)               Blade Outline Shape

This outline may be symmetrical about the blade centerline or skewed where the outline is asymmetrical about the blade centerline.

(2)               Blade Section Shape

These may be:

(a)               Flat face segmental type where the driving face is flat and the back is a circular segment.

(b)               Aerofoil section where the sections resemble the shape of an airplane wing.

(c)               Composite sections where the inner blade sections are aerofoil shape and the outer sections are segmental.

(3)               Blade Area Ratio (B.A.R)

This is the measure of the blade size and is the ratio of the total blade developed area and the swept area.

The Taylor and Gawn-Burrill series are of the symmetrical type with flat face segmental sections. The Troost B – Series are of the skewed type with composite sections.

The following propeller drawings Figures 3 and 4 illustrate the features of these two types:

FIGURE 3
FIGURE 4


Any one of these types are suitable for Trawler Yacht propellers however the Troost “B” Series shows a slightly higher efficiency and is usually the type we specify despite their higher cost.  The higher cost is due to the more complex blade section shapes that have to be cast and finish machined.

Some propeller manufacturers offer proprietary types for which claims of superior performance are made. Such claims should be treated with a degree of skepticism unless the manufacturer can show proper test results for his propeller.

Choice of Propeller.

Single screw installations are best fitted with four blade propellers with some degree of blade skew as they are better suited to the disturbed inflow conditions experienced in centerline propulsive systems. For twin screw installations three blade propellers are the best option although if accommodation spaces are located above four blade screws should be considered, as they are less prone to vibration.

Once the blade number is decided and having calculated the Power coefficient the designer is able to calculate the Pitch, Diameter and efficiency from the Standard Series Chart. Usually the designer must design the screw using a higher power coefficient than that derived for the calm water speed of the vessel. This is to provide a weather margin for the vessel. If there is no weather margin the engine may become overloaded in increased sea states. The designer must us his judgement in this respect but typically a weather margin of 5-10% should be allowed for in a Trawler Yacht.

Having determined the main particulars of the screw a cavitation check has to be made on the chosen propeller. Cavitation is a very complex phenomenon where the trust loading peaks on the blade surface exceed the vapour pressure of the water. Cavities form in the zone of the lowest negative pressure and continuously collapse and reform. This can lead to noise and erosion of the blade surface and in some cases severely effect the performance of the propeller. The only practical approach to this problem is to adjust the blade area to limit the mean blade pressure to a figure the designer judges sufficient to provide a margin against cavitation. This blade pressure depends mainly on the rpm and depth of immersion of the screw in each particular case. Figure 5 illustrates a cavitating screw captured by high-speed camera in a test tank.

image10 image12
image14 Photographs from cavitation tunnel showing a progressive decrease in amount of cavitation, at a constant cavitation number and speed, with increase of blade area.

FIGURE 5.

Propeller Manufacture.

Propellers should always be manufactured to ISO 484 Class 1. This standard stipulates close tolerances for diameter, pitch, blade section shape and balance. Time and again we have seen poor performing propellers where on close inspection the cause has been bad tolerancing rather than mistakes in the original design.

Propeller and Hull Interactions

The propeller efficiency we have discussed up to now is for the screw running in open water in a test tank, ie behind a “phantom” ship. When the propeller is placed behind the hull loses occur through the interaction between propeller and hull. Not only does the propeller itself suffer a loss in efficiency because it is now rotating in a disturbed water flow but the low pressure in front of the propeller imparts a low pressure on the hull surface immediately ahead of it. This creates an imbalance between propeller thrust and hull resistance. In other words the propeller must develop a greater thrust than the hull resistance to overcome this imbalance. The increased flow of water over the rudder due to propeller action has a similar effect by greatly increasing the resistance of the rudder.

The exact losses due to propeller – hull interactions are not directly calculable but can amount to 10% of the installed BHP. The following precautions should be taken to minimize them:

(1)               Avoid high block coefficient (Cb) hull forms.

(2)               Avoid thick unfaired deadwoods in front of propellers

(3)               Never use crude flat plate rudders. Always fit double plate rudders with streamlined aerofoil sections.

(4)               Ensure propeller tip and sternpost clearances are generous.

Shaft and Reduction Gear Losses

In the example used we have shown that for a conventional shaft drive and reduction gearbox 5% of the BHP is lost in transmitting the power to the propeller. This has resulted in a propulsive coefficient (P.C) of 0.42

Conventional shaft drives are still by far the most efficient system for power transmission. What effect do the more novel forms of transmission have on the propulsive coefficient?

(1)        Hydraulic Drives

As well as being inordinately complex these systems can result in a 20% loss in BHP in transmitting the power to the propeller. This would result in a fall in P.C. from 0.42 to 0.35. In other words in our example the installed BHP would have to be increased from 250 to 290 BHP.

(2)        Z Drives

These stern leg drives were primarily designed for harbour tugs and the like where maneourability considerations outweigh any question of propulsive efficiency. Although their mechanical efficiency approaches that of a conventional shaft drive their great disadvantage is that their stern leg and large propeller pod, which are rarely properly streamlined, add greatly to the hull resistance. In this case our P.C. would probably fall from 0.42 to 0.38.

(2)        Variable Pitch Propellers

There are no propulsive efficiencies to be gained with the use of these. They have lower propeller efficiencies than the equivalent fixed pitch propeller. The principal causes of this are the large boss diameter needed to house the pitch actuators and the helicoidal blade surface is only correct for one particular pitch setting. These propellers are excellent for their intended purpose i.e. in tugs and fishing vessels were full engine power is required at both towing and free running speeds, or in installations where the main engine speed is fixed and vessel speed is varied with pitch control. None of these considerations really apply to a long-range trawler yacht. The mechanical efficiency of V.P. propellers the same as our conventional system but the poorer propeller efficiency could lower our PC from 0.42 to 0.39.

In conclusion it should be pointed out that hydraulic drives, Z drives and V.P. propellers are very much more expensive devises than a conventional shaft drive.

Conclusion

Readers may well ask what does all this mean in dollar terms?

We have demonstrated that the ingredients for good propulsive economy of a trawler yacht are large diameter slow turning propellers which implies deeper draft and propulsive economy go hand in hand.

Therefore if we take an actual example we can demonstrate the penalty to be paid for restricting the draft of a vessel.

Our W72ft trawler yacht, hull No1 (Figure 6.) has the following propulsion characteristics:

B.H.P                                       230 @ 1150 rpm

D.H.P                                      218

N                                              278 rpm

V                                              9.665 knots

Va                                            8.70 knots

Bp                                            18.40

Propeller Efficiency                 63%

P.C Propulsive Coeff.             0.51

Draft                                        10ft

Propeller Dia.                          64”

FIGURE 6

Note the independent wing “get home” propulsion with self feathering propeller.

Say for instance that a prospective client wants the draft reduced from 10ft to 8ft. This will require a reduction in propeller diameter from 64” to 48”.

From studying the propeller chart applicable to this vessel we find that to reduce the diameter to 48” we must increase the rpm from 278 to 500rpm. This corresponds to a new Bp of 35 and from the efficiency table we find the propeller efficiency has fallen from 63% to 54%.

Calculating the Propulsive Coefficient (P.C) in the normal manner we see this has fallen from 0.51 to 0.436. In other words the installed B.H.P must be increased by 0.51 divided by .436 (17% from 230HP to 268HP) to maintain the same steaming speed of 9.65 knots. This means an extra 1.84 gal/hr of fuel consumption. This adds up to an extra 1840 gallons over a 1000-hour annual running or 55,200 gallons over a 30-year life. Also the range of the vessel is seriously reduced, in this case from 9,000nm to 7724nm.

Hopefully, after reading this the prospective trawler owner is not put off powerboats for life, but instead if his intentions are to be ocean cruising it enables him to better judge the merits of different designs from the point of view of propulsive economy.

Where it is an owners intention to use his vessel for coastal cruising with annual usage of less than say 200 hours none of this really matters. It would be pointless to strive after the last ounce of propulsive economy in this circumstance.

Losses on Power Generation.

Since the invention of the internal combustion engine 100 years ago there has been little improvement in its thermal efficiency. Turbo-charging and after-cooling has resulted in improvements of around 10-11% but their thermal efficiencies still hover around 43% for heavy slow turning diesels and 38% for high speed types. The bigger diesels mitigate some of this inefficiency by operating on cheaper heavy oil fuel. For various reasons this option is not open to high speed diesels typically fitted in trawler yachts.

A small amount of this lost heat is recoverable in a trawler yacht by fitting distillation type water-makers and hot water heaters using the engine coolant heat. However these are not significant savings.

In about 1920 the Still Engine Company of Hull, England, decided to use the waste exhaust heat, which we have seen amounts to a 25% loss, in a new design of heavy marine diesel. Instead of exhausting this gas straight into the atmosphere they passed it through a steam boiler. The top of the engine cylinder was designed to operate on the diesel cycle as usual but the bottom of the piston was arranged to operate simultaneously on the steam cycle thereby creating a hybrid diesel/steam engine.

The steam was also used as a cylinder coolant thus eliminating a separate water cooling circuit and at the same time capturing some of the diesel generated cylinder heat. The engine was started by raising steam in the ships boiler and cranking the engine on the steam cycle.

Higher thermal efficiencies were achieved than equivalent diesels of that era but the system suffered from the disadvantage that the ship had to be fitted with dual steam and diesel plant with its attendant complications. A few ships were fitted with this engine but the concept abandoned in the late 1920s.

No doubt it would raise a few eyebrows if a modern diesel engine manufacturer were asked to produce a hybrid diesel/steam engine for today’s modern trawler yacht.

Losses in power generation are pretty much beyond our control and rather than dwell on it, it is instead better to concentrate on those areas were real improvements can be made.

© Copyright 2000. T.C. Watson & Sons Ltd.

All rights reserved.

Last Updated (Saturday, 31 October 2015 05:00)