
ContinuousWave Whaler Moderated Discussion Areas ContinuousWave: Whaler Performance Propeller Shaft Gear Ratio in Relation to Propeller Diameter, Pitch

Author  Topic: Propeller Shaft Gear Ratio in Relation to Propeller Diameter, Pitch 
fodimi 
posted 03062010 01:55 PM ET (US)
How can it be explained that the lower turning speed of a prop shaft causes a large diameter propeller? 
jimh 
posted 03062010 02:55 PM ET (US)
From Gerr's PROPELLER HANDBOOK, page 51: "Two major factors control propeller diameterengine horsepower in relation to shaft RPM, and hull resistance." Gerr goes on to give several charts which graph a relationship between propeller diameter and shaft RPM for various power levels, and he also gives an equation describing the relationship. I found Gerr's work interesting and tested his chart against my own experience. I have an engine rated 225HP with a gear reduction of 1.85:1. My maximum engine speed is 5,700RPM. The shaft speed is therefore 5700/1.85 = 3081 RPM For shaft horsepower I use the rated horsepower, which is measured at the propeller shaft. I then enter the data into Gerr's formula for diameter as follows: Diameter in inches = (632.7 x SHP^0.2) / RPM^0.6 Evaluating this at RPM = 3081 yields a calculated diameter of 15.1inches. In actual fact, I have been using several propellers with very good results whose diameters are in the range of 15.0 to 15.25 inches. This anecdotal observation seems to confirm the validity of Gerr's propeller diameter selection method using SHP and shaft RPM. 
jimh 
posted 03062010 02:57 PM ET (US)
As for the "why" that explains this, I am afraid you may have to undertake some serious study of propellers and naval architecture to develop the necessary understanding. Gerr does not provide a derivation or explanation for the relationship. 
fodimi 
posted 03072010 02:46 AM ET (US)
Dear Jim, First of all, i would like to say hello from Greece and the Meditterenean sea, and therefore forgive me for my moderate English talking. I own a RIB lengthed 8.60 m and i am about to put a motor of Suzuki DF 300 on it, topic on which i'll become later on that. I would like you to make the relation of'gear ratio in concerns of torque on propshaft' clear to me. How can the great torque on the propshaft be explained having a gear ratio of about 2.30:1.,when motors with a gear reduction of 1.87:1 have smaller torque on the propshaft. I would like you to explain this to me in two ways: a)technically and b)practically(meaninq quite easily and step by step in practice). Looking forward in hearing from you Thank you in advance 
jimh 
posted 03072010 10:36 AM ET (US)
Thomas I will try to explain the relationship between engine crankshaft speed, engine torque, propeller shaft speed, and propeller shaft torque with respect to your Suzuki 300HP engine. The Suzuki 300HP engine is rated to produce 300HP at a crankshaft speed of 5,700 to 6,300RPM. Source: http://suzukimarine.com/sr_10/df300/features/ We will use the median, 6,000RPM. Horsepower, torque, and output shaft speed are related as follows: POWER = TORQUE x RPM/5252 A good explanation can be found at http://en.wikipedia.org/wiki/Horsepower#Relationship_with_torque
Shaft Speed = 6000/2.08 The engine horsepower rating is given at the propeller shaft. This includes any loss in the gear reduction. Now we can find the torque: TORQUE = POWER x 5252/RPM If we had the same motor but it had a gear reduction of 1.87:1, the shaft speed would be 6000/1.87 or 3208, and the torque at the propeller shaft would be TORQUE = POWER x 5252/RPM 
jimh 
posted 03072010 11:00 AM ET (US)
In general we can say that the more torque at the propeller shaft the greater propeller pitch can be turned. As the motor gear reduction is increased (numerically), the propeller shaft speed decreases, but the torque is increased. For a given horsepower engine, a slower turning propeller shaft will be able to turn a higher pitch propeller, and vice versa. This makes good sense because we know that boat speed is related to horsepower. The horsepower does not change. A boat should be able to reach the same speed with either: a higher pitch propeller turning more slowly, or a lower pitch propeller turning more rapidly To fit a propeller for a particular application we can use several methods. Here is one method: select propeller diameter according to Gerr's formula above If you tell me the speed range you expect to reach with your boat, motor, and loading conditions, we can estimate the pitch and diameter of a suitable propeller. I will proceed with an example using the assumption that your boat can reach 50MPH, you have 300HP, and the propeller shaft speed will be a 2.08:1 reduction of the engine speed of 6,000RPM Calculate propeller diameter according to Gerr: Diameter in inches = (632.7 x SHP^0.2) / RPM^0.6 Evaluate at Diameter = 16.6inch Calculate propeller pitch using a SLIP of 0.15 or 15percent as an estimate and using the CONTINUOUSWAVE Propeller Calculator at RPM = 6000 Pitch is calculated as 21.5inch. These calculations suggest that an initial propeller trial might be to use a threebladed propeller with a diameter of 16.6inches and a pitch of 21.5inches. Most outboard motors have their propeller in an aperture which limits the diameter. I do not know the precise details of your Suzuki motor, but the maximum diameter may be limited to less than 16.6inches. If so, you will have to adjust the diameter of the propeller to fit the aperture. The figure I used for your boat speed, 50MPH, was just pulled out of the air. You need to estimate the expected boat speed you will reach with your boat and 300HP, taking into account the weight and hull. You can use another calculator to do this, Crouch's calculator. See: http://continuouswave.com/cgibin/crouchcalc.pl Other considerations that apply in your case are the nature of the boat. An RIB boat will probably be lighter than most other types, and therefore it will be faster. You might need to recalculate for a higher speed, which will imply more pitch. 
jimh 
posted 03072010 11:26 AM ET (US)
I just noticed in the specification for the DF300 that Suzuki says: "Swings a 16inch Diameter Prop" The calculated propeller diameter of 16.6inches will be too large. You will have to limit propeller selection to 16inch diameter. In general when propeller diameter is limited to a smaller diameter than optimum, the efficiency may suffer. The topic of propeller selection fills whole books, so it is complex. 
fodimi 
posted 03072010 02:35 PM ET (US)
Jimh I will try to analyze you my thought step by step and i would like you to tell me the point i am wrong. Having concerned what we've told so far: 
jimh 
posted 03072010 04:47 PM ET (US)
Fuel economy is dependent on two factors: the engine efficiency and the propeller efficiency. The engine efficiency varies with its throttle setting. Engine efficiency is measured by brake specific fuel consumption (BSFC). In general you can say that BSFC will be best (lowest) at midthrottle settings of engine. The variation of BSFC with engine speed and load is a complex function, so you cannot simply analyze it. But most engines will show their best economy at throttle settings around 1/2 to 3/4throttle. Propeller efficiency will vary at different shaft speeds and boat speeds. Propeller efficiency is a function of many variables. For a given propeller diameter, pitch, blade number, blade configuration, rake, cup, and other propeller geometries, and in combination with boat hull and load, the efficiency will vary at different speeds. Usually a propeller designer will have an idea of the expected speed that his propeller will be useful, and the design of the propeller will be optimized for that speed range. For example, a propeller with low pitch like 15inchpitch is likely to be optimized for slower speeds, around 30MPH. A propeller with high pitch like a 27inchpitch is likely optimized for higher speeds, around 60MPH. Propeller designers generally describe the type of use they expect for the propeller. This description will give you a clue for what speed range and what load range the propeller has been optimized. Selection of a proper propeller will usually involve onthewater testing. You really cannot just pick the perfect propeller on paper or with a formula. 
fodimi 
posted 03072010 05:32 PM ET (US)
I agree about of BSFC. I would like your opinion for the rest of my thoughts. 
number9 
posted 03072010 07:51 PM ET (US)
Most of what was explained about the relationship of gear ratio, torque and potential efficiency/performance would seem to favor higher gear ratios. We often see up to a 233:1 on midrange and HT/BF motors but most larger OBs uses less gear reduction. Is this primarily to keep the gear cases smaller resulting in less drag and higher potential WOT speeds or costs associated with building the drive to handle the increased torque? 
fodimi 
posted 03082010 02:50 PM ET (US)
Jim, Let's come to the basic point: 
jimh 
posted 03092010 12:51 AM ET (US)
The first step in propeller selection must be: Estimate the top speed expected with the power, weight, and hull type. 
fodimi 
posted 03092010 06:40 PM ET (US)
Jim, I hope not to be tired with all of these of my questions, but i'm really confusion... top speed at 5800 rpm : 54 mph 1. Can I use a propeller with a diameter about 15'' or 14'' ? it will be bad for the engine? what is the disadvantages of this use? 
jimh 
posted 03102010 12:34 AM ET (US)
Using your data HP = 300 with Crouch's Calculator, the hull constant or hull factor works out to 238. A hull constant or hull factor of 238 is quite high, almost in the race boat category, but I suppose that for a speedy RIB that might be a reasonable value. Using your data RPM = 5800 and estimating SLIP to 10percent, I enter that data in the Propeller Calculator to find PITCH = 22.7inches. Rounding up the pitch to 23 would be appropriate. The starting point for a propeller would be a diameter of 16inches and a pitch of 22 to 23 inches. I would begin testing with a threeblade propeller 16 x22 or 16 x 23. 
fodimi 
posted 03102010 03:06 AM ET (US)
Jim, If i test a 3blade propeller 16''x 22'' or 23'', the engine speed will reduce more. So, i wiil not be in the proper limits of RPM given by the manufacturer. Isn't that harmfull for the engine? 
jimh 
posted 03102010 08:22 AM ET (US)
I did not understand the data to be actual results. If your actual results are PITCH = 21 your propeller selection is good. The engine is reaching very near to the maximum engine speed. The calculated SLIP value is 2.6percent, which is very low. The actual pitch of the propeller may be higher than stated. If you want to try a fourblade propeller instead of a threeblade, Gerr suggests the following conversion factors: Diamter = 0.94 To go from a 16 x 21 three blade the conversion would suggest a fourblade with dimensions 15 x 20.6 
Mic Chr 
posted 07312010 06:38 AM ET (US)
HiI read your posts, since I have a Cobra 8.6meter rigidbottom inflatible boat (RIB) with a DF300 Suzuki. I have tried several propellers and up to now I found that the Lexor SOLAS 3x16x19 is the more suitable. My higher speed is: RPM = 5950 My cruise speed with this propeller is: RPM = 4500 I am not satisfied with this propeller since I have high consumption, and I am looking to install a Mercury REVOLUTION4 fourblade propeller, which has a diameter of 145/8inches. What pitch are you suggesting to try? Thanks 
jimh 
posted 07312010 08:27 AM ET (US)
Let me first convert the data into units of MPH and GPH, which will be more familiar to North American readers. We have at full throttle for this Suzuki DF300 on a 28.2foot RIB the following: RPM = 5950 Which becomes RPM = 5950 The value of fuel flow, 21.1GPH, seems low for a 300HP engine. We would expect that a gasoline engine producing 300HP would consume more fuel per hour. If we have a brake specific fuel consumption of 0.5lbs/HPhour, at 300HP we should be burning gasoline at a rate of 300HP x 0.5lbs/HPhour x 1gallon/6.25lbs = 24gallons/hour There are two inferences to be drawn from the fuel flow. Either the SUZUKI DF300 is marvelously fuel efficient at fullthrottle, or it is not producing full power output of 300HP. Turning to the propeller, we next look at the data and assess the SLIP. Into the Propeller Calculator, I enter this data: RPM = 5950 http://continuouswave.com/cgibin/propcalc.pl I calculate SLIP as 10.6, which is a reasonable number for a large boat with a single propeller. This result tends to confirm that the current threeblade propeller with 19pitch and 16inch diameter is working reasonably well. For estimation of the pitch of a REVOLUTION4 fourblade propeller, I would use the the following logic. First, using Gerr's guidelines, I calculate the fourblade dimension from the current threeblade: Three blade Fourblade Because the REVOLUTION4 diameter of 14.625inch is less than the estimated diameter, 15inch, and because the REVOLUTION4 is only available in 15, 17, 19, 21, 23, and 25 pitch, my suggestion is to try the REVOLUTION4 19pitch propeller. 
Tohsgib 
posted 07312010 11:24 AM ET (US)
[First], your DF300 redlines at 6300 [or has a maximum engine speed of 6,300RPM], and you are not reaching 6,000RPM. Suzuki fourcycle motors like to be run [at] high [engine speed] and get close to their redline [or maximum rated engine speed], or they will not perform well. Suzuki also pioneered the bigmoney fourcycle and if they recommend a 16inch diameter [propeller], I would stick with it. I think their engineers know more than you or I do about propellers. Suzuki, makes fourblade propellers; see if they have one for your engine in a 17pitch. I would assume that a 28foot boat with a single screw would require a fourblade. An Airship R8 with a Verado 300HP does 68MPH or 59NMPH running a 15.25 x 24 fourblade with a 1:1.75 ratio, 5,900lbs, and burns 29.8GPH. You might have something aloof with your engine with your numbers and GPH. Lastly, I went to Suzuki's website and all the boats they tested ran threeblade 16inch diameter propellers, no fourblade propellers, no smaller diameters. Also most with a 18.5pitch ran faster than you and burned more fuel. One ran 5,800RPM at 51.6MPH burning 26.75GPH. The other ran 6,200 with same prop at 49.7MPH burning 24GPH. 
jimh 
posted 07312010 12:28 PM ET (US)
If a gasoline internal combustion motor burns 26.75GPH, and we consider that its brake specific fuel consumption (BSFC) was around 0.55a value typical for such an engine running near its maximum throttle and loadwe could estimate the horsepower as HP = HPhour/0.55lbs x 26.75gallons/1hour x 6.25lbs/1gallon If the engine were a gas guzzler, and its BSFC was 0.6, this would still imply the horsepower was HP = 279HP In comparison, the Suzuki DF300 under discussion is reported to consume only 21.2GPH at full throttle, and if judged as we estimate above, appears to only be making 220 to 239HP. I suggest you check the engine rigging to be certain that the throttle is opening to fullthrottle. A SUZUKI DF300 should be burning more fuel at fullthrottle than your engine. 
jimh 
posted 07312010 12:31 PM ET (US)
It is also typical in boat, motor, and propeller performance analysis that a lot of speed is gained in the last 500RPM of the engine speed increase. This is because most of the boat is out of the water at that speed and the propeller is becoming more effective as speed increases. As a result, getting that extra 500RPM from the engine can mean an increase in top speed of 5MPH. 
Mic Chr 
posted 08012010 07:16 AM ET (US)
Thanks guys for all of your info. I had talked with the Suzuki represented engineer in Greece and he propose to me to use a four blade propeller but since Suzuki does not have he told me to try the Merc Rev 4 but with a pitch of 21. Also he proposes me to run the engine close to red line, near at 6300 rpm. I found the proposed pitch too high, looking at your analysis (Gerrâ€™s Guidelines) and therefore I asked you. With a 20 pitch propeller, and more load (5 persons)I have seen consumptions of 86 lph (22,7 GPH) at a max of 5500 rpm. My rib full load should be weight around 1800 kg (4000 pounds) 
Tohsgib 
posted 08022010 11:00 AM ET (US)
I would run a 17" 4 blade or a 18" 3 blade. 
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