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Author Topic:   Propeller Shaft Gear Ratio in Relation to Propeller Diameter, Pitch
fodimi posted 03-06-2010 01:55 PM ET (US)   Profile for fodimi   Send Email to fodimi  
How can it be explained that the lower turning speed of a prop shaft causes a large diameter propeller?
jimh posted 03-06-2010 02:55 PM ET (US)     Profile for jimh  Send Email to jimh     
From Gerr's PROPELLER HANDBOOK, page 51:

"Two major factors control propeller diameter--engine 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 225-HP with a gear reduction of 1.85:1. My maximum engine speed is 5,700-RPM. 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
SHP =225

yields a calculated diameter of 15.1-inches. 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 03-06-2010 02:57 PM ET (US)     Profile for jimh  Send Email to jimh     
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 03-07-2010 02:46 AM ET (US)     Profile for fodimi  Send Email to fodimi     
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 03-07-2010 10:36 AM ET (US)     Profile for jimh  Send Email to jimh     

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 300-HP engine.

The Suzuki 300-HP engine is rated to produce 300-HP at a crankshaft speed of 5,700 to 6,300-RPM.


We will use the median, 6,000-RPM.

Horsepower, torque, and output shaft speed are related as follows:

where POWER is in horsepower, TORQUE in foot-lbs.

A good explanation can be found at

The effect of the gearing in the outboard motor gear case is to reduce the shaft speed. In your engine the gear ratio is a reduction of 2.08:1. When the engine crankshaft speed is 6,000-RPM, the propeller shaft speed will be

Shaft Speed = 6000/2.08
Shaft Speed = 2885

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 = 300 x 5252/2885
TORQUE = 546 ft-lbs.

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 = 300 x 5252/3208
TORQUE = 491 ft-lbs.

jimh posted 03-07-2010 11:00 AM ET (US)     Profile for jimh  Send Email to jimh     
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
--select pitch to produce boat speed expected by horsepower, weight, and hull form calculations

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 50-MPH, you have 300-HP, and the propeller shaft speed will be a 2.08:1 reduction of the engine speed of 6,000-RPM

Calculate propeller diameter according to Gerr:

Diameter in inches = (632.7 x SHP^0.2) / RPM^0.6

Evaluate at
SHP = 300
RPM = 2885

Diameter = 16.6-inch

Calculate propeller pitch using a SLIP of 0.15 or 15-percent as an estimate and using the CONTINUOUSWAVE Propeller Calculator at :

RPM = 6000
RATIO = 2.08
SLIP = 15
MPH = 50

Pitch is calculated as 21.5-inch.

These calculations suggest that an initial propeller trial might be to use a three-bladed propeller with a diameter of 16.6-inches and a pitch of 21.5-inches.

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 16.6-inches. If so, you will have to adjust the diameter of the propeller to fit the aperture.

The figure I used for your boat speed, 50-MPH, was just pulled out of the air. You need to estimate the expected boat speed you will reach with your boat and 300-HP, taking into account the weight and hull. You can use another calculator to do this, Crouch's calculator. See:

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 03-07-2010 11:26 AM ET (US)     Profile for jimh  Send Email to jimh     
I just noticed in the specification for the DF300 that Suzuki says:

"Swings a 16-inch Diameter Prop"

The calculated propeller diameter of 16.6-inches will be too large. You will have to limit propeller selection to 16-inch 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 03-07-2010 02:35 PM ET (US)     Profile for fodimi  Send Email to fodimi     

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:
gear ratio = 2.08:1 means greater torque at propeller shaft, means
have the ability to use propeller with bigger diameter, means,
have bigger thrust.
Therefore my boat with big load will move easier. So, a propeller of big diameter should be able to approach two goals (which are of my interest):
1. better navigation in rough seas,
2. smaller fuel consumtion(and so economy)in a range of 3.500-4.500rpm(please explain to me, as simply as you can, why better economy is given at the medium rpm).
And all i've mentioned, compared to a propeller with smaller diameter.

jimh posted 03-07-2010 04:47 PM ET (US)     Profile for jimh  Send Email to jimh     
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 mid-throttle 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 3/4-throttle.

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 15-inch-pitch is likely to be optimized for slower speeds, around 30-MPH. A propeller with high pitch like a 27-inch-pitch is likely optimized for higher speeds, around 60-MPH.

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 on-the-water testing. You really cannot just pick the perfect propeller on paper or with a formula.

fodimi posted 03-07-2010 05:32 PM ET (US)     Profile for fodimi  Send Email to fodimi     
I agree about of BSFC.
I would like your opinion for the rest of my thoughts.
number9 posted 03-07-2010 07:51 PM ET (US)     Profile for number9  Send Email to number9     
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 mid-range 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 03-08-2010 02:50 PM ET (US)     Profile for fodimi  Send Email to fodimi     

Let's come to the basic point:
I have a RIB lengthed 8.60 and i am about to put a Suzuki DF 300 on it.
Which thoughts (step by step) should someone make, so as to pick up the candidate propeller, without using any calculators? Especially before the first test of the boat on the sea?
STEP 1 : Should we always start from estimating the propeller's diameter? if not, which is this certain feature of a propeller with which we should start?
STEP 2 : Which is the second feature of the propeller i should think of afterwards? maybe the pitch?

jimh posted 03-09-2010 12:51 AM ET (US)     Profile for jimh  Send Email to jimh     
The first step in propeller selection must be: Estimate the top speed expected with the power, weight, and hull type.
fodimi posted 03-09-2010 06:40 PM ET (US)     Profile for fodimi  Send Email to fodimi     

I hope not to be tired with all of these of my questions, but i'm really confusion...
As a result of testing on the sea, it is very possible to have:

top speed at 5800 rpm : 54 mph
diameter of propeller : 16''
pitch of propeller : 21'
weight of boat(full load) : 5800lbs

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?
2. it's better to try more pitch?
3. If i try with a 4 blade's propeller, what pitch and diameter should i prefere? and for what reasons?

jimh posted 03-10-2010 12:34 AM ET (US)     Profile for jimh  Send Email to jimh     
Using your data

HP = 300
MPH = 54
LBS =5,800

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
RATIO =2.08
MPH = 54

and estimating SLIP to 10-percent, I enter that data in the Propeller Calculator to find PITCH = 22.7-inches. Rounding up the pitch to 23 would be appropriate.

The starting point for a propeller would be a diameter of 16-inches and a pitch of 22 to 23 inches.

I would begin testing with a three-blade propeller 16 x22 or 16 x 23.

fodimi posted 03-10-2010 03:06 AM ET (US)     Profile for fodimi  Send Email to fodimi     

If i test a 3-blade 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 03-10-2010 08:22 AM ET (US)     Profile for jimh  Send Email to jimh     
I did not understand the data to be actual results. If your actual results are

PITCH = 21
RPM = 5800
MPH = 54

your propeller selection is good. The engine is reaching very near to the maximum engine speed. The calculated SLIP value is 2.6-percent, which is very low. The actual pitch of the propeller may be higher than stated.

If you want to try a four-blade propeller instead of a three-blade, Gerr suggests the following conversion factors:

Diamter = 0.94
Pitch = 0.98

To go from a 16 x 21 three blade the conversion would suggest a four-blade with dimensions 15 x 20.6

Mic Chr posted 07-31-2010 06:38 AM ET (US)     Profile for Mic Chr  Send Email to Mic Chr     
Hi--I read your posts, since I have a Cobra 8.6-meter rigid-bottom 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
NMPH = 40
LPH = 80

My cruise speed with this propeller is:

RPM = 4500
NMPH = 29
LPH = 42

I am not satisfied with this propeller since I have high consumption, and I am looking to install a Mercury REVOLUTION4 four-blade propeller, which has a diameter of 14-5/8-inches. What pitch are you suggesting to try?


jimh posted 07-31-2010 08:27 AM ET (US)     Profile for jimh  Send Email to jimh     
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 28.2-foot RIB the following:

RPM = 5950
NMPH = 40
LPH = 80

Which becomes

RPM = 5950
MPH = 45.6
GPH = 21.2

The value of fuel flow, 21.1-GPH, seems low for a 300-HP engine. We would expect that a gasoline engine producing 300-HP would consume more fuel per hour. If we have a brake specific fuel consumption of 0.5-lbs/HP-hour, at 300-HP we should be burning gasoline at a rate of

300-HP x 0.5-lbs/HP-hour x 1-gallon/6.25-lbs = 24-gallons/hour

There are two inferences to be drawn from the fuel flow. Either the SUZUKI DF300 is marvelously fuel efficient at full-throttle, or it is not producing full power output of 300-HP.

Turning to the propeller, we next look at the data and assess the SLIP. Into the Propeller Calculator, I enter this data:

RPM = 5950
RATIO = 2.08:1 (I believe this is correct for a DF300)
PITCH = 19
NMPH =40

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 three-blade propeller with 19-pitch and 16-inch diameter is working reasonably well.

For estimation of the pitch of a REVOLUTION4 four-blade propeller, I would use the the following logic. First, using Gerr's guidelines, I calculate the four-blade dimension from the current three-blade:

Three blade
PITCH = 19

PITCH = 19 x 0.98 = 18.6
DIAMETER = 16 x 0.94 = 15

Because the REVOLUTION4 diameter of 14.625-inch is less than the estimated diameter, 15-inch, and because the REVOLUTION4 is only available in 15, 17, 19, 21, 23, and 25 pitch, my suggestion is to try the REVOLUTION4 19-pitch propeller.

Tohsgib posted 07-31-2010 11:24 AM ET (US)     Profile for Tohsgib  Send Email to Tohsgib     
[First], your DF300 redlines at 6300 [or has a maximum engine speed of 6,300-RPM], and you are not reaching 6,000-RPM. Suzuki four-cycle 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 big-money four-cycle and if they recommend a 16-inch diameter [propeller], I would stick with it. I think their engineers know more than you or I do about propellers.

Suzuki, makes four-blade propellers; see if they have one for your engine in a 17-pitch. I would assume that a 28-foot boat with a single screw would require a four-blade. An Airship R8 with a Verado 300-HP does 68-MPH or 59-NMPH running a 15.25 x 24 four-blade with a 1:1.75 ratio, 5,900-lbs, and burns 29.8-GPH. 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 three-blade 16-inch diameter propellers, no four-blade propellers, no smaller diameters. Also most with a 18.5-pitch ran faster than you and burned more fuel. One ran 5,800-RPM at 51.6-MPH burning 26.75-GPH. The other ran 6,200 with same prop at 49.7-MPH burning 24-GPH.

jimh posted 07-31-2010 12:28 PM ET (US)     Profile for jimh  Send Email to jimh     
If a gasoline internal combustion motor burns 26.75-GPH, and we consider that its brake specific fuel consumption (BSFC) was around 0.55--a value typical for such an engine running near its maximum throttle and load--we could estimate the horsepower as

HP = HP-hour/0.55-lbs x 26.75-gallons/1-hour x 6.25-lbs/1-gallon
HP = 304-HP

If the engine were a gas guzzler, and its BSFC was 0.6, this would still imply the horsepower was

HP = 279-HP

In comparison, the Suzuki DF300 under discussion is reported to consume only 21.2-GPH at full throttle, and if judged as we estimate above, appears to only be making 220- to 239-HP.

I suggest you check the engine rigging to be certain that the throttle is opening to full-throttle. A SUZUKI DF300 should be burning more fuel at full-throttle than your engine.

jimh posted 07-31-2010 12:31 PM ET (US)     Profile for jimh  Send Email to jimh     
It is also typical in boat, motor, and propeller performance analysis that a lot of speed is gained in the last 500-RPM 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 500-RPM from the engine can mean an increase in top speed of 5-MPH.
Mic Chr posted 08-01-2010 07:16 AM ET (US)     Profile for Mic Chr  Send Email to Mic Chr     
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 (Gerr’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 08-02-2010 11:00 AM ET (US)     Profile for Tohsgib  Send Email to Tohsgib     
I would run a 17" 4 blade or a 18" 3 blade.

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