posted 09-17-2012 04:30 PM ET (US)        

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What is the largest propeller diameter that somebody could fix in so as not to damage the [VERADO 300-HP I6] gearcase?

Hi everybody! It has been a few days that I entered your forum.I'd like to congratulate you for the very interesting posts and useful opinions. I'd like to open a thorrow discussion over prop's diameters. To start with, I would like to tell you that I have the new Verado 300-HP with 1.85:1 gear ratio. I do not want to connect the conversation with the hull length neither the boat's or cargo's weight. Thanx in advance

posted 09-18-2012 08:38 AM ET (US)            

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I am not familiar with the formula you mention for determining the proper diameter of a propeller from the horsepower applied to the propeller and the propeller shaft speed. I suspect that the formula is probably from the work of naval architects in designing larger vessels which do not operate at planing speeds.

In the case of an outboard engine the propeller diameter is constrained by the arrangement of the outboard engine gear case aperture. Propellers designed for outboard engines tend to have their diameter adjusted in relation to their pitch, with the limitation that the diameter is not too big to fit into the propeller aperture on the outboard.

posted 09-18-2012 10:58 AM ET (US)            

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You should be wary is simple formulas or rule-of-thumb's. They often fail to account for the many subtle parameters of a specific application.

The maximum diameter propeller for a given outboard is simply controlled by the distance from the centerline of the propeller shaft to the anti-ventilation plate, which in the Verado 300 is just over 8 inches as well as the availability of propellers.

The largest propeller that Mercury makes for the Verado 300 is the 13" x 16" MIRAGEplus. All of the other propeller models as well as all the other pitches of the MIRAGEplus have smaller diameters, from 15-3/4" down to 13-1/4". I do not know of any aftermarket propellers for the Verado 300 that have larger or smaller diameters.

For the most part, you do not concern yourself with diameter by itself; you choose the model of propeller you want to use for your application, select the appropriate pitch and the propeller designer has already decided the diameter.

posted 09-19-2012 03:54 PM ET (US)            

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Jim-
The formula that I mentioned is from <<PROPELLER HANDBOOK>> by Dave Gerr, page 52, formula 5-3. I think this formula is proper for vessels which operate at planning speeds. If I am somewhere wrong,please correct me.
I believe that apart from the arrangement of the propeller aperture, the gear case also defines the diameter of the propeller which can be used. That’s why the outboard engines have different gear ratios.
Τhe gear ratio of Verado 300 hp is at 1,85:1. The gear ratio of Suzuki 300 hp is at 2,08:1.
The gear ratio of Suzuki have a greater reduction (higher number ratio), which means the gear case is bigger in order to spin larger diameter’s propellers. We all know that a larger diameter propeller is always harder to turn.

Tom-
Gerr’s formula is going well as far as Suzuki is concerned and gives us D=16΄΄, that is exactly what Suzuki suggests. Therefore, I wonder why the same formula that gives D=14, 9΄΄ for the Verado motor, could not operate properly.
<<The slower the prop-shaft speed, the larger the diameter an engine can turn>>. Is this something that you agree with?

posted 09-20-2012 12:44 AM ET (US)            

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Yes, I have a copy of Gerr's PROPELLER HANDBOOK right here too.

You asked what the maximum diameter propeller for a Verado 300 is. The answer is simple: 16 inches.

Formula 5-3 does not give the maximum diameter propeller. It gives diameters for "...three-bladed propellers of the standard elliptical contour and ogival (flat-faced) section, with blade widths of about 0.33 mean-width ratio. This propeller type will be found to give good results for almost all ordinary installations."

I think you will find that 14.9 inches, the diameter that Formula 5-3 yields for the Verado 300, is squarely in the middle of the various diameters of propellers available for that motor. Propellers used on the Verado 300 with diameters significantly greater than or less than 14.9 inches may well represent propellers that are good fits for unusual applications and/or propellers with more than three blades and/or blades with more complex contours and sections.

By the way, Suzuki does not recommend propellers with 16 inches of diameter for their DF300. They merely offer two models of propellers, and only two, one with a 15 inch diameter, the other with a 16 inch diameter. There are numerous aftermarket propellers for the DF300 that have different diameters than those two.

posted 09-20-2012 08:26 AM ET (US)            

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Jim,
I honestly had no intention to offend anyone, especially you and Tom, who I deeply respect and appreciate for your work. I just mentioned Gerr’s formula, because I thought that this formula give us the maximum propeller diameter.

Tom,
Thank you really much for your explanation and the absolutely coherent documentation. I had the mistaken impression that the above formula gives us the maximum propeller diameter for an outboard engine. Could you please tell me your opinion about whether you agree with my statement?
“Eventually, a gear ratio of 2.30:1 can move a propeller of 16’’ diameter with less distress, compared to a gear ratio of 1.75:1, since the slower the prop-shaft speed, the larger the diameter an engine can turn.” However, is this difference of distress between the two gear ratios actually so small, and thus insignificant, for the propeller size of diameters in question?

posted 09-20-2012 08:47 AM ET (US)            

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Propeller design--as far as I can tell--is an extremely complex engineering problem. In the case of outboard engines my feeling is that propeller design is probably done by a lot of trial and error experimenting. It does not cost very much to make a propeller for an outboard engine, grind it, bend it, cup it, and experiment with it. You can change the propeller on an outboard engine in about five minutes with the boat in the water. On the other hand, propellers for large ships are much more expensive to fabricate. One ship propeller probably costs as much as several thousand outboard propellers. It is prohibitively expensive to experiment with full scale ship propellers. Just hauling the ship to change propellers costs a small fortune and takes the ship out of service for several days, a further loss of money. As a result, I suspect that much more theoretical engineering and calculation goes into large ship propeller design than into outboard engine propellers. I believe that the very best data about modern propeller design is proprietary. You won't find it in $35 books about propellers. You'll have to pay $100,000--or more--to a design firm to get that knowledge. However, if you are the owner of a modern ship that uses $5-million in fuel every year, you could easily recoup the expense of the propeller design fee in fuel savings in one year of operation.

With regard to a "proper" diameter for an outboard engine propeller, it is best left to the propeller manufacturers. Pick a propeller based on its pitch and its recommended application, and leave the diameter to the propeller manufacturer.

posted 09-20-2012 09:58 AM ET (US)            

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quote:

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"Eventually, a gear ratio of 2.30:1 can move a propeller of 16’’ diameter with less distress, compared to a gear ratio of 1.75:1, since the slower the prop-shaft speed, the larger the diameter an engine can turn.” However, is this difference of distress between the two gear ratios actually so small, and thus insignificant, for the propeller size of diameters in question?

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I do not understand what you are asking. Why is the first sentence in quotation marks? Did you mean Evidently when you wrote Eventually? Did you mean to write stress where you wrote distress?

Gear ratios do not move propellers, the engine moves the propeller. The great ratio simply trades torque for speed.

posted 09-21-2012 04:51 AM ET (US)            

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This data was collected on my 33-foot offshore v-hull boat with a single 300 hp Verado (gear ratio 1,75:1).

Mirage Plus 15-1/4΄΄ X 19΄΄

RPM KNOTS
3000 19,4
3500 24
4000 29
4500 33
5000 38
5500 42
6000 46
6200 47,3

I still have the same boat but now with a new 300 hp Verado (gear ratio 1,85:1).
This engine has greater gear ratio, so I’m thinking to buy a Mirage Plus 21΄΄. Would you suggest to go ahead with this purchace or should I prefer the Mirage Plus 19΄΄?
My purpose is to increase my cruise speed as well as to increase the final speed, but without decreasing the engine’s rpm.

posted 09-21-2012 08:18 AM ET (US)            

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A 21" MIRAGEplus will probably drop your WOT engine speed to around 6000 RPM.

You might also consider the 21" Tempest Plus which should put you back at 6200 RPM.

Jim's Propeller Calculator is very helpful in selecting pitch:

http://continuouswave.com/cgi-bin/propcalc.pl

posted 09-21-2012 08:21 AM ET (US)            

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From your data, we can compute the propeller shaft speed for your original engine:

6,2000-Revolutions-crankshaft/1-minute x 1-revolution-propeller shaft/1.75-revolutions crankshaft = 3,543-revolution-propeller-shaft

If the new engine reaches the same crankshaft speed, its propeller shaft speed with a 1.85 gear ratio will be only 3,351.

The propeller shaft speed will decrease by 192-RPM. To compensate, the propeller pitch should increase in proportion to

19-inch x 3543/3351 = 20-inch

posted 09-21-2012 03:58 PM ET (US)            

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Tom ,
According to your suggestion, I’m thinking to try a Τempest Plus 21΄΄.
By the way, have you got in mind a particular propeller which will be able to increase the final speed of my boat ?

Jim,
My engineer claims that now that Verado has greater gear ratio we are able to use longer pitch , but of course without decreasing the engine’ s rpm. According to that, do you believe that the greater the reduction, the bigger (concerning the pitch) the propeller can be used, without of course decreasing the engine’s rpm;
Is this thought reasonable or not ?

posted 09-21-2012 04:37 PM ET (US)            

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Tom says:

"There is no such thing as a 20 inch pitch MIRAGEplus."

That's gonna be a problem if you want to use a MIRAGEplus because the proportional pitch dictates a 20-inch pitch. Perhaps you can experiment with the orignal 19-inch pitch propeller, or a different style of propeller, or try the 21-inch and have it pitched-down a bit by a good propeller shop.