Prop pitch vs gear ratio...

[wca_tim]

OK fellas. which is the most important variable in choosing a gear ratio / prop pitch combination? Assume merc alpha one with under 300 hp with max operating rpm of 4800. Is there any real difference between say a gear ratio of 1.47 and 19 pitch prop and a gear ratio of 1.84 and a prop pitch of 24, or even 1.65 prop pitch of 21. For WOT rpm each combination would give a theoretical top speed of 59-60 mph and actual top end of 52-53 assuming a 10% slip. Is one end of the scale generally more effective and turning torque / horsepower into forward thrust?

Or is the answer: it's much more complicated than that? I know it is more complicated, but I guess what I'm getting at is whether or not if there is a clear winner in terms of these two particular variables when it comes to performance? My gut feeling is that prop design probably has more to do with it than actual pitch or gear ratio, that some prop designs are better at high shaft rpms and others perhaps better at lower shaft rpms.

At first glance / oversimplified perspective, they're just two different "gear ratios", one coupling the engine to the prop shaft (outdrive gears) the other the prop shaft to the water / forward thrust (the prop). each is going to have some losses and special considerations, but which is more important in the shaft rpm ranges for say 50-70 mph

Any insights or experience is appreciated...

 

[Frank Acampora]

Re: Prop pitch vs gear ratio...

My only experience is with outboards and I don't know if it would be applicable to I/O ---BUT--

On Chrysler and Force outboard engines, both 3 and 4 cylinder, 85, 90, 120, and 125 horsepower, there are two lower unit gear ratios: 1.78 and 2.0 to 1.


1.78 to 1 with a 17 pitch prop has just about the same theoretical top speed as the 2.0 to 1 with a 19 pitch prop. Yet, the 2.0 to 1 with the 19 pitch prop is usually about 3 -4 MPH faster, and with one particular engine 6 mph faster, on the same boat and engine combination as the 1.78.

Yes, I change lower units and engines more often than I change underwear. And neither of them smells bad.

Don't know why it works, it just does. Engine RPM is close to the same --say within 100--so it must be something else. Perhaps turning the higher pitch prop slower results in either less drag or less slip or both, resulting in more available horsepower being used to drive the boat forward.

 

[Silvertip]

Re: Prop pitch vs gear ratio...

There are three basic factors at work here (ignoring the hull design and load it carries). The first is engine power and RPM. The more power one has available, the taller the gears (lower numerical ratio) the engine is capable of turning and consequently the prop pitch the combination is capable of twisting within the RPM range of the power plant. Outdrives in the case of I/O's are generally somewhat RPM limited so that dictates to a degree, the gear ratios that setup will reliably handle. To get prop speed up, gear ratios in the 1.6 range are common. Props fall at the end of the line and transfer the twist to forward motion within the limits of the engine and gearing. Outboards spin a little faster the I/Os but the same principles apply. You have so much power and so many rpms to deal with. Select gear ratios that work best with that power plant and the props again transfer that combination into forward motion. Mercury for example as a marketing thing with their "Bigfoot" motors. Those are simply lower units off a midrange engine that contains a 2.3 gearset rather than the 1.83 normally found on 40, 50, and 60 HP engines. It allows the engine to spin larger diameter props when these engines are attached to pontoons or barges.

 

[Texasmark]

Re: Prop pitch vs gear ratio...

This is a very interesting thread to me. I have also wondered the same thing for a very long time.

I am running a 90 hp Merc which is at the mid-lower hp end of Merc's midrange 4 1/4 inch gearbox. It and the 75 are at 2.3 and the other engines 100 to 125 hp are at 2:1.

Real confusing setup to me. Only thing I can presume is that Merc attempts to put a large dia prop on a lower hp engine hoping to get the pulling performance out of a lower priced engine. Then they do an about face with the next smaller engine, the 60 hp where they go to the smaller dia, lower hp gearbox, running a much smaller prop and spin it at a dazzling 1.64:1.

Would like to hear from a propulsion specific, nautical engineer on this one.

Mark

 

[TilliamWe]

Re: Prop pitch vs gear ratio...

In short I don't think there is any difference to your original question.
It seems to me that on sterndives, Mercruiser changes the gears so as to prop it with a very common pitch, but still have room for increase or decrease, with readily available props.
Example, my dad changed a 4.3 to a 5.7 and kept the same 1.81 ratio. He could not get a big enough pitch prop easily. So he switched to a 1.65 ratio drive (Mercruiser would have put a 1.47 with it) and was able to use a 26 pitch prop that he had. If performed just fine.

 

[domains]

Re: Prop pitch vs gear ratio...

I know on atvs you change the clutch basket gears instead of diff gears ...lowering them housing will not withstand pressures. just a thought .

 

[Texasmark]

Re: Prop pitch vs gear ratio...

Now, I never thought of that but that makes sense......propping for readily available props. That really has merit and makes sense. Could be as simple as that.

Thanks Tilliam,

Mark

 

[wca_tim]

Re: Prop pitch vs gear ratio...

I agree that in general that's what we do - choose a gear ratio that allows use of readily available prop pitches. Intuitively, I would think that there is a range in pitch where props are more efficient. Perhaps over the range that is reasonable using currently available gear cases and prop materials and design the difference is negligible and the specific prop design and hull combination far outweighs differences of a few inches in prop pitch.

If that is the case, then is the primary reason that a 350 behind a mercruiser outdrive in a lighter boat isn't run with a 1.8 gear ratio and a 27+ pitch prop? What about a 4.3 run with a 1.47 gear ratio and a 19 pitch prop? My 4.3 in current trim, fully loaded (2 adults 2 kids, tool box fishing gear 25 gal gas, cooler, etc...) is running 53-54 mph gps with a 23 pitch laser 2 at 4800 rpms and seems to run out of cam at that point (even trimming up to where the prop ventilates only nets an increase of 100-200 rpms). If I assume a pitch of 24 thats below 10% slip.

based on the above, a 1.47 gear ratio a 19 pitch prop would be about the same in terms of performance.

It is clear that prop set-up is a black art as far as most are concerned, and that although there are some simple rules that apply, a lot of the variables are largely left to trial and error on a case by case situation. Does anyone have more to add or a good reference I can dig through?

Thanks for the input... I'll let ya know anything I learn on my own.

 

[QC]

Re: Prop pitch vs gear ratio...

There are a couple of general things to consider here that complicate this discussion a lot.

1) Different speed ranges change the answer, if we are truly talking light boats and high speeds then the answers are on race forums

2) Mass marketing (the prop pitch range discussion of Tilliam's) changes the answer for most regular boaters too.

3) Torque within the drive, and prop components. If a 25" was the "best" then a boat that could only reach 20 MPH with a 3.0 liter could potentially result in more torque mechanically on the gear sets and prop hub with a 4/1 ratio than a big block with a 1/1 drive at 80 MPH . . . (I didn't do the actual math, but the point is that torque is multiplied with gear reduction)

 

[EricR]

Re: Prop pitch vs gear ratio...

My experience which may not fully apply here is with large boats with inboard diesels.

Viking yachts produced a 53' Sportfish convertible with twin 820 hp V10 MAN diesels. The first 15 to 20 off the line were equipped with 1.5:1 reduction gears and turned four blade props of something near a 28 diameter X 30-32" pitch.

I worked for an MAN dealer at the time- the boats that people had large fishing towers put on or had say a 13' Boston Whaler with the big davit mounted on the bow or boats where they loaded up on marble countertops in the inside had lots of low power complaints, they lost RPMs with added wieght, makes sense.

The engineers decided to go with 2.0:1 reduction and turn bigger pitch props slower, I do not recall what they went to.

The subsequent boats were much less sensitive to added wieght as the boats were rigged.

 

[domains]

Re: Prop pitch vs gear ratio...

You would get more Torque, but lowering the gear would only even off with more prop pitch. just like a street bike lowering front sprocket = torque but only one gear to play with.

my guess is smaller gear bigger pitch that just evens off. putting more strain on gear case.

 

[QC]

Re: Prop pitch vs gear ratio...

putting more strain on gear case.

If you were referring to my post, that was my point . . .

 

[Texasmark]

Re: Prop pitch vs gear ratio...

Well, I still think about the simplicity of setting up the gearbox to get the powerhead to run with currently available props and I really think that's the answer......after all these years of wondering.

I mean, you have to increase the size of the lower unit to install larger parts to support the added thrust of larger engines.

In your engine lineup you have to cross over somewhere. So, the engines close to the crossover may be too high or low in hp to run with the readily available props that fit that gearcase, so they tweak the gear ratio to overcome that.

Whatta deal.....someone did something that makes sense for a change.

My 2c again.

Mark

 

[SuperNova]

Re: Prop pitch vs gear ratio...

Me and lime4x4 had a chance to play around with gear ratios and an extensive array of props this summer and I learned a couple of things.

1. Theory is great, and it even works sometimes--to an extent. The real problem with balancing prop pitch and gear ratio is not top-end, it's getting the hull up on plane. Just before you break over is the greatest load you place on the whole drivetrain and it is also where the most prop slip occurs; mostly due to the load. This slip mostly negates the bigger prop advantage, so if your gear ratio is such that it allows the engine to rev too high at this point, you will never get the boat up on plane, regardless of what "theory" dictates. Once the boat is on plane, things will work according to theory because the load drops significantly and you will hit your top speed.

2. The ideal situation is to gear the outdrive for the engine/boat combination, and then use the prop selection to fine-tune. It is very difficult to use the prop to overcome any shortfalls in gear ratio selection.

Tim is right about it basically being two gear ratios, the problem being the second set of "gears" (the prop and the water) are always changing ratios due to the variable nature of prop slip. Comparing car gear ratio and tire combinations to boat gear ratio and prop selection is a very common trap that theorists fall into.

 

[Tail_Gunner]

Re: Prop pitch vs gear ratio...

Actually nova i would think slip is pretty constant for a given prop design and hull type, basic mechanic's are going to stay the same. If a lower gear ratio is used in the drive and a larger prop is used to get it to max rpm @ Wot it will be sluggish out of the hole or but should achieve a higher speed..maybe..:redface:

Hmmm ok lets use a 4.3 220 hp motor 19' boat 3000lbs
1. 2.1 gears 23 pitch prop yeild's 4800 rpm
2. 1.81 gear's 22 pitch prop yeild's 4800 rpm
3. 1.66 gear's 20 pitch prop yeild's 4800 rpm

I would be schocked if the 23 or 22 climbed out of the hole faster than the 20.....it has less surface area (drag is a big factor)

The magic is finding a gear,prop combo that the motor can turn over @ wot in the max rpm range and where the motor has a rapdily declining tourqe curve

 

[tommays]

Re: Prop pitch vs gear ratio...

I goota find that last boat test i read it was really bad


Single engine DUO-PROP drive on a 26' that needed to turn 4000 RPM and fell off plane at 3400


Tommays

 

[SuperNova]

Re: Prop pitch vs gear ratio...

Actually nova i would think slip is pretty constant for a given prop design and hull type, basic mechanic's are going to stay the same.

No, that is wrong. Slip varies with load. And water temp. And water quality, etc.

 

[stevens]

Re: Prop pitch vs gear ratio...

Yes, slip varies with speed. Here are some published numbers from a test of a boat like my own (Uttern D66) equipped with a 175 Verado:

rpm 2500 3000 3500 4000 4500 5000 5500 6000
mph 8.63 9.78 13.11 19.91 28.19 34.52 40.28 44.99
mpg 3.09 2.58 2.56 3.28 3.86 3.35 2.72 2.38
slip 61 % 62 % 56 % 43 % 28 % 22 % 16 % 13 %

Or, as a chart:



The boat in the test:

 

[John_S]

Re: Prop pitch vs gear ratio...

Now this thread is getting interesting!


SuperNova,

I would be very interested in any data you collected from the testing, boat and engine pkg, especially if it was an alpha drive. My assuption upto this point was, if you were off by only one increment of the gear ratio, you could probably compensate with prop pitch. Sounds like your testing might have proved that wrong.

 

[Tail_Gunner]

Re: Prop pitch vs gear ratio...

No Nova is just is just illuminating external factor's. that is water density chop etc. Now if you will the water is a constant 65 degree's no chop and no current.........keep the mediun simple as it is.

Over simplified yes and no..........actucally no, water temp mean's little even fresh or salt and there is a difference there.

Use the K.I.S.S. princple, after you find a gear prop combo that give's you a good hole shot and top speed your there...

Ahh now you might ask the qestion what give the boat the best weight balance, stern lift or bow lift that is what dives's the boat fastest, and has no bearing on this thread..

Hey Ken can you come out of retirement for a breif momenet or 2......


Opp's Steven's of course speed is going to reflect different slip value's aka how much hull is int the water....... where the motor develop's top tourqe.......but that is not in question here.........just gearing and what give's the best result's..........waves water temp current's bow lift stern lift are all secondary question's...