Prop pitch vs gear ratio...

[Texasmark]

Re: Prop pitch vs gear ratio...

I too disagree on constant slip. I took data on my current rig and "on-plane" breakover point was 40% slip whereas at WOT it dropped to 11%.

I has to vary as you guys have said. The load on the engine changes as a function of what the boat is doing. Since water is not a solid, it moves out of the way (prop slip) more as the load on the prop increases....like coming outta da hole.

Mark

 

[SuperNova]

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.

In a large number of cases where the boat has plenty of power your assumption would be correct, you can use prop size to adjust for gear ratio. Think of it this way: Gear ratio is the large or rough adjustment and prop size is the fine adjustment. Prop size can correct for improper gear ratio, but only up to a point. The general idea is to gear it so the prop selection falls into a relatively normal range. Our boat was a 26' 80's SeaRay Sundancer with a single 260 horse 350 ci and Alpha I. We initially used a 1.5 ratio (1.47) and had dropped down to a 13" pitch and still couldn't get the boat on plane (and there were plenty of test props used with varying designs and pitches) and then we installed a 1.8 gear ratio and went back through our prop selection again--all the way up to a 4-blade 21 pitch. With the 1.8 ratio and a 17 pitch we could just get her to roll over onto plane by using the trim tabs all the way down and getting all the weight forward, then once she planed off she would over-rev and the throttle had to be backed down, thus limiting her top speed to 18 mph. What she really needs, we believe, is a 1.6 ratio and then we'll figure out props again. There is more to it, and there are a couple of threads from June and July and August devoted to our testing and trying to figure this out, but this at least gives you an idea. In this case the boat is minimally powered, and that is when the gear ratio vs. prop size really show up magnified.
--
Stan

 

[Silvertip]

Re: Prop pitch vs gear ratio...

Here comes the old phart again. Boat drive train selection can be compared to the process of drive train selection for a car or truck. We obviously have an engine of any given horsepower with a torque curve of who cares! The engineers pop that engine into a car of a given size, mate that engine to a transmission with a specific set of gear ratios and feed that power to wheels with a given diameter. So we also have an engine in the boat (doesn't matter how its mounted), its mated to a drive containing a one gear transmission, and that power must feed a wheel -- in this case a prop. Now then, the car has all sorts of mechanical advantage via the tranny, the but the boat does not. If you look at transmission and final drive ratios for cars and trucks you rarely find two of them identical and in the case of trucks, multiple offerings are possible to fit the intended duty for the vehicle. So final drive selection is key, whether its a boat, car, truck, or ocean liner. If one intends to tow at maximum rating with a truck, one best select the highest ratio (don't confuse high gear and high ratio as they are opposite). However if one wants a vehicle that gets the best economy, one picks the lowest ratio, especially if one will never tow anything. Although that also gives the theoretical highest speed, wind resistance and engine power will come up short so a higher ratio (somewhere between the two) will likely provide that result. Now back to the boat comparison. If you compare lower unit ratios, you find the same variations exist from one manufacturer to the other, but they generally fall into a specific range within a few decimal points depending on engine size. Again, the engineers job is to pick a gear set that works well MOST OF THE TIME with the amount of power available, on the AVERAGE BOAT the engine will be installed on, under ANTICIPATED operating conditions, with a given diameter and pitch range of available props. All of the "averages" and "most of the times" that don't work out for those of us who seek the optimum setup, are compensated for by changine prop design and pitch. Just like a car, if a performance issue arises, there are only two basic ways to compensate. 1) more power or 2) alter gearing. In very rare cases, changing tire sizes (equivalent to the prop) will that compensate enough to overcome a power shortfall or a gearing problem. It can in a very marginal situation but its generally a too-big boat or too small motor that's the issue. I like to use Mercs standard midrange and big foot legs as an example. A 50 HP standard gearcase has 1.83:1 gear set and spins a small diameter prop. Provides fair hole shot and good top end. The same power head slapped onto a big foot gear case has a 2.33:1 gear set and can spin 13 - 14 inch props. Why have two legs. Simple, the 1.83 is used on planing hulls. The big foot primarily on pontoons and work boats that need the push from a large blade area prop. And then there's the ocean liner that spins the prop at a dizzying hundred RPM give or take a little. There's no magic here folks -- numbers are used to a point in the selection. Testing does the fine tuning. Phew -- I'm done.

 

[Texasmark]

Re: Prop pitch vs gear ratio...

Nice job Silver. Looks like you pretty well covered it and I agree. ( I know you were chewing your nails waiting for that.....grin)

Mark

 

[Silvertip]

Re: Prop pitch vs gear ratio...

Naaahhhh! I only chew my nails when someone asks what prop they need and give us nothing to work with.

 

[45Auto]

Re: Prop pitch vs gear ratio...

I thought the original question was "why the eqivelent ratios?" and is there any advantage to one over the other:

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?

To relate this to automotive gearing it would be the same as a manufacturer offering 14" wheels with 3.27 gearing, 15" wheels with a 3.50 rear end, and 16" wheels with a 3.73 rear end. The end result is the exact same when the rubber hits the road, the engine can't tell any difference. 10 revolutions of the engine will move the car 134.7". Each case would have the same pulling power, same acceleration, and same top speed. Is there any advantage to one over the other?

I have no knowledge of boat propeller engineering (some background in aircraft props though), but my guess would be that there is some limit on practical tip speed (cavitation maybe?) that limits the usable RPM of the prop. Seems most pleasure boats are geared to turn the prop around 3000 - 3500 RPM. If not, you could eliminate all the gear reduction losses and just spin a prop with half the pitch at engine RPM. Maybe the lower the prop RPM the more efficient it is? There's several good "readable" books on prop theory like this one: http://astore.amazon.com/mwallis-20/detail/0071381767 that supposedly don't require you to do the differential equations to understand it. My factory set up has always worked well enough for me that I've never worried about it!

 

[QC]

Re: Prop pitch vs gear ratio...

To relate this to automotive gearing it would be the same as a manufacturer offering 14" wheels with 3.27 gearing, 15" wheels with a 3.50 rear end, and 16" wheels with a 3.73 rear end. The end result is the exact same when the rubber hits the road, the engine can't tell any difference. 10 revolutions of the engine will move the car 134.7". Each case would have the same pulling power, same acceleration, and same top speed. Is there any advantage to one over the other?

Yes, the one with the largest wheels and the lowest profile tires will suffer the least from tire deflection and theoretically be more efficient and waste less power . . .

Seems most pleasure boats are geared to turn the prop around 3000 - 3500 RPM. If not, you could eliminate all the gear reduction losses and just spin a prop with half the pitch at engine RPM. Maybe the lower the prop RPM the more efficient it is?

I am curious about this right now as I am looking at buying an I/B boat with a 1:1 ratio. SB Chevy, so prolly 4800 RPMish. It will have a 13" diameter prop and 13" pitch. Will be my first I/B but it seems like it will be screaming . . . Maybe with the hull as one big AV plate it is different?

 

[Silvertip]

Re: Prop pitch vs gear ratio...

QC -- that combo gives you about 51 MPH using a slip figure of 14%. How well it comes out of the hole will be the test. Yes -- propeller diameter reaches a point where there are diminishing returns with RPM. Small diameter props seem to work well with small displacement engines and somewhere between 1.8 - 2.0 gears. Mid range engines handle 13 - 14 inch props with 2.3 gears. Bigger engines get bigger props and gear sets drop below 2.0 again. On the other hand, look at ocean liners. Huge props, but they turn very slowly in comparison. Two blade cleavers like high rpm (as on drag boats). It's all voodoo once you get past the basic numbers.

 

[SuperNova]

Re: Prop pitch vs gear ratio...

I thought the original question was "why the eqivelent ratios?" and is there any advantage to one over the other:



To relate this to automotive gearing it would be the same as a manufacturer offering 14" wheels with 3.27 gearing, 15" wheels with a 3.50 rear end, and 16" wheels with a 3.73 rear end. The end result is the exact same when the rubber hits the road, the engine can't tell any difference. 10 revolutions of the engine will move the car 134.7". Each case would have the same pulling power, same acceleration, and same top speed. Is there any advantage to one over the other?

As said before, this is a common trap that people fall into. This does not take prop slip and bite into account at all.

And to answer the question posed by this response: Yes there most certainly is a difference. With the 16's and 3.73's you will turn your differential much slower which will turn your axles much slower which will turn you tires much slower...all resulting in reduced stress and wear and increased component life. But on the other hand you need to take unsprung weight into account as well as inertia of the wheel/tire assembly--if it is much heavier than the 14" wheels, your suspension life will be reduced and acceleration will be reduced due to inertia of the heavier wheels.....should I go on???

 

[QC]

Re: Prop pitch vs gear ratio...

QC -- that combo gives you about 51 MPH using a slip figure of 14%.

Yeah, I got 50 MPH with 15% slip . . . Anybody know what acceptable slip is for an I/B? This is a Chris replica, straight I/B, very similar to a standard direct drive ski boat, 3 blade bronze propeller, very shallow V.

It's all voodoo once you get past the basic numbers.

And on this, Sir Silvertongue, we completely agree

 

[45Auto]

Re: Prop pitch vs gear ratio...

Yes there most certainly is a difference. With the 16's and 3.73's you will turn your differential much slower which will turn your axles much slower which will turn you tires much slower...all resulting in reduced stress and wear and increased component life.

This is a common trap that's not readily apparent. Actually the components on the 16's and 3.73's are higher stressed, assuming equal friction losses, equal diameter axles, bearings, etc. In all three cases the engine is turning at the same rpm and delivering the same power to the rear end. The pinion gears are all turning at the same speed. But with the 16's the engine torque is multiplied by 3.73 (the rear end ratio) and transmitted to the wheel through the axle. The 3.73 axles are seeing 14% more stress (3.73/3.27 = 1.14) than the 3.27 axles on the 14" tires. All components (bearings, housings, etc) should be sized to take this greater load.

As I said earlier, as far as the motor is concerned all these combinations are equal (ignoring minor variables like moment of inertia, sidewall stiffness (assume equal aspect ratio), etc). Since the motor is turning an equal RPM and outputting an equal amount of power in each case, it is readily apparent that the bigger components are more highly loaded by simply looking at the definition of power. Since Power = Torque x RPM / 5252, and the bigger tires are turning at a slower RPM, it takes more torque to transmit the same amount of power through them as is transmitted through the smaller tires at the higher RPM. This is why you can't keep putting in smaller and smaller rear end components as tires get bigger and bigger! (Maybe I could put some 44's on the old F150 and twist'em with toothpicks .......NOPE!)

But on the other hand you need to take unsprung weight into account as well as inertia of the wheel/tire assembly--if it is much heavier than the 14" wheels, your suspension life will be reduced and acceleration will be reduced due to inertia of the heavier wheels.....should I go on???

No argument that heavier components will reduce suspension life. I was ignoring minor differences. I doubt that the difference in weight between a 14" and 16" wheel would even be noticable over a typical component's lifetime. No suspension component (shocks, springs, etc) suppliers that I'm aware of even bother to ask for a wheel weight when specifying parts, or reduce the warranty if you're using a heavier wheel!

A higher moment of inertia will definitly reduce your acceleration. I seriously doubt if it would be enough to worry about in a street vehicle. If it was significant I bet all the drag racers would be running 6" diameter go-kart tires!

Please go on - always looking to learn more!

 

[Frank Acampora]

Re: Prop pitch vs gear ratio...

Been reading this post for a while and the theory is fine. But it still doesn't explain to me --and I have actually done this on my force 125 on a 21 foot cuddy-- why, with the original 1.78 to 1 gearing and a 17 inch pitch prop the engine will drive the boat at about 33-34 mph at 5,000 rpm. With 1.78 and a 19 pitch prop RPM fall off to 4,000 and top speed drops also. With 2 to 1 gearing and a 19 pitch prop it will drive the boat at 38 mph with the same engine RPM. (around 5,000)
Note that 1.78 with a 17 pitch is exactly the same theoretical forward travel as 2.0 with a 19 pitch. and engine WOT rpm is the same also.
SO: You would think that if RPM is the same and theoretical forward travel is the same then top speed should also be the same--but it's not!

Come on! Lucy, you got some splaining to do. Use small words--big ones make my head hurt.

PS. I also had the same result on a 15 foot deep vee Glastron with a 90 hp Chrysler--lower gear ratio and larger pitch give higher top speed at the same engine rpm--so it's not just that one hull and engine combo.

 

[Tail_Gunner]

Re: Prop pitch vs gear ratio...

As said before, this is a common trap that people fall into. This does not take prop slip and bite into account at all.

And to answer the question posed by this response: Yes there most certainly is a difference. With the 16's and 3.73's you will turn your differential much slower which will turn your axles much slower which will turn you tires much slower...all resulting in reduced stress and wear and increased component life. But on the other hand you need to take unsprung weight into account as well as inertia of the wheel/tire assembly--if it is much heavier than the 14" wheels, your suspension life will be reduced and acceleration will be reduced due to inertia of the heavier wheels.....should I go on???

Nova...Nova.... First you get the boat out of the hole, then you get it holding a acceptable plane speed and finally you get your final top rpm
and speed.......then ya try a bigger prop........then ya cant get out of the ....hole........it's a vicious cycle

As to gearing it's all what your looking for 2/1 1.8 for lower powerd 3.0.......1.8-1.60 for 200hp v6 and 1.5 for high hp and i believe it's all about how much tourqe the system will handle.

Now when ya get out of the hole and holding plane and hit wot at a good rpm.....well ok let's talk bow lift stern lift and slip ratio's aka that's when slip play's a big role........fine tuning..

 

[45Auto]

Re: Prop pitch vs gear ratio...

Wish I could explain it to you Frank! That's why I've been following this thread myself. Hoping somebody could explain your results. Only way I could see that you could pick up 10% more speed at the same RPM at WOT is if the 2.0/19 combo had 10% less slip or less drag. Wouldn't have to be 10% less drag because the drag climbs exponentionally (fast!) with speed. So a little less drag can make a big difference in speed. Were the 2.0 and 1.78 lowers the same except for gearing? If they were, were the props the same or were they different styles or materials?

 

[Tail_Gunner]

Re: Prop pitch vs gear ratio...

Been reading this post for a while and the theory is fine. But it still doesn't explain to me --and I have actually done this on my force 125 on a 21 foot cuddy-- why, with the original 1.78 to 1 gearing and a 17 inch pitch prop the engine will drive the boat at about 33-34 mph at 5,000 rpm. With 1.78 and a 19 pitch prop RPM fall off to 4,000 and top speed drops also. With 2 to 1 gearing and a 19 pitch prop it will drive the boat at 38 mph with the same engine RPM. (around 5,000)
Note that 1.78 with a 17 pitch is exactly the same theoretical forward travel as 2.0 with a 19 pitch. and engine WOT rpm is the same also.
SO: You would think that if RPM is the same and theoretical forward travel is the same then top speed should also be the same--but it's not!

Come on! Lucy, you got some splaining to do. Use small words--big ones make my head hurt.

PS. I also had the same result on a 15 foot deep vee Glastron with a 90 hp Chrysler--lower gear ratio and larger pitch give higher top speed at the same engine rpm--so it's not just that one hull and engine combo.

1.78 17 5000 34 = 24% slip
2.1 19 5000 38 = 12% slip

Were both prop's new and identical make and stlye, that has to be the issue......different type's can make a huge diff........the gear's were mechanical.......the prop's were the only variable aka alum ss cupped rake big blade's small blade's worn or old hub's?


Last but not least was there any difference in your hole shot and planing speed's?

 

[domains]

Re: Prop pitch vs gear ratio...

Best mod would be get a 17 1/4 ,3/8,1/2 pitch expl. Or go metric 123456789.

 

[John_S]

Re: Prop pitch vs gear ratio...

Nova, thanks for the additional info.


From the additional posts, my head is starting to hurt, too!



Frank,

Was your 17 and 19 pitch prop the same, brand, style, blades, and material? The 1000rpm drop would lead me to believe the 19 had more cupping, SS vs alum, or something else going on. Cupping alone, could make a 19 pitch prop act more like a 20 or 21 pitch.

 

[Frank Acampora]

Re: Prop pitch vs gear ratio...

On my 21 footer the 17 and 19 pitch props were both Michigan SS cupped. Both had just been repaired by the same repair shop so both were identical except for pitch.

On the 15 footer, both props were Chrysler stock aluminum (actually made by Michigan) and both were used but in excellent shape. Again, both props were essentially identical except for pitch.

In each case, gearcases were identical except for gear ratio.

But now, I'm thinking: Next season, I'm going to run this as a test and actually write down exact RPM and GPS speeds at several points of throttle travel. Then I'll have hard facts and not just my memory.

 

[wca_tim]

Re: Prop pitch vs gear ratio...

WOW! Great stuff. I have thoroughly enjoyed reading the posts above and need to think a little more about it myself. I was considering changing the gear ratio in my outdrive, but after the thoughts above and some playing over the past couple of weeks with different props, I think that the 1.84 with a higher pitch prop will be the better set-up.

I'm awful tired right now, and will post more info tomrrow or Sunday, especially since I may go test 2-3 more props tomrrow if get motivated. I;ve been working through a few props I've bought and borrowed and am really impressed with the results.

My reference point was a 23 pitch laser 2, good holeshot (heavy skiers right out of the water, etc...), top end of around 55 depending on the day. The best now is a 3 bladed 26 pitch Turbo, that was customported by a previous owner and has a significant amount of cupping.... significantly better holeshot than the laser 2, top end of 61 on the best run. both behind merc alpha 1, 4.3, 1.84 gear ratio, etc...

The turbo has flatter blades and less rake than the laser did, I was really surprised that it pops the boat out of the water the way it does. WOT is at about 100-200 rpm lower with wot at about 4600 or so...

thanks for the insights. I'll have more to add at some point...

I'm enjoying trying to better understsand this, but probably want to avoid the fluid dynamics simulations or math that requires dif eq.... I've been there before, but even mulling over the myriad of potential issues / variables in prop design is already making my head hurt...

and that's before even considering the hull design and other related factors... ouch...

 

[domains]

Re: Prop pitch vs gear ratio...

Does prop slip mean that rubber thang lol