Gear ratio....i'm so lost

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JoeM
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Gear ratio....i'm so lost

Postby JoeM » Fri Nov 17, 2017 8:39 am

I may be putting the cart in front of the horse as I don't even have lumber to saw yet. But I've been trying to wrap my head around different transmission gear ratios.

I THINK I understand the concept of marine gearing. In a 2:1 ratio the Engine runs at 2,000rpm while the prop shaft spin at 1,000rpm.

I THINK I understand the change in torque in relation to gear ratio. I'm becoming familiar with the HP formula. In that same 2:1 ratio I THINK the torque increases at the shaft even though the engine is turning the same rpm.

In a separate yet related note, I understand that props can be a way of fine tuning for improper gearing. That through changing pitch you can theoretically get the same performance out of different sized props. Unfortunately I get lost when they start talking about which is best for hole-shot, slow speed, etc.

What i'm completely lost on is why to choose one over another? I've read posts on this and other forums about this topic and it honestly confused me more. Higher ratio, turning higher prop slower is better for larger cruising type vessels? Lower ratio, turning smaller prop faster is better for fast planing type boats? Higher ratio, bigger prop better for slow and efficient yet little hole shot ability?

I am going to build the 25' stretched to 27' True Grit semi-displacement hull, trying to set up to be efficient at 10kts but hope to be able to be pushed to 16kts or more occassionally. I'm currently thinking about using a new 3.0L Mercruiser with 135hp rated set up as a central straight shaft inboard setup, I do realize I won't be getting the full HP at the prop and that the 135hp is more like 120hp. It's max rpm is 4,400-4,800. Plans estimate a 16" prop. My current thinking is that I do NOT want a 1:1 and at minimum want a 1.5:1 so I can turn a bigger prop and be more efficient at slower speeds? I realize that I can make up for some improper gearing through playing with different size and pitched props. But I don't even know if my thinking makes sense and would like reassurance that i'm on the right track or some advice on how to get on the right track.

Thanks all!
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steveh41
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Re: Gear ratio....i'm so lost

Postby steveh41 » Fri Nov 17, 2017 9:24 am

Joe,

This is a great reference on boat propellers with charts and formulas for choosing the correct prop in a given setting...

http://www.gerrmarine.com/PROPELLER_HANDBOOK.html

Steve
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Jimbob
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Re: Gear ratio....i'm so lost

Postby Jimbob » Fri Nov 17, 2017 10:36 am

Hi Joe,
This is what I remember when I had my runabout with an i/o.
I had a prop with a 21 pitch which meant that for each revolution it should advance the boat 21" in the water.
I also had a prop with a 19 pitch which meant that for each revolution, it should advance the boat 19" in the water.
The 19 pitch prop was like a low gear. And is the one I would use for pulling skiers. I could pull a stump out with that prop.
I used my 21 pitch prop when I was crusing and wanted to get the maximum top end speed. (up to 70 mph)
Diameter of the prop also matters. I had two props of the same diameter.
The thing that you want to do is select a prop that will max out the rpm's at full throttle. Sometimes a shop will let you try different props out to see what is best for you. Prop shops can sometimes rework the prop changing the cup and pitch. That's how I ended up with the 19 pitch prop. I had a damaged prop reworked.
Jim
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Bill Edmundson
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Re: Gear ratio....i'm so lost

Postby Bill Edmundson » Fri Nov 17, 2017 12:41 pm

Joe

I'm no expert on these hulls, for sure. I don't know about HP on these boats. But, you will be looking for a shaft RPM of 3000 w/16" prop. That gets you to a 1.5:1 gear. A 16" prop you will probably need a 1 1/4" shaft, maybe 1 1/8". I think a starting point on the prop would be a 16x6.25. If you have the HP, that might get you to 16 knots at WOT. Ten knots would be about 2800 RPM.

Again, I'm not a prop expert. I don't know about prop rake or cupping. Rake should not be an issue for you. Cupping is tuning. I think this will get you close. This was all based on a !5% slip ratio.

Bill
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Re: Gear ratio....i'm so lost

Postby JoeM » Fri Nov 17, 2017 1:05 pm

steveh41 wrote:Joe,

This is a great reference on boat propellers with charts and formulas for choosing the correct prop in a given setting...

http://www.gerrmarine.com/PROPELLER_HANDBOOK.html

Steve


Thanks for the recommendation, I have added this to my wishlist.

Jimbob wrote:Hi Joe,
This is what I remember when I had my runabout with an i/o.
I had a prop with a 21 pitch which meant that for each revolution it should advance the boat 21" in the water.
I also had a prop with a 19 pitch which meant that for each revolution, it should advance the boat 19" in the water.
The 19 pitch prop was like a low gear. And is the one I would use for pulling skiers. I could pull a stump out with that prop.
I used my 21 pitch prop when I was crusing and wanted to get the maximum top end speed. (up to 70 mph)
Diameter of the prop also matters. I had two props of the same diameter.
The thing that you want to do is select a prop that will max out the rpm's at full throttle. Sometimes a shop will let you try different props out to see what is best for you. Prop shops can sometimes rework the prop changing the cup and pitch. That's how I ended up with the 19 pitch prop. I had a damaged prop reworked.
Jim


That helps me understand prop pitch a little better, thanks!

Bill Edmundson wrote:Joe

I'm no expert on these hulls, for sure. I don't know about HP on these boats. But, you will be looking for a shaft RPM of 3000 w/16" prop. That gets you to a 1.5:1 gear. A 16" prop you will probably need a 1 1/4" shaft, maybe 1 1/8". I think a starting point on the prop would be a 16x6.25. If you have the HP, that might get you to 16 knots at WOT. Ten knots would be about 2800 RPM.

Again, I'm not a prop expert. I don't know about prop rake or cupping. Rake should not be an issue for you. Cupping is tuning. I think this will get you close. This was all based on a !5% slip ratio.

Bill


The plans call for a 1 1/8" minimum diameter prop shaft. I had read some people had problems finding a props for the larger shafts, is that true?

Thanks Bill, I think this is closer to answering my question and reinforced my thinking about needing at least a 1.5:1 ratio.

I'm still a little fuzzy on the WHY to choose a transmission ratio over another but perhaps after reading more on props i'll understand. For now I'll just continue with the plan being 1.5:1 ratio and choose prop according to that.
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Re: Gear ratio....i'm so lost

Postby Bill Edmundson » Fri Nov 17, 2017 1:25 pm

Joe

Check with the prop-shops on the shaft dia. 1 1/8 may be plenty. 3000 RPM gives a blade tip speed near 200 MPH. After that you can get blade tip cavitation on a 16" prop.

Bill
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kens
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Re: Gear ratio....i'm so lost

Postby kens » Fri Nov 17, 2017 6:44 pm

The deeper the gear ratio, the bigger a boat you can push with the SAME engine, albeit slower.
Your OP about prop size tuning for improper gear ratio, Hmmmmm, no.
If you have the improper gear ratio installed, then no prop can recover from that.
However, you choose the closest gear ratio to perfect, within the boundaries of transmission manufacturer and type of tranny; then choose prop as the final item at sea trials.

Typical gear ratios are in .5:1 increments. example 1:1 1.5:1 2:1 2.5:1 3:1 you choose the closest gear ratio from this list and prop to suit. there is a 'window' of gear ratio for your boat, and you work within that window. Very few of the tranny manufacturers have the exact same gear ratios, and there is variations in tranny types, so you get a wide selection of ratios all things considered.

As you go to a deeper gear ratio all the boat & shaft & strut & rudder componants get bigger as well. (because of the increase in torque) the boat gets bigger too, because of the bigger strut to hang the bigger prop, to maintain the same shaft angle, the engine moves forward, to maintain the same CG, the boat gets longer. to turn the bigger boat, the rudder gets bigger. The gear ratio is a 'package deal' that is built into the boat.

I am running the 3.0L with the VelvetDrive 2:1 in a 25' Double Eagle. If you build the 27" True Grit, with 3.0L, and 16" prop, you would also look at a 2:1 gear or similar ratio. No shallower than 1.5:1 and no deeper than 2:1. Shaft diamter is 1/14 of prop diameter.
Oak is over rated, everything about it takes extra time; then it warps, splits or checks !!! :roll:

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Re: Gear ratio....i'm so lost

Postby Bill Edmundson » Fri Nov 17, 2017 7:28 pm

What Ken said. Plus, depending the exposed shaft length you may want to consider a whip strut.

Bill
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Re: Gear ratio....i'm so lost

Postby JoeM » Fri Nov 17, 2017 7:57 pm

kens wrote:The deeper the gear ratio, the bigger a boat you can push with the SAME engine, albeit slower.
Your OP about prop size tuning for improper gear ratio, Hmmmmm, no.
If you have the improper gear ratio installed, then no prop can recover from that.
However, you choose the closest gear ratio to perfect, within the boundaries of transmission manufacturer and type of tranny; then choose prop as the final item at sea trials.

Typical gear ratios are in .5:1 increments. example 1:1 1.5:1 2:1 2.5:1 3:1 you choose the closest gear ratio from this list and prop to suit. there is a 'window' of gear ratio for your boat, and you work within that window. Very few of the tranny manufacturers have the exact same gear ratios, and there is variations in tranny types, so you get a wide selection of ratios all things considered.

As you go to a deeper gear ratio all the boat & shaft & strut & rudder componants get bigger as well. (because of the increase in torque) the boat gets bigger too, because of the bigger strut to hang the bigger prop, to maintain the same shaft angle, the engine moves forward, to maintain the same CG, the boat gets longer. to turn the bigger boat, the rudder gets bigger. The gear ratio is a 'package deal' that is built into the boat.

I am running the 3.0L with the VelvetDrive 2:1 in a 25' Double Eagle. If you build the 27" True Grit, with 3.0L, and 16" prop, you would also look at a 2:1 gear or similar ratio. No shallower than 1.5:1 and no deeper than 2:1. Shaft diamter is 1/14 of prop diameter.


Thank you Kens!

That cleared up a lot for me and reinforced my thinking in regards to my application! Are there any books on this subject that you would recommend? Expanding my boat building library as I delve deeper into the choices.

So the prop can fine tune to a very small degree but will not make up for a larger discrepancy in ratio so get it as close to right the first time. How does one determine the window? I sometimes dislike the fact that I am not simply satisfied with knowing the answer but need to know how it was reached...Am I broaching into territory where if I really want to know I should just get books from Naval Architect programs?

I read your thread where you tried to find the right prop, and it had a lot of good information that i'm still digesting.

Bill Edmundson wrote:What Ken said. Plus, depending the exposed shaft length you may want to consider a whip strut.

Bill


Thanks Bill! The plans don't call for it but I was wondering if I should. At what length would one want to install a whip strut? I know it's not much but would rather not have the efficiency loss of another bearing if not needed.
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Re: Gear ratio....i'm so lost

Postby kens » Sat Nov 18, 2017 6:09 am

The books by Gerr are good. one is mentioned above.
You get the gear ratio by figuring the size, weight, shape of the boat plus the engine HP installed.
Your speed will be a function of HP vs weight.
When you get your speed you use engine rpm to get gear ratio.
When you got HP and gear ratio, you can get prop diameter.
When you get prop diameter you can get pitch.

The best efficiency will come with the biggest prop diameter, which comes with the deepest gear ratio.
I know that my boat is geared 2:1, and I am on the deep end of the ratio window. If your boat is a little bigger and a little heavier, then the 2:1 would fall into the window.
The books by David Gerr are good, and Glen-L has a good book on inboard motor installations.
Oak is over rated, everything about it takes extra time; then it warps, splits or checks !!! :roll:

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Re: Gear ratio....i'm so lost

Postby JoeM » Sat Nov 18, 2017 11:28 am

kens wrote:The books by Gerr are good. one is mentioned above.
You get the gear ratio by figuring the size, weight, shape of the boat plus the engine HP installed.
Your speed will be a function of HP vs weight.
When you get your speed you use engine rpm to get gear ratio.
When you got HP and gear ratio, you can get prop diameter.
When you get prop diameter you can get pitch.

The best efficiency will come with the biggest prop diameter, which comes with the deepest gear ratio.
I know that my boat is geared 2:1, and I am on the deep end of the ratio window. If your boat is a little bigger and a little heavier, then the 2:1 would fall into the window.
The books by David Gerr are good, and Glen-L has a good book on inboard motor installations.


I appreciate all the help! The picture is coming into focus for me now.

I have the Inboard Motor installation book by Glen-L and am glad I got it. The motor/running gear will be my biggest learning hurdle and while I am not an expert by any means, I feel I'm going to be able to do the install now. Likely by the time i'm ready to actually do it i'll be even more confident about it. I have added the David Gerr book on props to my wishlist, Christmas and my birthday are right around the corner. My wife is already itching to put decorations up which means i'll be on the roof putting up lights soon enough.
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Re: Gear ratio....i'm so lost

Postby Bill Edmundson » Sat Nov 18, 2017 1:10 pm

Joe

With a 2:1 ratio and 4600 RPM, I'm seeing a 16x11 prop. 16x10 at maybe 4800 and 16x12 at maybe 4400.

Let me try a different approach. What RPM is the max Torque and continuous operating RPM? That should be near the cruising speed. Boats are really about torque not ultimate HP.

Bill
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Re: Gear ratio....i'm so lost

Postby JoeM » Sat Nov 18, 2017 3:38 pm

Bill Edmundson wrote:Joe

With a 2:1 ratio and 4600 RPM, I'm seeing a 16x11 prop. 16x10 at maybe 4800 and 16x12 at maybe 4400.

Let me try a different approach. What RPM is the max Torque and continuous operating RPM? That should be near the cruising speed. Boats are really about torque not ultimate HP.

Bill


Thanks for the numbers!

I honestly don't know. I've looked for a performance chart with torque, rpm, and hp but have yet to find one. I've only found tests for specific hulls where they focus on hp, speed, mpg. Can I plug and chug into the HP formula to generate the curve on my own, though I don't know the specifications for continuous operating RPM? I assume I can and I will play around with that later and report back. Off to a family thing now.
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Re: Gear ratio....i'm so lost

Postby Bill Edmundson » Sat Nov 18, 2017 7:18 pm

Joe

If your engine will turn 4800, I assume it's not diesel. At 10 knots, that's about 65% of WOT. That is about right for gasoline. Diesels at 80-85%. The prop estimates should be good as starting point. Cruise RPM should bee about 2875.

Bill
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Re: Gear ratio....i'm so lost

Postby kens » Sat Nov 18, 2017 10:40 pm

There is no real performance curve nor numbers for the little 3.0l mercruiser, it doesnt exist.
I have already researched that. It is not there, and, that little engine does not make 135hp. nor 140hp, nor 130hp.
it is reliable, and makes torque, it is a good engine, but the HP numbers don't exist.
that's because it doesnt make much HP. but, it will make torque.
I have the old version (non Vortec) and it makes about 112HP.
That is, if you assume for gasoline to provide about 10HP/per gallon/per hour. and that is generous.
My 3.0L engine burns 8.0 gallons per hour balls-to-the-walls.
That indicates to me 80HP.
At cruise, I burn 5.0 gallons per hour, and that tells me 50HP.
There is only so much BTU's of energy per gallon of gasoline, and as such, my little 3.0L engine is telling me that it is 112HP at the flywheel.
If you start doing the math with 112HP at the flywheel, then DEDUCT transmission losses, then DEDUCT strut bearing losses, and DEDUCT all the other losses, what you end up with is the numbers posted by my fuel flow measurements, AND that of Glen-L who designed it.
And at the end of the day, it is about 80HP AT THE PROP. hence my numbers of 8.0 gallons per hour, @ 10 HP per gallon per hour.

If you build your 27' True Grit, and install a 3.0L VORTEC engine, you may go up to 115HP @ flywheel, but not much beyond that.
It is a good engine, BUT, don't let the salesman sell you into 130HP. it doesn't exist.
It is a good engine, but only buy into 115HP.
It will make torque though.
Oak is over rated, everything about it takes extra time; then it warps, splits or checks !!! :roll:


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