LU gear ratios?

[Techno]

Whats the best here if top speed is your choice? I noticed that with 1.87 or whatever it would need like 32+ pitch. Do they sell a more reasonable gear set so you can run a lower pitch and still get the top numbers? I seem to remember that there is a limit due to the bullet size and thus the gear sizes which would break if too small.
Even if it was an empty case that was rebuilt, just what are the choices?
Then there are those tiny suckers speed masters? Aren't they geared more for reasonable speeds rather than tugboat speeds?:eek:

When I get my boat finished and the engine broke in I'll need to replace the welded LU and need a new prop. It seems that it would be wiser to get a correct LU and the prop to match that route- rather than a prop to match the LUs gear set your stuck with.

[150aintenuff]

there are 1.62, 1.75, 1.87, 2.0, and 2.3 ratios for the fullsize 2.5 and 3.0 L cases.

[Little Jon]

People will say different things. i would think the 1.87 would be better but the gear ratios are 1.62, 1.75, 1.87, 2.1, and 1.1 ( speedmaster ) Those are the only lower unit gear ratios i have heard of

[150aintenuff]

People will say different things. i would think the 1.87 would be better but the gear ratios are 1.62, 1.75, 1.87, 2.1, and 1.1 ( speedmaster ) Those are the only lower unit gear ratios i have heard of
the 2.3 ratio is a high altitude 135 gearcase ratio.. not reall common...

[Little Jon]

Okay thanks. I have never heard of that gear ratio. But on top speed, i will think the 1.87 would be the best because it is closer to 1.1? But for drag racing i heard people use the 2.1 gear ratio... Am I correct? OR mistakenly wrong

[Techno]

Thanks all.
Little Jon I think your wrong and right.
1.87:1 is close to 2:1 and 2:1 means the engine is spinning twice the prop speed.
1.67:1 is closer to 1:1.
Lots here seem to use the 1.87 but not sure why since this takes a large prop.

[oldschoolltv]

Merc's like to rev especially the 2.4-2.5 the omc's cant rev like merc's so we like the merc cases with 1.62's al;ong with the 3l merc's since they have rev limiters so 1.62 and big wheel equal top-end you need a torqey motor for 1.62-1.75's, 1.87 are best all around and more plentyfull to find as well, Matt

[Techno]

Whats the diff between running a 28" prop with a 1.62 and a 32" with 1.87? Isn't this torque the same?
Running the same prop does make a difference but if you negate the ratios out with the proper prop it seems like no diff at all except for prop pitch chosen.
1.87 with 32" no slip is 136 mph
1.62-28"=136
Same torque isn't it? Now I don't mean if you put a 28" on a 1.87

It seems like most use a 1.87 not because they give you more torque but because there are more of them for the hoggy fishin boats.:D

[1BadAction]

The torque at the prop isnt the same. even with the same "effective pitch" the 2:1 is going to be slightly better accelerating. you'll notice that the heavier the boat "generally" have the lower gear. if the torque could be had with a smaller pitch prop then whats the point of all the different gearsets?

and 1.62s arent for 2.5s, they are only in 3.0 cases.

[Little Jon]

Thanks for clearing that up techno.

[150aintenuff]

Okay thanks. I have never heard of that gear ratio. But on top speed, i will think the 1.87 would be the best because it is closer to 1.1? But for drag racing i heard people use the 2.1 gear ratio... Am I correct? OR mistakenly wrong

they use 2:1 because of the holeshot ability of the gearing.the actual difference physically from 1.87 to 2.0:1 is 1 tooth.

[150aintenuff]

and 1.62s arent for 2.5s, they are only in 3.0 cases.


i think inspectorlance would disagree there jim... he has a 1.62 equiped 2.5 case. granted it was custom assembled...

[1BadAction]

i think inspectorlance would disagree there jim... he has a 1.62 equiped 2.5 case. granted it was custom assembled...
nobody asked you, know it all asshat. people have inline cases with 1.87s, doesnt mean that merc made 1.87s for them. :rolleyes:

no go off yourself, the whole S&F community would be a much happier place.

[150aintenuff]

nub you said it.... i simply stated a 1.62 would work in a 2.5 case...

[1BadAction]

http://img340.imageshack.us/img340/3897/offyourself2xy5.jpg

Make like nike and just do it.

[150aintenuff]

http://img340.imageshack.us/img340/3897/offyourself2xy5.jpg

Make like nike and just do it.



dude every time i try someone frikken stops me for some dumb reason...

[Techno]

1BadAction Could you explain that? This makes no sense to me.
A 2:1 has more torque of course. But how much torque the prop shaft has isn't in question. Its how much thrust or torque is being transmitted to the water.
At least this is the way I see it.:confused:
Or to put it in gear language.
2:1 driving a 24 inch gear is the same as using a 1.62:1 driving a 19" gear.
The first is gear up then down.
The second is geared down then up.
If there was no gear box at all then a 12" gear would be needed.

The reason for different gear sets is to stay away from 12" pitch or 35" pitch props and nearer the optimum pitch. I would assume.

[1BadAction]

a prop isn't a gear thats creating a driving force against a solid object, so the comparison isn't valid. I don't even think I would call a prop a "gear" - regardless, theres way more to it than I really want to get in here, but basically, as you spin that 19" prop on the 1.62 faster than the 24" prop on the 1.87 to get the same MPH, the drag goes up exponentially, therefore sucking up more engine TQ.

[Techno]

I think you have it backwards. a 1.87 gear set spins faster. It should be using more already. The engine gear is spinning 1.87 times for every 1.62 times of the other. Both prop shafts are turning once.
Equalizer: the less spinning less lossy 1.62 now has to spin faster and assumes the same loss as the 1.87, or close enough not to matter. Compromise. If anything, increasing leverage increases loss not the other way around.

If you don't want to call a prop a gear just call it a lever. A gear is a lever and so is a prop. The only thing that matters about the prop not driving a solid surface is going outside its preferred pitch, which is pretty much what I'm driving at.:D The final drive is a prop on any gear ratio so they can be compared to gears.

I'll take it to the extreme. The torquey tug boat gear set now has to run a 50" pitch prop. This will plain suck. But why run a tug boat at 100 mph?
On the other gas sipping torque cheap ratio you have to run a 1" pitch. That sucks too. Again why run a high speed gear set as a tug boat?
Somewhere between 50" and 1" there is a best prop pitch and the gear ratio should enable it, not the prop enable the gear ratio.
I'm not trying to be an A:eek: but I don't see why I would want to run a 32" prop when I could run a 26" prop for my purposes, or why someone would run a 19" prop when they could run a 26" for thier purposes. This is what I'm wondering, getting at, being an A:eek: about.:D

I think I failed to mention the golden ratio prop thing. Thanks David.;) I had assumed it. :o

[1BadAction]

1.87 gear set spins faster. a 1.87 gearset spins the propshaft SLOWER to get the same speed from a bigger prop. we're not talking about engine rpm because that has absolutely ZERO to do with prop speed and hydrodynamic drag. you can talk all you want about the better efficiency from a 28 VS a 32, but unless your boat is a potato chip or has a big CID or high TQ DFI engine on the transom, its going to be a pig. The better efficiency (which I question if this is true, if it is, its VERY slight) of the 28 over the 32 is not enough to make up for the need to spin it faster. I guess the STV guys down here with bone stock 280s running 125 to 130mph with 34p cleavers don't know what they are doing with setups right? :rolleyes:

we aren't talking about levers or tugboats, wild changes in prop pitch/hull weight, or how something works in "theory", we are talking about THE REAL WORLD and 2.4/2.5L lake weight fast boats with pitches that are VERY close to the optimum ratio. (which, btw isn't much more than a rule of thumb for bench racers.) and for that, the 1.87s are about perfect. Just look at the gearing on the factory engines, I-3s and 4s get 2.3s mostly, 2.0s get 2:1s usually, 2.5s get 1.87s, and 3.0+ usually get 1.75s or high perfs get 1.62s.

now, if you want to run a lower revving 3.0L merc or omc that makes more TQ and doesnt spin as high, then the 1.75s or 1.62s might be a better choice- or a high strung 2.0 and 2:1s on the other end of the spectrum... but thats a whole different subject.

[oldschoolltv]

when you get up to 32 pitch props they act like paddle wheels and try and walk sideways more then forward, I was told along time ago that a 28 pitch is about the most efficent big size you want to go to, just imagine if they mad a 45 pitch prop do you think it would work or would it go sideways instead, yes they are putting up some great numbers with 32-34 props but I wonder if they had a 1.75 ratio could they go a little faster? I was also told if someone offers you to try a prop try it, as you never know what will work until you try it as therory's have holes in them, Matt

[1BadAction]

when you get up to 32 pitch props they act like paddle wheels and try and walk sideways more then forward, I was told along time ago that a 28 pitch is about the most efficent big size you want to go to, just imagine if they mad a 45 pitch prop do you think it would work or would it go sideways instead, yes they are putting up some great numbers with 32-34 props but I wonder if they had a 1.75 ratio could they go a little faster? I was also told if someone offers you to try a prop try it, as you never know what will work until you try it as therory's have holes in them, Matt

yep thats right as far as "paddlewheel" effect. according to both keys Dave and some of the other guys, the 1.75s just don't work good on the 2.5s. You gotta keep in mind these props arent straight pitch, they have both cup and progressively more pitch from the base of the blade up. thats where the prop shop comes in. :D

Of course a 45 pitch prop thats 14.5" diameter would be crap no matter who works it, but thats 11" of pitch more than the biggest wheel we can run, and about 17" pitch sizes bigger than "optimum"

[150aintenuff]

the Offshore guyts are turning upwords of 40P and arent whining about it... but they also get 19" of diameter

[Techno]

After thinking about shep70057 post, 19-22" props I think merc builds these things for those people and not us, us meaning me.;)
Yes I do have a potato chip. I wonder if any of the Stvers running the large pitchs spent the same time on the lower pitchs. What works for others is usually what you do since it works and saves you time. That don't mean its best. Have they run 1.62s instead of these paddle wheels? Or are they using 1.87s because this is what came with thier fishin motor and others told them to use it?

I'm still lost, your saying that 1.87 gears produce more torque. Yes they do on the SHAFT and then they lose it with the 34" prop. So how does a 32-34" prop not reduce the torque a 26-28" prop would provide? Your totally ignoring that the prop has the final word on what is finaly coupled to the water. I don't care what a dyno on the prop shaft is reading I care what the lake is reading... or more acurately the speedo.;)

[1BadAction]

I have explained it multiple times and you still are clueless. The faster you spin these props, the higher the TIP SPEED is, exponentially increasing the DRAG in the water, thus REDUCING THE OVERALL POWER that is available to push the boat forward. Of course there is a point where more gear is better than more pitch in higher pitches, like 36 or so, thats why you don't see them manufactured. The 32 and 34 cleavers are also unique designs, and im sure that has something to do with the lost eff. over the smaller props. But, whatever, you've already made your mind up about what you think is better, so I might as well be talking to the garbage can sitting next to me...

what I don't get, is these guys have ran every case and prop combo under the sun on their lightweight stvs with 260+ HIGH PERF motors for more hours than you will ever think of running, and you think you know more about setup than what they do. :rolleyes: you and 150istoomuch must be related.

PS- your boat aint a potato chip. a boat thats 1000lbs RTR with driver and fuel is a chip.

[oldschoolltv]

1bad what slip numbers or rpms do they turn the 32-34's to get the 120's? I have wanted to try a big 32or 34 cleaver since my motor isnt a high rev'r but if the slip goes up to get the good speed It would'nt help as much for me.

[rpm racing]

Or are they using 1.87s because this is what came with thier fishin motor and others told them to use it?

Techno, I don't think anyone that is running over 130 is doing what others are telling them.

It amazes me when people who have little or no set up experience ask questions and then are lucky enough to get the right answers still are determined that there "theroy" is right and not follow the advice?

WHY BOTHER ASKING IF YOU KNOW WHAT YOU ARE GOING TO DO.

I boat with a guy every weekend that will spend any amount of $$ to get that last few mph out of his River Rocket and he has tried all the gears and will tell you that the only ones that will work are 1.75 and 1.87 on a STV with strong 2.5 on it. He has been running 1.87 for the last few years.

And I highly doubht that there is anything light weight about your STV.

I have a 915 pound boat ready to run and it cost big $$$ to get these things together this light.

[1BadAction]

1bad what slip numbers or rpms do they turn the 32-34's to get the 120's? I have wanted to try a big 32or 34 cleaver since my motor isnt a high rev'r but if the slip goes up to get the good speed It would'nt help as much for me.

I think around 12%, but the cleavers are much better at the higher speeds than what they are at 110 or so. Might want to see what Dave ran on hotshots boat to get it up there, I believe it ran 122ish.

[Mark75H]

a prop isn't a gear thats creating a driving force against a solid object, so the comparison isn't valid. I don't even think I would call a prop a "gear" - regardless, theres way more to it than I really want to get in here, but basically, as you spin that 19" prop on the 1.62 faster than the 24" prop on the 1.87 to get the same MPH, the drag goes up exponentially, therefore sucking up more engine TQ.

Yep, there is a lot more to it (and I don't really want to go into it either). Here's a little sampe: Drag goes up if they are the same diameter, but if the boat is light, they don't have to be the same diameter and you can shed some of that drag with smaller diameter

If you aren't able and ready to consider 4 or 5 dimensions at once, it's not for you ...

The 40 X 19 Offshore props probably aren't turning 4,000 rpm very often; that's why they can get away with so much pitch (and diameter) ... again it is the tip speed and drag thing

[Techno]

I asked a question and got opinions. Why shouldn't I question these opinions.

THEN FINALY he says tip speed on the prop when through all the posts he was dissin the prop. No where did he say tip speed of the prop. In fact he discounted the prop as being anything in this torque equation.
I can see tip speed as being a something.:)

So. What happens when an STV runs this 1.62? Or am I not allowed to ask?:D

BTW mine is a heavy weight at 1,250 rigged. Although this isn't complete so could go up or down a smidgen.

[1BadAction]

I said prop speed in post #18- which includes the tip. :p

[Hottrucks]

to things to think about your car
1st gear with spin the tires
2nd will get you rolling
3rd you can hardly move why ???

same amount of torque???

But if I put smaller tires ( props) on I can still burn the tires in 1 and 2 and don't have to smoke the clutch in 3???
But can I go as fast???

if a small prop spunfast isn't as efficient than why do the V 8 drag boats and boat like miss Budwieser use such small ones??? I would guess they are just wasting $$$

Just food for thought!!!!!

[The Big Al]

if a small prop spunfast isn't as efficient than why do the V 8 drag boats and boat like miss Budwieser use such small ones??? I would guess they are just wasting $$$

Just food for thought!!!!!

They are over driven, ratio like 1 to 3

Engine 1
Prop 3

[1BadAction]

They are over driven, ratio like 1 to 3

Engine 1
Prop 3

yep, and their HP per LB is also something around 6hp per 1lb of weight (in the case of drag boats) I'm pretty sure none of us come close to that.

[Instigator]

I don't know.

About the time I think I have prop pitch vs dia. vs gear ratio figured out I try a combination that does something totally opposite of what all the experts (including me) "know" it will do.

Here's a couple of things I know through experience and or observations.

Lot's to be learned through formula's and theories (I use them all) but doesn't always equate at the water!

On the light/fast boats (I think this applies regardless of design) if it's a 2.4 or 2.5 Merc you run 1.87's.
Seems to be to get the most from those motors yoou need to let them spin 8500 and up.
In doing so you give up toque to get there.
If you take one of those motors and put 1.75's or 1.62's in them it would be like like putting freeway gears (3.23's) in a car with a big cam in it.

The 3.0 liter Mercs and JohnRudes are grunt motors and work better in the 7 to 8K range which limits you on top speed with 1.87's in the Merc and 1.86's in the OMC.

That being said, I learned a long time ago that a lot of 120 MPH boats will lose to a 100 MPH boat on the lake.
The guys that are really fast (they got that way by trying EVERYTHING regardless of what the experts told them) know that it takes perfect conditions and a lot of water to get there.

If on the other hand your boat runs 95 - 105 but gets there from 40 or 50 in a hurry, you will win a lot of races against the guys claming 115+.

The boat/motor combo will like what it likes regardless of what we think/know.

I know on my old Vector project I ran everything from 22" to 32" on it.

The motor RPM limited out with the 22" and didn't have the grunt to pull the 32" (1.86 gears).
One of the most surprising parts of the above though was that there was only 2 or 3 MPH difference on top with this hude range in pitch.
I wouldn't have believed if it not for doing it myself.

The 30 was the fastest (by 1 MPH) but as already said, everything had to be perfect and it took a long time to get there. (meaning worthless on the lake)

The best all around prop was a 26" drag copy. Boat was only 543lbs so it made a great drag prop and still ran 98/99 MPH but would do it any time anywhere and made the boat cake to drive.
Another impportant point w/running the 26 is it is way better for the motor than running a 30 or 32 which keeps the motor highly loaded the whole time.

Gary

[terry taylor]

HI. I think your last two sentences provide many answers with this thread thanks

[jphii]

On the light/fast boats (I think this applies regardless of design) if it's a 2.4 or 2.5 Merc you run 1.87's.
Seems to be to get the most from those motors yoou need to let them spin 8500 and up. In doing so you give up toque to get there.
If you take one of those motors and put 1.75's or 1.62's in them it would be like like putting freeway gears (3.23's) in a car with a big cam in it.

The 3.0 liter Mercs and JohnRudes are grunt motors and work better in the 7 to 8K range which limits you on top speed with 1.87's in the Merc and 1.86's in the OMC.

Several OMC drag racers run 1.75's and turn over 9k. Joed here has a T3 with a 3L Merc and it spins 6200 and runs 111 with a 1.62 and 32 Soddy-Daisy, and it's a great all around prop. I'm going to try a 1.65 OMC lower. There are all kinds of people running all kinds of different ratios. You never know until you TRY IT YOURSELF. It all depends what you're looking for.

That said, with the 1.86 gears in mine I've run everything from a 20 to a 34. The fastest prop I've ran was a 30 Spinelli with a ton of work done to it.

[David - WI]

if a small prop spunfast isn't as efficient than why do the V 8 drag boats and boat like miss Budwieser use such small ones??? I would guess they are just wasting $$$

Just food for thought!!!!!

I think it's because being "efficient" isn't the most important thing for those boats... they need to be "quick" rather than "fast". They never reach their "ultimate" top speed while racing anyway... that's not what they're built for.

Those engines need to turn high rpm's to make power, and a large diameter prop has too much "bite" in the water. A smaller diameter prop "slips" more off the line or out of the corner, letting the motor spin up into it's useful powerband more easily.

It's kind of like running a high stall-speed torque convertor in drag car... it's not very efficient but it lets the motor spin up quickly and start making useful power earlier.

They are "wasting" some horsepower on the top-end in exchange for better acceleration. Efficiency just isn't their main priority.

[Techno]

Instigator If I change your lake to river or assume your talking drag racing then what you said makes sense. But when some ones says lake. Drag racing is the last thing I'm thinking of.
I've noticed many people have a picture in their head and this is the way they boat. This picture doesn't always translate to other peoples mental picture.

I picture this when I think "lake":D
http://upload.wikimedia.org/wikipedia/commons/thumb/c/c1/New_York%27s_Finger_Lakes.jpg/300px-New_York%27s_Finger_Lakes.jpg
The last thing on my mind is not overtaking a slower boat or constant face shredding launches.:p Although those are fun.
For scale the 2 long ones in the middle are about 40 miles long.
I think I'll start a thread on that.:)