Performance Boats Forum banner

1 - 20 of 82 Posts

·
Resident Ford Nut
Joined
·
10,075 Posts
Discussion Starter #1
lets say you have a low reving engine that makes 800 lbs ft. at 3,500 rpm. If you put a 2:1 gear box behind it and the out-put shaft is spinning 7,000 would that be the same as having 1,066 hp ? ( less any loss to spin the gear-box)

S CP
 

·
Premium Member
Joined
·
25,975 Posts
lets say you have a low reving engine that makes 800 lbs ft. at 3,500 rpm. If you put a 2:1 gear box behind it and the out-put shaft is spinning 7,000 would that be the same as having 1,066 hp ? ( less any loss to spin the gear-box)

S CP
Nope. any time you gear uup you gain RPM but lose torque, same goes the other way. Its why you car pulls like hell in low gear. Boat loads of torque traded for
RPM. My V-drive has less TORQUE at the prop than at the crank because of the overdrive. But it stills has the same HP minus frictional loss from the drive train. You don't lose HP(again except frictional losses) fro the gearing, you lose torque and gain RPM. Reverse it, and you gain torque and lose RPM. At the end, the Torque X RPM / 5252 still hold true. Make sense?



100% free webcam site! | Awesome chicks and it is absolutely free! | Watch free live sex cam - easy as 1-2-3
 

·
Highaboosta
Joined
·
3,431 Posts
That's a good explanation Bob.
I understand about torque multiplication and torque division but I never considered that the HP would remain the same, it just changed RPM.
 

·
Resident Ford Nut
Joined
·
10,075 Posts
Discussion Starter #4 (Edited)
Nope. any time you gear uup you gain RPM but lose torque, same goes the other way. Its why you car pulls like hell in low gear. Boat loads of torque traded for
RPM. My V-drive has less TORQUE at the prop than at the crank because of the overdrive. But it stills has the same HP minus frictional loss from the drive train. You don't lose HP(again except frictional losses) fro the gearing, you lose torque and gain RPM. Reverse it, and you gain torque and lose RPM. At the end, the Torque X RPM / 5252 still hold true. Make sense?

No............ maybe bp can explain it to me ............ :)bulb

Power plant is turning at it's max load/power 800 lbs ft at 3,500 if the gear box is spinning at 7,000 how am I losing power ?? other than the drive-line loss ?

if I am losing power ( or trq) because of rpm how much would it be ?


Something told me it wasn't straight across but I don't get it........at least not yet.

Edit: So if the gear box takes 85 lbs ft to spin at 3,500 so the net at the out-put shaft would be 715 lbs ft that would be 953 hp vs 1,066 at 7,000?

S CP
 

·
Registered
Joined
·
6,573 Posts
No............ maybe bp can explain it to me ............ :)bulb

Power plant is turning at it's max load/power 800 lbs ft at 3,500 if the gear box is spinning at 7,000 how am I losing power ?? other than the drive-line loss ?

if I am losing power ( or trq) because of rpm how much would it be ?


Something told me it wasn't straight across but I don't get it........at least not yet.

Edit: So if the gear box takes 85 lbs ft to spin at 3,500 so the net at the out-put shaft would be 715 lbs ft that would be 953 hp vs 1,066 at 7,000?

S CP
sorry jon, can't explain it any better than gn did.

your "edit" example is confusing. i could guess what you're asking, but not gonna do that. i do not understand your questions about "losing" power.
 

·
Highaboosta
Joined
·
3,431 Posts
Look at it this way,
Say you have an I/O which is 2:1 gear ratio underdriven.
5000 rpm and 500 ft lb input is only 2500 rpm output rpm but torque is multiplied by 2 to 1000 ft lbs.
The input gear has more leverage against the output gear.

Now consider a vdrive with 2:1 overdriven gears.
5000 rpm and 500 ft lb input gives 10000 rpm output but torque is divided by 50% to 250 ft lbs.
 

·
Registered
Joined
·
6,573 Posts
Look at it this way,
Say you have an I/O which is 2:1 gear ratio underdriven.
5000 rpm and 500 ft lb input is only 2500 rpm output rpm but torque is multiplied by 2 to 1000 ft lbs.
The input gear has more leverage against the output gear.

Now consider a vdrive with 2:1 overdriven gears.
5000 rpm and 500 ft lb input gives 10000 rpm output but torque is divided by 50% to 250 ft lbs.
... with no frictional losses...
 

·
Banned
Joined
·
1,263 Posts
Gears multiply power inputed to them by their ratio.
Gears divide rpm inputed to them by their ratio.


EX:
400ftlbs from engine into a 4:00 gear ratio = output of 1600ft/lbs
5000rpm from engine into a 4:00 gear ratio = output of 1250 rpm


Hope that makes it easier.
 

·
Highaboosta
Joined
·
3,431 Posts

·
Banned
Joined
·
1,263 Posts
No............ maybe bp can explain it to me ............ :)bulb

Power plant is turning at it's max load/power 800 lbs ft at 3,500 if the gear box is spinning at 7,000 how am I losing power ?? other than the drive-line loss ?

S CP
Forget about frictional hp for a minute to a grab a hold of what is going on.

Your engine is not losing power. It is what it is. However, actual shaft power, with your gear ratio example, the power after the gears will be less than the motors.

Gear Ratio = Power Multiplier (and output shaft rpm)
2:1 gear ratio = 2 (shaft rpm will be 1/2 the engine's rpm)
1:1 gear ratio = 1 (shaft rpm will equal the engine's rpm)
1:2 gear ratio = 1/2 (shaft rpm will be double the engine's rpm)

Don't think about cars in respect to driveshaft rpm because the driveshaft is before the rear-end gears. This may trip a boat owner up.

800ft/lbs at 3500 engine rpm
800ft/lbs X 2 = 1600ft/lbs at 1750 output shaft rpm
800 X 1 = 800ft/lbs at 3500 output shaft rpm
800 x 1/2 = 400ft/lbs at 7000 output shaft rpm

Don't confuse engine tq with output shaft torque. Don't confuse engine rpm with output shaft rpm. They will only be the same if a 1:1 gear ratio.

So, the question if one is talking prop driven boats is, do I add more output shaft torque and run a bigger prop slower, or do I reduce output shaft torque and run a smaller prop faster ? The answer to this is definately application specific. Many variables. Type of drive system, boat weight/design/intended use, style of prop, engine, and etc.
 

·
Banned
Joined
·
1,263 Posts
Nope. any time you gear uup you gain RPM but lose torque, same goes the other way. Its why you car pulls like hell in low gear. Boat loads of torque traded for
RPM. My V-drive has less TORQUE at the prop than at the crank because of the overdrive. But it stills has the same HP minus frictional loss from the drive train. You don't lose HP(again except frictional losses) fro the gearing, you lose torque and gain RPM. Reverse it, and you gain torque and lose RPM. At the end, the Torque X RPM / 5252 still hold true. Make sense?
Are you saying the following about shaft horsepower?

Since we know the following equal horsepower

TqXRPM
------
5252

and using a 2:1 gear ratio for an example:

(2 X TQ) X (1/2 X RPM)
----------------------
5252

Math says it's the same hp being (Tq X RPM) / 5252


If so, I can say I really never looked at this part of it either but have followed those guidelines...if you know what I mean.
 

·
Registered
Joined
·
3,868 Posts
This is cool, but I an not getting caught in this trap again!!!! It boils down to a designed weight-mass-movement. I will say torque gets it moving and H/P gets it going. Then toss in gearing, prop, impeller and weight of what is being moved. M
 

·
Resident Ford Nut
Joined
·
10,075 Posts
Discussion Starter #13
I was just thinking out loud that if someone wanted to build an impeller dyno ....would you be better off using a trq monster diesel engine and then over-driving it to get to 7,000 rpm or would you build a blown or TT gasoline engine to make 1,100 -1,200 hp at 7,000- 7,200. :)bulb

Just a "junk yard wars" thought.

S CP :)bulb
 

·
Registered
Joined
·
3,868 Posts
I was just thinking out loud that if someone wanted to build an impeller dyno ....would you be better off using a trq monster diesel engine and then over-driving it to get to 7,000 rpm or would you build a blown or TT gasoline engine to make 1,100 -1,200 hp at 7,000- 7,200. :)bulb

Just a "junk yard wars" thought.

S CP :)bulb
Hey sleeper! It's actually a good topic and very disregarded ! I have gotten beat up over this before for different applications. Building a impeller dyno would be tough unless you had a computer to factor in every aspect of the designed useage- Weight-drag coeficient effect and intended design. Then you need to factor in the horsepower and torgue rateings -then toss in intended usage. Which I think you have done quite well with your'e G/W. You seem to be getting the most of both worlds and maybe giving up a touch on one or the other! It seems to me you have built a boat that meets your're needs and added a sh$t pile of money to get there! JMO M
 

·
Premium Member
Joined
·
25,975 Posts
Are you saying the following about shaft horsepower?

Since we know the following equal horsepower

TqXRPM
------
5252

and using a 2:1 gear ratio for an example:

(2 X TQ) X (1/2 X RPM)
----------------------
5252

Math says it's the same hp being (Tq X RPM) / 5252


If so, I can say I really never looked at this part of it either but have followed those guidelines...if you know what I mean.
Exactly!


I was just thinking out loud that if someone wanted to build an impeller dyno ....would you be better off using a trq monster diesel engine and then over-driving it to get to 7,000 rpm or would you build a blown or TT gasoline engine to make 1,100 -1,200 hp at 7,000- 7,200. :)bulb

Just a "junk yard wars" thought.

S CP :)bulb
Jon, your idea WAS sound at one time. It was the premus of Rudy Ramos' and the Pattersons' Allison marathon boats. Low RPM torque thru a v-drive with 300% gears, compare to a smaller higher revving V8 with 18% gears. Rudys allison made 850 hp at 3000 rpm. Any idea how fast you have to spin a 427 ford to equal that? No way they could do that in 1964 and make it live for 500 miles. It worked great, THEN! Now, we spin engines to RPM they could dream of, and do it for longer periods of time. There is no way in hell the Allison, or the Ford tank engine could ever hope to hang in there today.

Steelcomp said it 100% correctly on mdsheppies' thread on torque VS horsepower thread. Stop thinking torque or HP and think POWER. Its about the area under the curve that kicks ass. How you take the power to the wheel, impeller, or prop, will not change the HP, or the POWER at all. It comes down to the power across a range. If the engine makes insane torque at low R's, it may seem viable with the right gearing. BUT, if it occurs to narrow of a range, its a slug. This is the trouble they are having with the diesel applications in sport recreational boats. They almost require a 2 speed be cause the power range is not as wide as a gasoline engine. It will pull a ton if gear, but with the narrow rpm range, it does 25 mph at an idle with the right gear for max top speed.

You talking jets. I would think, and I know very little about jets, but I wouldn't GEAR the jet, I would change the impeller for the power. Now maybe with tha diesel, you would have to gear it. But I would think you would have the proper impeller for the blown deal to keep it at or just above max torque RPM.

This is Bill Millers nitro dyno he bought from Berstein that Armstrong built with Louie Hammel at 10,000 RPM Speed. It uses a 4 to 1 gear reduction because the absorber is from a tug marine dyno and can handle the RPM. But the calculations are the same. They just take the torque reading from the dyno, divide it by 4, multimply by engine RPM and divide by 5252. Its all about MEASURED torque, and math. Contrary to popular belief, dynos DO NOT measure HP.





100% free webcam site! | Awesome chicks and it is absolutely free! | Watch free live sex cam - easy as 1-2-3
 

·
Registered
Joined
·
1,535 Posts
I was just thinking out loud that if someone wanted to build an impeller dyno ....would you be better off using a trq monster diesel engine and then over-driving it to get to 7,000 rpm or would you build a blown or TT gasoline engine to make 1,100 -1,200 hp at 7,000- 7,200. :)bulb

Just a "junk yard wars" thought.

S CP :)bulb
Jon, this is an exact question that came up on offshoreonly a short time ago. There are a few builders that have won world championships with diesel boats. What they and Buzzi ( world class Italian boat builder/racer ) found out, was even though they were using the big torque monster diesels, it ended up being all about prop speed. They had to use big over drive reduction boxes to get the prop speeds to be what the gas engines were turning. Two speed transmissions seems to be the way to go with the diesel boats.


Darrell.
 

·
Registered
Joined
·
3,868 Posts
Jon, this is an exact question that came up on offshoreonly a short time ago. There are a few builders that have won world championships with diesel boats. What they and Buzzi ( world class Italian boat builder/racer ) found out, was even though they were using the big torque monster diesels, it ended up being all about prop speed. They had to use big over drive reduction boxes to get the prop speeds to be what the gas engines were turning. Two speed transmissions seems to be the way to go with the diesel boats.


Darrell.
The point is perfect! M
 

·
Premium Member
Joined
·
25,975 Posts
Jon, this is an exact question that came up on offshoreonly a short time ago. There are a few builders that have won world championships with diesel boats. What they and Buzzi ( world class Italian boat builder/racer ) found out, was even though they were using the big torque monster diesels, it ended up being all about prop speed. They had to use big over drive reduction boxes to get the prop speeds to be what the gas engines were turning. Two speed transmissions seems to be the way to go with the diesel boats.


Darrell.
Which gets back to Steelcomps statement about POWER and POWER under the curve. Its about the range of power as well as the peak power. Peak torque, or peak HP is usless.
A wide powerband is just as important as a peak number. BUT, you can create a artificial power band thru multiple gears. Look at formula 1. Look at small displacement 2 stroke road race bike. Zip squat torque in a very narrow band, but 12 speeds in the gear box.
Like DMoore said about the diesel boats. Lots of torque but to narrow of a range. so they widen the range with a 2 speed. Nordic boats is finding the same thing with boats they are testing with the Duramax. They really need a 2 speed.



100% free webcam site! | Awesome chicks and it is absolutely free! | Watch free live sex cam - easy as 1-2-3
 

·
Registered
Joined
·
3,868 Posts
Which gets back to Steelcomps statement about POWER and POWER under the curve. Its about the range of power as well as the peak power. Peak torque, or peak HP is usless.
A wide powerband is just as important as a peak number. BUT, you can create a artificial power band thru multiple gears. Look at formula 1. Look at small displacement 2 stroke road race bike. Zip squat torque in a very narrow band, but 12 speeds in the gear box.
Like DMoore said about the diesel boats. Lots of torque but to narrow of a range. so they widen the range with a 2 speed. Nordic boats is finding the same thing with boats they are testing with the Duramax. They really need a 2 speed.
This is why I was reluctant to get caught in this arguement! It also answers Sleepers question in a very roundabout way. gearing, impeller dyno! build a peak power range and gear accordingly to stay in the power range. If that makes sense to all. If you build an engine with a very wide power range the industry has called the low end part "torque" and the upper end "H/P". As stated in many threads a dyno reads torque only. so how we relate the terms as to how we run an engine is WHAT?? I have been thru this before and I am still waiting for a perfect explanation. According to engine specs derived by the engineers at EVERY engine manufacturer they achieve a torque rating and a horse power rating. M
 

·
Premium Member
Joined
·
25,975 Posts
I am not certain exactly what you getting at ol guy, but I will repeat this again and again. Dyno DO NOT measure hp. HP is a math calculation of MEASURED torque at a given RPM. Scale measure weight, but they cannot determine body fat. That requires a calculation
You can have an engine that makes a peak of 500lb tq @ 3500, and you will get your ass handed to you badly by an one peaking at 500 lbs @ 5500. So tell be ol guy, what wins, torque or HP. And no, I seriously doubt you going to GEAR the 3500 engine to stay anywhere near the 5500 one. Its why formula 1 cars don't use broad torque curves and 2 speeds.



100% free webcam site! | Awesome chicks and it is absolutely free! | Watch free live sex cam - easy as 1-2-3
 
1 - 20 of 82 Posts
Top