hp and tork and the new engines

[Tim Powell]

Issue with new engines Cummings , Cats , Mans.

I have stumbled on a issue. What is the problem here you guys with motor knowledge.
The new cats 12.9 is rated at 1050 hp but they will not perform as their counter parts. For example a detroit 892ti hp of about 735 to 725 will push a 61 S
Spencer cruse at 21 to 22 knots. A set of 1050 hp cats the new 12.9 just reach a cruse speed of 24 knots. There is 325hp difference X 2 equals 650 hp difference
what is up here ?? One would believe that the cat would release tork numbers but there are none that i can fine. Is the statement true THERE IS NO REPLACEMENT FOR DISPLACEMENT?

Ok Scott right down your alley. So does hp alone help the speed or is it a combination.

[Pascal]

3 kts extra speed is just 15% more speed but it tskes a lot more than 15% more hp to get that...

Hp vs speed curve is far from linear as drag (both air and water) increses exponentially

[Boatsb]

Torque on a 2 stroke is usually higher than a 4 stroke HP for HP. That being said the displacement of an 8v92 is about 12 liters so its kind of close.

Remember the amount of hp to increase 1 MPH is going to get larger as you go faster. Its exponential. so when you get moving at 25 Kts and want to go to 26 it takes more HP increase than from 20 to 21.

Id also look at the hull shape and weight as factors. If the weight is moved forward or aft it may change the drive line angle and the efficiency of the hull.

[eze2bme]

Wouldn't gear ratios play into this also?

[jim rosenthal]

I would think that all the comparisons would be for engines the drivelines of which were optimized for those engines, in terms of reduction ratio, shaft dimensions, and propeller diameter, pitch, cup and blade number. I have the impression that over the last several years the trend has been towards deeper reduction gears, thicker shafts, and larger multi-blade propellers turning at lower speeds.This seems to have achieved greater thrust with less cavitation and better prop performance.

If an older boat were repowered with modern diesels, the repower might included a deeper gear ratio and larger, slower-turning propellers. If there was a performance improvement, which you'd hope there would be, it might be hard to tell which part of the performance improvement came from the new engines and which from the deeper gears and newer-designed wheels.

I think horsepower is a derived figure, from torque, and it's torque that turns propellers. Maybe the CAT engine in Tim's original example wasn't propped for maximum performance. Or perhaps the performance is similar, or slightly better, but the fuel economy is much better- so the actual performance is really improved.

[Tim Powell]

i would think that all the comparisons would be for engines the drivelines of which were optimized for those engines, in terms of reduction ratio, shaft dimensions, and propeller diameter, pitch, cup and blade number. I have the impression that over the last several years the trend has been towards deeper reduction gears, thicker shafts, and larger multi-blade propellers turning at lower speeds.this seems to have achieved greater thrust with less cavitation and better prop performance.

If an older boat were repowered with modern diesels, the repower might included a deeper gear ratio and larger, slower-turning propellers. If there was a performance improvement, which you'd hope there would be, it might be hard to tell which part of the performance improvement came from the new engines and which from the deeper gears and newer-designed wheels.

I think horsepower is a derived figure, from torque, and it's torque that turns propellers. Maybe the cat engine in tim's original example wasn't propped for maximum performance. Or perhaps the performance is similar, or slightly better, but the fuel economy is much better- so the actual performance is really improved.

cat installed and tested. Does not mean it is or was correct.

[jim rosenthal]

Unfortunately, you are exactly right. Just because it's in there and working doesn't mean it is working as well as it could. Sadly.

[fedu]

I think CAT 12.9 are FPT/IVECO re-badged also think they replaced the C9 with an FTP (C8.7 ?) to make Tier 3

[madhatter1]

Here's a thought on some equal but different engines. I have Cummins 270's. Runs 27 MPH at 2400, 2600 max for RPM. If I go to a 330 I need to reduce pitch to make the engine reach top RPM so now I have a lower speed at 2400 but will reach a slightly higher top end with extra HP and RPM. And so on with the 370. At the end of the day it would seem I would be much faster with 200 more HP but no so with reduced pitch to allow the higher HP engine with equal "tork" at lower RPM's to reach top RPM.

Are these newer CAT's higher RPM engines? Common practice for todays high output engines. I like my low RPM engines.

[krush]

HP is the only number you need to discuss. Ignore the torque talk. It is a distraction.

[Boatsb]

Diameter and pitch will be critical to getting the rpm and prop load correct. The transmission ratio may have more impact than you think