hp or torque?

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« on: November 08, 2019, 05:15:50 PM »
prolly been discussed before but..
which one matters most?
to the simple minded (me) I know the faster the pump shaft spins the faster my jet boat goes.  easy
but do I need more hp..  or more torque to go faster?
reading the 'definitions' doesn't help this simple mind.
need a 'now i got it' explanation please

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« Reply #1 on: November 08, 2019, 08:56:01 PM »

Itís kind of always been a big debate. Here are a couple of good posts from some pretty knowledgeable guys.

What are you goals?

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« Reply #2 on: November 11, 2019, 06:12:00 AM »
Wanted to clarify, torque is a rotational force and is what is actually measured by a dyno.  Horsepower is a unit of work that  is calculated.  The formula is ((Tq * RPM) / 5252).

Scott Foxwell and Sleeper CP posted some good information. 

Also, from GN7, "...torque tends to be a product of CID. The HP the engine is designed and built to make will determine where in the RPM the torque curve falls. As you build for more power, the torque band will shift up the RPM scale. But the max torque remains relatively the same, the curve just shifts.

I think it was Ken Duttweiler that once said, build it for the desired power and RPM, and the torque will take care of itself.
Translation: like Steel (Foxwell) said,, you can't make power without making the required torque. Because torque is IN the equation, HP is simply the product of that equation."

A jet boat is essentially a single RPM engine and does not need or want low RPM torque.  The pump impeller will stall the engine at whatever RPM the cut matches output of the engine

It's not like in a car where you are pulling gears and need a long flat torque curve.  When you mash the throttle from a dig, the impeller slips.  This is referred to as "the burn out."  If the engine is producing enough power, the RPM will bounce off the rev limiter and it takes some time for  the pump to recover as water is starting to be fed to the impeller.

This problem is exaggerated in a positive displacement supercharger application because so much bottom end torque is produced by these engines.  A larger cut impeller is typically recommended for positive displacement applications to launch better at the sacrifice of top speed.  The smaller cut impellers just can't the excess torque.  For this reason, I feel that centrifugal superchargers are better suited to jet boats.

One statement that has never sat well with me is "a jet drive is the only propulsion system that is fully loaded at all RPM."  The following numbers are torque and horsepower measurements for an American Turbine AA impeller:

2,000   84.9   32.3
3,000 191.2 109.2
4,000 339.9 259.0
5,000 531.1 505.8

You can see how the power requirements to turn the impeller impeller quickly ramp up with RPM.  These numbers only represent the resistance that the impeller loads against the engine.  If the torque required to turn the impeller exceeds the output of the engine, the pump will stall the engine at that particular RPM.

An important point mentioned by Scott Foxwell is the engine torque curve needs to stay ahead of the impeller torque requirements.  This allows the engine to RPM up quickly.

If you are building a river hotrod or a race boat, shifting peak torque closer to peak power is beneficial.  You need to figure out what is a realistic peak power RPM for your combination of parts and build for that RPM.  It important to note that when looking at dyno sheets, you want to look at uncorrected torque and horsepower numbers.  The RPM that your boat will turn is whatever power your engine is producing in the weather conditions on that specific day and time.

If you are building a cruiser or for towing skiers, wakeboards, tubes, etc., I would recommend a large impeller (AA) and build the engine so that your typical operating RPM is approximately 80% of your peak power RPM.  Since top speed is not the primary requirement, a better choice is to maximize your fuel consumption rate.  In this case, i would like the operating RPM to fall between peak torque and peak horsepower.
"I want to roll with my brother Joe" - Joe Bateman - January 29, 1950 ~ November 27, 2013


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