HP vs. RPM

[mdaspit]

I don't understand the relationship between horsepower and RPM.
I hear that a 302 when it is "on" should turn about 7100 rpm.
But when I look at the chart of the 302 performance curves, it shows peak horsepower at about 6500 rpm, then as the rpm goes up to 7100, the horsepower drops. I don't get it.

So... really, you don't want the motor to develop maximum horsepower?

[Dave M]

Lots Of Answers

You are sure to have many varying opinions and stated facts.

Basically, every engine from every make, has a peak HP/RPM factor. It has to do with everything internally working in harmony. This means from the port timing, to the exhaust layout.

Every engine is different. There is no one magic number.

Have you ever been in your boat testing, and the throttle hook up is allowing the butterfly to open past center ? You back off a little and it speeds up. Think of it this way. Everything is postioned for a reason, port height, width, exhaust, intake, etc. You will find the max potential by experimenting or running the numbers. We usualyl experiement. I have found massive differences between paper and reality. You can't take into account the weights of the boat and driver, and course layout with a dyno. They are only good for straight line speed and how fast it will get to that speed. Good info, and not a bad place to start. But don't take it as gospel.

[B Walker]

HP vs RPM vs MPH simplified

HP vs RPM vs MPH simplified

RPM and speed are directly related. In other words,the higher the RPM, the higher the speed.

HP just changes the time it takes to increase RPM. In other words, on a 302 torque curve at around 6500 RPM the engine will be making peak HP and will increase in RPM at the fastest rate. As you move higher or lower on that torque curve the rate of change in RPM will be slower than at its peak.

"long course" propellors will generally turn higher RPM, therefore higher speeds, but take longer to get to top speed than a "short course" wheel that wont turn up as high in RPM but gets through the powercurve faster.

Clear as mud?

Good luck Mark, sounds like your on the right track thinking of this stuff.

Tip O the Day: figure out how test for lap times, not top end speed.

BW

[Blackhawkguy]

no shifting in boats

Also remember that a boat does not have a gearbox to keep the engine in peak HP RPM range so when you exit a corner the RPM may be below peak HP, then as you travel down the strait it builds, once you are moving it does not take as much HP to move the boat so RPM's will play a role based on the prop pitch, it's a combination that cannot be duplicated an a dyno.

Snowmobiles use a auto shifting clutch so you set them up to go to peak HP RPM as soon as you peg the throttle, the clutch keeps the rpm there and the belt moves up and down based on torque required to move the sled, once they get to 1:1 the rpm will also build a little but most people will never get to run that long in a straight line.

Ever listen to a formula one car shift, they are constatly trying to keep the thing in peak HP range

Do not get to wrapped up in perfect max HP RPM, it is good to know, use a stop watch and try diferent lenth straightaway's, base your prop selection on witch gets you there the quickest for that lenth of strait, then at the races check the straight or course times and select your best prop off your test data-its not the fastest but the quickest in any given distance that winns.

Kerry

P.S. higher pitch prop- lower RPM, lower pitch prop-higher RPM in same amount of distance. Lower pitch for acceleration-higher pitch for more top end-still the stop watch on a particular coarse size is the key

[Islandmon]

"Tip O the Day: figure out how test for lap times, not top end speed."
Good advise......thanks !

Sam,

[ZUL8TR]

Hp vs RPM some basics

Here is a start on some basics but testing is the final key:

The simple relation is Hp = Torque x RPM/5252

Hp = hprsepower a calculated quantity
Torque in ft-lbs a measured quantity
RPM in revs per minute a measured quantity
5252 is a conversion for the units used and the mechanical equivalent of power at 33,000 ft/min = 746 watts = 1 Hp.

see here for details on Hp and Torque

http://en.wikipedia.org/wiki/Horsepower

http://en.wikipedia.org/wiki/Torque

Engines develop torque and that is what would be measured with a engine dino. Torque is a twisting leverage measurement that the connecting rods exert on the crankshaft which in turn is transferred to the prop via the gear case. The water only feels the forces that the torque provides to the prop.

Horse power is the rate of doing work (Torque/time) and is a calculated quantity based on the measured torque. As rpm increases the amount of air that can be ingested becomes limited by many factors in the intake and the reduced time for the fuel to transfer to the combustion cnamber thus torque and Hp fall off. Based on engine design Hp and torque peaks do not occur at the same rpm. Usually torque reaches a peak before Hp does. Like you note for the 302 in your post.

The greatest force to the prop would theoretically be at the max torque it receives but that would not necessarily be the max speed because boat speed is related to prop average pitch, engine RPM, gear ratio and all the various drag components that create prop slip.

Operating at an rpm above the max Hp and torque is a way of keeping the engine momentum up as speeds vary at various parts of the course. Similiar to reving beyond the max Hp in a car with a manual tranny to keep the revs up as much as possible in the next higher gear. This keeps the engine closer to its max performance rpm operation range. Best lap time not max speed is the desired result.

[Richard Hearn]

HP is only important so long as it generates additional RPM with respect to propeller pitch. Speed is determined by RPM, Gearbox Ratio, Propeller pitch and propeller slip. Here is a website that helps you determine it: http://www.rbbi.com/folders/prop/propcalc.htm.

HP is VERY important when understanding punch (coming out of the first turn) as it helps you get a bigger pitched prop up to flatline RPM, faster.

[ZUL8TR]

Hp and rpm

HP is only important so long as it generates additional RPM with respect to propeller pitch. Speed is determined by RPM, Gearbox Ratio, Propeller pitch and propeller slip. Here is a website that helps you determine it: http://www.rbbi.com/folders/prop/propcalc.htm.

HP is VERY important when understanding punch (coming out of the first turn) as it helps you get a bigger pitched prop up to flatline RPM, faster.

Well it is Torque that provides the acceleration from the twisting force of the engine to the prop that is then transferred to the water by the force from the blades. Hp is related to torque and rpm as a calculated value as explained above.

[sam]

Torque provides no acceleration as it has no time component. It comes from an idea that is about a paragraph long and looses its context when reduced this much.

The answers that account for acceleration being the key here are correct. If you were propping for solely for top speed you would want to be propped exactly at the horsepower peak.

For closed course racing, you have to choose a prop that gives a balance of top speed and acceleration. Working that balance compromises a little bit of the fastest possible speed in favor of more acceleration.

Let me fabricate some numbers for demonstration

Let's say the fastest speed your motor can push your boat with the fastest prop is 75 mph. That same prop, when called to accelerate off the turn at 45 mph takes 10 seconds to reach 65 mph.

A better closed course prop tops out at 69 mph, but only takes 5 seconds to reach 65 from 45 off the turns. By the time the guy using the top speed prop gets to 65, you have already been going 69 for 3 or 4 seconds. By the time the speed prop gets to 69, you have been going 69 for almost 20 seconds ... you are way ahead of him before he starts to catch up with you. Within another few seconds it is time to turn and scrub off to 45 mph again; the speed prop never even made it to the full 75 mph before it ran out of straight. >>>>>>> Since you are propped for a compromise of speed and acceleration, your top rpm is above the 6,500 rpm power peak ... 7,100 sounds OK to me

[pat]

Here is another perspective. Always try to match your prop to maximum peak HP in the corners. Since races are usually won or lost in the turns, those who go through the turns faster than the rest are likely to wind up in the lead. So, get your boat running at 7200 RPM on the straight-a-way (where speed is more dependent on RPM) and get your RPM working at 6500 in the turns where HP is maximized. You may have to go through a number of props to hit that magic number. One good thing about Yamato engines is that you actually get a published RPM/HP curve. If you run a Sidewinder, OMC or Merc you need to go to the Dyno to get that data. Agreed that not all engines are created equal and that you will get HP gain/loss depending on the quality of engine that you have.

Kants Racing 83-CE

[johnsonm50]

All very good information. With probably no exception our race boats qualify as light load conditions, therefore enabling a motor like a 302 [I hear they have kick] to easily run off the curve graph. You dont need every last HP to make more RPM equaling more speed. Unless as stated you were going only for all out straight line speed then a [good] prop restricting the motor to its own max point would be fastest at the cost of acceleration.

[mdaspit]

Einsteins...every dam* one of you.

Thanks

[Larry M]

There is a lot of things that come into play when playing with props. If the water is super rough go with one that will accelerate. You may not to be able to hang on in rough water with your usual prop. I'll tell you a story that this worked out perfect. Years ago Oklahoma had a dash for cash and it was for the Super E class but anybody could step up. We didn't have a Super E so we ran our E Merc.The water was very rough, at the start every body was way early on the clock and was just about off plane. We had a little prop on and when the flag dropped Kevin out accelerated to the first turn and that was the race. The OMC SE motors had at least 5to 7 mph on us but the water was so rough they couldn't use the speed. Now on the flip side I would say 75 % of the races are won at the start. If you get a good start and get to the first pin first with a top end prop that is a huge advantage so I would give up a little acceleration to get there first. It's been said and I don't think it's far off that you need 3 mph to pass someone.
Larry Mac

[Dave M]

Best Said

You can read a book and try to understand it and maybe even beleive it.

You want the real deal ? Go test. Learn your rig. All that book is gonna do for you is get you going in the right direction.

Another good thing to do is become friends with your favorite prop builder, and stay in his/her good graces ! They already understand all this stuff and usually will get you the right wheel eventually. It takes feedback from you though.

Someone said test for lap times and not top end. Might be some truth to that...........

[ZUL8TR]

Torque and acceleration

[QUOTE=sam;150099]Torque provides no acceleration as it has no time component. It comes from an idea that is about a paragraph long and looses its context when reduced this much. .....................................

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As you note torque does not have a time component but the fact that the engine is developing power and converting it to torque of the crankshaft and to the prop shaft directly implies that time is present because of the rpm of the crankshaft. Engine power is developed by the linear motion of the pistons and forward motion of the is developed by the angular rotation as torque delivered over time to the propellor (or wheels).

The simplified discussions presented in this thread certainly lack the detailed physics regarding acceleration of a boat with a propellor when all forces are considered. However a section in Dave Greer's book in Chapter 1 under the torque subsection on page 2 is noted:

http://books.google.com/books?id=8w0...age&q=&f=false

Geer notes that to propel the boat [and accelerate it] the power has to be delivered as torque [at rpm] to the propellor. The forward constant linear speed of the boat would be developed by the reaction to the forces delivered to the water by the propellor at a given rpm such that thrust force equals all drag forces. The acceleration of the boat would be developed by the angular acceleration of the propellor as it changes rpm delivered by the engine power as it changes rpm and the corresponding time variable force (thrust) delivered to the water would result in acceleration that exceeds drag forces until a constant speed is reached where thrust = all drag forces.

A simple analog would be rowing a boat. The human is creating power by the strokes per time of the oars in the water. The forward boat movement is developed by the force from the oar area in the water and pushing against it and the leverage arm is the distance from this oar water force to the oar chock (actually the oar is a lever that multiplies the torque provided by the human). This oar lever distance x water force would be the torque delivered to the water and the stroke rate is the time component to yield torque/time. The boat propellor is like the oar but in a rotating condition.

The situation is certainly more completated than this when drag and surfacing propellors are considered but the same principal holds that power yields torque at an rpm (or stroke rate) that does the moving.