Horsepower vs RPM???

[WATERWINGS]

I know that stock motors (200 HP) for instance are supposed to run out of the power curve around 6000 RPM.

Suppose they do, but if the motor will turn 7000 RPM, sure, its out of its "peak HP" but if it will turn that, it will run faster (with the same prop), right?

I realise that it may not have the "punch" over 6000, but if it is still turning higher R's, it is still gonna increase speed!!!

Please settle this old argument I had with someone a few months back.

Tell me I am right!!

[Reese]

anytime you can spin more rpm's your speed will increase...but a stock 200 or any other mota will only hit top speed when pitched to peak hp.

[Raceman]

You'd probably be surprised how many boats will run faster when you lay the pitch to em. As far as a stock 200 Merc, there's no question that the boat'll run a good bit faster if it's pitched where it can't turn the blade over 6K as opposed to pitched where it can turn 7K. The 7K blade will definately accelerate harder. Back when I ran 1500XS's on 15' Ally's, I ran a 30, while the other guys ran 26's and sometimes even 24's. (2:1 gear ratio) At a roundy round race, they'd have the advantage, but for playin' I'd have over 5 MPH on them on top.

When I was runnin' kilos with the early V6 race motors, I ran an 18 on a 1:1 Speedmaster (roughly the equivalent of a 36 with 2:1 gears) I'd have run a 19 if Merc made one back then. Dropping back to a 17 or 16 hurt the top end several MPH.

I think that even in the case of the 300 drags, with their high port timing etc, the guys that are screamin' em on lake boats are foolin' themselves or either they just like to feel the acceleration.

[WATERWINGS]

If I already turn a 30 chopper @ 6800,

and a 32 cleaver @ 6700,

and a 28 chopper @ 7000,

I would have to run a 36 pitch to get the R's down to 6300 or so, then I would have no hole shot and no acceration at all.

The kind of "playing" we do on my narrow and windy river, where you stop and talk, then everybody takes off to the next beach, I would be the last in line with such a tall prop!

Plus meeting all the ski boats, and god forbid DAMN WAKEBOARDERS with there 3'+ waves where you have to almost come to a stop, take back off again for the next 200 yards then almost stop again, a prop like that would kill my boat.

My point to this guy was the faster you could turn a prop, say 26 or 28, the faster you would go. No mater where the torque curve was.

Break is over, I will check back later!

[Raceman]

"My point to this guy was the faster you could turn a prop, say 26 or 28, the faster you would go. No mater where the torque curve was."

That fact in itself is true, but probably is a skewed argument, depending on the context. To use random numbers for the sake of making a point, and again assuming the stock 200 engine, if you can turn a 26 6500, the boat will run faster if you hang a 28 on it. If you did something to the setup of the boat where it had a potential to run faster, adding still more pitch would likely increase speed more so than just turning the 26 harder.

Where people get lost in the shadows on this situation where the torque peaks are concerned thinking that anything that increases RPM is good, even if it's only reducing pitch. Of course there's a point where the load (pitch) is added to the point that the motor just can't pull it efficiently.

[Techno]

Anything will go faster if the hp is enough to overcome drag. If the boat has less total drag by going faster then it needs less hp. So as the hp curve goes down and the drag curve goes down the speed increases, untill the air drag starts kicking in and beating up the remaining hp.

The outlawed porshe 917-30 can am could achieve ungodly acceleration, until it got to a fairly high speed. Then it took for ever to reach top speed. The engine still had HP to accelerate, just not at the same rate it had. It must have been going fastest at a lower hp level than maximum.

Even easier. Take WATERWINGS example with the 28" running at 6500 instead of 7000. Or wherever his engines top rpm is produced. Spin it to 7000. The engine ain't spinning 500 more rpm without some change.

[Reese]

with your statement about drag and hp.

I only wish the boat gods allowed us to go faster with less hp as you're implying in your statement.

There is no free lunch in this case...drag does not deminish with speed...on the contrary drag increases exponentially with speed...regardless of how efficient you make a padded v-bottom or air entrapment boat.

Just because you reduce drag from reducing wetted surface does nothing to reduce drag from lift...which is what is causing you to reduce wetted surface. Reducing wetted surface is always a better trade off in terms of drag but since most of us find it nearly impossible to break a slooooow 125 mph with 350 hp...you get my point about drag...it's always present and never deminishes with speed.

[WATERWINGS]

Help me understand this,

Two inline choppers, a 30 and a 28, both were in near perfect condition,

I turned the 30 @ 6800 for a speed of 91.6 GPS,

I turned the 28 @ 7000 for a speed of 94.1 GPS.

[Raceman]

If you're talking a stock 200 horse Merc, I think it's unexplainable. The only thing I could guess is that the 28 is a lot better blade than the 30. It happens, but usually not to anything like that degree.

[Raceman]

I fooled with the numbers you posted above and came up with an answer to the unexplainable, and support for my theory that one blade was far superior to the other. Assuming 1.87 gear ratio, the slip with the 30 in the numbers you posted above was 11%. With your numbers for the 28, the slip was only 5%.

[WATERWINGS]

WOW!

Is 5% an achievable slip #?

I noticed on the chart I have, that the actual speed was better than was listed on the chart, for the 28, (the chart said 89 MPH)

The chart listing for the 30 was 93 MPH.

I think if someone (not a guru, but joe average) told me that his slip was that low I would have a hard time believing it!:eek: :D

[Raceman]

All kinds of things effect prop efficiency, plus there's always the chance of mis labeling. Heavier cup would also make a difference in effective pitch.

[WATERWINGS]

neither had much cup, I plan on rolling some into the 28 though.

The 30 was not mine, but with the RPM drop, it would make since that it was a higher pitch than the 28.

[bfly]

I'll give you something else you may want to think about. I've been running a 2.5 245 7000rpm for the last 2 years...or so i thought. I replaced all my teleflex factory gauges this spring with gaffrig because I wanted a gps speedo and my new tach says 6200 rpm. Yes the switch is set for a V6!!!800 rpm's difference!! now I have to see which one is right and maybe re-prop. Bye the way my holeshot is not spectacular either. My bet is the gaffrig is correct.

[WATERWINGS]

If anything, my tach is reading lower that actual, because I am running (speedwise) faster than the chart shows, or my slip is really much lower than 10%

How about checking it with one of those tachs made for rc boats, where you wrap the tach wire around the #1 plug wire, it will hold max RPM and will go to at least 20,000 RPM.

[heath brinkley]

Waterwings, if the prop was not yours was it possible that somewhere down the line somebody had some pitch added or taken out of the prop, that would account for the unexplainable difference in speed calculations. About the 5% slip, I was talking to Rusty Campbell (guru, not joe av.) about my STV. I don't have GPS and was asking him if he thought the speeds I was coming up with were acheivable ( I figured at 8% ) he told me every time they tested the kind of prop I was using on that STV bottom it came up at 6%. I would have never guessed that, although that is a pretty efficeint hull design. I'm with you on the pitch thing tho' props over 30" are usually so boring to drive even with the extra top end I can't stand it. I'd rather give up some mph on top end and be able to give myself whiplash at ANY speed. In fact the glass bucket seat on my STV is split down the middle as we speak :D

[WATERWINGS]

so true

[Bill Rogers]

The best way to check a tach is with a signal generator. I do this for some folks on a routine basis.

I like to make a chart showing actual vs indicated rpm, and if the tach has a calibration adjustment, I'll put it right on the money at whatever rpm the owner wants.

I'll be glad to furnish details on the test setup and the procedure I use ...

[WATERWINGS]

Bill,

Please do!

[Bill Rogers]

An outboard tach measures the frequency of the AC voltage generated by the alternator (before it's rectified and converted to DC). Besides a power supply and voltmeter, you need an audio oscillator and a way to accurately measure frequency.

The frequency/rpm conversion is very convenient for 12-pole alternators. 12-poles produce 6 AC cycles per revolution of the crank. You have to divide by 60 to convert cycles per minute to cycles per second. So that comes out to 0.1 hertz per rpm, or 10 rpm per hertz. Therefore 100 Hz = 1000 rpm, 200 Hz = 2000 rpm, etc.

You need a frequency source capable of putting out at least 15v rms. Most modern signal generators won't do that. I use an old HP tube-type audio oscillator that will put out up to 25v rms.

The power supply is connected to the "ign" and "gnd" terminals on the tach. The osc. is connected to the "send" or "pulse" terminal (ref to "gnd"). You can use a battery for the power source, but I like to vary the voltage and make sure the tach maintains accuracy over a wide voltage range.

I start by setting the power supply to 12v. The tach should go to 0 rpm. I set the osc. frequency to 400 Hz (which represents 4000 rpm, mid-scale on an 8k tach). Then I start bringing the osc. output voltage up slowly until the tach begins to work.

Most tachs will begin to read accurately at 4.5 to 5 volts rms. This is an important test because some regulators only put out about 6 volts rms on the gray lead. I then run the osc. output up to about 17 volts. It could get this high if the tach's connected to an older unregulated charging system. All the while the tach reading should be constant.

I like to do the same thing with the power supply voltage. The reading should remain constant from 10v up to 17v DC.

Once it passes these tests, I set the power supply to 14v DC and the osc. ouput voltage to 14v rms. Then I start checking and recording the accuracy of the tach in 500 rpm increments across the scale. Finally, if the tach has a calibration adjustment, I'll calibrate it at whatever rpm makes the most sense for the application. Most tachs seem to be calibrated from the factory at mid scale. They'll be off sometimes a couple of hundred rpm at full scale!

I have one to do this weekend. I'll be glad to post some photos of the setup. Do you want them in this thread or would it be better to start a new one?

Comments, questions, suggestions for improvement are welcome.

[Raceman]

Could we get a definition on "burning out the engine"?????

[heath brinkley]

Bill, if this is a service you provide, do you have a web site or phone #. I have a tach that has some issues, I'm finishing up a boat this next week or so and I'm going to hook the tach up to it and see if it was just a wiring problem. If not I'd like to be able to get in touch with you.

[Bill Rogers]

Heath, right now I just mess around with this stuff in my spare time. Eventually I'll be in the service & repair business full-time, but I'm about 5 years away from being able to retire from my day job.

In the meantime, I'll be glad to help you and other S&F members any way I can. You can PM me, email me , or call. If you call, you'll probably get my answering machine - just leave a message and I'll get back to you as quick as I can (be sure to identify yourself as an S&F member).

Bill Rogers
brogersal@hisurfer.net
(334) 388-4709

[heath brinkley]

Thanks Bill, I'll get with you if this thing doesn't work with the new wiring

[Gerben]

Bill,

How's this for a crude test for someone not trusting their tacho:

A little 12v transformer, a rectifier (to double the freq.)
As AC current should be 60Hz (not sure how stable though), after it's doubled with a rectifier, there is a 120Hz signal, so tach should read 1200rpm?

Gerben

[sho305]

http://www.esitest.com/cart/325.html

I bought one of these. I checked my new teleflex tach on my inline up to 4,000rpm and it was spot on. It would not work at first, I had to get an old plug wire and attach it so the wire was away from the others; then it was fine.

[mattyc]

Here's my comments on your idea:

"A little 12v transformer, a rectifier (to double the freq.)"

This will work for the the IGN pin -- assuming you're talking about one of those little wall plug-in AC-DC converters. Still, check the voltage though. Under little to no load, they may put out anywhere from 9 to 15 VDC.

"As AC current should be 60Hz (not sure how stable though), after it's doubled with a rectifier, there is a 120Hz signal, so tach should read 1200rpm?"

The way I understand it, the waveform sent to the tach is not rectified. It is an AC signal. So for starters you wouldn't want to rectify it.

And secondly, your wall electricity is going to give you about 115 VACrms, which is WAY TOO HIGH to run directly into the tach. You could turn this down by winding your own transformer (if you do, please put a fuse inline on the primary side!). (If you really want to do this, let me know and I'll tell ya how)

Thirdly, you are only ever going to get 60 Hz out (and it is pretty stable). You'd need a PLL to start multiplying the frequency... and that's probably not gonna happen in anyone's garage.

[WATERWINGS]

SHO, Thats a cool tool,is there a chance that a tach could be ok @ 2000 and off @6000 or 7000?

How would you check it that way?

[sho305]

I have not tested too many with it, but like said earlier they seem to be on at some point and get worse the farther you go from that point/rpm. This tool will read at any speed in spec. I could run it up and down and it updates pretty fast.

The cheapy white Teleflex, the sportsman I think, was +/-100rpm idle up to 4k. Did not want to rev it anymore on the hose, even though it sounded cool:D the one in my avatar now, a '73 1500 Merc.

[Bill Rogers]

Heath, anytime; just let me know if you need any help.

Mattyc, Gerben's idea is sound. The output of a full-wave bridge rectifier has twice as many positive pulses as the input waveform, ie, the negative part of the waveform is flipped over and made positive. If it's not filtered in any way, the frequency will be twice that of the input. In marine charging systems, the battery serves as the filter. There was another thread that talked about the danger of disconnecting the battery while the engine's running. When you do that, you remove the filter and it floods the system with AC.

Gerben, I assume you're using the boat's battery to power the tach. For the tach drive signal, you're right: If you drop the powerline voltage to 12v with a transformer and run it through a full-wave rectifier, the tach should read 1200 rpm. BTW, powerline frequency in the US is very stable (most of the time), usually between 59.95 and 60.05 Hz - not sure how it is in your time zone, but I bet it's fairly constant. That's not a bad idea for a quick and dirty check.

Waterwings, this is your thread, we sorta got off topic here.:D Hope you don't mind. Maybe we should start a new thread?

Sho305, once again you've come with an interesting tool. I wouldn't mind having one of those! New tachs, even the cheaper ones, are surprisingly accurate. But if Mr. Offshore does his thing a few times they can sure take a beating and start acting strange. I think a lot of the damage is physical rather than electrical.

Wings - yep, a tach can be right on the money at 2000 rpm and be off a couple hundred at 6 or 7k. That's why I check them all across the scale. I usually calibrate a tach for a stock motor at 6000 rpm. Afterwards it may be off 100 or so at the lower end, but that's usually no big deal.

[sho305]

I wish it had a timing light type lead so I could take a spin with it in hand, but once I set/verify the tach I guess I don't need to. Maybe I'll check it every year or so or if it reads funny. I can also set my lawnmower rpm with it too:cool:

They have that tiny tach also....might have to get one for that oddball chryco Force of mine just to prop it:rolleyes:

[mattyc]

Yeah, I see where Gerben was going with that now. I was just envisioning someone essentially plugging their tach into the wall...