Using a dyno to help prop

[outboardnut]

Ok so I've got a dyno. It's the international dyno, some call it K and O.

Trying to learn and figure everything out with it.

One thing I don't quite get is peak output. According to the manual it comes with they talk about peak output being less than peak RPM.

So if I dyno a motor, lets say a 150 HP engine, and it starts generating 3500 PSI at 5000 RPM, and drops to 3400 PSI at 5500 RPM, then 3300 PSI at 6000 RPM, should shoot for a prop that will load this engine @5000RPM and call it right?

I have always thought more RPMS will equal more speed regardless of whatever PSI the thing is generating on the dyno. Isn't this more of a torque reading they are giving?

My only conventional thinking on proping a boat is if the mfg's listed operating range is 5k to 6k, prop the boat to 6K and you are getting every pony she has.

According to internatinal dyno, they are saying you want to hit it at the peak output which may be less than peak RPMS?

 

[4JawChuck]

Re: Using a dyno to help prop

Peak output will be at the torque peak...always. Propping the boat should not lower the rpm below the torque peak under full load...that means fully loaded with fuel, fish, tackle and beer including people+head wind+current etc. Ideally you will find the optimum performance by propping the boat to exceed peak rpm by a few hundred even fully loaded so there is some overhead rpm to prevent lugging the motor below the torque peak. Your acceleration results will be optimized by propping in this manner but mileage will suffer slightly, better to underprop than over prop for engine life. The mfg lists maximum rpm so you don't overrev the motor but most properly propped boats exceed this rpm slightly for best performance on the water when completely unloaded.

For example I ran four identical props on my 90Hp Merc, a 20,21,22,23 pitch Laser II. The 22 had the best all around performance for a full load and only exceeded the max rpm by 600rpm unloaded, with a healthy tailwind and current in my favor I have seen 6100 rpm and a max speed of 55mph. Loaded with people fuel and gear and going upwind against the current I usually hit 5400 rpm which is just under the max of 5500 recommended for this motor.

If I pull adult skiers I found the 20" pitch to be a better prop so I have two props to choose from depending on usage that day, I get the same top speed with either prop and much better mileage from the 22" but my peak rpm was over 6500 at WOT unloaded with the 20" which is unacceptable for all around use or the wife driving it. The 23 was slightly better for mileage but just slightly, it also lugged the motor below the torque peak when fully loaded and accelerated very poorly compared to the other lower pitch props.

Its always trial and error either way you look at it, dynos for outboards are mostly for testing running characteristics and proper tune than for prop selection. In general a two stroke motor needs to operate in the curved portion above the torque peak to avoid falling off it, operating the motor under the torque peak at full throttle is considered lugging the motor and unhealthy from a bearing and piston/ring life point of view.

 

[Chris's CVX16]

Re: Using a dyno to help prop

Also remember that Horse Power is a calculation based on RPM and the torque produced at that RPM.

So if your motor makes 157.6 ft-lbs at 5000 and 145 ft-lbs at 6000 the torque is obviously less but the RPM number makes up for it a bit. The Dyno should have a pressure-torque chart.

In my example it is 150HP @ 5000 & 165.7HP @ 6000

Remember horsepower = (torque x RPM)/5252

 

[Texasmark]

Re: Using a dyno to help prop

Nice replies and numbers guys. I love it.

Most of the time it is really hard to get the data you are dealing with (wanting to deal with) from OEM suppliers. Seems they are happier blowing smoke up your ......... rather than supplying you with usable data that you need to make decisions.

That data is really important when you are trying to prop out and find yourself over or under the recommended rpm range. Obviously, you can't load up the rig with a boat full of Sunday afternoon fun seekers and using the same boat and prop, expect to get maxed out with WOT speed and performance when you are alone or with a single companion wanting to get max range on a fishing trip.

I personally don't know what OEM's think they are hiding but personally I think it absolutely stinks and even sucks............so to you OEM's...............the bird.

And for you referees, (moderators) you can just leave this alone. It has a ton of creditable merit and has needed to be addressed for 10's of years.

You know, having had a half hour or so to contemplate what I said, being retired, bored with nothing to do (of my own choosing), and all, I may just start a testing company to supply the data that OEM's don't have the (whatever reason) to not publish. I may just do it as a public boating service to all my friends (?) on here and new acquirees. Hmmmmmmmm food for thought.......geez, the redneck is glowing. Won't need an electric blanket tonight.

Mark

 

[QC]

Re: Using a dyno to help prop

Chuck, you simply either do not understand this stuff or do not understand the semantics. You blow off my posts, so I am going to have to be emphatic. Annnnd, in case you are still wondering, I am not making this stuff up.

Peak output will be at the torque peak...always.

If you can provide one engine rating chart that proves this I will provide 100 that proves the opposite. This is an absolutely FALSE statement! In the thread where you and I started our little dispute all of h's charts show peak torque RPM is below peak horsepower RPM. For some unintelligible reason you have now decided to use the word output. Please let me know what output is in your book. My best guess is you are referring to horsepower, but require a euphemism to keep this silliness up.

Propping the boat should not lower the rpm below the torque peak under full load...

There is not one manufacturer that agrees with this statement. WOT RPM recommendations are based on peak horsepower (output for you). What you describe is peak horsepower . . .

operating the motor under the torque peak at full throttle is considered lugging the motor and unhealthy from a bearing and piston/ring life point of view.

Actually, a lug condition is when an engine is operating at wide open throttle (in the case of an Otto cycle engine) and it either won't accelerate or engine speed drops (the latter is rarely encountered in a marine pleasurecraft application). Almost all marine engines, if propped properly, are only in a lug condition at WOT RPM i.e. the engine won't accelerate any further. These are technically correct definitions and not what is typically used by the general public, but it is factually correct. Sort of consistent with your statement, is that if an engine is overpropped and could not reach planing speed, it would be in a true lug condition. Many of us routinely operate our engines at WOT temporarily below peak torque, that's how we get them to plane . . .

 

[Texasmark]

Re: Using a dyno to help prop

One might merit from a farm tractor design when it comes to torque curves. When tilling the soil, conditions change and one finds oneself in a situation where the load on the tractor becomes excessive. One could downshift to a lower gear for more torque, or one might purchase a tractor that was properly designed.

The properly designed tractor would have a torque peak below the normal operating rpm of the engine. If a lugging situation were to evolve, in lugging the engine it would drop in rpm's whereas the torque curve would increase which counteracted the load. I had a Massey Ferguson with a 3 cylinder Perkins Diesel that was designed just like that. It would dare you to try to load it up.

With that said, I like to run on the outside of the torque peak with my boat which puts me at or above the max operating rpm's. Being propped like that with a ported (air over the blades at low speed) prop made for a variety of functionality with one prop and kept me from having to change every time I wanted to fish or play.

Mark

 

[45Auto]

Re: Using a dyno to help prop

Peak output will be at the torque peak...always

Wow, I'm off the forums for a few weeks and people start posting all kinds of silly stuff. What's wrong with you, QC, you can't keep these guys under control???



Here's the power curve for probably the most common marine engine out there, a 5.7L GM. Chuck, notice how it has the torque peak of 370 lb-ft at 3200 RPM and max power of 292 HP at 4800 RPM? At peak torque it's only making about 2/3 of it's max power. Not sure what this "output" thing you're talking about is, but it obviously isn't power (which is the key to performance).

I've been gone for a while, sounds like I missed some "interesting" discussions!

Outboardnut, in answer to your original question, if you're looking for maximum SPEED you need to prop your engine where it's showing the most pressure on your dyno, regardless of RPM. That's the most power your motor can generate. If you're looking for max acceleration, that's another discussion. You can't maximize speed AND acceleration with a single output ratio.

To use a car analogy, do you think your car is faster at 8000 RPM in first gear (probably 20 MPH?) or 5000 RPM in top gear (probably 100 MPH?).

For single point operation (max top speed) RPM and torque are irrelevent. Proper gearing (or propping) takes care of that. Power is the bottom line.

 

[QC]

Re: Using a dyno to help prop

Yeah, the timing of your disappearance sucked . . . check out this thread

 

[Texasmark]

Re: Using a dyno to help prop

Yeah, the timing of your disappearance sucked . . . check out this thread

Geez, I'm glad I wasn't part of that. My head is still spinning.

Mark