HORSEPOWER,DYNO'S AND PROPS!

A general, non-powerplant specific, discussion on airboat technology, ie., hulls, rigging, polymer, etc..
Twisted humor
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Re: HORSEPOWER,DYNO'S AND PROPS!

Post by Twisted humor »

muffled500hp maybe you can humor me and answer a few basic questions.

First, if you have dyno sheets figured out so well. At what point in the torque or hp curve should I set my max prop speed at?

Second, can you tell me the most basic way to tell if a dyno sheet has be adjusted to show numbers desired than actual numbers? While your at it, think you could explain why that is a good way to check?

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Re: HORSEPOWER,DYNO'S AND PROPS!

Post by cntry141iq »

Like plum says the prop is the ultimate dyno test ... however a honest set of dyno pulls will let you figure out what kind of gear and prop you want to run based on the engines power curve and the props power curve as it relates to the type of running you wish to do.

As for if the dyno sheet is honest well ... I always suggest having a dis-interested party make the dyno run ... no sense in letting the fox guard the chicken coop. If the person making the pull has no dog in the hunt an dyou don't tell them what you expect or who built the motor then you will probably get as honest a pull as the dyno is capable of.

If you buy 600 hp then you should get 600 hp. If you buy a prop rating instead of a hp rating then you should get it as well. You should get what you contract for ... plain and simple. You should get it the first time out and not have to go back and get it tuned up or tweaked. The engine should do what it was bought to do as a minimum not a maximum.

I like the saying ... if you don't dyno ... you don't know. Well with daves new program the prop is the dyno and if you are buying a setting of say .. 2.5 on a 3 blade 80" well then you should get it ... makes it pretty simple and easy to understand to me.

Now will it work at any altitude ... I think not ... most engines lose power as altitude increases and as humidity increases as well you can't burn without air and you can't burn water.
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Re: HORSEPOWER,DYNO'S AND PROPS!

Post by Bob Burnside »

If you buy 600 hp then you should get 600 hp. If you buy a prop rating instead of a hp rating then you should get it as well. You should get what you contract for ... plain and simple. You should get it the first time out and not have to go back and get it tuned up or tweaked. The engine should do what it was bought to do as a minimum not a maximum.

I like the saying ... if you don't dyno ... you don't know. Well with daves new program the prop is the dyno and if you are buying a setting of say .. 2.5 on a 3 blade 80" well then you should get it ... makes it pretty simple and easy to understand to me.

Now will it work at any altitude ... I think not ... most engines lose power as altitude increases and as humidity increases as well you can't burn without air and you can't burn water.[/quote]

That's what I am talking about, HP at sea level is easy. Try the same set up at 4200' that's another story.

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Re: HORSEPOWER,DYNO'S AND PROPS!

Post by Waterthunder »

Making horsepower at altitude is easy to you just gotta add atmosphere pressure! I do have a few motors in Alaska and Idaho I just built them a little different to make power at a higher altitude! I will say you can't run them motors here in Florida they wouldn't live long!
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Re: HORSEPOWER,DYNO'S AND PROPS!

Post by cont520 »

Dave can u answer this for me I know I am on a a/c boat but why do I get more speed out of my 72 inch prop vs my 74 whether its streached out over 700 feet or in a 400 ft race. I have clocked it both ways and gpsed and my 72 inch bustes my 74 out of the water now on the hill its a diff. story . Just curious on your take
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Re: HORSEPOWER,DYNO'S AND PROPS!

Post by cntry141iq »

I aint dave but I will try ... longer props give more low end thrust and cost you on the top end. smaller dia is faster but doesn't run ground as well and your cruise rpms will or should be more with the smaller prop at least that is what I have found ...
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Re: HORSEPOWER,DYNO'S AND PROPS!

Post by Waterthunder »

In a drag race more thrust doesn't mean your boat will be faster! There are so many different dynamics that are involved! Case in point the bigger dia prop will almost always be faster on the hill unless you put so much prop your motor can't handle it. This is not the case on water right now I have sooo much more motor then I have prop I must do this to band aid something! Think about it it takes a 4 blade to hold back our new 550HP EFI motors don't you think me and the Ground Rattler make more power then that on motor let alone on nitrous??????
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Re: HORSEPOWER,DYNO'S AND PROPS!

Post by cntry141iq »

sorta seems like a classic case of wanting one engine/prop setup to do everything ... no dis respect meant now ... but it aint gonna happen there is no free lunch as the saying goes.
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Re: HORSEPOWER,DYNO'S AND PROPS!

Post by Waterthunder »

I have said this for as long as I can remember or at least since high school. There is never one combination that is best for everything and there is never a magic combination! Also You can almost say every combination is the best at something! I will say without a doubt I have learned the Sensenich R blade is the best all around prop it does everything well! You may find a prop with more bottom or maybe more top but you will not find one that the average is as good everywhere!
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Re: HORSEPOWER,DYNO'S AND PROPS!

Post by BenBad »

I have found all of this very interesting but being a newbie I would like some opinons on my next prop for this next summer. I have a stock sbc 400 DD on a heavy hull 14 ft boat. I have room for up to a 72" prop. I need to find something that will help me get on and off of the sandbars up here in Nebraska. I am not into top speed, I'll get there sooner or later. I have to live with the boat I have for a couple of years while I save up to get what I would like.
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Re: HORSEPOWER,DYNO'S AND PROPS!

Post by plumcrazy »

the best running dd small block ive see had a 6 blade warp cut down to 66 inchs i was impressed and it was a 350

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Re: HORSEPOWER,DYNO'S AND PROPS!

Post by Twisted humor »

cntry141iq wrote:Like plum says the prop is the ultimate dyno test ... however a honest set of dyno pulls will let you figure out what kind of gear and prop you want to run based on the engines power curve and the props power curve as it relates to the type of running you wish to do.

As for if the dyno sheet is honest well ... I always suggest having a dis-interested party make the dyno run ... no sense in letting the fox guard the chicken coop. If the person making the pull has no dog in the hunt an dyou don't tell them what you expect or who built the motor then you will probably get as honest a pull as the dyno is capable of.

If you buy 600 hp then you should get 600 hp. If you buy a prop rating instead of a hp rating then you should get it as well. You should get what you contract for ... plain and simple. You should get it the first time out and not have to go back and get it tuned up or tweaked. The engine should do what it was bought to do as a minimum not a maximum.

I like the saying ... if you don't dyno ... you don't know. Well with daves new program the prop is the dyno and if you are buying a setting of say .. 2.5 on a 3 blade 80" well then you should get it ... makes it pretty simple and easy to understand to me.

Now will it work at any altitude ... I think not ... most engines lose power as altitude increases and as humidity increases as well you can't burn without air and you can't burn water.
I've read a few times that a dyno will help you pick your combination, but still no answer to my question. Sound like some bench racer talk to me. That would be B.S. in case you didn't know.

As for the accuracy of the dyno, also no answer. Guess not to many on here understand the math of it. I'll help you a little you can only calculate HP. It can't be measured.

Of course, you should get 600HP if you pay for it. You should also get 700HP if you pay for it. Not sure where that came from, but true.

Now for the settings involved as far as altitude. The pilot I spoke with last night told me that it takes less and less power or throttle to spin the prop as you go up. That is until you go above the optimum point of his plane is 8000 feet. After that it starts to take more throttle to maintain the prop speed as you go up. So, the thinner air does have an affect on the motor as far as it builds less power, but the drag on the prop goes down faster than the power of the motor. Since most won't be going 8000 feet up. What you will most likely see is that if your jetting is right for the altitude you will spin the same prop.
Last edited by Twisted humor on Thu Dec 17, 2009 9:10 pm, edited 1 time in total.

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Re: HORSEPOWER,DYNO'S AND PROPS!

Post by Dbell »

Good post twisted, its amazing how many people dont know horsepower is calculated by torqe and rpm. on an airboat engine you might want to be more concerned about peak torqe. and of course at what rpm its making it.

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Re: HORSEPOWER,DYNO'S AND PROPS!

Post by jdotson »

I've posted it before but torque is the work of the engine and horsepower is the marketing of the engine.
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Re: HORSEPOWER,DYNO'S AND PROPS!

Post by crowhater »

During the summer months I can spin my motor with ease to 5000+rpms but this time of year on the coast I drop to 48-4900rpm. The cold wet air makes it much harder to spin a prop. If you are driving in the rain you will lose 100rpm pretty easy. Fog also kills 100 rpm. Keeping your prop clean and waxed helps but you still lose rpm.
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Re: HORSEPOWER,DYNO'S AND PROPS!

Post by Corey Harris »

Twisted humor wrote:
cntry141iq wrote: So, the thinner air does have an affect on the motor as far as it builds less power, but the drag on the prop goes down fast that the power of the motor. Since most won't be going 8000 feet up. What you will most likely see is that if your jetting is right for the altitude you will spin the same prop.
I'll agree with this. Felber told me this would be true, i'd have less HP at 4200' but the resistance is less on the prop so i'd roughly be turning the same prop/pitch as it would in FL. (less push/thrust though because the air is thinner)

I turn a 82" 3 Blade R, 2.68 at 5200 RPM.

Interesting observation though, as we dropped below 32F and the air became thicker and more humid; I lost 200 RPMS and some push. I assume this is a double whammy - more humid/less HP, also thicker air more resistance on the prop?
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Re: HORSEPOWER,DYNO'S AND PROPS!

Post by Waterthunder »

I'm just a dumb @$$ redneck but I think if you go up in altitude the motor and the prop are both denied the same amount of air! So if the motor makes less power because there is less oxygen then I would think the prop would also turn easier because it's swinging thru less oxygen! The major outside factor would be humidity the more humid it is the less power a motor makes but the harder it is to swing the prop this is a time when the work get's harder and the motor makes less power. Once again a dyno uses a correction factor who know what amount of humidity or altitude the dyno operator corrected for. In other words if you have a motor that really made 500HP on the dyno at sea level and you changed the correction factor as if you tested it at 3,000ft or at a 100% humidity you can add 40HP to you dyno sheet!
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Re: HORSEPOWER,DYNO'S AND PROPS!

Post by Whitebear »

Of course unless you have an induction system that compensates for altitude, ya need to lean the mix as altitude increases. Less air (O2) to burn the fuel at altitudes.

Yup the power decrease of the engine at altitude is proportional to the loss of prop load. Thats why complex aircraft have controllable pitch props. Fine for ground level take off and you can set coarser as altitude increases.

Generally the engine is loaded using the CHT, AND Manifold pressure. By doing the dance with these two along with mixture the engine is tailored for max performance at (N) RPM

Even though a turbo will extend the altitude ability ya still have to keep temps and pressure in balance.

Its old technology, BUT most automotive folks will never run into an altitude problem thats very severe.

With 1HP being 742 watts and an engine rated in kilowatts, its simple to calculate HP at any given altitude with any chart that shows engine efficency/RPM/Performance-vs-altitude. You can even use fuel burn rate at altitude to derive the ratios.

I have seen Dave post before that a dyno can be made to read just about what the operator wants it to. I'm not an engine engineer but you can't fudge what load the engine will carry at what RPM. That's how performance characteristic charts are developed. Its done with aircraft every day when calculating density altitude-vs- runway length required. Its all the same physics and the ratios are the same. A prop is nothing but a wing just running in a circle. So the idea that the prop is the best dyno, indeed has merit. Hard to fudge physics.

Temperature and humidity are recorded at any airport with an ATIS system at I believe it is 30 minute intervals and broadcast on radio in the vicinity of the airport. Most also have phone numbers to a tape loop with this info as well. It will be labeled with a letter, like Weather A/B/C and so on throughout the day. So the info to accurately correct a dyno with is accurately available with a simple phone call within 30 minutes of the dyno run. Any error then become human error either true error or intentionally induced.
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Re: HORSEPOWER,DYNO'S AND PROPS!

Post by swampdragster »

I always turn to a dyno when a customer ... much more!

JUST CURIOUS DAVE, WHAT DID THE DYNO TELL YOU WAYNES MOTOR MADE BEFOR IT HAD THE UPGRADES ?????

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Re: HORSEPOWER,DYNO'S AND PROPS!

Post by Waterthunder »

I guess I better say it makes over 550HP after the upgrades. Here is where it shows my point I KNOW WITH OUT A DOUBT some changes we made to Waynes motor LOST HP! When I first built Waynes motor I recommended to run a certain intake however Wayne preferred the nasty looking spyder instead! I told him the spider will make more HP but the one I wanted would run better for him! Wayne said it best you have to prove it to me. His motor is the perfect example he started off with a 2.55 and a intake not best for his application! A year later he switched to the 2.68 I preferred and to an intake that better suited his needs! And walla he now has a well matched combination that swings a 4 blade R.
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Re: HORSEPOWER,DYNO'S AND PROPS!

Post by Twisted humor »

jdotson wrote:I've posted it before but torque is the work of the engine and horsepower is the marketing of the engine.
If that's true then watts rating are just for the marketing of electric motors. :lol:

So, that means the future selling tool of performance cars with electric motors will be, "it makes 350KW" or would it be, "it has 350KW?"

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Re: HORSEPOWER,DYNO'S AND PROPS!

Post by jdotson »

That's what I'd be looking for if I was in the market for an electric car!!! :lol: :lol: :lol: :lol: :lol: :lol: As an employee of a power company....I'm always interested in kW ratings....the higher kW the faster the meter will spin!!!!
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Re: HORSEPOWER,DYNO'S AND PROPS!

Post by Whitebear »

Gas engines are measured in Kilowatts all over Europe. A watt is a measure of power. Actually it can be a measurement of heat since everything boils down to heat anyway.

if 746 watts is 1 HP then a 300 HP engine is a 223.8 Kilowatt engine.

An electric motor develops power just like an engine does. The current drain off the source will be somewhat different than the output rating of the motor. But still a 1HP electric motor is a 746 watt motor.
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Re: HORSEPOWER,DYNO'S AND PROPS!

Post by jdotson »

Hmmmmm....since you mentioned kW's I'm wondering if KVAR's could be considered as nitrous for the electric powered vehicles? :D :D
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Re: HORSEPOWER,DYNO'S AND PROPS!

Post by Whitebear »

A VAR is defined as:
VAR - abbreviation for volt ampere reactive. Unit of ac reactive power.

Now using reactance you can get one hell of a kick for a circuit. I never worked in VARs or KVARS but its the same old electronics. But knowing the reactive kick you can get I would assume its likely a bit more of a jolt than Nitrous ! At least instantaneously. If you can continue to repeat it LOOK OUT !

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