Anyone ever see actual propeller efficiency curves for the props we all play with, Mirage, Bravo, Hydromotive, etc.? It might help us when we are trying to optimize a setup. We all think about power and torque and RPM and slip, but all the boat knows is thrust and drag.

Prop efficiency = Thrust HP/propshaft HP

Thrust HP = Speed x Thrust (ft/sec x lbs)/550

1 HP = 550 ft-lbs/sec

Mercury says that 80% is about the best efficiency you can get and that value varies according to slip(angle of attack). If slip is too high, then the amount of propshaft HP that actually gets converted into forward thrust is reduced. If slip is too low, the amount of forward thrust is also reduced, as you use up too much power trying to force too much blade area through the water.

I have attached a chart from Mercury's propeller book. This chart shows efficiency dropping to 70% in a fairly narrow range of slip(angle of attack). This could make a big difference in speed since it is ultimately prop thrust that overcomes the drag of the hull and drive.

In this example Mercury says the maximum efficiency occured at 3-4 degrees angle of attack which works out to about 10% slip, what you might expect with a Mirage on a vee hull.

I have a feeling that some of the "mysteries" of prop selection could be better understood if we had access to the real curves from the prop manufacturers. Anyone out there with this info?

You guys labbing props have anything to say about this?

Look at the prop efficiency curve and tell me, if the prop you were using hit maximum efficiency at 5% slip and your test data told you that you were at 10% slip, what would you do? Would you try something to reduce slip or change to a prop that had it's best efficiency at 10%? I don't have the answer, I'm just trying to figure this out.

Don't say lab the prop, because the prop would then have a different curve. It makes sense that thinning the blades should increase overall efficiency by reducing the power required to move the blades through the water, a good thing to do in any case, but it doesn't address the original problem. Or does it? I don't know.

For more on this topic check out the Bravo vs. Hydro thread in the Testing and Upgrades Forum.

What do you make of this graph?

Used with permission www.aerodyn.org.

Or this one?

Tomcat.....(just because this is your more current post). Did you every hear more about the other excal and speed. That still is almost hard to believe....

Tomcat,

I don't know if i understand your question, but as you know the way the efficiency of a propeller is calculated is the actual speed as compared to (divided by) the theoretical speed if the prop were "screwed" through a solid (times 100). There are prop efficiency calculators all over the place.

Graphs for compressible fluid flow (air) have absolutely no bearing on props in water, unless you are trying to get familiar with terminology. Water for our purposes is considered incompressible.

Thrust of the prop is effectively impossible to calculate unless you know a lot about the boat's bottom, ie all of the coefficients of drag etc. You can "back" into it as you have shown if you know the propshaft horsepower, however it is not a factor that we need to know directly, of concern obviously, but not necessary in our efficiency numbers.

Angle of attack is a little understood ingredient in our world as while pitch is a simple concept, trim is not as it changes the entering blade one way and the exiting blade another. Additionally, the boat bottom changes angle with waves etc. These kinds of sciences are called empirical because experimentation is often the only reliable way to see what is happening on an system wide basis.

Does this help???!!!

Ted

Kaamacat - I put the two callers in touch with one another. The first caller gave up on the boat; he felt the salesman was playing dumb when it came to questions about the boat's performance problem. Others that have looked at the boat felt that the salesman wasn't that interested in talking to them. There's a funny vibe going on.

My own paranoid opinion is that either the boat weighs an extra 1000 lbs for some reason, or the engines are 7.4L - 310 HP with 502 MPI stickers. That's the only way I can get the numbers to make sense in my power vs. speed program. The drives they are advertising as Speedy #3s look like TRS with the lowercases painted, so the sticker theory has to be checked.

It's too bad, the boat looks great and the price is a steal, maybe just too good to be true.

Hello Ted - Thanks for the response, I will address your points in order, then ask the question again.

What you describe as propeller efficiency is the equation for slip. It's prop slip calculators that are all over the place. Efficiency is power out/power in. It's only related to slip in that maximum efficiency occurs at a certain slip for a given prop.

The terminology for air propellers and water propellers is the same, the difference is the density of the fluid. It's even OK to compare them since air flow is essentially non-compressible in the operating range of air propellers. The small pressure differences that can be generated by a propeller fan can be ignored and the flow treated as non-compressible with very small errors. This isn't true of high speed blowers and screw compressors, but it' s OK for propeller fans and even low speed radial fans. The second and third curves I have shown are for air propellers. I would love to see some for water propellers, but I expect them to be similar.

Thrust...without it nothing happens, so it is important, but hard for us to measure. So we watch our speedo and prop for max speed. With the equations listed above, we could calculate thrust if we knew propshaft HP and efficiency but I agree that the absolute value of thrust is not important. What I want to know is "Have I got the most possible thrust out of the power I put into the prop?" For this I need to be able to plot my test data on a graph of the prop efficiency to see where I am. Check the thread on Bravo vs. Hydromotive for more on this point.

Angle of attack and slip are very closely related. In the first prop efficiency curve attached above, Mercury plots efficiency against angle of attack or slip. They make no distinction, so let's just concentrate on slip. (For a good explanation of the relationship between angle of attack and slip see Mercury's book "Everything You Wanted to Know About Propellers.")

To keep things simple let's leave trim out of it and say that the prop thrust is directed parallel to the surface of the water and the correct angle of the hull for minimum wetted surface is provided by the aerodynamic lift of the tunnel. Sorry vee guys, but this is why cats are faster; they're not wasting thrust to hold the bow up. According to Mercury, in this situation a fast cat would have an ideal slip of 5%. They have the experience and the prop efficiency data so I assume they know what they're talking about.

So here's the question: What do you do if you are measuring 10% slip in your tests?

I am new to this; please help with drag co, and angle of attack, ?, rake
changes, weight of props and so on. and of course bottom eff

THRUST AND DRAG
 

The hull and drive hanging in the water produce drag. The hull is planing and the drag results from the friction of water on the wetted surface of the hull bottom and from the displacement of water. The drive is partially submerged and the drag results from friction and from the displacement of water. Just to give you some idea, when Mercury was testing their Blackhawk surface piercing drive they said that dragging a Bravo drive through the water at 60 mph created 350 lbs of drag, while the Blackhawk only created 35 lbs of drag at the same speed. There is also aerodynamic drag from the hull and deck moving through the air, but this is a lot less than the hydrodynamic drag.

So drag can be measured in lbs. If you wanted to you could drag a boat behind a tow boat with a spring scale on the tow rope and measure the lbs of drag at different speeds. This would create a hull curve of drag vs. speed. This curve is a parabola; drag goes up in proportion to the square of speed.

The propeller creates thrust, also measured in lbs. Since you have already measured the drag of the hull and drive at different speeds you know how much thrust the prop is delivering when the boat is moving at those different speeds.

Propellers produce thrust in proportion to their RPM, diameter or blade area, and the density of water. It takes power to do this. As RPM increases, the thrust increases, but because the propeller is being spun by an engine that has a horsepower curve that rises and falls when you pass the peak horsepower, the engine and prop together produce a thrust curve that looks like a hump.

When you plot these two curves on the same graph, top speed, maximum RPM and maximum thrust all occur at the intersection of the two curves. This is shown on the attached graph. The only difference is we don't tow boats around with a spring scale in the tow rope. We put engines of different known HP into the boat and see what different speeds we get. After doing this many times we develop empirical equations that make the hull curve. As a result the attached graph is propshaft HP vs. speed, not thrust/drag vs. speed. This approach works just as well for predicting speed. The exception is when you change drive heights to reduce drive drag, and change props (in particular blade area) to maximize the conversion of propshaft HP to thrust. But that's another story.

The attached curve shows two different engines in the same hull.

Tomcat,

Sorry if my original post was condecending, i didn't realize how much effort you had put into the topic and how knowlegeable you were. I thought you were mixing up efficiency and slip. Dumb me.

I did not take Compressible Fluids at WPI, but did take lots of Fluid Dynamics. I thought that aircraft propeller designers routinely let the tips go super critical which made all of the numbers end up being described by a lot of differential equations. I never heard anyone say that the media could be considered the same, but it could very well be correct for sub critical flow, it really doesn't concern us anyway except to understand the various graphical presentations.

The real issue is your question, how do you improve slip and can you improve it by "homing" in on better propeller efficiency.

I believe that you could but the practical problem is that you would need a wall of various number of bladed propellers of various pith and diameter. Lake X comes close to having that including various ratio drives. Still it is an awesome equipment problem.

We "hook up" at about 8% slip. We would prefer 7% and have never seen a 5% slip but the Lake X folks say they have.

I believe, and i know this is a cop out, that testing is the only hope. I know the right combination could improve the slip number you have now. If you had a slip vs. efficiency curve for your boat, it is still likely that the equipment doesn't exist (ratio, diameter, pitch) to hit an optimum. Having custom props made is fine if you are close and know exactly what direction to go. For high speed boats, prop efficiency is going to be an elusive factor and slip calculations are likely to be the only way to know if changes are sucessful and therfore which direction to go with additional modifications.

Enjoyed the discussion with you.

Ted

So if we basically have to leave props alone and their only variable is currently available props plus or minus labbing, The other variables to change to increase efficiency- decrease slip is:

1-lighten the boat...throw overboard the junk, lighter materials
2-decrease hull drag...?these hull coatings
3-decrease drive drag...raising drive, IMCO of Merc. lower units
4-decrease wind drag...hard to do
5-decrease driveline parasidic loss...use synthetic lube

What else do we have?

IMHO the major factors are drive height and design, propeller selection and centerline of prop distance from the tunnel. When prop selection and drive height are optimized it now becomes quite a chore to consider how far outside of the tunnel one should be. I know of outboard boats that have done a lot of testing on the topic and Big John has an opinion that is likely well informed and accurate. Is this ok for all boats and drives?? Not likely.

The manufacturer should know an optimum drive location, but this is not going to be for all out speed unless the manufacturer is very committed to racing and has a varying setup for the two different uses.

Testing and more expensive testing!

Ted

Ted - No problem, I didn't think that your post was condescending. Since we can't see one another when we are talking here, I never jump to any conclusions about the poster's "tone". I'm just happy to be discussing my favourite tech subjects with other enthusiasts. Talking it through helps me think.

I never took fluid dynamics, so you are way ahead of me. I just look at the equations and graphs and try to understand what they mean in practical terms. I think you're right about Lake X. A wall of props etc. is necessary, and the guys doing the most testing are usually the guys winning in any motorsport. There was an article a few years ago where Lake X finetuned a 42 Fountain. Didn't touch the engines, just changed drive heights, ratios, props and prop rotation. They increased mph from 65 to 70. That's huge! Minimize drag and maximize thrust.

Since every hull/drive/engine combination is different, there is no one size fits all prop. When you have already done everything else on Marty's list, then cutting props, maybe losing a few props in the process, is an acceptable risk. What I would like to do is get a prop efficiency curve for my Bravo 28s, find the current slip and see where I am. This should help me decide whether I need more or less slip, and would direct me to a shorter list of things to do. If, for example, I need less slip, I would add blade area. Reducing slip by lowering the drive increases drag, throwing the baby out with the bath water. If I need more slip (does this ever happen in real life?) I can't raise the drive any more, so reducing blade area is about all I can do. By the way, with Hydromotive four blades (less area) the boat is faster, but runs too flat.

It has occurred to me that if these prop efficiency curves exist for the Bravo and Mirage props they may be for running submerged. Semi-surfacing would be a different situation, different curve, another damn variable.

In any case, I have raised the drive as much as I can without major surgery. I can't change drive spacing for the same reason. I can't do much about the weight and I don't believe in the speed coatings. So I'm left with getting as much thrust out of the prop as possible. I'm faster with three blades but can't live with the blowout, so here I am with four blades. But I suspect I now have too much blade area. I think a larger diameter three blade would be a better answer, but the 29 Mirage Plus is the only one with 15" as opposed to ~14.5" diameter. I doubt of I can spin that much pitch at anymore than 4600 RPM, so heating and banging is necessary.

So I'm testing like everyone else is. But I still want to see those efficiency curves!

Tomcat, The first part of that curve doesn't make sense.. If there is 0 slip wouldn't HP in + hp out (thrust in = thrust out) and 100% efficient. The curve would look flat and then drop off as it does with increase slip. Maybe the scale is a log 10 and the upcurve is with 0-0.5% slip and the flat is 0.5-15% slip and >15% slip is the downcurve. What abut turning down the diameter of the bravo prop by 1/4" at a time?

good Q cobra marty

i'm trying to understand the details better too. the graph doesn't have a scale on the x axis. if the beggining IS 0 (zero) , then it appears 25% of the energy goes to "free spinning" a props frontal projected area (leading edge area) through the water. a graph after labbing SHOULD then show this initial load reduces. Are we thinking right Tomcat. ?

That curve is copied out of Mercury's propeller book, and I wish that it had a scale on the x axis. Here's what Mercury says:

No Slip - Can only occur when the propeller is windmilling (boat is coasting to a slower speed or being towed)

Too Little Slip - Too much diameter for the engine. A wasteful amount of power is being used up in blade friction. rather than in producing thrust.

Slip is Correct - Most efficient use of power at the propeller shaft.

Too Much Slip - Too little diameter for the power and load. Can also happen when there is too much cavitation or ventilation. This has the effect of reducing the blade area in contact with the water, thus creating a propeller that in effect is too small for the power and load.

So you're right. At zero slip, HP in = HP out because they both equal zero. You could say that power efficiency is 100% but it doesn't make sense, because there is no power. We don't really care about this situation, because when you're being towed you have other things on your mind, like what caused your engine's HP to go to zero! It's the other conditions described above that concern us.

This characteristic of the propeller, it's power efficiency, is not calculated from slip, somebody measured it and plotted the results against slip. They are related by observation only. Later on engineers developed equations that could define the relationship mathematically.

I don't care about the math, I just want the best bang for the buck. Just like we want to adjust pitch so that the engine is operating at the best RPM, we want to adjust blade area so that there is just enough to handle the power and load, no more, no less. This is where the prop is most efficient.

I think you could turn down the diameter of the Bravo prop. Of course, you don't know when you've gone too far until you get there, so props may be scrapped. I'm going to try the 15" diameter 29 Mirage Plus first. I may not be able to pull the RPM but it should tell me if blowout on planing and turns is better/liveable. Like you I may be carrying two sets of props.

Tough in the real world "adjust blade area...just enough to handle the power and load, no more no less" All good until you add another passenger, use up or fill up with fuel, etc. Most of our boating is under such varying variables that the most we can ask for is a comprimise at best. Well we can allways ask for more.

You all have the curve figured out right...but to pin it down a little more...

The curve is a qualitative representation only. It shows that with increasing slip the prop exhibits a best efficiency point. The x axis does not start at zero but at some point differing for different props.

Tomcat's verbal descriptions from the Mercury handbook put in words what the curve represents. Too little slip is a concept that is hard to describe in the way that they have. Remember no slip equals no thrust. To understand it about the best you can do is mentally lock the prop up to the fluid so tight that no water gets "pushed." That is the no slip point but it is not on the graph as that would be zero efficiency, an unrealistic point to graph.

Ted

Zero slip- OK if the prop is in a solid even cement there is no thrust and 100% slip then there is no thrust for both. The curve is probably exagerated to show this and is not a true mathematical curve. That is why there are no values.

The curve from the Mercury book does looks like a made up example to teach the concept. Lookng at a real curve with the x axis labelled would help. Just want to know which side of the peak I'm on.

Ted - I just looked at your profile. You know more than you're saying! ;) Don't hold out on us, man. How about some details on the Skater and how you dialed it in.

Tomcat,

I wouldn't hold out...

On the 36 Skater (Old Ocean Outboard boat) we took out about 800 pounds, got the engine heights right, the right props, got water to the engines and de-trinketized some of the goofy stuff that was on the boat.

We are not done yet...will be testing at Lake X first week in April.

Thanks again for starting this thread...very interesting topic.

Ted

Ted, What happened to the cig with the turbos and the arnesons? The Ironman.

Gave it to Michael...he got it going and sold it...bought a 30 Spectre, then we sold that to buy the Skater.

That Cigarette was a great hull.

Thanks for asking.

Ted

I love that boat. Can you post any pictures? We'll need at least one shot of the boat running like a scalded cat on Lake X. Good luck with it.:D

For what they are worth...there are lots of pictures on our web site www.critical-lift.com

Ted

Sorry...meant to post this one!

Ted

Rambunctious - I got your email, but having trouble with ISP right now when sending out. So I'll answer here.

I looked at the equations for drag and the website you linked. Very informative. Skin drag, form (or eddy) drag and wave drag. The equations do suggest power is proportional to speed cubed, but it looks like these are all geared towards displacement hulls, and the flow of water along the hull is likely laminar. What happens with a planing hull? There isn't much wake when you're really moving so skin drag should predominate, and I'll bet it's turbulent flow.

A point to ponder. Years ago, in the Turbocharger bible written by Hugh MacInnes, there was a short section on Marine turbocharging. There was a graph showing power required vs. speed. The curve showed a distinct hump as the boat began moving in displacement mode with a rapidly increasing power requirement (cubed?). Then as the boat fell over onto plane the power required dropped a bit and started a much gentler rise (squared?). I lost my copy of this book so I can't scan the graph and post it, but you know this effect is real. Once on plane it takes less power to go faster than "plowing speed".

This is as good as anyplace for this...

I got my Poker Runs America in yesterday and i was dying to read Steve Sandler's article "Round Trip Ticket" on prop rotation.

I don't know Steve Sandler. I'm sure he is a fine guy but the article was worse than awful, it was trivial. You can sum the article up in 4 words. "You need to test."

His assumptions on loading on the props in my opinion are just plain wrong. His assumptions on portions of the props being blocked by Vee hulls is nonsense. In-water props are set back enough to be in the water, and surface piercing props couldn't possibly have the force vectors he describes as blades are out of the water. Talking about docking i put under the "who cares" category (maybe that is just me).

Conclusions were tentative...basically wishy washy. PRA could have found many experts to "get into" this topic from a performance boat point of view. They didn't.

Ted

tomcat, I 'm going to try something here, posting a screen grab.

jsut to show the equations to others.

the point of the equations was skin drag WAS proportional to v squared, jsut like the drag through air for the hull frontal area and drag of the outdrive through the water (displacement). My explanation for the drop in force upon plaining is that the wetted area is part of the constant in front of the v squared functions, upon the transition to planing, the hull creates lift to significantly reduce the wetted are. I agree at this "zone" the pure v cubed curve doesn't fit. But, Upon planing, where the wetted area does not change significantly upon speed, and in the range we are discussing for performance, this is where the aireo drag constant x velocity^2 + the outdrive drag constant x velocy^2 + the wetted surface area constant x velocity^2 ( as stated in the attached equations) all sum to the total force. By the associative properties of algebra, the net force simplified is (Aero factor + outdrive factor + skin drag factor) X Velocty^2.


This equation simply states my understanding that skin frinction is a funtion of V^2.

Therefore the POWER, = Force x Velocity = a function of V^3 !!


I know I'm wearing you down Tomcat. come on over the the V^3 rules side :D

seriously, I would love to try and demonstrate these fundamentals with your program and real testing. I still reserve the right to get smarter. maybe I'm all wet (no pun)

try again to post a screen grab

i give up for now attaching a screen grab, here is the websight i am referring to

http://www.kent.k12.wa.us/staff/trob...tml#Resistance

ok tomcat, I know I've about beat all the life outta this topic.

but i found a source, he's not a rocket scientist, but he IS a boat scientist :)

Dr. John Batenh. PhD in Marine Engineering. His PhD topic was performance prediction of high performance planing watercraft.

He confirms my statement that the drag is a function of velocity squared thus the power is a function of velocity cubed.

YES:D

BTY,The reynolds number in the equation is a dimensionless number that ends up being a constant for a given boat lenth, water density, etc..

here is his reply:

Hi Rob,
Planing hulls and non-planing hulls produce a combination of dynamic lift and
static lift. The proportions vary, and the resistance is a function of
whether the vessel is going over the water or through it. The two primary
components of resistance are wavemaking and frictional resistance (mainly
frictional for a planing craft). in all cases the frictional drag is a
function of: (1/[Reynolds number (V.L/viscosity) squared]). The effective
power comprises the various frictional resistance values summed and
multiplied by the speed of the vessel through the water. The power required
to drive the vessel depends on the efficiency of the hull form, and hence the
laminar/turbulent flow around the hull. It also depends on the efficiency of
the propulsor and the efficiency of the combination of the propulsor with the
hull, i.e. hull, rotative and 'open-water' efficiencies multiplied together -
called the Quasi Propulsive Coefficient (QPC). Divide the effective power by
the QPC and multiply by the shaft/gearbox efficiency (losses) and you get the
power to drive the vessel through the water. Power is therefore proportional
to velocity cubed.
Got it?
Regards, John Batehn


Hey....We now have a new term to throw at the gear heads, " hey.... what is your quasi propulsive Coefficient".... (said in a nerdie tone)

So v^3 Rules!! right???????
i can't find any more proof than a rocket ... i mean boat scientist


Rambunctions

Yea but power is TORQUE and not HP. TORQUE is what turns the prop. MORE TORQUE = MORE POWER!! = MORE SPEED.

So, Power(Torque) is a function of V ^ 3!!

TORQUE RULES!

Cobra Marty

See post 19346 Speed vs Power for more discussions. but....

By definition, Work (energy) = moving an object with a given force a given distance. (ex 1 foot- lbs)

Power, by definition = rate at which energy is produced per unit time
= (ft x lbs) / second

or lbs x (ft/sec)

or force x velocity

1 horse can lift 550 lbs at 1 ft/sec. that same horse can lift 1100 lbs at .5 ft/sec ( through a pully system) = both are one horsepower. the force is doubled in one case, while it's velocity is halved.

550 ft-lbs/ sec is the defininition of one horse power (1 Hp)

Torque x rpm is the rotating equivelent to force x velocity. 600 ft-lbs of torque at 3000 rpm is 1/2 the power as the same 600 ft-lbs turning at 6000 rpm.

think of your outdrive gearing. say 1.5:1 for a bravo. if your engine makes 600 ft-lbs at 6000 rpm, then through the outdrive gearing, the prop is making 900 ft-lbs of torque and is turning 4000 rpm. (neglecting drivetrain losses for now)

(600x6000) = (900x4000)

the power is the same (energy is conserved)

Cobra Marty, Your statement is true in the context when one is talking about a particular rpm. at that rpm, more torque = more power.

Rambunctious,
can you put some numbers in your equation to simulate a real world situation & do a 5 or 6 point data entry that will substantiate your hypotheses?

I love a good equation as much as the next guy, BUT...sometimes the facts don't fit the theory. Here are some points on the curve.

A 36 Skater with a pair of HP500s will run about 110 mph under ideal conditions. The same hull with a pair of 900SCs will run about 150 mph under ideal conditions. Compare these numbers and you will see a velocity squared relationship for power. Take the same hull and put in 2 X 1500HP and you have the new kilo record of 190 mph, again squared. When I put these numbers into Mercury's (squared) equation I get very similar hull constants and curves that align almost perfectly. I don't know why and I can't explain it, but these actual test results don't agree with the power = velocity cubed relationship.

Interesting in another thread that for a speedo which is really a pressure guage but MPH = 8.1 times square root of pressure reading. So is mph related to square and not cubed?

Cobra marty,
pressure of water is related to square of mph (velocity ) . Kinetic energy (energy of motion) = 1/2mV^2. thewater rushing at a certain velocity is "Stopped" thus creating a pressure proportional to the velocity squared (transforming all kinetic energy of the water onto potential energy (pressure))

POWER is Force X velocity. (add another velocity term) getting the velocity cubed function for Power.

Tomcat, Dean Campbell, I looked on Powerquest's website ( they build their boats here in Holland) to get speed vs power. Im afraid their speed data is marketing driven ( i.e. 5 mph for each engine upgrade) but they give a range in speed for each engine package. because of their large range, their data fits both curves. Your data would be a good example. my only rebutal to that is like you mentioned when we first discussed this. the hull in the cat may be lifting more and making it more efficient (through the reduction in wetted area).


I would love to find good data for a vhull to answer Dean Campbell. The Powerquest data isn't accurate enough.

ANYONE HAVE BEFORE AND AFTER POWER UPGRADES. LEAVING OUTDRIVE AND POSSIBLY PROP ALONE

Tomcat, could you also overlay the v^3 function to see the difference from the V^2 line you have just for fun?. What are you using for a graph? I was messing with excell for the Powerquest study I was doing to answer Dean C.