Ted Zoli

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

cobra marty

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?

Ted Zoli

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

tomcat

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!

cobra marty

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?

Rambunctious

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. ?

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.

cobra marty

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.

Ted Zoli

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

cobra marty

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.