prop doing bad

[Rick Willoughby]

Originally Posted by baeckmo

Bblagonic,

The problem you face when adressing a real world problem on a forum like this, is that you have to discriminate the input from those who are misunderstanding and misinterpreting plus the ethernal wannabee scharlatans, from those who have factual knowledge. There is only one way that will tell; final results and experience.

I am an enthusiastic advocate of the principle of free speak, but must admit that sometimes it becomes frustrating to see skilled persons like daiquiri and others having to spend so much effort in negotiating pure incompetent nonsense that pops up from wannabees in each and every thread. I wish that Apex1's WROM were a reality.........!

Now to your problem, adding to your dilemma as to wether I am thrustworthy or not....:

Checking the data of the setup you have described against Wageningen B peformance, using a wake factor of 15%, we get the following results for noncavitating operation:

Ja=0.69, SHP=257 hp, Thrust=11034 N. With a depth to shaft cl of 0.7 m, the cavitation coefficient ("cavitation number", Sigma07) at 0.7 radius is 0.17 and in order to limit cavitation to 10 % of the blade area, you need a BAR of 1.92! Obviously, this does not work in reality. To understand what actually goes on, please see the engine power diagram attached, where I have added some data.

Your engine is very close to this variant. You have the line of nominal power, below that a shaft power line. If you look at the point 257 hp/3500 rpm (the noncavitating result from prop calculation), you will see that it is far above what this engine can produce. The maximum noncavitating rpm possible would be 3100 rpm.

With an engineering guess of hull drag, we can check cavitating performance with reduced rpm and speed of advance. We do not enter the 10 % limit until about 2400-2500 rpm, so from there and upwards the cavitation intensity is increasing (thrust and efficiency going down). This is illustrated by the broken line, starting at 2400 rpm/96 hp and crossing the available shaft power at 3500 rpm/198 hp. This is the explanation why your engine is working at 3500 rpm and still cavitating heavily, as Daiquiri noted.

The Sigma07 value corresponds to the lowest acceptable pressure coefficient (-Cp) anywhere locally on the blade profile at 0.7 radius. If foil analysis (test or numerical) indicate anything lower than -0.17 on the suction side, there will be cavitation from there on. A value of -0.3, as mentioned somewhere, is telling us either that the profile studied is not suitable for this application, or that the method of analysis is incorrect; the selection is yours!

If I may give a piece of advice: listen to Daiquiri here, he knows what he is talking about (....as long as he is not contradicting me of course...).

Bojan
So there you have it. Two weeks later after juggling the curves this result is an alternative means of arriving at what was provided in post #19 - always useful to have verification with independent methods.

I prefer understanding and applying the basic physics as it gives better insight than playing with curve fitting and applying fudge factors.

I should also point out that the 5% section I used to determine Cp was given at 0.7R for an AU type prop. I think the MAU type is more common and I do not have any blade section information on this. From memory the 3-bladed Wageningen prop has a 3.5% section at 0.7R.

Irrespective, trying to stuff 220HP though a 17" disc at 20 to 25kts is going to result in partial cavitation no matter how well designed/manufactured the prop is. It is a matter of whether the efficiency gain will be good enough to get the speed you expect.

Increasing the BAR and the number of blades will do better for sure. Be mindful of the cavitation and look for blade sections that will operate at lower Cp - meaning thinner sections.

There may be some useful information on this thread for you to peruse:
Hull modifications
If nothing else it gives you an idea of the relative size of prop for reasonably efficient operation. Heavily loaded props waste a lot of power.

Rick W