prop doing bad

[Guest625101138]

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

[bblagonic]

Hi guys!
I feel responsable to inform you about the project developments.
I made some further test, made consultations with producer, made the sketches and finally I have two spray rails of 60mm width and 150cm long (see pictures - white area on antifouling surface).

The boat went in the water last weekend and the results are 20kn at 3450rpm!!!
I have more 150rpm to reach the motor max rpm (3600) and hopeful some speed.
The trick is that this is all with the old prop (17"x23").

My intention is to put the prop with bigger diameter and greater surface.

Measuring on board, it come out that I have enough space to put prop of dia.18" and dia.19".
With dia.18", there is abt.20%of dia. clearance to the hull.
With dia.19", the clearance is abt.15%.

My intention is to install prop of 18"x22".

But as there is enough space to install even bigger prop (19"), should I go here with max. instalable prop diameter of 19"?!

I`m asking that not just because the clearance to the hull is only 15% of dia. but also beacuse increasing the diameter I`m reducing the pitch and all of that is not advisable on higher speeds 20+ knots (if I´m right )
Is it advisable allways to put the max. instalable prop diameter even on boat with higher speeds?!

[Joakim]

The problem you have is a too small propeller, which causes cavitation and excessive slip and thus poor efficiency. As Baeckmo calculated you can only use about 100 hp from your engine at the cavitation limit.

Since your propeller is cavitating badly, it is difficult to estimate the thrust it is producing, but I'm guessing it is in the order of 8 kN, which would mean ~50% efficiency.

You should increase the blade area and the diameter. 20% tip clearance is fine and even 15% should be OK. Once you do that, the propeller is not cavitating at all or much less. Thus it will also have much less slip and you should decrease pitch.

Changing to a 4-5 bladed 105% 17x22 would allow you to use ~150 hp at the cavitation limit, but you will still have too much cavitation at 8 kN thrust. You will likely reach 20 kn at lower rpm in spite of lower pitch.

A 18x22 propeller would likely be over-pitched unless top speed is increased to above 25 kn. You still need maximum blade area to avoid cavitation.

I would think a 4-5 bladed ~100% 19x20 or 18X20-21 is what you need. These should have over 60% efficiency at 8 kN/20 kn, thus leaving some room for higher speed. If you expect to reach 25 kn, then I would add an inch to the pitch, but that may cause problems getting over 20 kn, since rpm at that point are lower.