prop doing bad

[bblagonic]

Hi guys,
I`m calculating the right prop for a boat of my friend.
The boat`s performance is quite bad in comparision to a boatbuilder chart and the trials data I saw in one article (but with different engines).

There are few things that I`m quite concern about:

1. How could I predict the wake and thrust deduction factor for a planing power craft with a single prop?
I´ve found on this forum a diagram but didn`t understand what Wt and Wq means? I need only W as a factor. Those diagrams are form Hadler 1971. paper and are used for twin prop configuration.
See the diagram in att.

2. When reading the Kt and Kq factors from KtKq diagram of Wageningen B-series, should I enter into the diagram with J as advance coeff. or with apparent advance coeff. Ja (Ja=J(1-w)?:confused:

and the third

3. What would be the best suitable calculation for checking if my prop will cavitate or not?


Sorry for so much questions but I`m spending weeks trying to get inside the world of propellers.

Thank you in advance.:)

Bojan

[gonzo]

Cavitation will be produced by too high a propeller speed or too small a blade area

[bblagonic]

tnx Gonzo,
the thing is that I would like to listen from you your experience about the way of accurate prediction of cavitation.
Is this from the Burrill charts or using the Gerr`s equation which include the MWR (mean width ratio), or some else empirical formula?:cool:

regarding the thrust deduction factor and wake factor...
If somenone has the pdf of Hadler and Hubble paper: Prediction of Power performance of the series 62 planning hull forms" 1971,
I`ll be very grateful sending me this.:o

thanks guys
Bojan

[gonzo]

Gerr's book is simple enough. Just plug in the numbers and it gives you propeller dimensions.

[baeckmo]

Bojan, here my personal view:

1/ For the planing region, monohull, ~constant deadrise, use (1-w)=0.97.
2/ Understand Ja as the working environment for the prop; Va=Vship*(1-w), ie.; Ja=Va/(n*Dp).
3/ As I am unfamiliar with Gerr's algorithms, I suggest you use Burrill's limits. If it is a leisureboat you may accept a 10 % cavitation level according to B's calculation method.
4/ The Hadler/Hubble paper is a recalculation of the original Clement/Blount test results; if you can't locate it, try to find the original paper:
"Resistance Tests of a Systematic Series of Planing Hull Forms" SNAME 1963.

[Guest625101138]

Hi guys,
I`m calculating the right prop for a boat of my friend.
The boat`s performance is quite bad in comparision to a boatbuilder chart and the trials data I saw in one article (but with different engines).

There are few things that I`m quite concern about:

1. How could I predict the wake and thrust deduction factor for a planing power craft with a single prop?
I´ve found on this forum a diagram but didn`t understand what Wt and Wq means? I need only W as a factor. Those diagrams are form Hadler 1971. paper and are used for twin prop configuration.
See the diagram in att.

2. When reading the Kt and Kq factors from KtKq diagram of Wageningen B-series, should I enter into the diagram with J as advance coeff. or with apparent advance coeff. Ja (Ja=J(1-w)?:confused:

and the third

3. What would be the best suitable calculation for checking if my prop will cavitate or not?


Sorry for so much questions but I`m spending weeks trying to get inside the world of propellers.

Thank you in advance.:)

Bojan

Bojan
If you really want to understand what is going on you should make use of a good analytical tool like JavaProp. It will take a few hours to appreciate how it works but it will give give you much greater insight into what is happening than looking over empirical charts. You can run the Applet from this page:
http://www.mh-aerotools.de/airfoils/javaprop.htm

You will not be able to get the exact foil of the prop being considered but there will be inbuilt ones that are close to the one you are looking at.

The thing with JavaProp is that it can give you Cp for the prop. From this you can determine if the blades are cavitating by considering the flow velocity over the blades at various radial positions at your speed of interest.

If it is a typical inclined shaft installation then the shaft inclination will have much greater influence on cavitation than any wake factor on the planing hull.

Another thing that often gets scant consideration is appendage drag. Are protrusions minimised and streamlined. Also how does the actual weight compare with the boats you are comparing it with.

Rick W

[daiquiri]

The thing with JavaProp is that it can give you Cp for the prop. From this you can determine if the blades are cavitating by considering the flow velocity over the blades at various radial positions at your speed of interest.

Hello, may I ask you please to elaborate a bit more on the following (not very clear) passage: from Cp obtained from JavaProp to the cavitation estimate?

[Ad Hoc]

Well Rick, are you going to reply to daiquiri's simple question??..or doesn't JavaProp know how to answer simple questions?

[baeckmo]

Bojan,

To gain the required "engineering feel" for the variables in propeller selection, I strongly recommend that you first continue along the path you are following now, before you start using computer programmes! The graphical overview you have when working along the manual way will give an understanding of the optimizing process, that is not a natural result from just flipping numbers in a software.

I understand that you are using diagrammes in the (kt, kq, eta) versus Ja format. A better way to get preliminary results, and to see the possible alternatives, is to use the Taylor diagramme, ie P/D over load factor Bp. This format is available for a range of configurations for the Wageningen B-series propellers. This series covers most applications with a reasonable precision; special props are a bit further down the road for you right now, I guess.

As for the cavitation limits, the Burril method can easily be transferred to an Excel sheet, becuse it just involves routine arithmetics based on loading figures from the propeller selection. Of course, it doesn't hurt to have a clue on what is meant by the basic terminology (cavitation, NPSH, cavitation number a.s.o).

[apex1]

Bodo,
would you mind to have a look here too?

http://www.boatdesign.net/forums/props/propeller-shaft-bertram-29887.html

cheers Richard

[daiquiri]

I think it would be really about the time to start educating people about the concept of VALIDATION of the software before using it for the evaluation of new designs, perhaps in a new thread dedicated only to that issue.
And that thread should (imho) pop-up each time a person connects to the forum.

Now, there is a page with validation data at JavaProp site. It can be found here:
http://www.mh-aerotools.de/airfoils/jp_validation.htm
coloured lines are test data from NACA Technical Report n.594, thin black lines are results from JavaProp.
How to read diagrams:
- Fix the advance ratio J on the horizontal axis, draw the vertical line through it, up to the curve of interest.
- Read the corresponding values of Ct, Cp and Eta (efficiency).
Ok, so let's read what the Author says about the results of the validation:

1) Efficiency vs. Advance Ratio:
He says: "It can be seen that the overall characteristics are met quite well; typical deviations are in the order of 5%"?
Well sorry but, based on the graphs I can see, I have to disagree. All the lines start to diverge severely above the point of max. efficiency, quickly becoming completely useless. Below that point, it seems that anything can happen.
The orange line follows pretty closely the test results up to J=0.5 and becomes very imprecise above that value.
The magenta line is reliable only in the range between J=0.8 and J=1.2. Below it there is a 10+% deviation and above it the error goes skyrocketing.
The blue line is out of target by nearly 10% along it's enire length, diminishing the error to some 6-7% only in the region of max efficiency.
So not only the error is not constant, it also follows a very irregular pattern with respect to J - depending on blade angle of attack. So, with such a herratic behaviour of the error function, how can you use this software for a reliable prediction of design chracteristics of a new prop?

2) Thrust Coefficient vs. Advance Ratio (but the same is valid for Power Coefficient vs. Advance Ratio curves):
The Author says: "Here we can see a good representation of the region where the curves are almost straight"
Where is that? The only thing well-rappresented is the slope of the curves, in the linear region. But all the calculated curves are shifted to the right, and that shift induces errors of the order of 30%.

The errors observed above are due to the fact that JavaProp uses a strip-theory (which becomes "blade-element theory" in case of props) method for the analysis, combined with empirical airfoil data. It means that the flow is esentially considered to be bi-dimensional at every station, thus neglecting all the very important effects of radial flow components along the blades (particularly near the blade root or hub). It also relies on the the correction factor for tip losses (Prandtl's, Goldstein's or else), which generally becomes less accurate with increased blade loading or Advance Ratio. That is the main reason for the big errors observed in the region of low Advance Ratios and for the shift of the curves at high J's. Back in my university days I had developped a very similar algorithm (written in Matlab, for a good old i386 :) ) for direct and inverse aircraft propeller analysis, the same strip-theory and (consequently) exactly the same kind of errors encountered. I'm looking here at the graphs at the JavaProp's validation page and it seems to me too look at those old printouts of Matlab graphs... We are talking about 1997-1998, so its nice to know that fundamental physics have not changed since then, and the algorithm errors have not become smaller just because graphs are calculated and plotted much faster and in colors now.

Now, the authors of JavaProp (dr. Martin Hepperle et al.) have put a big effort and a considerable ammount of their time into that site. Big respect to them for the job done. But they have been a little bit optimistic when claiming the precision of JavaProp's output results - at least from what can be seen in the page above. And I'm pretty sure that JavaProp was never really intended to be a primary tool for prop design, if not only at a preliminary stage.

So I would like to advise averyone to be careful when using the numerical analysis software for design purpose, be it JavaProp, JavaFoil or whatever. You better always check out the validation results first and try to understand the limits of validity of mathematical models used by the software. Unless you intend to use it just for playing around with some numbers in your spare time.

P.S.
Bojan, if I were you I would stick to the much more reliable propeller charts and to Baeckmo's sound advices - Taylor's chart for prop sizing and Burril's chart for cavitation estimate.

[Ad Hoc]

The method described by baeckmo is exactly the method i use when designing a prop. I ahve a "fist full" of charts and as he rightly points out
...The graphical overview you have when working along the manual way will give an understanding of the optimizing process, that is not a natural result from just flipping numbers in a software.....nuff said!

As for validation.
Whole heartily agree with Daiquiri. Without validation from an independent source, once is ok, twice is much better, the output is as much use as a chocolate tea pot!...nice pretty colour plots though :P

[baeckmo]

Very good observation daiquiri; and a very important note on validation!!! While you were writing I was in fact checking some of the values in the JavaProp validation section. If we just look at the 30 degree setting, which equals a P/D of about 1.26 (depending on definition), and take values for Ct at J= 0.75, 1.0, and 1.25, we get for JProp Ct~ 0.13, 0.1, and 0.042. The corresponding test data are: ~0.155, 0.112, and 0.065.

The absolute difference is then 0.025, 0.012 and 0.023; the error compared to the test is then 16%, 11% and 35%. The error is systematic and far to great to be acceptable even in preliminary calculations. And dreaming of the use of any kind of pressure coefficient to predict cavitation is just nonsense!

Although I like the working principles of the JavaProp and admire the effort put into it, it still has to see some further development before it is of any use. The observed deviations from test data suggest to me that the lattice/solidity effect of the vane configuration is not fully taken in. Both the fact that the JProp data show a stall at about 75% of the J values and the too low circulation values after passing the vanes, point in that direction.

When you test a rotor (pump, propeller or turbine) with fewer vanes (less solidity) than optimum, you will see deviations in performance, very similar to the deviations between calc and test in this validation example.

If this is correct, it should not be too difficult to introduce a better compensation for the lattice effect into the program, making it a very useful tool for us, instead of a toy.

[pistnbroke]

maybe I am stupid .but if this boat exists can we know the length and the engines its got on it them maybe someone can tell you what prop to put on it ...as a ball park figure

[bblagonic]

well I can`t say I`m surprised about the discussion arrised from my questions as I had clear view about the knowledge of members of this forum...tnx

now...back on subject

baeckmo...I will follow your suggestion to first introduce myself in theory and empirical equations dealing with propulsion
the fact is that I have the basic knowledge in mechanics and hydrodyinamcs as I graduate as Marine Engineer but till know I was dealing mainly with mechanic system of boat and ships:cool:

pistnbroke...your question is far from stupid but my intention is to fully understand the problematic and to be capable resolving the future problems that will arrise in my career

as your advice is to go intro the Bp delta diagram for better estimation could you also advice me where can I found (buy, download) all the these diagrams. I have few of them but those ones are far from all the chart numbers that Taylor made?


the boat is a round chine, round stern, single screw (5° angle shaft) and dimensions are as follows:

Lwl = 6,5m
Bwl = 2,45m
BHP = 220 hp at 3600rpm
screw DIA. 17"
screw PITCH 23"

the screw is light scewed, AU series, three bladed with B.A.R. of 0,73

DISPLACEMENT (supposed) - 4t = 4.000 kg

the goal is to reach the max speed of 23-24 knots
Now the boat is performing only 20 kts at 3500rpm what is the max. achievable

producer trials data says that with 220hp and displacement of 3,4 tons (3400kg) the boat max speed is 26knots

these days I will try to measure the exact displacement :idea:

As the eninge is old, I`m not quite sure if it still develops the design power. Do you have some advice how to check this? I`m also ready to buy a device (dynamometer) in order to be capable of checking the real power the engine is generating. (Some advice about the model or producer of a small power meter?)

once more time my acknowledgments to all of you
I`m sure we will collaborate well in future

[bblagonic]

sorry...forgot to mention two more things:
gear ratio is 2:1
and
max. prop diam. is 17˝

[Ad Hoc]

bblagonic

With the basic figures you have posted, the max speed you'll get is around 17~18knots, assuming the 220BHP and 4tonne max displacement, for your hull. The hull sounds like a classic semi-displacement hull, and will get a bit more draggy near its top end and lots of wash. Not to mention may be a bit "tippy" at high speeds, especially when turning. The round bilge causes instability at high speeds.

[baeckmo]

In addition to my notes on solidity earlier, it would be interesting to see if a "fake" increase of solidity in Heppler's validation example could reduce the error levels. This might lead back to the specific section of the programme, where an underestimation of the solidity effects is introduced.

What do you say Rick? Some time when you have the JavaProp up and running, could you recreate the validation propeller and run some calcs with the three blade settings in the validation calc, and see what increase of solidity is needed to reduce the error levels around the eta max advance ratios (if at all possible)??

The interaction effects between vanes in a rotating lattice has always been a problem for the aerofoil theory of turbomachinery, and I think we have to live with a clever combination of numerical and empirical analysis for a long time yet.

And, finally, we should be aware of the difference between "propeller design" and "propeller selection"; Bojan started the thread with a selection problem.

[Guest625101138]

........
Lwl = 6,5m
Bwl = 2,45m
BHP = 220 hp at 3600rpm
screw DIA. 17"
screw PITCH 23"

the screw is light scewed, AU series, three bladed with B.A.R. of 0,73

DISPLACEMENT (supposed) - 4t = 4.000 kg

the goal is to reach the max speed of 23-24 knots
Now the boat is performing only 20 kts at 3500rpm what is the max. achievable

producer trials data says that with 220hp and displacement of 3,4 tons (3400kg) the boat max speed is 26knots

....

Bojan
I have attached the JavaProp design info for a prop of similar proportions to the AU series you have. The power and thrust figures are too high as JavaProp has no ability to allow for cavitation and a large proportion of the blades are in cavitation.

I have also included the Cp plot for a 5% foil of similar shape to the AU series. The maximum value of Cp is around 0.33. When you look at the flow vectors at the various radii it shows that the portion of the blades beyond 130mm radius are in various stages of cavitation at 20kts and 1750rpm.

I use JavaProp as a first pass for prop calculations but I have my own spreadsheet with in-built macros for more comprehensive analysis. I have attached a portion of it that has the key data related to the conditions you are considering. The key values are that the prop will be producing around 8900N thrust, power is 153kW (204HP) and prop efficiency is 60%.

This data is primarily derived from first principles using basic physics. I adopt this approach to engineering wherever possible as I seek to understand what is happening. It opens a much wider spectrum than just applying empirical relationships.

Empiricists often have limited understanding of fundamentals and put great faith in the measurement processes that were used to produce the original data. They will always claim discrepancies between an analytical solution and the measurements to be a flaw in the analysis, relying on the bold assumption that the original measurements using instruments of the vintage have no errors.

Rick W

[bblagonic]

Can anyone check if I`m going on right way...

Situation:
The three blades prop installed 17"x23" runing on 220hp@1800RPM and with B.A.R.0,73 is high into cavitation.
Further to avoid cavitation I also need a thrust of cca. 9500N for 25kts.

Results:
I calculated (assuming the props I can easly get from "Mikado") that following prop would be suitable:

5 blades
B.A.R. 1,06
17"x20"
skewed (as it is installed behind the strut)
type: AU

I know that even B.A.R. 1,06 is not enough to be on 10% back cavitation but this is the great B.A.R. of prop producer I`m dealing with.

I`m litle bit scared of five blades prop? Is it unusal for a small pleasure craft of these dimensions?

Could anyone recheck with some software if my engine would run the prop selected ?
tnx

[daiquiri]

Can anyone check if I`m going on right way...
Situation:
The three blades prop installed 17"x23" runing on 220hp@1800RPM and with B.A.R.0,73 is high into cavitation.
Further to avoid cavitation I also need a thrust of cca. 9500N for 25kts.
Results:
I calculated (assuming the props I can easly get from "Mikado") that following prop would be suitable:
5 blades
B.A.R. 1,06
17"x20"
skewed (as it is installed behind the strut)
type: AU
I know that even B.A.R. 1,06 is not enough to be on 10% back cavitation but this is the great B.A.R. of prop producer I`m dealing with.
I`m litle bit scared of five blades prop? Is it unusal for a small pleasure craft of these dimensions?
Could anyone recheck with some software if my engine would run the prop selected ?
tnx

5-bladed prop is not very usual for boats of that size, but is nothing to be feared of. If you have a heavily loaded prop at risk of cavitation, it is one of solutions.

I was rather checking your input data and there is something wrong, imho:
9500 kN at 25 kts gives some 164 HP effective power.
Having an engine which gives max 220 HP (maybe less), and taking into account the transmission drive losses, it means that you are seeking a prop with some 78% efficiency, which will be impossible to satisfy with commercial props of that size, particularly if you will be running with some cavitation.

You need to determine more precizely the actual working point. What is the exact boat's weight? Is the actual speed of 20 kts claimed on the previous page reliable? Do you know the engine model and do you have the power curve for that engine?

[bblagonic]

that`s what I call brain storming...thank you Daiquiri...

as Daiquiri made me attention on probably wrong thrust required made following:

The way I find out the required thrust of 9500N was by entering the actual prop and vessel performance data into the Kt Kq diagrams.
(With actual prop the boat is performing 20kts but with thrust reduced because of a high cavitation!!!)
If I put the actual RPM and delivered power into those diagrams, I find out the thrust that this actual prop can make...but without consideration that in a real life the thrust is much lessbecause of a big amount of cavitation!!!
Finnaly entering the actual data, the diagrams shows that at this speed of vessel and this RPM the prop is delivering 9000N of thrust but this is far away of truth because of cavitation occuring.

To recheck the actual vessel resistance (and thrust required) I made the two following calculations:

1) I know that this model of boat (and the same weight) performed on sea trials
vs=26kts
with
Pb=200HP engine
If I assume the
OPC = 0,6
the effective power will be
Pe=132HP
as
Pe=R*vs
leading to R=7255N
Assuming the thrust deduction factor to be
(1-w)=0,97
Thrust will be
T = R/(1-w) = 7480N

2) in acknowledgment to Dingo`s generosity I`ve used the excel program based on Savitsky method
I made a calculation estimating vessel parameters as correct as possible
The program calculated that for 26kts I need a thrust T= 7100N...this is very close to the thrust calculated in point 1

What do you think about the methods I`ve used?!
It is difficult to find out the correct thrust needed when you have a poor propeller installed and there are no specific data of the vessel. I know that these data I can measure from the boat and this is probably the next thing I should do the first time the boat will be out of the water.


regarding the vessel we are talking about: it is a 15 year old Calafuria(Catarsi)
LOA=7m
Lwl= 6,2m
weight is cca.3500kg
engine is VM motori 220hp@3600rpm

[Guest625101138]

Can anyone check if I`m going on right way...

Situation:
The three blades prop installed 17"x23" runing on 220hp@1800RPM and with B.A.R.0,73 is high into cavitation.
Further to avoid cavitation I also need a thrust of cca. 9500N for 25kts.

Results:
I calculated (assuming the props I can easly get from "Mikado") that following prop would be suitable:

5 blades
B.A.R. 1,06
17"x20"
skewed (as it is installed behind the strut)
type: AU

I know that even B.A.R. 1,06 is not enough to be on 10% back cavitation but this is the great B.A.R. of prop producer I`m dealing with.

I`m litle bit scared of five blades prop? Is it unusal for a small pleasure craft of these dimensions?

Could anyone recheck with some software if my engine would run the prop selected ?
tnx

From this I gather you cannot increase the diameter. Is this correct? Best way to improve efficiency is to increase the diameter.

Increasing the BAR will reduce the amount of blade in cavitation. So some improvement there.

You need to increase the pitch if you stay with the same diameter as the motor is near full rpm now at 20kts. The reduction in cavitation does not alter slip much. Why did you select 20" when the existing prop is 23"?

If you need 9500N to get to 25kts you will not quite make it with the power you have on a 17" diameter prop.

There is some likelihood that the hull drag is still reducing up to 25kts. So it might be lower than the 8900N I calculated at 20kts. Gives some prospect of actually getting 25kts.

There is an easy to use Savitsky model here that you can use to cross check Dingo's spreadsheet:
http://www.electricmotorsport.com/store/ems_ev_parts_batteries_thunder_sky.php
You will see that the shape of the drag hump is highly dependent on the location of the LCG.

Rick W

[daiquiri]

Hold on Rick, before giving numbers. I am still not convinced that the initial data here is all correct.
I have found this test of the new Calafuria, with very similar dimensions and weight:
http://www.nautica.it/boatshow/catarsi/7eng.htm
26 kts max speed has been recorded at 3800 rpm and 3400 kg (half-loaded condition). The prop installed is Mikado ZL 16"x21".

Then I checked the site of VM Motori's marine division (http://www.vmmotori.it/en/02/03/index.jsp) and discovered that they have 2 engine models giving 220 HP: mod. MD706LH and mod. MR 704LH/704LX, both delivering at 3800 max rpm, which is in line with the above test.

So Bojan, are you really sure your engine has 3600 rpm max? Did you notice particular exhaust fumes during the boat test run? If the max rpm is 3800, like the engines above, than you are probably currently over-pitched and will need to diminish the prop pitch to let the rpm increase so the engine will deliver more power.
Next, is the gear ratio exactly 2.0 or a tad less/more...?

[bblagonic]

well these data are 100% sure:
my engine develop max. 220hp @ 3600...this is an older type of engine than those ones on the vm site. I have all the data of motor from VM in original.
max. prop diameter that could be installed is 17". Bigger would be about 11% of the diameter near the hull. I think it would be too much.

the half load displacement should be near 3,4 tons...so as from the test on the web

shortly I will come back with new data. It seems to me now that
17x24
BAR1,06,
5 blades
would be appropriate.
I know that I will not look serious giving different prop type each replybut I´m still new in this and I like to understand things!

[bblagonic]

further...gear ratio is exact 2,0 :1
as far as I see...the great deal is to define the exact thrust needed for each speed but do you have some other method to estimate the thrust (resistance) of the actual boat other than above mentioned...

[Guest625101138]

Bojan
If you have to pay good money for a new prop it is best to sneak up on the best solution by collecting as much information as easily as you can. You have only just started and it is not always easy to get a consistent data set.

With regard to your proposed new prop. The 5-bladed prop with greater BAR gives an improvement in efficiency.

Going up in pitch will get higher boat speed ONLY IF the motor has the power to achieve it. My calculations show you will need 220HP on the shaft to get to 25kts if the drag is the same at 25kts as I calculated it to be at 20kts. The current prop is absorbing just over 200HP at 20kts.

As noted earlier there is a reasonable chance that the drag is still coming down the hump. However the fact that the motor does not get to rated speed of 3600rpm makes we wonder about this. It could be an engine problem related to age but 200+HP on the shaft with a 220HP rated engine is quite good.

One of the measurements you can make to better understand the drag curve is the angle of the trim at various speeds. The trim angle goes hand in hand with the drag. Attached charts show trim and drag at different position of LCG for the same boat weight. Getting the trim angle at various speed should provide a clue about the shape of the drag curve. Do you know the position of the LCG? (These curves do not have the exact boat data but you can do the same thing with your Savitsky spreadsheet. They also are ideal without any appendage drag.)

If there is severe drag hump then anything you can do to shift the LCG forward reduces the hump. Obviously reducing weight will make it easier to plane providing it does not shift the LCG aft. These things are obvious in an outboard powered dinghy where you can shift weight easily but not quite so obvious in a larger boat.

So I suggest you try to gather more data before you jump in and buy a new prop.

Do some measurements of engine speed and trim against boat speed from about 12kts upwards to build the picture. If you can remove some weight and do another run it will provide another useful data set.

You may also find a prop supplier who can provide tests props rather than buying before trying.

Rick W

[daiquiri]

I'm affraid Savitsky method is pretty useless in this case, because Catarsi Calafuria is not a prismatic hull. Here are some pictures of the type of boat we are talking about:

http://livorno.bakeca.it/be/file/976/976048b960f653f6b76d79d34fdad206.upload

http://cache.secondamano.it/mmo/2/501/911/2_376564004.jpg

http://cache.secondamano.it/mmo/2/501/911/2_-148183660.jpg

It is a double-ended, round-chine hull. Planing ability has been obtained by applying the additional horizontal surfaces (wedge-type) in the transom area, plus two wide lateral chines. Designed for seakeeping, not for planing efficiency. Kind of boats used by port pilots.

I would like to hear Bojan confirm or make a correction to this description, because it is fundamental to create the correct initial picture of the problem.
I'm sorry if it might sound like avoiding to face the numerical solution of the problem, but that's exactly what it is. I have learned in these years not to give numbers before I'm sure that the essence of the problem has been understood well.

It means that all the resistance figures given in the graphs above need to be multiplied by an unknown, but substantial, correction factor. Could be anything between 1 to 1.5 or even more. So it is not a reliable way to go, imho. Actually, I'm really not sure if any reliable theoretical calculation method exists for this type of planing hull.
The calculation needs to be performed based on the known test data only, imho. It will be an incremental process - knowing what we have now, let's figure out what we can do to improve it, bit by bit.

Bojan, could you please post the engine HP vs. RPM curve, if you have it?

[Ad Hoc]

If the pictures of your boat, are as posted above by daiquiri, then there is no point as he noted using any Savitsky method, or otherwise. The results will be totally meaningless, and to try and use such results, just indications a total lack of understanding of not just what the Savistky program is and its limitations, but basic naval architecture.

The link to these boats also does not state what condition that those speed which are quoted on the website are taken at...what displacement??. It does not say. So caution is advised..

Since the figures quoted, implies half load and quotes a value, but the performance speeds quoted do not state catagorically the displacement at the trial speeds given.

So, even more misleading and consequently confusing data.

More hard facts before anyone needs to push a button..or make a mark with a pencil!

Sorry, more leg work, less pushing buttons...for now at least anyway.

[bblagonic]

Bojan
I have attached the JavaProp design info for a prop of similar proportions to the AU series you have. The power and thrust figures are too high as JavaProp has no ability to allow for cavitation and a large proportion of the blades are in cavitation.

I have also included the Cp plot for a 5% foil of similar shape to the AU series. The maximum value of Cp is around 0.33. When you look at the flow vectors at the various radii it shows that the portion of the blades beyond 130mm radius are in various stages of cavitation at 20kts and 1750rpm.

I use JavaProp as a first pass for prop calculations but I have my own spreadsheet with in-built macros for more comprehensive analysis. I have attached a portion of it that has the key data related to the conditions you are considering. The key values are that the prop will be producing around 8900N thrust, power is 153kW (204HP) and prop efficiency is 60%.

This data is primarily derived from first principles using basic physics. I adopt this approach to engineering wherever possible as I seek to understand what is happening. It opens a much wider spectrum than just applying empirical relationships.

Empiricists often have limited understanding of fundamentals and put great faith in the measurement processes that were used to produce the original data. They will always claim discrepancies between an analytical solution and the measurements to be a flaw in the analysis, relying on the bold assumption that the original measurements using instruments of the vintage have no errors.

Rick W


well, I said from beginning that the boat is not hard chine...OK my apologise, I shouldn´t use Savitsky as method.
I start from beginning. I know the performance of the boat with actual prop (20kts @ 3500rev). Using KtKq diagrams It comes out that with this numbers the prop is producing thrust of 10700N with SHP of 203HP.
So it is teoretical thrust the prop would produce if there would be everything ideal.
But in the real world large proporsion of blades are in cavitation (load cav. nr =0,235 and thrust loading coeff. = 0,279). Burrill chart shows point left of 50% line back cav.
My question now is following:
Is there a possibility to find out how much thrust I`m loosing because of cavitation of these proportion?!:D

I`m also wondering about the results that Rick listed. In the first photo the teoretical thrust is cca.10500N (it is close to my calculation using KtKq diagrams which is 10700N) but in the photo showing excel file the thrust is reduced to 8900N. Please explain me why is it so?!

regarding the shape of the vessel and pictures uploaded. The pictures shows two different boat size. The boat I`m talking about has no big keel as the boat on the last picture even it doesn`t have a stern like arrow like on the first picture. The problem is that I don`t have a picture of the boat bellow water but I will try to make it.

Bojan

[Guest625101138]

......
I`m also wondering about the results that Rick listed. In the first photo the teoretical thrust is cca.10500N (it is close to my calculation using KtKq diagrams which is 10700N) but in the photo showing excel file the thrust is reduced to 8900N. Please explain me why is it so?!

regarding the shape of the vessel and pictures uploaded. The pictures shows two different boat size. The boat I`m talking about has no big keel as the boat on the last picture even it doesn`t have a stern like arrow like on the first picture. The problem is that I don`t have a picture of the boat bellow water but I will try to make it.

Bojan
Bojan
JavaProp determines what is occurring from the basic physics. It is primarily an analytical approach. The prop curves you are using are based on extensive test data. They are empirical. A good analytical model should have close agreement with good test data for given conditions. As you rightly point out if there were no cavitation you would have the actual thrust being achieved by either method.

My model is similar in construct to JavaFoil only I have more detail that can automatically calculate for different Re# and cavitation. So my model takes more real world factors into account.

The basis for my calculation for cavitation limits the lifting face pressure as actually occurs in the real situation. JavaProp does not have this capability.

I determine the point of cavitation using the Cp for the approximate blade shape and the velocity over the blade at the various radial positions. I provided you with the Cp curve for a 5% foil as is the AU series. This curve can be produced using JavaFoil. It shows the Cp on the lifting face is around 0.3.

As far as the hull goes it has to be operating essentially in the planing regime at 20kts given the length, beam and weight. Although the curves I attached to the last post were not intended to represent the hull you are looking at, they do indicate what you can expect as the hull goes from displacement mode to full planing. The lift to drag ratio is increasingly related to the angle of trim as you get into planing mode. The trim is easy to measure at various speeds. From this you may be able to determine where the boat is on the drag hump. If it is under 5 degrees then it likely over the hump. If it is above 6 degrees it is likely still coming off the hump.

I have my doubts that the motor has sufficient power to get the boat much faster with a different prop having the same diameter. The risk is that the motor becomes torque limited and you actually get lower top speed.

You are better off doing more testing so you get more data. This will build a more comprehensive picture. Trim, speed and engine revs are all easy to measure and help build a picture of the drag curve. At slower speeds you will not have prop cavitation so this removes one of the difficult variables.

Getting rid of weight and moving any moveable weight forward usually helps getting fully on the plane. So have a look at what can be done in this regard.

The aim of further testing is to get confidence that the engine will have the power to get a higher speed with a different prop.

The reason I like analytical methods is that they lead to understanding the basic physics rather than blind faith in test data that might not relate well to your case. In the case of the curves you are using they are related to a particular blade section and plan shape and at a Re# only applicable to the scale and speed used for testing. They are bound to have errors or need correction for shape and Re# even without allowing for any hull interference and cavitation.

I have seen some useful analytical tools for determining drag on planing hulls but I have not found one I can afford. Hence Savitsky has to suffice although it is not ideal and it will help if you understand how the dynamic lift interplays with wave drag and viscous drag.

Rick W

[Ad Hoc]

Bojan

If you wish to fully understand your problem, you ahve to be able to "think" like a naval architect/engineer too. For example:

"..JavaProp determines what is occurring from the basic physics. It is primarily an analytical approach...The prop curves you are using are based on extensive test data.... They are empirical".

So looking at this statement. Starts off by saying the approach is analytical. Oh wait..then it is taken from testing (ie not analytical)...then..oh wait, more....it is empirical.

This is a classic statement made by those that are not professional engineers, nor posses the mental analytical tools to dissect data to establish what is what objectively, as one is trained to do. Since a procedure cannot be purely analytical and empirical at the same time, it is one or the other.

Since
"...I determine the point of cavitation using the Cp for the approximate blade shape and the velocity over the blade at the various radial positions.."
How is this analytical..it aint...it is pure guess work.

There are currently no 100% correct analytical tools that can give exactly prop designs, such as any RANS programs. There are some which are getting closer, but there is far far too much going on, in the region, in respect to fluid dynamics, to say analytically what is correct.

You may want to pursue some aspects further by reading:

"Design of Propeller Geometry Using Streamline-Adapted Blade Sections", by Kim, Kim, Pyo, Suh, 2009
"simulation of unsteady flow free-surface flow around a ship hull using a fully coupled multi phase flow method", by Zwart, Godin, Penrose, Rhee, 2008,
"Investigation on the Vortex structure of propeller wake influenced by loading on the blade", by Paik, KIm, KIm, 2007
or even
"Basic design of a series propeller with vibration consideration by generic algorithm" by Chen, Shih, 2007.

These if you wish to peruse an analytical method, will provide more background.

But first, look at the basics. What is the real true displacement of your hull (website has just 'sales' data, ie not reliable)...the real true power delivered, and the type/shape of boat. Without this basic data,....you will go no where fast..unless, you wish to pursue just an analytical understanding of prop's.

[baeckmo]

Sorry Bojan, but basic info on hull shape is missing. Speculating on drag etc (not to mention some unvalidated "numerical analysis"), is a waste of time until we know more of her shape and have correct figures on displacement and projected frontal area for air drag.

And regarding the amount of cavitation, I feel inclined to doubt the test data you refer to as well, unless the AU prop has quite extraordinary cavitating qualities.

Cavitation performance is very individual for the various propeller series. As per today, there is no realistic numerical method available for engineering prediction. In order to get an idea of the reduction as f of cavitation number, you must have test results, either in the usual "kt/Ja" format with sigma as parameter, or in the "tau/sigma07" format as seen in Burrill's cavitation diagram. Unfortunately, the usual annotation of sigma as f of advance velocity is hiding the real physical effects of the governing velocity, which is the local relative velocity. With this in mind, the Burrill format should be preferred.

Attached you will find a compilation of data for some propellers that have "crossed my trail" (real world trial data). The Gawn-Burrill series behaves quite similarly to the Wageningen props, while the Newton-Rader is a typical high speed transcavitating propeller with the cambered pressure face, necessary for good cavitating performance. It is just to demonstrate the variations and general trend (veeery empirical...), so use with caution!

[bblagonic]

Producer of the boat in subject told me that if cavitation was my problem than the engine would go to overspeed.

But my engine is not reaching even max rpm ?!?!:confused:

What do you think about that? Theoretically looking it seems to have right and I think that I read in some article that the cavitation would result with overspeeding of the engine.
But as I told, the actual engine is not reaching even max rpm insted my calculations give results that the propeller is in high (more than 50%) back cavitation area.

[daiquiri]

If you cannot reach the max. rpm, it means either that the engine is under-performant or that the prop is over-pitched.

[Submarine Tom]

Change the prop and see what happens...

[Guest625101138]

Producer of the boat in subject told me that if cavitation was my problem than the engine would go to overspeed.

But my engine is not reaching even max rpm ?!?!:confused:

What do you think about that? Theoretically looking it seems to have right and I think that I read in some article that the cavitation would result with overspeeding of the engine.
But as I told, the actual engine is not reaching even max rpm insted my calculations give results that the propeller is in high (more than 50%) back cavitation area.

Many people use the term cavitation when they are referring to aeration. I expect this is what the boat producer thinks you are talking about.

Props can have localised areas of cavitation. I determine that the prop you are looking at is in partial cavitation. This is a localised process over a portion of the blades that lowers efficiency. Some props are designed to work in cavitating condition.

This might give you some better understanding:
http://en.wikipedia.org/wiki/Cavitation

I roughly agree with your calculation. Cavitation does not kill the prop performance it causes an increase in slip and consequential reduction in efficiency.

Rick W

[Submarine Tom]

Some people use the word aeration when they mean ventilation.

Tom

[Ad Hoc]

quite right Tom.

A prop with a hull 'above' it...this is cavitation.
A prop with no hull above..it is ventilation, the air is literally sucked down from the surface, which cannot occur when the prop is under a hull (possibly cavitation too, but that depends on the load factor on the props, ie the principal causes of cavitation).

[daiquiri]

Many people use the term cavitation when they are referring to aeration. I expect this is what the boat producer thinks you are talking about.
Rick, I am sure that mr. Vicenzo Catarsi, after many glorious decades of boat design and building, knows well the difference between cavitation and ventilation.
And, more important, knows very well the performance of his best-selling boat and what steps should be taken to fix this problem.

[Guest625101138]

Bojan
The attached photo compares a CFD model prediction of cavitation and an actual photo showing the partial cavitation of the blade. As you can see it is only affecting the outer portion of the blade.

Cavitation makes a normal lifting type blade less effective than it would otherwise be. It only affects the lifting face (suction side). The pressure face is still working more or less as normal. The suction pressure is limited by local water boiling at the low pressure.

Who ever you were talking to that said the motor would suddenly speed up when the prop cavitates does not understand cavitation.

Rick W

[Ad Hoc]

Oh dear, yet again, here we have Rick copying and pasting and as always not understanding what it is he is talking about. Well, he only recently learnt what KM and GM are:
http://www.boatdesign.net/forums/stability/metacentric-height-free-ship-michlet-15582.html
so can't expect too much really!..i could list more, but you get the point!

"...Who ever you were talking to that said the motor would suddenly speed up when the prop cavitates does not understand cavitation. .."

This is utter nonsense!
Spoken, yet again, like a someone trying to impress, but doesn't know what they are on about.

So, what is cavitation? In very simplistic terms it is thus:
This occurs when the prop is overloaded, ie it is asked to produce too much thrust for the area of its blades. The water on the forward face of the blades literally 'boils'. The fwd face of a prop is the suction (or work done). Hence when this suction pressure falls to zero, ie a vacuum, cavitation begins. So, since no suction, there is no longer any thrust being produced by the prop.

When there is a breakdown in thrust, what occurs??.....the engine races, why, because there is no longer any resistance/thrust being produced it is free to rotate merely....

For someone who constantly plugs numbers in to programs and gives endless prop advice, I'm not surprised he doesn't understand cavitation. A program wont tell him the theory, it is just numbers.

PS
No doubt i'll get endless neg hits, as always, for pointing out the obvious....'tis a compliment really :)

[daiquiri]

Well Rick, you are completely wrong on this one...

First, it is not true that cavitation affects only the outer portions of the blade. There are several types of cavitation, some of them develop at the blade root, some at central portions of the blade, some at the tips, some at the leading edge, some near the trailing edge, some in the space between blade tips and the hull, some at the hub.

Second, it is not true that it only affects the suction face. It affects any point of the blade where the pressure drops to nearly vapour pressure of the water at the given temperature. It can develop on the ventral side (pressure side) of the blade too, if the effective angle of attack of the considered foil is sufficiently small to provoke a severe local drop of pressure at some point of the ventral suface. It usually can happen in the proximity of (or immediately behind) the leading edge.

Third, when cavitation happens, there is a sudden drop in both Kt and Kq prop coefficients, so the torque required to turn the prop becomes smaller than the torque available at the shaft, and the prop revs up.

If you would like to learn more on this subject, I would suggest you to buy and read this excellent book: "Marine Propellers and Propulsion" by John Carlton.
All these things are also very-well explained, with sample propeller charts for various cavitation numbers, in the book "Hydrodynamics of High-Speed Marine Vessels" by Odd M. Faltinsen

I would also invite you not to rely too much on Wikipedia articles or on other non-verified Internet sources for learning about these things. This is a very specialistic area of study and not many reliable data sources are publicly available. So books and verified academic articles are to be preferred instead. The article you have cited gives a very basic information only and doesn't get any deeper into the specific aspects of the cavitation aplied to marine propellers. In general, too many internet articles are just a copy-and-paste work based on some second or third-hand information found at other internet sites. Unfortunately very few info from the original sources (research institutions) is readily or freely available. This is a kind of problem where empirical observations in cavitation tunnels are of fundamental importance as we still don't have a comprehensive and reliable all-round theory to explain and predict every type of cavitation observed (though a remarcable progress in this direction has been made lately by CFD).

[Ad Hoc]

Quite right daiquiri, there are many forms of cavitation. Locations are as I am sure you are fully aware, unlike Rick, of the cavitation bucket, which is dependent upon the blade thickness, camber and AoA.
The wider the bucket, the greater the range of J.

As for Wiki, fully concur. That stuff is for kids, not serious academic reference or research.

[daiquiri]

As for Wiki, fully concur. That stuff is for kids, not serious academic reference or research.
Yes, but let alone academics, which requires much more rigour than is probably needed for the purposes of this forum...
Wikipedia is a fine site if you search for some broad qualitative info. If, for example, you are an ignorant in some field (but you don't want to die ignorant) then Wiki is probably a good source of general info on a particular subject. But as such it is clearly not suitable for engineering purposes, where a detailed insight is important. The Wiki page about cavitation, or about airfoils (for example) clearly shows these limitations.

[bit]

Rick, you are completely wrong on this one.

First, it is not true that cavitation affects only the outer portions of the blade. There are several types of cavitation, some of them develop at the blade root, some at central portions of the blade, some at the tips, some at the leading edge, some near the trailing edge, some in the space between blade tips and the hull, some at the hub.


hi. A design ok.

[daiquiri]

hi. A design ok.
Nice illustration, thank you.

[Ad Hoc]

Nice schematic diagram.
That about sums it up..nice one, Bit :)

PS..i've just notice that image is from the paper "some current issues with cavitation and propeller rudder interaction"
nice catch :)

[Guest625101138]

Bojan
If you are still interested in gaining a better understanding of cavitation as it pertains to the situation you are looking at then this paper will be of interest:
http://www.ichd2010.org.cn/ICHD-EC/ICHD/eighth/papers/ICHD2008_2B-02_50.pdf

The prop modelling in section 4.3 and 4.4 will be of particular interest as it shows how cavitation zones are progressive and can shift around on the blades depending on their operating condition. In the case of an inclined blade the cavitation zone can be fully collapsing and reforming every revolution as shown in the example.

Sheet cavitation is not something that happens suddenly causing a prop to instantly run unloaded as laymen think - that is aeration or ventilation. It is progressive and will occur on certain parts of the blades depending on loading condition, blade shape, blade depth and blade orientaion to flow.

It is only reducing efficiency over what could otherwise be achieved with a better design/manufactured blade working under lighter loaded condition. As you want to get a higher speed for the same power you need to improve prop efficiency. One way is to reduce the amount of cavitation.

A blade that has surface imperfections could have localised cavitation at those points. So examination of the blades and cleaning up if necessary could improve a little. If the leading edge has been blunted through impact this will have a detrimental affect.

The paper demonstrates how Cp of the foil is tied into determining where cavitation is going to occur over the blade. This is what I mentioned in one of the earlier posts.

The point of this is that understanding what is going on and being able to model it reasonably accurately from basic principles leads to much better knowledge than simply applying empirical corrections that are unlikely to match your application given the range of props and variation in application.


Rick W

[Ad Hoc]

Ahh..i see Rick is reduced to giving me negative hits again. (That's 100 neg hit points this week, i must be really hitting the "truth" nerves :):P)...it is the kettle calling the pot black "...dimwitted arguments.."...hahaha

It is very very sad when someone -who claims to be an engineer- is so ignorant of facts and blinded by such belief in themselves that they do not wish to debate publicly and become silent when their flaws are exposed, or get their "big brother" to wade in...your only retort is to throw pathetic weak stones as neg hit points as a form of debate, shockingly shockingly pathetic and weak Rick. The most terribly immature and unprofessional behaviour as one can ever be....but there it is, you are not a professional engineer nor naval architect, so what can one expect, really??..honestly...just sad!

It sums you up I'm afraid Rick. All mouth and no trousers....just weak and immature, very very sad way to attempt to take a higher ground, as if anyone really cares. It is not surprising you have no idea how to design real boats. Stay with the little models you do...

Your typical MO is thus, make a bold statement, appear impressive and knowledgeable, thus (post#41):

"..Cavitation makes a normal lifting type blade less effective than it would otherwise be. It only affects the lifting face (suction side)..."

and, as always real professional engineers and naval architects point out your flaws. But, wait, what is your standard reply...., oh yeah, silence, and then you retort by saying (post #49):

"...cavitation zones are progressive and can shift around on the blades depending on their operating condition..

WOW, you go from one breath trying to appear impressive then, when countered, as always, you suddenly flip-flop and say the 100% opposite of your bold statement, before being questioned. Amazing!!!!

He could have said "oh ok, thanks for letting me know, i didn't realise that", or something similar, as anyone wishing to learn or seek the truth on a subject would and even be grateful to those that point out the error. But naaah, just ignores the faux pas and and facts, and blindly carries on.

You chop and change your position when questioned, ignore the flaws in your statements and then brazenly plod on expecting no one to notice your 100% about-face, clearly demonstrating your lack of comprehension and understanding of the subject at hand.

What kind of pseudo-science-religion do you aspire to??

I leave you with this:

" Nothing is more dangerous than the certainly one is right.
Nothing is potentially so destructive as the obsession with a truth once considers absolute"
Francois Jacob.

[daiquiri]

This is the comment Rick Willoughby left me as motivation for negative reps he has just given me:


"post #43 direct attacks rather than offering an alternative opinion. I do not like showing you up as a fool. Rick W"


I invite everyone to read my post #43 (http://www.boatdesign.net/forums/props/prop-doing-bad-29864-3.html#post313898) and judge by themselves if my post is a personal attack on Rick Willoughby or is it rather a motivated technical explanation of cavitation, with pertinent bibliographical reference, and a note on usage of Wikipedia as a source of technical info.

I have submitted this to the attention of the moderator, and have invited him to take a decision on this arrogant, patronizing and uncivil kind of treating persons who are trying to give help at the forum.
I am appaled by this behaviour. You should be ashamed of yourself Rick Willoughby. I have never given you a single negative point, though I did have occasions to do it for the technically inaccurate and inconsistent stuff you have written on several threads.

[Ad Hoc]

daiquiri

There is none, non at all in your post #43. It is a statement of fact, unlike Rick's #41.

Rick just does not want to be questioned, he does not want to be shown as a "dabbler" or "amateur" to those he is trying to preach too. He is not open to reasoned argument nor when shown the error of his ways he ignores and childishly hits with endless neg point ...it is very irrational behaviour and suggest a deep rooted mental issue with "authority figures" and "control"

Whatever it is...it ain't engineering and it ain't professional.

How someone like Rick can be allowed to be called a "guru" is laughable. He wouldn't last 5mins in the real world of design/engineering. But this "cyber world" of his is probably all he has and covets attention and the need to be seen as a "pro"....just pathetic immature behaviour. Totally agree daiquiri, as always :)

[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...).

[Guest625101138]

Hello, may I ask you please to elaborate a bit more on the following (not very clear) passage: from Cp obtained from JavaProp to the cavitation estimate?

Slavi
I delayed my response to you on this as I felt it was simply a cynical question with the intention of you making good on your threat highlighted in the attached. You and I know you readily accept misinformation from others without offering your view. We also know you have a propensity to rush in without giving things much thought - as you do occasionally like this particular question.

To make this clear for you, the Cp can be determined from JavaFoil for the particular prop section being considered. The method used to determine the area of cavitation using Cp is explained in post #53 above or in the paper linked to post #49.

And I suppose you do disagree with the information given in post #53 as it discusses cavitation as being progressive. There is nothing here suggesting the motor runs away at the point of cavitation! So lets see you take a stance against this as you have made it clear you believe the engine will suddenly speed up when the prop goes into cavitation.

My response to your silly comments are shown in red-

Well Rick, you are completely wrong on this one...No I'm not - per usual you jump in without reading or thinking as fools are apt to do.

First, it is not true that cavitation affects only the outer portions of the blade. There are several types of cavitation, some of them develop at the blade root, some at central portions of the blade, some at the tips, some at the leading edge, some near the trailing edge, some in the space between blade tips and the hull, some at the hub.
We are talking about a specific propeller here - as noted earlier the lifting front face is in partial cavitation from the 130mm radial line on average but will cover more of the downgoing blade and less on the upgoing blade due to shaft inclination.

Second, it is not true that it only affects the suction face. It affects any point of the blade where the pressure drops to nearly vapour pressure of the water at the given temperature. It can develop on the ventral side (pressure side) of the blade too, if the effective angle of attack of the considered foil is sufficiently small to provoke a severe local drop of pressure at some point of the ventral suface. It usually can happen in the proximity of (or immediately behind) the leading edge. Again I am discussing a the specific situation relevant to this thread on the reasonable assumptions that the prop is made to specification and undamaged. There will be a small area of cavitation on the outer tip of the back thrust face but this acts to reduce losses to induced drag.

Third, when cavitation happens, there is a sudden drop in both Kt and Kq prop coefficients, so the torque required to turn the prop becomes smaller than the torque available at the shaft, and the prop revs up. It is not sudden - it is progressive. Anyone who thinks there is a SUDDEN reduction in thrust resulting in engine speeding up has no idea of cavitation on an open water boat prop.

...

Rick W

[Frosty]

Ad hoc you are mistaken when you suggest that someone can repeatedly hit you with neg points. The forums computer will not allow neg or pro rep points twice in a set period of time - 3 months.

If you have 100 neg points in this one week as you say you have then they must have all be from separate individuals.

[Ad Hoc]

i didn't say i have been hit by the same person in a short period of time. Just by the same person again, but within the time allowed on this system.....still weak and pathetic regardless.!
All saying the same thing....sad really. Just like the poster above...seeking redemption.....!

[baeckmo]

Regarding thrust breakdown due to cavitation, it is a well known fact that the steepness, or gradient is varying primarily with rotor specific speed. A high specific speed unit, like a low solidity propeller will have a gradual performance reduction with decreasing cavitation coefficient, while a radial pump (=low specific speed) has a sharp threshold.

So, in bblagonics case, we must expect thrust and efficiency gradually decreasing with propeller load, as shown in my graph.

[daiquiri]

Rick, it looks like you have decided to take aim on me and you don't want to let go. Well, no problem...

I delayed my response to you on this as I felt it was simply a cynical question with the intention of you making good on your threat highlighted in the attached.

The words you have highlighted were not a threat. I was simply telling you that I would not stay silent (I know you would like everyone to bow down to your feet but... sorry, I'm not that kind of person) if I saw that you were writing untrue or unproven information. If you see me saying some crap in some thread, and you can prove it, it is your right (or better, an obligation - if we want this forum to inform and educate) to do the same.


You and I know you readily accept misinformation from others without offering your view. We also know you have a propensity to rush in without giving things much thought - as you do occasionally like this particular question.

This funny patronizing style of writing had made me laugh. Thank you. :D :D :D

Now let's go technical.

To make this clear for you, the Cp can be determined from JavaFoil for the particular prop section being considered. The method used to determine the area of cavitation using Cp is explained in post #53 above or in the paper linked to post #49.
As far as my knowledge goes, the ONLY thing you can estimate with JavaFoil is the probability that the cavitation will start in a given point, via Cp value. Once you have decided that Cp at a given point of the foil is low enough to allow you to assume that the cavitation will begin there, you have no means of estimating neither the extent of the cavitation nor the type of cavitation, nor the quantitative loss of foil's performance.
That is, unless you have an access to the source code of JavaFoil and you know how to modify it.
I'm citing from the paper you have linked in the post #49:
"...the sheet cavitation model presented here uses transpiration velocities to deviate the flow as to represent the cavitation sheet. These transpiration velocities v* are equivalent to additional sources strengths which are simply added to the part of the surface carrying the cavitation sheet. As a consequence, the implementation in the code is relatively simple since it is only a matter of modifying the slip condition on the body surface where the cavitation sheet is located. The problem is how to determine the strengths distribution of the additional sources needed to correctly simulate the cavitation sheet. The core of the model is the function used to determine this distribution and it is the topic of the next section."
Translation: once the cavitation starts (based on some Cp criterion), the model of the flow field around the foil will have to be modified by adding a number of discrete sources of unknown strengths. For the purpose of that research they have been modelled by a function the Authors have called "transpiration velocity v*". That function is a part of their computational algorhitm and, in case of JavaFoil, would need to be implemented at the level of source Java code. Are you telling us that you are able to do it? Sorry if I have my doubts that you have access to the source code of JavaFoil...


And I suppose you do disagree with the information given in post #53 as it discusses cavitation as being progressive. There is nothing here suggesting the motor runs away at the point of cavitation! So lets see you take a stance against this as you have made it clear you believe the engine will suddenly speed up when the prop goes into cavitation.
There is nothing in Baeckmo's explanation that goes against my knowledge on cavitation, why should I disagree? The cavitation is a progressive phenomenon, in a sense that it grows with blade loading. The graphs given by Baeckmo should, imho, be inserted in the "Hall of Fame" section of the forum, if there is anything similar to that. He has demonstrated, in various threads, to have rock-solid balls when it comes to hydrodynamics and propulsion.

I have used the expression "sudden drop in Kt and Kq coefficients" and I admit that the word "sudden" should not have been used in that context. It is my fault.

But I should also have known that you would end-up playing a lawyer, desperately trying to hang to every word said, just to get yourself out of this unpleasant situation which has been created.

What a pitty that every discussion where someone disputes your claims has to take the same ugly direction.

[Ad Hoc]

Daiquiri

Just thought i would add the "typical" graph of performance one obtains showing the effects of cavitation on Kt, Kq and efficiency, as you have rightly noted.

This is pic attached, primarily for those that are unfamiliar and wish to understand a bit more. Daiquiri and baeckmo have posted very detailed stuff above already, and the 'words' are shown in this pic of prop performance.

Rick, as always shall ignore, then chop and change his position....not worth responding to the "RickW website I am always right don't question me" forum D, waste of time, his computers cannot be questioned.

[apex1]

To Bblagonic,

unfortunately I did not manage the WROM* going into production by now. You otherwise should transfer Ricks statements and Play Station numbers directly towards it.
Ad Hoc, Baeckmo, daiquiri give you proper and sound advice, as always. Rick makes bubbles, as always!

Regards
Richard
* WROM = WRite Only Memory

[Guest625101138]

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:
http://www.boatdesign.net/forums/projects-proposals/hull-modifications-23205.html
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 :confused: )
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.