engine question

Please nautical send me only in this thread your questions or comments, because so doing the matter may be of interest for our friends, thank you!

What do you mean concerning the rpm too low?

N = rpm of engine
n = rpm of propeller

 

Thanks Ranchi. Seems to have solved the problem

Maximum Engine RPM 2600
Propeller RPM 1020
Screw B 4.55
Efficiency 56 % (behind hull efficiency)

 

How much is the diameter? It's the opt.? Because the efficiency seams to be low.....

 

The diameter is 40 Inches. I cant get anything better than that. IF the redutions ratios were highr maybe i could get a lower propeller RPM and get better results. . But the highest reduction ratios available seem to be about 2.5 but 56 % is greater the value I used when calculating power so i think its ok.

 

Something strange...
How much is the pitch, the Rtot and the engine power for 14 knots?

Did you try the ZF 350?
It absorbs the max. power in light duty with i=2.63 i.e. n=875 rpm

 

well I have changed the speed

Lets say here are the details

LWL 30 m
BWL 7.6 m
Draft 1.55 m
Displacement = 124 tons
Service speed 13.3 knots
Total Resistance = 611 HP

Engine details
Maxmimum power per engine 500 hp
Maximum RPM 2600

Propeller
BAR 0.55 Blade no. 4
P/D 0.7 D 46 inch P 32 inch

is this ok

 

My opinion:
- power marge in rough sea to low, the engine get in overload.
My opinion is to install more power and get speed at 60/70% of max. power for a 124 t vessel.
- P/D to low (0.7)
- ETA0 = 0.56 to low

Perhaps better :

BHP = 775
N = 2300
i= 2.63 (ZF 350)
n=875
Z=4
EAR=0.65
P/D= near as possible to 1.2 but > 1
ETA0= + 0.600

Is too much?
Never be satisfied if you reach an ETA0 of 0.56 at 14 knots speed


I've the feeling that this is not the right propeller

 

Why wood?
Aluminum? much better and lighter!!

 

well i totally agree with you. ranchi, but the folks who will be looking at this will be asking why i have taken a larger engine when the power required is less than that.

 

well i totally agree with you. ranchi, but the folks who will be looking at this will be asking why i have taken a larger engine when the power required is less than that. The reason i changed my speed is also to stay closer to the theoretical value

 

do u have the bp delta chart for EAR 0.65

 

The "folks" that has the necessary money to buy a 30m yacht doesn't have problems if you put 500 or 700 hp....but you save in fuel consumption because the engine is running with lower rpm....

 

Sorry, I don't have by hand the diagrams.

A suggestion...instead to use the open water diagrams there is a publication in an old Marine Technology (SNAME) where the ETA0 values are plotted according to the Sigma (cavitation number).

Easy to use but the model screws have been tested in cavitation tunnel and not openwater, this means that the ETA0 have the loss due to cavitation.
You can so check the correct EAR.

For the publication try to contact Jeff, the moderator...

 

The '' Folks" I meant are the academic folks remeber this is academic.
Anyway I have changed a little bit.

took 615 hp engine with a lower maximum RPM and found a gear box that had a ratio of 3. ZF 500 1 IV

took a EAR of 0.7 efficiency of 60 %

Diameter 42 inches
Pitch 38 Inches

P/D about 0.9 Did a Cavitation Control too. Seems ok.

So i guess it ok. Thanks again I will contcat Jeff

But i have one question. I followed the method in Dave Gerrs book and The diameter and all are select based on the maximum RPM. He says that we should do that because if we based the culculations on a lower RPM than we will be holding on engine RPM down and this would damage the engine.

If we base the culculation on Max RPM what will happen to the efficiency of the propeller when we operate at 85 % Maximum Rating. ( I have been thiking about it but I am not sure what will happen )

 

Now the choiche seams to be much better.
For the rpm I don't see any problem.

Work well...!