# engine-propeller matching

Discussion in 'Props' started by Walid, Oct 21, 2020.

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1. Joined: Aug 2020
Posts: 20
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Location: algeria

### WalidJunior Member

hello
it's my first threard in this side and I hope things will going alright

so,I will begin by the keller criterion for the"CAVITATION"
the K factor depend on the number of popeller,type of the vessel,they mention:
k=0 for high speed twin screw
k=0.1 for Twin screw
k=0.2 for single screw

to be in the safe side regarding the cavitation issue,I will use k=0.2 for a single screw Xboat.

in order to select the optimum propulsive system for Xboat I found the folowing path is the most suitable when it comes to the optimum shaft-RPM and the required DHP for a given diametre and thrust,therefore we can select the right engine and gear-ratio which can suit our propeller.
firstley,we have:
t=0.2"by assuming the resistance augmentation factor"
w=0.2"by assuming wake fraction"
V=15knot=...m/s
Rtot=...KN"bare hull resistance"

output data;
the required thrust to reach the desiride speed is:
T=R/(1-t)

we can apply the cavitation criterion;
EAR=AE/A0=0.55+0.2=0.75 "just an example"
now,we must plot the curve of the polynomial Kt=cont.J^2 on the B4-75 chart.
so:
the required shaft-RPM:
the higest efficiency=...
P/D=...
J=...
n=Va/J.D=...=...rpm=...rps
and:
the required delivred power:
KQ=...
Q=...KN.m
Pd=2pi.Q.rps=...kw=...HP
and by assuming the mechanical losses:
shaft efficiency=0.98
gear efficiency=0.98
we can find the BHP requied and the suitable gear-ration

as we can see above,there is a clear influence of the K,t,w factors on the final results.

Any suggestions regarding this factors!!!

2. Joined: Jun 2009
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### baeckmoHydrodynamics

Personally, I prefer the Burrill method for the basic estimate, because it gives an indication of the amount of blade area that is cavitating. You may have to live with a certain amount, but then a basic knowledge of the pattern gives possibilities to adjust blade shape locally to improve performance/operating life. When you have the results (rpms, propeller dia, pitch, blade number, blade area ratio aso) from the first turn of the optimizing spiral, you check the thrust coefficient vs cavitation number (both according to Burrill). Then you have a preliminary idea about the cavitational state, and what measures to take to avoid cavitation to a reasonable degree (or promote/stabilize in the case of trans- or supercavitating propellers).

In reality, given the huge variations of cavitation qualities in the water, the relevant question is not "why does it cavitate", but rather "how come this one does not cavitate". The answer must come from an understanding of the first principles, not by picking a more or less random constant. The better your estimate of the local pressure/velocity field is, the better the final outcome. Remember that too much blade area reduces the efficiency!

Edit:
Now when satisfied with the performance at the design operating point, you do it all over again for at least two off-design points. You then have to check possible engine overload situations due to high resistance or wave/hull interaction in particular speed range(s). Again, cavitation performance may be critical due to changes in inflow velocity and leading edge angle of attack. It is not uncommon to find that the full speed design condition is conflicting with low speed/high resistance conditions.

Last edited: Oct 22, 2020
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3. Joined: Aug 2020
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Location: algeria