Summary of Power & Propeller Design Steps

Hello to all!

Could someone pls give me a simple step-by-step summary for determining the power of the engine and the design of propeller (preferrably 3-bladed) for yachts 17 to 30 meter length and above 20 knots speed?

I would appreciate your response.

Thank you.

JohnHeart

 

Hi JohnHeart,

The integrated circuit will look as follows:

1. Calculation of resistance and towing power.
2. Proceeding from geometry of a aft part of the hull - definition of as much as possible allowable diameter of the propeller.
3. Choice of propeller’s series. It will give you infomation about blade’s geometry.
4. Calculation of parameters of the propeller(s) and engine's(s) power, providing to your yacht desirable speed.
5. A choice of model of the main engine(s) from existing in the market and, probably, a choice a reverser-reducer.
6. Definition of parameters of the optimum propeller(s), corresponding to main engine(s) and max possible speed of your yacht.

All calculations of the propeller can be made both for the new painting hull, and in view of the subsequent biofouling (that is preferable). The power on capacity about 10-15 % in this case undertakes.

Good luck, Dim.

 

Thanks Dim.

I will follow your 6 steps. I'm new to designing/determining the correct engine and propeller for yachts so I need to study one-by-one all the steps you have given.

It would be spoon-feeding if you will give me the details of those steps but if you can give it I would appreciate. At least I will know where's the limitations of it.

JohnHeart

 

Dear JohnHeart
I have shown you, if so it is possible to say, a skeleton of the scheme on which you can act. I agree, that you can not do without the help of the expert. I do not attend by realization of similar calculations, as for a long time I work in the other direction.
Maximum, that I can afford is to pass all steps, specified by me, for enough slow-speed hulls and for own pleasure. I shall show as an example calculation of main parameters of props and main engines for a yacht "Juno" from my gallery. In calculation the datas for props of old series "B" were used. The a resistance's curve of a "Juno" hull represented by me in "Additional information".

I have issued all calculations for this series of propellers in the same own xls-file.

So:
Desirable speed: 16 knots ( 8.23 meter per second ).
Number of props: 2
Number of blades: 4
The greatest possible diameter of the prop: 1.75 m.
Taylor wake fraction coefficient for twin-screw vessels: 0.08435
Thrust deduction fraction under the Taylor's formulas for twin-screw vessels with propeller-shaft strut: 0.11905

General view of the table and the schedules are exhibited on the first picture.

Outcome of the first stage:
Power of each main engine: 3.3 x 200 = 660 kW
Same with allowance for marine biofouling: 759 kW
Rotational speed of the propeller: 5.8 rps
P/D : 0.98
ADR : 0.66
ETAO : 0.66
ETAB : 0.62

We choose the engine and a reverser-reducer.
Main Engine: CAT 3508B-TA
783 kW at 1600 rpm
RRG: ZF 4550 C
gear-ratio: 4.11
(For example. I know, that the choice of the engine can cause the whole discussion. My opinion in this occasion, that without consultations of experts on engines here to not do without.)

We determine parameters of the prop which corresponds to the chosen engine.
All calculation is made for engine power 665 kW ( 783kW - 15% x 783kW ).
General view of the table and the schedules are exhibited on the second picture.

Outcome of the second stage:
Vmax : 15.97 knots
Ne : 783 kW
Rotational speed of the propeller: 6.5 rps
Dopt : 1.648 m
P/D : 1.01
ADR : 0.71
ETAO : 0.67
ETAB : 0.65

In summary I want to tell, that me learned to make these calculations, but in my speciality they not the main thing. Now I make these calculations to receive an estimation for speed, propeller's diameter and to choose model of the engine. Once again I repeat - only for own pleasure.

Dim.

 

Step one. (About prop)

Dim.

 

Step two. (About prop)

Dim.

 

Thanks Dim,

I think you have labor a lot to make that formula and graph in excel.

I will wait for other calc you can show. Is that applicable to yachts with particulars I've mentioned?

JohnHeart

 

John Heart says:
>I will wait for other calc you can show.

John,
Please bear in mind that in all probability, this is what Dim does to earn a iliving. While he obviously does not mind the occasional freebie (in common with many of us), it is bad manners to ask for too much.
Steve

 

Thank You, Steve.

Dim.