propeller design

[NavArch007]

Hi everyone,

Ive been working on this project for 40m crewboat vessel. The situation is as follows:
Vessel spec:
L=40m, B=7.2m, T=1.75m
Design point: 21knots at 180tonn displacement.

The vessel has a restricted propeller diameter of 1.1m due to hull clearance. So to produced the required speed, 3 propeller were necessary. Unfortunately, the gearbox ratio chosen cannot be altered due to space constrictions and a gearbox ratio of 2.95 was already chosen. Also, the vessel will have 3 engines producing 1400bhp at 2300rpm.

To make the propulsion unit more efficient, the shaft angle was increase from 8 to 10 degrees in order to accomodate a larger propeller; and now it is possible to fit a 1.35 m diameter propeller.

I would be interested to hear any comment and suggestions and also if possible, some calculation on expected operating rpm, BAR and pitch ratio to compare with my results. I have found that for 4 bladed 1.35m diameter:

BAR=0.85, P/D=1.005, at an rpm of 530.

I have been using Autopwr program. Any suggestions regarding a good propeller design program for personal use at a reasonable price.


thanx.

[baeckmo]

First of all, how did you arrive at 530 shaft rpm? In my book 2300/2.95=779.7!

That is the primary dimensioning rpm. Your Froude nr (by volume) is 1.4 and she has a slenderness ratio of ~6.5 depending on WL length. This means that there should be no hump to worry about, the top speed is actually head-on the little hump there is. All provided there are no special "tricks" in hull design. In this case propellers must primarily be selected to satisfy either of two basic conditions:

A/ Max power-max speed,
B/ Cruising condition, added resistance due to weather conditions, allowable continuous power output.

I guess your speed estimate of 21 knots is referring to condition "A"? What's cond "B" like?

[NavArch007]

thanks for your reply baeckmo.

The design is basically a conversion of a previous vessel. The previous designer had opted for a gearbox ratio before specifying the propeller rpm!

When i looked at the design, i thought that to get the best out of our propeller, we would need a bigger gearbox ratio; however such a procedure would not be possible due to space restriction on the vessel which would not enable it to accomodate a bigger gearbox.

Condition B is 130ton displacement at 25knot.

[pamarine]

Wouldnt "B" be below "A" in terms of speed and heavier in terms of displacement?

Just reading the post here I would guess the speed for "B" to be in the 18 knot area but not sure what the displacement would be (add passengers, fuel, stores, etc)

Originally Posted by NavArch007

Condition B is 130ton displacement at 25knot.

You misunderstood baeckmo.
Condition B is at the same displacement, but continous (always lower) speed.

And gearbox dimension is not necessarily related to ratio! The ratio can be choosen within a certain range in the same housing!

Regards
Richard

[Çemberci]

Here is our results
* 4 Bladed Wageningen B
Diameter 49.00 inç
Pitch 48.00 inç
Dar = % 80.74
Speed = 26.00 knot
Price MnAlBrz 7450 Euro
* 4 Bladed Master
Diameter 45.81 inç
Pitch 47.00 inç
Dar = % 95.07
Speed = 26.00 knot
Price MnAlBrz 7580 Euro

Oktay Çemberci
istanbul/turkey
oktay@cembercidenizcilik.com

[conceptia]

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