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)

[apex1]

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