Rudder Design

John, with a career like yours, "dumby" couldn't be an appropriately descriptive word used for you.

 

Thank you all, I think the information below would help.

PROPOSED SPECIFICATION FOR RUDDER & PROPELLER /APERTURE.
TYPE: FISHING TRAWLER
LOR: 57FT
LWL: 52FT
BEAM: 16FT
DRAFT: 5FT
ESTIMATED WEIGHT: 40 – 45 TONS
ENGINE: CUMMINS 6CTA8.3-M220 (220HP, 1800 RPM, CONTONIOUS)
PROPELLER
PROPELLER SIZE: 40 X 27, DAR 60%, RH, 3 BLADE, (MANGANESE BRONZE).
SHAFT: 2.5 INCHES DIAMETER 316 SS, OR 2 INCHES DIAMETER AQUAMET 17
PROPELLER SHAFT FROM DEADWOOD: 1 X SHAFT DIAMETER. ADJUSTMENT CAN BE MADE TO ACHIEVE 6 – 8 INC CLEARANCE B/W PROP TIP AND HULL

RUDDER
RUDDER PROFILE: NACA-0012
CHORD LENGTH: 2.6FT
HEIGHT; 40-48 INCHES (1.2 X PROP DIAMETER)
ASPECT RATIO: 1.6 (span/chord length)
RUDDER STOCK: 5.3 INCHES (From leading edge, 17% of chord length)
SIZE OF RUDDER SHAFT: 2” AQUAMET 22, AISI 630 OR ALU 6082
SURFACE BLADE MATERIAL; Mg BRONZE, 316L SS, AISI 630..
RUDDER THICKNESS: 3.7 INCHES ( 12% of chord length)
END PLATES: SHOULD BE 2 X THICKNESS OF BLADE- 2 X 3.7 INCHES = 7.4INCHES

CLEARANCES- THREE BLADE PROPELLER
(% OF PROPELLER DIAMETER)
MINIMUM CLEARANCE B/W PROP TIP AND HULL – 17% OF PROP DIAMETER (40”) 6 - 6.8 INCHES. (Dependent on aperture)
MINIMUM CLEARANCE B/W KEEL AND TIP (4%) – 1.6”
DISTANCE FROM DEADWOOD TO PROP (27%) – 10.8”
MAXIMUM FROM PROP TO RUDDER (15%) – 6”

Thanks
Edington

 

I don't understand your needs. It appears you have a 0012 section, with a 17% balance and fairly conventional end plates. What's to design? Not knowing the hull form, underwater areas and gear arrangements, I'd say 17% balance might be a few percent too much. I'd be more comfortable with 15%, but without an idea of what type of hull, gear, etc., this is a guess.

What is the make, model and designer of this vessel?

 

PAR, Well, no naval architect is involved here, it is a wooden displacement hull being built by local guys. They have been doing this for years. Their technical expertise is a bit questionable, but that is what we have There are no proper laid down structures as in you have in the US, Europe and else where for building boats.

Edington

 

It still appears you have enough information for your rudder, though I'll bet you could just use a flat plate and have a rudder that's within 98% of the efficiency of a NACA section. I guess what I'm asking is what do you need? You have the physical dimensions, the sectional shape and configuration (balance and plates), so what more is necessary?

 

PAR, I think you and other contributors have answered my question. You see I am new to this and what I really want to know is how efficient is the flat plate compared to NACA section. My mind is made up now. When I take into consideration the cost of producing (NACA & Flate plate)and also experiences others have had with flat plate with. Do you think I should be concerned about the round rudder stock which runs outside down the height of the flat rudder plate?

My regards

 

You either carry an oversize stock or a tube which becomes the main strength member. It also has an effective in improving the flat plate hydrodynamics.
Horizontal stiffeners are usually required and the trailing edge wedge stiffens the edge and improves the stall characteristics.

Classification societies include rules for construction of flat plate rudders and you can download them from links provided here:http://www.boatdesign.net/forums/classification/


You could work through the rule based design here with some feedback it's pretty straightforward.

 

It depends on the speeds you'll be moving, but no a displacement trawler, shouldn't be bothered with a shaft bulging out of the side of the plate.

 

Several boats, similar to the figure (LOA=13,80 m), are working on the coast of Angola. See you at the rudder they have.

 

Define efficiency?

Q..what is the purpose of a rudder
A..to steer the vessel and provide directional stability

Thus which will do the job....both.

Everything comes down to cost or performance. Since both can do the job..it is a cost issue. That is for you to decide.

Whether one turns 0.5sec faster or 1 degree per second faster...who cares?...does it do what is says on the tin...yes or no?

Rudders are not rocket science...they either work, or they don't. They either cost a lot of money or they don't. Choice is yours.

 

IF you look at the NACA graphs , look carefully at the flat plate.

It does provide very good lift at small angles and is really EZ to build/repair.

IF you were building a 12 Meter or a new J boat , a complex build would be OK.

2 seconds a mile might be a big deal,

For a powered workboat , Flat is fine.

 

I totally agree.

 

I see many flat plate rudders with the wedge on the trailing edge (made from a few pieces of flat bar or angle). Why is it there? How important is it?

I realize that it greatly stiffens the trailing edge, what about flow? Does it help increase lift? I have heard about preventing stall at large angles, it seems like the leading edge would be more important for stall issues.

Can anyone discuss the reasons for and usefullness of these trailing edge wedges.

C.O.

 

Thanks to everyone for their advice, I do not think a 10 knots trawler would have to worry about rudder stock sticking out along the height of the rudder blade.

Anyway I am interested in the topic Cor raised about the wedge on the rudder.

 

The links you directed me to, ie, www.webstore.Ir.org in the boatdesign forum has either been moved or longer available. Any ideas?