Rudder area for dual rudder

[teoman]

Hey,
I read a lot of threads about rudder area.
My boat is a 37 feet lobster boat. I will have twin 150 HP on that.
From what I read, I need to have about 5% of the lateral under water area.

But what about with dual rudder ? is it 5 + 5 or 2.5 + 2.5 % ???

thanks a lot..

[daiquiri]

No, hold on. I guess you are referring to the thread rudder design, where E. sponberg has given you a rule-of-thumb area for sailboats.

Since you have a motorboat with 2 props here, I presume that there will be 2 rudders placed behind the props. In that case your recommended total rudder area will be much smaller, between 2.5 and 3.5% of the Lateral Hull Area (Ahl). The lower value is common for a good directional authority, the higher value is recommended for boats which often need to perform low-speed maneuvers.
If you believe you will often have to perform long backwards maneuvers (in tight channels, marinas or similar), when rudders will be working out of the prop slipstream, then you should increase this value to 5% of the Ahl.

It is intended as a total area, thus a sum of areas of the two rudders.

Cheers!

[teoman]

Thanks a lot.
This is exactly what I ment.
I will probably need rudder effectiveness when maneuvring in marinas.
My lateral plane area is 7 M2. so 5% would be 0.35 M2 total.
And that divided into 2.

That looks very small... is that normal ??

[daiquiri]

It is not that small. Means that each rudder will be approximately something like 60x30 cm, for example. You definitely don't need more, it would create eccessive resistance when cruising. In very tight spaces you will use engines for maneuvring.
Cheers

[teoman]

I am not sure if I understoos correctly :
My lateral plane area is 7 M2. 5% = 0.35 M2
0.35 M2 % 2 ( 2 rudder) = 0,175 m2 for each rudder

wouldn t that make about 10 cm x 20 cm ??? that would equal 0.2 m2

I was very bad in math, is that correct ??

[teoman]

ooppppsss sorryy hehe,
please forget what I said.. just getting late here heheheh

[teoman]

One last thing then,
is it better to have it deep or long ??

50 depth x 40 long ??
or
60 depth x 30 long as you mentionned ??

[daiquiri]

Originally Posted by teoman

I am not sure if I understoos correctly :
My lateral plane area is 7 M2. 5% = 0.35 M2
0.35 M2 / 2 ( 2 rudder) = 0,175 m2 for each rudder
wouldn t that make about 10 cm x 20 cm ??? that would equal 0.2 m2

No, no...

1 m2 = 100 cm x 100 cm = 10000 cm2

So 0.175 m2 = 0.175 x 10000 = 1750 cm2

60 cm x 30 cm (my example) gives 1800 cm2, very close to the target.
10 cm x 20 cm (your example) gives only 200 cm2.

Originally Posted by teoman

I was very bad in math, is that correct ??

Or just bad in unit conversions, which is a very common thing.

Cheers

[daiquiri]

Originally Posted by teoman

One last thing then,
is it better to have it deep or long ??
50 depth x 40 long ??
or
60 depth x 30 long as you mentionned ??

Ok, just saw that you have corrected yourself while I was posting my reply. It's ok now.

Depth or width - it will depend on how you will use the rudder.
For the same rudder area, a deep rudder will give more lift and less drag at the same tiller angle. Hence, it will be more efficient. But, since it will have a smaller foil chord (what you call "length"), it will stall more easily, particularly at low speeds. It will also give higher bending loads at the rudder stock.
Conversely, a shallow rudder with wide chord will be less efficient (less lift and more drag for the same tiller angle), but will be more stall-resistant and will give lower bending loads.

But, unless you plan to spend your days at low-speed maneuvering in small spaces, it is generally preferable to have a deeper rudder. That's a hydrodynamic side of the medal - you also have to consider draft restrictions, for example. I'm leaving that part to you.

Hope that helps.

Cheers.

[teoman]

thanks a lot for those precious information...
Thats very important for me )

now I can work on that..

Thanks..

[tunnels]

Originally Posted by teoman

Thanks a lot.
This is exactly what I ment.
I will probably need rudder effectiveness when maneuvring in marinas.
My lateral plane area is 7 M2. so 5% would be 0.35 M2 total.
And that divided into 2.

That looks very small... is that normal ??

Ok so you now have the blade area, but what about the actual blade shape and deepth and what about offset (inboard or out board ) to the CL of the propeller shaft ?? you are only part of the way there !!!

[teoman]

Acutally I was trying to define the cost of the equipment (cylinder, pump, etc) that I would need...

But off course I would appreciate your thoughts about blade shape, deepth and offset to the CL of the propeller shaft.
Those are actually not my area of expertise

If you can give me some advice on that, I will work on it when I start the work.

Thanks