# Rudder Area Calculation

Discussion in 'Boat Design' started by ravenousrob, Oct 24, 2015.

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### ravenousrobNew Member

Hi,

I am in the process of building a small 5 meter steel workboat and require some help on sizing my rudder.

How do I work out the surface area of the rudder needed for my boat and how do I work out the best placement of it with relation to the prop?

Cheers
Rob

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### SailplanJunior Member

Hi Rob,
It'll come down to a few factors.

What weight is your workboat, how big is the engine and what speed will your workboat travel at typically.

In terms of rudder placement, the rudder will work most efficiently when it has flow over it.

Do you have a General Arrangement of your workboat. This would show where the engine is to be placed and the shaft length, prop position.

You would need all of this information before attempting to answer your question.

Kind regards
Paul

3. ### OleboynowPrevious Member

I would try avoid the temptation to use a flat plate, a balanced foil rudder will give your boat control
best to transom hang with a tiller
For your lil boat 16 inches span and maybe 9 or 10 chord
This is one I designed for a disp river craft, ignore the engineering as the rudder was designed with all the shafting etc inside the hull
the rudder is so important
ships have changed rudders and managed to dock without tugs, whereas before the change tugs were needed

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### TANSLSenior Member

It seems to me that a boat of 5 m should have a flat plate rudder, traditional, not a balanced rudder, some 100 or 120 mm behind the propeller.
Area? : 1/30 of the projected area of the hull.

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A rough rule of thumb is 2% of the under water profile area.

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### PARYacht Designer/Builder

Welcome to the forum Rob.

If your boat is going to be restricted to displacement speeds (4 - 6 MPH or 6 - 9 KPH), just about anything will do. Make it as big as practical and what looks about right for other craft of it's general dimensions and use.

If on the other hand you'll be powering her up above these speeds, you'll probably want to put some thought into area, plan form, sectional shape and location. As a rule, the faster you go, the more true this becomes, but if it's what I suspect (a hefty 15' workboat), just about any plate, welded to a shaft will get it done. Yeah, it's not going to be as efficient, as a rudder with lots of math and head scratching behind it's design, but it'll be pretty damn close in spite of the head scratching and number crunching. I say go down to the local marina and look at other small workboats and see about what size they are and make something similar. You'll be done welding her up before lunch and maybe working her the next day, while the number mashing is still being worked out for someone else's "fancy" rudder.

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### TANSLSenior Member

It does not seem right, too small for this type of boat. I totally agree with PAR, "Make it as big as practical".

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So why say 1/30 which is only 3%?...

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### TANSLSenior Member

You seem to be a good mathematician and soon you will notice that 3.33% is more than 50% higher than the 2%.

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### JSLSenior Member

Let's see a drawing of the boat. What is the propeller size??

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So, here we go again. Either a lack of comprehension or just a constant desire to post on every thread regardless of the value to the posting.

so how does this statement equal this:

It doesn't. yet you seem happy to cling to it despite agreeing with the contrary....go figure!

So either you agree to make it as big as possible..or you wish to limit it to a smaller area....Thus, either your first statement is nonsense since 1/30 does not equal as big as possible or your second statement is. It can't be both.....so, which is it?

All you are doing is demonstrating that you are a number cruncher/programmer type of person, rather than a naval architect. A Naval architect is guided by trends not absolutes such as 3.333% when 3% is suffice.

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### TANSLSenior Member

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### ravenousrobNew Member

To all that have contributed thank you for the input!

I have attached a rough sketch of what I hope the rudder to look like I was thinking, as "PAR" suggests, to make it as big as practicable, however I was going to put in some balancing so tiller work is not arduous! The prop shaft angle is estimated as I need to get the boat and engine in the water to trim the boat.

Some numbers

Waterline Length 4350mm
Waterline Beam 1575mm
Draft 300mm (ish)
Displacement around 750KG
Looking at an 11" prop
20 HP @ 2300RPM from Lister ST2
Displacement speed for the boat is around 5.5 knots however I was hoping to get around 7-8 out of her as she is quite streamlined.

I have also attached a few photos so you can get an idea of the boat, if anyone can see any glaring errors with my sketch please let me know

Cheers Rob

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14. ### OleboynowPrevious Member

gee Rob, yes that is a pretty boat, congrats
Sorry for the arguing in this thread, it does appear that people want to post for the sakes of it, in this respect I have to agree with Ad Hoc
My fancy rudder was as much for others as you
Flat plate will be ok with some balance
if you have a dfx read capability I,ll do a dwg,
That boat is so nice, I think you need it to not be transom hung but up through bottom with a heavy wall tube and simple brgs and simple O ring seal at top with engineering plastic thrust plate
The is a steel called valuric hollow bar, which is ideal for stern and rudder tubes(maybe valeric)
by the way, you cannot ever get that speed
ever) unless you plow the water with huge motor, or make her plane remember speed of a disp craft, sq root waterline x 1.3 approx

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### PARYacht Designer/Builder

Agreed, she's not "streamlined" well enough, though I'll suspect some pretty WL's, she just has way too much volume to move, to get the 7-8 knots you desire.

It looks like you're going to put some type of fairing above the prop. I'd recommend you skip this, as the tip clearance will be too tight.

Place the shaft on a strut if you mist and make the rudder top as close as practical to the hull bottom, but other wise, don't fill the aperture any more then you need to. Lastly stay shy on the amount of balance, say 10% or less. Your rough drawing shows over 20% and with the prop that close, you might find steering troublesome. You can always add more if you think it's necessary, though solid steering gear can ease helm pressure, which shouldn't be excessive with the speeds and hull form you've chosen. I'd like to see some lines on this hull, can you post them?

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