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Author Topic: Curious question on rudders and keels  (Read 1876 times)

dlancast

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Curious question on rudders and keels
« on: January 10, 2012, 05:48:46 PM »

Howdy folks,

I am no engineer on the subject of fluid mechanics and hydrodynamics, but I have always been curious about why scale models require an increase in rudder size and the addition of heavy keels to help stabilize model sailboats.  I think it  has something to do with molecule vs. mass.  Is this true?  Can someone enlighten me here?  How do you know how much to increase in rudder size and keel?

I must have alot of time on my hands in retirement, but inquiring minds just need to know.

Thanks,

Dennis
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rmaddock

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Re: Curious question on rudders and keels
« Reply #1 on: January 10, 2012, 05:51:27 PM »

With sails it's because hulls are a volume and sails an area. Volume reduces much faster than area so the sails rapidly overpower the hull.

Ither way round I can get my head round the maths.

You have a boat, you double it's size. The sail area doubles but the volume quadruples.....I think.

Clear as mud.
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boatmadman

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Re: Curious question on rudders and keels
« Reply #2 on: January 10, 2012, 05:59:05 PM »

The other main factor at play here is the fact that the model boats are scaled down, but, you cannot scale the water and its effects down as well.

Consequently, the water has a disproportionate effect on the model.

Ian
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HS93 (RIP)

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Re: Curious question on rudders and keels
« Reply #3 on: January 10, 2012, 05:59:17 PM »

its the above deck to bellow deck weight distribution the same, I am not a yacht person but would the sails/ mast etc be heavier than of a full sized boat.

also I think a model may sail a bit faster than a full sized boat (scale speed). and is the wind scalable

just my thoughts from someone who has only sailed once or twice with a borrowed boat.

Peter

er we both posted at the same time
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Colin Bishop

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Re: Curious question on rudders and keels
« Reply #4 on: January 10, 2012, 06:01:32 PM »

It's easier to look at it the other way round. When you scale down a full size boat the sail area reduces by the square root and the hull volume by the cube root. So the proportion of sail to displacement increases quite dramatically. Which is why you don't see many working model square riggers!

The fact that water and weight of construction cannot be scaled down pro rata does have some effect but nowhere near as much as the change in the sail/displacement ratio

Colin
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dlancast

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Re: Curious question on rudders and keels
« Reply #5 on: January 10, 2012, 06:03:11 PM »

Years ago, I built an RC model of the "Emma C. Berry".  She was about 4ft long.  The model was ballasted with lead shot incased in epoxy, inside, low.  I was able to sail her in a good breeze without reefing.  I believe that her stability had more to do with her wide beam.  But, she did quite well without adding a long or heavy bolt-on keel.  Her rudder was large.

Dennis
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dlancast

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Re: Curious question on rudders and keels
« Reply #6 on: January 10, 2012, 06:08:40 PM »

Ok, I think I understand the dynamics, but I'm still confused about the true effects about what is going on.  I have found that these principles are true for model aircraft... typically they have larger control surfaces.  I'm still wondering if this has to do with molecule and mass relationships, or should I just not worry about this and go sailing :embarrassed:

Dennis
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malcolmfrary

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Re: Curious question on rudders and keels
« Reply #7 on: January 10, 2012, 06:21:23 PM »

Quote
just not worry about this and go sailing
Top idea. :-))
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Umi_Ryuzuki

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Re: Curious question on rudders and keels
« Reply #8 on: January 10, 2012, 11:51:23 PM »


How about this, a 1 mile per hour wind equals 24 mile per hour wind at 1/24 scale.

 :-)
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malcolmfrary

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Re: Curious question on rudders and keels
« Reply #9 on: January 11, 2012, 11:06:41 AM »

Because of the forces acting are related to the areas involved, the wind speed acting on the 1:24 model equates to about 5mph (sqrt24 = near enough 5) on the full size one.
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carlmt

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Re: Curious question on rudders and keels
« Reply #10 on: January 11, 2012, 12:01:31 PM »

Found this on Deans Marine website...............

REAL WIND SPEED TO A SCALE OF 1/96.
----------------------------------------------------------------------
KNOTS             SCALE WIND SPEED            FORCE OF THE WIND BY
                                                                           BEAUFORT NOTATION = 1/96
....                                   ........                                         ...........
5                         48.9                                         Force 10   gale
10                       97.8                                         Force 15 4th degree hurricane                                                                                                                               
20                       195.96                                      Force ? never recorded
30                       293.00                                      Force ? Mach .4 ?
.......                   ........                                         ...................

1 Knot = 1.15 mph
1 Knot = 0.514 m/s

I guess you could proprtion this for relative scales ie - halve the scale wind speed for 1:48 then halve again for 1:24?
Carl

malcolmfrary

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Re: Curious question on rudders and keels
« Reply #11 on: January 11, 2012, 09:23:30 PM »

Quote
I guess you could proprtion this for relative scales ie - halve the scale wind speed for 1:48 then halve again for 1:24?
Yes and No.
Deans information is based on the square root of scale formula, so sqrt96 is just under 10, which shows with the figures given (i.e. original divided by 10).  To keep the maths finger-simple, sqrt24 is near enough 5, so divide by that.  Sqrt48 is near enough 7, so 7 is what you divide by for that scale.
1:100, divide by 10
1:49, divide by 7
1:24, divide by 5
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tobyker

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Re: Curious question on rudders and keels
« Reply #12 on: January 11, 2012, 10:38:18 PM »

I suspect so far as rudders are concerned, a. we have less space to manoeuvre in, and b. unless you have a servo for each sail, you cannot steer by the wind - ie let main go but keep jib sheeted in to pull the bow round when you go about - so you need a bigger rudder to increase the turning moment. If in doubt - check out the chord of the rudder on the VASA - it's tiny and is only for trimming  - they must have steered by the spanker and the foresails.
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Bryan Young

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Re: Curious question on rudders and keels
« Reply #13 on: January 12, 2012, 04:40:51 PM »

Ever seen a single screw ship turning? If so you'll know that the actual turning arc is pretty big compared to the length of the ship. Also, on a real ship. you get transverse displacement...ie the ship goes sideways. Most modellers seem to want the model to turn very rapidly...nowhere near what happens with the full size ship. Also...a bit of contention here...many models have the turning point (the position within the model) either too far forward or too far aft for the hull length and balance. In a perfect scenario the slightest movement of the rudder will result in an alteration of course/direction.
This is easily visible if a ship (or model) is down by the head...the rudder has to work harder. Similarly, if the vessel as excessive stern trim then she will be overly sensitive to the rudder and be very difficult to steer.
So it's all a question of hull balance really. BY.
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