What shape should the rocker be?

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Jamestj
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Joined: 25 Aug 2004, 17:50
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What shape should the rocker be?

Post by Jamestj » 08 Feb 2005, 09:36

The hull of a sailing boat really has to have its maximum depth where the fin is attatched. The fins of IOMs are invariably more than 500 mm aft of the stem. Since fish and birds and sailing boats are usually wider forward and taper towards the back, why should IOMs be different?

Probably a very simple answer but I am trying to design my own boat and new to this.

Thanks

James

awallin
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Re: What shape should the rocker be?

Post by awallin » 08 Feb 2005, 15:17

Jamestj wrote:The hull of a sailing boat really has to have its maximum depth where the fin is attatched. The fins of IOMs are invariably more than 500 mm aft of the stem. Since fish and birds and sailing boats are usually wider forward and taper towards the back, why should IOMs be different?
fish and birds are different in that they move in only one medium: air or water. Boats move at the boundary of water and air so you can't just copy a bird or a fish...
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Anders Wallin

Nigel
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Post by Nigel » 08 Feb 2005, 15:45

How about a flying fish :lol: :roll:
Nigel Winkley
GER 87

spaldi01
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Post by spaldi01 » 09 Feb 2005, 10:06

Hi James

Complex question!
Current thinking appears to be to place the max rocker and consequently the fin in the middle with almost straight lines to the stem and stern. This can be seen in designs like the Erica and Stealth. If you move the point of maximum fullness further forward trying to achieve the least drag aerofoil shape then you will also have to move the fin and rig further forward. This may put the front of the jib boom over the stem and also present other problems resulting in a poor design. Today’s IOMs represent very highly evolved shapes that represent good compromises between the conflicting design requirements, and the straight rocker profile mentioned appears to result in a hull shape that tacks well and resists nose diving when going down wind.

It is surprising that when you consider the wide variations found amongst IOM hulls and the very small difference in performance it illustrates the difficulty in designing a hull with any significant advantage. But that doesn’t stop us trying.

JohnB
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Post by JohnB » 13 Feb 2005, 22:19

Jamestj

A bit of the question but in the 1970's Bob Miller (Sydney) now Ben Lexcan, designed a series of boats I think the range of one tonners where first (The One Ton Cup was held in Syney 1972) called Ceil III (won the 1973 Sydney Hobart overall)
Followed by Ginkgo & Appollo II menbers of the aussie Admiral Cup team 1973)
And in 1975 another one tonner Rampage won the Sydney Horbart.
1983 the America's Cup was history.

All these boats appeared to have their max hull depth forward of the keel, looking like a fish.

Here's a link about the 1973 Admirals Cup with a photo of Ginkgo with a typical early 70's IOR boat for comparison.

www.rorc.org/admiralscup/history/1973/default.html

JohnB

Even
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Post by Even » 14 Feb 2005, 09:29

All these boats appeared to have their max hull depth forward of the keel, looking like a fish.
Two comments: The shape of yachts designed to the IOR rule were significantly influenced by the IOR rule itself. The mode of exploitation varied over the years, but in the seventies deep forefoots (and nearly flat rocker was favoured.

Secondly, the wave drag of yacht depends on the volume distribution rather than rocker. It is the 3-dimensional shape that is important, the rocker is only a part of it.

Jamestj
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Joined: 25 Aug 2004, 17:50
Location: Building one at the moment

Post by Jamestj » 14 Feb 2005, 13:49

Yes but ....

Broad bows create drag. This drag pushes the water down rather than sideways. An equal and opposite reaction would raise the bows - perhaps causing the vessel to plane. Do IOM's gain a greater advantage by going through waves rather than rising over them?

Received wisdom is that IOM hull should be narrow. Presumably narrow boats win races so everyone buids narrow hulls. What is the physics behind this advantage?

My present inclination is just to build an attractive hull and hope that it works!

Nigel28
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Sail number: CAN 28
Design: Vancouver
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Post by Nigel28 » 14 Feb 2005, 18:40

James
You wouldn't be far wrong with that approach :!:
If narrow boats are winning what about the TS2 :?:
IMHO model sailboats can not really be compared to most full size yachts as the dynamics are just so different. For example look at the CoG, about 20cm under the hull for a model (3+ times max rocker) and probably some where around the hull for a full size yacht.
Therefore form stability (beam) does not have the same importance it does with full size yachts :!:
Remember its not the hull that wins but the Guy or Girl sailing it.
So my advice is to build your good looking boat and get on the water sailing as more time on the water will turn out to be more important than design details :lol:
Nigel Ashman
CAN 328

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