Traditional rudder design query

[Sgf]

Hi all

Im looking at the rudder design of an old English square sterned coble, as pictured below, and am also reading Skene's Elements Of Yacht Design

This rudder is

• Deep and narrow
• Forward-raked
• Hung on the transom without a skeg or fin ahead of it
  

From what I understand of Skene’s recommendations the optimum rudder is generally:

• Of moderate aspect ratio
• With maximum chord at or just below mid-depth
• Mounted upright or with slight aft rake
• Ideally faired by a skeg, to improve flow attachment and raise the stall angle

Skene emphasises smooth water flow, delayed stall, and effective lift at small angles of attack

So my question is: why did traditional builders of the coble do the opposite?

• A forward-raked rudder tends to lower the stall angle, especially at the lower blade
• There's no skeg, so flow hits the blade directly, increasing the risk of separation
• It's very deep and narrow, meaning a likely stall at ~12–14° unless mitigated
  

Yet, these boats were widely used for fishing under sail and oar in rough conditions, and needed to be manoeuvrable, stable, and I assume beachable.

So, is the forward rake mainly about improving tiller balance, by placing more area ahead of the pivot? Or could it be a case of functional trade-offs: compromising hydrodynamic efficiency in favour of simplicity? Or does the depth of the blade make up for early stall by keeping it immersed and giving good control at low deflection angles?

The book the design is in (inshore craft by Basildon Green hill) suggests it is to act almost as a centreboard as well as a rudder, and this boat was rigged with a lug and jib.

I’d love to hear from those with traditional boat experience, designers, or anyone who has sailed these or similar boats. Is this a case of form following function in a very specific working context — or am I missing something hydrodynamically?

Cheers,
Sam

 

[peter radclyffe]

if the boat grounds going fwd, the rudder should lift rather than break, the boat was grounded stern first with rudder lifted off

 

[peter radclyffe]

skene did not fish in a winter gale off a north east england beach

 

[Sgf]

if the boat grounds going fwd, the rudder should lift rather than break, the boat was grounded stern first with rudder lifted off

As in it would lift off the pintles? Makes sense.

Do you think the quasi centreboard idea has any merit too at all?

 

[gonzo]

Skene would not have covered work boats. His book is about yachts.

 

[wet feet]

All the posts contain parts of the truth,which is all they can do being quite brief.One misapprehension that seems to be present is that there is any area of rudder forward of the pivot,there is no appreciable area ahead of the pivot as the conventional rudder fittings are raked at the same angle as the transom.Additionally,trying to calculate the stall angle of the traditional rudder is a bit more involved than taking the overall length of the immersed part and comparing it to the perpendicular distance from leading edge to trailing edge and then referring to a table illustrating the trends for stall angle.A better set of measurements would be the effective draft of the rudder and the chord-measured parallel to the waterline-as that would be comparing the effective span to the effective chord.

Another aspect of the boat that isn't universally understood is that the boat is built in a very particular way and when afloat,the keel line rises from stem to stern.Which means that the lateral area of the hull is unusually dispersed,by the standards of other boats.The entire boat and even the naming of the parts was evolved to suit local conditions and worked very well,but making comparisons to a book covering the general design of traditional yachts is perhaps not the most appropriate way to determine whether it ought to have functioned well.Even the revised version of the book will include some advice on fin and skeg cruiser/racers,but won't stray into the realms of coble practice.

By way of illustrating what was around in Skene's time,here are the lines of a J class,and it's rudder has some of the characteristics of the coble equivalent.It worked.

 

[myszek]

Coble is a very unusual boat, which "futuristic" solutions fascinated me for years.
She doesn't have any centerboard nor protruding keel. All the side resistance is provided by the deep bow part of the hull and by the rudder. Thus, the rudder has to be deep, to be an efficient foil. It is also raked forward to maintain the reasonable position of CLR.
There are some boats with similar solution of the lateral resistance in Mediterranean region. I remember one in the naval museum in Barcelona, with the same forward-raked rudder. There is also Croatian Bragoc (or Bragozzo in Italian), which big rudder is not raked forward, but the CE of the schooner rig is shifted far aft.
Coble has, however, an unique hull shape, with a deep bow and very flat rear part of the hull. When the engine was added, the tunnel for the propeller appeared, and the stern began to look like a catamaran.
So, we have a hull with a single, deep bow, wide transom, concave bottom at the stern, no keel, and deep, forward-raked rudder to provide side resistance. Very avant-garde design, even for today. I would like to se a sailing dinghy shaped like this.

regards
krzys

 

[Sgf]

Ah so, to check I understand you, the rudder shape/position is compensating for the unbalanced underwater shape of the hull (as in lateral resistance against leeway is concentrated nearer the bows because there is less hull in the water the further towards the stern you go
Interesting. I take it the reason for the hull shape evolved for beaching stern first, and high bows to punch out through surf to get to sea?

 

[CarlosK2]

That Rudder is AR < 3

And forward raked

Stall at 25-30° (?)

 

[CarlosK2]

(CA Marchaj)

The deeper part of the rudder is less loaded and therefore can withstand a greater angle of attack

 

[CarlosK2]

"it is to act almost as a centreboard"

---

Yes

This is seen in traditional boats: the rudder wants to be a daggerboard

"Dorna"

 

[Sgf]

Excellent thankyou for the info

 

[CarlosK2]

One man show
One free man, One boat (-1964)

(It is surprising because already in 1910 these boats were threatened by industrialization that lowered the price of sardines)

 

[seasquirt]

Looking at many photos and some sketches / drawings, the rudder would probably have reached around the same depth as the deep fine bow, but seeming to extend deep due to the rising of the bottom toward the stern, as wet feet notes above. With no centreboard, or keel, or lee board, they look to use the deep fine bow, and the two rub rails (don't know proper term) either side of the ram (the flat wide central plank aft of bow section), to limit leeway under sail. Probably not pointing very well, or steering quickly. Nothing below to entangle nets, and the rudder can be lifted out of any problems, then replaced. The rudder probably had a strong tether to prevent it from accidentally drifting away if/when lifted off the pintles when beaching in surf, or while clearing nets. Nothing to worry about when sailing into the beach, except not to broach, then drop the sails at the last moment, and grab the rudder out of the surf.
The model 'Javiota' held by the boy doesn't have the traditional deep fine bow; possibly a copy of a later coble with a motor, or a style from a different region. I'm not sure that the photo of V13 1,397 is a coble either, with its pointy stern. Guy is enjoying it anyway,

 

[skaraborgcraft]

Generations of "working boat people", found what worked from doing, they did not read books written by people who knew nothing about their craft, literally. As above, the reasons given are for the purpose and conditions the Coble was used in, rather than some academic formula written in a book.