Effect of moving max thickness on a keel or rudder

Hi all:
I’m planning some work on my J22 keel. It’s a low aspect fin keel, and a one design class with pretty tight limits on what can be done, but there appears to be an advantage to moving the point of max thickness forward a bit. I’ve heard anecdotal notes about how that affects the boat balance and speed, and how some designs are better for lighter winds, or better pointing etc.
Does anyone have any thoughts on the effect of relocating max thickness?

 

The class rules specifically prohibit changing the shape of the keel, so moving the max thickness would be cheating.

 

Could you expand on that information, please?. Thanks.

 

The class rules prohibit changing the shape of the hull, but only require that the rudder and keel conform to the range of dimensions provided in the class rules. Most fast boats have legal keels, that have been modified from stock. There is a set of minimum dimensions for the foil, which somewhat approximate a NACA 00XX (varies depending on which section you measure), and a method to limit keel chord length and fore/aft location on the hull.

 

My choices appear to be adding some volume forward of the 30/35% as compared to the class standard, to approximate the NACA curve, or to add even more volume forward than is in the NACA form.
The run to the trailing edge is very linear in the class minimum as compared to the NACA form, which is visibly curved. I'm wondering how these features affect drag and foil stalling behavior.
Hollows are prohibited.

 

http://j22.org/wp-content/uploads/2022/02/J22-Class-Rules-2022.pdf

MODIFICATIONS
The keel shall be of moulded lead to the building specifications and cast in a mould supplied by J Boats, Inc. The external dimensions and configuration of the keel shall comply with the table of offsets contained in official H.1 Plan C. The keel may be overcoated in any base liquid or paste protective material or fibreglass and faired; provided that the fairing does not change the design shape of the keel and that the keel still meets all class measurement requirements.

 

The design shape is not always the as built shape.

 

And the current shape of a 36 year old keel may not have any relation to either the design or built shape.

 

Whether what I plan to do is permitted under the class rules should really not be of any interest. I'm a big boy, and can read the rules myself, so thanks for the information. If you are going to be reading the rules, rather than flinging them at passersby, you might want to get the current version, there were a few changes this year.

If you have any insights regarding the original question, I'd love to hear them. I'll try to rephrase: What impact on sailing, such as keel stalling, excessive drag, ability to point, and boat speed in various conditions and points of sail, are affected by what particular keel features? Linear versus curved trailing surfaces? More volume ahead of the point of max thickness?

I was hoping, not having the benefit of an advanced degree in hydrology or fluid dynamics, to engage the thoughts of those that do, and discuss what features of various keels are intended to do, so that I could make a better judgement before I dive in to repair the already non-compliant keel that is hanging under my boat at the moment.

 

Ok, back to the original question....and welcome to the Forums....
The following generalizations can be made....YMMV
For a well shaped foil....
Moving the maximum thickness forward will improve stall and increase drag slightly at most operational AoAs
Moving the maximum thickness aft will lead to earlier stall but decrase drag at low AoA (And no...a J22 will not be in the drag bucket of most extreme foils)

Pick your posion....

EDIT: FWIW, a NACA 00xx max section is 30% aft, a TMB-EPH min vortex shape is 43.6% aft

 

Thanks!
I very much appreciate the practical application.
One other question. It appears to me that the keel was partially faired at some point, the trailing edge is curved a bit, the chord lengths are at or slightly below minimum length, but close to max forward, and the leading edge is well short of max chord and max forward. To wrangle it into a legal and competitive shape, I think I'll need to add a bunch of volume at the leading edge, and thin out the rear third or so of the foil. The class foil has a very close to linear run from the maybe 50% back to the trailing edge, the NACA shapes have a slight but definite curve the entire way. What advantage does the curve or lack thereof in the rear third or so of the foil bring?

Chris

 

I took a look at the J22 class rules. https://www.j22.org/wp-content/uploads/2020/12/J22-Class-Rules-2020.pdf

The keel is very detailed in Sections E and H. And if it is as you described, then perhaps someone tried to minimize wetted surface (and if you race on light wind lakes...not a bad idea). On the other hand, if the surface was near minimum to begin with, too aggressive sanding could also get you there.

For a foil in water, the run aft of the max thickness should have a very slight convex shape to it increasing towards the TE (i.e. the Hyperbolic part in EPH). At the real world hull speeds of a J22, if it is straight...just don't bring that extra gear & beer. We are talking very subtle shapes here that are totally overwhelmed by your average sea state effects. But the trailing edge should be at the minimum 6mm and square cut. Also note that MPE and MPF (the approximate keel stub joint I guess) have minimum thicknesses assigned to them (E.3.3 f and j) but the keel stub could be faired with a larger radius and a dillet which doesn't seem to be measured. FWIW, the rudder section seems a typical 30% aft shape close to a NACA 0013.

 

Once upon a time, before asymmetrical downwind sails, the J22 was a hot class, with lots of "optimizing" going on, resulting in lots of language in the measurement section of the rules.

There was originally a prohibition on fairing the foils, but the wide variety of keel sizes and locations coming out of the TPI plant made it hard to police.
The MPE/MPF thickness was a later attempt to limit fairing of the keel stub for safety reasons I believe.
The keel plan offsets vary at each section, closer to an 0010 at section one on the keel design plan, and 0014 at section 4, possibly to get enough lead low enough.
We sail in current and lighter air most of the time, so maybe a smaller foil doesn't hurt. At least one measurement is out of compliance though, so I'll have to do something.

All of this will ultimately be overshadowed by the usual errors at the organic tiller interface...

 

<shrug>...Typical for most foils to have one end a thicker foil than the other. Additionally, the interfrence drag is proportional to the t/c at the hull/foil root intersection....so we have the "tomahawk" keels on the 12m or the "bulb" tip keels like the Gary Mull Ranger 26.

 

So, if you are going to shorten the chord, do it at the hull end? The common knowledge is that max chord keel is fastest, but mine is on the small end of the range, and we have no boat speed issues. It could accelerate better, but overall if it wasn’t already out of spec I wouldn’t change it