Stern extension

[Alik]

Chickadee is right.
Also, Your boat's Froude numbers are higher than ones for IACC boats. As we know, higher Froudes allways require more transom volume. Enclose diagram from one Russian book on boat design, that concerns optimum transom draft. Here TTR is transom draft, T is hull canoe body draft, FrL=Fn=v/(g*LWL)^0.5 Check Your boat, probably she is OK?

[Tactic]

Interestingly I know of several yachts similar to mine that have up to 1m added at the stern.Always it seems with a positive outcome.
Will have to Think on this problem some more!
Thanks for the advice and learning experiences.

Cheers
Tactic

[PAR]

Yes, you can add to the length of your transom and get some benefit, but a lot more will need be done then tacking on some tail fins. If you boat has some weather helm the additional drag and plane area may be beneficial, but it's likely your boat had reasonable balance so this addition would need to have it's rig and/or other underwater areas explored.

Your transom seems well clear of the static LWL and the plan and sectional shapes suggests you'll not gain much in squatting or heel, so this extension wouldn't provide any bearing, which I assume is the idea behind it. The notched shape under a decent press may hurt your down wind performance quite a bit, but the length, if it gets wet and takes some loading could help in upwind work, assuming the helm has a reasonable balance.

The Froude figures for this boat are substansualy different then the IACC experiments and comparison isn't practical.

The seemly positive results of other boats may be a result of a lot more effort then they are willing to release for competition consumption. They may also be driving her harder with a false sense of advantage.

I'd spend my money on insuring the appendages are really smooth and symmetrical, per the designer's intent, maybe a better sectional shape if the rules permit. I'll bet you'll get more speed out of perfect foils and flawlessly buffed underwater areas then tinkering with the butt of your boat. In the end, this extension idea is a crap shoot and additional weight in an area you don't really want any.

[PAR]

If I recognize the hull shape correctly, it's late 60's maybe early 70's thinking and typical of the "V" forward sections popular then. If you want her to go faster, then pop the deck cap off her, remove the interior liner if equipped, take out the open celled foam that is holding several hundred pounds of moisture by now. Replace it with closed cell. Then when the hull is bare, strip half to 3/4 of the chopper layup out of the inside of the hull. Reskin the inside with a high strength fabric and reassemble the boat.

She'll be lighter by a third or more and will be a much more lively boat, which can be reballasted and rigged to take advantage of the new lower CG and taller (and stronger by necessity with the lower CG) sail plan. She'll still be limited by her reasonably burdened belly (by today's standards) but she'll do the best she can considering.

[Tactic]

I doubt she could be lighter.
She is 3 skins of 3mm NZ Kauri ,Cold moulded.No excess weight in any of the deck or structure,(6mm Okume)
This aint no chopper gun hack
There is no foam or anything inside that dosent hold it together.
She is dry sailed and doubt if the hull and deck weigh more than 200kg.
She is in as new condition. Perfect NACA dagger board(retrofitted) and rudder..about to get a new rig and rags.(her 4th rig)

Designed in 1967..not a bad guess there.
We are coming under pressure from more modern lighter boats which also tend to be longer these days,hence the re hashing we are doing.
Trying to get the best from this old girl.
Thanks for the advice


Edit, Looking at the water coming off the transom sailing upwind in heavy air I am convinced that a extension is going to help.Waterline length seems to be limiting upwind performance in a blow.
Downwind we are ok.
I agree that the V cutout is not ideal,Looking into some variation of the Vara rudder or simply mounting a rudder on a new transom further aft.
Or off course not doing anything at all.....

Another edit,
I wonder why it is then that r/c model racing yachts with similar "formula" type rules to the ACC seem to have adopted the same types of overhangs?
ACC yachts benifit from them,Model racing yachts also benefit from them.
I really must study all this more carefully.

Cheers
Tactic

[PAR]

I wouldn't look to the RC pond yachts for answers, the scale doesn't translate well (unless there really big puddle boats), though it's a cheap place to test radical ideas. IACC craft are rule built boats and the shapes indicate just that. The use of overhangs with an appropriately designed hull is an old trick, most racing rules tended to penalize it, so you don't see it near as much now, though it once was used commonly. IACC boats doesn't benefit from them, but use them to circumvent some rule limitations when the static LWL is measured in calm water next to a dock, inside the breakwaters. Under a press, she's displays and lives in a very different set of conditions. The bustle attached to the butt of the old CCA racers did the same thing, with less success. Then there's the Hula . . . tricks to beat rules, not necessarily good things to have on your boat, usually having a limited range of usefulness.

Your boat has likely a number of issues compared to current trends, beam to far forward or not drawn out aft as far, to much belly, an entry that will plunge fast and bluffer then today's standards, weight, NACA 00 foils rather then the 63's commonly used in production craft and other items of interest.

Your lee side wave train is probably more limiting then LWL length upwind with a boat shaped like that. You may have to accept the old girl may be past her front of the fleet days. Grind her back and finish her bright, she can be the prettiest at the dance instead.

[Mark 42]

I was thinking of something similar... essentially like having a daggerboard
trunk that rotates (a little bit like a railroad roundhouse turntable).
But it would require a very long seal... probably a greased O-Ring.

A large O-Ring can work... look at the Space Shuttle SRB sections.
It can be a challenge (no pun intended) too.

Have you considered twin rudders off of the swallowtails?
But then, at that point you may as well move the single rudder.

The issue I see with a swallowtail is a possible tendency to dig
in if it isn't swept up enough... but too much upsweep and you
lose what you sought to gain.

Originally Posted by Sketch

One solution to maintaining trailerability while extending the stern and keeping the rudder in it's current position involves a bit of precise building - but then again so does extending the stern

I wish I could remember where I read about this configuration in order to give thanks and a link to more detailed info. The setup involves a hollow carbon tube, the diameter of which is greater than the width of the rudder chord. The tube sits in a bearing that allows it to rotate. The tiller, or other steering mechanism is attached directly to carbon tube. Now imagine taking your existing rudder, but adding a cylinder that fits the inside of the carbon tube, on top (where the rudder stock would be).

Here's the cool part - drop the rudder into the hollow carbon tube and voila, retractable rudder, with little drag (the base of the cylinder should be faired into the shape of the bottom of the boat), in the same position as the original.

If you have a spare, you can even change your rudder without getting wet, say, to experiment with different foil sections, lengths, etc...

Kevin

Kevin, both time's you've described your rudder system it has sounded identical to the VARA system(see the thread-"CBTF and VARA)". There are variations of the VARA around too.
A simpler solution that sacrifices the ability to vary the rudder area but may be substantially lighter and retractable is a sort of daggerboard trunk within a trunk. The smaller trunk has the rudder tube attached to the front end and is as long as the chord of the rudder. That trunk slides in another trunk so that when in place the rudder is free to turn.
Simple and retractable(for trailering) and keeps the rudder in the original location-for what that may be worth.

[Tactic]

I have been thinking and drawing a little more.
In this version I have trimmed the hull by the stern by 1 inch..the stern extension is now 1/2 wet.
Transom hung rudder,no swallow tail arrangment.
Has narrowed waterline fwd,shifted max wl beam slightly aft and lcb etc more in line with modern trends.Has made a major change to wl length also
The new bigger more efficent rig should take care of the extra wetted surface(marginal )

edit,The waterline in my original line drawing is with the boat unloaded.In reality my transom is much closer the the water with the crew aboard.
In fact on our last cruise for the first week the transom was immersed.

Cheers
Tactic

[Mark 42]

Make a model of your boat... maybe 6 ft or so, and try some tow
testing. You can put a spring scale (fish weighing scale) on the
towline, tow on a boom out to the side (like a barefoot pole) and
see if the drag increases or decreases at various speeds and
heel angles when you make changes.

Basically it's the same as flow tank testing if you can do it on
smooth windless days and get the model away from your wake.

You can go smaller or larger on the model; it's a trade off between
cost and accuracy.

If you have a river that flows fast, smooth and steady, then you
already have a flow tank (like in the movie "Wind").

The problem with a large scale modification (full size) is you
can't really compare it to the "what if" configurations unless
you have time and resources to try several ideas.

[Tactic]

Final(I think!!) before and after drawing.
Seems to have given the old girl a facelift if nothing else.
Cheers
Tactic

[Mark 42]

Adding a lot of roach in the main w/o more headsail may make
canting the mast forward a bit (less rake) desirable to offset
increased weather helm. Hull change (aft extension) will also
influence this, possible offsetting the increased roach.

[Tactic]

modifications are finally almost complete.Rudder will be transom hung but with the "hole" sealed in a cunning way which i may show later.
Stern extention weighs 5kg and is 840mm long,built from 10mm foam with light glass skins.
Bow extention shown with cream primer, and is approx 10kg.A new cabin roof was made that is 5kg lighter than original.
Some other yachts of similar type have had these alterations done with noticable performance increase.

[messabout]

Tactic:
Your aft extension is presumed to follow the lines of the aft sections smoothly. If the extension follows the aft rocker lines, the whole of the extension will not be in the water. That statement is based on the premise that your boat is running on its' designed lines and the transom does not drag. The extension, as in the pictures, will also make the boat a bit dicey in a following sea.

These things are useful on a power boat. They used to be called squat boards. Some wizard decided to make them adjustable and they are now called trim plates or some other term, depending on geography. Fine for power boats, not worthwhile on a sailing boat.

Spend your time and money on some other kind of improvement. Before you do, determine whether your boat can be forced to go faster. If it is heavily rockered, or it is carrying a lot of weight, or it is short and beamy, then it won't go much more than hull speed no matter what you do.

[Guillermo]

Originally Posted by messabout

...If the extension follows the aft rocker lines, the whole of the extension will not be in the water...

Well, increasing hull length astern makes the boat somewhat faster because when aproaching hull speed, the stern wave brings the overhang into the water, thus increasing waterline length and so hull speed. Quite common effect in old sailing boats with long overhangs.
Not to talk about the probable increase in lifting area when running or reaching, favouring coming into planning or at least semi-planning speeds.
I do not know if this last will happen in this boat, but has been tried succesfully in others.
If the added scoop is not closed, it can become dangerous in a following sea.