Stern extension

[Mark 42]

As the boat heels I would expect more of the stern addition to immerse,
which would increase waterline length and increase the hull speed.

I'm thinking it'd be fun to mess around with something like this... if
I had time for a science project. Some sort of inflatable extension
comes to mind... old discarded rafts and duct tape could make for
a lot of crazy experiments.

But, I'd be happy if I could just find time to get out to my boat and
check the mooring rigging and bail the rainwater out before my cabin
sole substructure rots away!

Anyone wanna buy a Cal 25 project boat? I need to get it off of the
trailer so I can bring my other boat home for rehab.

(The trailer is NOT FOR SALE)

[gypsiemarine]

Hi, if you want to extend the waterline aft then extend the cockpit also and hang the rudder there. You might need to have a look at the rudder balance but it will work fine there. As to increased performance, you have a longer waterline....

[Tactic]

Nearly ready for splash day.A few more pics of the mods.
First pic is from 1981.

[Guillermo]

I remark that scoop dangerous when in a following sea...
Would you please post some photos of how the rudder works trough the scoop, if possible?

[Tactic]

A deck will be built in on the scoop if it becomes permament.At this stage it can be removed and discarded if it does not work out.
The rudder will simply go through a cut out in the bottom,I hope to seal it with some sort of mylar flap or similar.The rudder maybe shifted to the back of the scoop in the future.All components have been made removable at this stage.
After this boat was designed in 1967 more where built off the same mould with 2 feet added to the stern,These boats have been very popular with over 300 built.Hence my confidence that this scoop will work out.I ran all these modifications past the original designer as well.
Hole cut in the scoop today,spalsh day tommorrow.....

[Guillermo]

Originally Posted by Tactic

...The rudder will simply go through a cut out in the bottom,I hope to seal it with some sort of mylar flap or similar...

I think this is going to bring you problems, because the cut needs to be big (45º each side?) avoiding annoying water entrance at high speeds may prove difficult. You're also going to loss dynamic support, maybe even neutralizing the gains due to the added length.
In my opinion you should bring rudder right to the stern now and forget about that cutting through idea.
Cheers.

[Tactic]

rudder chord is 260mm,40 degrees each side.Cut out is not very big.
Rudder "hole"will not be in the wake of the boat when heeled.(study of this boat and others like it sailing to weather)
As I said I hope to seal it.
No problem blocking the hole and moving the rudder aft in the future.
Rudder will be inclined to "let go" earlier if mounted on the scoop.
I have done my homework on all of this....time to suck it and see.

[Roly]

G'luck with it! Impressive foil on the plumb-er bow too! I for one will be very interested in the result.

I was considering a removable pivoting extension on my S34 on two stainless tabs on sheer at transom,locking down on the transom at keel-end. Serve as longer water line to windward And a swim platform cruising!
Take it off for class. Kinda a fun thing to do seeing as we are rebuilding.

You could be in for a bit of a blow tomorrow ....depression coming in from the NW.

Very curious to your hole sealing device

[Tactic]

First sail in light airs seems positive.Both bow and stern wakes appear far less than before.The hull now makes very little fuss through the water.It was strange to see such little wake behind the boat.
Even in 3 knots of breeze the scoop is partially immersed.My gut feeling is that there is a increase in boatspeed,though of course the proof will be on the race course.I will have a better idea after this coming weekend.

[LP]

Sounds like you might have a winner on your hands. I've been following your thread sporadically. I've built a boat that drags it's stern more than I care. So, I've got a definite interest in the outcome of your endeavour.

I'm glad you've stuck to your guns and kept with your intentions.

My boat's stern makes such a ruckas. I keep looking at my plans and yes the transom is supposed to be immersed. A 1-2 foot extention would make for a much cleaner exit and very likely, improvements in performance. You're results sound very promising. Posts some finished pics.

[Tactic]

Ok ,first results are in.
race day didn't bring much wind and we ended up sailing very slowly for 7 hrs.
Anyway we finnished alongside all the 25 footers that should beat us by quite a a margin.The only wind we got for the day was a 15 minute burst of about 10 knots 2 sail reaching.We were in company with a group of bigger boats and we slowly pulled away from them on this reach.
Again the bow and stern wake on this yacht is now very clean indeed.
I am already convinced the improvments are significant.

[Roly]

That must be real gratifying! To me the forward entry section looks cleaner
and more like newer surfing hulls. I bet the increase in WL is somewhat greater than 840mm on some points of sail.

[LP]

Tactic,

Here is the file I promised.

For anybody else that is interested, this is a proposed extension for my own craft in Freeship form.

[LL2]

That extension looks like it could work! I bet you'll be cruisin' on that when you get it done! I would love to take a ride and feel the wind on my face!

[mojounwin]

Tactic,
How has the further testing gone? Do you think it is a viable option for increasing speed on your design boat?

Have you decided to leave the rudder where it is?

Cheers
Mojo