Some how though I am not too happy with the existing Aluminum coupler. I think there is a chance of misalignment. 
Would be exploring springs suggested by Maharana as coupler.

Use of Antenna tubes of Transmitters may be quite suitable to fix 2 mm / 3 mm shafts with a 4 mm U-Joint. You get a bunch of tubes with diff. dia by splitting an antenna. Don't spoil a new transmitter's Antenna, collect one from TV/Radio repairing shop cost you around Rs.50/-

Maharana your suggestion for using a nylon tube is apt. I tried using a thick cable wire outer sheath as a coupler and it is working fine.The problem of vibrations and misalignment are all taken care off. 
I cut the inner of a cable wire such that it left enough space for the shaft in the outer sheath at both the ends. Then with the inner piece in place I coupled both the shaft. It works absolutely fine.
I ran the motor for 3-4 minutes at 50% throttle and the motor is not getting hot, neither is there any wear and tear in the coupler. Will give it a longer trial run this weekend.

On no-load these tubes work fine, but how far it can withstand on full load is doubtful. First insert right kind of tube inside a spring to protect the tube as well as a solid grip on driving & driven shaft. See the fig.
 

I am back after a long break of 2 month. But first the good news... I have been blessed with a baby girl on 27th August.Fatherhood has been keeping me busy. Got to sneak a few hours to progress on this project.
Close to completion.

Maharana thanks a lot for your suggestions on the coupler.
I have made it exactly as per your recommendations.. .It is amazingly stable and takes care of the slight misalignment of the shaft and motor.

I completed the rudder assembly  and the electronics placement in the boat. Also glued the top covers of the boat. Struggled a lot with making a cooling coil for the motor with a SS pipe.Just couldnt bend it to size. Finally have decided to make the cooling coil from copper pipe. Will install the same by tomorrow. Intend to glass the boat as the next step. I think will be able to complete the same the next week end.
Here are some pics of the coupler and the boat.

some pics of boat

This looks great!!!!

hope u will get some time to finally complete this!!!!
cheers
Ashish

Finally completed the cooling for the motor. Used a copper 4mm pipe for this .Crimped one end. filled up the complete pipe with table salt. Used a paper cone to fill up the salt.once filled crimped the other end of the pipe as well. Since the pipe was very malleable could easily turn it on the motor. In case the pipe is not very malleable then it has to be turned on a bar with dia equal to that of the motor. Managed to make this coil after one failed attempt to make this coil out of steel pipe.
Also installed the water pickup. As of now the water pickup is placed between the transom and the prop.
planning to shift it to behind the prop. water outlet is on the left side of the boat.

here are some pics.

Managed to fibreglass the boat this Diwali vacation. Also completed the top covers of the boat.
Fibreglassed with 50:1:0.1 ratio of Resin: hardener:cobalt. Too much of caution proved otherwise for me. I feared the resin might harden up too fast and reduced the hardener.It took a  longer time to cure. Some part took almost a day.
First cut the fibre cloth to size. It can be easily be cut by hand. But gloves are a must.The  resin is too sticky to come. Although not as bad as a feviquick.
Glassed one part at a time and prepared the resin mixture accordingly.Cloth is to be put in place with spots of resin and then using a paint brush to apply resin mixture all over. Then moved on to other parts. Repeated this with another layer of cloth.
Would now be sanding this and then painting and making the canopy.
Should be able to sail in two weeks.

Instead of a underwater propeller, can we use a propeller above the water to push the boat. and this pusher propeller can be steered with a servo. Don't you think that would be a cheaper and a easy option.

hey frnd,
where did u get prop, shaft, radar and all...
pl respond soon....
i am also making a boat u can see in my recent post...

As for the prop. I got the same for India Hobby centre Mumbai. Since you are in Bangalore.. you can also try Rcforall . there are good props available there as well. I made the shaft and the stuffing tube myself at a local workshop.The shaft is a steel rod of 4mm dia. Threaded M4 threads at both ends. also got a tube to suit this.
A better option to the shaft is a cycle spoke. It is also 4 mm and has thread at one end with a nut.I did not use this since the length didn't suit me. What is the motor that you are using. I see you already have a prop.Do you want to change the prop.
The rudder is again made from scrap metal plates from a PC. The rudder guide is made from a broken stapler. a closer look , will give you the idea.

Thanks a lot...
i have a outrun brushless motor! it high torque motor....
ya i bot prop and a u joint from RCFORALL thats going good a i have a shaft of 4mm...
all is i have to fix ball bearings to run the shaft properly.
and will that prop half out of water work? and your out radar is really good...
But how did u joint that shaft and prop i din get u!

Why are you using ball bearing and where are you installing it?
I have made a drive dog.
http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXMWA7&P=0
This drive dog slips over the shaft. I glued it to the shaft with epoxy and prop slides on the shaft and locks itself on the dog the other side you then tighten it with nut.Check out the water pick up picture on the 32nd post on this thread, should give you a clear idea.

Finally managed to complete and sail it yesterday. Following were the outcome of the maiden sail.
It seems to loose power since the nose dived with higher throttle... Reason: seems that the prop is thrusting the motor than the stuffing tube. Would be rectifying the same with a spacer between the prop and the stuffing tube.
Cooling seems to be OK. As suggested by maharana would be changing the water outlet from the side to the back inside water. This would ensure that air does not enter the cooling channel.
Also would be adding cooling for ESC. It seems to get hot.

Would post video of the same with the above changes.

it looks great!!!!!!! congrats after so many efforts and time, u were able to maiden it!!!!!!!!

cheers

Sreekant sir,
Looking good..At last you are done with it....

looking forward to your video.....

hey nice!!
great job on the prop n rudder...i've made the same boat too
how have you coupled the motor and propellor shaft???i always have a hard time with the coupler
if only i had seen this thread earlier...

Just a thought.

Since the boat is 'nose diving' at higher speeds, could it be possible that your horizontal stab is generating a lift at the aft (tail)? That would result in the bow (nose) being pushed below the water line. The higher the speed, the more lift would be created at the tail.

What do the experts feel?

hi mate.
nose diving..........place the battery behind the half way of the hull.
water cooling...this 'should not' be necessary with the brush less motor especially 'with a heat sink on'.
looks good though and nice and cheap build 
a video of this would be nice :-)

regards
don

Thanks guys for your views.I changed the battery position on my second run and the nose diving problem is solved. Now is the craving for more speed hence I am now admist clanging the motor to a 3050kv brushless and a 5000mah battery and a 42mm prop.
Guess this should make it faster.  Just that no one to take videos of my runs
But would be asking ashish to help me with the videos. Will post shortly.

Quote from: @k@sh on April 28, 2011, 06:16:27 PM
hey nice!!
great job on the prop n rudder...i've made the same boat too
how have you coupled the motor and propellor shaft???i always have a hard time with the coupler
if only i had seen this thread earlier...

I have used a Spring as a coupler. To connect it with the drive shaft I have inserted a Cycle spoke nut into one side. And on the other side is a M4 screw. I am using a EDF motor and it has a M4 tap at the centre. Hence this M4 screw from the coupler spring directly gets screw onto it. will post closer pics of the same.

hi mate,
glad it's sorted 
good luck with the new motor fitting.
look forward to video 
regards
don

good boat

Was not too happy with the fixed shaft drive. Hence decided to change it to a flex shaft drive. My second attempt at using a flex drive. Ensured correct machining of the components to avoid all the mistakes done the first time. Found that the coupler machined the first time had a lot of deviations. Used a speedometer wire for the flex shaft. The direction of windings on the flex shaft make apt for a counterclockwise rotation.  Will be  also changing the motor to a 3060kv water cooled motor with a 3S 5000 mah Lipo. Have also upgraded the ESC to a Hobbyking SS 70 amp water cooled ESC.
Got the Strut shaft and the strut holder all machined in SS hence would take care of the corrosion. The Coupler is made of hex brass . Would be now driving a 42 prop on this , from the previous 32 prop.

Drive dog is pending to be machined.  Will post detailed photos of the shaft in some here. Here is the partial assembly.