I broaden the front part of the hull so there it could may be possible to drive it also with an air propeller.
To day I tested the air propeller but I had only a 2 cells lipo and that wasn't enough to make really speed.
The powerful inboard outrunner was driven with 4 cells lipo and made the boat by far much faster.
https://youtu.be/qRDI_Ux5wrI
Update 07.31.2015
To day I tested the air propeller with another motor (STORM 1806 2300kv) and with a 4 cells lipo. Its much faster now:
https://youtu.be/yqHt3sAv-RI
But it isn't optimized yet. Full throttle pushes the bow down and breaks the speed.
Attention
Use the propeller mount with the protection ring only in still air, otherwise it will capsize for sure!
That's why I designed the slim version "propeller mount slim".

The hull consists of 4 parts, the longest is 180mm, or 140 mm if you take another combination. Anyway, I uploaded the complete hull, so you can slice it in pieces as long as you like. This is very easy to do with netfabb or another program. If you have difficulties, I can do it for you, but the smallest possible part is 130mm.

https://youtu.be/VMiSrvlvjjw

Update 08.24.2015
The third try was made with two propellers
http://youtu.be/bLIIah6aCVQ
https://youtu.be/sKNVVHtIEF8

Update 09.25.2015
A new video from a boat prited in PET by abcona:
https://youtu.be/PcmQxUKALkk

Update 12.20.2015
Uploaded sterntube test 7.4mm
This is for a test print if you want to use a sterntube with 6mm OD.

Update 01.03.2016
I added "hull back 190". You can still use a part of the interface. Just cut off about 25mm of the more norrow part of the interface.
So you don't need to print the "hull back 140" and the "hull middle 50".

Update 04.08.2016
"Nanafalke" did split the hull front 180 in two pieces (hull_front_40 and hull front 140) for smaller printers.
The parts are now available for download.
Thank you Sami

Update 06.11.2016
I uploaded hull_front_270

Update Oct 24, 2016
I uploaded hull_bow_129 and deleted hull_front _40

So if you can only print up to 140mm you can use this combination:
hull bow 129
hull front 140
hull middle 50
hull back 140

Update Nov 7, 2016
I uploaded hull_front_140_mod and deleted hull_front_140

Instructions
Printing
Hull:
The best is, if you print it with spiral vase methode. Then you will get a neat surface without any noses from layer change.
I printed with layer hight: 0.3 mm and width: 0.6 mm.
Anyway, you have to print it hollow with one perimeter otherwise it will not fit inside the interface part.
The transom is 2.1 mm thick, that means, you need at least 7 solid layers each 0,3 mm high.

Decks: spiral vase
layer height: 0.3 mm
width: 0.5 mm

Hull interface: spiral vase
layer height: 0.3 mm
width: 0.5 - 0,6 mm
The solid part of the hull interface is to print in spiral vase mode. You will get a closed ring profile. Afterwards cut off the deck opening.
In case it is not going through the middle hull part, so you have to cut it also across and put it in from both sides.
I prefer solids instead of parts with walls, even if it requires some rework, because they are easy to slice with every slicer program.

The interfaces are supposed to put them fifty-fifty in the gap of each hull part. In this way the the hull parts are aligned and easy to glue.
Before gluing you should mask the edges of the hulls with tape to keep them clean from glue.

Holes in support
The holes in the support to fasten the motor mount must be melted out by a slightly modified bowed soldering bolt ( as shown in the photograph)
Don't use a drill, it will damage the material.
The holes in the support to fasten the motor mount must be melted out by a slightly modified bowed soldering bolt ( as shown in the photograph)
Don't use a drill, it will damage the material!
The holes are not designed because holes are impossible to print in spiral vase mode.

Motor
Robbe Roxxy BL Outrunner 2834/8
Unfortunately, the Roxxy motor is not available anymore.
In principle you can take every motor between 26 mm and 28 mm diameter, and the screw holes must be on the shaft side.
It should not have more than 1100kv, otherwise the motor may become hot because it is not cooled by water or airflow of a propeller.

The most comparable to my motor I could find on HobbyKing , is this one:
http://www.hobbyking.com/hobbyking/store/__42501__NTM_Prop_Drive_Series_28_30_1100KV_Motor.html
Maybe also the smaller one could do it, though not so fast:
http://www.hobbyking.com/hobbyking/store/__25076__NTM_Prop_Drive_Series_28_26_1100kv_252w.html

Anyway, if you have a motor already, just give me the measurements and I will look if I can modify the motor mount.

Air propeller motor
STORM 1806 2300kv
http://flyingfolk.com/storm-1806-2300kv-4s-LiPo-flyingfolk-outrunner-brushless-motor
and
http://www.hobbyking.com/hobbyking/store/__61433__DYS_BE1806_2300kv_Brushless_Multirotor_Motor_2S_3S.html

Controller
RockAmp 5A XBEC
http://www.parkflieger.eu/antrieb-und-motor/regler-esc/rockamp/5645/rockamp-40a-regler-5a-xbec-v3-mit-ec3

Receiver
OrangeRx R615 DSM2
http://www.hobbyking.com/hobbyking/store/__28554__OrangeRx_R615_Spektrum_JR_DSM2_Compatible_6Ch_2_4Ghz_Receiver.html

Servo
TGY-R5180MG

Sterntube
4/3 mm, 120 mm long

Driving shaft:
3 mm rod 160 mm long with 11 mm M3 thread on one end.

Propeller
Diameter 35 mm
M4 propellers need a bushing from M4 to M3.
Or you make an adapter as in drawing,

Overall weight
With inboard motor: 570g
With 1 air propeller: 425g
With 2 air propeller: 500g

Coupling
The easiest way is to screw the printed coupling with M2 screws. But it holds not very long.
A better solution is to use rubber shock absorber like this:
https://www.banggood.com/M4-Series-Rubber-Shock-Absorber-Rubber-Vibration-Isolator-Mounts-p-948213.html
Additional you need two 8 mm aluminium rods 15 mm long.
In this rods you drill a hole from the one side and cut a thread M4 from the other side. The hole must be for one the diameter of the motor shaft (3.75mm) and for the other the diameter of the propeller shaft (3.0mm).
You can easiely drill a centered hole if you clamp the drill in a vice and the aluminium rod in a column drill.
Each rod needs two cross bore M2 to clamp the shaft.
I uploaded photographs of the coupling now.