I have think that hybrid shaft too, aluminium can handle torque and power easily, only problem is longitunal bending strenght. Carbon UD tube inside aluminium tube could work, but carbon-aluminium do somekind of eletric corrosion effect. There have to be aome inulation between, like fiberglas layer. Maybe carbon tube don't have to be bond to aluminium, then their different mechanical properties doesn't fight each other, who knows.
aluminium shafts give me base calculation line too, all this "knowledge" is just calculations and thinking, no real data yet. I can't be sure is 3500rpm limit real or not, it may be more or less. Next summer we will be wiser.
I have 18x28, 17x33 and 16.5x34 props, all four blades.
​​17x33 should be top speed props if this fat girl get on plane with them. Four speed transmission make really big help for everything, you don't have to worry spooling or anything at lower speeds.

Put 2x45 g aramid layer betwen Al and carbon for insulation.

3D CAD design had begin in cabin. CAD means CArdboard Design. I can't make this complicated planning just inside head, I have to see and feel how you can move, how much free space, etc there will be.

https://cimg6.ibsrv.net/gimg/www.off...5ba83aa655.jpg

https://cimg9.ibsrv.net/gimg/www.off...799c6c5030.jpg
Under sofa I wonder to fix roller blind mechanism so cloth can be stretched to roof making own cabin for Minnie (the little girl). Backrest could be lift up so if sofa is sofa backrest is down and when sofa is as bed, backrest can be lifted up so bed get wider and backrest cover hull windows to get cabin darker to sleep.

https://cimg0.ibsrv.net/gimg/www.off...6867d2a1e5.jpg

https://cimg5.ibsrv.net/gimg/www.off...56b63841c9.jpg
Next winter I blow that sh#tty design toilet off and build better with real shower possibility. It may be 15cm longer too but there is still good space for storage between sofa/ little girl cabin. Toilet have to get good heating and ventilation so it can be used as a drying cabinet too. It will be nice to get clothes dry as quick as possible if trip get rainy.

https://cimg1.ibsrv.net/gimg/www.off...07bf96ca9e.jpg

I really want dinner table and seats, table is too small and little too low now but close to good. Storage is more important than fancy round shapes, Real one won't be this square but close so I can make drawers in every empty space. I want convert this boat from dayboat to real cruiser so I can make two week trip with my family on the Finland coast or across nordic countries.

https://cimg2.ibsrv.net/gimg/www.off...e8e3bdbf2b.jpg
Floors weight 2kg a piece, not bad for 2meters long boards, they feel rock solid.

I think Fountain 47' could be convert to fast, good rough weather cruiser boat where you can live vacation time onboard. What you think? 120mph home on water, maybe, maybe not, who knows, but nobody had ever tried build one before this.

This thing will be soooo cool.

Early Cig cabins were well setup.


https://cimg4.ibsrv.net/gimg/www.off...19ab0b008c.jpg
That is what I'm looking to do. Have the table move down to become a berth.

One thing I found out...the little ones don't stay little for long. Better to plan for future size needs.

I almost forgot to ask:

How is it handled in Finland regarding the registration of a vessel that uses a non-certified engine?

I’m asking because I had major difficulties registering a speedboat that has a home-marinized LS 6.2-480 engine installed.

Thats no problem this old boar, I don’t be sure how it works for newer boats but you can just write what engine you have in registeration net site. I register this for Polish Wola V12 engine, it’s same than transmash, these engines have any serial numbers anyway. Insurance is different story.

I’m asking because I personally had real difficulties, and perhaps the following words may help. I resolved the complication by obtaining a German temporary residence, joining a German nautical club, and then not registering the vessel as older than 2008. I obtained permanent German registration, and German insurance companies insure it for me.