It's the fun of doing these projects.
Some people race bikes/boats/horses/cars etc. Some spend ££££s on exotic holidays with nothing more than a few photos to show for it. Over 10 years £75k isn't a major spend at £600 a month (a few meals and beers each month and that's easy done)
At the end of it you have an awesome bespoke boat that you can enjoy with friends and family (yours can even double up as accommodation/ apartment at weekends in new marinas) Selling at £25k is great as you even get some equity back out of it. (Unlike a friends once £80k 10year old Range Rover) Somewhere in the 400s p/m I reckon is excellent value to have a boat like yours.
You can probably guess from the above how many times I've tried to justify a project
Cheaper than therapy