diesel repower question

i want to put a small diesel and a direct drive into a 1975 skipjack24
and have an economical, long distance costal cruiser. i would
like to cruise at about 25mph.

i've used the equation " The Square Root of (Total Shaft Horsepower/Weigh)
x Hull Rating = Speed" from the site http://www.racedaymarine.com/speed.htm. i think this speed equation is for gas engines.

i have guessed my weight being 4500-5500lb (the boat will probably be closer to 4500) and hull rating being 225. at 5500lb the hp requirements are 68hp.
at 4500lb 56hp.

i am looking at an isuzu 4jbi 70hp, a cat 3054t/perkins 1004t and a mb m617t.
the cat/ perkins and mercedes rate at about 110-120hp. they should have plenty of power. they are avaliable and affordable. the isuzu and the cat/perkins are new surplus. i plan to do my own marinazation with a heat exchanger and a dry exhaust.

would i do well with a 70hp isuzu?

as far as tranmissions go, what rpm do i need to aim for?

i have a lot of repair work to do to stringers, plywood & glass. i included
two pictures of sister ships. sort of what the boat looks like now & what
i am aiming for.

thanks for any help

don

 

you need H.P. to cruse at 25 mph.I think you should look into yanmars 4LH line of motors they are availible with out drives also to fill that hole in the back of your boat.Home built marinizations are nasty just scrounge up the money and do it the white way and be done with it.all that crap you build your self will be riped out and replaced buy the next owner any way.Have your self a nice boat with a nice motor under warinty.