Two replacement engines even of around 100HP will likely run $12,000 - 15,000 each. Gearboxes may be included but if not then add another $2,000-3,000 each. With small gearboxes deep ratios are usually not available so the props will need to be replaced at another 3,000 each. There will be no way to practically modify them enough to make them workable.
Just the machinery changes will run:
engines 25,000 - 30,000
gears 6,000
props 6,000
Engine mounts will have to be redone as they will not line up/fit.
There will be lots of other things that once you get into it will need to be done such as electrical and controls may not be adaptable.
Then the labour and all the other costs from the other changes will start.
I am guessing but another 25,000 to 30, 000
I could see you running into well north of $60,000
Plus to boot the boat will be out of commission for a LONG period, suggest 3 months or more.
There will be no way to recuperate that cost on fuel savings unless you run 1,000 hr yearly or more.
I am not a mechanic but have seen enough of these threads to remember the costs suggested by the good mechanic members who do know.
My BIL 25 yrs ago replaced his 50HP engine with a 100HP engine. Had it replaced although he helped where he could to try to keep some of the expense down. The bill was $25,000 give or take a bit.
I have known others who were forced to change and it was a far larger bill than they envisioned.
So believe those who suggest get someone down to assess the engines you have. They may not be tired but just need some help. If they are run at hull speed revs with an occasion foray at higher revs for a cleanup then they may give you long and good service.
Otherwise find another boat with engines more suited to your wants.
