It's not as simple as just changing engines. Because diesel engines operate at a much lower rpm, in order to achieve similar performance, you will also have to change the transmission ratio and probably the propellers as well. Your current gas engines probably produce their best torque at between approx 3000-4000 rpm, a diesel produces it's best torque at a much lower range, probably 1500-2000 rpm.
To achieve similar performance, you will still need engines that produce similar torque, But a much smaller displacement diesel will produce the same torque as a larger gas engine. For instance, your current, 225 hp gas motors are probably V8s in the 5-6 litre displacement range. A 4 cylinder marine diesel in the 3 litre range will produce the same torque.
Ah Merc 225 Ford 302 engine. Your conversion to diesel will be very very expensive ! You don't tell us why you want to go to diesel? Frankly I think that you should just go to newer larger gas engines, much easier and cheaper to get the job done. If you want diesel then buy a different boat!
Well you don't mention the intended application and performance characteristics.
If you don't care how fast your boat goes, you can feel free to replace them with a 25hp diesel, it will motor around the harbor just fine but you won't get anywhere fast.
Your capacity limits may pertain more to weight and draft. Diesel engines are heavier than gas engines, so you may not have capacity to keep afloat with the heavier 225hp diesels, but chances are you can find some 150hp diesels around the same weight as the 225hp gas motors so the boat remains balanced, but loses some performance in the transition.
it's a 30' trojan sport fish…i know i don't need 225hp diesels.. what i'm looking for is the same performance and i'm not sure what hp diesel is comparable
they would need to have a lot of horsepower.