Nice research on the 7.3.
The one thing you will notice, the 6.9 will be a ton of weight in a 25' boat. ...
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Just know you won't go anywhere in a hurry.
The few boats I've seen with the marinized 7.3's complained about lack of speed, never lack of power or reliability.
The engine I'm replacing with the outdrive weighs about 800lbs and is directly at the back of the boat. The 6.9 will probably weigh around 1200lbs in marine trim, and will be 8' forward of the transom. I suspect balance won't be a problem.
As far as the boat being slow, I guess it depends what you mean by slow. I consider a 20 knot cruise to be more than adequate, and with the drivetrain in question, that won't be a problem. I don't know if you've even gone 40 knots on the ocean before, but in all but the calmest of days, your back couldn't take it, no matter what the hull shape.
I've done the hydro calculations (rough) for my particular hull, and at a 7000lb loaded displacement I'll require about 100hp to cruise at 20 knots. The IDI with a turbo should burn around 1 gal of diesel per hour for every 18 or so HP generated at cruise. So at 20 knots and 100hp, I should be burning around 5.5 gals of diesel per hour. 20 knots is 23 statute miles per hour, so my rough calculated cruise efficiency should be in the neighborhood of 4.2 MPG. I can live with that.
I've just acquired a 6.5L GM marine diesel (total garbage) with all of the pumps, coolers, and a water cooled turbo. I'm parting out all of the 6.5L specific stuff right now, but have nearly everything I need for the actual marine conversion besides the manifolds which I will be fabricating.
This is a great thread.