No at 58 mph my best so far I am at 4100 RPM, my no load Max RPM is 4300, why do you ask? Or it was 4300 before I started screwing with the injection pumps I should test it again and see if it has increased.

That is really close calculator, plugged in my numbers,came out exactly what the gps said at WOT

So it looks like for every 1000lbs the speed drops by 3mph.
A 38Fountain is 3000#`s lighter than my Flatdeck so it should be 9mph faster.
I think that`s pretty accurate.

With a diesel if you run close to the rpm limit its not using all the HP the engine makes. That's most likely why it don't slow up much with the extra weight.

+ a faster bottom.

I think I understand what you are saying but I'm not sure its entirely accurate, since it is Turbo it will make max boost and max power and lots of different RPM's just depends on the load, for example I can get up on one engine and if I leave it wide open it will rev up to about 3600 RPM and making the same boost as both engines make when I run wide open with both at 3900. Max RPM at WOT was only 3862 when I got the boat. I have adjusted the aneroid and max Rpm screw.

Feel free to educate me further always interested to learn more.

Nope the bottom is the same I just cracked the code, its all about the weight

Not good at explaining things. So here goes if diesels are under proped it might come real close to the rpm limit with out hitting the no load #. 3862 may be as close as you can get. That would give a advantadge to more weight to the boat with out loosing mph. Example a mack truck no load rpm is 2300. With a full load it might only go 2150.

Some smarter than me could probably ball park the number, but I would think it's actually extremely hard to calculate an exact number. Similar to any powered vehicle the weight will cost you the most when you need to accelerate the mass. For cars this is stop and go traffic. For boats using my imagination I would guess the time to plane will increase and cost fuel in that transition as well as any time you are trying to gain speed. Energy use at cruise is the sum of drag and thrust... So you need to calculate how much extra drag results from the weight. This is where I don't really know how to proceed. I guess there is a sliding coefficient of friction between the water and the hull, so additional pressure against the water would result in more drag there. The other harder to calculate values would be if shifting the CG of the boat resulted in other changes in drag. Would you now have to run at a more or less optimal drive or tab trim to maintain the ride you had before? I'm not really a physicist, so maybe all these factors are easy to simplify.

If he snorted the 1500#'s of cocaine just before he left the dock would it still weigh 1500#'s? I'm thinking it would at least make you go faster.