New 55 Hatteras SF, 1271TI's

Sounds like you're right on the money and have 50 rpms to spare for more weight, fish, growth or whatever. What speed does she cruise at? The one I ran cruised 22knots at 1950rpms, no owner items, light fuel, and just a hard top.

The formula: "1.3 x square root of waterline" comes to mind. Not sure if that applies to displacement hulls only.

That's all the number ever is, is a rough guide. Might as well just pull an approximation and go with it. What is it you really want to know? Your ideal speed for fuel economy? If so, you'll need to try and measure different speeds.

I agree I would like to know the waterline length but not to get more precision in hull speed as it's already such an imprecise and crazy estimate unless you have naval architects somewhere taking the exact shape and design and doing simulations on advanced equipment. When you find out it's true Froude number. Even then I don't know what you'd do with the number.

1.3, 1.34, 1.35, 1.2 x....Then is your ideal speed for economy 1 knot less or 2 or exactly or what? How does it change with load?

You cannot calculate hull speed as you have a planing hull or semi-displacement hull. Sportfish usually have a much higher "hull speed" than a motoryacht of the same size as they're designed to drag lures and such up to 10,12 knots and have a clean stern wake. What, you're going to have to do is put the boat on autopilot in a fairly calm sea, and then bring it up 50 rpm's at a time and record your GPS speed after a minute when the speed settles, 9-10 knots should be do-able on that boat without starting to raise the bow and push too much water, and 900-1000 rpms should get you there. With 12v71's TI's 800 rpms a pair will burn 7-8 GPH, 1000 rpms 12-13 gph, 1200 rpms 25 GPH, 1400 rpms 40 gph 1900 rpms 80 gph. Somewhere along the way, you'll notice that another 50 and then 100 rpm's doesn't give you hardly anymore speed until you get the boat out of the water.

You can use the waterline displacement formula to calculate theoretical hull speed for any mono-hull form, planning, displacement, semi-displacement.

It is just that, theoretical, as stated by others. With a planning hull, I would suggest you go to wide open throttle first and work your numbers back down, with a two way average. It is better than pushing the hull to a number.

Assuming your bow is raked at 45 degrees (probably less for your Hatt, and your freeboard is 7 feet), than an approximate waterline is 55 - 7 = 48'. Run the numbers between 47 - 49, the results won't vary much.

How much does your Bertram weigh? I would have guessed the Hatt would be heavier; shaving a little off my 56MY, I'd guess the Hatt 55SF at around 70-72,ooo with fuel and water and not much else. Your speed/RPM numbers are close to mine, (650hp 8v92ti) but just tad higher.

This..... 1.3xsqr waterline between 45 and 50 is tenths of a knot. Negligible on this side of the bar.

Sportfish from my experience tend to defy the standard hull speed rule of efficiency a bit and many times a 55' Sportfish will still be pretty efficient at 2 knots over the general hull speed formula. I would guess that 1000 rpms will get you around 9.5-10 knots whereas on a motoryacht or my express hull it will be right around 8.5 knots. Most Sportfish tend to have a very flat aft section which helps them provide more foward momentum before trying to dig the stern in and bow up trying to get onto plane. But, go out and try it and let us know your results. From what I've seen 1000 rpms with 12v71 TIs is a compromise between burning too much fuel and running under thermostat temp and not burning clean. 800 rpms tends to get you almost twice the range but coolant temps run much lower and tend to run dirtier over time (DDEC's are an exception here but doubt you have DDEC's).