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New 55 Hatteras SF, 1271TI's

Discussion in 'Hatteras Yacht' started by g collis, Oct 26, 2015.

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  1. Capt J

    Capt J Senior Member

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    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.
  2. g collis

    g collis Member

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    I'm not sure about her cruising speed at 1950. I ran her at 1600 all the way back, with the exception of two wot runs and I was making 15 knots at 1600. Had to return home after I got her back to the marina. We are going back down right after Xmas or a few weeks and I'll glean more info.
  3. g collis

    g collis Member

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    Hey Ralph, we will move more clothing, food etc. I have most of the tools on board already. I figure that with a full fuel load, she is about as heavy as she'll get. Few hundred pounds of clothing and food won't amount to much. I am delighted with how she is running. According to the tacs, her no load idle is approx. 600-650 rpm
  4. Capt Ralph

    Capt Ralph Senior Member

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    I may have typed wrong.
    Un-loaded (not in gear), max rpm. I think you will find 2350 also.
    Old shops use to make tune up reports for future comparisons.

    Please keep in touch on what that loaded 2350rpm does on your way home full of cloths, provisions, up hill, against the wind. If it comes down to 2300, that's perfect. If it stays up at 2350, get more beer, bait & rum on board and let us know where your at....:)
  5. g collis

    g collis Member

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    Will do Ralph. Are you still having issues with the "goo" in your tanks? I have an idea on how to get it out. Give me a ring when you have a chance. Phone Number Removed Gary
    Last edited by a moderator: Dec 11, 2015
  6. g collis

    g collis Member

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    Anyone know what the Water line Length is on a 1980, 55' Hatteras convert?
  7. Capt Ralph

    Capt Ralph Senior Member

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    Not a clue.. What's up?
    Please don't say you trying to calculate hull speed...
  8. g collis

    g collis Member

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    Yep, trying to calculate hull spped.
  9. Trinimax

    Trinimax Senior Member

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  10. Capt Ralph

    Capt Ralph Senior Member

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    I asked you not to say that. LOL

    For a varying hull form, designed for planing, the resulting number may be a rough guide rather than an exact number.

    Maybe some old ad may state the WLL.

    Your surveyor may have a resource for an exact wll.
    Last edited: Dec 18, 2015
  11. Oscarvan

    Oscarvan Senior Member

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

    olderboater Senior Member

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    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?
  13. Capt J

    Capt J Senior Member

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    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.
  14. Capt Ralph

    Capt Ralph Senior Member

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    Were a good bit heavier than your 55. But this may be a helpful guide.
    Bert Fuel 001a.JPG
  15. PacBlue

    PacBlue Senior Member

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    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.
  16. Caltexflanc

    Caltexflanc Senior Member

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    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.
  17. Capt Ralph

    Capt Ralph Senior Member

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    The specs are;
    65000 base dry, 87500 loaded & wet. Were also 18 wide.
    Deep V and floating, draws 5.5 feet.

    Were live-a-boards with clothes, tools, spares and 3 kats. Also bridge hard top, second gen-set and lots more batteries.
    Been on for over 12 years now. lots of stuff (junk).
    It takes a while to make the last couple of knots, but that sheet is not to far off.
    We have only seen 2300 a couple of times, usually near bingo. We can turn 2200, 2225 sometimes 2250 in reasonable time.
    Were LSD and only push these old DDs when the inlets are bad and once a day (long runs) to blow some slobber out.

    Yes, your 650 hp - 92s, should turn the same numbers under load. You should be moving faster at the top.
  18. Oscarvan

    Oscarvan Senior Member

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

    g collis Member

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    Many thanks for all this good info. Been under the weather for about 3 days and haven't looked at the computer. I appreciate all of you guys providing good ideas.
  20. Capt J

    Capt J Senior Member

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    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).