hull speed

Discussion in 'Sailboats' started by whitepointer23, Nov 24, 2015.

  1. whitepointer23

    whitepointer23 Previous Member

    Can i have an approximate hull speed please. My ketch is 33ft waterline. 10ft beam double ender. 5'6 draft full keel. Hoping for 7 knts. Am i dreaming. Pulling 40 hp perkins out . Have 70 hp volvo ready to go in.
  2. daiquiri
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    daiquiri Engineering and Design

    With a 70 HP engine you should have no problems to attain 7 kt, if the boat is well-balanced and the prop gives at least 50-55% efficiency.
  3. whitepointer23

    whitepointer23 Previous Member

    i haven't slipped the boat yet to see the prop size but it had a lot of thrust with the perkins. Most sailboats the size of mine seem to get hull speed easy with 30 hp so i should have plenty in reserve.
  4. daiquiri
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    daiquiri Engineering and Design

    In fact I was a bit perplexed when I read that you are substituting a 40 HP with a 70 HP. Thought that you had an underperforming boat with 40 HP power, which sounds odd.
    But anyways, a 70 HP will surely get you there with a nice margin for rougher seas too. Of coourse, it will also depend on the displacement of the boat. Do you have that data?
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    FAST FRED Senior Member

    70 Hp if use will pull a huge wake as the stern slowly sinks in the water.

    Double enders can not use Sq Rt LWL x 1.34 as the bow climbing the bow wave tips the stern further down into the water.SL x 1.15 is closer.

    Herishoff tried side fins as did a few Euro boat builders.

    If the 40 hp is sinking the stern now , limiting speed,70 hp will not be much of a help, except to heat the ocean.

    Few boats want to go "hull speed" under power as 1K slower will usually cut fuel consumption in HALF!
  6. daiquiri
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    daiquiri Engineering and Design

    Perhaps you have discussed this case in another thread so I am missing some info... For example "If the 40 hp is sinking the stern now" - where does it come from? Perhaps form some older discussion I am unaware of?

    Anyways... Regarding the 1.34 vs. 1.15 coefficient, it is not necessarily as you say. A lots will depend on the displacement and hull shape.
    For example, the boat in this thread: has LWL=12 m, Displ = 12 t, and was attaing 8.5 kt max speed before modifications, which gives a SLR of 1.34-1.35 . Yes, it was probably dragging a nice stern wave behind it, but the calculated speed was reached with that power.

  7. gggGuest
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    gggGuest ...

    Well, it always depends on hull shape displacement etc.

    A lot of people tend to think of 1.34 as being a magic number that applies to all boats. Of course its not. its always dependent on hull shape etc. But all else being equal, which of course it never is, a boat with a fine canoe stern will start losing buoyancy from the stern wave system (and thus effectively climbing up its own bow wave) at a lower speed than one with a broad flat transom stern.

    The 1.34 is really a rule of thumb approximation for the speed region at which wave drag increases rapidly, and it will differ for every hull shape.
  8. daiquiri
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    daiquiri Engineering and Design

    Sure. But although just a convention and not a physical barrier for max. speed, the SLR does have a practical scope when it comes to design of classic displacement boats. It is an idealization, a number which shows how much are your speed expectations within boundaries of what is sound, or how much have you ventured beyond that boundary. It also tells a person with knowledge and experience where should he approximately expect "sweet spots" of resistance for a given hull, in order to make a ballpark choice of the cruise speed. So, it is one of those well-established rule-of-thumb parameters which are actually very handy.
  9. whitepointer23

    whitepointer23 Previous Member

    I am not planning on trying to push the boat any harder with the bigger motor . I realise it will only dig a hole not increase speed. The perkins needs a new crank and full rebuild and parts are all special order now for a 4107. The volvo is not much bigger in weight and external dimensions and is in excellent condition. If i run it at 2000 rpm cruise like the perkins it will be putting out around 50 to 60 hp. But will have a lot of reserve power for pushing into the wind and tide. I just wanted to know if 7 knts was feasible. I will know for sure once the motor goes in. Thanks for all the replys.
  10. whitepointer23

    whitepointer23 Previous Member

    Other displacement power boat hulls i have owned were as follows. 40 ft fishing boat. 9.5knts cruise with large wake. 48ft tug 7.5 knts no wake. 30 ft fishing boat 7 knts low wake. 30 ft ferro sailboat 5knts no wake. 25ft motor sailer 8knts low wake. 30ft work boat 8knts low wake. So i do have a bit of experience with disp hulls but not much with full keel sailboats.
  11. Barry
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    Barry Senior Member

    What speed were you getting with the Perkins?
  12. whitepointer23

    whitepointer23 Previous Member

    No idea barry. I only idled it around the jetty and it spun a bearing before i had a chance for a good run. But tied up in the pen it had heaps of thrust so i think it must have a good size prop. I bought the boat from a deceased estate so i have no information on it.
  13. DCockey
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    DCockey Senior Member

    It can put out at maximum of 50 to 60 hp at 2000 rpm. The actual power it will be putting out will match the load on the engine, and will generally be a fraction of the maximum available power.
  14. whitepointer23

    whitepointer23 Previous Member

    Well its always max output for the rpm . It obviously varies with load . Should i have said it puts out 0 to 60 hp at 2000 rpm.

  15. JSL
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    JSL Senior Member

    Seems like a lot of power will go to waste and soot up the engine : You may have to de-rate it.
    The speed length ratio of 1.34 can vary depending on the hull for, displacement, etc.
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