Wii it exceed calculator top speed?

Discussion in 'Boat Design' started by eyschulman, Nov 20, 2011.

  1. eyschulman
    Joined: Jul 2011
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    eyschulman Senior Member

    Will it exceed calculator top speed?

    Fast semi dispacement hull. Loa 47.4 ft beam 13.4 draft 3.6 LWL 45.8 WT loaded 31,000lb power twin deisels 350 HP props 22x25. Boatdiesel.com calculator tops out at 17K. useing slightly more than half power available(399HP). at 23.5 Gal/Hr. I don't intend to burn that kind of fuel often and will run at 8-11K at 3-6 gal/hr. So what will happen if I use the remaing 300HP? I know there is no specific answer just looking for some educated guesses.
     

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  2. Submarine Tom

    Submarine Tom Previous Member

    It'll go a bit faster.

    Time will tell.

    Be sure and let us know.

    -Tom
     
  3. MechaNik
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    MechaNik Senior Member

    Use the calculator to plot a power curve (hp/speed). Then predict the curve up past the 17kt top end. You'll see it gets a pretty ugly increase in needed hp.
    Focus your efforts towards the speed range you want the most economy in, as Tom says the rest will be what it is.
     
  4. gwboats
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    gwboats Naval Architect

    eyschulman,

    Talk to your designer, he should give you all the information you need using more comprehensive prediction methods. Website calculators are rough and ready, your numbers look off to me.
    Good luck with your project.

    Graham
     
  5. Ad Hoc
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    Ad Hoc Naval Architect

    Not 100% sure what your question is...??

    But based upon your figures, she'll do around 19knots in good calm seas fair weather and clean hull bottom. Assuming you have selected the correct props etc. But looking at your GA..props look way too close to the hull bottom (looks like they're are almost touching), thus that 19knots could be several knots lower.
     
    Last edited: Nov 21, 2011
  6. DCockey
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    DCockey Senior Member

    What hull shape and dimension data does the BoatDiesel.com calculator use? Simple web calculators, cookbook type formulas from handbooks and similar may be reasonable for boats similar to the ones they are based on, but I wouldn't put much confidence in their results for anything else.

    The boat shown appears similar to a stretched "down-east lobster boat" shaped hull, (commonly called "semi-displacment") though not quite as flat at the stern. The lobster boats tend to have much less of a "hump" in the resistance curve than many modern planing hulls and resistance doesn't rise as fast as "displacement" hulls near the so-called hull speed.

    Looking closely at the GA and the construction photos I think there will be considerably more clearance between the props and hull than is apparent at first glance.

    I wonder if the twin 350 HP engines were specified by the owner. It's interesting that Devlin doesn't show any speed estimates for this boat on his site.
     
  7. eyschulman
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    eyschulman Senior Member

    Yes it is a streched version of a down-east type that Sam built( a 34ft hull lobstyer type) with modest twin power. That boat does perform quite well. The build goals for this boat were comfortable cruise at 8-10K with the ability to pump up to 16k when needed for rapids and getting wife to open markets on time. Certainly we would prefer 26" props but with room limitations 22" will have to do. The Boatdiesel calculator does take hull type into account and I used fast semidysplacement. The exact HP of motors and make is still up in the air I used mid range QSB 5.9 cummins for the calculator. Many of the new motors have silding HP and ratings depending on chip and adjustments. Sam thinks the boat will top the 17K mark but no idea if it will be 2K or 8K more. As for overpowering a down east hull is there such a thing?
     
  8. DCockey
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    DCockey Senior Member

    eyschulman, could you clarify your position with regard to this design since your are using "we"? Are you the designer, employee of Sam's, friend of Sams, fan of Sam's boats? Are you speaking for Devlin Designing Boat Builders?

    What other input is used for the Boatdiesel calculator?
     
  9. DCockey
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    DCockey Senior Member

    Lightbulb went off. Are you the owner?
     
  10. eyschulman
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    eyschulman Senior Member

    Sorry if I confused you I am the owner. The input for calculator included WT-WLL-motor output(rated) various crusing rpms-Trany ratio-prop size and # of blades- type of hull from choices given I used fast semidisplacement. I do not speak for Devlin but this is a one off custom design and I have considerable input from the get go. This is not a design and build a boat for me it is more like here are a lot of parameters can you fill them and if not why. Since I am intimatly involved with many of the details I use the concept of we.
     
  11. DCockey
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    DCockey Senior Member

    Thanks for the reply. I read your note on Devlin's website about the boat. It's an interesting concept and while it may not "mass market" appeal I believe there are others with similar desires.

    Have you thought about reducing the size of the engines? It looks like your requirement is for 16+ knots, and the Boatdiesel calculator is suggesting 19 knots with slightly over half the current power. Smaller engines would reduce both mass and cost.

    As far as "too much" power for a semi-displacement hull goes I think that depends on the specifics of the design including hull shape and CG location. Lobsterboats racing in Maine exceed 60 mph but those boats are considerably wider for their length than your design and probably lighter, and one rolled rather spectacularly several years ago.
     

  12. eyschulman
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    eyschulman Senior Member

    Yes we are considering cutting HP but not mass of motors. Looking at(heavy iron) John Deer Hp range 230-330. There are lighter motors but we are having issues in our area with some of those suppliers. My best guess the boat needs 400 Hp with a good torque curve. I like to throw an extra 100+ Hp into the mix. While > 75% of miles will be done at 8-12K we do want the 16K option and many of the criteria of this design hinge on that. At lower speeds its one gal/20hp one or two motors still need the same Hp the economy is not that bad. This would be a better design with one motor and if on the east coast it would be, but my personal experience with a lost single up in northwest out back forces me to live with the twins.
     
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