Long - Skinny Power Boats

Discussion in 'Boat Design' started by SAQuestor, Sep 24, 2004.

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  1. Chris Ostlind

    Chris Ostlind Previous Member

    Thanks for the comments.

    The overall beam of the vaka hull is 4.5', allowing for plenty of room for a side by side seating arrangement. There is a nice spray chine detail running from the bow to transom, that kick starts the small bit of flare in the upper hull panel. All this gives the boat a very slender hull in the water while still presenting a comfortable seating setup in the passenger compartment.

    The fairly high Cp amas remain very slender, yet have sufficient buoyancy for the times when they will contact the water. During the design process, it came up that this design, while principally created for short hop coastal cruising, would also make for a very interesting harbor launch. Something along the lines of a classic car being used for double dates for dinner.

    All we need now are 50's style diners at water's edge with carhops cruising out with the cheeseburgers and cherry cokes.
  2. charmc
    Joined: Jan 2007
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    charmc Senior Member

    OK, the bow on view you added to #88 illustrates that nicely. She just might fit in with grilled salmon and a dry Chardonnay, too. :)
  3. big-boss
    Joined: Mar 2008
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    big-boss Junior Member

    Tad, that is beautiful. Will it be plans you have for sale? Company going to produce them? Very nice. Any more info would be appriciated.
  4. paulewill
    Joined: Nov 2008
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    paulewill New Member

    long and skinny with a flat flat bottom.

    does it have trouble turning?
  5. dobsong
    Joined: May 2009
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    dobsong Junior Member

    Motor Canoe - Any further developments?

  6. Guest625101138

    Guest625101138 Previous Member

    I have been working on a retirement project for a solar boat. Something suitable for coastal hops. I started out with the idea of a long slender hull but it had certain limitations.

    I have been working on a concept (provided by a forum member), which I have called a faux-tri. It is really a stabilised monohull. It behaves like a slender monohull from powering perspective, has the initial stability of a narrow trimaran, is self-righting and has the cabin of a decent size monohull.

    I have done the hull shape to check power requirements and have a rough rendering of the proposed design per attached.

    If I ever build it, it will be built very light with modern materials, probably carbon fibre on Corecell, in flatpack construction. With the target displacement of 500kg it will do 8kts with 900W in calm conditions. The electric motor I have is rated at 4.5kW on 48V and this should get it to 14kts in calm conditions.

    I am really pushing the limits of efficiency to get the best from solar power but the concept could be applied to a conventionally powered vessel that would give good speed at very good economy.

    I am in the process of building a 1/5th scale model to see if it has any vices in a seaway that are not obvious.

    There are similar boats around so the idea is not new but it looks very good.

    Rick W

    Attached Files:

  7. Boston

    Boston Previous Member

    so whats the magic formula to calculate long skinny hull speed
    Im not after 1.34x the square root of lwl
    that one is kinda broad for what Im interested in
    Im after hull drag mostly and what it takes to get something up to a certain speed


    this things got three long skinny hulls rather than one fat one
    should go a lot faster but
    whats the math to figure that out from
    one of you folks is bound to have it right off the top of your head
    or at least know where to find it

    something tells me its more a mater of surface friction in a long skinny hull than wave resistance
    so there must be an average coefficient of friction for say marine paint
    resistance goes up as a square of speed if I remember
    seems like it would have been mentioned in the thread somewhere but I didnt see it
    thanks folks

    never mind
    I just found a hull resistance calculator
    interesting thread though
    was a good read
  8. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    "...so whats the magic formula to calculate long skinny hull speed..."

    There isn't one.

    You'll get empirical data from 'similar' boats and their hp/ton giving certain S/L ratios..but that's it.

    Just do like the rest of us, tank test to get proper results...otherwise, use lots of fudge factors from your 'basic' calculator!
  9. Guest625101138

    Guest625101138 Previous Member

    Your gut feel is correct. The wave drag for the lowest drag hull for any given speed will be around 5% of the total drag if you do not have a length constraint.

    The best examples are rowing sculls. Have a look at the wake they leave.

    There is a friction formula based on empirical measurement termed the ITTC friction line. You will need to determine the Reynolds number to apply it.

    Sadly Michlet is no longer freely available but it will give you drag within 2% of actual in calm water conditions. If you want the underwater shape for the lowest drag hull for your particular parameters then I can do it for you.

    The faux-tri concept looks good to me. I have not been able to arrive at anything with lower drag with adequate stability.

    I keep evolving the idea. My latest variation requires 750W to do 8kts. Should get to 15kts with the 4.5kW motor I have.

    Rick W

    Attached Files:

  10. Boston

    Boston Previous Member

    750 watts is like one horse power
    you could have the kids swim behind and push if you wanted
    that pretty bloody incredible Rick

    I been copying stuff like a mad man into a file called boat building tips and I got a whole section on calculating the Reynolds number
    its slow going but Ill get a decent design outline eventually

    I looked for MIchlet a while ago when I saw it mentioned in several places
    was a futile endeavor
    drove me nuts
    thanks for the tip but I could have used it about three months ago :)

    Ive always kinda liked that thing Rick ( Faux Tri )
    I think you may have a salable item there if you smooth it out some and it looks like your aiming in that direction

    I found this groovy hull resistance calculator
    you guys probably all have it but I thought Ild throw it up just so you could see which one I found

    its kinda odd but it does tell me the hull resistance in lbs pr knot
    so its a good comparison tool for different weights and beams on my 60' project

    I like wood
    and Im loving steam
    Im limiting power to 250 hp max 80 hp cruise
    I want some of those Vawt generators for my passive power regeneration
    ( you can keep solar cells a simple cost analysis places VAWT generators way out ahead of the curve )
    now I want a decent hull form
    Ill worry about getting the look back once I establish the form

    so whats better
    box keel type or trimeran in the say 15 knot range or whatever I can get out of 80 hp and 1000 rpm

    and whats that formula
    ITTC friction line

    Add man
    Ill definitely be tank testing something eventually I just want to math out as much as possible
    my skills are rusty but they're coming back little by little
    been building to long I guess
    and all the engineering that goes into a house is canned
    no there was little opportunity to practice
    this naval engineering is a bit overwhelming ( has this nasty intuition component to it that took some getting used to ) but I got time
    the tank will come in handy eventually but for now
    its all going to be done on paper till I can get to the point were I need to hire someone to make sure Im not just building an expensive coffin
  11. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    "...The wave drag for the lowest drag hull for any given speed will be around 5% of the total drag.."

    Yet another baseless statement from the none naval architect.

    At slow speeds the resistance is wholly viscous, as speed increase it affects the total resistance, since the frictional resistance varies at the power of V^1.825. As the speed increases a series of 'humps' and hollows' in the energy system of the waves occur. This increases the overall resistance, and is known as residuary resistance. The total resistance is a summation of the two.
  12. Guest625101138

    Guest625101138 Previous Member

    The lines of my faux-tri are simply for the next stage before I front my friendly local NA to make it look pretty and determine constructability.

    I will set GODZILLA to produce the the lowest drag monohull for 15kts and a displacement of 5t. It will be quite long. This forms the basis of a faux-tri because the amas just skim the water surface and offer little drag. It will give you an idea of the feasibility and a starting point for design iteration.

    Rick W
  13. Boston

    Boston Previous Member

    well make it happen cause on flat water that thing is going to be a step ahead in boating from what I can see so far

    why is is more beneficial to not submerge the amas and make em carry a share of the weight
    seems like
    wait dumb question
    the volume is more efficiently held with the least surface area by one submerged and the others just balancing

    how do you prevent sponson walking type instabilities at speed
    wouldnt you have to submerge the sponsons at least enough to accommodate people moving around topsides

    I still need to look up box keels
  14. Ad Hoc
    Joined: Oct 2008
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    Location: Japan

    Ad Hoc Naval Architect


    "...Ill definitely be tank testing something eventually.." Hope you have a cheap local place??

    "..why is is more beneficial to not submerge the amas and make em carry a share of the weight.." Resistance!

    "..how do you prevent sponson walking type instabilities at speed.."
    The constant motion of the boat when being exposed to waves will cause a slight roll moment, to either port or stbd. Depending upon how you have designed your ama's, and where you have placed them, will dictate whether the restoring force of one ama is enough to right itself or not, and whether there is sufficient dampening to prevent oscillations.

    "..wouldnt you have to submerge the sponsons at least enough to accommodate people moving around topsides.."
    Basically, yes, you can design your ama's to be 'kissing' the water at DWL, rather than being fully submerged. For example, you can have your ama's "Y" shaped. So a thin section is constantly submerged. Only the fatter part of the Y comes into to play at larger heel angles..

  15. Guest625101138

    Guest625101138 Previous Member

    Attached shows the lowest drag hull for the set parameters of 5t and 15kts.

    The hull is 29.5m long, 870mm beam and 365mm draft.

    It requires 14kW on the hull to do 15kts.

    Now if you think 100ft is too long you need to suggest a maximum length for the next iteration. Constraining the length to something around 80ft should not cause a huge increase in power. The other option is to set a lower cruising speed and accept the penalty running at higher speed. Normayll the drag would be minimised for the target cruising speed.

    Rick W

    Attached Files:

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