The perfect 10 knot boat/Lowest Cost per Mile?

Discussion in 'Motorsailers' started by RHough, Aug 29, 2008.

  1. RHough
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    RHough Retro Dude

    I've read many posts here and not found a thread on the total cost per mile for motorsailers compares to either pure power boats or pure sailing boats.

    At the large end of the scale there is Mirabella V ... 200ft and 795 tonnes. No doubt a great sailboat 18 knots under sail and 14 knots at S/L = 1. She also has 2 1,000 HP engines.

    She makes her passages under power ... because sailing her is more expensive!

    It seems to me that a good design would return the lowest cost per ton/mile.

    Why add sails to an efficient power boat if the net running costs are not lower?

    Pure sailing vessels have operating expenses too. If the rig has to drive the boat in all sorts of weather a selection of sails must be on hand. The sails and rigging are an expense. At 10 knots the rigging has a 3000 hour service life, the sails perhaps 1000 hours.

    It would seem that not having a huge rig for light air power by using the engine for propulsion would save money.

    Does motorsailing ever result in lower overall costs? Can the sails save enough fuel to justify the added expense?

    It would seem that a motorsailer and modern power assisted sail handling systems would make an ideal passage maker, are these boats limited to displacement speeds?

    If the goal was a nearly all weather 10 knot boat, for the lowest cost per mile, would it have sails or not?

    How big would it be?
     
  2. Knut Sand
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    Knut Sand Senior Member

    Aaaannd here comes one of my favorites (linked to on many an occasion....):

    http://www.setsail.com/dashew/do_paradigm.html

    :D

    Edit... But initially I would expect that sails would be cheaper, if you do not expect 100% performance/ stiffness out of them, accept to repair from time to time, replace them when _really_ worn out.... The boat linked to, have moderate engine installations, compared to the size of the boat, which again give less cost per seamile.
     
    Last edited: Aug 29, 2008
  3. lazeyjack

    lazeyjack Guest

    sailing is not cheap, , yes when you have finished the high cost of the build it become so, gpm, but 200000 k of above deck gear, masts, sails rig, winches on a 55 ft boat buys a lot of fuel
    here is a site
    we have just finished new design same principal different use see the economy
    http://www.boattest.com/oem/general-info.aspx?ID=476&lp_id=295#OverView
    the red boat is the model, successful, that we based our idea on, the render is by bhnautika, an idea by myself and him
    hope to go into production soon
    Mirabella, economical for such a size
    you take a Riviera, aust design, 55 foot, twox1015 shp cats, 390 litre per hour at rated hp, 5000l tanks, , in my view ought to be banned, using more than their fair share of worlds resource
     

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  4. northerncat
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    northerncat Senior Member

    cant go past a 40 ft displaning powercat in my humble op, peter kerr is cruising around the whitsundayas in one of his new cats designed around this concept from all accounts he gets 20 knots flat out 14 on one motor and economical cruising at around 12 -14 knots
    sean
     
  5. Tad
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    Tad Boat Designer

    There is a huge thread somewhere here on sail vs power and which costs more per mile. What the argument boils down to is, "what is the design criteria?"

    Adding a modern high-tech aluminum or carbon spar, and dozens of expensive rigging and deck fittings, the structural reinforcements as required, plus a keel and rudder up to the job, will never pay off. Except when the cost of diesel goes to perhaps $100 a gallon! (wild guess)

    But you can turn down a telephone pole to make a mast, and sew a lug sail out of plastic tarps, and the payback will start very quickly.

    The most efficient 10 knot boat might be a pure power trimaran. The main hull will be perhaps 100' long and 10' wide, ocean crossing weight will be 65,000 pounds, in flat water with no wind she would make 10 knots with 58 HP. About 2.7 gph.
     
  6. TeddyDiver
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    TeddyDiver Gollywobbler

    What's the hurry .. I mean if there's time limit's so why not to fly to destination and stay in a hotel..

    Seriously.. 10 knot boats aren't motorsailers...
     
  7. the1much
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    the1much hippie dreams

    sails are only more economicle if the winds blowing.
    Lazyz,,,i like that first pic.,, reminds me of the mail boats we had in maine. :)
     
  8. RHough
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    RHough Retro Dude

    I thought I had read that. I did a few searches and must have missed it. I don't like starting new threads if I can learn from existing ones. :(

    To TeddyDiver,

    What's the hurry? LOL!!!!

    10 knots is pretty slow. If a passage is planned, there is a trade off in weight of stores and fuel. If the extra weight of provisions exceeds the weight of the fuel to get there sooner, you might as well drive the boat faster. ;)

    Speed is also a safety factor. Weather routing is easier at higher speeds for two reasons, shorter passages do not require as long a range forecast and higher speed gives more escape options. 10 knots vs 8 knots is more than a 50% gain in area you can cover in 24 hours.

    The idea that the costs of the rig and sails are never recovered wold seem to be based on economical cruise speed under power ...

    The goal is 10 knots cruise. That is a S/L ratio of 1.5 on a 44.5 ft LWL.

    If the fuel usage is based on that, is should be much higher than at best economy in displacement mode. Now the cost of the rig is used to offset the extra fuel required by the speed.

    The total cost per mile has to include more than just direct operating costs.

    Allow me to change the question.

    The design brief is 3000 miles per year at 10 knots for the lowest total cost. Range is to be 3000 miles on trade winds routes (average wind 14-15)

    The cost of the boat is factored in on a 10 or 15 year depreciation schedule.

    10 knots at S/L = 1 means 100ft LWL

    Will a 100ft boat burn less fuel at S/L = 1 than a 45 ft boat at S/L 1.5?

    The 100 ft LWL boat might cost 11 times the 45 ft LWL boat.

    At $5 per gallon how many miles will it take to make up the difference in cost?

    Could a 45 ft LWL boat carry enough fuel to have 3000 mile range @ S/L 1.5?

    Will the choice of trade winds routes with higher speed and more reliable wind make having a good reaching rig worthwhile?

    If someone has already looked at this, I apologize ... just point me to the information. :)
     
  9. FAST FRED
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    FAST FRED Senior Member

    That SL of 1.5 on a sail hull will require tons of power , even if the boat is very skinny.

    Simply make the boat longer , but NOT wider and higher , and plan on a SL of .9 under power for low fuel use.


    A conventional old fashoned alluminum mast and SS rigging is well proven , and perhaps could be found used .

    The Parachute kite folks can also make an argument , but I'm not sure how reliable it would be all night with the SS driving.

    A 100 ft lwl boat does NOT have to cost 11 times a 50 ft boat, tripple maybe , but that's it IF its the same beam and hight.

    FF
     
  10. RHough
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    RHough Retro Dude

    Before I crunch numbers to check ...

    Dave Gerr has a revised "Hull Speed" formula:

    The formula is: V = 1.24*L^1.433 / D^0.311

    V= Speed Knots
    L= LWL feet
    D= Displacement pounds

    His formula gives S/L = 1.34 at a D/L of about 350

    For D/L below 350 the Gerr formula gives a higher number for displacement limited speed.

    .9 S/L is 10 knots on a 69 ft LWL
    1.34 = 11.1
    As it happens 10 knots is 90% of "hull speed"

    At D/L = 100 Gerr predicts 16.4 as the limit. 90% of that is 14.7 or S/L 1.32 ...

    Any idea how the D/L effects the fuel curve? Gerr's formula suggests that the near linear part of the drag curve extends past S/L = 1.34 for lighter hulls.

    Just so we all understand my scale for economy ... the boat I take delivery on in December gets almost 1 MPG at cruise speed of 29 knots, and at 30,000 lbs and 850 hp, she is considered an economical boat. :(

    I suspect that no hull designed to run S/L 1.5 as "economy cruise" will sail worth a damn. And no sailing hull will be effecient under power above S/L = 1

    Just kicking ideas around ...

    Mac65 with a 200 hp engine ... looks close? Cabo to Hawaii at 10 knots for 600 gallons?
     
  11. TeddyDiver
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    TeddyDiver Gollywobbler

    :D Well it's so slow in the world of aeroplanes that no harm done if it's halfed down once more.. I'm counting on continous 5 knots with my ms (hull speed 7.5) just becouse you can't go straight and there's allways something to see. (taking a dive with dolphins..) More than a safety factor I consider speed more an uncomforty factor and believe having the boat and it's crew being better prepared against the weather when running away isn't the main strategy.
    And maybe the most important reason.. Building a boat alone takes a lot of time and with limited funds ones gotto accept certian limitations, like moderate speed:cool:
     
  12. Tad
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    Tad Boat Designer

    Adding length is the cheapest speed you can buy.

    The 45' boat will have to weigh 30,000 pounds, be 11' wide, and will require at least 90hp at the prop (in flat water, no wind, or current) to manage 10 knots. That's about 4.4gph, or a minimum of 1320 usg for the 3000 mile voyage.

    Make the hull 60' long, S/L of 1.34 at 10 knots. Weight can go up to 40,000 and beam drop to 10', with required power at 76hp for 10 knots. That's about 3.7gph, or 1100 usg for the voyage.

    Say the sail rig knocked 20% off the fuel consumption, about 220 gallons less (per year) for the 60' boat. At $5/gal that's $1100 per year or $11,000 over 10 years. The addition of a sail rig may or may not cost more than $11,000.
     
  13. Tad
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    Tad Boat Designer

    Both statements above make no sense. Your suspicions are incorrect. There are far too many variables involved to make such blanket declarations.

    Certainly many (most?) sailing hulls can be powered to S/L 1.34 at better than 1mpg (your scale).

    How does one evaluate "worth a damn" as a measure of performance. TP 52's and VOR 70 hulls are optimized for far higher speeds than S/L 1.5. I would suggest these hulls would make great performance motorsailers, but of course there are some other considerations.
     
  14. RHough
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    RHough Retro Dude

    Sorry ... yes ... the statement was not qualified and I should have either shut up or thought it out ... :(
     

  15. FAST FRED
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    FAST FRED Senior Member

    "The addition of a sail rig may or may not cost more than $11,000."

    By NOT requiring a fully found sailing vessel, giving up the ability to "beat off " the proverbial lee shore in a gale , sailing gear becomes CHEAP.

    The mast you require are found holding lamps by the side of the road , or if you need really tall ones at the airport. Since there heavy wall aluminum far less rigging would be required .

    The old square sail was quite efficient ( Nelsons tacks were 90deg) especially if simply use for reaching and running.

    To simply cut fuel use and do away with a hugely expensive and complex stability system, a square sail rig would do fine.

    So long as its considered the secondary propulsion , although it would be a fine get home.

    Biggest added expense would be for a controllable pitch propeller ,CPP , at the newbuild stage to actually take advantage of the sails.That might be $10.000 but would help 100% of the time under power OR sail.

    FF
     
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