Power boat design for economy.

Discussion in 'Boat Design' started by Frosty, Apr 11, 2011.

  1. longcours62
    Joined: Jul 2011
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    Location: France

    longcours62 Junior Member

    Power trimaran

    When we were in the phase of the concept of our actual 'passagemaker' we though "multis"
    catamaran like Santorini 65 (Nivelt design) and even we hesitate with a power trimaran.
    If we sold our actual boat we will try to built one 'for see'.
    But we just know the metal work and for a 20 m length 'passagemaker' with some reasonable hull structure and plating thickness I am not able to built light enough :?:
    Lot of type of trimaran exist from the very light Ilan
    http://www.nigelirens.com/ldl/yachts-product.php?id=14&idcat=3
    http://www.trawlerforum.com/forums/s3/power-trimaran-5988.html#post84578

    For us the problem of the overall size it is not a main problem because the last time we went in a Marina it was 7 years ago :D
    For a couple 21 m lenght beam of the main hull at the deck level 3 m at waterline a small 2m , draft less than 1 m, beam overall 7/7,5 m air draft betwen 2,8m , one single engine 250 hp and the weight ....it is my problem I can doing that at less than 44000 pds
    Sure it is a (too ?) big surface ,but for voyaging it could be interesting, for nearly the loa of our actual boat we get (minimum) 40% less volume but it still enough for a couple and a (big) dog.
    Compared with our boat we can save some tones on the alloy, ballast, engine, arrangements(less cabins !! and wood less tick)
    The target would be 10 kts for one Liter per NM (44% less than our actual boat 1,78 lt per NM at 9,85 kts) and the possibility to reach a higher cruising speed than our 11,3 kts at 2300 rpm.(15/16 kts ??)
    We check carefully compared to our boat we could save 12500 kgs BUT still 20500 kgs
    for a power tri it still a lot (not a lot for passagemaker)
    Frequently I got same answer than Frosty from professionals : "not possible" except one time 15 years ago a builder and motor maker said : we are interested by power tri for make promotion of our products" until we understand they are interested by sold engine and the hull at full price AND get some publicity .
    Clearly a power tri you built her and must keep her ....all your life :p

    But even at 20000kg with Lw 20 m a beam at wl of 2m it must be more economical than our boat , to built (less alloy, ballast, wood, engine,smaller rope chain anchor etc) and under way if ti is possible to reach 1nm per liter at 10 kts
     
  2. Frosty

    Frosty Previous Member


    Could some one explain what FnV 2.2 actually means and why it is relative and in what area does this affect.

    Ad Hoc?

    I am an engineer not a mathamatician, so any numbers I have to admit are meaningless to me but the front of my boat with all its superstructure and flybridge must be like pushing 4 or even 5 8x4 sheets of plywood through the air. Now at 20kts I have difficulty in engineering 'imagioning' that it would be only 5% of power required.

    If you could rig up 5 sheets of ply on the front of your car I doubt you could exceed 30MPH.

    So un- aerodynamic is it that the air can not get down the tunnel and will pick up any spray from the bow and aim it at my face 12 feet up on the bridge.

    Typical power boat superstructure yet are we sure that is only 5% of power.

    A lifting foil device would make the tunnel bigger.
     
  3. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    FnV is the volumetric Froude number. It differs slightly from the standard Froude number, based upon length. Each has their own merits and some researchers & NAs prefer one notation over another.

    FnV is simply = volume (of hull) / Sq.Rt of [ g.Vol ^(1/3)]

    So your boat if it is roughly 40 foot waterline = 12.2m
    If she displaced around 20 tonne, so the volume is roughly 20m^3 (actually just 2.5% of you bother for the SW density, but for this example, not worth it).
    If she is doing say 20 knots = roughly 10 m/s

    So the FnV of your hull is 10/(9.81x20^{1/3})^0.5 = 1.9

    If you did this the Fn (length - not volume- way) = 10/(9.81x12.2)^0.5 = 0.9
     
  4. Frosty

    Frosty Previous Member

    So what FnV numbers do you need for a foil.
     
  5. Ad Hoc
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    Ad Hoc Naval Architect

    Accordingly to sottorf, he quotes excess of 2.2, but you'll need to ask him. It is his figure that is quoted, not mine.

    So if your hull is 14 tonne, if i recall (??), can't recall the length, but lets say 40foot = 12.2m

    Then the speed must be greater than 20.8 knots.
     
  6. sottorf
    Joined: Sep 2007
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    sottorf member

    For foils to work Fv > 2.2 as a rule of thumb. It is possible to make HYSUCAT work for lower speeds in some cases but those are exceptions to the rule.

    The reason for this is: the hydrofoils do not create enough lift at low speed to be able lift the hull enough out of the water (lift proportional to the square of the speed). The added resistance of the hydrofoils is thus not offset by reduced hull resistance. For Fv<2.2 total resistance is usually between 2-10% higher than the same hull with out foils.

    The best solution for making an economical power cruiser is to keep the boat as light as possible and use the most efficient propulsion possible.
     
    1 person likes this.
  7. vandutch
    Joined: May 2012
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    vandutch Junior Member

    Powerboat.

    What is reasonable for you in economy?
     

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  8. Frosty

    Frosty Previous Member

    yes your spot on,-- 40 foot water line 12 tons( 14 wet), top speed is 24kts
     
  9. vandutch
    Joined: May 2012
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    vandutch Junior Member

    It took 4 pages

    I was reading commentaries "elegant and swift hull", "light weight construction aerodynamic efficiency" "clean and trim with design" without the "windy air tunnel" where one can lose their hat, with fuel consumption around 20 kts @35L/H and I thought VanDutch might have a slot but reading more thoroughly its not a catamaran, and that is what is missing on the market. VanDutch is avant guard so who knows perhaps they'll take on the challenge...
     
  10. Frosty

    Frosty Previous Member

    What do you mean by making them work. Surley a foil of any shape would improve a bit but not necessarily by enough to make it worth while.

    Some would work but maybe not as well as others.

    So I assume you only tolerate a certain improvement to use the word 'work'

    So then--to work, what is your measurement of improvement.
     
  11. masalai
    Joined: Oct 2007
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    Location: cruising, Australia

    masalai masalai

    Hello I have been "off-line" :D :D cannot afford satellite link... :p

    Oh dear stuck in a rut still... Foils are used in High-traffic and "safe regions" - too many floating (or semi-floating) logs and containers and so forth and fifth - - - - - Been tried in Melanesia with not pleasant consequences - even a basking turtle is a significant risk... 'Hu-suck-cat' - is that the correct pronunciation? - - - - Would work very nicely in some cases / regions but not necessarily where Frosty and I "play'... a bit too much flotsam & jetsam....
     
  12. sottorf
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    sottorf member

    Frosty: be making them work, I mean a resistance reduction and thus an improvement in efficiency. I have tried many different foil systems and configurations. To get hydrofoils to work for lower speeds requires mounting them below the keel in order ot lift the hull really high out of the water. This is not practical for most leisure boat applications.

    Ma-suck-lai: What happens when the foils hit someting is a common question. I have no idea what kind of HYSUCAT you have experience with but if properly designed, you can run them into most debris without sustaining damage that would not normally damage the hull. Recently I saw an aluminium HYSUCAT running up on rocks at about 30 knots. They floated the boat off at high tide and the boat went back into service the next day after some checks -no damage to foils.

    Secondly a properly designed HYSUCAT passenger vessel can get Class approval to carrying paying passengers in debris infested waters such as those of Hong Kong. Do you understand what that means?

    HYSUCATs work well if designed properly. If they did not there would not be over a 1000 boats and counting that have been built in sizes up to 36m.
     
  13. masalai
    Joined: Oct 2007
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    masalai masalai

    Then come and ply the waters between Kavieng, Rabaul, Madang and Lae in PNG or Western Province in Solomon Islands - - - I DARE you - - - as earlier efforts by high speed vessels reliant on foils were adversely affected by 30+M x 1+m diameter logs and equally sized trees floating around after regular flooding rains - encountered along the routes Also try in areas renown for marginally floating containers (North NZ is a recent example)... PNG now uses old 'displacement' boats for domestic runs... Even then, with regular "accidents" in areas renown for rough seas....

    Besides, you are off topic - "Powerboart design for economy" - and that - in the 100 tonne range is what is needed in Melanesia as the locals cannot afford air-fares.... Does your 'sortof husuckat' remedy that need? - - I doubt it ever would...... My boat does 10knots at 6Litres per hour for a little less thanr 1.7N Miles per litre . . . 40ft loa, 21ft beam and load potential around 2 tonnes for short 100 mile runs and a passage making capacity of around 2000 miles.... But it is my home and where I live...
     
  14. FMS
    Joined: Jul 2011
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    Location: united states

    FMS Senior Member

    If you don't know how to even spell the name ... :confused:
    The SOR certainly has changed from an efficient powerboat where 'aerodynamics must be a consideration' to a 100 ton yacht now.
     

  15. bernd1972
    Joined: Mar 2011
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    Location: Laboe, Germany

    bernd1972 Holzwurm

    Well, if we┬┤re talking fuel economy, what do you think about 65ft. travelling with:
    1l per nm @ 8,5kts
    1.6l per nm @ 12kts
    3.3l per nm @ 16kts

    still good?
     

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