Power boat design for economy.

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

  1. erik818
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    erik818 Senior Member

    I don't know of any 10 m - 12 m 4 tonnes monohulls that I would say are economical to run at 12 knots, but there are lots of stuff I don't know. 2 NM/liter diesel or better is good economy, and anything less than 1 NM/liter is unacceptable. I want 12 knots to be affordable also when using it for a full day. In addition to that it must behave well at about 6 knots. I think that a box keel type hull could do the trick but have not yet seen any that is taylored to my specific parameters. I cat would fit the bill, except for not being a monohull.

    Par wrote it earlier in this thread. Those looking for a 20 m powerboat might not put priority on fuel economy. If I were to make fuel efficiency the highest priority I would as suggested modify my behaviour and slow down to 6 - 7 knots and get a boat optimised for that speed. Unfortunately I also want to go 12 knots with reasonable economy which complicates the issue.
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    FAST FRED Senior Member

    "No matter how much money I had, I would be most interested in a boat that is as efficient as possible.

    Efficiency is not mere fuel cost."

    Purchase price ,, loss of money to work

    slip costs ,,ongoing expense

    maint ,, KISS is cheapest,

    and finally resale value.

    Add them all up and some old GRP common boat of the right size will usually be the cheapest most Efficient !!!
    the total coat from steeping aboard to finally stepping off the last time.

  3. sabahcat
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    sabahcat Senior Member


    In this article here http://multimarket.multihull.com.au/index.php?option=com_content&task=view&id=26&Itemid=45 it says that cruising between 13 to 14 knots they achieved 1.3L/nm economy.

    I have engines about half the hp size (65 x 2) in my 50 fter and am hoping for 1nm/L at around 9 knots.

    Recently I bought the tender for it, a 4 metre tinny with a 20hp on back, it does around 18knots with 2 on board but uses approx 1litre of fuel for every 1.3nm travelled.

    Putting that into perspective, it makes the catamarans shifting a house around for similar fuel burn look very efficient.
  4. Don H
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    Don H Junior Member

    What is generally considered as good economy? Frosty's boat drinks 35liters per 20nm or .57nm/liter. (pretty good compared to my 200hp 2 stroke outboard!! i have no idea on whats realistic here other than what i read.)
    Is 1nm/liter reasonably achievable? is 2nm/liter a dream?? i realise there are many factors involved but say useing the boat Sabahcat has pictured. smaller motors? slower speed (8 knots) is 2nm/l a possibility?
    Just like a boat reaches its hull speed and extra HP is just a waste, does a boat reach its most ecconomical and it becomes very hard to save any more?

    Thanks Don
  5. eyschulman
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    eyschulman Senior Member

    Eric I think there may be some false economy thinking here. Say you buy a 15-20 year old semi dysplacement type that can get 12k- 12gal or say 7k at 2-3gal and this boat costs 150,000$. Then you compare the overall costs including fuel for 7years to a custom one off trying to achive max fuel efficency. More than likly the custom boat will cost much more. That is the same sinario usually found with comparing more expensive fuel efficent cars with run of the mill efficent cars. Its a cool dream but the numbers don't add up even if it were possible. For that kind of fuel efficency to mean something the boat has to be run like a comercial unit 24/24/7
  6. groper
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    groper Senior Member

    I don't get it, there's already heaps of fuel efficient cats on the market... it's not like frostys is the only one, open your eyes there's plenty new boats to fill that market... what am I missing here?
  7. whitepointer23

    whitepointer23 Previous Member

    i think tad roberts on the right track, he has some nice fuel efficient designs.
  8. eyschulman
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    eyschulman Senior Member

    I am begining to think that the desired fuel efficency is in real life not very realitic thus few or no boats 40-50ft size with a burn of 6 or 12 li for 12 knots. Is that like asking for a full sized family car at 80miles/gal. It may be technicaly possible but not yet practical.Some years back Yanmar did a promotion on their diesel outboards with a trimaran that went around the world. Very fuel effient but not a practical boat.For real efficency driven people look to the wheel and the pedal powered bike. On the other hand if you wnat to take a few bikes and the rest of your household on the water 4nm/gal at 8k is very good as is .8-1gal/nm at 12k.
  9. Frosty

    Frosty Previous Member

    An efficient engine is not the end or beginning of economy. Economy is a package of the entire machine.

    While some are trying to accept this concept perhaps a surface leg that clear the water like an outboard or mercruiser but a sea going version could be included. Thye idea is a simple one but it does not exist.

    If some one was to make a leg I suppose similar to an Arneson that cleared the water and I did not have to clean the prop but push a button and lower it in, then I would have difficulty if finding an excuse not to to buy one, or 2. Thing is this concept is soo close to being in existence, just needs to move up some more till its clear.

    As it is now I am bagging and yet loosing a never ending battle with prop cleaning. A days notice at least is required to get out, another reason power boats dont move much, and for me is the main problem.

    An idea I would like to be included in this yet fictional boat that seems hard to Imagine by many but not all.

    But wait, tis not just clean props that would be of benefit, corrosion , anode replacement reduced with less electrolysis and all that nasty stuff.

    Radical? possibly,---but does not mean impossible, sounds more sensible to me.
  10. CatBuilder

    CatBuilder Previous Member

    Uh, that's exactly how the props work on the sailing cat I'm building, Frosty. Lots of sailing cats have props that clear the water when not in use and go up or down at the touch of a button on the helm.
  11. masalai
    Joined: Oct 2007
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    masalai masalai

    About Foreign Affair.
    "Foreign Affair is a 47ft Motor Catamaran, designed by Robin Chamberlin. Launched in June 2003, she has travelled more than 22000 nautical miles (40000km), taking Bill & Sandy from far NQ to the bottom of Tasmania and out to New Caledonia and the Loyalty Islands.
    Powered by two 100hp Yanmar diesels, Foreign Affair has a range of 1700nm with 10% reserve." - - - borrowed from here http://foreignaffairskimberleyvoyage.blogspot.com.au/ but many of those interesting designs are hard to find as the designers have retired...

    So from the above with a very nice picture, - - For Frosty,
    . . . . Modify the design to take either sterndrive legs that will lift clear of the water or surface piercing systems likewise (latter part is engineers play) - re configure aft section to handle drives is hull design (Leo?)... - - - - B/L ratio around 13:1 to 15:1 displacement "style" - 20 knot top cruise? 12knot economical cruise - seems logical to go stern-drive and tilt?

    for me, - reduce the weight wherever possible but no "common rail" engines, as they cannot be serviced in remote regions? I am happy with 10 to 12 knot cruise... I am worried about logs and other debris and not really keen on night passages at speed... I stop at night, (mostly), so engine noise is not an issue, as much as it seems to bother Frosty... Longest run for me is Cairns to Samarai of about 480 N Miles (48 hours - - - 2 nights)
  12. Frosty

    Frosty Previous Member

  13. philSweet
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    philSweet Senior Member

    They are called boat lifts. Entire boat goes up and down at the touch of a button. We have marinas full of them. Cheaper than engineering a retractable drive system, and it lives outside the boat, doesn't add weight, doesn't take up space, easy to service. Keeps the entire boat clean. If it breaks, you still have a boat.
    1 person likes this.
  14. CatBuilder

    CatBuilder Previous Member

  15. masalai
    Joined: Oct 2007
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    masalai masalai

    Nice Catbuilder,
    Sadly those are not made now (in the high thrust slow rotational-speed large propellers) and I considered 20hp or thereabouts and all were high revving screamers... Even the 27hp diesel outboards (rebuilt second hand) screamed at 4100rpm and had fairly high revving propellers.... at 3000rpm from the engines, my propellers rotate at about 1200rpm... which is nice, but sub 1000rpm max (3600 engine rpm) would be better...

    I would like a pair of stern-drive legs that could accept up to 50 diesel hp and geared to reduce a 50hp kubota diesel from max continuous rpm to sub 1000rpm at the propeller and spin a 17 spline, 15" propeller but then such a propeller would be almost impossible to find? or horribly expensive? - - The stern-drive / motor could be separated by a double universal jointed shaft. (no steering needed as tight manoeuvring is by engine, and on passages, by rudders hung forward of the propellers), so steering is available when under sail with the legs up... To clear the rudder posts the legs would be offset slightly outboard of the rudders... Fitting would mean chopping off a bit of the tail to accommodate and strengthen the mounting for stern-drive assembly. I could also position the depth sounder/sonar-velocity device on a bracket inboard and away from the prop wash...

    http://www.boatdesign.net/forums/boat-building/my-little-piece-peace-25962-113.html#post486300 current propellers on CNO are 3 blade 15" diameter x 12" pitch - dementia seems to be confusing this little vegemite... I am confused...
    also here http://www.boatdesign.net/forums/boat-building/my-little-piece-peace-25962-115.html#post487692 "'Bit the bullet', ordered and paid in full, for a pair of fixed 3 blade propellers 15 x 12 as nearest to requirement in stock and available in Australia... for Au$1232 including GST... "

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