Hull shape design advice for 11.5m, 8kts, Fn=0.4

Discussion in 'Boat Design' started by ram68ocean, Jul 12, 2021.

  1. ram68ocean
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    ram68ocean Junior Member

    Gday, I have done a few hull concept designs for a vessel with these particulars:
    LOA max. = 11.5m
    Beam max. = 2.2m
    Service speed = 8 knots (averaged from operational speeds ranging from 5 to 12 knots)
    Thus, Froude number = 0.4 approx.
    Estimated initial displacement required at DWL = 8500-10500 kg
    The next step would be to rank the different hull designs and select a few for the next design stages.
    For ranking the hulls I would ponder fuel efficiency, reduced motions and internal space.

    My questions are:
    1. - What methods can I use to compare and rank the different hull shapes?
    2. What methods can I use to estimate required power?
    3. - From your experience what hull shapes would work better? Any examples? For info, I am doing a similar vessel research.
    4. Any suggestions to take in consideration such as typical C_b, C_p, draft, etc.?
    Thanks!
     
  2. Ad Hoc
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    Ad Hoc Naval Architect

    Ok..so you have a hull that is 11.50m...check
    Displacement, let's use worst case = 10.5 tonne..check.
    That gives you a length-displacement ratio of 5.25...this is not ideal, as it is on the "low" side.

    So, the first thing you need to understand before going any further...the LD ratio of 5.25 limits what you can or cannot do to improve whatever performance aspects you seek.

    Thus, you really need to DESIGN your boat rather than focusing on an absolute or illusion that hull shape will play a significant part. If the finished deign, works, then it works. The notion of optimising the hull shape is a red herring.

    In simple terms, what is your SOR....your statement of requirements?

    Since if the hull shape, whatever it ends up being, satisfies the SOR... box ticked.. check!

    But then you'll say, oh.. but surely hull shape B is better than A?...well, if you look at small decimal places then of course (just like on any calculator, two computed numbers will always be different!). But in the real world, with a moderate to low LD ratio, there is very little you can do. Other than make sure the front is pointy, the back end has a nice gradual shape for good flow into the prop. That's about it. Anything else is a waste of time chasing for next to zero gain.
     
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  3. Mr Efficiency
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    Mr Efficiency Senior Member

    How do you get a 11.5m x 2.2m boat weighing 10 tons ? Answer, hire a bricklayer !
     
  4. ram68ocean
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    ram68ocean Junior Member

    Thank you for the feedback. Appreciate what you say that optimising the hull shape is 'red herring'. Is this translating into 'do not bother spending too much effort in getting a nice hull'? I would rather spend time on it, even if it was as a personal exercise.
    Things I should have added: The max. LOA and Beam are because it is desired to be transported inside a 40 ft container. So if 10m long works, then 10m shall be.
    There's an existing base boat of 9m, and I have scaled up to 11.5m using Barkla method (from Principles of yacht design).
    Main requirement is long endurance (500 hours) on diesel at about 8 knots in open waters, it's a survey vessel.
    I have roughly estimated as a first guess a total fuel capacity of 3.5 - 4.8 m^3. About 1 tonne for the Aluminium structure. About 2 tonnes for ship equipment. About 2 tonnes for payload.
    First estimate for total (minimum) installed power 120 hp. Gives 13.5hp or 10kW per tonne (89 kW/9 tonnes).
    I am designing the boat by creating the GA, 3D model, equipment list and weights estimate. Just looking for more experienced advice on the hull shape so that I can start with right foot.

    For example, what of these hulls looks more like a good starting point?
    http://fab.cba.mit.edu/classes/863.12/people/colley/PressFit_files/image006.jpg
    Boats for sale Spain, boats for sale, used boat sales, Commercial Vessels For Sale 2016 WORK BOAT - Apollo Duck https://www.apolloduck.com/boat/commercial-vessels-work-boat-for-sale/492907
    Kewatec WORK 1100 Trawler - Kewatec Aluboat https://kewatec.com/boats/kewatec-work-1100-trawler-aluminium-fishing-boat/
    Also, Maxsurf and Delftship offer to start a new design with different type of parent hulls. Which ones suits best?
     
  5. ram68ocean
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    ram68ocean Junior Member

    So what's your suggestion Sir?
     
  6. Mr Efficiency
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    Mr Efficiency Senior Member

    Why does it need to be so heavy ? Half that would seem enough to deal with
     
  7. Mr Efficiency
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    Mr Efficiency Senior Member

    OK just noticed your reply above to Ad Hoc, 4000 nautical miles endurance ? What mandates that ? Three weeks continuous ?
     
  8. Ad Hoc
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    Ad Hoc Naval Architect

    Correct

    Your prerogative :D

    That's 1.2 litres per hour!!??

    That cannot be correct. Fuel, is part of your payload!

    Finish that first, before doing anything else!
     
  9. bajansailor
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    bajansailor Marine Surveyor

    Can you post a copy of the GA that you have drawn so far please?

    They all appear to be much wider than 2.2 metres though.

    Have a look at the Far Harbor 39 sailing yacht, aka 'The Container Yacht' - here is one for sale.
    Far Harbor 39 2006 | Used Boats | RCR Yachts https://www.rcryachts.com/used/far-harbor-39-2006

    Far-Harbour 39 GA.jpg

    Note that her quoted displacement of 13,000 lbs / 5,900 kg is much less than your estimated range of displacement.

    Your 2.2 metre beam is going to be very restrictive. Is this absolutely a requirement?

    Rather than shipping 'inside' a 40' container, why not ship it on a 40' flat-rack when needed?
    Then you have a bit more latitude re beam and depth.
    OK, the shipping company might charge you extra for 'overflowing' into the adjacent container spaces on the hatch cover, but that would be rather sadistic.
     
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  10. Mr Efficiency
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    Mr Efficiency Senior Member

    Those boats you provided links to are not remotely like a skinny 2.2 m boat.
     
  11. Mr Efficiency
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    Mr Efficiency Senior Member

    Anyway, if you can come up with this 11m boat that will go 4000 NM at 8 knots, the world will beat a path to your door
     
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  12. bajansailor
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    bajansailor Marine Surveyor

    Yes, you will have achieved the holy grail of boat design! :)
     
  13. ram68ocean
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    ram68ocean Junior Member

    That's right, 3 weeks at sea!
     
  14. Mr Efficiency
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    Mr Efficiency Senior Member

    What is the nature of the task ?
     

  15. bajansailor
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    bajansailor Marine Surveyor

    Have a look at Nigel Irens' Rangeboat - she is slightly bigger (12 metres) than your maximum length, but she would appear to satisfy some of your other requirements.
    Note though that her displacement is 'only' 5 tonnes.
    Rangeboat - Nigel Irens Design https://nigelirens.com/boats/power-boats/rangeboat/
    The webpage notes that she consumes 1.2 litres / mile at 14 knots with an 80 hp engine, so she should get much better fuel economy at 8 knots.
    I still think though that a range of 4,000 miles would be rather optimistic.
     
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