Hull for power catamaran

Discussion in 'Boat Design' started by Cacciatore, Jun 1, 2020.

  1. Cacciatore
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    Cacciatore Junior Member

    Hi guys, a general question... what is for you the ideal hull for a power cat about 50ft (inboard engine). I would see some drawings to have an overview. In several powercat I have seen some troubles of propeller choice and relative cativation mostly with subtraction on hull of the propeller well and not so deep skeg.
     
  2. bajansailor
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    bajansailor Marine Surveyor

    I don't think that you can generalise very easily - there are so many factors that have to be taken into consideration.

    What are the requirements that you are looking for in your ideal 50' power cat with inboard engines?
    Such as what cruising speed, range, how much accommodation, displacement or planing hulls, type of construction, budget available........
     
  3. Cacciatore
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    Cacciatore Junior Member

    only to have a general overview of different hulls. Anyway it is a planing cat, workboat, material fiberglass hand layup, superstructure and underneath cabins approx 10 tons dry weight with engines.
     
  4. Ad Hoc
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    Ad Hoc Naval Architect

    As bajansailor notes, you can't generalise so easily like that...You're putting the cart before the horse.

    For example... a good efficient power cat would have a length-displacement (LD) ratio of around 7.
    With your 50foot - 15.3m length this means the displacement would be around 10.5 tonne. Oh... but wait you still need a payload... fuel/water etc etc...so this is not a complete picture.

    Oh ok.. so you want a planning hull... ok.. so the payload to lightship ratio will be roughly 20%. ... ok.. so that means your full load will be around 12 tonne.

    So, what power do i need, well, you can use this overly simplified graph:

    upload_2020-6-2_7-56-5.png

    So, let's say you wish to go at around 20 knots.... so roughly 36 hp/tonne x 12 = 432 hp, or since a cat = 220 hp per side.
    For peace of mind an unrestricted rating would allow you to do what you wanted with the engine....say take a Cat C7 that weights roughly 800kg.. add a box = roughly 1 tonne.
    Then add shafting/props etc...and the mech weights = roughly 3-3.5 tonne.

    That only leaves around 7 tonne for the lightship... that is not much for a 15m boat. We have designed cats in the 9-12m range that have their lightships more than this...albeit is commercial, but it suggests the values are light!
    The structure would be roughly 50% of the lightship for this type/size = say 3.5 tonne... this just leaves 3.5 tonne for the electrics the outfitting etc etc.. not much, it is already getting tight.
    Boats always grow in weight, never get lighter!!

    So, you need to define what it is you want - your SOR. What speed... how many pax... how long endurance... how many cabins... size of galley etc etc. That defines your full load weight...with then defines the rest of the boat... and if it will work with the SOR. Without the SOR, the purpose and duty of your boat defined, you're putting the cart before the horse... since the hull form that is used, is a result of the SOR, not the other way around.

    So, whilst you may wish to "generalise"... it serves no purpose other than... you need to establish and write your SOR to get a real feel of what is or is not possible.
     
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  5. Cacciatore
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    Cacciatore Junior Member

    Sorry for misunderstanding I only ask to see some drawings of the side view for inboard power cat around 50 ft
     
  6. Mr Efficiency
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    Mr Efficiency Senior Member

    This may be a translation issue, but can you explain that statement better ?
     
  7. Ad Hoc
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    Ad Hoc Naval Architect

    As Mr.E notes - that statement, is at variance with your request for the "ideal hull" for a power cat.
    It appears you have 2 very different questions, and they are not necessarily related.
     
  8. Mr Efficiency
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    Mr Efficiency Senior Member

    He may be seeking illustrations of different set-ups for shaft drive cats. With the Tennant displacement power cats, the canoe stern with the prop protected behind it, seems a beaut solution for reduced draft and good protection for the propeller, but adapting that to a planing cat could test the imagination.
     
  9. Cacciatore
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    Cacciatore Junior Member

    Thanks for reply guys , I only want to see illustrations of several set-ups for power catamaran . Direct to shaft tilted 12-15deg , with joint straight like an oil tanker , v-drive or surface propeller etcc..
    Regarding the statement , I have noticed that in several catamaran having propeller well subtracted on hull have cavitation issues and difficulty on choosing the right propeller .
     
  10. Dejay
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    Dejay Senior Newbie

    Ideal really depends on what you want. How fast do you want to go? How heavy?

    For me the ideal hull for a power catamaran would be a slow and ultra efficient displacement hull with a big prop on an electric outboarder motor that can be lifted out. But actually the ideal power catamaran for me would be a power trimaran ;)

    I assume the cavitation issues are from having to use smaller fast rotating props because of lack of space? I'm also not sure if catamarans can really plane. Or what kind of inboard setup works for planing.

    Sorry I can't help you with illustrations. But I assume the basic set-ups for drives are the same as for monohull powerboats. Just more slender. You'd just have to search around for pictures.
     
  11. Cacciatore
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    Cacciatore Junior Member

    Thanks for reply , what do you think about the use of a surface piercing propeller on a commercial catamaran (fishing boat )? The target is to build the boat around the available 2x 360 kw MAN engines of the old boat achieving the velocity of 40 knots . Easy to reach in my opinion only some doubts about the use of traditional propeller shaft system...sorry I have only passed and doubled 40 knots only with outboards :D
     
  12. gonzo
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    gonzo Senior Member

    There are two caused for cavitation. One is blade area too small. The other is turning too fast. Sometimes they happen at the same time. I suppose you could build an ultralight catamaran and get 40 knots with the available power. Look at the chart Ad Hoc posted. At the far right end, 30 knots require 68HP (51Kw) per ton. Even at that speed, your maximum loaded weight/displacement would be 10.5 tons for the available power.
     
  13. Cacciatore
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    Cacciatore Junior Member

    Thanks for reply the 10 tons for the dry displacement as you can see on my previous post (2nd ) are already planned and I think to be better. Anyway due low clearance space for the propellers , troubles on draft and variations of geometry on the keels ( that can cause difference of pressure and turbolence on propeller ) I think that could be easy use directly surface propeller but it is realiable for a workboat ??
     
  14. gonzo
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    gonzo Senior Member

    For your planned speed, the unladen displacement will have to be half of what you say.
     

  15. Ad Hoc
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    Ad Hoc Naval Architect

    Hmmm...ok..so that is roughly 1000hp total. So, at 40 knots that equates to (if you extrapolate the hp/tonne chart...)

    upload_2020-6-10_8-14-56.png

    That is roughly 100hp/tonne.
    This means your boat cannot weigh any more than 10 tonnes to get the 40knots.

    Exactly..

    The numbers don't add up.

    You need to define and refine your SOR.
     
    Last edited: Jun 9, 2020
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