Very long and skinny pontoon hulls

Discussion in 'Boat Design' started by Saqa, Jun 9, 2021.

  1. Saqa
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    Saqa Senior Member

    Hi all
    I am wondering about the speed potential off a pair of 40' torpedo type hulls about 40cm in diameter (roughly 3000L total) running a prop at the back of each hull, maybe surface piercing types

    The intent is to have a small pod on the cross-beams as the cockpit. Most of the payload capacity dedicated to the power plant (custom generator running the pair of electrical motors inside the hulls)

    Vessel needs to carry 200kg of people and 30kg of bring on stuff

    1 - Can something like this be accelerated to very high speeds using electric motors available today, what kind of speed potential is possible?
    2 - Would something like that be safe if it gets rough, assuming no weakness in the build?
    3 - Would a slight angle of attack help in keeping the nose up and tackle waves better?
    4 - If the 40cm wide hulls had planning capable bottoms, would it have higher top speed?
     
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    They could be powered to very high speeds with available electric/battery technology, but for about as long as it takes to make a cup of tea ! To have anything running underwater at high speed, that isn't a fish or an aquatic mammal, is offensive to the God of skin friction. You must have got that impression yourself, if you ever tried to tow a lure at even moderate speeds, even a small lead-head feather lure towed at 20 mph for tuna, bends the rod noticeably. The fish have no trouble catching the lure, they are blessed by nature with the ability to greatly reduce skin friction.
     
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  3. Mr Efficiency
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    Mr Efficiency Senior Member

    There were some posts on this forum this week about foil assisted pontoon boats, using something branded as Hydrofin, worth a look. There seems little doubt that foils have a lot to offer, because they get fast boats further out of the water, into the much less retarding medium of air.
     
  4. Ad Hoc
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    Ad Hoc Naval Architect

    Well, before you embark on your flight of fancy, you need to consider some very basic facts.

    1. Define "high speeds" in relation to this 12m length tube.
    2. Once that has been defined, do a quick google search for PEM motors that are less than 40cm in diameter that have the power you require...does it fit.
    I can short circuit this...no!
    3 The size of batteries required and will they fit..again, short circuit this..no.
    4 The weight of the PEM and the batteries, is this less that that allowed for in your total available make up of 3000l...again, I can short circuit this..no.

    And that is even before addressing

    FoF are fine, but it shall remain there until you address some basic facts first.
     
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  5. bajansailor
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    bajansailor Marine Surveyor

    Torpedo type hulls work best underwater, where they wont have to suffer surface wave making resistance.

    On the surface, I think they will just be like another typical pontoon boat.
    Yes, apply brute power to it, as per some of the pontoon boats available today, and they will go 'fast' (relatively) - but they will not be as efficient as a surface hull that is optimised in shape (re prismatic coefficient et al) for the speed desired.
    Look at how much power these 'fast' pontoon boats need - if one had (say) 2 x 100 hp O/B motors, then you will still need at least that power (approx 2 x 75 kw) from your electric motors and generator - and at least a 150 KW generator, which is going to take up a sizeable chunk of your payload capacity (along with the fuel required to power it....)

    Re your total buoyancy of approx 3,000 kg for the two pontoons - this is only if both are completely submerged.
    Is this the intention, to have them in a kind of SWATH configuration?
    Usually with pontoon boats you don't want to go past immersing the hulls more than halfway - in this case you only have a total buoyancy of 1,500 kg available for everything.
    And this is not a lot really, when you add up the weight of the pontoons, cross beam structure, the custom generator, fuel for this generator, the two electric drive motors and propellers, and the payload.

    And I just saw that Ad Hoc has explained it all much more simply and succintly above!
    In a nut shell - it ain't gonna work. :)
     
  6. Saqa
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    Saqa Senior Member

    For me, very high speed for a boat is over 60 kilometres per hour

    Not a SWATH, intention is wave straddling and piercing ability

    Doesn't have to be round or 40cm diameter (two 6mx1.2mx3mm sheets conveniently rolls to about 40cm diameter and length and material cost under 1000 each hull, hence reason for picking 40cm value)

    I think its roughly 250kg for the pair of hulls if done in 3mm sheet. Only an engineer could tell me how much the connecting structure would weigh for a 3m beam

    If something like this is feasible, then I will approach an NA. My 6m hire boat build will be full survey and the 3.5m sportfisher hire boat under 2D exemption thingy so intending to use the same NA after the completion of the hire boats. The bay crosser is purely personal craft

    Power to the props - I can only think of
    1 - Electric motors
    2 - Car fan hydraulic type motors running off a pod mounted engine
    3 - Pod mounted engine and air prop

    Just trying to come up with some options to explore in further detail. While I ponder this, I am trying to use a program called hulls to draw up a RIB type alternative concept using my not very extensive DIY kite making experience

    Use would be a high speed bay crosser covering about 140km return point to point on a good day and along the inside shore of Fraser Island if unexpected worsening of conditions occur
    Ability to demount and store on a 25' trailer
     
  7. bajansailor
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    bajansailor Marine Surveyor

    Sorry mate, but pretty much everything you mention above is in conflict with each other - I cannot see this proposal working.

    60 km/h is about 35 knots - your pontoon boat is going to need a lot of grunt to get up there, even if you try to tweak the hulls to improve them re prismatic and wavemaking.
    Electric motors are relatively small, but a 75 kw motor is still going to be a sizeable beast - Add another one, and the generator to power them, and the fuel for a 150 km range, and your boat is going to start complaining......
     
  8. Mr Efficiency
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    Mr Efficiency Senior Member

    fast, long range, open water capable battery-electric boats are still off over the horizon, if/when they do arrive, it will be widely trumpeted. Electric cars are pretty impressive already with power and range, but the powering requirements of fast boats are a tier higher. That they don't need brakes, is a hint to that.
     
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  9. Mr Efficiency
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    Mr Efficiency Senior Member

    I saw the other might on TV that a fast, fully foil supported, minimal wake electric powered runabout is being touted for use around Venice, where the wash from boats is a problem, I guess if they can swap out batteries quickly or charge them quickly it would work, if always in reasonable proximity to base.
     
  10. Saqa
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    Saqa Senior Member

    What about hydraulic types motors in the hulls with lines running to a deck mounted engine?

    Or air boat with pontoon hulls instead of jon boat type?

    Or even a stabilised mono hull type affair as in a very narrow planning main hull with outboard or drive from a jetski
     
  11. Saqa
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    Saqa Senior Member

    I like the look of the foiling main hull on the updated "bucket list" concept too. Looks like it can be very light all up with a pair of floats and suitable propulsion. Would something like that scale okayish down to about 20' for testing the concept in as a 1 seater? Or, is the waterline length between the fore and aft foils not very scalable?
     
  12. bajansailor
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    bajansailor Marine Surveyor

    Re your three questions above in post #10, for what you want to do - I think that if they were thought to be viable, somebody would have already 'been there and done that'.

    Re the bucket list concept, which thread is this - can you post a link please?
     
  13. Saqa
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    Saqa Senior Member

    I must say that we haven't for sure run out of new types of things to make, just have to play with ideas to fit these particular scenario

    Re bucket list
    The “Bucket List” Prototype Building Blog – Harryproa http://harryproa.com/?p=424
     
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  14. Mr Efficiency
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    Mr Efficiency Senior Member

    Hydraulic drives involve losses that make them less attractive
     

  15. Saqa
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    Saqa Senior Member

    The losses are there is fairly widely known, but, I still wonder about a pair of those running of a suitable engine to deliver those 75kw to each prop after the losses vs electric motors and generator in terms of weight and cost

    Question to all
    Would a stabilised monohull based on the main hull like the bucket list wearing an air prop be a better option to the pontoon boat? I list the air prop as getting a prop in the water on a foiler is giving me headaches :)
     
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