an unpractical hull design only for ddwfttw boat

Discussion in 'Boat Design' started by sailor2, Jan 14, 2009.

  1. sailor2
    Joined: Jan 2009
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    sailor2 Senior Member

    Finally I managed to pass the being human, not a robot test for registrating into this forum so can post by my self :)

    This was originally in wind-powered-sail-less-boat thread
    but this is a side trip for that discussion so perhaps belongs to another thread.
    A direct link to the post in the mention thread this begin :
    Only 2 posts on the subject there, nothing in between them.

    I originally wrote :
    And Rick W. replyed :
    Did you miss these 2 sentencies of mine or did you just say you agree in an other way ?
    I suggested that a huge wheel would decrease total drag opposing movement compared to a hull for ddw course only. I certainly don't see much merit for that idea at all for any practical purpose, as I can't figure out any reason why someone would want a boat incapable of going to any other direction safely or at all as that is what I think my idea would lead to. Not necessarily the case with normal hull shape with prop&turbine propulsion system.

    So would you please limit your response to ddw course only as anything other is considered irrelevant in this thread in general and certainly with my idea.

    I surely can't see why windage would kill that idea while going ddw until perhaps exceeding 2 times windspeed, something I would't see reason to expect happening anyway.
    Can't figure out why you think tubular ring having any less aerodrag in ddw course either if that's even what you meant.
    My assumtion was & currently is that windage could be calculated similarly as foil analyses can, starting with nonviscous analyses. That would give very small drag as frontal area can be small due to being thin and flow speed here is just apparent wind to axis being near zero therefore. Then moving on to viscous case one should now calculate local airspeed for each surface element as variable of both circular speed (as a function of local radius & angular position) & linear speed as in nonviscous case. And then integrate with respect to both radius & circumference. My analyses lead to result this being much less than reduction of viscous drag on water. Therefore total drag reduction as wave making drag can be also be smaller as well due to being narrower on wl allowed by assumed double wetted area compared to optimised normal hull having same lwl & displacement.

    If you think I went wrong somewhere can you or someone else point it to me ?
    Sure they are , but if my analyses is correct the difference would be much smaller than I have thought before realising this idea.
  2. Guest625101138

    Guest625101138 Previous Member

    Draw a picture so I am clear.

    My understanding is it will need huge wheels to get the required waterline length. I understand the part in the water will not be moving. The axle will be doing say 12kts in a 10kt tail wind. The top of the wheel will be doing 24 kts with 14kts apparent wind. It will be an aerodynamic shape but still a huge area to force through the air.

    You also have to react the moment created by the prop and turbine. This will require a third wheel.

    My modeling indicates I could get one of my 7.2m hulls to 12.5kts in 10kt breeze sailing DDW. I doubt that wheels will achieve this as my hull has a draft of 100mm and beam of 270mm. Your wheels will be hard pressed to match the low wave drag of this plus any reduction in water drag is likely to be offset by the windage on the top of the wheels and lifting of water due to surface tension off the trailing edge of the wheels.

    The idea with a tube is to run it around a caterpillar type frame. This avoids the large area exposed to the hugh wind velocity.

    Maybe I have not got the right understanding of what you propose. A drawing might help. I think if you start to engineer it you will find it has some serious flaws.

    Rick W
  3. sailor2
    Joined: Jan 2009
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    sailor2 Senior Member

    Thanks for the response.
    Propably a good idea, but I'm not very good at doing drawings. Would math description of the shape do ?
    There is a torque balance to be met if free wheeling is assumed. The rotation speed automatically adjusts to whatever is needed to have that balance. This also automatically somewhat minimizes total drag reasonably closely, in fact it will rotate too fast which can be slowed down by braking or even taken some of the power from it to prop to accheave the same more efficiently. The center of effort of aero drag on the wheel is closer to the axis than hydrodrag, so aero drag is greater in magnitude to have the balance.
    But like I said it pays off to change that balance by slowing the rotation speed down, so that aero drag doesn't exceed hydro drag too much.

    Make them even if you like, would be closer to the optimum than assuming zero waterspeed like unslipping wheel. Then compare hydrodrag of this idea multiplied by 2 to your hulls and result is what ? That's the question. I assume wavemaking drag of your hull is under 20% of total hulldrag. If that's reasonable then this wheel needs to have wetted drag 50% less than that of wetted drag of conventional hull and this wheel has less total drag. That means 6 knots apparent water speed against 12 knots for conventional hull for same drag if this has 2 times wetted area. And that's only valid if both have same displacement & lwl and distribution of displacement reasonably same as well.

    So try this : tailwind 10 knots, boatspeed 12 knots, max speed due to rotation 7 knots. Max apparent wind speed top of the wheel 9 knots. Apparent water speed 5 knots. If total windage is less than water drag, this is beter than a hull having:
    same lwl, same displacement and 50% less wetted area. If not, this idea is proven dead.

    You presumably have valuable practical experience about props & turbines, so I see no point trying to re-event that part with lowsy theoretical assumptions. Would be more sensible to use same data to make any comparison as valid as possible. This includes weights or rather displacements for the hulls. My initial assumptions seemed to assume far too heavy craft so would have to recalculate. You gave measurements below for one hull, except displacement, but that doesn't provide all stability needed. Are you using a trimaran configuration or what ?
    It would clearly be more easier to match conventional cat hulls with this idea than a tri with amas very lightly loaded.

    Another issue is your statement that it would still be a huge area to force through the air. That huge area is just the smooth surface area (630 sq meters) and that only creates the viscous part of air drag. That area is not forced through the air any way as long as going ddw. Not sure how Re-number for this part should be calculated as air sees infinitely long path while boat speed matches truewind speed.

    The usually more important inviscous profile part only is related to apparent wind speed related to center of mass, rotation doesn't change that drag at all as far as I can see. And that area is the cross sectional area which can be much smaller than surface area. Below case about 6 m^2 for each wheel. Assumed apparent wind just 2 knots, so this drag is small. We could be assuming 0.2 knots and it still represents ddwfttw. That would benefit this idea compared to regular hull shapes, like I said this is unpractical idea afterall, just theoretical stuff to make a point on ddwfttw on the water and nothing else.

    I initially assumed 20m diameter with 0.3m width and 0.603m immersed draft leading lwl = 6.84m. Clearly much more displacement than your hull has.
    My suggestion above includes 3rd hull, not 3rd wheel, and assume it dimensioned so it lifts off the water (at least almost so) at top speed at design conditions by pitching moment.
    You are correct about water lifting off, however as this water is mostly directed behind rather than up, it becomes a thrust force canseling out part of the drag it creates. Remember, the wheel is not rolling without slip on the water.

    It could very well have serious flaws, that's why I want to discuss it to see those.

    So far I'll have ignored all structural engineering and just assumed it has same weight overall as craft based on hulls would have. If major part of weight comes from other parts than hulls this would be close. Just keep in mind it also makes tower holding the prop loads unnecessary as the beam connecting hulls is already high enough to allow direct connection of prop loads to that. Only need one smaller strut for turbine connection, but those has less loading than tower for prop support would need to handle. That difference allows wheels being heavier than hulls would be without over all weight increase. Is this difference great enough to do that is another unknown factor to me.

    Based on your estimations, it's clear that this idea would need bigger airprop to match bigger drag, but it allows more displacement to carry it all.

    Top speed of any waterbourne ddwfttw vessel is really:
    (excess thrust of prop & turbine system) / (drag due to carrying weight)
    If efficiences of the drive system are the same, and why wouldn't they be. Actually bigger craft have some advantage on that one.

  4. Guest625101138

    Guest625101138 Previous Member

    The idea certainly favours large scale.

    The prop design I did started out aiming for a boat speed of 5m/s. This meant I was designing for quite low apparent wind. It might be possible to get a smaller prop if you were to design for a boat that needed at least 30kts wind to achieve DDWFTTW. It has the power potential to operate in a planing hull at these speeds. Could have three widely spaced hulls to handle changing loading conditions all designed to plane.

    I think my design more or less shows Bauers stuff to be highly optimistic.

    Rick W
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