Are squared hull/transoms essential for a planing catamaran hull?

Discussion in 'Hydrodynamics and Aerodynamics' started by NoviceJoe, Jul 22, 2020.

  1. NoviceJoe
    Joined: Jul 2020
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    NoviceJoe Junior Member

    Hi, I am trying to make a planing catamaran from a windsurfer board cut in half and a 2.5hp outboard.
    The board has a rounded stern and I am wondering if it would work better if I was to square it off. It currently has a top speed of just over 8mph and doesn't quite plane. I'm thinking of a cut between the two toe strap fastening plates.
    Any thoughts?

    NB: I have the recent original thread about other aspects of the design in the 'design' section entitled
    2.5hp Catamaran - power delivery issue

    FullSizeRender.jpg
     
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    Squaring it off, is more likely to hinder, than help. The real problem is the engine is way too small to be going faster than what you are doing. If squaring it off would make it faster, it would have been made that way in the first place, and the speed would have been much more as a windsurfer, when wind conditions allowed, than 8 mph.
     
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  3. Ad Hoc
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    Ad Hoc Naval Architect

    It'll sure help. Since the flow needs to separate cleanly to prevent the squatting, negative pressure, pulling the stern down.
    Removing the large negative aft pressure, created by a 'typical' ship shape, is the main source why displacement hulls do not plan....hence all transoms are generally vertical and have clean sharp edges.
    Just look at all planning hull transoms... they are like that for a reason...
     
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  4. Rumars
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    Rumars Senior Member

    Boat x kg, engine+fuel 13,5kg, clothed human y kg. (x+y+13,5)/2.5=?
     
  5. Mr Efficiency
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    Mr Efficiency Senior Member

    I wouldn't be so sure, in the case of this board, which seems pretty flat aft. This forum seems to have become home base for people who want unrealistically high performance from drastically underpowered vessels of various kinds, planing boats will not run on "fumes" like slower displacement types potentially can, but avoiding headwinds ! A 2.5 HP outboard is designed to get your tender 100 metres to shore without rowing, not planing anything.
     
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  6. Will Gilmore
    Joined: Aug 2017
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    Will Gilmore Senior Member

    That's how I understand it, but with regard to the 'z' axis. As I imagine it, and is funny because I was just doing that very thing earlier today before seeing this thread. If a bost is up on a plane and has a flat squared transom with an essentially rockerless bottom, would it really adversely change the dynamics of detachment by extending the stern out in a wedge or round-off transom? Bottom still flat, transition vertically still squared.
    [​IMG]

    -Will (Dragonfly)
     
    Last edited: Jul 23, 2020
  7. Ad Hoc
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    Ad Hoc Naval Architect

    To be 100% clear YOUR z-axis is what... the vertical plane?

    Is a 'bost' the trade name of a boat?


    Well yes.
    Because the centre of lift has now changed, and not necessarily for the better. Since whilst the length may be longer... (wetted)... the CoL has moved fwd relative to its original position, which can become a problem and initiate excess trim even porpoising.
    Thus, these small 'add ons'... must be treated with caution and it is always - a case by case - basis.

    But the rounded shape transom shown in the image, is not ideal at all...a square/flat shape is required... since being rounded, will merely encourage the flow of water to move from the sides going aft to a round and lap up the aft end, the transom, which is the opposite of what you wish to occur, since this does not cleanly separate the flow, in an aft direction, as required.
     
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  8. Barry
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    Barry Senior Member

  9. NoviceJoe
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    NoviceJoe Junior Member

    x = 26, y = 75
    ? = 45.6
     
  10. NoviceJoe
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    NoviceJoe Junior Member

    Thank you, but that is the challenge. Your comment seems strange considering your username! I have seen videos of people planing with 2.5hp so it is possible (maybe not with a catamaran but we shall see).
    My aim is to go faster than 10mph so not far off.
     
  11. NoviceJoe
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    NoviceJoe Junior Member

  12. Rumars
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    Rumars Senior Member

    45,6kg/hp is a little bit much for what you desire. Drop to 20kg/hp or under and we can talk about square transoms, prop pitch and so on.
     
  13. CT249
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    CT249 Senior Member

    It depends.....

    There's not just one type of windsurfing board, any more than there is one type of car. One can't be simplistic in either case.

    The board in your pic appears to be an older board of about 9-10ft length from the '80s or '90s. Such boards were only designed for planing whereas most even older boards were longer so that they could perform better in light winds. However, boards of those days suffered from control issues and therefore they had narrow tails (I won't go into the details). More modern shortboards (as mentioned in the article) are designed with wider, shorter shapes that plane at lower windspeeds and have square tails, but they can get away with it because of rig and skeg improvements. However, when they are well below planing speed they have even higher drag.

    The problem is that a longer hull is more efficient at lower speeds and therefore can achieve a certain speed with less power - but that long skinny hull is then less efficient at planing although they can reach 30+ knots. A short, fat hull (like a Formula board which is 3' wide and about 7' long) is very inefficient at low speeds but if you can summon the power to drive it fast enough to reach its planing threshold, it abruptly starts to plane at lower speeds and more efficiently than the long skinny hull. The jump from "displacement" to planing speeds and vice versa is really dramatic with the very short, fat planing windsurfers, and so is their lack of performance when not planing.

    So there can be three ways to go. You either go to long, skinny hulls that perform efficiently in displacement speed but won't plane as well, or short fat hulls that will plane well IF you can get them to go fast enough to reach planing speed, or somewhere in the middle. As a windsurfer and an owner of a 2.5(?) and 3.3hp outboards as well as bigger ones, I'd tend to think that a windsurfer like yours could be the worst of both worlds (or less likely, the best of both). It's moderate in length but fairly skinny compared to Formula boards or modern slalom boards. I'd go for either a Formula board (very short, but a metre wide and a demon when planing but needing lots of power to get to the planing speed) or an old style 12'+ board that would perform well in displacement mode and therefore possibly reach planing speed. However, at a guess your setup may well be in a critical power/drag zone where there is no clear best choice among the various approaches.

    BTW planing cats have some basic physics issues. Look up planing hull aspect ratio and Savitsky.

    PPS - I didn't realise you'd already cut the board in half. There are windsurfers that are so small that they are called "sinkers" for a reason. If you chop your current board down you may find out why. I think you may have to look for new hulls, but look up the whole issue of aspect ratio and planing, and hull drag and speed at planing thresholds, first.
     
  14. NoviceJoe
    Joined: Jul 2020
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    NoviceJoe Junior Member

    This is turning into a very interesting discussion for me. Many of you clearly think that this is just not going to work for various reasons, such as; lack of power, too small an area for planing, rocker, hull efficiency, too narrow, weight distribution etc. But, some of you obviously think that there may be a way (with some tweaks) to get this boat to go over 10mph which is the goal. I have learnt a lot about boat design in the last few weeks and that is as much fun as anything else. I am already pleased that it floats, is stable and can do 8mph. Thanks to all of you who have contributed thus far.

    The pedestal idea I had that someone else suggested proved to work really well for alleviating my prop ventilation issue, it also has a little plate on the bottom. As mentioned before it didn't translate into higher speeds but was much smoother. I have had another idea that I could integrate a tertiary planing hull, roughly along the lines of the drawing below. Please excuse the crudeness of the illustration I'm completely new to CAD as well! The idea of this is obviously to increase the planing area of the hulls. I envision it to be flat with the hull bottoms but have a slight rise in the front of it. I am thinking of raising slightly (maybe an inch or two) from the height in the illustration so it is just below the natural waterline. As soon as the boat moves forward it will clear itself of water and contribute significantly to lift.

    What do you think? Should the whole thing be at an angle? Does it need to have its own bow to deflect water over the hull decks? Will it just be continuously water logged and have a negative effect?

    I have a higher pitched prop coming later this week and am still considering cutting about a foot off the stern of each hull (perhaps between the toe strap fastening plates). This will be the last resort.

    upload_2020-7-26_16-30-56.png
    upload_2020-7-26_16-58-17.png
    upload_2020-7-26_17-1-39.png
    upload_2020-7-26_17-5-18.png
    NB: The aft 140cm of the hulls are virtually flat, so I am not too concerned about the 'rocker' effect.
     

  15. tlouth7
    Joined: Jun 2013
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    tlouth7 Senior Member

    Rather than cutting material (and planing area) off the back of the board, could you not build up a sharp edge somehow? You just need to fill in the fillet to a sharp corner.
     
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