2.5hp Catamaran - power delivery issue

Discussion in 'Boat Design' started by NoviceJoe, Jul 16, 2020.

  1. NoviceJoe
    Joined: Jul 2020
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    Location: Dorset

    NoviceJoe Junior Member

    Thank you. The boat is made from a BIC Veloce 328 windsurfer board (12.5kg) that has obviously been cut in half.
    It cost £30. The aim was to save a lot of work, time and money - but maybe it wasn't the best board to choose. I have learnt a lot from a very small investment so far.
    Thanks again for your interest.
     
  2. portacruise
    Joined: Jun 2009
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    Location: USA

    portacruise Senior Member

    Can you increase the planning area by sitting further forward just for a test of surface area? But the small freeboard might mean a tendency to catch water on the bow if there are any ripples or waves, not helpful. If your cuts to the board create a square stern, that is likely to cause more drag.
     
  3. NoviceJoe
    Joined: Jul 2020
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    Location: Dorset

    NoviceJoe Junior Member

    Thank you. I have experimented with weight distribution as much as possible but you are correct in thinking that there is not enough buoyancy to get too far forward.
    I would be very interested to hear other views on potentially creating a square stern. All planing hulls I have seen have a square stern/transom. My understanding is that it is required for easier release of wash (ie creates less backwash and eddies at speed), and I guess it also creates a pivot point beyond which there is no hull to dig into the wash. It is a big decision to make in terms of modification so I need to be sure its not going to make things worse.
     
  4. BlueBell
    Joined: May 2017
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    Location: Victoria BC Canada

    BlueBell Ahhhhh...

    While you've got the board in two, cut out a hole for the engine leg and glue them back together.
    A nice snug fit with zero movement, water tight.

    Don't cut any length off, you need all the planing area you can get!
    Now stand up on that thing and make 'er go, WOT!
    A stand up paddle board paddle could really help here.

    Once you get bored with that we can look at fitting hydrofoils.
    But you're going to need a longer leg... or are you...
     
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  5. portacruise
    Joined: Jun 2009
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    Location: USA

    portacruise Senior Member

    My opinion is that going on plane or even hydrofoil with around the minimum power possible- is different than having a mostly full-time planing craft design, with plenty of power. Given copious power, design can be changed to ignore displacement efficiency rules (and fine-tune the planing experience) which might otherwise be needed to get up to planing speed. Given Copious power means just about anything is possible- like, for example, barefoot water skiing.
     
  6. NoviceJoe
    Joined: Jul 2020
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    Location: Dorset

    NoviceJoe Junior Member

    Copious power is not what I have with a 2.5hp outboard! The aim is to build a car roof topable boat that is easily handled and launched that can also reach a decent cruising speed (10mph+), mainly for just a bit of fun or perhaps a simple fishing platform for an hour or two.
     
  7. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    You better make sure the wind doesn't come up, or turn direction, during your outing, or it could be a long trip home. Seriously, there is no "hack" that is going to turn this into a speed machine.
     
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  8. portacruise
    Joined: Jun 2009
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    portacruise Senior Member

    "Copious power is not what I have with a 2.5hp outboard!" Yes, that is why I don't think a planing Hull design (Wide Hull, square stern, Etc) meant for copious power, or a draggy catamaran hull are likely to work.

    Many of the virtues such as a wide Hull and square stern which help while planing, work mostly against getting out of the hole when below planing speed. Once a boat is on top planing, it takes slightly less power to just barely keep it there than it did to get over the hump. With a minimal power of 2 .5 HP it may not be possible to reach the planing speed unless an efficient hull is used when below plane speed. So I tend to agree with Blue Bell suggestion in post 19, because the efficient sailboard Hull which works below planing speed and also at planing speed as a sailboard -became less efficient when it was made into a catamaran.
     
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  9. kerosene
    Joined: Jul 2006
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    Location: finland

    kerosene Senior Member

    Uffa Fox did a planing boat comparable to a rowing 8. It was at best on Par with the displacement model - but at much shorter hull.

    It is an interesting reference for a low power planing craft anyway.

    I would think starting from insulation xps would be cheap. 3 layers of 30mm glued together would allow for setting the rocker shape easily.
    At 2.5hp you are at about 2kw, so maybe 5-6 strong rowers but much less weight. (Maybe 4 if the oars beat in efficiency with big margin)




    IMG_20200724_232015.jpg IMG_20200724_232030.jpg IMG_20200724_232130.jpg IMG_20200724_232108.jpg
     
  10. messabout
    Joined: Jan 2006
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    Location: Lakeland Fl USA

    messabout Senior Member

    NoviceJoe, I strongly suspect that you are beating a dead horse. The experiment with the board made into a catamaran was a noble try.

    Now why not regroup and build a purposeful boat that will do what you seek. You need some flat surface area to support the total weight of the boat when under power. You can easily enough build a scow bow type boat, not unlike the ubiquitous Jon Boat, with two sheets of Ocumee ply. It will be light and have enough bottom area to plane with the modest horsepower that you intend to use. It can be small enough to cartop easily and stable enough to avoid drowning. It will be easy enough to design and build...........It can be a much more comfortable fishing boat if that is also in your SOR.
     
  11. CocoonCruisers
    Joined: Dec 2015
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    Location: Marseille & BuenosAires

    CocoonCruisers Junior Member

    NoviceJoe, what you are trying to achieve is entirely possible. I had the Honda 2.5 hp on this light skin-on-frame tender CORACLE 250 - Nautiraid https://nautiraid.com/en/products/coracle-250/ .
    Alone, i could easily reach planing and speeds around 11 knots. But when you added a second person things became borderline at best.

    What you have proven is not that there is a problem with the power, but that your hull is not efficient enough to get on plane.
    Low-speed planing works better with wider hulls (a higher aspect-ratio lifting surface is more efficient).
    That's why BlueBell's idea to glue the thing back together around your outboard's leg makes sense. It will get the drag down at least 30-40%.

    One thing that kills lift very effectively though is even slight rocker, and some sup/surf/etc boards have some for reasons that have nothing to do with low-speed lift. You might want to have a close look at your board regarding that.
    Why ? Because rocker creates downward lift. Just hold the back of a spoon under a water tap to get a feel for it:

    If you're only after lift, a scow is good but this is even better:
    [​IMG]
    And if you want to take things to the extreme, put the outboard on the long edge instead of the short one, to maximise the aspect ratio... but drive carefully as it will become quite unstable. It will be very easy to submarine the contraption !
    Good luck :)
     
  12. BlueBell
    Joined: May 2017
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    BlueBell Ahhhhh...

    Come on NoviceJoe, indulge me, show me how it doesn't work!

    It'll work! You've got 18 times more power than the average Joe!
    And, it's sustained power for as long as you keep fuel in the tank!

    Use the SUP paddle to keep your balance.
     
  13. NoviceJoe
    Joined: Jul 2020
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    Location: Dorset

    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.

    View attachment 158909
    View attachment 158910
    View attachment 158911
    View attachment 158912
    NB: The aft 140cm of the hulls are virtually flat, so I am not too concerned about the 'rocker' effect.

    Sorry if I have broken forum etiquette but this is also posted in the hydrodynamics section on this site (I have two concurrent threads going) oooups.
     
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  14. kerosene
    Joined: Jul 2006
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    Location: finland

    kerosene Senior Member

    Wanna do a test on a budget?

    30mm XPS board is here euro 7.50/Board (60cm x 250)
    Cover the in between area with XPS foam. two layers glued together (use 2 layers of 30mm or 25mm so you can make it retain the curve in the bow). Cover with layer(s) of glass and epoxy.

    Epoxy is the largest cost.





    [​IMG]

    upload_2020-7-26_22-36-31.png
     
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  15. Rurudyne
    Joined: Mar 2014
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    Location: North Texas

    Rurudyne Senior Member

    Suggestion: those table legs would make a great place to mount extra structure, supporting a pair of upside down buffet/serving tables either side of the main table and carefully positioned to serve both as stabilizing amas and for wave drag reduction effects.

    Also, for wave system resistance reduction it may help to use an old executive board room table, as they are much longer, affording better opportunity to place the buffet tables farther aft from the bow.
     
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