Low powered. planing tinny copy

Discussion in 'Boat Design' started by Horsley-Anarak, Aug 17, 2014.

  1. Horsley-Anarak
    Joined: Jan 2009
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    Horsley-Anarak Junior Member

    Have just built a 1/4" ply boat, car top-able, light that planes with a 3.3 Mariner.

    Have not weighed it yet, but would guess 30-35KG.

    Have I missed any obvious things that may improve the performance, but will not greatly increase the weight?

    I looked at pictures of aluminium boats, and used that for the basic shape.

    Would additional strakes help, and add additional lift?

    I would be grateful for input on how to improve the hull.

    Thanks

    H-A
     

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  2. lewisboats
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    lewisboats Obsessed Member

    Wow... good job. Planes with a 3.3? I don't think you can beat that. I don't expect that any fine tuning of the hull will get you more than a 1/2 mph more. You could try different props on the motor, or go with a 5 hp if you really need to go faster.
     
  3. PAR
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    PAR Yacht Designer/Builder

    The picture doesn't show the boat up on plane, but in displacement mode (bow seemingly fully engaged, quarter wave still attached, etc.). What speeds are you seeing at WOT? Without more power, strakes will be on no benefit, just additional drag.
     
  4. Horsley-Anarak
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    Horsley-Anarak Junior Member

  5. PAR
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    PAR Yacht Designer/Builder

    Full plane speed should be 3 times the sq. root of the LWL, times 1.3. At 2.5 times this speed, you're climbing over the hump and the bow is thinking about coming down again. At 2 times this speed, you're scooting along, but you're on the back of the bow wave and not really in full plane mode.
     
  6. tom28571
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    tom28571 Senior Member

    We had a long running discussion/argument thread on here last year that never resulted in any consensus definition of when a boat was planing or not. There are almost as many opinions as people looking at the issue.

    Your boat looks to be very efficient with the 2hp and I doubt you can do much, if any, better with it. It does not meet with some people's definition but is certainly well beyond the displacement as well as the hump stage. Hull speed for your boat at LWL of approx 10.5' is about 5mph and it appears to be well beyond that. The reason I don't particularly like one of the usual definitions as given by PAR is that there are too many types of boat that don't fit in the narrow range where that definition might make some sense apply. SWATHS, catamarans, foilers and really long narrow hulls to name some of them. I find the whole idea of "hull speed" to be too simplistic and misleading when applied to planing craft.

    My personal definition of planing is when at least half of the displacement is supported by dynamic lift. Even that gets sticky in some cases like foilers. The video shows it may not be quite there based on how far the hull rises to my eye but it is inconclusive.
     
  7. PAR
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    PAR Yacht Designer/Builder

    I understand the general argument, but we're looking at a boat with what appears a relatively continuous and quite modest deadrise, from midship aft. This type of hull form actually requires more power to climb the hump then a warped or flat bottom hull form. The stern wave is still attached and though this hull form wouldn't have as much bow rise as other forms with more initial dynamic potential, shows maybe a single degree of bow up trim. Given enough power, I suspect this hull would adopt the typical 3 to 4 degrees of trim and scoot off nicely, given the limited views of the hull, a reasonable assumption. If this hull is as I think it's shaped, she'd probably hold this trim angle throughout the acceleration portion and well into her operational range. So, with these assumptions, she'll be at a S/L ratio of ~2.5 when her stern waves leaves and ~3.0 when she settles and the quarter wave rolls over. This isn't what I see in the picture.

    What I see in the picture is a good bit of compression under the bow, that isn't even touching the chine strake yet, very little squat, which would be untypical of even a modest monohedren, a visible dip in the LWL in the last half of the hull, suggesting the quarter wave has a long way to go yet and the stern wave still attached, though clearly a hole is developing, mostly from the prop wash, not the transom.

    As to when full plane is achieved, well that argument will go on long after we're dead, but 2.5 - 3.0S/L is fairly well accepted. Maybe you can cheat this with some hull forms, but a hull like this is pretty easy to access. She'd do well with a 10 HP outboard, easily achieving full plane and likely running in the high teens, low 20 MPH range, but she has to get up and clean off first.
     
  8. Mr Efficiency
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    Mr Efficiency Senior Member

    With a 3.3 hp motor, being able to disappear out of sight on a dark night would be difficult ! Try a 9.9 and feel the fresh air rush past !
     
  9. Horsley-Anarak
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    Horsley-Anarak Junior Member

    Thanks for the replies.

    Will do some more testing, with Go-Pro and GPS.

    With only me in the boat and the 3.3 it does go will, surprisingly well I thought.

    I know the planing issue is a very grey area, but I still believe there is more to be learnt about light hulls using small engines that achieve twice or more than their calculated displacement hull speed.

    I have a number of 6hp twins so will give it a go with one of them as well, I feel it should do high teens as Par suggests.

    Fitting a 10 HP will make it go faster, but then I would need to strengthen most of the boat, thus increasing weight.

    As being "car topable" was one of the main design criteria, keeping the weight low was a challenge, I am looking for best performance with a small engine.

    Here is another short clip http://s269.photobucket.com/user/Horsley-Anarak/media/DSCN1438_zpsa36cdbce.mp4.html

    H-A
     
  10. PAR
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    PAR Yacht Designer/Builder

    If you use a 9.9 and assuming you'll have a 500 - 600 pound load (engine, fuel, you, cooler full of beer, etc.), you probably will not experience enough speed to test her structure all that much. Maybe some local deformation occasionally, if you fly off a wave, but mostly you'd be okay. Much more then the low 20's and yes, you'd have to consider some bottom reinforcement. Of course, a prudent skipper will back her down when conditions insist.
     
  11. myark
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    myark Senior Member

    Great stuff, even planes with a 2 hp
    A boat when it does not plane will pushe a wall of water in front of it as yours the water is clearly slipping under the front.
     
  12. rwatson
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    rwatson Senior Member

    While you have your Go pro running, stick it over the stern and see where the water level comes to while running at speed.

    My bet is that it wont be very high up the transom, so that would be planing for most definitions
     
  13. Horsley-Anarak
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    Horsley-Anarak Junior Member

    That was what I was thinking, will do that.

    H-A
     
  14. lewisboats
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    lewisboats Obsessed Member

    I built a little 10 footer similar to yours. I was clocked at 23 mph by another boat with my 9.5 SeaHorse Johnson pancake motor. It was built of 1/4" Luan stitch and glued and weighed only a bit more than yours. You needed to sit on cushions or else you needed to enjoy a good "Paddling"! :p

    [​IMG]

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  15. Horsley-Anarak
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    Horsley-Anarak Junior Member

    Wow 23 MPH, did it flex at all?

    I did notice a bit of flexing on mine, but nothing worrying.

    No offence, but I think mine is a little prettier :)

    H-A
     
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