Displacement powercat single 40hp

Discussion in 'Boat Design' started by Saqa, Oct 8, 2014.

  1. Saqa
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    Saqa Senior Member

    I should add that electric start is a must as I hate the thought of having to pull start two outboards over ten times a day
     
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    If you can extend the drive leg ( kits are sold after-market for some engines) you are better placed for single engine operation, but still difficult to make it work. If the budget would allow it, 25hp 4-stroke Yamahas (electric start) would be a suitable engine for a twin engine set-up, with a good gear ratio and prop size.
     
  3. messabout
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    messabout Senior Member

    You have a better chance of getting 25 knots, or somewhere close, with a 20-25 foot monohull in combination with the 40 HP outboard. The mono has more interior space, is not as wide, is maneuverable, and presents less complexity in design and build.

    Why a cat?
     
  4. Saqa
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    Saqa Senior Member

    A pair of smaller motors are easily fitted, a number of them do have electric start. That is a fall back option that is proven and reliable.

    Whats missing is power trim/tilt which would be nice to have on a daily use craft. There are aftermarket trim units made for small motors but they are on the other side of the globe from Fiji/Aus and shipping a pair to the Pacific would not be very cost effective. The power to weight ratios offered by the 2 stroke units as well as self servicing in remote areas is an advantage

    Taking that into consideration, if a 40hp main engine can be made to work well, the it can be pretty good all around. A 15hp or something can be carried on board for 'get me home' in case of main unit failure

    I would like to explore the theory behind the performance issues suffered by a motor on a cat. Please let me know if there are any holes in my understanding of the issues

    Prop ventilation and cavitation with the motor
    Wave slamming issues with the pod

    Prop issues - Am I correct in understanding that the prop pulls air down from the surface instead of the denser water in front of the prop? That the pod helps by forming a barrier in the vertical water column in front of the prop so fluid from in front of the prop will be grabbed instead of above the props plane?

    If that is the case, will a decent sized custom foil mounted on the cavitation plate, which extends forward of the prop help? To prevent stress on the cast alloy motor cavitation plate, a foil pretending to be a boat bottom back end can be mounted to the boat just in front of the motor at the same depth as the cav plate

    Does a foil there offer less drag then a pod?

    Since the leading edges of the foil stays below the water line at all times, does it suffer to same extant as the pod with wave slam?

    I visualise a 5mm plate alloy sting ray type shape mounted under the deck using a leg or two made from plate. The trailing edge of the plate in the same plane as the motors cav plate. The 22" shaft motor mounted using a bracket to the deck and at the same height above water as would be on a regular monohull

    The foil of a size sufficient to provide lift to decrease overall draft so decreasing drag but using power trim to keep nose down and boat flat on the water to keep all the waterline length in the water

    Again, I dont know if I actually correctly understand the issue or if my logic is correct
     
  5. Saqa
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    Saqa Senior Member

    Hi mate, you are right in that. Its my location that is making look at cats. I am located in the far northwest of the main Island in Fiji. There is a persistent wind from one direction that blows hard most of the time with constant chop about 2' on the very wide lagoon making any trip out to the breakers on the barrier reefs hell, for me anyway. I have been on monohulls from 14' to 28' and have always been in discomfort from the pounding. I want to relax on the water even on the ride out or in. A cat seems to be the way to go gleaned from prior threads
     
  6. Mr Efficiency
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    Mr Efficiency Senior Member

    A displacement cat would make an efficient platform to troll low to mid-teen (knots) for those Wahoo, which like fast-moving lures and baits ! Run for hours at low fuel use compared to a planing hull which would cost a bomb at those speeds.
     
  7. Saqa
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    Saqa Senior Member

    Something I am curious about. Does a trimaran offer the same benefits as a cat when it comes to narrow displacement hulls getting over the speed limit restricting a mono?

    Is that a more viable option for a single 40hp?
     
  8. Richard Woods
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    Richard Woods Woods Designs

    I am late to this thread as right now I am involved with SailOK in Oklahoma.

    I agree with groper. I think it would be a mistake to use a single engine unless you really had to. Far better to have two, better manouverability, more protection from flotsam, more reliable and safer, you can run at slow speed on one engine more efficiently

    If you had two 40hp you could get near 25knots in lightship mode, not sure of the exact figure without knowing the exact engines, prop sizes etc. You have seen the video showing 16+knots with twin 20hp. But resistance is not linear so you don't double speed by doubling hp

    the pod needs careful design to work properly if you use a single central outboard

    Richard Woods of Woods Designs

    www.sailingcatamarans.com
     
  9. Saqa
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    Saqa Senior Member

    Great to hear from you Richard, thanks for the info. Do you know anything about the foil instead of pod that I mentioned a few replies ago on this page?

    Also trying to learn if a Tri would be the hull choice for single motor use

    I listed the model number of the motor in the original post, cant remember it now. Running a pair of those is an option that I can take if single proves too troublesome to implement during tests
     
  10. groper
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    groper Senior Member

    I'm not sure which engines have factory controls, but I've seen anything from honda 15hp 4str to Yamaha 9.9high thrust to tohatsu 20hp on Richards skoota with forward controls. I think most of these have electric start option also. I know nothing about 2 strokes these days, no one seems to be using them anymore, and quite frankly, they're a pain in ***. The 4str is the way of the future mate :)

    A trimaran is doable, however it will be a lot more time consuming to build. If you search out stabilised monohull, you will find the types of boat that would suit your purposes in this regard, and yes it would work well with your single 40hp.

    The problem is not only wave slap and cavitation but in certain conditions the prop can come out of the water entirely between waves. It depends on the length of the boat, it's pitching characteristic, the pod location, the wavelength and the boat speed. This is why it requires careful design. If ya simply drop the engine deeper into the water, you'll pay a drag penalty from dragging the entire leg along rather than just the cav plate and below. This is why transom mounting is more favourable for this type of outboard installation, and you'll get much better maneuverability aswell.

    What did you think of the camera boat I spoke of earlier?
     
  11. Saqa
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    Saqa Senior Member

    The camera boat looks fantastic. How do you reckon a pair of similar hulls would go if:
    • around four or so alloy box, 'T', 'C' or 'L' extrusion alloy beams bolted for full deck frame work
    • alloy mesh covering front and rear thirds of the deck with thin sheet alloy over the rear mesh deck
    • Middle third dedicated to 8' wide x 8' long x 6' tall cuddy with further 6' of canopy overhang
    • Alloy railing all around
    Powered by pod mounted 40hp

    or

    Three hulls even more slender decked out the same with the 40hp on mid hull

    Will appreciate some guidance
     
  12. groper
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    groper Senior Member

    All sounds good, have seen all this done before and it works. Only thing I don't like is the single motor on the pod. Keep the tunnel clear. If you want to run through that chop at decent speed, you gotta keep the tunnel clearance high, displacement cats don't run on a foam and spray cushion like a shark cat type of planing hull.

    Those types of hulls are super easy to build using tortured plywood. As the compound curvature is so minimal, you van simply bend the ply into shape, clamp and glue. Super easy.

    Alloy beams, yep, or make your own box beams from ply, whatever takes your fancy.
     
  13. Saqa
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    Saqa Senior Member

    At 30' long and 10-14' beam, what do you think of the speed potential with a single pod mounted 40hp?

    and twin 40s?
     
  14. Saqa
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    Saqa Senior Member

    For popping with 8' sticks, I am thinking a max deck height of 1.2m above water
     

  15. groper
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    groper Senior Member

    Not even a professional could answer that with the available Info you given. It completely depends on how heavy it ends up and that depends on the level of comfort and interior volume you build into her. :idea:
     
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