Prop-Tunnel, Diesel/Electric

Discussion in 'Hybrid' started by micah719, Mar 19, 2013.

  1. micah719
    Joined: Jul 2012
    Posts: 30
    Likes: 7, Points: 0, Legacy Rep: 92
    Location: Somewhere in Germany

    micah719 Plotting Dreamer

    Hmmm, trying to nut out a suitable system concept is like the old "There's a hole in the bucket, dear Liza" song...tailchasing around in circles.

    Hull shape & size:

    LWL 35 ft, LOA 40ft, waterline beam 11ft, draft 3ft, displacement 20k lbs +. Flatbottomed steel Junk with fine entry, full lines amidships with twin chines and modest bilge keels, moderate rocker on bottom. Lifting fenestrated barndoor rudder and bow daggerboard. 100% prop tunnel. This is an extreme shoal-draft setup with excellent beaching ability, and trying to retain reasonable openwater passagemaking and heavy weather safety. I'm playing with the the idea of a mid-topside chine to add bouyancy as heel increases (especially into the browntrouser arc), with a deckhouse. The ultimate would be self-righting, but close enough is ok since whatever tips her over should be enough to get her back into positive righting moment.

    Propulsion parameters:

    Main engine use would be harbour/river/canal maneuvering, with power reserve for emergencies. Figuring on 7 knot max speed under engine alone. More would be welcome, but not essential.

    House loads are minimal, though the ability to run a decent welder/compressor/pump setup is important.

    Rough guesstimating gets me to a 20 inch prop with max 4% tip clearance to tunnel, driven by a high voltage (ballpark 120V) DC e-motor with DD battery bank/diesel genset supply, backup charge from solar panels/towed generator.

    My guesstimating is running into a brick wall with a suitable battery bank size. Replacing as much dead lead with useful batteries as ballast seems a good idea to me. The hull has oodles of space along either side of a central bilge channel. Subdivided, vented, battery compartments wouldn't be too hard to build.

    Questions:

    Are my estimates of a 30kW e-motor, slow running 30kW diesel genset on the right track?

    Is the efficiency gain of a prop tunnel combined with e-motor torque and optimum genset rev-range a viable concept? Desired advantages are reliability and low running costs.

    Is it feasible to split the generator capacity into two smaller units, and can these be run in series when maximum power is needed? Advantages sought here would be redundancy, economy at cruising speeds, and options in weight distribution/space efficiency.

    Of course, I could eschew the motors altogether and go all-sail, but set up the hull with provision for adding system components later.

    Let the brainpicking commence, and thank you in advance for any replies!
     
  2. micah719
    Joined: Jul 2012
    Posts: 30
    Likes: 7, Points: 0, Legacy Rep: 92
    Location: Somewhere in Germany

    micah719 Plotting Dreamer

    Ok, I've been dredging the mass of info on this site and elsewhere, and have realised that 30kW off the genset is a different kind of measurement than 30kW out of the e-motor. Revising downward to a 20kW motor with a 20kW genset puts me in confident territory with some room to pare off even more.

    Prop size also seems to be able to grow, though I'm not sure whether a fixed blade or a variable pitch prop is best...given that she'll be under sail as much as possible, I'm curious as to how draggy the prop is when it's in a tunnel and if the expense and complication of a variable pitch prop is worth it.

    Also, for constructing a scale model I want to know tunnel dimension fairly accurately, though I guess I could first build the tunnel oversize and fill in layers for smaller sizes when the high-tech fishing rod tow-testing begins. (Thanks to Mr BATAAN for the tips about scaling models, btw...).

    Another concern is bouyancy at the stern...I was originally thinking of a free-flooding stern well like the original Chinese junks, but losing the bouyancy of the tunnel suggests to me that I can't have both. So, a tunnel it is, with a JSD for the browntrouser downwind stunts.


    Still wondering about these two queries, though...

    Is the efficiency gain of a prop tunnel combined with e-motor torque and optimum genset rev-range a viable concept? Desired advantages are reliability and low running costs.

    Is it feasible to split the generator capacity into two smaller units, and can these be run in series when maximum power is needed? Advantages sought here would be redundancy, economy at cruising speeds, and options in weight distribution/space efficiency.
     
  3. FAST FRED
    Joined: Oct 2002
    Posts: 4,519
    Likes: 109, Points: 63, Legacy Rep: 1009
    Location: Conn in summers , Ortona FL in winter , with big d

    FAST FRED Senior Member

    I suggest you locate and read Nigel Calders article in Professional Boat Builder .#142 April and May. The current issue.

    Even if you have to pay for it a couple of bucks might save a $100,000 mistake.
     
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