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Thread: Powering a Dory?

  1. #1
    Join Date
    May 2018
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    Charlottetown, PE, Canada
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    Default Powering a Dory?

    I have been reading The Dory Book because I have been taking an interest in Dories lately, as opposed to sail boats, and I got to wondering about power. The plans I have seen all seem to be roughly the same for a 27' St. Pierre; 6 to 14 knot cruise, 20hp to 50hp, 2400#-2800# hull weight, etc... You can find designs in the book or online in several places. They are advertised as economical long range cruisers, Jeff Spira says on his website and one of his videos 8hp auxiliary diesel could net you a 6 or 7 knot cruise at 1/4 gallon an hour on his 27' design. Then I got wondering about electric motors and outboards; if the hull is that economical would it be suitable for an EV boat? How would you pick the power? Straight hp to kw or is there nuances that need to be considered? What about safety and emergency?
    What I was thinking was an EV outboard and a battery with solar panels to trickle charge and a back up generator in case the sun doesn't shine and you run out of power (this is assuming you start each trip with a fully charged system). But now that I have 2 systems wouldn't I be better off just going with a gas outboard, a small deep cycle batter for systems, and a on board generator\solar to top up the battery? Or do you do an EV boat system with solar and store a backup gas motor with fuel in a locker?
    I like the idea of quiet clean trips around the bay and to close islands but what if after some experience I decide to go to NFLD? or the Magdalenes? How do you balance power needs, range needs, and emergency safety? It would be extremely expensive to have 100kw/h 48v battery on board to ensure you have the power in an emergency likewise a 6.65kw/h 48v batter isn't necessarily cheap and what do you do if you hit big seas and your battery goes dead? generator? back up motor?

    Or since you can sail a dory would it be better to have a economical outboard with a simple sail rig? I know some dories cannot be sailed so I would have to pick a design that fits that use. If I definitively knew a 14Kw motor, or 6Kw motor, would provide sufficient power then I would be able to figure some of this out fairly easily. If a 6Kw is good enough I can likely repower an old evenrude 9.9 with a golf cart motor, or other small EV motor, to make a make shift EV outboard; then it is just a matter of 48v batteries and charging them.

  2. #2
    Join Date
    Jan 2013
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    Richmond, Virginia
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    12,169

    Default Re: Powering a Dory?

    My two cents: Small hi-thrust outboard would give you good and reliable service. New small 4 stroke motors around 25 hp will be very economical to run and would be surprisingly quiet in a covered motor well.

    So much less to worry about: do I have enough charge? should I wait until the battery bank is charged full?

    I have a 20 hp Suzuki, it only came with a 3 gallon tank. It sips fuel.
    Skip

    ---This post is delivered with righteous passion and with a solemn southern directness --
    ...........fighting against the deliberate polarization of politics...

  3. #3
    Join Date
    Dec 2003
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    Sound Beach, NY
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    4,618

    Default Re: Powering a Dory?

    I had a one cylinder diesel in a 26' yawl, plenty of power for hull speed.

  4. #4

    Default Re: Powering a Dory?

    I built "basically" a Ken Hankinson "Ultra Pierre" 26' sailing version, admitadly, modified it a bit ( I have some threads for BooBooToo). I put a Yamaha High Thrust 9.9 with an 11-3/4 x 5-3/4 prop ina well in the stern. I am measuring 3 hours per gallon fuel consumption at hull speed (6 knots) running about half throttle. She sails fine for me and I've been very pleased with her overall performance. Still making tweaks on a few things but am pleased with the results.

    mike

  5. #5
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    Apr 2010
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    Fairfield, CA
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    Default Powering a Dory?

    I recently put a solar electric drive in an 18 foot skiff, and am considering solar electric for a bigger boat. It is clear that gas motors are still the least expensive way to go. An equivalent electric motor is 1.5 to 2 times the price, and batteries sized for reasonable range are many times that. That said, I may still do it. A quiet, fume free motor is addictive.
    - Rick

  6. #6
    Join Date
    May 2018
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    Charlottetown, PE, Canada
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    Default Re: Powering a Dory?

    What are you going to use as a backup in case the system fails or the sun stops shining with not enough batter to get home? Are you going to a fuel generator for emergency purposes or carry an extra emergency outboard? I was thinking about doing electric now because even if it isn't cheap I am not comfortable going too far. Figured by the time I was technology and pricing would have changed. On the other hand, I could go with a cheaper ICE outboard and just wait until the tech is more affordable.
    What size battery pack and range are you planning for? What engine size?

  7. #7
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    May 2018
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    Default Re: Powering a Dory?

    From the responses it sounds to me that I don't need a 20hp engine, I could start with a smaller 10hp or 14hp engine and be going fast enough. My goal is range and not so much speed, if I can motor along at a sail boats pace economically I will be happy. That said has anyone tried a sailing rig on the Pierre dories and did they need a keel or running board? I am assuming your sail area would be quite a bit less than say what could be handled by a sail boat designed to sail, and special attention would need to be paid to wind speed to ensure you don't get into something that topples your boat.

  8. #8
    Join Date
    Jun 2003
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    Default Re: Powering a Dory?

    A dory can have quite a bit of windage and a "snails pace" on a calm day is going to be downwind or backwards in a breeze. Which is when you really want your engine to have "enough". Sailing a dory into a hard chance is not going to work, there is simply not enough power in that hull form.

  9. #9
    Join Date
    Apr 1999
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    Hyannis, MA, USA
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    Default Re: Powering a Dory?

    Dories are most efficient at low speeds and if properly laden have low to acceptable windage. However, as they get up to about three fourths of "hull speed", dories' lack of buoyancy in the stern causes that big hole in the water that takes too much horsepower to climb out with any efficiency.

    The dory shape makes hanging an outboard very tricky and a motor well removes buoyancy right where it's needed. I'd go with a small inboard diesel with a geared down well-pitched low RPM prop, or electric with genset.

  10. #10
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    May 2018
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    Charlottetown, PE, Canada
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    Default Re: Powering a Dory?

    Well I'm not looking to go fast lol... so doing an electric motor and generator, kinda like a hybrid car, isn't a bad idea then? I don't like the idea of a inboard because I want to beach easily and I worry about the prop shaft leaking where it goes through the hull. That said there are haul-up shafts and through hull shafts have been used for a while now lol. The well also takes up space in the boat so I was actually thinking about fabricating a mounting plate that attaches to the transom at a corrective angle more suitable to mounting a outboard. Not sure if that is a good idea though...

  11. #11
    Join Date
    Apr 1999
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    Hyannis, MA, USA
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    Default Re: Powering a Dory?

    Most power dories in the range you cite - 27' and well over a ton - have an outboard rudder and skeg to protect the prop. No place for an outboard. And huge space for an engine, especially a small diesel.

    When dried out the skeg makes them sit bow down but on enough of the flat bottom that they stay upright.

  12. #12
    Join Date
    Jan 2000
    Location
    Cushing, Maine
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    4,162

    Default Re: Powering a Dory?

    There are some small dories that have been successful with wells and outboards. The Dory Shop in Lunenberg does one if you want to see one.
    Ben Fuller
    Ran Tan, Liten Kuhling, Tipsy, Tippy, Josef W., Merry Mouth, Imp, Macavity, Look Far, Flash and a quiver of other 'yaks.
    "Bound fast is boatless man."

  13. #13

    Default Re: Powering a Dory?

    I designed a skew into BooBooToo from just aft of the centerboard trunk (900# steel centerboard) to just forward of the well I designed into the very stern (didn't want to place an outboard well amidships). The prop is just under the transom to bottom intersection and the skeg is just low enough that a 1/2" bar welded to it sits just under the skew on the outboard. While the prop tip is roughly 4" below the waterline, it's a foot above the bottom of the hull due to the rocker. I can easily beach the hull and with the centerboard up, I draw less than 14" of water.

    mike

  14. #14

    Default Re: Powering a Dory?

    My apologies, my auto spell check alters my typing sometimes before I can catch it. In my previous post, skew should be skeg.

  15. #15
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    May 2018
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    Charlottetown, PE, Canada
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    Default Re: Powering a Dory?

    The 2 plans I have been looking at are Spira's Newfie https://spirainternational.com/hp_newf.php and Nexus' St. Pierre https://www.nexusmarine.com/st_pierre.html. They are both really similar with the Newfie being narrower. As a pocket cabin cruiser for 2, or a day boat for 4, what should I expect for performance? I am looking for economy over speed but still want to be fast enough to get there. I might want to try, when I am ready, a longer trip solo. For the most part it is going to be used like a ocean going pickup truck.
    Another plan I was looking at is https://spirainternational.com/hp_clem.php but it is listed to require a bigger engine to power the hull.
    Side note, I also found a first edition Building a St. Pierre Dory by Mark White for $75 Canadian, is it worth picking up? I have a digital copy of the Dory Book I am reading already.

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