Sharpie Schooner 42 with an outboard motor

Discussion in 'Boat Design' started by valery gaulin, Apr 6, 2018.

  1. valery gaulin
    Joined: Jan 2017
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    valery gaulin Senior Member

    Is this a bad idea, a really bad idea, a not so bad idea???

    The center of the propeller is about 15 inch below the waterline. I fitted a standard 20HP or a 25HP high trust both seams to fit.

    Remember that the rudder is quite big and there is boyancy helping when heeling.

    Seams to solve a lot of problem from fitting the outboard inside of the cockpit. Will it cavitate badly???

    Need your input. Thank you
     

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  2. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Mounted like that, you'll likely need a full set of controls and power trim as well, because you'd never be able to reach the puppy. Why are you trying to make this harder than it is? As small splashwell, cut into the transom will do and you can reach it for controls, starting etc. With a shaft, coupling it to the rudder itself, it'll remain clear while underway and you'd have vectored steering under power. It would be much closer to the end of the LWL, so it wouldn't vent near as much. Additionally, this splash well could be moved slightly forward (making it a well) and do exactly the same thing, except it would be still closer to the end of the LWL, further preventing venting.
     
  3. chinaseapirate

    chinaseapirate Previous Member

    I give up. If KISS is overiding factor then that is the best you can do (in the well, not the rudder mount- ridiculous in my opinion). Get the 12x7 prop and don't rev past 2/3 throttle. You wont cavitate the prop until you suck most of the reserve power out of your engine . You would get about 50% better mileage if you could stick a 12x8.5 prop on a 15 hsp but you can't because it WONT FIT. A 9x7 prop would fit on a 10 hsp and get you your 6.5 in flat water. You could get twice the mileage if you went with a 12 hsp inboard diesel with a gear reduction drive(with reverse) and sail drive type prop. Just switch to a center cockpit and put a cabin where you outboard well is. Yanmar makes a 27hsp diesel outboard but the prop still too small and way to powerful a set up unless you want to push 8knots+. All of these setups lose to a 7 hsp aircooled chinese throwaway vertical diesel engine with 3:1 gearing on it coupled to the lower unit of a big broken motor outboard - or tohatsu style where its easy to remove prop size restrictions...
     
    Last edited: Apr 19, 2018
  4. chinaseapirate

    chinaseapirate Previous Member

    Actually, how about this compromise... Buy three trolling motors, 6 props, three different sizes 3 for 3 motor operation 2 for 2 motor operation and one for single. all three will fit in your well. At least double the engine life, 1/5 the cost, better milage, and still "off the shelf".
     
  5. goodwilltoall
    Joined: Jul 2010
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    goodwilltoall Senior Member

    Way better inboard just fwd of rudder, all thats needed is the vertical lift since rudder will steer, looks better, plus all servicing on deck at waist if u build the lift to go that high.
     
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  6. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    Overpropping an engine so it doesn't reach its target RPM is a bad idea. If you don't need the total power, then install a smaller engine. Otherwise, the combustion chamber's pressure and temperature will wear out and/or damage the engine.
     
  7. chinaseapirate

    chinaseapirate Previous Member

    True but those are market based engineering flaws. You can have a larger diameter, lighter weight, lower blade area, lower pitch ratio prop installed on outboard pushing a lower resistance hull even at the ridiculously high prop gearing, and cruise at 1/3 to 2/3 throttle because of OP's lower resistance hull at 6.5 knots compared to to a boat 1/8 the displacement of hers moving 2-3 times the speed which the outboard was originally designed for. The engine if it weren't inanimate would love you for it. I will agree its not really worth it - she should get a 9.9 hsp merc longshaft and live with 6 knots max, marginal efficiency and zero severe condition performance, if she doesn't like the obviously better performing inboard diesel. There is a large space shown in the profile line drawing...fill it up, maybe with a 20" X 21" prop at 350 RPM, some how some way. A continuous duty 22 Amp single phase 220V AC complete with variable speed, reverse wiring, and 5:1 gear reduction, a 90 degree angle drive (with shafts), a couple of sealed bearings and a bit of fabrication for $1000 or less ??
     
    Last edited: Apr 19, 2018
  8. M&M Ovenden
    Joined: Jan 2006
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    M&M Ovenden Senior Member

    Hi Valery,

    Leaving the engine aside for a moment, do you think this boat fits your SOR ?


    1. Cruising and sailing from Montreal, Canada, St-Laurence River, the canals, Lake Champlain, The great Lake area, maybe eventually the great Loop. The ICW, down to the caraïbe. When the weather permits it crossing to the island of Cuba, sailing around the island, island hopping in the Caraïbes, If everything is still going well at this stage sailing the coast of south America.

    2. The sailboat needs to be large enought for a family of 5, my wife, me and 3 kids.


    While I think it's a very attractive design for coastal cruising, is it really big enough ? It just seems too small to me for a family of 5.

    Cheers,
    Mark
     
  9. valery gaulin
    Joined: Jan 2017
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    valery gaulin Senior Member

    @M&M Ovenden

    Is it big enough for a family of 5??? It sure is on the small size. But remember that for coastal crusing provisionning every week minimum is in the planification.

    Also this sailboat needs to be single handed when the family is on vacation from the vacation!!!

    It also needs to be in a size that it is a reasonable building project even if it is never reasonable to build a boat!!! LOL

    It is all about compromise, size is a compromise! Also for coastal cruising with a family it might be necessary to stop more often than not at marina for comfort. This will not be my dexision , but wife decision. I don't want to be charge and arm and a leg ecerytime with a too big of a boat.

    I get your point. When I will be drawing the interior layout thing might be more clear regarding to size.
     
  10. Angélique
    Joined: Feb 2009
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    Angélique aka Angel (only by name)

    No, what you're advocating is impossible to mount in the limited prop mounting space of the outboards on the market, in fact it's no installation option on any outboard with a in height fixed anti-ventilation plate. I've mentioned this in post #25...

    ‘‘ . . . . to mount the larger diameter prop you also need an enlarged prop mounting window on the leg. . . . . ’’

    You're restricted by the current design set up for a max prop diameter on each outboard, which makes that the max to mount prop diameter is limited by the distance (×2) between the center of the propshaft to the underside of the anti-ventilation plate, there's no or almost no room for a larger prop diameter above the manufacturer's prescription.

    Hence there's almost no variation in available prop diameters for each outboard. So the options to alter thrust are near limited to choosing another pitch, and/or another blade area, and/or another number of blades, since the max mountable prop diameter, or near to it, is already the standard in what's generally used.

    [​IMG]
    (pic source)
     
    Last edited: Apr 20, 2018
  11. Angélique
    Joined: Feb 2009
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    Angélique aka Angel (only by name)

    The needed kid space onboard depends a lot on their age, as well as the time to spend onboard.

    A lot of adolescents can't stand to have their parents close around them all the time, and often that's mutual.

    So how old are the children during the anticipated time of cruising ?

    P.S.​

    Best don't go uninvited to their quarters, allow them to have their own space there, that's if you want to keep the atmosphere on board kinda pleasant for everyone.
     
    Last edited: Apr 20, 2018
  12. chinaseapirate

    chinaseapirate Previous Member

    >Angelique Again...all true, except the word "NO" at the beginning of your clarification...thank you for the diagram labeling the anatomy of a outboard motor. I have noted your usage of bold italics and and certainly not going to argue your position there as I have ZERO experience of having anything "installed" by a factory authorized dealership or certified mechanic, and I have not ignored your previous post #25. Without the italics, why can't I recommend installing a larger diameter prop that doesn't NECESSARILY overload the engine. Part of the "window" , already visualized by the installer, is created by whacking the prop restriction/props size "governor"/ anti-ventilation plate off on most smaller mid range and smaller Tohatsu outboard motors with a grinder. If you wanted to increase the skeg depth, that is no big deal either. If the motor sucks air in its final installed position JB weld something above it. I hate the stupid things(anti- cavitation plates). I bought a used 55hsp "commercial" johnson once because it had a lower gear (not ratio) than anything comparable. It had a big fat "prop diameter restriction" on it. I thought I could replace the 15 x 14 with a 13,5 x 17 (why they didn't have a 15 x 17 or 18 in first place for it I can't fathom), so I did and nothing, same speed resulted both 1/2 throttle and full throttle. I think the lower unit in general was just too large to let the smaller prop get enough clean water... of coarse my boat was a 32 foot long Filipino surfboard weighed about 3000lbs. I needed two props - 15 x 18 for full throttle and a 17 x 22 for cruising and a way around that particular models anti-cavitation plate, which contained exhaust or water cooling plumbing in If I seem to dismiss or disrespect the marine industry or architects/designers catered to it, I don't, other than just a "non-applicable.. N/A". I don't design glorified rowboats, floating bathtubs (even if some of them have sharp pointy ends them), double hulled barges (typical "power cat"), or leadbellied sailing craft. The design here in question I am am considering as a small cargo ship.
     
    Last edited: Apr 20, 2018
  13. Angélique
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    Angélique aka Angel (only by name)

    In my view the anti-ventilation plate is a functional thing, to lower the leg's drag it enables to mount the leg higher without the prop getting to ventilate air from the surface.

    An anti-cavitation plate is a non existing item, since cavitation is a different problem that needs to be dealt with in a different way than mounting a plate near the surface.

    However "anti-cavitation" plate is a often used misnomer for an "anti-ventilation" plate, by people who don't know the difference between cavitation and ventilation, or they're just being sloppy in the used names, and so unintentionally cause confusion about what's really going on, and what to do about the negative effects that are caused by each of the different problems of cavitation and ventilation.
     
    Last edited: Apr 20, 2018
  14. Angélique
    Joined: Feb 2009
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    Angélique aka Angel (only by name)

    In reality grinding off the anti-ventilation plate to create more prop space isn't as much an option as you claim there.

    [​IMG]

    [​IMG]
     
    Last edited: Apr 20, 2018

  15. chinaseapirate

    chinaseapirate Previous Member

    Are you kidding? The word "most" is not that difficult to understand. You will LOSE if you want to count pictures (waste of space):cool:.[​IMG] [​IMG]
     
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