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Thread: Outboard well design in flat bottom displacement speed boat

  1. #1
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    Default Outboard well design in flat bottom displacement speed boat

    Looking at drawings and photos of outboard wells for banks style dories, I see a wide range of outboards being set to different depths, often below the cavitation plates. Is the idea to get the prop deeper than the belly of the bottoms rocker and into clean water. Does it even matter at 5.5 knots. I’m sure all designs differ in their optimum depth but is there a rule of thumb like on transom mounted planing hull boats? Can the prop be mostly “shadowed” by the boats rocker and still perform well?

    I’m coming at this in terms of shallow draft and simplified haul up mechanisms for beaching a 27 foot dory with a vertical well and a 9.9 high thrust outboard.

    Thanks

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    Default Re: Outboard well design in flat bottom displacement speed boat

    Something of a misnomer in the thread title isn't there?A displacement speed boat is hardly a logical connection.I have seen a few wells that were lengthened to permit the motor to swing up quite a lot and some that had the bottom planking astern of the well cutout removed and a substitute bottom a few inches higher so the aft face of the well wasn't there to act as a brake.

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    Default Re: Outboard well design in flat bottom displacement speed boat

    Search Google images for "dory with ouboard well". There's a ton of pics.

    A straight drop-in well is to be avoided if possible. The best option is a widened transom with an open well aft, to reduce drag and to vent exhaust gasses.


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    Default

    Quote Originally Posted by John Meachen View Post
    Something of a misnomer in the thread title isn't there?A displacement speed boat is hardly a logical connection.I have seen a few wells that were lengthened to permit the motor to swing up quite a lot and some that had the bottom planking astern of the well cutout removed and a substitute bottom a few inches higher so the aft face of the well wasn't there to act as a brake.


    I read him to mean a displacement speed design as opposed to a planing speed design.


    Kevin


    Sent from my iPhone using Tapatalk Pro
    There are two kinds of boaters: those who have run aground, and those who lie about it.

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    Default Re: Outboard well design in flat bottom displacement speed boat

    I saw an incredibly clever well in a little dory. Since the motor was above the lip of the well, it only had to be a bit wider than the prop diameter. Vertical front and sides, sloping back.

    The motor itself clamped to a separate piece of wood. At the top and bottom of the port and starboard faces a bolt with bushing stuck out and this rode in a trough at first vertical but then radiused to horizontal.

    The idea was that you first lift the motor straight up, then the upper what amount to little wheels start the turn and the back of the motor begins to rotate up. Eventually the motor is horizontal and since the motor itself has traveled forward, the actual hole in the boat has much less fore and aft length than a well that just allows tilt up.

    The watertight part of the well is right in front of the moving mount plank with the sides and front of the well extending over the bottom as far as needed. A plank dropped over that space when the motor was down making for ample stern sheets and stowage under.

    It would take some time to noodle out the exact dimensions for any boat and motor but shouldn't be too hard.

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    Default Re: Outboard well design in flat bottom displacement speed boat

    27 feet. St. Pierre dory?

    Make it high sided, like either 20 or 25 inches for a long or extra long shaft outboard, in order to have enough freeboard in heavy seas.

  7. #7
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    Default Re: Outboard well design in flat bottom displacement speed boat

    Yes, poor choice of words, how about: “hull speed limited heavy displacement flat bottom boat”?

    I grew up around boats with wells that opened through the transom and some that tilted up inside the well forward of midship but these were planing Carolina dories where the cavitation plate was flush or just below the hull or tunnel bottom.

    I’m asking specifically about a displacement speed limited flat bottom boat with a vertical well.

    A tilt well and open transom not so suited to the narrow bottom, narrow tombstone transom and limited aft cockpit of a banks-ish dory with a forward cabin.

    Just to get this out of the way: I understand the limitations of a cranky unballasted dory and how vertical wells are sub optimal. I’m just curious about the prop depth and placement of the cavitation plate in regards to 10 inch prop behind 11 or so inches of rocker. I guess it could be easily tuned with trial an effort.
    Last edited by Close Call; 04-10-2021 at 05:22 PM. Reason: Fat thumbs

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    Default Re: Outboard well design in flat bottom displacement speed boat

    You're not likely to get any cavitation with an engine that will drive that boat at hull speed, but you are likely to get significant ventilation if the motor is set too high. So go with a 25" shaft motor and set the motor as low as possible that still keeps the water from overtopping the well. If you hit anything with the lower unit, then you're snookered in a major way. A drop-in well in that boat is a terrible idea, but if you're committed to it, then set the cav plates low to keep water from being sucked out of the well and ventilating the prop. Been there, done that, regretted it totally.

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    Default Re: Outboard well design in flat bottom displacement speed boat

    Ian- that is interesting. I can almost picture it, it would still take 20 or so inches of room aft of the forward face of the well. I’ll have to draw that out. To see if there is room.

    Terry- Thanks for the info on the dry well, I was wondering if the prop would pump it clear it and whether the cav plate would stop the aeration without it being way below the bottom. Did you have a thick rubber gasket at the well orifice or a bomb bay door?

    I have a really nice 9.9 fi Tohatsu with a 25” leg that is starting this.

    thanks
    Last edited by Close Call; 04-10-2021 at 05:52 PM. Reason: Fingers

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    Default Re: Outboard well design in flat bottom displacement speed boat

    I had a 23' St. Pierre style dory that I ran up the Inside Passage from Seattle to Sitka. It had a longish well that allowed the OB to be raised enough for beaching, or to kick up when hitting the rocks, which happened way too often. It had a 9.9 HP 4-stroke Honda with a 25" shaft. Ventilation and cavitation was never an issue, but the exhaust cloud coming out of the well nearly killed me. So, go with the wider transom with the stern opening to get those gases vented out the rear. Nexus Marine has a great design for the boat you have in mind.




    http://www.nexusmarine.com/st_pierre.html

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    Default Re: Outboard well design in flat bottom displacement speed boat

    A straight drop-in well is to be avoided if possible. The best option is a widened transom with an open well aft, to reduce drag and to vent exhaust gasses.
    A drop-in well in that boat is a terrible idea, but if you're committed to it, then set the cav plates low to keep water from being sucked out of the well and ventilating the prop. Been there, done that, regretted it totally.]
    So, go with the wider transom with the stern opening to get those gases vented out the rear.
    TerryLL is absolutely right - it's the exhaust gases that you should be worried about! I've had to spend 5 hours with my head as far over the gunnel as I could get it when the wind matched our speed in a Banks dory with a drop in well to keep breathing clean air. You've been warned! Moe

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    Default Re: Outboard well design in flat bottom displacement speed boat

    Quote Originally Posted by MoePorter View Post
    ...t's the exhaust gases that you should be worried about! I've had to spend 5 hours with my head as far over the gunnel as I could get it when the wind matched our speed in a Banks dory with a drop in well to keep breathing clean air. You've been warned! Moe
    MoePorter is absolutely right. Running downwind at 5kn with a 5kn wind will put you in a permanent cloud of exhaust gas. Everyone on board will be
    poisoned to some extent.

  13. #13
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    Default Re: Outboard well design in flat bottom displacement speed boat

    OK duly noted I’ll walk away from the idea. Too bad I was really liking the idea of a shoal draft, quick to build displacement pocket cruiser that could take some rough water (regardless of the quick motion)

    I operate a a couple of 700-400 horse alloy beasts for work, just trying to make amends and do the exact opposite. I’m a sailer, but my home port is 99% un-conducive to that.

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    Default Re: Outboard well design in flat bottom displacement speed boat

    You sure don't need to give up your plans to build a shoal draft, quick to build pocket cruiser like a St. Pierre. Just rethink the outboard well. Widen the transom a bit and use a flow-through well and you'll have it all. I'd build it in aluminum fer sure.

  15. #15
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    Default Re: Outboard well design in flat bottom displacement speed boat

    And don't be afeard o' a little ol' exhaust, just change course a bit. I've had the same thing happen while trolling for salmon with a transom mounted outboard maybe 100 times. Change course, voila!

    I spent many years working part time out of a 21 foot straight sided dory with a well long enough to allow tilting the outboard when going ashore. No problem. You just need to add a removable sloped baffle to avoid plowing with the after bulkhead of the well. Make the well tapered, as in narrower aft, add a cleat on each side for the baffle to sit on, drop it in and away you go. You just have to yank it out to tilt.

  16. #16
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    Default Re: Outboard well design in flat bottom displacement speed boat

    I’ve got a half dozen or more sets of st pierre plans but was looking hard at the ones by Tom MacNaughton. It would be months faster for me to build than some of the others I have. I have short summers to build here and only outdoor/un-heated space, so getting the boat dried in before winter is critical. The MacNaughton design due to it’s construction technique wouldn’t be suited to tweaking the transom width.

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    Default Re: Outboard well design in flat bottom displacement speed boat

    And don't be afeard o' a little ol' exhaust, just change course a bit. I've had the same thing happen while trolling for salmon with a transom mounted outboard maybe 100 times. Change course, voila!

    I spent many years working part time out of a 21 foot straight sided dory with a well long enough to allow tilting the outboard when going ashore. No problem. You just need to add a removable sloped baffle to avoid plowing with the after bulkhead of the well. Make the well tapered, as in narrower aft, add a cleat on each side for the baffle to sit on, drop it in and away you go. You just have to yank it out to tilt.
    The value of your post are design details in a well that might result in avoiding carbon monoxide exposure. Your light hearted bit about just changing course is unfortunate. A well in a banks dory can result in a very dangerous condition. In the one I have experience with the shape of the hull created a low pressure area in the aft area that sucked the exhaust into the boat - it was an interesting phenomenon - just like a open back window in some station wagons. In any case it's a potential hazard that two of us have experience with & felt a responsibility to detail.

  18. #18
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    Default Re: Outboard well design in flat bottom displacement speed boat

    I think one of the things you're saying is that the baffle helped to keep the CO out of the boat by changing the flow pattern and not encouraging it to enter. That may very well be the case, it sure does make sense. I don't know because we never ran it without the baffle. Well, we did once but since the well had been designed too low aft water came rolling in over the aft bulkhead, so we raised it a few inches and added the baffle and worked it hard for years.

    I know what you mean by the back of the station wagon. I was the one who got to ride there any time the family went somewhere and I got very sick every time.

    I think that it works differently with the well, that the CO rich water get's sucked up against the back wall and churns around releasing the gas. The baffle keeps it down and under the boat. Anyway, it worked.
    Last edited by Gib Etheridge; 04-11-2021 at 08:58 PM.

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