Blackrock 24

Discussion in 'Boat Design' started by LP, Mar 12, 2013.

  1. Tad
    Joined: Mar 2002
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    Location: Flattop Islands

    Tad Boat Designer

    LP....

    I would make the cockpit seats flat, all that crown will make them very uncomfortable. Seat to sole can be anything down to 10-12" but 16" is decent, up to 20" is okay if it's a really narrow footwell. Shallower must be wider. Higher backrests are nice, about 15" is minimum to feel like you're down inside the boat and not sitting on top......

    Here a 9.8HP drawing from another thread on another board. I usually pull these into my 2D CAD drawing as a "Reference Image" and scale to suit, then trace the image and add it directly into the drawing. Note that the bracket is not vertical but angled.....

    mfs9.8-2009.jpg
     
  2. philSweet
    Joined: May 2008
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    Location: Beaufort, SC and H'ville, NC

    philSweet Senior Member

    Lp, your OB well needs help.

    1. A 15 hp bigfoot can push about 6 tons around. You can't use anything close to that. A Tohatsu 3 (all 3's are Tohatsu, I like the little Merc the best out of the lot) will push and maneuver a day sailer version of your boat just fine, the trouble lies in keeping the prop in the water. From a design perspective, I'd size the well to accomodate a range of six hp ob's. My personal choice would be an old two cyclinder 4 hp from Johnson or Evinrude.

    2. The next thing is to get the leg in the water. The ob clamp needs to go right down to the bottom of the hull. Locate the bottomplate of the powerhead about 4" above the waterline. Use a 20" shaft. I've attached a photo of my old Cal 28 showing the ob leg sticking down. That's a 25" leg 15hp Merc bigfoot and this boat had a long and storied history running drugs all over the Carribean for 20 years before I cleaned her reputation up. The waterline was about a foot too high when I bought the boat.

    The simplest way do do this is add a six inch high board on the bottom of the hull a suitable distance behind your bulkhead for the ob clamp, frame carlins back to the transom, put small shelves either side for gas jugs, and drop a six right down to the bottom. You'll need remote throttle and electric start is very nice since you will have a lid over all. My Merc lived for fifteen years in tropical salt water this way. I pulled it once a year for routine maintenance.

    There is no particular reason to have the outboard kick up if it a six or smaller. You just need to route all controls and fuel line so that you can lift it clear for trailering or beaching or sailing without the drag. If you wanted a semiserious beach camper and pocket cruiser and you wanted to go to the trouble, the way to get these to kick up is to put the clamp board at an angle transversely so it swings up and to one side, and mount the rudder about 4" off center so the tiller attaches to the side of one cheek and runs up the centerline. Then you carve a relief in the transom to let the leg slide up with the prop outboard of the transom and turned away from the rudder. Sometimes you have to block the rudder hinge out a couple inches to retain enough rudder clearance. The leg may not completely come out of the water, but it's ten times better than dragging the whole thing around.

    The arrangement you show is sometimes used on a planing motor skiff, but basically doesn't work on a sailboat bobbing around. The deeper you can get the prop, the quieter the water is and efficiency goes up fast.
     

    Attached Files:

  3. philSweet
    Joined: May 2008
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    Location: Beaufort, SC and H'ville, NC

    philSweet Senior Member

    Little details-

    I think the rudder arrangement is going to feel funny in the hand and the rudder skeg is going to interfere with with the well's ability to stay empty. I'd loose everything below the extension of the hull. Rudder cheeks trimmed to the line of hull extension. There is no structural problem with doing this. I'd also get rid of all the balance in the rudder. Little racers do this to make their tillers as light as possible, but it feels dead and unstable. Just run the leading edge of the foil straight down parallel to the hinge line and however far aft the front of the cheeks are - 1" or so. This works like caster on bike wheels, it gives the boat a desirable tracking stability and you can now cruise a course for hours with just a bungy on the tiller. It's not like it's going to tire you out. The problem with a balanced rudder is that the whole boat heaves and pitches and you feel the inertia of the rudder system in your hand. You don't want these perturbations to overwhelm the hydro feedback. So try to balance the tiller and rudder system statically about the hinge and let some hydro forces through. That gives the boat a chance to adapt it's new owner to it's whims.
     
  4. LP
    Joined: Jul 2005
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    Location: 26 36.9 N, 82 07.3 W

    LP Flying Boatman

    Thanks for the reply Tad. (and the drawing of the motor)

    It reconfirms my dimensioning, but tells me I may have the motor positioned too high. My current sailboat has flat seating in it and as soon as she heels, your wanting to slide down the seats unless you brace yourself. My thinking here was that at 10 deg of heel your still sitting flat and tendency to slide would not occur until double that value. That and maybe a textured paint should help keep bums planted. Other considerations for the design were proper drainage and greater volume inside while keeping the sheer reasonably low. I'm pretty tied to the design at this point (build in progress), but I appreciate your comments. The top of my coamings are around 12" above the seats aft and go higher, forward. I'll have to see if I can work in a little more height.
     
  5. LP
    Joined: Jul 2005
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    Location: 26 36.9 N, 82 07.3 W

    LP Flying Boatman

    Phil, thanks for your input. Lots of useful tidbits to chew on.

    I drew in the larger motor with the thought that it would work less hard and be quieter than a smaller motor that is having to work at a higher capacity. I could swing a bigger prop at a lower speed and only work it at a fraction of it's capacity, plus the reserve of the larger motor. I can see that when not in use, the is a higher drag penalty if it's left in the water. I can see the advantages of going smaller though.

    Your clamp suggestion is noted and stored for future reference. Ultimately, I'm going to have to fiddle with this with the actual pieces when I get to that point. I'm trying to avoid cutting into existing structure when the time comes for the power plant installation. So for now, I'm trying to get the big picture right and hopefully I've had enough foresight that the little things work out well. Fortunately, I already have a "carlin" system that is an inherent part of the design. The angled clamp is a great idea for getting the motor leg to rise to one side of centerline.

    To All:

    I'm curious about at what point a genset becomes available on smaller motors. I would like to have the ability to charge the ship's battery with the motor.
     

  6. philSweet
    Joined: May 2008
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    Location: Beaufort, SC and H'ville, NC

    philSweet Senior Member

    Anything with electric start has some ability to charge batts. That's why I suggested you design for a six. There are usually at least two versions of 6's w/wo electric start and recharge. They are pretty puny, but they can keep an anchor light lit, run a depth sounder and vhf, and recharge your cell phone and gps batteries. I prefer to treat all electronics as portable on this size boat. I'd remove everything, including the battery, at the end of each day.
     
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