How to design an outboard on an Atkin Seabright Skiff???

Discussion in 'Boat Design' started by SalmonMan, Mar 30, 2007.

  1. SalmonMan
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    SalmonMan Junior Member

    Thanks to everyone that has responded to this post. I have learned a lot from them.

    What I like most about the Atkin Seabright hull is it’s an “excellent model for rough water service”, “for carrying a heavy load without increasing the draft greatly”, “is the ideal form for use in shallow water” and “especially as to unusual speed with modest power”. Those quotes come from the description of the Rescue Minor http://www.boat-links.com/Atkinco/Utilities/RescueMinor.html written by William Atkin. That is what I am looking for in a boat, but I like the simplicity of an outboard motor verses installing an inboard.

    Sal’s Dad has offered up an interesting solution with his outboard. I will be very interested in learning how his boat performs when he gets it in the water. Please keep us posted.

    I’ve corresponded with Renn Tolman regarding the Tolman Seabright hull and he is planning to run an inboard on his boat. The outboard cutout is for a trolling motor.

    So, maybe I’m barking up the wrong tree. What other hull designs offer semi-displacement speeds at a low horsepower requirement, while at the same time offering a seaworthy, rough water capability? I’ve looked at some boat design plans but by no means all of them.

    Thanks in advance for your replies.
     
  2. Willallison
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    Willallison Senior Member

    FF - sorry, I meant why bother with the SB's complexity if using an outboard. It's attributes when built as designed appear to be well documented - and worthy of further examination

    Tom - The shape you describe has (unless I misunderstand you) already been tried - and with much success. Many displacement catamarans employ it. The only difference being that the 'box keel' is replaced with parabolic or circular sections, that are more efficient in the displacement realm.
    I've been tinkering with a monohulled version along similar lines, which I've called the monomaran. In this incarnation, the L/B ratio is around 12:1, which is considered the minimum in order to be really efficient. It requires some stabilisation, which I've added in the form of inflatable tubes.
     
  3. FAST FRED
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    FAST FRED Senior Member

    "I find this whole realm of the box keel very interesting. I wish someone would build some of the versions with high displacement in a narrow canoe body keel and a low deadrise upper hull. That is, with the keel L/B ratio over 10:1 and 75% of the displacement married to a planing hull bottom that supported only 25% of the displacement.

    It might be possible to allow the box keel to have the plaining angle of attack , so the hull above would still be fairly level.

    The trim might be changed by venting the aft negative deadrise section with an OTS trim tab set.

    While a 10-1 L-B boat would be interesting , the Old Camper and Nicholson pre-war 50 ft launches ran 20K on 100HP, so its a proven concept.

    Adding the light weight of todays materials and engines should allow the LB ratio to fatten and still be cheap to push.

    We are planning on a 38.6 loa with about a 5.5 ft BWL, 7.6BOA (all to fit into a container), so it wont be as fine as 10-1.

    But I'm wondering if the box keel actually was 75% of the displacement , if ONLY the LB ratio of the box keel would predominate?

    Lighter weight equals cheaper high speeds, so the weight of the engine and fuel would be the key in displacement , at 1500 lbs for the inboard diesel and 500 miles of fuel at perhaps 4 nmpg , it won't be too light.

    Atkin had a pretty simple shape of the keel , today it would be easy to create a "more sophisticated" shape , for lower drag and less resistance.
    This would perhaps allow the boat to run aground harder , so might not be great for a designed beachable boat.

    Flat bottom on an airfoil shape?


    FF
     
  4. tom28571
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    tom28571 Senior Member

    Will,

    I would most definitely use a minimum surface area keel. The "box" term was just in keeping with our generic usage. I think a semi-circlular bottom with straight sides. The upper hull would be fairly normal as a low deadrise planing form with more warp than usual to allow a sharp entry. I don't think an inflatable upper hull could have or maintain the shape required for such a boat.

    Fred,

    I think a flat planing surface on the keel might defeat the purpose of the combination as I see it. If the designed displacement of the complete boat would allow it, the L/B ratio of the keel would go higher than 10:1. The idea is to have most of the displacement in the low drag non planing keel while the upper hull would be planing but with such a low bottom loading that it would tansition to plane with no significant "hump" and at a trim angle as low as one degree. If it could not do quite a bit better than 4mpg, I doubt it is worth further effort. My current boat already transitions at at no more than two degrees, so it seems very possible. The closest thing that I have seen is the "Displacement Glider" but that is not there either as they do have a keel shape that I think has too much drag and that funky hull shape.

    There are boats that carry all of the displacement in keels that operate fully submerged but they have to be cats or have fly by wire stabilization. Neither of these fit a low cost, moderate speed, very efficient vessel like I am thinking of.
     
  5. Willallison
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    Willallison Senior Member

    Apologies to the original poster of this thread - we seem to have hijacked it somewhat, so I'll be relatively brief.

    FF - as Tom suggests, there's no point in building any planing traits into the keel section as it operates in a displacement mode, albeit at much higher speeds than the traditional 1.34 SL, as a result of its high LB.

    Tom - Sorry, obviously didn't explain the monomaran very well. The basic hullform is rigid, with an inflatable collar that would ride with only the aft section just above the at speed WL. A pic says a thousand words, but all I have here is a rather shoddy image from 'paint', so maybe it's worth a hundred or so...:D
     

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  6. Willallison
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    Willallison Senior Member

    In reality, the 'keel' section would probably carry greater draft than indicated. The blue line represents that 'at speed' WL.
     
  7. kengrome
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    kengrome Senior Member

    Here's a little one I've been designing in the evenings while building my Tolman Seabright Skiff during the days. It's 12 feet long, 4 feet wide, 24 inches high at the bow, 18 inches high at the transom, 15.5 inches high at the lowest point along the gunwales, and should weigh 250 pounds or less including the engine.

    I plan to build it using mostly 1/4 inch plywood (maybe 3/8 inch on the box keel bottom panel) and power it with a cheap 6.5 HP industrial gas engine. The transmission will be a simple v-belt system with idler pulley for forward and neutral, and no reverse except via oars or pole or paddle of course.

    My concept here is a "Personal Seabright Skiff" ideally suited for one person to use as a recreational / fishing / hunting / coastal explorer, a boat with great fuel efficiency and enough seakeeping ability that the owner won't feel threatened if he gets caught out in the rough stuff. I also want this boat to be able to sneak into just about any shallow backwater areas a person can find, and to run at top speed in less than a foot of water, and to beach it anywhere there is a beach available -- all without the need for a jet drive and with virtually no risk of damage to the prop or drive train or rudder.

    At 6 inches of draft it has 9.5 inches of freeboard and its displacement is 500 pounds, 250 for the boat and 250 for a big person and some fuel and gear. A nice feature of this little boat is that it still has 6.5 inches of freeboard at a total displacement of 980 pounds -- which means it can actually haul 3-4 full-sized adults around safely when the seas are calm enough.

    Note that I colored the sides light teal, the garboard/tunnel sections orange, and the flat box keel bottom white. I also used white on some of the pix to show the submerged portion of the boat. The profile shows 500 pounds displacement (typical running load with one person) and the underbody pix show both 250 and 500 pounds displacement to illustrate the waterline difference between an empty boat and "running mode".
     

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  8. DaveH1
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    DaveH1 Junior Member

    Ken,

    I think this design speaks to a needed niche.

    Could reverse be added to the drive train by means of another couple pulleys and belt(s)? Reverse seems to be to important a feature to live without in remote areas. Getting hung on a rock in water too deep to easily push off or finding a way through a duck marsh with lots of dead ends would be examples of where I would want reverse gear.

    From a hunter's perspective, I suspect there might be a fair amount of waveslap noise generated by the tunnel portions when slipping along at displacement speeds or when still in the water. Maybe not....

    I like the idea of a tunnel-protected propellor as an alternative to a jet, and the low expense and user-friendly maintenance of the simple drive train.

    Do you have a speed estimate for this design?

    thanks,

    dave
     
  9. dick stave
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    dick stave Senior Member

    It occurs that such an elaborate bottom design would be better suited to fiberglass construction (and Im no fan of glass boats) for production. Once a plug and mold were built ,they could be produced inexpensively for a mass market. If you wanted to think big ,a stamped aluminum bottom with proprietary chine / sheer extrusions built in a production jig.
     
  10. tananaBrian
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    tananaBrian Junior Member

    Dave Gerr is likely referring to the fact that you lose buoyancy when you cut sections out of the bottom of a boat, e.g. a tunnel. As with all designs, you have to carefully iterate between CG, CB, and hull form design until you achieve the best-compromise answer. If you change the tunnel dimensions, then it will have an impact on the boat's waterline and lift characteristics when on plane. Get the waterline right and the lift will generally take care of itself on a boat like this.

    In any case, the boat referred to is a very interesting and good design and it would be neat to see it converted to an outboard version ...most likely a smallish (25 hp) outboard in a well and no high expectations for being the first guy on the fishing hole ...but what's wrong with 17-18 knots? Most sea conditions limit you to that anyway and this boat will save a lot of fuel for you so you can go twice for everybody else's once.

    It's very funny and coincidental that I've been beating up on these same questions, trying to come up with an idea that will result in high mileage, reasonable speed, very shallow draft, and good load capacity boat design ...for Interior Alaska's rivers. Most folks around here use shallow V hulls with very very modest (1" deep) tunnels and jet-conversion outboards. These things suck a lot of fuel but are tough aluminum and can slide right over gravel bars. If a guy can beat the efficiency equation, which generally means switching to a prop, and can get up gravel bar-ridden braided Alaskan streams, then he'll be onto something. Suddenly your river hunting range will outstrip the other guys' ... a good thing! I hope to get a design together this summer sometime and I'll be the guy that uses it :)

    Brian



     
  11. dick stave
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    dick stave Senior Member

    I agree an outboard in a well would be the simplest solution for power.The anti cavitation plate could be half of a gasketed sealing surface with motor down, and jacking vertically with a rack and pinion device or a foot operated lever as the old disappearing propeller boats had. For better prop protection, one of those encapsulated (ducted fan ) outboards the U.S. military uses could potentially be employed ("USED OUTBOARDS.COM") sells them at surplus prices. The lower unit sealing issue could be solved with a rubber bellows surrounding the leg and sealed at bottom with aforementioned mating gaskets. There is a greater demand in areas where weeds are encountered, as in the pacific northwest rivers and in alaska the jet drive will always rule.The 4 stroke jets are good on gas and nothing can run more shallow...
     
  12. tananaBrian
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    tananaBrian Junior Member

    To be honest though, I hate putting outboards in a well. There are too many issues that can arise, especially if trying to minimize the size of the well so it mates with a tunnel properly. Water sloshes up the well, exhaust can creep in and stink, lids cause motors to snuff out if exhause IS creeping in, props are hard to service if you get wound up in someone's line or in river gunk. I've never owned or operated a boat with an outboard in a well, but these are complaints that I've heard from friends. Maybe today's clean running 4-strokes would make a difference.

    Noting that the most efficient planing hull possible is going to be the good ol' flat bottomed boat since it doesn't waste energy pushing water out to the sides in trade for a softer ride. This is a fair trade for water that isn't too rough, e.g. rivers and bays.

    If someone forced me into a corner today and made me draft up an efficient design for shallow water use, here's what I'd do:

    - Use a TLDI (type) 2-stroke or modern 4-stroke for the lowest gallons per hour possible. I'd probably select the TLDI 2-stroke since 4-stroke motors can sometimes be higher-momentum motors and you damage more than just a sheer pin (or prop key) if you hit something.

    - Go with a smaller-diameter larger-pitch stainless steel prop (maybe a 4-blade? But probably a standard 3-blade cupped prop.)

    - Flat-bottomed boat with a minor double-chine and modest half-round type tunnel, the boat designed from the start for the tunnel so a) the waterline is appropriate, and b) planing works properly and in a stable manner.

    - Hydraulic jack plate and shallow-water nosecone on the outboard

    - Consider a prop guard on the motor

    Going in reverse in shallow rocky, gravel bar, or weedy conditions is always a pain, especially with a prop guard, but there isn't any perfect answer for the shallows. Most 'extreme shallow' boat designs really don't work in the shallowest of conditions, other than jets and jet conversions ...but that's the trade-off. Work in the shallowest of all conditions and burn too much fuel (jets), or work in not-quite-as-shallow conditions and save a little on fuel (prop). I think that if you design the boat to be light, then getting out and dragging it now and then isn't the worst situation (not dragging on dry ground, but with no people in it ...floating across the shallows with a bit of scraping along the way instead... else give up.)

    Good ol' boys down south with their bass boats use jack plates and shallow water nose cones on flat bottomed boats ...experience tells all when it comes to what works and what doesn't.

    Brian
     
  13. dick stave
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    dick stave Senior Member

    Were getting into a regional discussion here.The guy with the nitro wouldnt last the day where I run,and I would be pulling weeds out of the foot all day where he does. There are always going to be V8 jet sleds in the river and fountains with blown big blocks in the lakes. Moreover, these guys dont give a s--t about what the fuel costs are. That being said, I admire an effort to resurrect an old design and give it another try. I dont think it will gain mass appeal as there is no greatest all around boat and it may be perceived as a gimmick. Still its a pretty cool boat for the home builder.
     
  14. kengrome
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    kengrome Senior Member

    I hope you're right, I like it myself so I hope it performs well.

    Yes it could, although it would obviously cost more.

    I figured that most people would prefer to save the cost of reverse gear and just use the oars to row backwards (or push off other things) until they could turn around. Then again I understand the kind of use you're thinking about, and reverse would be very nice for your use of this kind of boat.

    Maybe when at rest, but I think I should build one and report on this first before guessing at it. From what I have learned about these boats, the tunnels fill with water almost immediately when the prop starts turning, even at the slowest prop speeds, so I don't think there will be any wave slap when you're moving -- at least not under the hull.

    Me too, that's one reason why I'm so interested in this particular hull style. Other reasons include very shallow draft, very fuel efficient, inexpensive inboard engines that any lawnmower shop can fix for you cheaply, etc.

    It will probably do 15+ MPH on 5-7 HP but this is just a guess. One thing William Atkin told his builders repeatedly is to not overpower these boats. I think it was Robb White who said the handling gets squirrelly if you try to run the boat too fast.
     

  15. kengrome
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    kengrome Senior Member

    Hi Dick,

    I'm no fan of fiberglass either, and before I built the hull of this Tolman Seabright Skiff:

    http://www.bagacayboatworks.com/images/seabright02/

    ... I was thinking that a fiberglass bottom would make everything a whole lot easier. But once I built this bottom in plywood I realized that it is only the first build that would create problems. The second and third and all future builds will be much easier, especially when a custom jig is used that makes the panel-twisting task easier. Now that I rebuilt my jig specifically to make twisting this panel easier, I don't even think about using a glass bottom.
     
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