Simple Steps

Discussion in 'Boat Design' started by JPigott, Oct 24, 2002.

  1. JPigott
    Joined: Oct 2002
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    JPigott Junior Member

    I'm designing and building a 23-24' 8'6" beam stitch and glue sportfishing boat.

    My needs are:

    Reasonably Shallow draft at or below 12 inches, shallow water deeper flat capabilities. Working with 12 degree deadrise and will give up rough water capabilities for lower horsepower reqs, shallower draft and reduced roll.

    Reasonable offshore capabilities, Northeast Atlantic, Tuna
    Decent and efficient speed 50 mph or below
    Outboard powered 140 to 200hp
    Hull weight 1600-1800 pounds or less. I'd like to vacuum bag as much as possible on Okume or possibly meranti

    I'm adapting a stock plan and recalculating scantlings and all the hydrodynamic and hydrostatic features.

    I've got the Edmunds, Larsson Eliason books, Fiberglass, Devlin book etc etc etc.

    I plan on building a small prototype. I have locked down all the primary design features. I'm still tweaking what I want below water and the best transom type for my needs.

    1) My question would be what could I use for reference for a simple/modest proven step. I have an idea on how to accomplish it with notched stringers and inset step planks and filleting.

    2) Also what are the advantages disadvantages of a inset transom/motor mount. I would imagine that bringing the motor in would improve draft in the rear slightly. What are the disadvantages.

    3) Also want to incorporate a door for diving and tuna. I want something that doesn't compromise hull strength significantly and blends well with the boat. A slide down side door dovetailed or something similar is what came to mind.

    4) I'm using Rhino and wondering what add ins might be helpful to quickly ascertain basic hull performance characteristics.

    It is not a center console but a trim cockpit cuddy with a gradual step up from stern to a large front casting deck. It's a low profile fish around. The front deck also slides up with canvas underneath to provide substantial headroom for this size of boat.

    Any suggestions or resources?
  2. duluthboats
    Joined: Mar 2002
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    duluthboats Senior Dreamer

    JPigott, welcome to the design forum. You have a very interesting project going. I know very little about stepped hulls so I can’t help you there. This link will take you to some marine plugins for Rhino. If you haven’t found them already.

  3. Guest

    Guest Guest


    As Gary said, a very interesting project.

    Question. If you are going stitch and glue, where does the vacuum bagging come in?

    1600 - 1800 lbs is doable but maybe pushing it for the power and speed you want in the open ocean. With no superstructure, the hull bottom and cockpit sole should be made into a torsion box with longitudinal bulkheads and the bottom should be at least 3/4 inches thick to absorb the shock.

    Bringing the engine inside the transom will increase draft aft on the same waterline.

    A step will greatly increase longitudinal stability (stiffness) which translates into a very rough ride in open water waves. The no free lunch thing.

    12 degrees aft deadrise is about the limit for the size and weight you want. You want to keep the aft chines immersed.

    No problem with the transom door provided the engine thrust moment is properly transferred to the hull.

    Keep us informed.
  4. JPigott
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    JPigott Junior Member


    Hopefully you mean interesting in a good way! ; )

    It is actually a very clean and good looking design. I'll post a wireframe or rendered version when I pin down a few thngs.

    It is a composite stitch and glue design. Using epoxy and biaxial glass.

    Similiarly it will use stringers and bulkheads in an egg crate fashion with the appropriate structural features to distribute loads effectively. Similarly the integrated cockpit, scalloped gunwale steps and front deck frame are monocoque and will provide additional structural assistance.

    Not sure if I understand the statement about the rear waterline. Perhaps what I'm saying is I'm bringing in some of the weight towards the lcg/lcb in order improve the trim angle in a resting state. This is all very preliminary. I am in the first turn of the design cycle but am looking to avoid reworking it too many times. To a large degree I'm little lost on the true effect thus the question. This particular aspect is not so critical and I realize that an outset outboard would potentially provide better rough water characteristics.

    3/4 of an inch sounds a little thick. I don't want to double the bottom boards, so if I can't achieve the stiffness required I will reduce my top speed expectation and perhaps increase the deadrise to deflect pounding forces. I will use some finite element analysis although the step seems a little problematic.

    Thanks for the plug in help. I still want to calculate everything by hand but I'm looking for some quick numbers for evaluation purposes.

    I will have it evaluated by a Naval Architect/Yacht designer familiar with this type of construction before building.

    Finally, I have read all the most recent relevant posts and see some information resources that would be of benefit.

    Thanks again.
  5. JPigott
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    JPigott Junior Member

    Also while I continue to read and reread many of the sections on design, there is nothing like experience. I can't say I truly know what I'm doing. I'm benefitting because the original hull design was thoughtfully and competently designed and evaluated based on experience, comprehensively calculated, and scantlings are based on finite and derived characteristics.

    However, since I'm departing somewhat from the original design I'm a little intimidated by the process.

    Boat design is as much art as it is technically demanding.

    I guess I'll just have to jump in and revise and learn as I go. I have two full months to finalize the design. The inshore offshore requirements are 75 % 25% respectively. I'm looking for short offshore trips and careful planning to reduce the need for a heavy duty offshore hull. Similarly, I don't expect to run at higher speeds unless conditions are perfect.
  6. Tom Lathrop
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    Tom Lathrop Junior Member

    If the waterline length is held constant, bringing the engine inside the transom will reduce the aft buoyancy because of the necessary cutout for the engine. Thus the stern will settle by some amount.
    I was referring to a plywood bottom when I mentioned 3/4 inch thickness.
    I finished schooling on such subjects a long time ago and avoid them now by building on past experience of what works and a few intuitive leaps. Happily, this is adequate for the kind of designing I do. There is plenty of calculating but no computer driven designs. Not because I think that these programs are bad, just that I enjoy the hands-on approach more. As someone said on this forum once, a pencil sketch on a napkin by a good designer is worth more than the slickest 3D picture from a computer design program if the input is flawed.
    By the time you are finished with the design, you may find that you are confident enough to go ahead on your own but having the blessings of a working designer can be a plus anyway.
  7. JPigott
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    JPigott Junior Member

    Thanks again Tom, good information. I can probably get some of the assistance I need from the original designer. To a large degree I have already learned much from him. He and some of his colleagues put me on to the right books and have shared a lot of insights into building and design.

    There is a lot of interesting terminology to contend with, i.e. abaft ship, athwart ship in these books.

    The implied catch is I have to do the majority of the work first, which is only fair.

    I am also approaching the limits of a single sheet composite for curved designs to my understanding.
  8. Bill the Cat
    Joined: Oct 2002
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    Bill the Cat Junior Member

    Sounds like a real neat project.

    At Devlins web site, he describes using the bending radius of 1/4" ply to make a composite turn and then doubling the the layer to get a 1/2" thickness (see the Godzilla plans and building area).

    Do you plan on developing a web site & sharing the fun?

    My project isn't nearly as adventurous - but I'm the way to building my own design Flats Boat.

    Good Luck!
  9. JPigott
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    JPigott Junior Member

    Bill the cat,

    checked out the site very nice. Looks like you have it all layed out nicely. Growing up in Texas, I can appreciate the design your pursuing. My uncle had one of the early ultra shallow flat bottom skiffs built in my grandfathers garage. Circa 1973.

    I've been wanting to build a boat for 10 years. Truthfully the Internet gives so much more breadth to the subject and a connection to both seasoned individuals and those in pursuit of building to there own needs. I wish you best results and certainly the education is worth the price of admission.

    I've dabbled in everthing, even web design so that is a great suggestion. I recognize the front page web.

    Sometimes it's the simplest things that are the most challenging. For example securing a suitable building location in colder climates. I have one hopefully I can secure for the period required to finish my boat.

    In this crazy world sometimes taking pride in your own creations is the greatest thing.

    Best Results.

    J. Michael Pigott

  10. Jimboat
    Joined: Feb 2002
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    Jimboat Senior Member

  11. Bill the Cat
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    Bill the Cat Junior Member

    Hey guys,

    Thanks for the encouraging words.

    the Simmons Sea Skiff uses an inset transom:

    Is this where your design is headed? It's certainly better than hanging out over a stern in choppy seas begging a sick outboard to kick.

    My forte is good web searching - the one I like the most for your defined needs is Devlin's Sea Chaser -

  12. JPigott
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    JPigott Junior Member


    I came in late last night so perhaps the wine was talking more than reason. Your multi hull design looks pretty adventurous.

    Good resources and thank you. Yes those were along the lines of consideration. I had forgotten about those boats that I believe were popularized in the Chesapeake area.

    Wellfleet is just short distance away for me. I am in Chatham Cape Cod. Might be interesting to check them out.

    After running an inboard a little bit this summer, I'm firmly interested outboards and there simplicity. However, at Tom said there is no free lunch. I prefer not to go twin engines so swamping the motor is a concern.

    Well someone once said to me about another subject a picture(drawing) is worth a thousand words I will attach one shortly

    Attached Files:

  13. JPigott
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    JPigott Junior Member

    This is a rough style drawing. Couldn't locate the top view. Probably have to make several more modifications but it contains most of the features I'm trying to design.

    From back to front. Decided to go with a standard transom to get as much space as possible out the boat.

    The aim of this boat is versatility.

    The poling platform is to slide down when not in use. Clearance for the motor is something I'm still pondering.

    The cockpit area is aluminum frame cage with fiberglass/composite facings forward. If I decide to move to Florida, I'm trying to avoid a greenhouse effect.

    On each side of the narrow cockpit area is a raised step/walkway with a slight incline forward before stepping up to the mostly flush front deck.

    I'm tweaking this space to accommodate two berths. Leaving more room for other things forward under the deck area.

    The front deck will raise up on a simple inverted yoke which slides on a coupling shape built into the bulkheads on both sides.

    Canvas will bridge the space from deck to gunwhale.

    The small cutouts shapes are tentative vents for circulation.

    I'll put a web site together. Unfortunately I'm in the process of moving to a place without distractions.
    Last edited: Nov 13, 2002
  14. JPigott
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    JPigott Junior Member

    One other thing, I don't expect to be able to pole this thing too efficiently. Instead, I would use something similar to the Lenco Trim N Troll. However, I would be looking for a raised platform that can serve multiple purposes in the back.

  15. JPigott
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    JPigott Junior Member

    Little less flat looking

    Attached Files:

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