Hull shape/DIY plans recomendations for planning 16' 40Hp boat?

Discussion in 'Boat Design' started by S V, Apr 11, 2019.

  1. S V
    Joined: Jan 2019
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    Location: Lithuania

    S V Junior Member

    Hello everyone,

    This is my first post, I am a total newbie in boatbuilding, and have a (not so distant anymore) dream to build a boat for myself.

    The requirements are:
    -Planning monohull (probably...)
    -40Hp outboard, as the bigger are a bit too much expensive. Maybe 30Hp, if planning is possible
    -Up to 5meters (16 feet and a little) in lenght, as i dont have so much space in shed
    -Up to 1.85m (6 feet) width
    -Mainly for very protected waters, it will not see swells bigger than 0.5m, the biggest water body it will be in is hydrodam reservoir of ~60 km^2 (~25 square miles)
    -Seat 4 people comfortably, in a car-style 2+2 seating
    -The max total displacement will be no more of 850kg (1900lbs)
    -Resonably fuel efficient, fast and stable.
    -"Sport" cabrio style, so only seats and windscreen
    -Will be built from hdpe sheets. This could be a separate discussion, but the principles are pretty much the same as plywood, so any projects for plywood are suitable.
    -Trailerable with general purpose trailers
    -No very shallow water capability is needed
    -Build simplicity is a priority

    What DIY plans could you recommend? What the hull shape could you recommend?

    Currently i am leaning to enlarge by ~10% popular Russian design "Sever 420" (4.20m LOA), like
    Катер «Север 420» | Наборы для постройки лодок — Boat-kits.ru https://boat-kits.ru/kits/kater-sever-420/ (text is russian, but there are pictures)

    or some double-chine design, like Keyhaven skiff from Sassdesign http://www.sassdesign.net/

    Also liked Bluejacket boat videos, but they are way too large for me.

    All types of advice are very welome ;)

    p.s. Yes i looked at many of diy plans, but it is pretty hard for me to evaluate them.
    p.p.s. Modification of the plans is not a problem, i dont need like 100% done boat, proper hull shape is enough.
     
  2. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    Welcome to the forum, I have read your specs, and you do have a pretty good sense of what you want, although I can see a couple of "optimistic" assumptions. Firstly, the requirement of HDPE construction is a little unorthodox. For what reason(s) have you chosen this material ? Do you have expertise in fabricating with it ? It could make it difficult to reach your speed and carrying capacity requirements. as it won't make for a very light structure. You say four passengers, are they all to be adults ? You certainly will struggle with 40 hp to have a 16 foot boat go anything approaching "fast", with four adults aboard.
     
  3. S V
    Joined: Jan 2019
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    Location: Lithuania

    S V Junior Member

    Thank you for your promp and... honest reply and thank you for you future opinions.

    Yes, i havent done the most basic weight calculations as for this moment as I need finished project to do this acuratelly. And your words worry me a little :/

    The HDPE sheet building method is popular in other industry i have experience with (water waste treatment). And yes, I do have the fabrication from HDPE, PP experience, and this part of building worries me least. There are some not so small and trivial chalenges as to make curved lines, but they are not deal breakers. The other reason for chosing HDPE is the other even more distant dream to start special purpose tough boat building business. Ok, I cannot call it business - maybe an extracurricular commercial activity. This will be a pilot boat to see if this idea is feasible. The third reason is that I live in north east Europe where recreational boating season is less than 4 months long, and the other part of the year weather is very humid and cold, and there is no way I can get dry and heated storage for the boat, that pretty much rules out plywood. The fourth reason, which also rules out plywood-epoxy is personal health - my shed does not have industrial grade ventilation, actually it does not have any at all, and the most time of the year you cannot work with open doors... Well that leaves me with welding alluminium, which I have no clue how to deal with, and plastics, which I have some experience with and have some really nice properties.

    The density of HDPE 300 is ~0.95g/cm^3, the alluminium is ~2.7g/cm^3, the starting point of the prediction is that the sheets for the bottom and sides of the hull will be 10-12mm thick. Alluminium is much much more stiff, but this is 0 offshore ambitions boat and I hope with proper scantling its weight/wobbliness ratio can be under control. My initial prediction is up to 280-300kg for boat w/0 engine. The speeds I am aiming are not insanelly fast - 20knots at 3/4-4/5 throttle is ok. Yes, the plan is to have total weight of the boat, outboard, fuel, 4 adults and some stuff up to 850kg.

    Most close HDPE boat example I found:

    padded V design, 4.20m (~14 feet) LOA, beam 1.7m, 6mm bottom, 5mm sides HDPE sheets, weight of the hull w/0 console is 130kg (~290 pounds), builder states that with 2 people aboard, I assume this is no more than 400kg, and 15Hp 40kmh (~22 knots) is possible. Also builder says 8mm HDPE sheets will be ~150kg (~330 pounds). I could bet, that the material is the softest HDPE, not the high quality, but this boat looks for me much stiffer and durable than most of the alluminium john boats.
     
    Last edited: Apr 11, 2019
  4. HJS
    Joined: Nov 2008
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    HJS Member

    You are definitely on the right path. Many boats have been built in this material.
    I have checked your numbers a first time. With the limited length you selected, your calculated total weight is the absolute maximum for the boat to work. I would choose a blunt bow at deck to get a long a bottom as possible.
    It is important that the planing surface has the right size and shape. That's what I worked with in my boats.

    JS
     
  5. S V
    Joined: Jan 2019
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    S V Junior Member

    Thank you for your opinion, HJS, as I can see you are the interceptor guy from the link I posted. It is an honor to have your opinion.

    Can you be more specific about blunt bow? Does it mean, that LWL shoud be as close as possible to LOA?

    What would be your approximate, but not optimistic, prediction for speed? 40Hp, 850kg, 160cm beam at waterline, 480cm LOA, ~5 degrees deadrise transom, warped V, single chine, 1 spray rail, almost no lifting strakes (only as thin as hdpe sheet + a little more, 2x2=4cm total), and no interceptor for start. Will it plane at all?...
     
  6. Mr Efficiency
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    Mr Efficiency Senior Member

    Being adept in fabricating the material would be a good start. I understand this material to be flexible, so you will need to pay particular attention to the scantlings, and select a shape that has a minimum of slab-like flat panels. You mentioned 5* deadrise, I would suggest 8 degrees at least, probably ten, and a full bow, as mentioned, does give more internal room. And seeing you are living at high latitudes, you don't want to be shipping much spray, so a well raked bow, with a not overly deep forefoot is preferred. Chances are you should be able to cruise at say 15-18 knots with a 40 hp motor, loaded, but it is probably the bottom end of powering for a boat of this duty, in terms of passengers etc
     
  7. Mr Efficiency
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    Mr Efficiency Senior Member

  8. Heimfried
    Joined: Apr 2015
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    Heimfried Senior Member

    Hi S V,
    as I understood you will use HDPE sheets to build the boat. In which way will you join the sheets/planks? Welding? (As far as I did read it is not possible to glue pe with normal epoxy and difficult and expencive to glue it with special glues.)
     
  9. Mr Efficiency
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    Mr Efficiency Senior Member

    Welded with hot air, or better still, hot nitrogen. Never done it, myself, I don't know how difficult.
     
  10. Dejay
    Joined: Mar 2018
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    Dejay Senior Newbie

    Does HDPE need to be painted against UV damage?
     
  11. Mr Efficiency
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    Mr Efficiency Senior Member

    I don't know that too much in the way of paint can be relied on to adhere to it. There was a builder who came to the forum a while back, who only built in black HDPE, saying it was the only thing that stood up to UV. I have seen quite a few around in green though, in a climate where UV can be severe. I guess they start looking a bit shabby over time, if seeing too much sun though. A boat kept under cover, the colour should not be a problem.
     
    Dejay likes this.
  12. S V
    Joined: Jan 2019
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    S V Junior Member

    Hemfried, thank you for your curiosity,

    For the start: "plastics" and "glue" in one sendence very rarelly fit together. In my knowledge it is impossible or almost impossible to glue hdpe with any other material than some form the *pe varieties itself. The joining will be made by welding, mostly by extrusion welder.
     
  13. S V
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    S V Junior Member

    You are correct. There is almost impossible to stick permanently glue, paint or almost anything to *pe plastic sheet. The plastics for outdoor building have to be uv resistant or uv stabilized. For the end users, like us, this means basically the same - it is suitable for outdoor building (boats!!!). Usually manufacturer mentions it in product specification. And mostly standard uv resistant *pe sheets will be carbon black. Myself I never built any outdoor plastic product as the experience is only from indoor equipment, so this will be new also for me.
     
    Last edited: Apr 12, 2019
  14. Dejay
    Joined: Mar 2018
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    Dejay Senior Newbie

    I guess white HDPE is less resilient to UV? Maybe some "ultraviolet" tinted HDPE would work to absorb UV but reflect the rest of the light to not get as hot.

    Sorry for just asking questions instead of being helpful for you build :)
     

  15. S V
    Joined: Jan 2019
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    Location: Lithuania

    S V Junior Member

    Thank you for example. Yes, good static stability, not much freeboard, and loooooots of wasted deck space. It looks around 8-12 degrees deadrise in transom.
    Is this the hull and bow shape worth to consider?

    From the same site i stumbled onto Sea Strike 16 https://bowdidgemarinedesigns.com/sea-strike-16/ - this is more or less the upper deck shape i am looking to: high freeboard, lots of space. Only "sports" type, without T top and not so much deadrise.

    EDIT: They also have Shoreline 16 https://bowdidgemarinedesigns.com/shoreline-16/ - almost exactly what i need, but this one is also has much wasted space.
     
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