hull performance at slow planing speeds

Discussion in 'Boat Design' started by erik818, Feb 26, 2007.

  1. erik818
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    erik818 Senior Member

    I’m about to design and build a boat meeting the following specifications:
    - Economical cruising speed at approximately 18 knots and at 12 knots
    - Maximum speed is of no concern.
    - An occasional trip on the open Baltic Sea is possible, but mostly it will be used inside an archipelago and on large lakes. I’m perfectly willing to slow down as needed and also to adjust the course to take the waves better when caught out in rough weather.
    - Length 8 – 12 m.
    - 18 knots at maximum 25 hp to meet fuel consumption criteria

    In my mind, to meet the above specification the boat will have to be a light weight design and has to start planing at speeds just above the maximum displacement speed.

    At planing, one formula I’ve seen indicates that the total weight shouldn’t exceed 1300 kg (=800 kg for the empty boat) to allow 18 knots at 25 hp. Are there any drawbacks with such a low weight or is lighter always better? (The weight should be possible to achieve with a sandwich technique I have tried.)

    I have omitted the reasons behind the specification to keep it short. Information in various threads in this forum definitely has improved my understanding, but not sufficiently regarding the effect of the bottom shape at speeds just above displacement speed.

    Any inputs to my problem are welcome.

    Erik
     
  2. messabout
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    messabout Senior Member

    Erik;
    You have an ambitious project but it can probably be done. You are looking for speed length ratios of about 3.0 plus/minus. The displacement length ratio looks to be in the low 40s. 25 Hp may be marginal but you can probably do it. The boat will need to be fairly narrow and have an afterplane that is flat or nearly so. Lightweight is essential for your recipe. There are drawbacks to the lightweight but if you are to get good speed with low horse power you must keep it light. Flat bottoms imply that the bottom supporting structure be robust. That will present weight problems of course. Working out the conflicting details is part of the fun with design work.
     
  3. fcfc
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    fcfc Senior Member

  4. Raggi_Thor
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    Raggi_Thor Nav.arch/Designer/Builder

    3.5 x (HP/ton)^0.5 is a good estimate for a quite efficient planing boat.
    Flat bottom from midship and aft is probably a good shape, or maybe a flat bottom with two chines per side?
    The weight you estimate shouldn't be hard to achieve with plywood over longitudinal stringers.
    Please let us know how it goes:)
     
  5. erik818
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    erik818 Senior Member

    Thank you for the comments.

    I was already thinking along the lines of a flat bottom with a longitudinal step. Flat is simple to build so I figured that maybe I'm just lazy. I appreciate the support for doing it the easy way.

    I tried a narrow flat bottom on my last 1/2 scale attempt and found that it worked rather well, except that I needed a small keel to make it steer well in a sidewind. From messabout's and Rangar's comments it dawns on me that the bottom only needs to be flat in the rear part so I can design in some "grip" forward.

    Another thing I tried was to make the bottom a sandwhich of XPS and plywood. This idea was a partial success; where the glue caught the bottom is very stiff but I had problems applying pressure over the large plywood sheets while the glue was curing. Next time I will use vacuum from inside instead of pressure. I include a picture of the crossection of the bottom to show the design idea for my next 1/2 scale prototype. XPS stands for "extruded polystyrene" (I hope that makes sense in English), a plastic foam sheet used as insulation e.g. between the soil and a concrete floor.

    It's probably more sensible to build the boat according to someone elses well-considered plans, but that would take out half the fun. Regarding the not quite ocean-crossing design of Idaho in the link, isn't the problem that the boat doesn't look like it's made to survive a wash-over by a large wave? I mean, it's not the flat bottom as such nor the fact that it is long and narrow?

    Erik
     

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  6. tom28571
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    tom28571 Senior Member

    Personally, I think it is a mistake to jump to a flat bottom for reasons of simplicity. Boatbuilding is not really simple anyway and adding a decent bottom on a boat is well worth the small extra effort. There are good rasons for a flat bottom but ease of building is much overstated as a good one.

    Look at your private messages at the top of the thread.
     
  7. SamSam
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    SamSam Senior Member

    I would think that that IS the reason. If you were broadside to a short steep wave, the flat bottom would not let the hull rotate on the wave but would resist and more or less take the same slant as the face of the wave. The narrow beam cuts down on any leverage the hull weight and shape would have to help keep itself upright. Being as how it would be shallow draft, the boats center of gravity would be high and would overtake the center of flotation quickly and ruin your day.
    Then too, if you were heading into or going with short steep waves, you might be alternately bridging two or straddeling one wave which would put a big strain on the hull. If the hull is built light, it might not be strong enough, and then again your day is ruined.
    Sam
     
  8. erik818
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    erik818 Senior Member

    Tom,
    I just read the rules for private messaging and as I'm new to this forum it will take another day before I can access the private message area. I'll check it asap.

    One shape or another of the bottom will not have any major impact on the total effort to build a boat, I can only agree on that. If there is a better bottom shape for my need (efficient planing at relatively slow speed) , I'm all ears.

    Erik
     
  9. messabout
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    messabout Senior Member

    Idaho is fine for its' purpose but excursions into the Baltic is not one of them. Claimed 30 mph speed of Idaho with 25 HP is stretching the truth.

    You have not told us whether your boat will have a cabin or accomodations for extended cruising. That will make a difference both in sea keeping and possible speed to be attained. If you are thinking of a day boat only, and you are adamant about keeping power limited to 25HP, then take a look at Phil Bolgers "Slicer". It is 29 feeet LOA and 5'-2" beam. It has a deep vee forward and transitions into flat at the transom. There are numerous iterations of the "Slicer" type from the boards of various people. Boats of that sort were, in the distant past, used as commuter vehicles for those who could afford that lifestyle. These are sexy looking boats that do very well with low power. They are capable in the hands of an experienced boatman but they are not intended for serious offshore work.

    Raggi-Thor has given you an equation that is a variation of the Crouch
    formula. That formula contains the three variables that are pertinent to the design. It can be seen that you can not have everything. If the weight goes up the power must go up in order to attain a target speed. The Crouch formula is not an absolute but it does give you some good estimates of what to expect.

    The longitudinal step that you have presented is likely to be more appropriate for higher speed vessels. It does provide some resistance to skewing or making leeway. There is a structural element that is favorable as well. I believe that a better arrangement is to have two strakes running longitudinally and seperated by about two thirds of the bottom beam. The strakes can be considered as skid shoes. They will trap a bit of air under the boat as well as dampen the spray. They will be easily replaceable in case of damage from grounding.

    You will find a lot of valuable information in Dave Gerrs' "Propeller Handbook" The book has much more than propeller information (ISBN 0-07-157323-2). One section of the book deals with what we are discussing here.
     
  10. Raggi_Thor
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    Raggi_Thor Nav.arch/Designer/Builder

    I think there are many hull shapes that can do the job.
    Here is one I find interesting, a flat after body, or with very small dead rise, but some twist forward to make it sharper.

    The foam/plywood sandwich is a bit risky.
    If the foam is not compatible with epoxy this will become a water trap and the ply may start to rot.
    I would prefer a single skin of plywood bent over stringers.
     

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  11. Pericles
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    Pericles Senior Member

  12. erik818
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    erik818 Senior Member

    I'm trying to combine two functions into one boat. I have access to a place in the archipelago about 50 NM from where i live and where I have my boat. My current boat is a traditional displacement wooden boat with an inboard diesel best described by "7"; 7 m, 7 knots, 7 hp and almost 7 NM/liter diesel. The boat is seaworthy, charming and has place for 2 - 4 people to sleep. Unfortunately the 50 NM trip takes 7 hours which rules out weekend commuting in this boat. I would like a boat that could make the trip in 3 hours with reasonable economy (0,5 liters/NM) and also allow 2 -4 people to camp in the boat for a few days. If I can't combine both roles, economy when commuting has priority. I can always keep my present boat and leave it out there during the summer, but two boats means more maintenance work.

    I appreciate the advice and ideas I'm getting and will dig more into litterature to better understand what's happening between boat and water. Any advice on litterature that would help me understand the hydrodynamics when going from displacement speeds to planing? Hydrodynamics unfortunately wasn't part of my engineering studies.

    Erik
     
  13. Raggi_Thor
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    Raggi_Thor Nav.arch/Designer/Builder

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  14. Raggi_Thor
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    Raggi_Thor Nav.arch/Designer/Builder

    0.5 liter/nm indicate a speed of for example 20 knots with a 25HP outboard run at 80% using 10 liters/hour?
    20HP/1250kg = 16HP/ton.
    This is probably to much weight or not enough power to reach 20 knots.
    You need 25HP/ton, I guess.
    Weight is a major factor here :)
    I have very old book by Lindsey Lords, "Nav. Arch. of planing hulls". It's probably outdated, but gives a lot of empirical data from a time when engines was less powerful than now.
    Savitsky is often mentioned in this forum.
    A search for him gave this interesting spreadsheets:
    http://personal.inet.fi/private/muu/plancat.htm
     

  15. Raggi_Thor
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    Raggi_Thor Nav.arch/Designer/Builder

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