50' LDL design project.

Discussion in 'Boat Design' started by Robjl, Jun 29, 2014.

  1. Robjl
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    Robjl Senior Member

    Attached is a DXF file for a 50' power cruiser hull. (the file is in metres)
    The vessel would have approx 100 HP diesel driving a 28" prop, designed speed of 7.5-8 knots. Displacement 12.5 tons lightship, 15 tons fully loaded. LOA is 50'. Max beam at rubbing strake is 13.12'
    At 15 tons LWL is 48.25', BWL is 10.8', Cp is .58, Canoe body draft is 2.75'. LCB is 4% aft of waterline centre.
    Active stabilizers would be fitted.
    This is a displacement hull not intended to plane.
    Fuel consumption should be miserly.
    Construction will be GRP but could easily be alloy.
    I have worked on the shape for too long and need an outside opinion.
    Any constructive comments appreciated.
     

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    Last edited: Jun 29, 2014
  2. gonzo
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    gonzo Senior Member

    I can't open the file. Could you post a PDF? Also, a design for fiberglass and alloy are usually quite different.
     
  3. Tad
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    Tad Boat Designer

    From a quick look I don't think I would do it like that. I want to see smooth (straight), gently rising buttock lines and no big bulge forward of the propeller. Your S/L is 1.07-1.15 so ideal prismatic will be lower, around .54. LCB will be further forward, about station 5.1. Immersed transom area will be about 15% of the largest (#5) section. Deepest point on the profile will be between stations 2-3, on a 10 station waterline with zero forward.
     
  4. JSL
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    JSL Senior Member

    Also check out your propeller blade loading - 28" dia seems a bit small for a 33,000 lb (15t) boat
    Years ago I did a 53 x 14' boat (semi-disp, hull @ 14.1 tons) and with a single 260 hp engine it topped out around 17 knots using a 34" 3 bl. prop.
    (as I could only see the 'plan' view I can't comment on the hull form.)
     
  5. nzboy
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    nzboy Senior Member

    I can only see a plan view as well .But reading Tad s comments I can guess a few things .Your idea is very similar to mine .I like Steve dashews designs and the way they are handling seas .I had been thinking of these ideas long before the dashews boats came along .Many of the early whale chaser boats had a similar design coming from a sailing and rowing backround . They surfed down waves and when engines came along they could plane no problem with buttock lines that weren't straight . I know Tad likes the deep forefoot and gentle rise of buttock lines .I ve hacked 2 of Steves boats around( for a 20m boat) first one is his 64 which has long flat bilge the other is his 115 which has a different profile(as yet untested) I would like to know what the differences would be
     

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  6. Robjl
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    Robjl Senior Member

    LDL project

    Gonzo, I will try to create a PDF file for you but neither of the programs I use seem to do it.(Prosurf & Caddsman). The hull shape is multi-chine, all flat panels, I think both GRP & Alloy are possible, though the scantlings and construction will be different.
    Tad, the reason for the bump (and it is really only about 6", though in profile it looks a lot more) is to allow the engine to be low and aft. It also allows a shorter prop shaft without intermediate bearings. The motor of choice is a John Deere 4045 (my advice is to go turbo but I really don't need the power, but my understanding is that it is more efficient ..?)
    I would use a ZF 63A 8 deg down angle gearbox with a 2.68 ratio.
    I have the Pc at .58 for SLR of 1.2 as that is what I see recommended by D. Phillips-Birt's "Naval Architecture of Small Craft"
    and also by Larsson&Eliasson "Principals of Yacht Design" (Pc .56 @ Fn 0.35)
    I have also relied on Larsson for the optimum location of the LGB which they recommend to be at 53.5% wl from bow.
    Am I missing something?
    I prefer to keep the Pc high to ovoid squatting at higher speed (up to SLR 1.4) if I choose to go faster, I understand that the extra knot and a half will double my fuel burn. But if I use Beebe's formula my Hp requirements at 8 knots are about 30HP, at 9.5 kts it doubles...but there are times when you just need as much speed as possible.
    The buttock angle at the transom chine is 6.2 deg, at 50% beam it is still only 6.5deg.
    The Length Displacement Ratio is 130 fully loaded and 111 in light trim.
    I have allowed for 2500 litres fuel and 1000 litres of water.
    The LCB is almost longitudinally fixed up to 20 deg heel.
    Beam length ratio is .22 at the waterline.
    JSL, the HP as mentioned above is modest, most would regard the 28" prop as large. I am chasing efficiency. The DXF file is 3D , try another view if you are using a cad program
    Nzboy, I too admire Steve's work. but I can't afford to buy, moor, fuel, insure etc a 64'.
    Thanks for your comments.
     
    Last edited: Jun 30, 2014
  7. nzboy
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    nzboy Senior Member

  8. TANSL
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    TANSL Senior Member

    Here, Gonzo, and as Robjl rightly says "both GRP & Alloy are possible, though the scantlings and construction will be different.". It seems that Robjlm knows what he is talking about.
    It's just an opinion but I think the shapes, details, knucles, of the ship could be simplified without substantially changing its overall look.
     

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  9. Tad
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    Tad Boat Designer

    I was reacting to this requirement. If that's not the requirement, and higher speed is, then another hull form is ideal. Ideal forms fit in very narrow boxes, and your job as designer is to rationalize the ideals....that's the design process.

    The usual chain of events is something like, "We want speed so install lots of power and give her a high prismatic." Then the boat gains weight and the price of fuel goes up, the speed is never achieved or used, and she's a sluggish, inefficient dog forever.

    Long, skinny, light boats suffer from trim changes far less than short fat ones. The Irens designed hull below runs up to 16 knots (S/L of 3.26), and she trims quite a bit at that high speed. She's also lighter (D/L is about 70) than your boat.

    Irenshull.jpg
     
  10. gonzo
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    gonzo Senior Member

    Tansl: thanks for the PDF. I like the tucked in propeller for shallow draft. The compound curves appear that they may be conical and developable. I would give the bow stem a little curve for aesthetics, but that is just my preference.
     
  11. Robjl
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    Robjl Senior Member

    More info...

    Tad, I can’t have explained myself well enough. This vessel is for my own use. I will build it myself. It will be my 5th build. It is my first power only vessel. My last build, my current boat has a few pics on my gallery. As someone smarter than me once said “I didn’t fall off the last turnip truck” and neither did I, but… I have limited experience in hull design, so I take my time and seek advice. The hull shape I have offered up for critiquing is the result of hundreds of iterations as I spiral towards a final compromise. I do see it as a compromise, but it is the best I can come up with.
    The boat will be our home for 6 month stretches. My wife and I cruise the SE & E coast of Australia and maybe further. We are just back from 2 x 6 months cruising. We have 30 years cruising experience, we know what we need.
    The vessel must be manageable, safe and comfortable. Accommodation for two with another smaller double cabin for guests.
    Air con, fridge, freezer, water-maker, dinghies, spares for everything, etc, etc.
    It will be fuel miserly, 1 litre / nautical mile or better at 200 miles a day (8 kts)
    Main engine John Deere 4045, ZF63A gearbox, a simple robust conventional driveline.
    A 150Amp x 24 volt alternator belted to the main engine.
    A wing engine, kuboto 20 HP with folding prop on a sail drive, with two 150A x 24V alternators. This is my DC power supply and “get home” motor.
    Batteries AGM 800 Amp Hr @ 24 volt.
    240 volt AC from two Victron 24/3000/70 Multiplus Inverter chargers.
    1500 watts solar panels on main cabin roof
    Hull construction is solid GRP, deck, cabin, roof, interior, all cored GRP.
    It is all flat panel construction, all surfaces are developable.
    The vessel will have active stabilizers, an extravagance I know but I think the hull shape will need them.
    Tad, you state that “Ideal forms fit in very narrow boxes” I agree, but a live-aboard cruising boat that has to cope with a variety of conditions at varying speeds, all the while carrying a substantial load needs a “wider box” it is inevitably an “optimized compromise”. Nigel Irens Electra is such a “narrow box” and as such is irrelevant to my design.
    With reference to my Pc at .58 I make the observation that conventional wisdom has it that a non optimum Pc has a greater (drag) penalty at low speed than at higher speed.
    I am interested that you suggest an immersed transom, 15% of the mid-ships section (immersed midsection I assume) I haven’t heard of that “rule” before, is there any empirical evidence that this is more efficient. I can see how it would keep the CB aft and also give a lower buttock angle at the stern. But I also see the ton of turbulent water that is dragged along behind the boat that only increases with speed…until the water starts to break away, if you have the speed. My transom as designed is just on the WL, but will immerse at SLR above 1.0.
    Any comments appreciated.
    Tansl, the shapes and knuckles are there for a reason, the lower knuckle at the bow neatly turns the bow spray away, the top knuckle (under the top strake) is there for appearance but it actually makes it easier to keep a fair line on the side plate junction. Some of the large power boats do this and it looks fine.
    Gonzo, as mentioned above, all plates are developable.
    Thanks all for your interest.
     
  12. Mr Efficiency
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    Mr Efficiency Senior Member

    Hull construction will be solid GRP ? So you intend to make a plug, a mould, then a hull ? Only way I know of to proceed to that end. There goes the next few years ! At the very least, before even starting the project, build a scale model and have it tested against a model of a known existing full scale design. My first reaction was to see a boat that, in plan view at least, looks more related to sail boats, I would want something with more stability built into the shape, for comfort's sake most of the time, and safety's sake on that day you should have stayed in port.
     
  13. TANSL
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    TANSL Senior Member

    Many boats have knuckles, we know what they are good for but, in my opinion, can be traced in a less complicated way.
    The surfaces appear to be "ruled", which does not mean they are developable. Specifically, the area marked with a blue ellipse, I'm pretty sure that is not developable.
     

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  14. nzboy
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    nzboy Senior Member

    I guess there are always compromises .Your Cp is normal for this size boat. Fitting a large prop is always a mission if you want it protected so this sacrifices length and hence speed .Tad s passengerlite 56 is also interesting as like dashew he has gone for a v drive with engines aft which then creates space for an aft cabin and hence a smaller boot top . Having a saildrive for an auxillary is also an idea Ive had and also fits well if it is aft. In tads 56 I would raise the kitchen up to wheelhouse level like dashews this leaves a nice storeroom space below and fuel space midships separate from engine room .With a narrow beam you want to avoid a large and high wheel house ,Keep things in the box ie. beam equals overall height .Its hard to know if stabilizers are needed with your hullform as they may be used to stop a snappy roll rather than excessive roll .Maybe the big iron ball would be better
     

  15. nzboy
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    nzboy Senior Member

    Yes I wondered about the solid GRP , probably a foam strip approach would be the way .As for stability I think a little more beam would help but then a full tank of diesel midships would also help .But with basically flat single chine stability is greater, also GRP is not ideal for this design as compared to a round bottom
     
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