Downeast hull design

Discussion in 'Boat Design' started by Marc78, May 17, 2006.

  1. Boston

    Boston Previous Member

    which is why I posted lines of the Downeast hull form, so folks could get a clear image of the type of hull commonly associated with a lobster boat.

    [​IMG]

    If your using a hull form of a traditional lobster boat people just might refer to is as such, specially if the top sides and sheer also resemble the layout.
    obviously the Downeaster has soft chines but its equally as easy to miss when a boat of similar sheer is pictured in its element.
    I can imagine the subtle differences would be easy to miss for a lot of folks but
    its the Downeast hull form this thread is all about and Ive yet to hear a clear consensus concerning the open water performance of this "hull form"

    my recollection is that they do really well in choppy seas or rollers and they are really popular or at least they were on Buzzards Bay, the cape side and out on the hook. No way they would be so popular among the fishermen if they were not well suited to it; not just the bay, but the open New England waters as well. Brant rock ( well off the south shore ) has got so many buoys floating around it you could walk across em and its out in the middle of nowhere, definitely not protected water and we fixed plenty of boats that worked there. They were no different than the ones in the bay.

    was hoping to hear from some folks who actually own a boat built on that hull form

    B
     
  2. Tad
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    Tad Boat Designer

    Once again Boston you have posted a question that could be addressed by writing 3-4 books.

    From Massachusetts to Nova Scotia there are hundreds of versions of Downeast Lobster Boat hulls, some good , some bad, some indifferent. The owner's/builder's, and designer's of these craft will all tell you that theirs is best and all others are junk....that's fishermen for you.

    Understand that these boats are built to make money, everything else is just gravy, if they don't make money they are put out to pasture. It doesn't matter what the boat performs like if it can run another 50 traps a day. Up until the past year or so there were good lobster prices and lots of lobsters, so boats got bigger and faster (more HP). Guys built 44' and 46' boats with 1000 HP in them, flat bottomed like the lines you have posted. This was to get maximum speed and carry lots of traps. Then fuel prices went through the roof and guy's started looking for smaller boats, or just stayed tied to the dock. Now fuel has moderated in cost, but lobster prices are lousy...and so it goes for fishermen.

    The lines you have posted are actually fairly radical. The lines below are more moderate. There are two main styles of Lobsterboat hull form, skeg built and built down. I include one of each below.

    See http://www.tadroberts.ca/about/pdf/power-boat-design-form-and-function.pdf for comparison resistance curves for displacement, semi-displacement (as some Lobsterboat hulls), and planing hulls. Every hull is designed for a given speed and design weight, none do well outside these criteria.

    boatlinessmall .jpg

    Whitepinecolour.jpg
     
  3. dobsong
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    dobsong Junior Member

    Tad,
    I do like the look and lines of White Pine especially if it could be stretched out by another metre and perhaps have a Yanmar 6CX-GTYE installed..... and maybe a few more centimetres of draught… what do you think?
     
  4. Tad
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    Tad Boat Designer

    dob,

    A stretched version of White Pine will produce a very easily driven hull, if the weight is kept in check. A 44' I 'm currently doing has 13'4" of beam and seems slightly narrow by today's standards. If one must travel, a narrower hull is faster and more comfortable. Why the additional draft?
     
  5. TollyWally
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    TollyWally Senior Member

    A picture's worth a thousand words. I've admired lobsterboats from afar for quite some time. The example I saw in Alaska gillnetting, was not ideally suited for that fishery. The hull seemed overburdened by the weight of the gear aboard her. But it was a handsome vessel that looked like it would perform well with lighter gear and lighter catches.

    I would imagine that often the boats are loaded down with pots only a few times a year, at least the inshore boats, the pots spending most of the time in the water. Maybe a string or two being moved to hotspots. Offshore boats might have to move greater amounts of gear longer distances even when most pots were soaking. This is all just a guess, as I've never observed east coast lobstering.

    There are more and more "lobster yachts" showing up in Puget Sound. But all I've seen are the springy lobster style sheer and house with big swim steps attached to the *** end. Looks like different engine location and layout from real ones and God knows what underbody details. This thread has shed a little illumination upon the subject, nice.
     
  6. dobsong
    Joined: May 2009
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    dobsong Junior Member

    Retirement Design?


    Tad,
    As per my email......

    I’m in the process of setting myself up for retirement and part of the plan is to upgrade my boat. I have recently joined the Royal Victorian Motor Yacht Club and this has given me the opportunity to look at a wide variety of boats and narrow down what I would like…..

    Mostly I enjoy longer 2-3 day trips (and up to a week away). My current boat is moored at Williamstown which is at the top end of Port Phillip Bay and I fish in the bay and sometimes cruise around it... trips number it 2-5 times a month but hope to increase the number of times I go out once I stop working full time.

    I also go out through the entrance (called The Rip) and head out into Bass Strait. Unfortunately my current boat only has a range of 50-60 nm @ 12 knots (with a 10-15% reserve) so these trips have to be carefully planned and are limited to short hops to where fuel is available.

    I would like to be able to go across to Tasmania (120 - 450 nm depending on landfall chosen) or coastal hop up to the Great Barrier Reef and on to Darwin (longest leg I'd do around 450 nm). Because I often go out alone I need the ability to cruise at 12 - 14 knots so legs of likely trips can be managable……

    Ideally I would like the following in my new boat:
    - Wooden construction (with epoxy sealing)
    - 36’ – 42’ LOA (possibly a foot or two shorter if other requirements met)
    - Easily handle by one person (a key requirment) with good access to the bow area
    - Maximum draft 4’
    - Cruising speed 12 - 14 knots
    - Range at cruise 600+ nm
    - Berth for a couple with ‘casual sleeping for 2 others
    - Good sized head (hate having to 'squeeze in')
    - Headroom 6’ 4”
    - Low profile (no flybridge, no excessive freeboard and engine down low)
    - Simple systems (don't want or need more than the basics... means no generator, no aircon, only 12 or 24 VDC electrics etc)
    - Slow revving diesel engine (Yanmar is my preference)
    - Space for a small dingy (say 9’ LOA) to be stored and easily deployed.
    - Full length keel that protects the prop and is able to withstand grounding
    - Low prop shaft angle and large propeller with a decent sized rudder
    - Good turning circle at low speed (forward and reverse) without the need to have thrusters
    - A hinged marlin platform
    - Easy motion at sea (slow roll motions that are well damped – also good tracking, hence my attraction to decent sized keels)
    - Low noise levels when cruising

    I do like the looks of White Pine but think it may be a little slow (her max is listed as 15 knots and I'd like to have some in reserve). I have looked at a locally built boat, a Cheviot 32, and it is lovely but just a bit to small for me, plus access to the bow is more difficult than I would like. (see http://www.woodenboatshop.com.au )

    The production boats available in Australia do not “tick all my boxes” so I am seriously looking at building here (importing is not very attractive at current exchange rates) and we have 3-4 competent wooden boat builders where I live so this seems like my best option.

    My time frame for completion is 2 - 2½ years.

    What are your thoughts? Do you have any plans that could be easily adapted to my requirements? Are you able to provide a summary of your usual fees for what I am seeking?

    I look forward to your response.
     
  7. Tad
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    Tad Boat Designer

    Dob...

    Lengthening the waterline to 40' drops the speed/length ration to 2.2 at 14 knots. This moves the LCB forward, makes the transom slightly finer, and moving engine and tanks forward. Holding the beam at 12'6" produces a nicely proportioned boat which will be comfortable in a sea.

    If she's kept simple (light) you could expect 14 knots with about 120 HP at the prop and 12 knots with 73 HP. That's about 22lph for 14 and 13.6 lph at 12 knots.

    Might look something like this.....galley could be upstairs or down.

    Dobsonsmall01.jpg
     
  8. Boston

    Boston Previous Member

    that's the info Im talking about
    did you use a hull efficiency calculator to derive the numbers or did you just get out on the water and start checking your rpm and millage

    if you have a hull efficiency calculator what one did you use and whats the result of a 60' lwl and a 13.5' beam with the same basic hull

    also
    I got some feed back on onother forum that the down east hull is stable to a degree but in heavier seas its stability drops off fast. whats your experience with the hull form in stormy weather

    thanks
    B
     
  9. dobsong
    Joined: May 2009
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    dobsong Junior Member

    Replacement for Snark

    Tad,

    Nice!!

    I like the way the beam narrows toward the stern and I also like the ‘raised’ floor and kitchen up in the main cabin area. The area of the aft deck looks about right for entertaining friends, fishing etc.

    The head could be a fraction bigger so the ‘shower’ has a bit more room (I’d use this for hanging wet gear as well). Maybe the storage area between the shower and forward berths could be sacrificed?

    I do like the Yanmar 6CH-THE3 engine (rated at 170 hp @ 2550 rpm) and it seems that this would get your design up to a cruise at 12 knots @ 1400 rpm and 14 knots @ 1700 – 1750 rpm (55% of max cont. rpm).

    Simple is good and what I want! I don’t want a generator, airconditioning or other ‘indulgences’. I am thinking a couple of larger solar panels, a 12 or 24 volt system, about 580 AH of battery capacity (480 house & 100 engine), a simple stove and a moderate sized fridge. Of course there would be the usual ‘safety’ gear (VHF & HF Radios, sounder/fishfinder & chart plotter as well as LED lighting. I may want to install one of the new electric ‘hooka’ dive compressors. Oh and an electric anchor winch.

    BTW what is the anticipated:
    - Headroom in engine compartment and amount of workspace for servicing?
    - Tankage capacity – Fuel, water & waste?
    - top speed with the above engine or is this ‘overpowering’ the design?

    Finally it would be nice to target 1 litre per nm @ 12 knots and 1.3 – 1.4 litres per nm @ 14 knots. Do you have any suggestions on how this design could be ‘improved’ to achieve (or exceed) these sort of targets while still meeting what I am after?

    Finally, last night I became aware of some select grade Alaskan Yellow Cedar planks (6” x 1” & 10” x 1”) that are available in near me, so I am thinking that this would be a good timber for planking the boat as I’m told it is light, strong and easy to work. Would this be a good choice? Or should I be considering another material to achieve the best outcome?
     
  10. Boston

    Boston Previous Member

    a small virtical wind turbine could keep you charged up while you are just sitting at the dock and its a lot cheaper and lasts longer than solar cells

    just a thought
    B
     
  11. dobsong
    Joined: May 2009
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    dobsong Junior Member

    For those interested - Snark details

    Name of Boat: Snark - Previously SuJane II
    Owner's Name: Graeme Dobson
    Boat Location: Williamstown, Victoria
    Registration No: RV204

    Overall Length - 24'
    Maximum Draft - 2' 6”
    Maximum Beam - 9’ 9”

    Construction
    Hull is clinker plywood, decking is planks of ply and cabin is Teak with ply roof.
    She has a round bilge clinker planked marine plywood hull with an open wheelhouse and a lower cabin with moderate flare forward and good size cockpit deck, strongly raked stem, transom stern and hinged timber slatted marlin platform. The lower cabin has a kitchenette, a separate toilet with wash basin and two bench seats that have been set up to convert to a basic double berth.

    Engine
    Chrysler 318 V8 Petrol 177 hp (direct drive originally with a Paragon gearbox recently replaced with a velvet drive because spares no longer obtainable for the Paragon)

    Designer & Builder – Halvorsen & Sons, Sydney

    History
    This 24' Sea Skiff was built for a Mr Wills and launched 20/12/61 with the name SuJane II. Her previous two owners were also members of the Royal Victorian Motor Yacht Club
     

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  12. Tad
    Joined: Mar 2002
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    Tad Boat Designer

    Yanmar6CHcurves.jpg

    The output curves above include a line called "Output(Cubic Curve)" as well as a "Output (Maximum)". Because we are using a fixed pitch prop, the propeller will only match engine power and RPM perfectly in one spot on the power curve. Usually this is at roughly maximum output. Note the two curves meet (cross) at the top of the graph. The "cubic curve" is sometimes referred to as the Theoretical Propeller Power Curve. It represents the power that will be absorbed by an average propeller (of dimensions matching the max output of this engine) at the stated RPM.

    So we need to look at the Cubic Curve to find power output and fuel consumption. We said the boat above will require 73 HP at 12 Knots, that's about 54 KW, from the graph 54 KW = 1900 RPM, the specific consumption at 1900 is 240 g/kw/h = 12.96 lph. The 120 HP (89KW) at 14 knots is about 2260 RPM, specific consumption at this engine speed is 230 g/kw/h = 20.47 lph.

    There is a very long way between these educated guesses and the real life finished boat, but the expectations are roughly in the range of possible. Fuel consumption can be reduced by making the boat smaller (lighter) for her length. The down side of this is motion gets quicker as the boat gets lighter for a given waterplane. So trimming size becomes a balancing act.

    The largest single weight in this boat will be the fuel load, so that capacity needs to be considered carefully. 600 miles at 12 knots is about 50 hours, 50 hours * 13 lph = 650 liters. So fuel tankage might be 700-800 liters. With a shower the water consumption goes up, water tankage will need to be 200-400 liters.
     
  13. Tad
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    Tad Boat Designer

    I'm also very jealous of the Yanmar engines available in Australia. We just don't get to even consider that type of slow-speed, heavy weight in North America. The 6CH appears to be an engine from the past.....simple and understandable...nice.
     
  14. Tad
    Joined: Mar 2002
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    Tad Boat Designer

    Boston....

    The most important factors in predicting the performance of any hull are two....length and weight (displacement). Everything else is relatively minor in comparison to these two. If you know the weight of your boat with some accuracy, you can predict her performance quite accurately as well. Without designing the boat (and incidentally building her) you don't know the weight.

    But we can make a WAG.....just for fun.

    We can base the guess on the size of the boat, actually on the "cubic number". Length * Beam * Depth (of hull, main deck to rabbet). So 60' * 13.5' * 7' = 5340, 5340/100 = 53.4 * .42 (moderate weight boats) = 23.4 long tons or about 53,400 pounds. That will be a first guess displacement.

    A 60' hull that displaces 53,400 pounds will require 44 Hp at the prop for 9 knots in flat water, no bearing looses, alternator drag, no current, wind, waves, no bottom growth, no appendages, etc. 10 knots requires 84 Hp, 11.4 knots is 135 HP, 12.5 is 184 HP, 13.75 is 240, 15 is 306 HP.

    The real challenge will be to build the boat at that weight and keep it there.
     

  15. dobsong
    Joined: May 2009
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    dobsong Junior Member

    Yanmar - You gota luv em!

    An Australian Yanmar repower that may interest - a 20 metre, 70 tonne displacement boat - 9.5 knots at 1400rpm and 12 litres per hour, 11.5 knots top speed!

    Yep north America is different... spent March there (St Paul, Chicago, Niagra, LA) and enjoyed... pity I didn't manage to visit Seattle to vivit my friend there... she tells me there are some great boats up that way....

    Slow can be good even if it weighs a lot!
     

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