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Thread: DWL (Designed Waterline) definition

  1. #1
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    Default DWL (Designed Waterline) definition

    The plans for my Jericho Bay Lobster Skiff has the Designed Waterline (DWL) drawn on it. Is this the same thing as LWL (Load Waterline), meaning where the boat sits in the water with a normal load?
    Is the DWL where you would paint your waterline or would you put the painted waterline an inch or above it to keep the topside paint out of the water?

    Slightly confused,
    Rich
    I was born on a wooden boat that I built myself.

  2. #2
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    Default Re: DWL (Designed Waterline) definition

    They can be the same thing.
    As to painting the waterline, The best bet is to float her with all gear, but no crew. Then paint the bottom paint an inch up, paint boot topping to deal with the scum and growth from the popple, then topsides paint. She will float at a different line when she is moving with you on board, and different again with passengers as well. Such is the way of small boats.
    It really is quite difficult to build an ugly wooden boat.

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  3. #3
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    Default Re: DWL (Designed Waterline) definition

    Quote Originally Posted by Peerie Maa View Post
    They can be the same thing.
    As to painting the waterline, The best bet is to float her with all gear, but no crew. Then paint the bottom paint an inch up, paint boot topping to deal with the scum and growth from the popple, then topsides paint. She will float at a different line when she is moving with you on board, and different again with passengers as well. Such is the way of small boats.
    They can be the same thing or are the same thing? I'd like to paint the bottom before I flip the boat off the building jig. It'll be a fresh water, trailered boat , so no need of anti-fouling bottom paint.
    I was born on a wooden boat that I built myself.

  4. #4
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    Default Re: DWL (Designed Waterline) definition

    Do a Google image search and copy one that you like


    or


    or
    It really is quite difficult to build an ugly wooden boat.

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  5. #5
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    Default Re: DWL (Designed Waterline) definition

    I just went through the same thing myself...
    I had offsets on my plans for the boot top, which gets quite wide up forward, less so aft.
    I sprung a batten to raise the bottom paint about an inch above LWL forward, and less than an inch aft.
    May not be right...may be...I'll find out when she gets launched.

  6. #6
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    Default Re: DWL (Designed Waterline) definition

    An easy way around the issue for a dry-sailed trailer boat is to paint the entire hull the same color, sans waterline.

    Tom
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    Default Re: DWL (Designed Waterline) definition

    Quote Originally Posted by WI-Tom View Post
    An easy way around the issue for a dry-sailed trailer boat is to paint the entire hull the same color, sans waterline.

    Tom
    That's what I did with my present plywood fishing boat and also the Hvalsoe13 I built. This boat, though, just begs for a proper painted waterline.
    I was born on a wooden boat that I built myself.

  8. #8
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    Default Re: DWL (Designed Waterline) definition

    Quote Originally Posted by Rich Jones View Post
    They can be the same thing or are the same thing?
    On a small boat, DWL is most likely an arbitrary line. Small boats LWL depends on where the crew sits, the weight of the OB motor, and lots of other variables. So the designer lays down a line for working purposes during the design.

    On a big ship, where all weights are accurately calculated, and cargo stores, fuel etc are known an LWL can be used as the DWL.

    What is the weight of the motor used by your boats designer, and does it differ from the motor that you will be using?
    It really is quite difficult to build an ugly wooden boat.

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  9. #9
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    Default Re: DWL (Designed Waterline) definition

    DWL is where the designer thinks the waterline will be, LWL is where it actually lands when the boat is floating... <grin>

    Although that is a bit tongue-in-cheek, it is not that far off. With a proven design such as the Jericho Bay Skiff, you can have pretty high confidence in the DWL being accurate as the LWL when the boat is afloat. With a new custom design, it is a function of the designer's experience with that type of boat and whether he/she has done a good weights-and-centres estimation during the design process.
    Hope for the best, but plan for the worst.

  10. #10
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    Default Re: DWL (Designed Waterline) definition

    DWL is the Datum Waterline – from which all heights (above and below) are measured.

    LWL is the Load Waterline under certain conditions. On larger boats this is often calculated at 0% load (Lightship), 10% load and 100% load and the stability calculations are done at those loads and and the data entered into the Stability Book.

    When you start a design it isn't possible to know exactly where the boat will float under given conditions. But you need a datum to work from. Some designers simply use a baseline, with all heights being given above it. But many designers (myself included) prefer to take a stab at where a working LWL is likely to be and use that as the Datum (DWL) for heights. I usually try to have my DWL at where I reckon the 50% LWL will be at – but many designers also use the Lightship. In any case it's rarely totally correct.

    The reason to use a reasonably realistic DWL is that stability calculations are done about the waterline – so for preliminary stability calculations one can use the DWL. Once all the structures etc. and other loads are known and the weights and centres of gravity of all the components are calculated, the the final stability calculations can be carried out at the different load conditions.

    This doesn't really apply to small (open) boats – as mmd says, you usually aims for a DWL that is roughly where the boat will float before the crew get aboard – so based on an early structure and gear (sail, spars, motor etc) weight and distribution calculation.

    Cheers -- George
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    Default Re: DWL (Designed Waterline) definition

    Quote Originally Posted by mmd View Post
    DWL is where the designer thinks the waterline will be, LWL is where it actually lands when the boat is floating... <grin>

    Although that is a bit tongue-in-cheek, it is not that far off. With a proven design such as the Jericho Bay Skiff, you can have pretty high confidence in the DWL being accurate as the LWL when the boat is afloat. With a new custom design, it is a function of the designer's experience with that type of boat and whether he/she has done a good weights-and-centres estimation during the design process.
    That's what I was hoping to hear. Thanks for that explanation.
    I was born on a wooden boat that I built myself.

  12. #12
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    Default Re: DWL (Designed Waterline) definition

    I've found that raising the design waterline two inches is about right for the top of the bottom paint for a 20 to 25 foot boat.

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    Default Re: DWL (Designed Waterline) definition

    Rich are you on a lake? She's no tanker, but there's a difference between salt and fresh lake water density and it's temperature if being meticulous

  14. #14
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    Default Re: DWL (Designed Waterline) definition

    I was told by a famous trimaran guy that DWL is half load.

  15. #15
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    Default Re: DWL (Designed Waterline) definition

    I agree a boat looks better with a painted waterline and/or boot. Visually, it stretches the boat out.

    I'd err on the low side, since its easier to raise it. Also, note Timo's comments about varying height( or width for a boot). An " even" paint line can appear hogged due to the shape of the boat.

    Kevin
    There are two kinds of boaters: those who have run aground, and those who lie about it.

  16. #16
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    Default Re: DWL (Designed Waterline) definition

    I thought the DWL was supposed to match the displacement number in the plans, which heavier than the weight of the vessel itself.

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