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Thread: Mast for 8' Lug Dinghy

  1. #1
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    Oct 2016
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    Default Mast for 8' Lug Dinghy

    Hi all,

    I'm just finishing off building an 8' Selway fisher lug rigged dinghy. In the plans it specifies building the unstayed mast out of 'pine or spruce' but provides no further detail. The specified spar is just over 9' in length and 50mm diameter for two-thirds of its length.

    I struggled to find decent timber locally (I'm in London in the UK) and ended up buying 'yellow pine', which I believe is also called pitch pine (feel free to correct me). It feels like a heavy and very stiff timber and probably not what was intended by the designer. Being a bit overexcited and impatient I ended up laminating two pieces with opposing and very straight grain together and building a mast. it came out really well and very straight and doesn't feel too heavy now that it's shaved down to size. I slimmed down the dimensions to 1.77 inch diameter on the basis that I was probably dealing with a heavier and stiffer timber than intended to save a bit of weight.

    So, my question is this. Have I ended up with something useable? Is 'yellow pine' (whatever that may be!) likely to be a suitable material? Will the extra weight I've likely ended up with be problematic or not? Does anyone have any idea of what kind of weight a mast for a lug-rigged dinghy of this size be when built of spruce or similar?

    Sorry for the long rambling question, thought I'd just outline some context! Thanks in advance.

  2. #2
    Join Date
    Jun 2006
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    Blacksburg, VA
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    Default Re: Mast for 8' Lug Dinghy

    If the nine-foot mast were two inches diameter over it's entire length the volume would be 339 cubic inches. If made of typical southern yellow pine (30 lb/cu ft) it would weigh 5.9 lb. If made of typical sitka spruce (the gold standard for spars, 28 lb/cubic foot) it would weigh 5.5 lb. So, the difference in weight is not something to worry about.

    Some yellow pine is as dense as 37 lb/cu ft, but that would be old-growth stuff not typically available in lumber yards today.
    I will beg you for advice, your reply will be concise, and I will listen very nicely and then go out and do exactly what I want! (Apologies to Lerner and Lowe.)

  3. #3
    Join Date
    Mar 2014
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    Ronneby, Blekinge, Sweden
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    Default Re: Mast for 8' Lug Dinghy

    1.77", is that metric imperial? No matter, it boggles my mind but I guess it's somewhere between 1 1/2" and 2" which would be 4 cm or so.
    There are many factors to consider when building a mast.
    To name a few
    species, growth ring distance, laminated, hollow or not, birdsmouth or cut in half and hollowed and glued together, solid and formed from the log, number and size of knots...
    It's a small boat so perhaps 4 cm will be ok, but I would probably go for 5 cm. This also depends on how you will be using the boat...
    Weight up high can be compensated by ballast, if you're not into racing it's no big deal.

    /Mats

    Elected Swedish Yourneyman of the Year 2019

  4. #4
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    Default Re: Mast for 8' Lug Dinghy

    Sounds like it should work out. What are you using for the yard and boom?
    "The enemies of reason have a certain blind look."
    Doctor Jacquin to Lieutenant D'Hubert, in Ridley Scott's first major film _The Duellists_.

  5. #5
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    Default Re: Mast for 8' Lug Dinghy

    As you have finished the spar all you can do is use it. You could test it by putting it on a saw horse, at the parners, holding the heel down by tieing it to something heavy like your cars tow bar, and pushing down on the top end to see what happens. Bearing in mind that the dingy will roll and inload the mast in a gust. If it creacks an makes cracking noises you have pushed too hard.
    It really is quite difficult to build an ugly wooden boat.

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  6. #6
    Join Date
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    central cal
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    Default Re: Mast for 8' Lug Dinghy

    I often use pine for wee spars such as that. Honestly, if I were concerned, Id sling it between two horses and bounce on it.

    I think youll be fine.

    Peace,
    Robert

    P.S. My opinion, based on loads of time in wee boats such as that, with wee sails, and pine spars...

  7. #7
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    Oct 2016
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    London, United Kingdom
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    Default Re: Mast for 8' Lug Dinghy

    Thanks all for the very useful responses. Sounds like it should work out ok. To confirm, it's two pieces of pine with opposing grain epoxied together. The grain is very straight and there are no knots throughout the length.

    Sorry for the weird metric/imperial measurements. I tend to work in metric but I'm aware most of the people on the forum are from the US. In metric, the plans stated 50mm diameter and I built it as 45mm. Incidentally, I found a website today with references from early-20th century boat building manuals that stated when building a spar out of Douglas Fir that is specified in spruce, you can reduce its diameter by 10%. Assuming yellow pine has broadly comparable characteristics with DF (as stated on a website I found) it would justify the 10% reduction to 45mm.

    I haven't started on the yard and boom yet but I'm keen to use something a little lighter. I should be able to get DF fairly easily and if I reduce the diameter by 10% as per above, it should achieve the same weight as fir in the slightly larger diameter.

  8. #8
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    Default Re: Mast for 8' Lug Dinghy

    Quote Originally Posted by Monzu View Post
    Thanks all for the very useful responses. Sounds like it should work out ok. To confirm, it's two pieces of pine with opposing grain epoxied together. The grain is very straight and there are no knots throughout the length.

    Sorry for the weird metric/imperial measurements. I tend to work in metric but I'm aware most of the people on the forum are from the US. In metric, the plans stated 50mm diameter and I built it as 45mm. Incidentally, I found a website today with references from early-20th century boat building manuals that stated when building a spar out of Douglas Fir that is specified in spruce, you can reduce its diameter by 10%. Assuming yellow pine has broadly comparable characteristics with DF (as stated on a website I found) it would justify the 10% reduction to 45mm.

    I haven't started on the yard and boom yet but I'm keen to use something a little lighter. I should be able to get DF fairly easily and if I reduce the diameter by 10% as per above, it should achieve the same weight as fir in the slightly larger diameter.
    look for an old scaffold plank from a salvage yard. Excelent quality timber and light enough.
    It really is quite difficult to build an ugly wooden boat.

    The power of the web: Anyone can post anything on the web
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  9. #9
    Join Date
    Oct 2005
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    The Netherlands
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    907

    Default Re: Mast for 8' Lug Dinghy

    I would use an old windsurfmast.

  10. #10
    Join Date
    Apr 2010
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    South Australia and Tasmania
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    18,061

    Default

    It will be fine. Next job......

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