Folkboat scaled up 16%

Discussion in 'Boat Design' started by Johnny Kadire, May 14, 2011.

  1. Johnny Kadire
    Joined: May 2011
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    Johnny Kadire New Member

    I want to build a Folkboat scaled up by 16%...

    I am considering building a hull to folkboat lines scaled up by 16%.
    loa 29',lwl 22'11",b 8'5",d 4'6".
    Bearing in mind that this is a nominal..ish scale up of a conservative traditional hullform what problems could I potentially encounter ??
     

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  2. Eric Sponberg
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    Eric Sponberg Senior Member

    Johnny,

    A lot depends on how you scale the boat. If you just want to stretch it longer, that's one thing, but if you want to scale it 16% in all directions, that's quite something else. You don't say what the displacement of the boat is, and I presume that the dimensions you have shown are for the original model. Therefore, the new model would have an LOA of 33.64'.

    If the weight of the original boat is say, 10,000 lbs, then scaling only the length is going to increase the displacement by length only, so the new displacement will be 16% greater, 11,600 lbs. But if you increase the dimensions in all directions, then the displacement is going to vary by length cubed, and the new displacement will be 15,609 lbs., more than 50% increase! That is quite a big difference. Your ballast would have to change accordingly. If the keel is a bolt-on casting, it will have to be totally reshaped and recalculated for the correct weight, center of gravity, and overall stability.

    Sail area is another factor. If you stretch the boat just by length, then basically the sail fore/aft dimensions will increase, but not necessarily the height. But, you can increase the height if you want to because the bigger boat will have an increase in stability, and so it will be able to carry a bigger sailplan than just a simple length stretch. If you increase size in all directions, then sail area will increase by length squared, LOA^2. You don't say what the sail area is, but let's use an example of 500 sq. ft. For a length stretch, the new sail area would be 16% larger, say 580 sq.ft. But if you increase size in all directions, then the sail area would be 672.8 sq. ft. Such a large increase in sail area is going to affect the design and size of the mast, the layout of the rigging and the sizes of all the shrouds, stays, and halyards. These are all calculated based on the boat's righting moment.

    For a simple length stretch, righting moment will likely go up by the increase in length, 16%. But for an increase in all directions, righting moment goes up much faster, by the fourth power of length. You do not say what the boat's righting moment is, but this is an extremely important number, so if you can, you should get it from the original designer, or hire a naval architect to calculate it for you from the plans. Let's say that the righting moment at 1 degree of heel is 600 ft-lbs, to pick a number. The maximum righting moment will then be about 30 times that, so about 18,000 ft-lbs. The maximum righting moment is used for the rigging design particularly, and may be used for other things related to boat structure. So if you increase size by 16% in all directions, the righting moment is going to go up to 32,592 ft-lbs, an increase of 81%! That will mean a big increase in mast and rigging sizes.

    Increase in displacement is also going to affect the size of the auxiliary engine, if the boat has one. A simple length stretch may not necessitate an increase in engine size, but a full scale increase in all directions might very well require a larger engine. With a larger engine goes a larger propeller and propeller shaft.

    So, there are a lot of ramifications to increasing boat size. Consultation with a naval architect can keep you focused and realistic on how the boat should be built. This type of change is not necessarily nominal-ish--the changes are NOT trivial. This is a major difference in scale.

    Good luck, I hope this helps.

    Eric
     
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  3. Johnny Kadire
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    Johnny Kadire New Member

    Thanks Eric. Your post was very helpful.To clarify.
    The original boat before increase is
    loa 25',lwl 19'8",b 7'3",d 3'11". disp 4800lbs,ballast 2300lbs and sail area of 240sqft in the form of a fractional rig...158 main and 82 jib.Dl is 279. I have attached a pic of an original on the hard to give an idea of her lines.
    It would be my intention to apply a uniform increase of 16% in all dimensions.The proposed change would give me a boat of loa 29',lwl 22'11",beam 8'6",depth 4'6",disp 7250lbs(est.) ballast 3475lbs(again est.)DL 269. On this hull I would suggest a masthead sloop of 350sqft with an appropriate mast section and rigging schedule (single spreader but as yet uncalculated) and a new ballast casting .
    I still have to do some research on the righting moment/rig/ballast but in terms of the hull lines alone is a my scale up an option ??
     

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    Last edited: May 15, 2011
  4. frank smith
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    frank smith Senior Member

    I like the Idea . It might be worth your time to look at the Mason 30 , a boat similar in concept . I think there was a write up in on of Rodger Taylors books .
     
  5. PAR
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    PAR Yacht Designer/Builder

    Johnny, as Eric has eluded to, you proportional scale up will need to be completely revised in regard to it's scantlings. You would be best advised to find a design of similar proportions to your enlarged desires. The questions you pose are the most common asked of a designer and often the most difficult to apply without a wholly new design being created. It is possible to find a design with similar looks to your folkboat, because the enlargement you're suggesting literally means recalculating every inch or surface area, every structural element and a whole new rig. How's your engineering skills?
     
  6. Eric Sponberg
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    Eric Sponberg Senior Member

    Johnny,

    It can certainly be done, but as PAR has mentioned, you really have to know what you are doing regarding EVERYTHING. You should not keep the same size frames--everything may scale a little differently. For example, an increase of 16% in all directions may not necessarily apply at the same rate to the scantlings (structural element dimensions)--they may be a little different scale in order to get the best design. I think the trickiest part would be the lofting--this boat was drawn by hand way back before computers, and so the lines and offsets are set for the original scale. To loft, you would have to literally remeasure and replot the lines over to the larger scale which would be an extremely tedious process and frought with possible errors. It may be best to find a 29'-30' folkboat design that already exists and is complete with lines and structural plans rather than try to scale this one up. However, if you are game for this project, you can do it, just realize it is going to be a lot of extra work and it would best to have a naval architect help you out.

    Eric
     
  7. daiquiri
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    daiquiri Engineering and Design

    Anyways, it's one lovely boat. :)
     
  8. Johnny Kadire
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    Johnny Kadire New Member

    Ok Briefly as I'm running out the door here..Thanks everybody for the input..
    I have already a revised outline scantling schedule drawn up.
    Weights ,admittedly,have yet to be calculated but I believe will be within acceptable parameters.
    With regard to lofting I planned on just applying a common,in this case 1.16 ,multiplying factor to my original offsets(one long evening with a calculator,)and lofting this onto the originally specified grid again increased by 1.16...or am I missing something..won't this work???
    I'm afraid the only architect in the budget is me and if its of my own misfortune I'll live with it..
    I should add,or confess, here that I have considered dropping the scale up to 12%.
     
  9. Eric Sponberg
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    Eric Sponberg Senior Member

    In order to scale the lines, you have to pick a reference point in 3D space to do a proper scaling. This can be station zero, on the hull centerline, on the baseline. The table of offsets gives heights and half breadths and you can scale those numbers directly. Actually, looking at your drawing, it appears to be of a different drafting convention where there is no station 0 (usually at the bow), but rather it starts with station 1 at the peak of the stern. Nevermind, you have to scale the positions of the waterlines and the waterline spacing, the positions of the stations and the station spacing, and the positions of the buttocks and the buttock spacing all accordingly as referenced to your original reference point. This applies no matter what scale factor you use, it's all the same.

    For example. Let's say that the first waterline is 1' above the baseline. In the new model, it will be 1.16' above the baseline. The 2' waterline will be at 2.32' above the waterline, and so on. Do the same for the stations and the buttocks.

    Because of the odd drafting convention, there may not be any baseline on the lines plan, but rather the reference waterline may be the design waterline. Heights would be measured above (+ numbers) and below (- numbers) from the design waterline. No matter. Pick one point in space from which to scale all numbers and scale accordingly. You will have to use the new lofted lines to calculate the new displacement, hull form coefficients (which should be the same, actually), and stability.

    Eric
     
  10. PAR
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    PAR Yacht Designer/Builder

    Eric has nailed it down as typical for him. I'm a little familiar with this type and it appears Eric is correct and the LWL is the reference and there are + or - signs to the related waterlines.

    I still think this isn't a wise course of action, on a vessel of this size. If it was me, I'd redraw the whole boat in it's current state, establishing modern conventions, which will make hydrostatics easier, then consider the scaling options.

    Lastly both Eric and I have been kind to you in regard to the idea, but frankly I don't think you have any idea how much these changes will affect the revised vessel (this isn't a personal dig at you, but a common reaction to a common question). We're not talking about 16% differences here, we're talking about huge differences, sufficient enough to require some professional help in all aspects of the design.
     

  11. Chuck Losness
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    Chuck Losness Senior Member

    Not to rain on your parade but have you thought of just buying an existing boat. One of my friends down here in Mexico has Camper & Nicholson (spelling???) 32 that looks just like a folkboat. Only bigger. You would be way ahead of the game in all respects.
     
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