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Thread: Penalties for scaling up plans?

  1. #1
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    Question Penalties for scaling up plans?

    My Iain Oughtred Sooty Tern is essentially a stretched Arctic Tern. About 10% was added to the length. She works. The penalty? She may be a tad tippier, but I've never sailed an Arctic T. to know for sure and too many other factors between subject boats may interplay to be definitive.

    Anyway, I was wondering, at what point does scaling up a boat design not work? A loaded question. Many factors, but let's say the example is a 17' sailing dinghy with flat lines aft to aid in planning. Consider Paul Gartside's latest in Watercraft, design #226 Centerboard Beach Cruiser.



    What happens if she is enlarged in all dimensions by 10%?

    What happens if you merely "stretch' her length by 10% and add 10% sail area?

    Design info is here:
    https://store.gartsideboats.com/coll...ser-desing-226


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    Default Re: Penalties for scaling up plans?

    My 39' ketch Wandering Star is a 10% enlargement all around of the 36' schooner Aries. I've never seen Aries or the two 36' ketches, but WS is a very good design. I do sometimes think she would go better into a chop if her beam had only been increased 5%, but she would also lose some stiffness.

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    Default Re: Penalties for scaling up plans?

    Its a touchy subject to some, but quite a few designers have routinely stretched or enlarged some of their designs, and in some cases by a lot more than the 10% that is often quoted as being the limit. Rigs do not square the same way a hull does, you might want to equate the current sq ft to the wetted area, and keep that number to find the increased area needed for the sail plan. No hard and fast rules with sail plans, depends where you you sail.

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    Default Re: Penalties for scaling up plans?

    Modest "xerox enlargements" often make small enough differences in performance that only a very good sailor will see. And there may be a few boats that are actually improved. However, most involve some sacrifice that would be better not to have made. A xerox blowup of 10% increases displacement by at least 22%.

    In contrast, modest stretching, essentially put the stations up to 10% further apart, quite often improves many displacement hulls. There are many instances where further improvement could be had by redrawing the rocker sweep of the bottom but such possibilities are hidden by the improvement of simple length.

    In general, simply stretching a boat has the effect of improving form stability.

  5. #5
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    Default Re: Penalties for scaling up plans?

    Just a comment on the current technology available to make modifications to design plans.

    By using computer drawing programs each dimension can be effectively changed proportionally without distortion. Each station can be manipulated to maintain fairness to match any change in beam or draft.
    Last edited by navydog; 08-31-2018 at 09:39 AM.

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    Default Re: Penalties for scaling up plans?

    I appreciate the replies.

    Without building a 100% and a 110% boat, not sure how you'd compare the sailing differences.

    With a 10% change there are likely ergonomic differences that come into play too.

    I would have thought lengthening the boat would make an easier rolling moment, but I understand how increased buoyancy might be the case within a reasonable "stretch".

    Sailing on the Chesapeake can serve up the full range of winds, and sometimes in the same day, but generally 5-10mph is the report.
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    Default Re: Penalties for scaling up plans?

    Here we go again... <grin>
    Hope for the best, but plan for the worst.

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    Default Re: Penalties for scaling up plans?

    Quote Originally Posted by Ian McColgin View Post
    Modest "xerox enlargements" often make small enough differences in performance that only a very good sailor will see. And there may be a few boats that are actually improved. However, most involve some sacrifice that would be better not to have made. A xerox blowup of 10% increases displacement by at least 22%.

    In contrast, modest stretching, essentially put the stations up to 10% further apart, quite often improves many displacement hulls. There are many instances where further improvement could be had by redrawing the rocker sweep of the bottom but such possibilities are hidden by the improvement of simple length.

    In general, simply stretching a boat has the effect of improving form stability.
    Area increases by the square of the scale, displacement by the cube, so a 10% increase in all dimensions would give you a boat with 21% more area and 33% more displacement.

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    Default Re: Penalties for scaling up plans?

    Again? Figured, but I don't see any pertinent results from such a search. Cough them up if you got them and I'll close this down. Thx-
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    Default Re: Penalties for scaling up plans?

    Well, here is a good part of what I was looking for:

    http://forum.woodenboat.com/showthre...nlarging+plans
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    Default Re: Penalties for scaling up plans?

    Yeah, again. This has been a recurring topic since I started participating in the Forum in 1999/2000, usually with the same results (and often with the same participants) each time. Essentially, the consensus is that, for wee small boats (under 10 - 12 feet) an across-the-board increase of 10% - 15% ("Xerox enlargement") doesn't cause many problems. For small boats (12 - 20 feet) the mob thinks that 10% - 15% increase in length only (setting up molds farther apart than in original plans) is OK. Beyond 20 feet, you start getting into scantlings and stability issues.

    There is always a vocal contingent who don't wish to be restricted in their freedom to mess with plans and 'experiment', and as long as they don't risk too much money on a failed experiment or risk the innocent lives of their friends & family, I'm mostly OK with that.

    My issue has always been with the "but quite a few designers have routinely stretched or enlarged some of their designs" (apologies to skaraborgcraft) viewpoint, which is trotted out to justify the practice among the unwashed masses. The difference is that the professional designers who do this know full well what the implications of the changes are, and will adjust the design and/or scantlings to compensate for the change in physical size of the boat. Amateur designers can do this too, provided that they have educated themselves, done their due diligence, and understand the pitfalls. But many do not, and the fact that it is entirely too easy to die out on the water causes me to caution those contemplating the process.
    Hope for the best, but plan for the worst.

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    Default Re: Penalties for scaling up plans?

    Quote Originally Posted by mmd View Post
    Yeah, again. This has been a recurring topic since I started participating in the Forum in 1999/2000, usually with the same results (and often with the same participants) each time. Essentially, the consensus is that, for wee small boats (under 10 - 12 feet) an across-the-board increase of 10% - 15% ("Xerox enlargement") doesn't cause many problems. For small boats (12 - 20 feet) the mob thinks that 10% - 15% increase in length only (setting up molds farther apart than in original plans) is OK. Beyond 20 feet, you start getting into scantlings and stability issues.

    There is always a vocal contingent who don't wish to be restricted in their freedom to mess with plans and 'experiment', and as long as they don't risk too much money on a failed experiment or risk the innocent lives of their friends & family, I'm mostly OK with that.

    My issue has always been with the "but quite a few designers have routinely stretched or enlarged some of their designs" (apologies to skaraborgcraft) viewpoint, which is trotted out to justify the practice among the unwashed masses. The difference is that the professional designers who do this know full well what the implications of the changes are, and will adjust the design and/or scantlings to compensate for the change in physical size of the boat. Amateur designers can do this too, provided that they have educated themselves, done their due diligence, and understand the pitfalls. But many do not, and the fact that it is entirely too easy to die out on the water causes me to caution those contemplating the process.
    Very good. I saw Paul Gartside's latest and knowing the relative success with my Sooty Tern, I wondered about the exercise in this instance. I've emailed Paul and expect I'll hear back soon enough.

    Over and out-
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    Default Re: Penalties for scaling up plans?

    I have no issue with Michals comment, and perhaps i should have added that yes designs have been well stretched, and often the scantlings are increased, if the extra displacement calls for it. I dont suggest anyone should increase size without doing their homework, maybe i took it for granted those contemplating doing so might know better.....

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    Default Re: Penalties for scaling up plans?

    If you started by making it longer (Mk1) it will have more wetted area drag at low speed and it will be heavier. That added heaviness makes it harder to get on the plane and gives a wider wind speed where it will still only go at hull speed, before it starts planing. You will drop back and stay in that displacement trough more often in variable marginal planing wind speeds, while your identical self in an alternative universe who built it as designed with a lighter boat keeps it planing or at least semi planing on a more linear speed 'curve'.

    So then add more sail area (Mk2)...well you can only hold so much sat on the gunwale 3ft out, unless you are an extra heavy person. You don't have a ballast keel secondary stability either. That limits what you can hold, but it would enable you to plane earlier if you wanted that but conversely you will also have to reef earlier otherwise you'll get too much heel, too much weather helm etc and end up in the water.

    It was already wide at 6'4" as drawn so now you'll now add more beam (Mk3)...that works until one day you have it over, it turtles and it's so form stable upside down you can't right it solo, and then your on the telly...that's forgetting the extra wetted area, wavemaking resistance, weight, heave, pitch and roll motion when its upright.

    There's a cluster of two man planing dinghy designs around 6ft x 16ft 10-12 sqm for good reason. Outside these parameters you can change a boat do something more, but it will be to the detriment of something else that's found to be important. Paul's 6ft beam boat has room to sleep 2 either side of the centercase, its got deckage, the extra form stability of 6ft beam/ 5ft waterline and stiffer aft sections compared to your ST as it rolls. Build as is, in WRC as shown. Solo planing and the cluster is around 14ft: without two on the rail its hard to power up a Wayfarer for example. Solo they're often reefed, and the guy in the 14ft'er with the same sail area has a lighter boat with less drag under him.

    Design parameters for good sea boats are usually moderate in all respects: not too much or too little of anything. 5ft beam boats cope with waves better. 6ft beam boats cope with wind better. 6ft is about max beam for self righting with some tankage. More than 16ft and its a trailer not a trolley to move it. Launch fees escalate when your boats bigger than normal family dinghy size usually.

    Being critical. Chined boats are more efficent than round bilge boats for planing, though people see them as more crude and are out of fashion on here. I'd prefer to have the mast inside the coaming closer to the center of the boat, not puncturing the deck and for leading lines aft, which would be possible if we binned the mizzen. More forward rake on the stem would give a slightly drier boat, but you can't take the Cornwall out of Paul. Love what he did with the aft keel for launching.
    Last edited by Edward Pearson; 08-31-2018 at 12:14 PM.

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    Default Re: Penalties for scaling up plans?

    this thread is awesome, following

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    Default Re: Penalties for scaling up plans?

    Quote Originally Posted by mmd View Post
    Yeah, again. This has been a recurring topic since I started participating in the Forum in 1999/2000, usually with the same results (and often with the same participants) each time. Essentially, the consensus is that, for wee small boats (under 10 - 12 feet) an across-the-board increase of 10% - 15% ("Xerox enlargement") doesn't cause many problems. For small boats (12 - 20 feet) the mob thinks that 10% - 15% increase in length only (setting up molds farther apart than in original plans) is OK. Beyond 20 feet, you start getting into scantlings and stability issues.

    There is always a vocal contingent who don't wish to be restricted in their freedom to mess with plans and 'experiment', and as long as they don't risk too much money on a failed experiment or risk the innocent lives of their friends & family, I'm mostly OK with that.

    My issue has always been with the "but quite a few designers have routinely stretched or enlarged some of their designs" (apologies to skaraborgcraft) viewpoint, which is trotted out to justify the practice among the unwashed masses. The difference is that the professional designers who do this know full well what the implications of the changes are, and will adjust the design and/or scantlings to compensate for the change in physical size of the boat. Amateur designers can do this too, provided that they have educated themselves, done their due diligence, and understand the pitfalls. But many do not, and the fact that it is entirely too easy to die out on the water causes me to caution those contemplating the process.
    I agree to a certain extent, what does annoy me though is when someone makes major changes to a design, which then doesn't work as well as expected and tells everyone who will listen that XYZ ( In this case, me) designed it. If I'm asked about a "stretch" or a "scale up" I'll run the numbers before agreeing to it. There are many factors in a hull and rig that don't increase at the same rate when scaled up, and if the design is already "near the edge" in search of a particular performance aspect, then it may take very little change to push it over into poor performance.

    John Welsford.
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    Default Re: Penalties for scaling up plans?

    JohnW [#8] is right. I don't know what I was thinking.

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    Default Re: Penalties for scaling up plans?

    Quote Originally Posted by mmd View Post
    Yeah, again. This has been a recurring topic since I started participating in the Forum in 1999/2000, usually with the same results (and often with the same participants) each time. Essentially, the consensus is that, for wee small boats (under 10 - 12 feet) an across-the-board increase of 10% - 15% ("Xerox enlargement") doesn't cause many problems. For small boats (12 - 20 feet) the mob thinks that 10% - 15% increase in length only (setting up molds farther apart than in original plans) is OK. Beyond 20 feet, you start getting into scantlings and stability issues.

    There is always a vocal contingent who don't wish to be restricted in their freedom to mess with plans and 'experiment', and as long as they don't risk too much money on a failed experiment or risk the innocent lives of their friends & family, I'm mostly OK with that.

    My issue has always been with the "but quite a few designers have routinely stretched or enlarged some of their designs" (apologies to skaraborgcraft) viewpoint, which is trotted out to justify the practice among the unwashed masses. The difference is that the professional designers who do this know full well what the implications of the changes are, and will adjust the design and/or scantlings to compensate for the change in physical size of the boat. Amateur designers can do this too, provided that they have educated themselves, done their due diligence, and understand the pitfalls. But many do not, and the fact that it is entirely too easy to die out on the water causes me to caution those contemplating the process.
    Can't quite recall, does stability increase by the 4th power of the scale? Of course, the skipper's weight doesn't change, so a larger boat may need a more stable hull, as it's not as dominated by the skipper's weight as a wee boat.

  19. #19
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    Default Re: Penalties for scaling up plans?

    Quote Originally Posted by Edward Pearson View Post
    If you started by making it longer (Mk1) it will have more wetted area drag at low speed and it will be heavier. That added heaviness makes it harder to get on the plane and gives a wider wind speed where it will still only go at hull speed, before it starts planing. You will drop back and stay in that displacement trough more often in variable marginal planing wind speeds, while your identical self in an alternative universe who built it as designed with a lighter boat keeps it planing or at least semi planing on a more linear speed 'curve'.

    So then add more sail area (Mk2)...well you can only hold so much sat on the gunwale 3ft out, unless you are an extra heavy person. You don't have a ballast keel secondary stability either. That limits what you can hold, but it would enable you to plane earlier if you wanted that but conversely you will also have to reef earlier otherwise you'll get too much heel, too much weather helm etc and end up in the water.

    It was already wide at 6'4" as drawn so now you'll now add more beam (Mk3)...that works until one day you have it over, it turtles and it's so form stable upside down you can't right it solo, and then your on the telly...that's forgetting the extra wetted area, wavemaking resistance, weight, heave, pitch and roll motion when its upright.

    There's a cluster of two man planing dinghy designs around 6ft x 16ft 10-12 sqm for good reason. Outside these parameters you can change a boat do something more, but it will be to the detriment of something else that's found to be important. Paul's 6ft beam boat has room to sleep 2 either side of the centercase, its got deckage, the extra form stability of 6ft beam/ 5ft waterline and stiffer aft sections compared to your ST as it rolls. Build as is, in WRC as shown. Solo planing and the cluster is around 14ft: without two on the rail its hard to power up a Wayfarer for example. Solo they're often reefed, and the guy in the 14ft'er with the same sail area has a lighter boat with less drag under him.

    Design parameters for good sea boats are usually moderate in all respects: not too much or too little of anything. 5ft beam boats cope with waves better. 6ft beam boats cope with wind better. 6ft is about max beam for self righting with some tankage. More than 16ft and its a trailer not a trolley to move it. Launch fees escalate when your boats bigger than normal family dinghy size usually.

    Being critical. Chined boats are more efficent than round bilge boats for planing, though people see them as more crude and are out of fashion on here. I'd prefer to have the mast inside the coaming closer to the center of the boat, not puncturing the deck and for leading lines aft, which would be possible if we binned the mizzen. More forward rake on the stem would give a slightly drier boat, but you can't take the Cornwall out of Paul. Love what he did with the aft keel for launching.
    Excellent overview. Thanks Ed. I've an itch to build another boat. Love my Sooty, UNA, don't want a slower boat, but would like more room for 2 aboard to overnight. The Caledonia Yawl is the same length, but 1' wider and almost 60 sf more sail. The volume difference is remarkable. We've too many (6) CY's in our TSCA now. Got to fight back. I've bought plans for Gartside's beautiful 6 Meter Lugger. Built a half hull of her and cut staves for the spars, but something has held me back. Maybe its that the cabin removes too much cockpit for kids and dogs. Perhaps I could build her as an open boat with Sooty features. I know every boat is a compromise. The Sooty doesn't lose to the CY to weather. Got to maintain that.
    Wonderful boat. Just considering more volume. Otherwise, the ST is perfect for one man camp aboard sailing. Just spent a week cruising in Maine with UNA. Delightful.

    As someone said, dreaming is cheap. Working toward the dream can be awesome.
    Thanks-
    Eddie
    Last edited by EeBe4; 09-01-2018 at 06:41 AM. Reason: add
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    Default Re: Penalties for scaling up plans?

    Quote Originally Posted by Edward Pearson View Post
    If you started by making it longer (Mk1) it will have more wetted area drag at low speed and it will be heavier. That added heaviness makes it harder to get on the plane and gives a wider wind speed where it will still only go at hull speed, before it starts planing. You will drop back and stay in that displacement trough more often in variable marginal planing wind speeds, while your identical self in an alternative universe who built it as designed with a lighter boat keeps it planing or at least semi planing on a more linear speed 'curve'.

    So then add more sail area (Mk2)...well you can only hold so much sat on the gunwale 3ft out, unless you are an extra heavy person. You don't have a ballast keel secondary stability either. That limits what you can hold, but it would enable you to plane earlier if you wanted that but conversely you will also have to reef earlier otherwise you'll get too much heel, too much weather helm etc and end up in the water.

    It was already wide at 6'4" as drawn so now you'll now add more beam (Mk3)...that works until one day you have it over, it turtles and it's so form stable upside down you can't right it solo, and then your on the telly...that's forgetting the extra wetted area, wavemaking resistance, weight, heave, pitch and roll motion when its upright.

    There's a cluster of two man planing dinghy designs around 6ft x 16ft 10-12 sqm for good reason. Outside these parameters you can change a boat do something more, but it will be to the detriment of something else that's found to be important. Paul's 6ft beam boat has room to sleep 2 either side of the centercase, its got deckage, the extra form stability of 6ft beam/ 5ft waterline and stiffer aft sections compared to your ST as it rolls. Build as is, in WRC as shown. Solo planing and the cluster is around 14ft: without two on the rail its hard to power up a Wayfarer for example. Solo they're often reefed, and the guy in the 14ft'er with the same sail area has a lighter boat with less drag under him.

    Design parameters for good sea boats are usually moderate in all respects: not too much or too little of anything. 5ft beam boats cope with waves better. 6ft beam boats cope with wind better. 6ft is about max beam for self righting with some tankage. More than 16ft and its a trailer not a trolley to move it. Launch fees escalate when your boats bigger than normal family dinghy size usually.

    Being critical. Chined boats are more efficent than round bilge boats for planing, though people see them as more crude and are out of fashion on here. I'd prefer to have the mast inside the coaming closer to the center of the boat, not puncturing the deck and for leading lines aft, which would be possible if we binned the mizzen. More forward rake on the stem would give a slightly drier boat, but you can't take the Cornwall out of Paul. Love what he did with the aft keel for launching.
    With respect, Ed, some of that goes completely against what lots of world-class dinghy designers, and the laws of physics like scaling effects, say.

    As you increase a dinghy's size weight goes up close to a linear fashion, while volume increases by the cube. Therefore, if you add length and leave everything else equal, the longer boat will proportionately much lighter. Compare the old 64kg International Canoe hull against the latest C9kg hull carbon Moth - the IC has a much lower (ie proportionately lighter) Displacement/Length Ratio than the Moth (about 32 to 72) because of the cubing effect.

    The lower DLR means that the longer boat has, all else being equal, less rocker and more slender lines overall - for example the very light but short Cherub racing dinghy has an entry angle of about 13 degrees while an 18 Footer uses its extra length to reduce the entry angle to 10 degrees or so.

    So if you add length and leave everything else the same, you add a minimal bit of extra weight and gain a proportionately lighter hull with finer, flatter hull lines. That hull is more easily driven at displacement speeds, so it goes faster. Planing lift increases with the square of speed - therefore the longer boat's extra speed has a positive feedback effect. The longer boat is going faster and therefore getting significantly more lift, which allows it to plane earlier.

    As Julian Bethwaite, for example, will tell you that's why the 29er planes earlier than the much lighter Cherub. The shorter boat runs hard up against the "speed limit" of its shorter waterline and proportionately fatter hull before it can develop sufficient lift to get onto the plane. Paul Bieker, Frank Bethwaite, Julian Bethwaite, Andy Paterson, John Claridge and others say the same thing - longer is faster almost all the time because length allows a finer shape, and that gives beneficial feedback from the hull speed and dynamic lift effects.

    Secondly, adding length while maintaining beam actually increases stability. Some of the volume in the extra length is away from the centreline. Heeling the boat means pushing that extra volume down, essentially. Because there is more volume away from the centreline, the boat becomes more stable.

    The extra speed efficiency of adding length can be seen by comparing the Laser Radial to the Europe. The Europe has much more sail (8m compared to 5.7), is dramatically lighter and has more sophisticated gear - yet it goes slower and is less stable, because it is shorter. Similarly, a 12 Foot Skiff's hull is about a third of the weight of a 49er and has pretty much the same sail area, but the 12 is significantly slower - that short hull is proportionately heavy and spends a lot of time hitting the hull speed wall.

    I think every racing dinghy designer I have ever interviewed (and that includes most of the world-class names) has said that longer boats are faster and more stable all-round in almost all conditions. The only exceptions are in extremely light winds and in very heavy air downwind. As one famous Kiwi dinghy builder and sailor once wrote, "length isn't everything - just 99%". If length wasn't faster, then you wouldn't see every development class dinghy (with the exception of one or two Fox ICs from the '30s) built to its maximum length, and you wouldn't see people like Julian and Phil Morrison adding extra length when they created production boats from development-class inspirations.

    And no, I'm NOT saying that speed is important. However, it is a factor we can look at and should do so the right way.
    Last edited by Chris249; 09-01-2018 at 07:21 AM.
    Has BigFella and SkyBlue on ignore.

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    Default Re: Penalties for scaling up plans?

    Getting back to the Gartside design. I don't think it was designed as a planing boat. I think it might plane given the right crew and a very strong wind. A stretched version probably wouldn't make much difference unless you stretched it by bringing the beam further aft, by inserting another station aft of Bmax.

    You might do better talking to Mr Welsford about his AWOL design

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    Default Re: Penalties for scaling up plans?

    Hey Gareth,
    I think PG mentions planing in his write up of #226. Certainly flatter aft, but not the Moth or Laser I sailed long ago. Not sure I want a 20' version of those. A "handful" is not my aim.

    AWOL: that has interested me. I'll take another look.

    Enjoying the discussion-
    E
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    Default Re: Penalties for scaling up plans?

    You could also look at Mr. Gartside’s #166.....once you scale up the #226 by 10% or so, you’ll be very close to this dimensionally, with the lug yawl rig already scaled to fit.
    I also like the AWOL, and Mr. Welsford’s Sweet Pea a lot. I asked him recently about a high aspect lug main for the Sweet Pea, and it sounds quite feasible. Might be worth asking about a lug for the AWOL.....
    Last edited by John hartmann; 09-01-2018 at 07:42 AM.

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    Default Re: Penalties for scaling up plans?

    Thx John. I do have the #166 plans. Built a 1/2 hull and real spars. Deliberating.
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    Default Re: Penalties for scaling up plans?

    Quote Originally Posted by John hartmann View Post
    You could also look at Mr. Gartside’s #166.....once you scale up the #226 by 10% or so, you’ll be very close to this dimensionally, with the lug yawl rig already scaled to fit.
    I also like the AWOL, and Mr. Welsford’s Sweet Pea a lot. I asked him recently about a high aspect lug main for the Sweet Pea, and it sounds quite feasible. Might be worth asking about a lug for the AWOL.....
    No problem.

    John Welsford
    An expert is but a beginner with experience.

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    Default Re: Penalties for scaling up plans?

    I don't know anything about the performance aspects of increasing beam and length, but I have noticed if you increase everything say 10%, the extra gunwale height will look more than 10% uglier, and really needs adjusting with a batten to get a good look.

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    Default Re: Penalties for scaling up plans?

    Quote Originally Posted by Chris249 View Post
    With respect, Ed, some of that goes completely against what lots of world-class dinghy designers, and the laws of physics like scaling effects, say.

    As you increase a dinghy's size weight goes up close to a linear fashion, while volume increases by the cube. Therefore, if you add length and leave everything else equal, the longer boat will proportionately much lighter. Compare the old 64kg International Canoe hull against the latest C9kg hull carbon Moth - the IC has a much lower (ie proportionately lighter) Displacement/Length Ratio than the Moth (about 32 to 72) because of the cubing effect.

    The lower DLR means that the longer boat has, all else being equal, less rocker and more slender lines overall - for example the very light but short Cherub racing dinghy has an entry angle of about 13 degrees while an 18 Footer uses its extra length to reduce the entry angle to 10 degrees or so.

    So if you add length and leave everything else the same, you add a minimal bit of extra weight and gain a proportionately lighter hull with finer, flatter hull lines. That hull is more easily driven at displacement speeds, so it goes faster. Planing lift increases with the square of speed - therefore the longer boat's extra speed has a positive feedback effect. The longer boat is going faster and therefore getting significantly more lift, which allows it to plane earlier.

    As Julian Bethwaite, for example, will tell you that's why the 29er planes earlier than the much lighter Cherub. The shorter boat runs hard up against the "speed limit" of its shorter waterline and proportionately fatter hull before it can develop sufficient lift to get onto the plane. Paul Bieker, Frank Bethwaite, Julian Bethwaite, Andy Paterson, John Claridge and others say the same thing - longer is faster almost all the time because length allows a finer shape, and that gives beneficial feedback from the hull speed and dynamic lift effects.

    Secondly, adding length while maintaining beam actually increases stability. Some of the volume in the extra length is away from the centreline. Heeling the boat means pushing that extra volume down, essentially. Because there is more volume away from the centreline, the boat becomes more stable.

    The extra speed efficiency of adding length can be seen by comparing the Laser Radial to the Europe. The Europe has much more sail (8m compared to 5.7), is dramatically lighter and has more sophisticated gear - yet it goes slower and is less stable, because it is shorter. Similarly, a 12 Foot Skiff's hull is about a third of the weight of a 49er and has pretty much the same sail area, but the 12 is significantly slower - that short hull is proportionately heavy and spends a lot of time hitting the hull speed wall.

    I think every racing dinghy designer I have ever interviewed (and that includes most of the world-class names) has said that longer boats are faster and more stable all-round in almost all conditions. The only exceptions are in extremely light winds and in very heavy air downwind. As one famous Kiwi dinghy builder and sailor once wrote, "length isn't everything - just 99%". If length wasn't faster, then you wouldn't see every development class dinghy (with the exception of one or two Fox ICs from the '30s) built to its maximum length, and you wouldn't see people like Julian and Phil Morrison adding extra length when they created production boats from development-class inspirations.

    And no, I'm NOT saying that speed is important. However, it is a factor we can look at and should do so the right way.
    If you don't change the scantlings, but scale all dimensions up by the same amount, weight of the structure will increase by the square of the scale, equivalent to area. Since the displacement on the designed waterline increases by the cube of the scale, that means if you don't ballast the boat down to the designed waterline, it will float higher and have different stability characteristics.

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    Default Re: Penalties for scaling up plans?

    /\

    Very true; the dinghy scaling references I've seen assumed that freeboard didn't increase by the same amount which would have kept weight down. However, the big boat floating lighter is unlikely to be slower.
    Has BigFella and SkyBlue on ignore.

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    Default Re: Penalties for scaling up plans?

    Quote Originally Posted by john welsford View Post
    No problem.

    John Welsford
    Good to know; thank you!

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    Default Re: Penalties for scaling up plans?

    Quote Originally Posted by Chris249 View Post
    /\

    Very true; the dinghy scaling references I've seen assumed that freeboard didn't increase by the same amount which would have kept weight down. However, the big boat floating lighter is unlikely to be slower.
    It is likely to have different stability characteristics.

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    Default Re: Penalties for scaling up plans?

    /\

    Yep. But the claim that a longer boat will be slower and therefore need more sail and therefore be tippier, as above, seems unlikely to be correct. In boats I have sailed with similar beams and weights, the longer ones are much more stable. Then again, I suppose I've never been unlucky enough to sail a boat that was scaled directly in all dimensions.
    Has BigFella and SkyBlue on ignore.

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    Default Re: Penalties for scaling up plans?

    www.sassdesign.net
    I'm not lost, I'm just uncertain of my position.
    I'm still confused, but on a higher level

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    Default Re: Penalties for scaling up plans?

    When I scale a boat up or down, (often done in the initial design process when playing with ideas) - I scale it, and then I sit back and look at it afresh, run all the numbers, check displacement, stability, sail area, blah blah blah. Typically that then suggests making some changes to the direct scaled design. Small changes/ big changes, all depends. : )
    The more you scale a boat, the more you're actually redesigning it - Who's ready to design a boat and put all the money and hours into building it? Ready for that, then scale away!

    p.s. Hi Eddie, (found you!)

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    Default Re: Penalties for scaling up plans?

    The scaling has been overdone when you have to stand on your toes to reach the ice box and the beer retrieved is so big and heavy as to require two hands.

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    Default Re: Penalties for scaling up plans?

    Hey, there's nothing wrong with the last part!
    Has BigFella and SkyBlue on ignore.

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