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Thread: Okay to stretch pram from 7-1/2 to 9 ft?

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    Default Okay to stretch pram from 7-1/2 to 9 ft?

    I purchased plans for a Ross Lillistone Alby dinghy and will pick up the plywood in a couple of days. After buying the plans, my wife has expressed an interest in coming with me (rowing, not sailing), so the boat now has to be big enough for the both of us. Spacing the stations from 12" to 14" would stretch the length from 7'7" to around 9 ft. If Ross had a design for a boat between Alby and Flint, I would buy it, but I'm not too interested in looking for another design(er). I'll either stretch Alby or keep it as designed.

    I can think of a few very minor solvable problems:
    1. Bow and stern transom true shapes may be slightly changed due to horizontal but not vertical elongation.
    2. Centerboard and sail spacing (CLR & CE) have to be per original drawings, and not stretched apart.
    3. Possibly less stability with light load, but since both chines are below the WL, I don't think this will be a problem.
    4. Slight weight increase of course.

    It seems that some designers routinely stretch their boats (Jim Michalak) and others sometimes caution against more than a 10% stretch (John Wellsford discussing Tender Behind) so I guess it depends on the circumstance.

    Your opinions would be appreciated.

    Jack - Seattle

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    Default Re: Okay to stretch pram from 7-1/2 to 9 ft?

    Go to the source. Ask Mr. Lillistone. He's accessible. I'm guessing he'll say no. My own guess would be no.

    There are lot of prams big enough to row 2. You mention Welsford. His 'Sherpa' at 9' is one example, and gets good reviews.
    David G
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    "It was a Sunday morning and Goddard gave thanks that there were still places where one could worship in temples not made by human hands." -- L. F. Herreshoff (The Compleat Cruiser)

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    Default Re: Okay to stretch pram from 7-1/2 to 9 ft?

    Thanks for your response David,

    Mr. Lillistone's website appears to be down (http://www.baysidewoodenboats.com.au/) and I didn't see an email address or other contact info in the plans packet. Maybe he's taking a break from correspondence?

    I do like the Welsford 'Sherpa' very much, but I'm accustomed to the Stitch & Tape method, and Sherpa's construction looks - to my eyes - like a longer and more complex project, with a (not sure of terminology) strongback, more frames, ribbands etc. If I'm wrong about Sherpa's complexity I'd definitely consider building it.

    Thanks again,
    Jack

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    Default Re: Okay to stretch pram from 7-1/2 to 9 ft?

    This is a big stretch but I don't think it will hurt. And with the extra length will improve stability.

    I would make two rowing stations, one for good balance with your wife, and one a bit ahead of that for solo. One thwart or better box seat that can move.

    But the definitive answer will be Mr. Lillistone.

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    Default Re: Okay to stretch pram from 7-1/2 to 9 ft?

    Jack if Mr. Lillistone’s unreachable I’d put your query to the folks at Duckworks. You’re close to ‘em, they sell his plans, likely have been exposed to this builders’ desire previously. Might even learn of others having taken this tack before you, or they might have some means of forwarding word of your wish to Mr Lillistone at your behest.

    (Seems Mr Lillistone has a blog too but with the last entry there dated early 2019, other than for existing content it may be a dead end.)

    Good luck in any event!
    Last edited by sp_clark; 09-18-2021 at 06:24 AM.

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    Default Re: Okay to stretch pram from 7-1/2 to 9 ft?

    Ian and SP Clark, thanks for your responses.

    Part of me is reluctant to contact Mr. Lillistone, because I only paid $54 for his plans and don't expect a lot of hand-holding, and respect his wish for privacy if that is his choice. Also, I expect that designers grow weary of people like me wanting to alter their designs, causing them to laboriously explain/defend their original design decisions, or give reluctant consent to a change they don't really want associated with their name.

    However, this is a simple plywood dink (though a great design IMO) and I can't for the life of me think of any meaningful drawbacks to a stretch, other than those noted in my original post.

    Jack

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    Default Re: Okay to stretch pram from 7-1/2 to 9 ft?

    Seems like I read someplace (maybe here?) that Ross had retired recently?
    I’m with Ian- seems reasonable enough to me to do as you propose.
    But only you can decide if you want to risk it.t
    I think I would, but then I would realize that there was a risk of ending up with a less than ideal result.

    I would recommend making a scale model first to prove the concept
    There have been some examples here of how to go about that but I can’t think of any right now.
    then, assuming you are happy with the model, make the forms and tack battens on the chines.

    see how that looks, make minor adjustments, and then start cutting plywood.

    I’m assuming this design does not provide developed panel shapes?
    Bill

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    Default Re: Okay to stretch pram from 7-1/2 to 9 ft?

    This 9ft Gartside pram was published in a past Watercraft and is in his book...







    Its a simplified version of his more 'rounder' prams. He also has a real beauty at 9ft (3.05) in metric with lots of shape. It has really nice lines No. 206A. Best I've ever seen, it's also in his book(s), but traditional construction as drawn. He reckoned this plywood one No. 172 was good and better than even he expected with alot of stability. Pretty quick and simple for decent 'user' pram tender.

    https://store.gartsideboats.com/coll...ram-design-127
    Last edited by Edward Pearson; 09-18-2021 at 05:52 PM.

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    Default Re: Okay to stretch pram from 7-1/2 to 9 ft?

    Thanks Bill,

    The plans do have developed panel shapes, and I would simply increase station spacing from 12" to 14", resulting in a length increase from 7'7" to about 8'10". This would give more room for two rowing, but also for a single rower give a few inches more legroom between the center (rowing) seat and the aft seat, as it looks a bit cramped in the drawings; a product of the 8 ft panel length limitation.

    A problem with starting from scratch with a scale model is I wouldn't know how much curve to put in the bottom to arrive at the right displacement. There is software for this but I'm not ready to go down that rabbit hole. The Alby drawings have a design waterline drawn, but no weight associated with that line. I would guess something around 400+ lbs (2 people + gear + boat weight). Though the one video of an Alby (sailing), it seemed to be near its DWL with one person plus sailing rig. Assuming a DWL of 400 lbs, a 16.7% stretch would take (I think) 67 more pounds to sink the boat to the DWL, which means it would float a bit higher in the water than the original for a given load. This is good for 2 people but the boat might be more tiddly for solo use. This is my only concern and I'm probably overthinking it.

    If nobody talks me out of it, this Monday I'll pick up 4 sheets of 6mm Okume ply and and start the layout at the increased spacing

    Jack

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    Default Re: Okay to stretch pram from 7-1/2 to 9 ft?

    Thanks Edward,

    That pram is about as nice-looking as they come, though even with its relatively simple shape, I'm afraid I don't have the skills (or patience!) to build one of Gartside's beauties

    Jack

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    Default Re: Okay to stretch pram from 7-1/2 to 9 ft?

    Only a flat bottom and two planks...it's just two stitch and tape joints either side then glass over the cheap ply (he was assuming box store Fir ply in the USA). It's drawn and intended for a quick and dirty non precious build.

    Even if you build the Lillistone, see how the transoms sit proud above the sheerstrake...it's so it can be turned over so it won't fill when left in winter and air still circulate underneath.
    Last edited by Edward Pearson; 09-18-2021 at 06:04 PM.

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    Default Re: Okay to stretch pram from 7-1/2 to 9 ft?

    Edward, truth is, I have my heart set on a v-bottom, and I particularly like how Alby is nearly flat in the center but fore & aft sections have a pronounced V. I don't care for the slap-slap when rowing a flat bottom boat and hope the V will help, while still retaining the other pram attributes. I'm thinking that Alby has some of the rowing traits of the Michalak Weevee but with better stability. Alby's chines are immersed (as opposed to Weevee's which are clear of the water), but are said to run parallel to the flow of water so don't slow the boat down. The video of Alby sailing is impressive - https://youtu.be/Sqlwmu4kPUk - and I'm hopeful that Alby will be one of the better-performing dinghies of her size, both rowing and sailing.

    Jack

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    Default Re: Okay to stretch pram from 7-1/2 to 9 ft?

    Didn't realise your sailing too. Yeah V front and back will help with slapping and reduce entry and exit angles, cost a bit of stability and motor performance but in that gybe she looks plenty stable and a fun little boat.

    Looking at Joel White's Nutshell he drew her at 7.5ft then again at 9 ft rather than 'space the molds' which must say something. I think always two up the extra proportional displacement pushing it down will make it work. If it was solo a better boat would be had drawn at 9 ft. I'd say an Oughted Auk would be the fastest boat you could build as it manages a shallow entry angle with a stem and the extra strakes refine the water plane shape to avoid excessive bends over 12-15 degrees which creates drag. with the same materials but its a bit more work if you're really looking for sail and oar efficiency. There's an argument for indulging a bit more work with a small boat though and if it had to be a pram the a stripped humble bee pram with the Gunter rig will be a delight.

    I remember getting annoyed with a lapstrake stem tender clacking away when I used one as a tender to a big boat and sitting in the cockpit, so I understand the noise thing when you're looking for peace and quiet. I'd only have a smooth hull one now, maybe just lap the top strake to knock off spray. The Lillistone tender looks a good little simple boat, and sometimes a boat that doesn't owe you too much are all the better for it.

    My only concern would be pulling the molds out. You'd have a more stable boat and a faster boat redrawing it at the same displacement and it'll sit to its lines properly. It'll sit like a beach ball empty without much difference in immersion as tender lines and pretty round, but most 'traditional tenders' were built at all different lengths without adjusting anything to be honest. A Nutshell would be worth more if you sold it and has a reputation for being a good little boat. That would be a 'free' boat if you were spending a bit.

    Last edited by Edward Pearson; 09-19-2021 at 07:21 AM.

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    Default Re: Okay to stretch pram from 7-1/2 to 9 ft?

    As designed, it’s fairly beamy and deep for an 8’ pram. As such, I’d think it’d carry 400 lbs. in moderate conditions.
    Stretching it will improve its capacity and stability, but will add the complication of scarfing plywood and might need heavier scantlings to maintain stiffness.
    I wish that designers would publish capacity figures. If they’re any good, they know it.

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    Default Re: Okay to stretch pram from 7-1/2 to 9 ft?

    Edward, you have given me more food for thought , just when I thought my mind was made up! Your 'beach ball' analogy gave clarity to my biggest concern and you said it far better than I did.

    Jack

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    Default Re: Okay to stretch pram from 7-1/2 to 9 ft?

    Thanks Jim, I agree with you, and will almost certainly (after your and Edward's comments) build as-designed. It will be primarily a solo boat, with my wife along infrequently, and I've often had 2 (and one time 4) adults in a dinghy less able than Alby.

    This has been an interesting thought process and I thank all of you for your input.

    Jack

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    Default Re: Okay to stretch pram from 7-1/2 to 9 ft?

    As a postscript, I found an email address and was able to reach Ross Lillistone. He is very generous with his time and knowledge, a fine writer, and all-around great guy. We had some interesting correspondence, the outcome of which will be an 'official' stretch of Alby! I feel honored and excited, and hope my middling skills will do justice to his design.

    Jack

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    Default Re: Okay to stretch pram from 7-1/2 to 9 ft?

    Quote Originally Posted by Jack Loudon View Post
    As a postscript, I found an email address and was able to reach Ross Lillistone. He is very generous with his time and knowledge, a fine writer, and all-around great guy. We had some interesting correspondence, the outcome of which will be an 'official' stretch of Alby! I feel honored and excited, and hope my middling skills will do justice to his design.

    Jack
    What did he say about how to go about it? Any caveats about handling/strength?
    David G
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    "It was a Sunday morning and Goddard gave thanks that there were still places where one could worship in temples not made by human hands." -- L. F. Herreshoff (The Compleat Cruiser)

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    Default Re: Okay to stretch pram from 7-1/2 to 9 ft?

    Quote Originally Posted by Edward Pearson View Post
    This 9ft Gartside pram was published in a past Watercraft and is in his book...
    That pram was designed for the previous owners of our Gartside cabin launch. When we bought it we left it out of the deal for Paul's children to use. They've graduated to these elegant craft.images-1.jpg

    images-2.jpg
    “Eventually, all things merge into one, and a river runs through it. The river was cut by the world's great flood and runs over rocks from the basement of time. On some of those rocks are timeless raindrops. Under the rocks are the words, and some of the words are theirs."

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    Default Re: Okay to stretch pram from 7-1/2 to 9 ft?

    Quote Originally Posted by Edward Pearson View Post
    This 9ft Gartside pram was published in a past Watercraft and is in his book...
    That pram was designed for the previous owners of our Gartside cabin launch. When we bought it we left it out of the deal for Paul's children to use. They've graduated to these elegant craft.
    images-1.jpg

    images-2.jpg
    “Eventually, all things merge into one, and a river runs through it. The river was cut by the world's great flood and runs over rocks from the basement of time. On some of those rocks are timeless raindrops. Under the rocks are the words, and some of the words are theirs."

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    Default Re: Okay to stretch pram from 7-1/2 to 9 ft?

    David,
    Nothing so far about handling or strength, though it's likely that more correspondence will follow. I hope to generally stay with the original Alby building instructions and specifications (particularly 6mm ply), as unsupported panel areas won't increase much. I do remember Phil Bolger commenting that a builder had destroyed a 1/4" ply Brick by hard sailing; presumably through repeatedly torquing the frame, so it's worth applying some thought to this.
    Jack
    Last edited by Jack Loudon; 09-20-2021 at 11:53 AM.

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    Default Re: Okay to stretch pram from 7-1/2 to 9 ft?

    I might mention that some dinks have magic lines and row easily whether light or laden. Spaulding Dunbar's unpretentious skiff is one example.

    https://i.pinimg.com/originals/83/c3...ec6462ab6d.jpg

    The LFH tender to his Marco Polo is another.

    https://www.duckworksmagazine.com/11...eshoff-002.jpg

    Both these little boats would suffer in carrying capacity if simply stretched as much of the secret is in the bottom curves.

    Most boats stretch ok because they are not that great anyway. Nor are their rowers. But I'd not tamper with either of these designs and I'd wand to more about Mr. Lillistone's dink before I'd stretch her. Stretching is usually doable within limits but this amount of stretch may be pushing it, or may be just fine.

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    Default Re: Okay to stretch pram from 7-1/2 to 9 ft?

    Ian,

    I agree with your comments, and I'm pretty sure my proposed bootleg stretch of Alby would have been 'okay' but any magic contained in the original would have been compromised. Designing a good boat isn't easy. A few years ago I read an interview with Jerry Montgomery, of the Montgomery 15, 17, and 23 sailboats. Lyle Hess had drawn the 17 & 23 and Jerry wanted to try his hand on the 15. If I'm remembering correctly, Jerry went to Lyle for advice and was humbled by how much Lyle knew that he didn't know, and how intricate the designing process actually was - and this coming from a highly successful boat builder and accomplished sailor.

    And I think that Alby has some of the magic you describe; it is more thoughtfully designed, and a better boat, than its appearance might suggest. Alby's chine has been drawn parallel to the waterflow (per Phil Bolger) for low drag, causing its slightly unusual appearance. There is a nearly flat bottom in the middle for stability that warps into V's at the ends for softer rowing into a chop, and it takes a big load to immerse those transoms. Alby is as good a pram as First Mate and Phoenix III are daysailers, and it deserves a wider interest than it has received. Okay; off my soapbox

    Jack

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    Default Re: Okay to stretch pram from 7-1/2 to 9 ft?

    I recently built a stretched Bolger Reubens Nymph, took it out from 8’ to 10’, 4’6” beam, built in 3/8” ply,
    It’s a bit heavy but rows well and it’s very stable.
    I have built a few Nymphs in the past, but learnt to add 6” to the bottom for a 4’ beam early on in the series, makes a big difference for a tender and rows fine. We have rowed with two men and two women in one in calm water.

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    Default Re: Okay to stretch pram from 7-1/2 to 9 ft?

    Paul, thanks for your input. That's a 25% stretch, more than my proposed 16.7%, so good to know it worked out well. I'm curious: In doing the stretch did you work from the table of offsets or did you stretch the pre-shaped panels?

    I thought that in stretching the developed panels the shapes would be distorted slightly, less elongated at the ends (where the panels bend inward) than in the middle. At least this is what I thought for Alby, that I would have to tweak the panels during assembly, to make everything fit. Maybe plane a bit off here and there to re-establish fairness. But then it wouldn't be an Alby, and the designer's knowledge and effort in massaging the lines to their optimum (the 'magic' per Ian McCoglin) would be lost or seriously diminished. My main concern would be in keeping the chine parallel to the water flow, which is an essential feature of the original design.

    But the designer has offered to 'officially' stretch the panels, so I will never know how my bootleg stretch would fare compared to the genuine version.

    It's encouraging to know that your stretches were a success.

    Thanks again,
    Jack
    Last edited by Jack Loudon; 09-22-2021 at 12:39 PM.

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    Default Re: Okay to stretch pram from 7-1/2 to 9 ft?

    I have played around with different sizes of Nymphs and Elegant punts for years, never found a problem matching panels, although with S&G you can fudge mismatches to a point.

    The nice thing about the Bolger designs is widening them just involves the bottom flat panel and both transoms, the panel profiles remain unchanged, I just lengthened the station distance

    Widening the bottom and transoms is also what Phil did when he widened the Nymph to make the Reubens Nymph..

    The Elegant Punt is a surprisingly useful dinghy as well..
    Last edited by Paul Fitzgerald; 09-22-2021 at 04:29 PM.

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    Default Re: Okay to stretch pram from 7-1/2 to 9 ft?

    Thank you Paul; very useful information. Based on your experiences, I appear to be overthinking this, and should just get on with it.

    By the way, I now know where the D'Entrecasteaux channel is located. Had no idea. Some Frenchmen sniffing around there awhile back, I suppose.

    Jack

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    Default Re: Okay to stretch pram from 7-1/2 to 9 ft?

    Yep, Bruny D’ Entrecasteau passed this way 1793 looking for the lost La Perouse expedition.

    James Cook anchored in Adventure Bay, outside the Channel in 1770.

    The French never made territorial claims, but they explored and named many features in the area. We could easily be eating baguettes for breakfast if they just raised their flag…
    Last edited by Paul Fitzgerald; 09-23-2021 at 05:58 AM.

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    Default Re: Okay to stretch pram from 7-1/2 to 9 ft?

    Didn't Phil Bolger write somewhere that designing a flat bottom boat that looked and performed well was one of the most difficult design challenges? Maybe someone else ?
    “Eventually, all things merge into one, and a river runs through it. The river was cut by the world's great flood and runs over rocks from the basement of time. On some of those rocks are timeless raindrops. Under the rocks are the words, and some of the words are theirs."

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    Default Re: Okay to stretch pram from 7-1/2 to 9 ft?

    I recently received revised drawings of Alby from Ross Lillistone, with plan & section views and the developed panel shapes showing a 25% length increase to 9.5 ft from the original 7.6 ft. This is a straight stretch with no revisions to the existing lines except their elongation, so displacement at DWL has gone from 414 lbs to 517 lbs, 25% exactly.

    Per David G's question, there were no caveats from Ross regarding construction changes from the original Alby, so I consider the new drawings to be a supplement to the originals, and will be mindful of the 1.25 increase in spacing for some pieces. The only change from the plans I'm considering is a slightly larger sail area, as the original 41 sf seems on the small side for the larger Alby. I may use the 55 sf sail (and possibly the daggerboard as well, to resist the larger sail) from the Lillistone Flint since I already have the plans.

    I've already bought some ply and other materials and will start as soon as clear a space in the shop by finishing an existing (non-boat) project. Once again I have to say what a generous and knowledgeable human being Ross Lillistone has been.

    Jack

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