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Thread: Pram Perimeters

  1. #36
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    Default Re: Pram Perimeters

    Quote Originally Posted by bheys View Post
    You asked a couple of questions about my pram build. The purpose - it's nothing more than a tender for a trailer sailor. The pram build started about a week before Christmas. I'm not keeping track of the time, but the intention was to make it a quick process. The strips were leftovers from a couple of kayaks and the ply was leftover from a recent sailboat build. It is built atop a ply strongback with the molds (1 foot spacing) attached egg crate style. I suspect that this boat is going to receive minimal TLC and maximum abuse. The interior is faired just enough to avoid air pockets under the cloth. Admittedly, it's a bit rough. Today I glassed the gunnels.

    There are some additional pictures at the end of the "Gartside 170" thread .
    I understand... Great work. I am thinking along the same lines... I am not building a piece of furniture to be cherished but a working, useful and fun tool.

    Your build looks great. I would think that you are skilled and not afraid of challenges. I take it that you are not going to add a rig, if this is going to be a tender for your sailboat. What kind of trailer sailboat did you build? Do you have any blogs of the progress of any of your boats? I'd love to read up on your works.

    Thanks again for the comments.

    Cheers...

  2. #37
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    Default Re: Pram Perimeters

    Quote Originally Posted by gypsie View Post
    Whats the initial stability going to be like with that lovely curvy bilge?

    And, look what someone did with a nutshell; http://urbanboatb1.web704.discountas...struction1.htm

    That is some crazy thinking outside the box... I really did like the read, Thanks.

  3. #38
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    Default Re: Pram Perimeters

    Getting ready to build the next model out of bass wood when it dawned on me that I would like to incorporate a few construction/design concepts during this build. Here is where I am at prior to changing the build method.



    I was going to just build a center-line strong-back to bend the bottom plank over, and build the hull upside down using a male mold. In the last model, I build a female mold and bending the bottom plank was a bit iffy.

    So here is my latest design/construction process issue. Instead of using a strong back (for the model, and real world boat, and wasting material on something that will not end up into the boat, why not build a backbone for the little boat into the design of the structure. I have seen this in many cases. Here are a few:





    With this thought in place, I was thinking that, like the owl pictured here above, I would use two main longitudinal backbones spaced far enough apart to provide spanning space for the sole (transverse planking between the backbones) for the boat and then using the backbones as the inward edge/support of longitudinal seats and flotation chambers along the sides of the boat. With the thought of a knock-down I would like to install flotation chambers close to the mid-point of the boat where it would lie in the water on it's side. (any thoughts on this?)

    I was also thinking about using two dagger boards (lee boards) or somewhat twin keels of a layout attached to these same longitudinal backbones. Another thought was (hold on to your hats) making a system to be able to change the bottom of the dagger boards to incorporate wheels for using the boat as a wagon and also adding foils for some fun sailing... keeping my options opens. Having a port and starboard dagger board does mean more work, but then the center of the boat is open for coolers, dive gear and hauling materials/provisions etc.

    This type of wheel (10" x 3" with nylon bearing) on a fork like a bicycle attached to a cap / plug from below?

    I like to owl system shown above, but I think I want to reverse the seat idea from the center to the sides, so that I can add the dual dagger boards/wheel concepts. (any thoughts there?)

    Many issues here to play with and I am thinking about getting deep into this final design now that I have the hull approximately where I want it...

    Any input at this stage would be great, as I am not an expert at anything, but I can fake it enough just to keep myself out of trouble.

    Thanks,
    Last edited by bjdbowman; 02-03-2019 at 11:23 AM.

  4. #39
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    Default Re: Pram Perimeters

    I think the "duel" dagger boards or wheels on a boat this size is just adding unneeded weight and complexity. Im not sure where the cross over point is between a small dinghy made from 4mm ply, and one of SOF with heavy frames and battens, often the solid skin ply boat can be lighter as its skin forms part of the structure.

  5. #40
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    Default Re: Pram Perimeters

    Quote Originally Posted by skaraborgcraft View Post
    I think the "duel" dagger boards or wheels on a boat this size is just adding unneeded weight and complexity. Im not sure where the cross over point is between a small dinghy made from 4mm ply, and one of SOF with heavy frames and battens, often the solid skin ply boat can be lighter as its skin forms part of the structure.
    I believe that the difference here is that the SOF can be stripped down to it's basic shell for lifting (by an old man) up onto a dock or other boat without the need of any assistance. Almost everything can and will be detachable so only the monocoque hull of the boat is left to deal with, which will be its skeleton and skin... This concept is way lighter than any plywood construction. Just a thought. I don't believe that complexity is really an issue here. I'm planning on this being a very basic build... Nothing too far fetched. I'm designing this build to eliminate all complexity ahead of time.

    Also, the wheels are only installed when needed, same as the dagger boards and the rigging. Otherwise it will be just a rowboat or dingy with a small outboard for everyday use as a tender. The other items are available for fun and utilitarian purposes, as needed, killing two or three birds with one stone.

    I know that this may seem alot to do for a little boat, but this is the first and experimental learning for the next few bigger boats.

    Thanks,

  6. #41
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    Default Re: Pram Perimeters

    I found the original boat plans from the very first post, the real parent hull...

    I believe that this is the exact parent hull that I was looking for. I found the designer and bought the plan book for $30 US. Here is the link!

    Plans for the Norwegian Sailing Pram http://simonwattsfurniture.com/norwegian-sailing-pram.html

    I just paid for the plans as a thanks for the inspiration... and hopefully a good read. Maybe I'll get some more inspiration and some help with the rig.



    Thanks...

  7. #42
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    Default Re: Pram Perimeters

    Some have commented above that the round bilge is an issue and that this was too complicated a design.
    Therefore: I have designed the worse case model to see if there are any improvements:


    I decided to try a single panel square bilge just for kicks. There are better initial stability and more interior volume, however that comes with some costs. Mainly in weight. The area of the hull is 50% LARGER than the last design, making a 100 lb build into a 150 lb build, assuming that the original would be that heavy.


    Here is the easy boxy design:






    This is the same size and general shape of the last design as shown below. My only issue is that I just don't like it nor do I feel that I could make a strong skin on frame build with this shape.

    This was the last design:


    So you can see that there is not a big difference. I do like the added stability and the volume, therefore, I decided to plump out this design a bit adding in some volume down low. I used a true radius along the chine making the bottom and the sheer 90 degrees and this did add to the volume and the stability.


    Here is the latest design:


    I also added to the bulwarks to increase the free board... I know that this is not the pretty little pram that I started with... but it's still moving forward... I want to add a small deck and locker on the bow so this will help with the aesthetics of the pram. I still like that last version better, however, I'm looking at more than just a pretty picture... What do you think?


    Any feedback?

    Thanks for the comments above...



    BJD

  8. #43
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    Default Re: Pram Perimeters

    I think even a flat sheer would be better than the slight reverse you have, and better yet just add a touch of height to the bow transom to get the slightest sweep into the sheer. Any reason for doing it like that?

  9. #44
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    Default Re: Pram Perimeters

    The new cross section seems to be a pretty good compromise between stability and ability to deal with waves. That flat part on the bottom invites the use of plywood down there where weight and stiffness would be beneficial.

    I think there is something to be said for a full plywood hull, which can be less heavy if the thwarts, etc. stay in the boat and help spread and support it. In skin and frame construction the frames have to be heavier if you want to be able to take the seats out. Seats can be lighter if there is a post underneath, which would also help tie the boat together, but that wouldn't work so well with removable seats. With skin on frame you would probably need removable floorboards, which you might not need for a plywood bottom/hull.

    The daggerboard-like insert for the wheel seems like a cool idea, something I'll be considering for moving the boats I am building. Such a small boat could probably balanced on a single wheel if you have some spread out handgrips like a wheelbarrow. The drawback to having the wheel underneath is having to tip the boat on its side to insert the wheelboard when you need it, and then getting it back upright. A cart with two wheels well spread would be most easily mounted to the stern but then you'd have to lift the entire front end.

    It looks like that top strake could probably come off. With the reverse sheer the boat is now so deepin the middle that you might need oarport holes to be able to get the oar blades at a reasonable angle to the water. That's at least part of the reason why most rowboats have a sheer that dips lower in the middle. The sheer as you drew it might allow a higher heel angle before getting water into the boat but that would mainly apply to flat water. In waves the boat wouldn't always lean down only exactly to the side, but it would pitch at the same time as well and water could get in over the lower ends.

    I imagine the reverse rake on the stern transom is to save weight, but it looks like it would be easy for a wave to just slop in.

    Your bow seems to be high enough out of the water, but you are giving up a lot of waterline and carrying capacity in the front end. I'm not entirely sure but my working hypothesis is that these long-bowed Norwegian prams were built that way just because they are easy to plank up with a minimum of shaping of the planks; a bit of taper at the ends is all you need. I doubt that they were built as prams for any sort of performance reason. To get the main benefit of a small pram, which is a high carrying capacity and stability per unit length, the designers of pram tenders usually rake the bow transom more and make the bow part above the water shorter. Doing this you could cut a foot or more off the length of this boat without diminishing its capacity.

    I have played with the design of little boats like that and found that the smaller you get the less you can accommodate multiple objectives and that unless you optimize a little boat for one thing it is easy to end up with some unholy compromises. In a 9.5' boat I'm designing I feel only slightly further down the road at best. I've finalized the hull shape and cut the hull plywood but am still tweaking the buoyancy tanks and other "furniture" and sailing parts. There just doesn't seem to be enough room to place everything in proper relation.

    The design aspect is both the most interesting and the most frustrating part of getting a boat. Theory and other people's comments can get you only so far and the rest needs to be learned from using different boats. The more you deviate from established designs, the higher the chances that you might have to start over after having gathered some data points from building and using your first design. At least in Florida you don't have a wall of snow and ice this time of the year. If you start now you could be on your third prototype of that boat before mine ever hits the water.

  10. #45
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    Default Re: Pram Perimeters

    BOI,

    Thanks for the kind words... all good advice. I am still trying to get to the ideal design and building method. This design has been in the computer for about a year and I am just now getting serious with finishing the design so that I can start building. I'm in the middle of a basswood model of an older hull, which is a good thing, because I am two revisions ahead already on the computer. My next model will be a larger skin on frame final prior to making the real thing. I'm searching locally for some wood to use. I have a system designed if I wanted to build entirely out of composite material, but I want to build this one in wood and Kevlar to get the feel of the light craft idea. I want to be able to carry this thing if needed... at least lift it onto the truck or larger boat.

    There is a lot to consider, and I think that the hull is finally moving closer to the final form.

    Here is the latest:






    Again, this here will be a work boat (tender dingy) and this is only the basic shape... the transom is going to be a small sugar scoop design so that I can board from in the water with gear on. Also there will be a rear bulkhead about station 1'6" that will be the rear of the cockpit, and there will be a small cuddi locker at around 7'-6" for locking up our gear when we are onshore and the tender is at the dingy dock.

    I'm staring on the computer 3d model of the skin on frame with all of the interior components added for final design.



    BOI, Have you posted pictures of your design or build? Thanks again for the input, as you can see, I am staring to address some of these concerns, and other comments.

    Thanks everyone.

    BJD

  11. #46
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    Default Re: Pram Perimeters

    Quote Originally Posted by skaraborgcraft View Post
    I think even a flat sheer would be better than the slight reverse you have, and better yet just add a touch of height to the bow transom to get the slightest sweep into the sheer. Any reason for doing it like that?
    Yes, I agree... I think that the bow was way to low... I raised the sheer up and made it flat. I wanted to add a small deck and locker in the bow, but I like the idea of this little boat sitting up-side down on a tri-trampoline.

    I was just trying to get some height into the bulwarks. Refer to the latest revisions.

    Thanks for the input.

    BJD

  12. #47
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    Default Re: Pram Perimeters

    Quote Originally Posted by BOI View Post

    Your bow seems to be high enough out of the water, but you are giving up a lot of waterline and carrying capacity in the front end. I'm not entirely sure but my working hypothesis is that these long-bowed Norwegian prams were built that way just because they are easy to plank up with a minimum of shaping of the planks; a bit of taper at the ends is all you need. I doubt that they were built as prams for any sort of performance reason. To get the main benefit of a small pram, which is a high carrying capacity and stability per unit length, the designers of pram tenders usually rake the bow transom more and make the bow part above the water shorter. Doing this you could cut a foot or more off the length of this boat without diminishing its capacity.
    BOI Yes, you are correct.

    The plans for the two different versions of Norwegian prams, that I have, were designed with simple planks. If you look at the design in post #41 you can see the rake on the bow due to the planking.

    Thanks again for the help.

    BJD

  13. #48
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    Default Re: Pram Perimeters

    I much prefer a hard-chine. Reason: when you row out to your sailboat, you have the dinghy filled with stuff, like a heavy toolbox, bags of ice, groveries, cases of beer, wet dogs, etc. Always too much stuff! And you have to get that up and onto the sailboat without anything falling in the water.

    That means you have to stand up, hold the sailboat with one hand, and sling the loads aboard with the other. You want a dinghy that is stable enough that you can stand in the chine without the thing dumping.

    http://forum.woodenboat.com/showthre...ed-ketch/page7

    I don't have any pictures of me doing that. Too busy! But here is the tender. It looks clunky. But it isn't. Fast, maneuverable, stable, tows lightly, and never fills when under way (in 15 years of Great Lakes cruising). And at 10ft, it's suitable as a lifeboat for these waters.



    As for gunwale guard, I use hot water pipe cladding. Not pretty. But very light and effective. And warm on the toes!



    Last edited by Dave Hadfield; 02-14-2019 at 09:46 AM.

  14. #49
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    Default Re: Pram Perimeters

    Quote Originally Posted by Dave Hadfield View Post
    I much prefer a hard-chine. Reason: when you row out to your sailboat, you have the dinghy filled with stuff, like a heavy toolbox, bags of ice, groveries, cases of beer, wet dogs, etc. Always too much stuff! And you have to get that up and onto the sailboat without anything falling in the water.

    That means you have to stand up, hold the sailboat with one hand, and sling the loads aboard with the other. You want a dinghy that is stable enough that you can stand in the chine without the thing dumping.

    http://forum.woodenboat.com/showthre...ed-ketch/page7

    I don't have any pictures of me doing that. Too busy! But here is the tender. It looks clunky. But it isn't. Fast, maneuverable, stable, tows lightly, and never fills when under way (in 15 years of Great Lakes cruising). And at 10ft, it's suitable as a lifeboat for these waters.

    As for gunwale guard, I use hot water pipe cladding. Not pretty. But very light and effective. And warm on the toes!
    Dave,

    I appreciate your comments and I do agree with you on many points. I like your little tender, great job. It looks like a Skylark 10 that I just ran across recently. If I would go that route, I would build completely out of plastic. Just my thoughts, however, my project is an exercise in skin on frame and light as possible design. With that in mind I really want to have the round bilge, but other than that, I good with what you have shared. Just for kicks, how much does you pram weigh? The Skylark's website say's 80 pounds. I am shooting for under 50 pounds.

    Thanks again, for the post.

    BJD

  15. #50
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    Default Re: Pram Perimeters

    I don't know -- I've never weighed it. I'd guess 75 lbs? -- I'll put it on a scale when I pull it out of my driveshed later this spring. I wanted it light enough to put on deck, but it tows so well that I've never been motivated to do that.

    But you're right. If you're going to haul a tender up on deck, being under 50 lbs is a good idea. I used to use an Elegant Punt for a dinghy. I found it too small and skittish at 8 ft (for a family of 4), but it was certainly easy to get onto the cabin roof. Weight was about 40 lbs.

    If you built my 10-footer as skin-on-frame, it would certainly weight less than that. But is that a good idea? Tenders get beaten-up, abused, run on shore. They find rocks. Kids take them away for an afternoon, exploring. Dogs thrash back and forth, with claws. Toolboxes fall into them.

    The one in the photos is getting old and will have to be replaced. I will not make it skin-on-frame. No need to change what it is. It has towed dry, and very lightly, for 15 years.

    The design "key" of it is the deadrise at the front, merging to flat-bottom astern, with lots of rocker.

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