27' Riva Aquarama Clone

Discussion in 'Boatbuilding' started by terra, Jul 14, 2021.

  1. terra
    Joined: Mar 2011
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    terra Junior Member

    Hi all,

    Looking for some guidance!

    Background: Mechanical Engineer working in advanced manufacturing. Hobby is rebuilding or restoring old things. Wife has green lit a boat project. Last year I bought the plans for a 27' Riva Aquarama on CWBP.com, converted them all into CAD, and now I'm here to learn.

    Question 1: For framing my plan was to forgo the traditional wood and jump to waterjet aluminum or steel. I prefer to stick to processes I'm familiar with and I could have all of my framing cut, coated, and delivered within 48 hours. Already adapted their design for mounting traditional stringers and mounting to the keel. Any obvious issues? Not really stuck to any particular design element.

    Question 2: Hull materials. I've been browsing pictures trying to see what the typical construction method would be. Strip planking? Carvel? What would be a good path forward and what would be a good material, both in shape and wood type? For strip planking would it be acceptable to buy wood and cut it down to strip size or is it typically purchased in strip form? What would be the better path for a beginner in the boat building game? I have about 40 gallons of West Systems 206 epoxy and probably 100' or so of CF and Kevlar fabric laying around to get me started, as well as 20 gallons of Titebond 2.

    I have an extensive background in composites and I'm still toying with the idea of 3D printing a female mold for the hull and top deck for a resin infusion of CF or Kevlar but haven't pushed down that path much.

    Many of my brackets and structural elements will be 3D printed in 17-4PH stainless and the exterior trim and controls will be printed in titanium, aluminum, and brass. Engine intakes will likely be a combo of printed polymer and carbon fiber ducting.

    Powerplants will be twin 6.3L V8's. Fresh water only (Texas lakes).

    I fully recognize that building a boat will not be a weekend undertaking but any help that people can offer in regards to hull fabrication will be very appreciated. Building the assembly jigs next week and prepping my workshop.

    Thanks all!
     
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  2. bajansailor
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    bajansailor Marine Surveyor

    What a wonderful project!
    Please do allow this thread to develop into your boatbuilding blog, and post lots of photos along the way.

    Although I would be rather dubious about making your frames out of aluminium, and especially so out of steel.
    How would you fasten the timber to these frames?
    Would the keel also then be made of metal?
    I think you would be better off having timber (plywood?) frames, perhaps even cut them out with a waterjet in a similar fashion to the steel or aluminium frames?
    Re hull planking, maybe best to research the availability of the different types of wood before deciding on eg strip planking or double diagonal cold moulding, or even 'traditional' carvel construction - I get the impression that even good quality plywood is rather scarce in North America now (?)
     
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  3. gonzo
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    gonzo Senior Member

    The shape of that hull does not work with aluminum or steel plate. Further, a boat that small in steel would be too heavy. The structure should not be in SS, which is also too heavy for that boat. You are choosing construction methods and trying to make a design fit them, which is backwards to how it should be. The materials and methods have to fit the design. You picked a design that is very complicated and requires a very high level of skill. You could change it to cold molded or strip planking if in wood, or to a sandwich construction. For aluminum, every part would have to be formed in an English wheel and sometimes hammered into shape. To get the finish that boat deserves, it would take you several years apprenticing to get the hang of it.
     
  4. gonzo
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    gonzo Senior Member

    It looks like I missed that only the framing was in metal. It is hard to make materials with large differences in expansion coefficients to work well together. Metal framing would be much heavier than wood, and also make it complicated. The face of the frames needs to have a rolling bevel to have a surface that follows the shape of the hull. Although not impossible, it is really hard to fabricate. Cold molded hulls can be built with little or no framing.
     
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  5. Rumars
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    Rumars Senior Member

    Changing the building material and technology is certainly possible, but you are effectively redesigning the boat from scratch. Weight, weight distribution, strength and stiffness must match the original if you want the boat to perform the same. The only thing that remains the same is the shape. Please keep in mind, your plans are for a Riva lookalike, adapted for home building, not the original boat.

    Now to your questions:

    1. While it's possible to convert to composite construction (the original one, metal framing with wooden skin) it's not a case of just cutting some plate. As your CAD model should show you, the frame has a rolling bevel where it meets the planking. Even if your cutting place can replicate this bevel, it's of no use to you because you need an actual flange there, you can't screw into the edge of the plate. There are several ways of doing this beveled flange, most commonly bending angle iron or welding the flange on.
    2. Original Rivas had plywood bottoms and hot molded sides, all african mahogany. The sides were made off the boat in three plies, two diagonals and one longitudinal, all material coming from the same tree.

    Your plans call for a cold molded plywood skin on plywood frames, all epoxy glued. The simplest option for you is to have the bulkheads CNC cut (3-axis is fine, the gap for the bevel will be filled with thickened epoxy), either in one piece, or in pieces and using knees. Keel and stringers can also be CNC cut with the appropiate bevels, the straight parts from solid stock, the curved parts from prelaminated blanks. This is what kit manufacturers do, take the finished shape from the CAD drawing and carving it from an appropriate sized blank (wich you draw in CAD around the finished item).
    The skin you laminate from plywood strips or solid wood, according to your preferences. The usual procedure is to staple the interior layers (composite staples preferred), and vacuum bag the last one on if the boat is to be lacquered. Combinations of plywood, solid wood layers and fiberglass are common.
    All african mahogany plywood is available, both khaya (wich is a true mahogany) and sipo (wich only looks like mahogany but is stronger and heavier), but not necessarily required, as is solid wood. Your boat was probably designed for okoume or meranti plywood.
     
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  6. terra
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    terra Junior Member

    Thanks for all the replies! In order of response:

    @gonzo, I think you got it the second time around. Yes, the idea being to replace just the wooden ribs with metal ribs. Not the hull skin at all. But you raise a good point about the bevel. That's exactly the kind of info I came here for. The bevel is purely there to ensure uniform contact with the stringer as opposed to coming in at an angle and only meeting at a corner?

    @Rumars Well aware the plan doesn't yield an actual original Riva. I have zero interest in originality. Would definitely prefer a modernized version. To answer your questions:

    1) Yes welding on a flange would be the simplest approach. I also considered making the ribs out of 6061 Al and doing 2 0.1" thick ribs spaced and welded to an internal flange between them so I can accommodate the bevel and the mounting flange on the same unit. But again I'm only here to ask questions and see what's a good idea and what isn't. Another option would also be to 3D print molds and make composite ribs from Kevlar or fiberglass, but then I have to consider how to edge drill for stringer and strip mounting.

    2) Good to know!

    So I'm just going to reiterate what I think you're telling me so that I can confirm my understanding. I could CNC the frame ribs (perhaps not accepted language, are all the "ribs" identified as bulkheads?) out of which material? Would that be sheet stock like marine ply or would it need to be actual timber? My plan if I used aluminum would be to cut them in pieces (L, R, Top) and weld them into a solid, or in wood I would likely cut them in the same manner but gusset plate each joint on the front and back sides. Still right so far?

    The keel I have a fairly good understanding of. I pulled that out in CAD months ago and have been working with a local shop on that.

    For the skin being plywood strips or solid wood strips, what are some ideas on strip dimensions? Especially ply vs. solid? Assembly with staples and epoxy is something I'm pretty familiar with. Skin strips would be stapled just to the bulkhead edges and stringers? I have a large vacuum bagging setup with hundreds of yards of CF and Kevlar sitting in the garage so I can definitely do the finishing work well.

    I looked up Meranti Ply and I'm seeing around $130 for a 12mm 4x8 sheet. That about right? Also seeing around $180 for a 4x8 3/4" african mahogany?


    Also a follow up question just to further understand the build process. Is this a correct, albeit very simplified, workflow?

    1) Keel
    2) Mount frame ribs to the keel
    3) Mount stringers to the ribs
    4) Skin over the stringers/ribs to form the shape
    5) Epoxy/FG coat?

    This month I'm working on building the motors and sourcing the correct bellhousings.

    Will be back with another set of questions I'm sure!

    Thanks all
     
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  7. Rumars
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    Rumars Senior Member

    Ribs/frames (yours would be normally called frames, ribs are smaller) can be made two ways, either you cut them directly out of a single plywood sheet, or you split them at the corners and cut individual pieces, then you glue on plywood gusset plates on both sides of the joint. The frames can be plywood or solid wood, as you prefer. If they are ply, when you cut them you use the smaller dimension as the controlling edge, this will result in a triangular gap where the bevel would have been. That gets filled with thickened epoxy when you fillet the frames to the skin (there are no screws, the fillets and if specified the fiberglass tabbing over it, keep the boat together). If you use the bigger dimension when cutting, the frames must be beveled, either directly from the CNC (5 axis needed), or by hand. Solid wood frames are usually gusseted and beveled, the gussets also get rivets or bolts in addition to the glue. Some boats used metal gusset plates.

    The workflow goes like this:
    1. Mount all frames to strongback upside down, insure proper alignment.
    2. Mount keel on frames, check alignment, glue.
    3. Mount stem, check alignment glue to keel.
    4. Mount stringers (chine, intermediate, deck), check alignment, glue to frames and stem.
    5. Final fairing by longboard. Even if everything is preshaped by CNC you must check fairness with a batten and adjust with the longboard.
    6. Glue on the first skin strips, this get glue everywhere they touch something else and staples or screws. Due to the nature of the very open mold, this first layer is usually plywood, as thick as will take the smallest bend (probably 4mm).
    7. Glue on next layers. If you want to use vacuum bagging for the second layer, you either must work carefull enough when joining the first layer to ensure an airtight surface, or hand laminate a light layer of fiberglass over it. After the second layer is on the surface is normally airtight, and vacuum can be used. If you finish bright, fairing must be done before glueing the last layer.
    8. Fair, fiberglass if specified, laquer or paint.
    9. Turn boat start on the interior.

    If you have any temporary frames, those must be covered with tape as to not glue the skin to them. Plywood thickness is specified in the plans, strip width varies with curvature, the more severe it is the narrower the strip. Some people simply cut all ply to 4-5" then adjust those to suit. In some areas the strips will end trapezoidal, or with convex edges.

    If you want to weld flanges on Al plates you must first build a jig to hold them to the correct angles. Normally the plate frames are errected then battens sprung around them to simulate the hull, then the flange is bent and clamped to the battens and welded. Then everything gets adjusted cold, because the welding distorted the whole thing.
    Angle iron is done differently, it's first bent to the appropiate curve, then clamped tight to a table and the bevel opened with a big lever to the precise angle.
    My advice is to forget about it, it's a lot of hassle and the result is a boat with wastly different expansion coefficient between frame and skin, plus the need for screws. Such boats have been done by the german navy, they were maintenance heavy even for them.
     
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  8. terra
    Joined: Mar 2011
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    terra Junior Member


    Well that's definitely the most helpful guidance I've received to date! I appreciate the breakdown, very easy to follow.

    Workflow seems reasonable. I'll be converting it into my standard build plan format and probably circling back with more questions.

    I'll stick with the plywood and planking method you've laid out and abandon metal work for the time being. I'll still metal print cosmetic components and mechanical brackets as necessary.

    A few intermediate questions as I continue to plan:

    1) Specific plywood for frames? Would that be marine ply as well or would a non-marine ply work given the location inside the hull?
    2) The 4mm thick ply example for the hull, I would assume marine ply but I'm going to reconfirm everything and not make any assumptions..
    3) If I CNC the frames to the large dimension and then bevel the other edge to allow for flush stringer and skin mounting what would be the ideal tool for that job? Most of my work is focused on automated assembly processes so I'm less familiar with the more traditional approaches.
    4) For glueing stringer to frame or skin to stringer/frame are we using "glue" to just mean adhesive here? I'd imagine that would be a 2 part epoxy or do you mean wood glue?
    5) If I didn't want to use vacuum bagging for the second layer of skin how would I adhere the second layer to the first outside of epoxy? Staples? Brass screws? Epoxy and clamp?

    Thanks again!
     
  9. Rumars
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    Rumars Senior Member

    The best use for your CAD skills is desiging a self aligning strongback to keep the frames and keel into position. It works like a 3d puzzle that can only be assembled one way.

    You use the best ply you can afford of the weight specified in the plans. Just take a minute and think how much those engines and a nice interior cost, then ask yourself why should you compromise on hull quality, wich is only about 30% of the total cost anyway. The difference between good and bad ply is a couple of gallons of fuel per sheet, enough for an hour at full speed. So what's in the plans, okoume, fir, meranti, etc.?

    You should read the Gougeons Manual, it's free and most of your questions (like 2,3,4,5 above) will be answered. Gougeon Brothers on Boat Construction book - WEST SYSTEM Epoxy https://www.westsystem.com/the-gougeon-brothers-on-boat-construction/
    Use your favorite search engine for "runabout construction" "cold molding" "epoxy wood boat" and similar wording, the web is full of blogs, videos, etc. about them. For example you don't have to reinvent the wheel with the strongback, just copy something you see online.

    1. Marine
    2. Marine or thick veneer
    3. 5-axis CNC, or plane, rasp, sandpaper or their equivalent powertools.
    4. Thickened epoxy
    5. Staples.
     
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  10. wet feet
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    wet feet Senior Member

    If you CNC the frames,it is entirely possible to get a good approximation to the bevels by cutting "steps" on the bevelled edges with a 3 axis machine.You simply project lines onto the edges in your 3D CAD version of the frames and cut along those lines at the heights they represent.The machining time increases accordingly but needs to be compared to the cost of all the epoxy saved.

    The advice to create a comprehensive strongback is very good-just make sure you can extract it without causing damage to the boat.A couple of removable sections near the middle might do it.

    With the plan including a scheme to install two huge V8's there will be some considerable forces involved and integrity of the engine beds and sterngear will need to be strong and well integrated into the hull.I hope the plans include a lot of detail as a novice may not have the depth of understanding that needs to be applied to every part of such a complex boat in order to make it beautiful and functional.
     
  11. gonzo
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    gonzo Senior Member

    A 3 axis CNC is able to cut the rolling bevel. The CADD is not too complicated. The forward and aft faces of the frames are designed at the proper location and the CNC will cut the bevel. What are the steps you propose for?
     
  12. TANSL
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    TANSL Senior Member

    I would not dare to say such a thing. Maybe I need some more experience in that field. In any case, could you explain what information the cnc machine needs and how to give it to it? I would be very interested to be able to incorporate this procedure into my software on cutting parts by numerical control. What are the steps you propose for?
     
  13. fallguy
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    fallguy Senior Member

    I think you owe Rumars a pint my friend.

    I only read the first three lines of his friendly work instruction.

    The frames are built with DWL holes/marks and then those lines are leveled if not clear. Believe it or not, watched a guy set it up using his strongback as the reference point. Then he said he was off by 2cm and couldn't get why....

    I won't argue hard here, but the CNC guy I worked with beveled nothing. He just ran a 1/2" bit and two axis.

    So, you may end up doing some handwork.
     
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  14. wet feet
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    wet feet Senior Member

    The steps are an aid to those operators who don't have experience of doing anything other than square edged panels,of which I have encountered a few. While the machine may be capable of cutting rolling bevels, it may not be the best way to introduce an inexperienced operator to the further reaches of what the machine can do and he may not have a CAM package that will accept bevelled surfaces..It calms apprehensive minds to cut a series of outlines at close vertical intervals as it keeps them in known territory.For my part,I'm happy to program either as I have many thousands of hours of 5 axis programming behind me.It will need a bit of CAD manipulation to flip some of the frame sections so that the bevels are "losing" bevels as you come up from the spoilboard. A 3 axis machine won't cope with undercut bevels and I suspect the OP realises this.
     

  15. gonzo
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    gonzo Senior Member

    The operator has to lay the plate on a vacuum table, set the zero reference and push start. The bevel is determined by the DXF file I give him.
     
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