Drive Hull Female Mold & Layup

I'd very much appreciate some help with the mechanics of building a female mold for a small drive hull concept. Having read many posts before joining the site, I'm very grateful for all the information I've learned. I'm hoping that someone can provide some feedback and direction for what I'm up to.

Attached is the developable hull form that I'd like to fabricate, essentially an open tube pontoon with a 2" flange along the gunwhale to through-bolt under an aluminum frame. TANSL has been very helpful to provide a hull model that can be used to CNC two half hull planks for the body/transom and bow.

I intend to build a set of frames in 1" ply. For the body/transom these are simple half circle pieces. Frames will be attached to a plywood base, which is reinforced with (2) 2x4's lengthwise on edge to provide overall stiffness to the mold. Once the frames are attached to the base, I'll make vertical stringers in 1x3 pine to attach/extend the frames up to the gunwale. The stringers will have a 4 degree bevel, when placed on the base will intersect a chord in the half circle just slightly lower than the full diameter. This will 'open up' the mold at the gunwhale 1/2" on each side, which I'm told is sufficient to properly release the hull from the mold. The top of the stringer, also with a 4 degree bevel, forms the horizontal surface for the flange. Is this clear so far?

2mm high-gloss PVC seems like a good choice for the mold itself, three sheets @ USD$59. There will be two planks per side, 4 planks total, based on the largest material dimension available, 4'x8'. For the hull/transom 2 halves each 31.5" x 8', for the bow 2 halves 31.5" x 4', shaped to the developable surface. I will temporarily attach a straight batten lengthwise along the keel line of the frames, butt the first plank along it, glue and clamp with epoxy. Once set, I'll remove the batten and butt the second plank on the opposite side of the mold to the first plank. This will leave the seam along the keel.

Before laying planks I'll scuff the portions that will meet the frames with 80 grit to give the epoxy some bite. I'll also reinforce the plank/frame attachment with glass and epoxy on the outside of mold. 3" strips will likewise be glued and clamped to the top of the stringers to form the flange, and likewise reinforced from the underside as well as to the outside of the planks.

I have some questions about filleting the keel and flange seams, particularly the flange, and also how much of a level difference in the butt joints can be taken up in the gelcoat and resolved after the fact. I would also like some feedback on layup schedule sufficient to carry the hull loading on the flange, 800 lbs of battery, motor, cables all in.

But for now, overall, does this approach make sense. I'm attracted to the fact that I won't have to make a plug, sand or fair the mold surface, and other than the fillets,I can go directly to wax and layup. This is a one-off (for now) but I expect I can get a few hulls off the PVC before it or the mold fail. Is that likely?

Thanks all for your input in advance, and Happy Thanksgiving.

-Jack

 

I hope I am mistaken in reading that your plan is to use 2mm PVC for the mould itself.It would be so flexible that merely wetting out the glass would flex it and probably cause the gelcoat and early layers to release.Thats before we get to the practicality of actually achieving anything like a fair inside surface.Blunt question,have you actually built a boat before?

 

Jangr, didn’t you start another thread on this?
I don’t see why you need a third hull at all, you can hang your drive and running gear under the deck, or inside one or both existing pontoon hulls.
For simplicity’s sake, You could buy an electric outboard, clamp it on and go!
Your approach is going to cost a lot, and if your mold building process is not radically rethought, it will be all for naught!
I understand and appreciate that you are focused on using solar power, and your application (short runs, infrequent with plenty of charge time between) makes sense.

 

Wet & Kap, thanks for the feedback.

Wet - please help me to understand your concerns better. I have never built a cold mold fiberglass hull, that is why I am here. Though I have spent 500+ hours peeling and replacing/fairing/sanding lots of alligatored/blistered glass on my 50' double planked Maine trawler. Bulkheads, cabin soles are all glass over 20mm bruynzeel, glass over toe-nailed mahogany plank on the inner bulwarks.

Choice of Vycom PVC is based on manufacturers spec, max bend radius of 8" for 2mm, vs 12" for 3mm. My max bend radius is 12" so I went lighter to be sure it would bend without deforming. Based on your feedback I'll run a 3mm test plank on junk frames, maybe bonded with 5200 would be easiest for frame edge to PVC, with glass and epoxy reinforced frame face to PVC face throughout.

I've wetted my share of light to medium weight woven glass with a chip brush. I'm thinking that putting enough pressure on a chip brush to flex 3mm sheet (1/8" luan equiv), on 1" thick frames, 12" centers - and I'll be wetting the glass with my knuckles. After 2 or three layers of glass over gel coat, the hull is basically holding the mold shape rather than vice versa, as I understand it. As for inner hull, its a glass bilge, one coat of high fill epoxy bilge coat and I'm done. I'm not looking for a fair bilge, only functional and neat, no fiber spikes to jab me.

Kap - I started with no hull, drive motor and batteries on deck, but both COG and shaft angle were too high for my liking. Complexity in custom strut and rudder framework (lots of AL welding $$) also discouraged me. I looked at simple electric outboard, but the cost per Kw is about the same as inboard, I lose 2' of usable deck space aft for a cutout, I can't resolve the inherent stability issues of shallow draft, thrust-based steering, and I can't optimize prop/RPM/Kw for hull speed on an outboard - it's basically one size fits all. You are right though, this will not be a cheap build, but that's ok. Its a matter of principle.

Please let me know is I'm still missing something basic in my remarks here. Some guidance on fillet technique, layup schedule, and flange loading would also be helpful.

Happy Thanksgiving!

Jack

 

What will you be carrying on deck? How much loading will your existing pontoons support?
An 8x12 pontoon does not offer much as a beast of burden, unless the pontoons are really big in diameter, and then that’s still a pretty short waterline.
Solar panels will necessarily be placed overhead as deck space is too limited, how will those and their supporting structure affect stability?
How much will the batteries and equipment weigh?
I think steering with the directed thrust of an outboard would be much better than a rudder behind a fixed prop, especially considering low power available.
You might consider as a low cost alternative, a half a canoe stuffed under the deck, or a whole one sticking out both ends would definitelyenable you to carry several batteries down low.

 

My concerns about the proposed solution are based on two factors.First there is the problem of achieving a fair surface when you join sheets.The second concern is the likelihood of a 2mm thick sheet flexing as you laminate since one of the common release techniques is to flex either the component or the mould to encourage them to separate.You may easily cause enough flexing while laminating and then you have a good chance of introducing crease lines or bubble like inversions in the surface.I am also a bit curious about how exactly you will frame the supporting structure at the forward end to hold the shape to it's intended form.I am not suggesting that it can't work,but I do think it may be less easy than it might appear to be.

 

Kapn & Wet, thank you for the follow-up posts. Your observations are helping me refine the concept, very helpful.

Kapn - I've made buoyancy and displacement calculations based on recommended pontoon wetting of 35%. The corresponding chord across the 23" diameter pontoon yields a waterline of 6.8" and 3150 lb displacement. These figures include the additional buoyancy of the drive pontoon, but disregard the marginal buoyancy of the three bow sections, which are difficult to model. So the figures are a little conservative by design.

Stripped to the frame, the two aluminum pontoons and cross members will be 600lbs. Without an actual layup schedule, I'm guesstimating the fiberglass drive pontoon, coosa bulkhead, strut, shaft, and bearings at 400 lbs. All in about 1000 lbs before I put anything aboard.

The battery pack, motor, controller, shaft linkage, shaft seal, and wiring will be another 750 lbs. That gets me to 1750 lbs. The largest load on deck is the deck itself, 5 sheets 3/4 marine ply @73 lbs per sheet, or 365lbs. Next the 4 solar panels with SS frames, mounted on the first six feet of the bow, 250 lbs. So I come in at 2365 lbs. The remaining 780 lb load is me, product, deck fixtures and two BLDC appliances - a rotary tumbler and a gantry winch. Another 5% of wetting, to 40% total, would still be acceptable from a stability POV and provides a further 500+lbs of payload buffer.

I had thought of placing the panels overhead, as you say, but the COG penalty for 250 lbs 8' above the waterline made me nervous. COG on a pontoon is above the waterline to start with, and I don't want to face the prospect of an unsafe pitching moment, e.g. crossing someones 2-3' wake on a diagonal. I think the hull design I have is basically a half canoe, 12' long, 23" wide, through bolted to the deck cross-members. Is this what you mean by half-canoe?

Wet - I'm trying to make the mold as simple as possible. I could easily fabricate the base of the frames for the tubular section with a radius jig on a router and my table saw. But for $100 I'll send the simple pattern out for CNC and get eight 1" thick bases identical down to the mm. Likewise, the butt seam on the tubular section is directly along the keel line. I'm not bothered if I have a visible .1 or .2mm seam when finished, as long as I can maintain a sufficiently thick layer of gel coat over the glass. This drive hull is buried under the deck anyway, only visible when I'm hauled out in the winter. Would be nice to have a shiny, seamless production hull, but not really necessary.

As you rightly point out, the bow section is where it gets tricky. The bow certainly requires a high fidelity CAD model. Fortunately TANSL has generously offered to help me and has provided a rough cut, developable hull form for us to iterate. This will get me CNC patterns for 1) two planks for the tubular section with a half ellipsoid aft for the 14 degree transom, and curved joint on the leading edge to conform a developable surface in the bow section, 2) two planks for the bow section, with matching curved joint to the tubular section, 3) three frames for the bow section, generated by passing normal planes through the CAD model of the bow hull form at 12" intervals. I intend to clamp the two bow planks together at the stem using wedges bonded with 5200 to the outside of the planks before glassing all the joints and frame surfaces on the outside of the mold.

The curved butt joint between the bow and tubular section will still be a challenge. I'm resigned to fabricating the two sections apart and butting them together, adding the 2" flange last. For each section I can clamp long battens on the interior to the frames, likewise shorter battens on the sides to join the two sections together. It may be that I need to use a freestanding interior 'station' sufficiently wide to clamp the entire joint between the two sections to a double wide frame. Looking at TANSL's model, its a pretty complex semi-ellipsoid curve where the two sections join.

I'm hoping the mold will release when I run a wooden wedge along the 2" gunwale flange, plus a few whacks with a rubber mallet on the mold planks. If the mold is a one-off, thats fine too. But I still don't have a good solution for fillets in the transom corners, the stem, for the joint between gunnel and flange (which must expose a non-glossy, cut edge).

I was thinking to use epoxy fairing compound and a wooden tongue depressor. What is the quarter radius I should try to achieve in the mold...1/4", 1/2"? If I let the fairing set up a little, then run an acetone rag to remove excess from the adjacent glossy plank surface, will I get a smooth enough surface in the fairing to go directly to wax? Or should I run a bead of top coat or epoxy over the fairing with a tiny brush and let it level out? Whatever I do I'm trying to avoid any sanding inside the mold which would scratch the glossy PVC. That would defeat the whole purpose of using this material in the first place. Likewise I thought to shape a bead of fairing with a quarter round formica 'trowel' and repeat the acetone cleaning as with the stem and transom fillets. Same question, will I need to top coat this before it is smooth enough to take the wax release and gelcoat?

Sorry for the long post, but I feel I'm getting some great advice here and I want to make sure I don't overlook anything.

Cheers!

Jack

 

This would be faster and cheaper:
pontoon boat logs | eBay https://www.ebay.com/i/372494999439?chn=ps

 

In Europe we would use radius balls and Plasticene to create the fillets.It would be a lot more compatible with a flexible surface.

 

Hi Gonzo, looked into that first, its a losing proposition. Pontoons are designed for buoyancy, not loading. Very thin walled tubes .018 max, often much less, rigid only because they're closed. Open the top to create a hull and you've opened up a can of worms...good aluminum guys in my area are $100-$150 an hour, booked solid if you can find one, but call it $100 and you have a guy. Its at least 10, more like 15 hrs in design, laser cut, & weld, plus $300 or $400 materials, LTL freight, etc, etc. Very tricky shaft log, strut, motor mount fab, easy to mess up. So $2000+ after you pay $1000-$1500 for a decent log not all bunged up. And you still have something that was never really designed for loading. New fiberglass will be (hopefully) less $$, engineered right the first time.

 

Do you let the clay harden? This is what I don't get. The fillets have to be waxed and polished before gelcoat, just like the rest of the mold surface, don't they? Or do you lay down gelcoat directly on the clay?! I don't mean to be dense, but I really don't understand.

 

I don't understand what you mean that it was not designed for loading. It is a log for a pontoon, ergo, it is designed for a load. Laser cutting is not necessary, a jigsaw will do just fine. There is nothing tricky about a shaft log. It is simply an aluminum tube welded to the bottom or the back of the log. Installing a motor in a log is routing and all parts are off the shelf. I think that you are complicating a simple thing way too much. A pod for the batteries installed below the deck and an outboard or sterndrive matched to an electric motor will be much simpler.

 

I've only some practical experience.

2x4s on edge are almost the least trustworthy timber possible as a strongback. At a minimum, use a 2x6 if mounting it to the floor or a 2x8 minimum for a freestanding frame....

The other thing is it looks like it would be far easier to build in foam, whether you use the foam as a plug or use the foam as the hull and just decore in areas of solid laminate. It seems simpler to me. Or you could build the round shoe solid and run core on the sides.

Also, recalling my own build, the cnc'd frames do not carry any bevel. So you try to build and if you forget the beveling; nothing fits at all in the tight areas forward.

The best way to build the front is to finish well short square and add a nosecone. You can setup the mould for a seam, etc. Then using a minimal amount of materials; you can craft a core or even glass nosecone. On my build, I wish I had done this as the front was a bear and did not repeat perfectly on each hull (catamaran build). When laminating, you will spend way too much time in a fine bow point and it will be poor as are many bow lamination jobs all over the world. You can use a stem mould from cnc precisely this way as well. My stem mould fabbed by cnc shop was never used to its full benefit and the reason is it was impossible. We finished a bit short of it for practicality. But had I finished well short to a flat stem say 2-6" wide, I could have built a nose using a stem mould laying flat. Instead everything was done using a chain jig and fairing and the work forward was a bear. In our case, we turned a 12mm piece of core on edge; well glued and inserted it at an inch wide and the continued with 9/16" all the way up to 2" laminations. In a non-core build, that much stuff would catch afire. It sounds tedious, but trust me, even for your small pontoon; split the nose off the main layup.

To setup the mould for a seam; just tape a piece of material into the mould to create a rebate. I like the wide shipping tapes.

This picture shows how I developed a square bow inside and if you look closely at the solid glass vee section in white, on the edges there is a 2.5" white strip under the glass. This forms the rebate and can be used in any direction. I would recommend doing a square to start versus building a square. Then you only attach a nose. This bow is about 7" of foam to get to 2" for inside glass. Ridiculous lots of work amd still very hard to build. If you look close, you can see some flying glass at the bottom of the bow. Almost impossible to do cleanly.

 

In post #5, I meant literally half a canoe, as cut an existing canoe to an appropriate length and glass in a transom and some mounting brackets, then fasten it under the deck as a center hull.
Craigslist always has old fiberglass canoes for cheap!

 

Two things,first if you use Plasticene or similar to to form fillets,it softens when warmed to hand temperature and you find it goes firm rather than setting solid which allows modest waxing and then after moulding you can scrape it off and use fine abrasive to bring up a shine on the component.

Secondly,I agree about using a small bulkhead a little aft of the forward extremity and having an easily replaced section.The laminating is so much easier and for hulls that are likely to lose a few inches you will be very happy to have the tooling to replace the damaged section.