Fiberglassing tubes?

[Zach]

Hi guys,

I'm wondering if you have any ideas for building a jig to fiberglass 12 inch diameter fiberglass tube. Tube will end up 5 feet long! It'll end up being a bow thruster tube.

I'd like to end up with a gel coat finish on the inside, and to spiral wrap it like a barber shop pole to keep from having horizontal seams/lumps.

Anyone seen how its done commercially?

Thanks!

Zach

[Rachel]

I don't know how the commercial places make them (and I'd imagine many/most boatyards just order them), but you might take an idea from CharlieJ on this forum.

He posted a few years ago (before he joined this forum, so it was on another one) showing how he made cockpit drain tubes for his Meridian. These were about 2" in diameter, and curved - they led from the after end of the cockpit out into the motor well.

He took a section of pipe -- I think it was PVC tubing, but I can't remember for sure now -- and wrapped it "barber-pole style" with fiberglass tape and epoxy resin (he must have put some kind of mold-release on the pipe - maybe PAM spray?).

When that cured, he sliced the whole thing in half lengthwise (like a sub sandwich), and removed the form (the pipe). Then he put the two halves back together and wrapped them again (on the outside) with the resin/tape. Voila! Custom tube :)

Maybe you could use PVC pipe or even Sonotube to do this? Of course I've left out the whole gelcoating issue, too.

I'll be interested to hear what others propose. Also, I don't know how important this is, but the bow-thruster tubes I've noticed have a pretty tight clearance to the props.

Rachel

PS: The bow thruster isn't going in your Triton, I trust...

[LazyGuy]

12" bow thruster? I hope you are putting it on a 50 footer.

If you are going to turn a triton, 4" would be more like it.

Vetus makes a thruster just under 12" it is 24 volts 760 amps.

If that is what you want, I would go with the PVC route. Make plugs for both ends out of wood then cut the tube in half or in quarters. Use the plugs and straps to keep it round while you are making the tube then remove the plug and start pulling out the pvc pieces after the fiberglass cures.

[Ceasar Choppy]

I just checked Mcmaster and the biggest inside diameter pre-made FRP they have is 2".

If you can find a cardboard tube (or something that will be destrictable in the end) to use as a mold, you just need to cover it with release agent or wax and lay-up the FG. Once cured, then you destroy the mold inside.

What I don't like about the PVC method Rachel mentioned is that it takes too many steps. And PVC is too rigid to use as a mold since it would be hard to remove even with proper release agent (its a male/female thing ;) ).

[Figment]

As far as I know, such shapes are formed commercially on metal mandrels which expand in the curing oven and then contract to release post-cure.

The difference between Rachel/Charlie's method and Dennis' suggestion is that PVC pipe is also prestressed. When you cut it, it jumps shape.

As weight and outward appearance are non-issues in this application, I like Charlie's method of forming, slicing, and reassembling. Yeah, you wind up with a pair of seams, but so what? That really should be the least of your issues with a large thruster installation.

If you use PVC, you might not need to cut both sides to get it to release. Make one cut, the pvc will constrict the distance of the saw kerf (careful, it'll bite the blade near the end), and that might just be enough to get it to separate. Then you only have one seam to tape.

Do you need your finished product to be EXACTLY 12" inside diameter???
12" sonotube is 12" on the INSIDE. I think the same holds for pipe, but can't recall with absolute certainty.

[CharlieJ]

I didn't cut the PVC- I cut the initial layup of glass- would work the same way with a 12 diameter chunk of PVC.

Mold release wax the pvc, spray on or roll the gel coat, let it tack, wind on tape to about 1/8 thick, cut it, pop it off the pipe, glue the cut edge back together ( used 5 minute epoxy) and lay up more tape ( or whatever) on the outside till it's as thick as you want.

Sometimes taking an extra step saves time in the long run. Since I wasn't that concerned with the inside as long as it was smooth, I wrapped plastic around the pipes tightly and taped it out past where my layup ended.

molds-



Wrapped with glass, no resin-



Finished, end view- You can JUST see the cut line on the far side- I used a dremel with a cut off wheel

[bcooke]

I never would have thought about the two stage tube buildup but that is a great idea Charlie.

Zach, you have not come clean with us. Is this a project for your Triton or something else? If you put a bow thruster in your boat you know you are going to be the talk of the town in the Triton community.

-Britton

[feetup]

We made some 8" FRP tubes a few years ago about 38" long. We used slightly undersized sono tubes (spiral wraped cardboard) and brushed several layers of melted parafin wax on them till the build up was about 1/4 inch. We then smoothed them out and polished them with scraps of burlap till they were quite fair, then polished them with a soft cotton cloth till they were almost shiny. The trick was to keep them cool and therefore the wax hard. This part took about three hours but materials cost was negligible.
We then layed up a few layers of cloth/mat/cloth, but not too much, and with care not to use any more catalyst than necessary, since we didn't want to melt the wax. When cured we just heated the whole thing up with a hot air gun till the wax softened and slid the tube out easily, almost like it was greased. We then sanded the outer surface and built up the layup, in this case to about 1 inch wall.
The job these were for didn't require any special finish other than being smooth and fair, but I would think putting down a layer of unwaxed gelcoat first and then the layup would yeild an even better job.
Adding a little stearic acid to the melted wax will raise the melting and particularily the softening point of the wax and minimize the concern about the heat of curing, allowing a thicker build up without worry. Gel coat would help that a lot too.

Feetup

[Zach]

Thanks guys! Some great ideas around here.

I like that two stage idea... Nice work Charlie. The inside diameter that would be perfect is a 11 and 13/16ths, but I haven't found any forms off the shelf. Otherwise it doesn't really matter, as Vetus wants 700 bucks for a half pound of fiber filled polycarbonate that is their prop.

The boat its going in is a 1942 83 foot Coast Guard cutter converted to a motor yacht. Building a one off bow and stern thruster for it, as Vetus wants 30k to do it in electric, and 45k to go hydraulic... and that is just crazy talk. They have a gorgeous office! The boat is being fitted with a hydraulic pump on the port engine for two anchor capstans. Its an oversized pump, and I'll be mounting it offset with belt drive so she turns over slow... in the event of docking at anything above idle, it won't change the local tides, or shoot a prop through a neighboring boat. (Grin)

Next question... Any of you guys and gals cut a hole for a bow thruster? Since the bow tapers its an eliptical hole in order for the tube to run straight. Vetus drills a hole through both sides, slide a rod through with a gigantic hole saw. Lewmar uses a steel rod, and a jig which a drill motor rotates around, looks like it works great on fiberglass (uses an abrasive cutter.)

The hull has a layer of glass, two lays of half inch ply, and double planked mahogany about two inches thick. (In other words, I don't want to be the one holding on to that hole saw! Grin)

[Tim Mertinooke]

Wow, green grass. I haven't seen my lawn since November...

[CharlieJ]

ROFL- that was several years ago, during summer time. But I DO have to mow this weekend. For the fourth time this winter too.

[catamount]

Now I've got to go think up some place on my boat where I just have to have some curved fiberglass tubes like those, they look so neat.