*might*have a chance of getting the compound curves where it attaches to the arch right).

New front curve, otherwise mostly the same. |

Wednesday afternoon we were finally able to get some supplies to the yard that were needed to start the build. These supplies included 4 sheets of 4 foot by 8 foot plywood, 8 foot long 2 by 4's, and a bundle of 8 foot long 1 inch by 3 inch wood strips. Right about now you might be wondering whether I'm building a fiberglass top or a small shed because it certainly feels a bit like the latter to me. But all the wood is to create the "mold" that I will need in order to create the top with the aforementioned curves.

Actually, not all of it is for the mold itself. The 2x4's and two sheets of the plywood were so I could build an 8 foot square table so I would have a flat place to construct the mold. The small chunks (crushed...almost) of granite that cover the yard don't really make for a good or level work surface, so before I can start on that, I need a place to work. This project is starting to feel like I am building a house by first going to out in the forest to cut trees for timber and mine ore for making nails.

So, Thursday morning I started by setting up the 12 foot square portable canopy that we purchased on sale at K-Mart while we were in Hampton. Shade is desperately needed in a boatyard in July after all. Then I began putting together the table. This is not as simple as it might sound due to the sad state of the lumber available at the big box hardware stores. It took a while digging through the plywood at Lowes to find the two flattest pieces they had. And they were by no means flat. When we laid the worst of the two on the floor at Lowes and stood on one end, the other end would lift up off the ground about 5 inches. The 2x4's were a little better but not much. So we got the best we could and hoped we could attach everything together so the result was a mostly-flat table. The result isn't perfect, but should be good enough for what we need. Oh, and my cordless drill battery died about 3/4 of the way through, so I still have a few bits to attach to really complete the table.

How one might normally draw an arc. |

But, the table was in good enough shape for the next task. The hard top has a curve from side to side and so I need to construct curved stringers so I can attach the foam board to it to get the desired curve before I apply the fiberglass. Thanks to the 3D model, we were able to determine the radius of the curve that we would need (since the arch sits at an angle, you can't just measure it). I needed to create an arc with a 38 foot, 6 and 3/4 inch radius. Of course, I don't have a drawing compass quite large enough to draw an arc that size, so I improvised. I took one of the boatyards straight boat stands and tied a piece of 40 foot line to its center post with a bowline. I then placed the stand about 35 feet from the edge of the table top and parallel with the seam between the two sheets of plywood that made up the top. I measured from the center of the stand post along the string out the 38' 6 3/4" and made a mark on the table. I then made a mark 6" back from that and drew a line perpendicular to the table seam at that distance. Then I took the plywood for making the stringers, ripped them into 6 inch strips and lined them up with the line on my table. Using the drawing, I marked where the center and end points of the arc should be on the stringer board and table and then lined up the board on the perpendicular line I created on my table. I then wrapped the line around a pencil so the tip of the pencil was at one of those marks when the line was pulled tight. I confirmed that if I moved the pencil and kept the line tight, it would intersect the marks. Viola, a crude 38+ foot long compass. I was able to draw the arc, paying particular attention to keeping the pencil as vertical as possible (ok, I ended up drawing several arcs and had to go back and darken small sections of line confirming the correct locations).

My 38+ foot long version of a drafting compass |

Out came the jigsaw and I cut the arc, leaving just a little excess. I then used a sander to finish off the edge of the stringer. Since my stringer needs to be 12 foot long and the plywood is only 8 foot, it is not a complete stringer and I will have to create a duplicate of my incomplete stringer, flip it so it is the mirror image, and attach the two together to create a complete stringer. We took the cut plywood to the boat to check our work, and the arc appears to be pretty close to perfect (best we can tell holding it up in what should be its correct position if it were holding the top on at the boat). So, tomorrow I should be able to construct the rest of the stringer pieces. In this heat, I'm not sure how far I will get, but I am very glad I have the canopy and a fan.

Great idea for the compass. That saying hold true...."necessity is the mother of invention"

ReplyDeleteOr maybe the saying "fortune favors the foolish" is more appropriate. Now, if I can only figure out how to transfer the compound curve and angle where the top meets the arch...still trying to come up with some inspiration there.

DeleteHi Mike, not sure if this technique will help, but it enables unusual shapes to be transferred. It is typically used when cutting out bulkheads. I have heard it called "fishing" as the shape cut on the stick looked like a fish, but any pattern will do.

DeleteThis link shows the technique:

http://www.builderbill-diy-help.com/joggle-stick.html

That is a neat trick...but my problem is that I need to be able to do it in 3 dimensions and I don't have an existing bimini with the correct curve to base it on...if that makes sense. The new top doesn't follow the lines of the soft top. I think I have an idea on how to proceed...just need to play around with it...and maybe find a laser pointer to project the lines.

DeleteI could just build the back edge of the top straight and then take it to the boat and scribe the curve...but that entails carying a 9 foot by 12 foot curved piece of fiberglass back and forth between the workspace and the boat a bunch of times...and I'm trying to reduce the number of those trips.

Is it feasible to cover the forward edge of the arch in plastic wrap, and then make a single layer glass mould from it? glass on a length of pvc electrical conduit to give it some stiffness.

ReplyDeleteIt could be done, but a partial casting of the arch doesn't help in this case. I'm probably not describing the problem correctly...and over-worried about it. The top I'm building is curved from side to side to match the curve of the arch. The problem is that the arch is canted backwards. So the curve of the top (which I know but doesn't exactly match the soft top that is there now) doesn't meet the arch at a 90 degree angle. So, in addition to the curve of the top (along the Y axis) there is a component of the curve along the Z axis in order to mate with the face of the arch. Then I need to add tabs parallel to the arch face surface so I can mount it. Add in the fact that I'm changing the angle the top will meet the arch and it just throws a bit of confusion into it all.

DeleteI think I have an idea on how to figure this out now. If I put a piece of wood at the proper angle of the arch, behind the frame I built, I think I can use a laser pointer to project the intersection. That's my current theory anyway.

It could be done, but a partial casting of the arch doesn't help in this case. I'm probably not describing the problem correctly...and over-worried about it. The top I'm building is curved from side to side to match the curve of the arch. The problem is that the arch is canted backwards. So the curve of the top (which I know but doesn't exactly match the soft top that is there now) doesn't meet the arch at a 90 degree angle. So, in addition to the curve of the top (along the Y axis) there is a component of the curve along the Z axis in order to mate with the face of the arch. Then I need to add tabs parallel to the arch face surface so I can mount it. Add in the fact that I'm changing the angle the top will meet the arch and it just throws a bit of confusion into it all.

DeleteI think I have an idea on how to figure this out now. If I put a piece of wood at the proper angle of the arch, behind the frame I built, I think I can use a laser pointer to project the intersection. That's my current theory anyway.