Building a small trolling boat.

[hoytedow]

Any news?

[Lurvio]

Yes Hoyt there are, just been too busy to tell them.

So, a bit of an update coming up. This is going to take a couple of posts.

The end of june and whole july have been busy months for farm work, so not much time for boat building. I had to clean the work area as it got filled with dry hay, 2800 small 6 kg bales. I got left a little corner to continue, so I was not kicked out yet.

Them pics, front deck strongback(?), quite a curve in it.


Front deck framing


Carlings started, gluing a small block to join it with the rear deck framing


And gluing the second layer


In the next update, how to make a composite propeller. Hopefully tonight, now I'm going to get the secont silt coating for today, harrowing.

Lurvio

[Lurvio]

Update #2

I first considered making the prop from aluminium, but I don't have welding tools for alu and I ditched the idea of a folding prop. I have had a chance to freshen my skills with fiberglass so it was a pretty obvious second choice for material. I didn't abandon aluminium completely though, as I made the prop core out of it.

The real problem was how to shape the blades as metals could just be twisted using a vise and a wrech. Also the amount of twist had to be pre-determined. After some brainwork I came up with a way to acquire the angles I needed.

The prop data kindly calculated by Rich Willoughby, thanks man.
-diameter 200 mm 8 ''
-pitch 230 mm 9 ''
-chord 40 mm 1,6 ''

-blades 3
-rotation Lefthand

To gain the needed angles I drew a rectangular that was 230 mm (pitch) wide and 628 mm high (diameter x pi). Next I drew a line through opposing corners and measured the blade tip angle, about 70 deg. Then I needed the root angle so I drew another rectangle, again 230 mm wide, but only 78 mm high (estimated core diameter (25) x pi), again I drew a line to opposing corners, the root angle is about 18 degrees.

Actually I drew four of the rectangulars side by side (dia's 200, 120, 40, 25 mm) but only the two above are important. Now I had the tip angle and root angle. Their difference is 52 degrees which is the amount of twist I needed in about 90 mm lenght of the blade.

First I needed a stable platform, a piece of 38 mm birch plywood fitted that bill. Then I cut two pieces of wood that both had a 26 deg. angle in them. I attached the blocks opposite ways to the base 100 mm (center to center) apart (90 mm would have been more correct but I figured it ain't that presise). Then I drew centerlines to the blocks and used files to fair the facing corners to an angle that matched the opposing side.

As a skin for my mold I had thought of using 4 mm plywood, but that idea was quickly abandoned as I tried to twist it into shape. Next I tried thick cardboard 80 mm wide, no fish. I cut the cardboard in two to make a 40 mm wide strip, it still didn't want to twist enough. One more try and I cut the strip in half again, finally success. I made more of the 20 mm wide strips and stapled four of them side by side, add some two sided tape and added three more strips overlapping the bottom layer. Some painters tape and I had a mold ready.

If you look closely enough for the first picture you'll see the twist is for a righthand blade, it was in this stage I finally managed to check which way the prop was SUPPOSED to rotate.


No big harm done, I disassembled the first attempt and tried again.


I drew a blade shape on the mold in hopes it would show through the glass. It did show through and I was able to transfer the drawing to the blade surface. That way I had a center line in all blades and the shape is identical. After covering the mold with a thin plastic film, I laid five layers of 290 gsm fabric. I lined the layers so all fibers from tip to root were parallel, but the longitudal fibers were diagonal and opposing between layers (does anyone understand this? ).


Three blades, one is already cut to size. I cut them a bit bigger than drawn to have a bit of extra to play with later on.


Now I had the blades, time to make a center piece to attach them to. As said earlier, I made the core from aluminium, a long bolt seemed like a good material. I milled a 10 mm hole in one end, 34 mm deep and a 5 mm hole the rest of the total depth on 45 mm. I beveled a space for the screw head at the back end (5 mm hole). I milled a two way groove to the surface of the piece to ensure fiberglass staying where it's supposed to.

Before glassing


And after, all faired and ready for blades


The next hurdle was attaching the blades to the core, and to get them all in the right position and angle. I made a jig that attached to the lathe, a T-shaped piece that the blade could be attached to. the angle for the jig was calculated using the method described earlier (seems we need three of the these), this time using 40 mm dia., the jig was placed 20 mm from the center of the core.

First blade getting attached. I used a mix of short fibers (<10 mm) and resin mixed together, using tweezers to get the fibers in place. The excess resin (you can't really mix small enough batches for this) was used for spare blades, I have three of them waiting. After all three blades were attached I repeated the short fiber and resin process to the back sides of the blades.

Pics of the process






After hardening, I used a sandpaper wound over a round piece of wood to shape the join area. Then it was time for reinforcement.


I shaped the propeller by attaching it to a cordless drill, and rotating it against a 60-grit sandpaper. This way I was able to shape all the blades symmetrical and not do any damage to the prop.

Finishing the propeller took a lot of sanding, some self mixed filler (resin and sanding dust (mostly fiberglass and resin), the brown bits on the prop) and more sanding...




...some primer...


...and some acrylic spray paint. Here you also see the first piece of the prop shaft, made of stainless steel, that larger part is the sliding surface against a nylon bushing and a water seal (outside the bushing), it was sanded down to 600 grit paper. The smaller part is to be glued inside a pipe that makes the next part of the prop shaft.


Well, there you have it folks, my way of making a propeller.

Lurvio

[Lurvio]

The blade thickness is a bit under 2 mm and the whole structure seems really solid, so I feel I did a fairly good job.

Making the prop tube is(was) the next stage. I'll have to take some more pics before telling about that one.

L

[hoytedow]

This is an excellent tutorial, Lurvio. Even I understood it. The blade result looks great. Well done.

[Lurvio]

Thank you. It's interesting to see how it works in the water. At 1400 rpm it moves air quite nicely.

L

[Lurvio]

Well, I have not been any less busy lately, but something has happened.

I got all the fiberglass (that I can think of) on the boat. I have attached the prop tube in place (used my own mix of (fiberglass) sanding dust and resin as a glue/filler) and made a styrofoam block to act as a core for the proptube's support. Two layers of wood glue proved to be sufficient cover against the resin as I attached the block in place with the dust/resin mix and shaped some fillets, then glassed over them. The small corners (the foam block is about 50 x 70 x 20 mm) were a real pain in the ass all through the process.

The front part of the bottom glassing had been undone for more than a month, but I finally got around to do it too. After that I sanded the whole boat and added a fill layer of resin. Next time I have to sand again and add at least one fill coat to get the hull surface level.

Not much to show, but I'll post some pics in the near future. I've been sitting inside on a computer for over a week making house drawings, calculating convection coefficients and all sort of fun stuff like that.

L

[hoytedow]

Using glue as a shield against resin was smart.

[Lurvio]

Kinda hard to glue something if it turns liquid in your hands.

L