Project 2 from EmptyNester: Category Off Road - custom tailpipe for Hovercraft

I recently rotated the engine and changed the mount on the hovercraft I built. I did this to correct problems I was having with one of the belts coming off freqently.

After one short 10 minute test flight (had lots of problems), the heat from the exhaust cracked the fiberglass.

Attachment 7603

I wasn't sure what to use to do the exhause but Zoama suggested a mandrel. I got one and am now getting ready to start the project. (I just want to make a little more progress on my CNC table first.

I haven't decided if I should use the second half of the mandrel to extend or redirect the exhaust.
Attachment 7604

At a minimum, I figured I'd want to extend it with the left over straight piece. Not sure it I want to leave the bend in to point it rearward.

i think the mandrel bend should work great to redirect the exhaust. I imagine it's mild steel not S/S? Will you need to incorporate some sort of support for the additional weight so it does stress crack the cylinder head from vibration?

Quote:

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Originally Posted by MoparCar

i think the mandrel bend should work great to redirect the exhaust. I imagine it's mild steel not S/S? Will you need to incorporate some sort of support for the additional weight so it does stress crack the cylinder head from vibration?

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One of the selling points was that it wasn't stainless, so its easy to weld.

I had been thinking about support. The weight isn't much, but I am concerned about the vibration. I'd have to figure out how to do it too. My only thought so far was to use rod to make a triangle/tripod support. I could then fasten the bottom of the triangle to the deck.

If I don't fasten it, I'm afraid the vibration will eventually damage the plywood deck (its only 1/8"). If I do, I'm concerned I might cause problems from flexing of the hull.

I thought it might be worth while to come up with some vibration dampening mount for the deck.

I am still a rookie welder. I have my good days and my bad days.

I decided to do a butt joint of the mandrel after it was cut in half. This would allow it to go further to the side and have a rear facing exit.

I had really bad problems with burn through. I had to set the welder lower than it probably should have been (slow buzz versus the normal one). But it was the only way I could keep it from burning through.

I got it done, but had to grind it down because it looked so bad.

Attachment 7703

This picture shows how I plan on mounting it. I will put some kind of support under the end of the tail pipe.

Attachment 7704

I usually use .030 wire. I had run out while I was working on my CNC table, so I switched to .035.

I didn't give it a second thought when I was welding the tailpipe and had the burn through issues. So, I think in addition to the tapered ends from the cut, I think the thicker wire may have contributed to my problems.

I switched to .030 wire when I welded the pipe to the muffler. That went much better.

Attachment 7795

That should help. I use .023 on thin metal.

At what thickness metal do you jump from .03 to .023?

Quote:

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Originally Posted by EmptyNester

At what thickness metal do you jump from .03 to .023?

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Here's a chart...
Attachment 7905

I got the engine mounted again.

Everything seems to be fine. I haven't put an additional support for the tailpipe in. I'm thinking it just needs a support that sits on the deck. I'm concerned that if I attach it, it may create more problems than it solves.

Attachment 8102

Unfortunately, while I was adjusting the belts, I fell and hit the thrust duct very hard. It broke the fiberglass joint on both of the duct supports, left and right). The duct support is the wood in the picture with the flag on it. You can see I've already sanded it so that I can put new fiberglass on. I've also sanded the area behind the muffler where the heat from the exhaust gas cracked the fiberglass (before I installed the tailpipe) .