Air Drive

While the duct is under construction I wanted to get the rudders built so I could know where to put the rails to support them since they will be inside the duct itself.

I was going to use bent aluminum sheets like the last - but aluminum has become expensive! Plus I finished these with 6oz cloth and two-part paint and they still weigh in at 1lb 13oz. They are 16x36 in size.

They will be mounted to two 3/4" x 0.063 aluminum (6061) tubes by some Delrin blocks. I made the blocks with the bandsaw and holesaw. some 10-24 screws will hold them to the rails.

For the airdrive frame I'll be using 1x1x0.120 stainless (308L) tubes. They are about $140 for 20 feet these days. I was going to mix some 1x2x0.063 tubes but decided against it.

My first attempt at making the air-drive frame resulted in severely distorted tubes after excessive welding.

So I remade the frame. Here are a couple of shots with the engine and fan mounted. You can see the Amphibious Marine fan extended hub and the 6" sheave for the propeller. It's nice as the belts sit above the deck (no cutouts) while the fan sits low in the cowling/shroud/lift duct. I am glad I followed Bryans advice here.

The engine supports are welded on. They rise another 1.5" above the frame. I could've probably used 1x1 and brought it down 0.5" but this keeps a nice clearance between belt and deck. I think I'll add a strip of 1/16" rubber under the frame where it touches the deck to keep the noise down. The bottom of the engine is alreay sprayed with CRC rust-inhibitor (again, thanks Bryan Phillips)

Third times a charm...

OK. I rebuilt the frame for a third time and this time I like it. No risers except for some spacers (1/8" aluminum sheet) under the engine mounts and some rubber/plastic ones to stand the frame off the deck a tad. It also lowers the fan to right at the bottom of the duct.

Idler swingarm

I had considered doing what I did with the last craft and making the tight side of the idlers fixed in their location - meaning only the slack side idler moves to tension. The other variation was both fixed idlers and tensioning done via moving the main bearings - this second one is very hard with a duct as the prop can not be moved up and down like it could if it were just caged with much greater clearances. So I stuck with the classic swingarm - with one exception. With the duct right behind the idlers it means there's no room for a pull-type tensioner at the bottom. It also means that either the frame legs go under the duct or terminate just behind the lift duct in front of the thrust duct (which is what I did - see later).

Using a central pivot the tensioning can be achieved by pulling back to a cross-bar on the other side. One downside of this is the force on the pivot point is quite high. Although I had built my swingarm out of 0.120 1x1 stainless and 3/8 round I chose to do it the way below.

Push tensioning

This way a 0.5" stainless threaded rod will be under compression from a lower cross-bar. It keeps things well out of the way too.

Final frame

It is all stainless 0.120 (1/8", 11ga) 1x1 and the main frame weighs 17.5lb and the swingarm with 5" steel idlers, 5/8" bolt and nuts weighs 5lb. I probably could save 5lb by using 0.82 (14ga) steel but at least this frame will be very strong. All the TIG welds have been pickled and polished.

To support the frame legs that no longer go all the way to the back of the hull I added a glassed-in crossmember. You can still see the original holes at the very back.

The prop weighs 7.75lb and the prop pulley, spacer, 1" stainless shaft (12" length) and the hardware (split taper bushing, bolts) weigh 16lb. This has to be one place where you could save some serious weight if they made a 16" A/BV sheave. This pulley is no where near its limits - especially as the shaft is not taking the torque. It would just need to be strong enough to hold the bolts for the prop.

The Idler is complete.

First time lineup for the duct and its center marker. I built an faux engine with a spare idler pulley for getting the belt in roughly the right position. Should work for my next craft build too ( :-) )

The first bit of actual assembly is complete. The fan sits nicely in the lift duct with under 1/8" clearance all round. I was surprised you can still lift the craft from the rear at this point.

Here's a couple of shots with the duct and the prop. Looks like the height of the prop is just right - although I might want to add another 1/8" spacer under the top bearings to raise it a tad. This way I can use a small pad of 1/8" rubber under the duct where it stands on the deck. I will have a bolt holding it in place but this will be a softer mount - although still not allowing it to move.

I added the touch blocks below to control belt movement.

Here's a quick shot of the steering wheel to steering cable linkage

And finally some footage of the first running up of the engine: http://www.youtube.com/watch?v=Pl6L6RcyhIc