Monday, September 5, 2011

The rest of the drivetrain

I can say now that the scope of my summer project has been changed slightly.  The boat is ready to hit the water (it already has actually, but I'm behind on posts), but it is not finished.  I have yet to do any final sanding on the hull; and no varnish yet, either.  I will do a few more things to prepare for the Hydrobowl this coming weekend, but I will focus on the prop and steering.  Varnish will probably wait until spring.  Right now I want to enjoy it for a few weeks before the weather turns.  And winters are long here...

The final area of the boat I have not focused on in posts is the rear end of the drive.  It is a variation of the flex shaft design Rick has refined lately.  I like a lot of advantages of the "standard" flex shaft, but I have had an idea in my head for a few years, and I really wanted to see if it would work.  Now that it is mostly done, it is easier to explain.  I am using a flexible shaft, for the full length of the boat, but it does not extend below the hull as in Rick's system.  I am running the shaft under the seat and along the deck line to the stern where it joins with a separate drive leg.  The drive leg uses a 1/4" pitch chain common on twisted chain drives, but the chain does not need to twist since the gearbox has already changed the pedal motion 90 degrees.

After a lot of searching I was able to find inexpensive stainless 9 tooth sprockets for this chain.  The max shaft size was 1/4" but I wanted to drill them out for 3/8" shafts.  This meant there wasn't enough material left in the hub for a cross pin, so I found a local bicycle fabricator who silver soldered the sprocket on, and it looked pretty secure.

I realize now that my photo collection is incomplete for the drive, but I can add photos later if there are any requests.  The shafts are mounted in stainless bearing and I leave it all open to water.  This is the same approach I used on my first boat, it is 7 years old now on the original parts with no corrosion on the stainless.  Good enough for me.  The chain runs through a 1 1/4" OD aluminum tube.  This is the smallest possible size drive leg I could figure out how to make.  The round tube is welded to a 1 1/4 rectangular tube which houses the prop shaft and bearing.

The drive leg tube has a foil fairing based on precise plywood pieces cut from the CNC batch.  I sandwiched foam and wood to maintain and accurate shape.  I precut a (slightly undersized) hole in the wood pieces, and once this was all together, I cut it into front and back halves along this hole centerline.  I then wrapped sandpaper around scrap tubing and sanded away the foam from the inside.  This allowed me to glue the fairing to the drive leg and essentially not add any width to the tube except for a layer of glass cloth.

 One of the key reasons this general design appealed to me is that the prop is steerable.  The drive leg tube nests inside a square tubing with flange bushings.  The drive leg tube has a slot for the upper drive shaft, and the clearance in the slot allows for about 20 degrees of rotation each direction.  It can be increased slightly if needed.  The square tubing is bolted to the inside of the hull.

 One of the primary concerns with drive legs is weed catching and collisions.  Generally, these are not issues where I will be boating for the near future, but I added a small strut to help deflect weeds and ramp over obstacles if I hit something.  I will eventually file this strut to a nice profile.  The twist isn't ideal because it will add slight drag, but it is the fastest approach for now.  The bottom of the strut is bolted with a shoulder bolt to the drive leg bottom so the leg can pivot about the strut.

There are plenty of downsides to this design relative to using only a flex shaft under water, and I was aware of them and willing to try it anyway.  I accept it is not an ideal solution for most home brew boats.  But there are a few advantages, and I will focus on these.  Because the shaft only has a small amount of curve, I can use 3/4" diameter (.058" wall) tubing the full distance, and maintain the best possible torsional stiffness with a minimum of weight.  I calculate the shaft at about 44 Nm/rad stiffness, significantly higher than cases where a thinner shaft is used underwater to accommodate the increased bending.  There is no need for a rudder of any kind, and the strut fairing can be more effective since it is not surface piercing.  This is in addition to the steering from prop thrust.

Beaching is more of a gray area.  The strut is currently fixed so it will hit when beaching.  The lake I am on is deep, so this is not a concern for me.  The downside is that the strut is fixed, the upside is that the shaft is protected so beaching will not bend the shaft, which can happen when not using spring steel.  In fact, the second reason this design was appealing was that I intended to make the drive leg beach-able.  My idea was to detach the stern portion of the hull and add a transom.  Then I could use the deck panel or a portion of it to flex up with the shaft.  It would essentially be a long hinge flexing/pivoting with the shaft.  Since the shaft does not flex a large degree in steady state use, it could accommodate more flex with beaching.

In the end I decided this would take more time, and I would prefer to proof the general concept and reliability before I cut into the boat.  If I decide not to continue with this drive leg it is currently easy to undo.  If it works out for a year or two, I can still use it as a test bed for a steerable and beach-able flex shaft drive leg.

The complete drivetrain is very low friction.  I can easily rotate the shaft with my fingers and maintain motion for about a prop rotation; however, I managed to bend the shaft slightly during my experimenting and it has a slight pulsing as it rotates due to this bend, so it always stops at the high resistance point when I try to spin it.  It is not something I can feel with my legs.  I replaced my original bushings with ball bearings since they are protected from water.  I would consider it relatively light weight as well.  The drive leg adds weight but there is no rudder and the shaft has only 12 inches of steel.  In the end, the full assembly with full shaft, shaft bearings, drive leg and rudder control is just under 4 lbs.

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