The Hidden, Never-resting Enemy

If your boat has an inboard engine, chances are that it has a shaft log - you know, that place where the prop shaft exits the hull.  This cannot be a mechanically rigid seal because it needs to accommodate movement of the engine on its mounts, and movement of the prop shaft in the cutlass bearing.  Typically then, regardless of the seal type, the shaft log includes a short length of rubber hose, one end of which is clamped onto the rigid hull fitting, and the other end onto the actual seal housing.

OK, so it's below the waterline and therefore needs to have two hose clamps on each end:  four hose clamps in total.  Also, typically, the shaft is slanted down, towards the aft end of the boat.  And it's in a typically hard-to-access location.

Add all this together and what do you have?  An opportunity for the bane of all boaters to work, silently, unobserved, trying to sink your boat.

No seal is perfect; this means that there will be the occasional drop of sea water exiting the seal.  Because the shaft is slanted aft, this lone drop will travel aft, along the bottom of the seal assembly and then the bottom of the hose.  In it's passage, it will travel over the bottom side of all four of those hose clamps. 

The bottom side of the hose clamps.

Where corrosion can work it's evil, undetected.

The NEW hose clamps

Last weekend I changed those four hose clamps on Eolian's shaft log.  All four of them showed corrosion; two were terrible.  And all four hose clamps looked almost new from the top, the only side that you can see.

If both hose clamps fail on one end of the hose (typically the aft end), there is a good chance that the torque applied to the hose via the seal assembly will cause the hose to rotate and eventually come off of the hull fitting.  When this happens, water starts coming into the boat.

How fast?  Well, that depends on your boat's construction.  On Eolian, the cutlass bearing is embedded in the hull, just aft of the hull fitting that the hose attaches to on the inside.  Therefore in our case, the water flow would be minimal - just that little bit that can squeeze past the clearances between the shaft and the cutlass bearing (and its lubrication grooves).

If, on the other hand, your boat has its shaft supported on a strut, then the flow area is huge - the difference between the shaft diameter and the hull fitting diameter - probably not less than a half inch.  There's gonna be a LOT of water coming in!

Due to the seriousness of a failure, and because visual inspection cannot disclose incipient failure, your shaft log hose clamps should be changed on a schedule, regardless of their appearance.

What frequency?  Well I can't tell you that.  If you haven't changed them in a while, go do so.  If they look bad, you waited too long. Cut your maintenance interval in half for the next time.  And if it's been more than 5 years since the last replacement, your should go change them now!

The Hidden, Never-resting Enemy

Yup, It Works!

Recently I was working with some West System epoxy (this was on a car project, not a boat project, but no matter).

Nobody who has worked with epoxy has ever, ever come out of the project with clean hands.  Come on, admit it.  Your hands were sticky when you were done.

In the past I've used a variety of solvents to clean off the mess.  Most worked, but most were also harsh. 

I had read in the past that vinegar (a 5% solution of acetic acid in water) was effective as an epoxy hand-cleaner.  But I never believed it.  Come on, a water-based solvent effective on an organic sticky mess?  Ha!

Well, the joke's on me. 

Universal goodness

I tried it, and it worked!  And by "worked", I mean that it literally washed away the black sticky, partially cured mess on my fingers and hands.  As effectively as if it had been maple syrup...  no!  More effectively!

(Of course, once the epoxy is fully cured, it is cross-linked.  That is, the entire bulk of the epoxy is essentially a single molecule of unimaginable molecular weight.  No solvent can dissolve it, tho some may infiltrate and swell it, weakening it.)

Yup, It Works!

More Propeller Thoughts

Some time back I did a mind dump of some thinking about boat propellers.  And one of the comments on that post hit a nerve - one that I have been thinking about for years.

Tip vortices.  What are these?  They are the spiraling water that slips off the ends of the prop blades when it is turning.  They come from the fact that water on one side of the prop is at a higher pressure than water on the other side.  This arrangement holds just fine until you get to the end of the blade, and then the high pressure water just spills off the blade and joins the low pressure on the other side, making a vortex.  For visualization, the same thing happens at the ends of an airplane wing, causing sometimes beautiful effects.  And drag.

Wing Tip Vortices

Making vortices uses energy - energy that could have been used to propel water astern giving thrust.  So, how to stop this waste?  On an airplane wing (or a keel...), one way is to put up a fence to stop the spill-over, thus the development of winglets and winged keels.

So what would a fence on a propeller blade look like?

• Start with a conventional propeller.  
• Add a ring that goes all the way around the ends of the blades.  
• Extend the blades profile to meet the ring.  

This is an interesting example - the ring here is being touted as a guard, which of course it is.  But it almost meets the purpose of a fence.  It falls short only in that the ring is not wide enough to fully cover the ends of the prop blades.

Stationary ring bolted to engine

Why doesn't this prop guard achieve the purpose?  In fact, this is probably worse than no ring at all.  The tips will still be forming vortices, which will then immediately impact the (stationary) ring, creating additional turbulence and drag.    It is important that the tips extend to and attach to the ring, and that the ring rotates with the propeller.

Ducted fans have been using (stationary, however) rings forever.  And the cross section of the rings is designed to minimize flow turbulence as the fluid enters the duct (look at the leading edge of a jet engine cowling for an example).  If the rotating ring had such a cross section, drag could be reduced even further.

Now, if only I had a bronze foundry and some propeller tooling to play with...

If someone out there wants  to do the experiment, I need a RH 20x14 prop to fit a 1.25" shaft...

More Propeller Thoughts

AIS - Should you have it?

AIS - Automatic Identification System - is a system whereby ships (boats, yachts) broadcast information about their location, speed, course, and much more on marine VHF channel 87 and 88 as digital data.  (Tune your VHF there if you want to hear what digital data sounds like.)

There are two classes of AIS devices, Class A and Class B.  Class A devices are intended for commercial vessels, and transmit with more power and more frequently than the Class B devices, which are primarily intended for recreational craft.

For either class, there are two kinds of devices, transceivers (transmitter/receivers, also incorrectly called 'transponders' - a term meaning transmitter/responder, a device that provides data when interrogated by an incoming signal), and receivers.  As you might suppose, transceivers both broadcast their ship's data and also receive data from other transceivers.  The other type of device, the receiver, collects data but does not broadcast it.  In practical terms, vessels with transceivers can see each other, but tho vessels with receivers can see those with transceivers, they are invisible to other vessels.

In order to provide position, speed, heading, etc data, transceivers need GPS information.  Many have a built-in GPS receiver, a few can use an external GPS via NMEA sentences.  Receivers do not need access to a GPS.  Typically, both transmitters and receivers make the data they collect available via NMEA 0183 and/or NMEA 2000.  Some also provide the data via RS-232 or even USB.

How is the data made available to a human?  Typically, a chartplotter will be connected to the AIS device via a NMEA bus and will display detected ships as icons on the display in their correct position, usually with a speed/direction vector.  The chartplotter will also note Point of Closest Approach, and if that is within a settable danger radius, will sound an alarm.  Tho few computers (laptops, tablets) are equipped to receive NMEA signals, adapters are available.  And of course almost all computers have RS-232 and USB capabilities.  This means that if you are running a navigation application on your computer, and it is one which is capable of receiving and interpreting NMEA sentences, then your computer will display nearby ships just like a chartplotter.

But... there are web applications which show AIS ship data world-wide...  why would you buy an AIS unit yourself?  Why not just use one of these apps on your phone?

The problem with the web-based AIS apps is one of reliability and delay.  Imagine how many AIS receivers are needed to provide world-wide coverage.  Now imagine the internet network that is required to collect and process all this data.  It is a huge system.  And there are processing delays - significant delays.  And there are system and network failures.  Further, making use of the web app that results from all of this collection and processing requires that you are not in a cell phone dead spot (there are plenty of dead spots in the San Juan islands, for example).  All of this combines to mean that the web display will be at best a picture of the situation from some time back.  I used a web app on my iPhone before we got our AIS receiver.  There were times when the display showed a Washington State ferry more than five miles away, when in fact that ferry was right next to us.  Kind of funny in bright sunlight, but not so funny in dense fog.  If you have your own AIS receiver, you won't be depending on someone else's system with all of its inherent delays and dependability issues, or your local ability to connect to the Internet.  Complexity is the Enemy of Reliability.

Coming also is the use of synthetic AToN (Aids To Navigation - buoys, etc.).  For this, a VHF station on shore broadcasts AIS data as if it were an AToN at a particular spot.  There is at least one of tow in Puget Sound, and you can expect more, particularly for situations where a buoy has gone missing or has drifted out of place.  The synthetic AToN will show up on your chartplotter, even if you can't see it with your own eyes.

The case of the out-of-place bouy is an interesting one...  suppose that a buoy has drifted 100 yeards into shallow water.  Would you believe your eyes, or would you believe the display on the chartplotter?

If you never are out and about in less than good visibility, then you won't need RADAR or AIS.  But if you find that you need to navigate at night or in poor visibility, then you should have both RADAR and, at the least, an AIS receiver on board.

Are they expensive?  Receivers are much less expensive than transceivers.  As a personal example, I traded an old iPhone for a used AIS receiver.  Watch eBay and craigslist - many boats that originally had just a receiver are now upgrading to a transceiver.

A number of VHF radios now are available with built-in AIS receivers, seemingly a natural blending of capabilities.  However as a standalone AIS receiver their displays are so small as to be almost ludicrous, and they provide so little information that they do little more than raise the anxiety level on board.  But connected to a chartplotter, this is a viable solution.

How about the transceivers?  Well, I just checked eBay, and found a new one, with a built-in GPS for $400.  There are a lot more available in the $500 range.  You can also spend thousands for a completely self-contained unit that includes its own display capability.  I find these to be ill-conceived, since if you have a chartplotter aboard, you won't be using the display.  And if you're investing this kind of money, you almost certainly have a first class chartplotter on board.

As for us aboard Eolian, we have that AIS receiver I got in trade for the old iPhone, and it is hooked up to our chartplotter.  I will strongly consider getting a transceiver when the prices fall more, or when some start appearing on the used market at a lower price.  In the mean time, it is wonderful to be able to see a ferry approaching Thatcher pass from the inside when I am approaching from the outside with no sightline.

(A tip of the hat to Jason for prompting me to write this!)

AIS - Should you have it?