To bond or not to bond is the
question in boating circles. With the electrical system gradually being
installed it is time to consider whether all or part, or none of the
fittings should be electrically bonded (grounded). There are arguments
for and against, especially in wooden boats, and the main ones for seem
to be threefold: for corrosion protection, for lightning protection,
and to protect against accidental short circuits. Against, is the fact
that bonding may in some circumstances make corrosion more likely, that
wooden hulls are sensitive to alkalainisation caused by bonding, and
that is should be possible to achieve the same effect as bonding by
installing a high quality electrical system, and insulate all fittings.
The fact that Ariadne will be trailered, and not left lying in the water, does not entirely preclude the necessity of considering bonding, although it does reduce the likelihood of sustained damage being caused by the sort of stray currents which may be encountered in say a poorly run marina. But let us first look at the arguments for bonding.
Corrosion Protection.
Corrosion is a different matter. That affects all boats, and can result is surprisingly rapid destruction of metal components. There is a particularly volatile mixture of metals at the stern, where the propeller, shaft and rudder are all of dissimilar composition, but these can be largely protected by a sacrificial zinc anode on the shaft. Stray current corrosion, which might be a consideration while moored at a wharf, say, could actually be worsened by bonding, but selective bonding, coupled with selective insulation will prevent its effects, and this is probably the strongest argument in favour of bonding.
Short Circuit Protection.
For such an event to be a problem there would have to be a current leakage not high enough to trip one of the breakers of blow the fuses. That is possible, especially in the slow blow fuses which have to be able to tolerate high momentary current inflows, such as might be encountered on a starter solenoid, for example. They are usually rated above the running amperage of the device they are protecting, so a small leak will not trip them. It will, however, be sufficient to induce its own circuit, and cause at the least corrosion problems, if not electrical damage. Bonding against this seems like a good idea.
But, consider the circuitry on the boat. The shaft, propeller, zinc complex is joined by the shaft seal, but they are all electrically isolated. The shaft ends at the sprocket, and it only contacts a rubber drive belt. There is no electrical communication with the motor. Similarly, the rudder, tiller and steering cable are all connected together and to the steering column, but no further. The peripheral accessories, such as navigation lights, etc. are all double wired back to the battery negative terminal. No devices use the motor casing as a return path. The through hull fittings are all fully insulated.
The only way a leak could establish its own circuit would be via a wet bilge, and for that to happen there would have to have been a water level so high that the bilge pump would have had to have failed. Admittedly, in the case of a catastrophic failure of the shaft seal the water level might go that high very quickly, but, if that were the case, bonding would not be enough to prevent a total electrical short, which, in any case, would be the least of my worries: except for the bilge pump. I am concerned to ensure that the bilge pump will continue to work for as long as the batteries are viable, and, with that end in mind, I propose to route the pump's wiring directly from the positive bus through waterproof conduit.
With all these considerations I feel that isolation and unbonding is going to be an adequate technique for protection in this boat. There will be no common ground point.
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In order not to starve the motor mount-to-hull bond of epoxy I have laid it on thickly and packed it in tightly around the adjoining surfaces.
Rather than try to form a
fillet all at once for the mount, I let it set first and then fillet it
the next day. This allows me time to work carefully around the mounting
baffle grooves which cross the mounts themselves, and which have to
remain free of epoxy. While I am doing that I don't want the bilge to
be slopping about with liquid glue.
Next day the fillets are put in. What a job! Crouched over in this tight compartment surrounded by fuming epoxy. I had forgotten what an unpleasant task it is to deal with the stuff.
The freshly laid fillets still with peel ply on the motor mounts.
The battery trays are only sufficient for two batteries at the moment. Two more can be accommodated beside them, and two more in front astride the bearers. Here, the third tray is being introduced.
These lateral trays will be epoxy filleted to the hull, and after that the incomplete fillet seen here between the dash bulkhead and
the hull will also be rounded off neatly.
A discarded car battery serves to test the trays for size.
Ventilation holes in bulkhead A.
It is also time to run the conduit through from the cockpit to the motor compartment, as the holes in the floor attached to the front of the dash bulkhead will soon become inaccessible beneath the battery trays. An elbow fitting redirects the conduit upwards when it reaches the end of the tray, and a second section will be fitted to it to bring it above the waterline.
Battery cables will be kept waterproof by conduit when passing beneath the waterline.
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The fact that Ariadne will be trailered, and not left lying in the water, does not entirely preclude the necessity of considering bonding, although it does reduce the likelihood of sustained damage being caused by the sort of stray currents which may be encountered in say a poorly run marina. But let us first look at the arguments for bonding.
Lightning Protection.
This can be dealt with quickly. The slipper launch is an open boat which will not be used in poor weather. A forecast of thunder storms will mean no boating for me. And, although lightning strikes are disproportionately high in water vessels, they are still exceedingly rare in this part of the world, so I am not inclined to bond the boat for protection against them. If I had a tall masted yacht which I lived on it would be a very different story.Corrosion Protection.
Corrosion is a different matter. That affects all boats, and can result is surprisingly rapid destruction of metal components. There is a particularly volatile mixture of metals at the stern, where the propeller, shaft and rudder are all of dissimilar composition, but these can be largely protected by a sacrificial zinc anode on the shaft. Stray current corrosion, which might be a consideration while moored at a wharf, say, could actually be worsened by bonding, but selective bonding, coupled with selective insulation will prevent its effects, and this is probably the strongest argument in favour of bonding.
Short Circuit Protection.
For such an event to be a problem there would have to be a current leakage not high enough to trip one of the breakers of blow the fuses. That is possible, especially in the slow blow fuses which have to be able to tolerate high momentary current inflows, such as might be encountered on a starter solenoid, for example. They are usually rated above the running amperage of the device they are protecting, so a small leak will not trip them. It will, however, be sufficient to induce its own circuit, and cause at the least corrosion problems, if not electrical damage. Bonding against this seems like a good idea.
But, consider the circuitry on the boat. The shaft, propeller, zinc complex is joined by the shaft seal, but they are all electrically isolated. The shaft ends at the sprocket, and it only contacts a rubber drive belt. There is no electrical communication with the motor. Similarly, the rudder, tiller and steering cable are all connected together and to the steering column, but no further. The peripheral accessories, such as navigation lights, etc. are all double wired back to the battery negative terminal. No devices use the motor casing as a return path. The through hull fittings are all fully insulated.
The only way a leak could establish its own circuit would be via a wet bilge, and for that to happen there would have to have been a water level so high that the bilge pump would have had to have failed. Admittedly, in the case of a catastrophic failure of the shaft seal the water level might go that high very quickly, but, if that were the case, bonding would not be enough to prevent a total electrical short, which, in any case, would be the least of my worries: except for the bilge pump. I am concerned to ensure that the bilge pump will continue to work for as long as the batteries are viable, and, with that end in mind, I propose to route the pump's wiring directly from the positive bus through waterproof conduit.
With all these considerations I feel that isolation and unbonding is going to be an adequate technique for protection in this boat. There will be no common ground point.
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72. Completion of the Motor Compartment
With the approach of the laying of the forward subdeck it is time to finish anything which can be done before the foredeck stringers are glued down. Up to now they have been left removable for ease of access to the motor compartment. So, the motor was taken off the bearers, so that they could be epoxied and filleted to the hull, and the few remaining keel bolts which have not yet been glued in are now added. Two more substantial stainless steel brackets are used to replace the small enamelled ones which served a temporary purpose, and the remaining battery accommodation is completed now that the location of the steering mechanism is finally decided. There will of course be occasions when the compartment has to be entered after the deck is laid, for example to do a lot of the wiring, but the less of that I have to do the better, as it is awkward to get in and out, and every movement risks damaging the windscreen, once it is returned to its proper position.
In order not to starve the motor mount-to-hull bond of epoxy I have laid it on thickly and packed it in tightly around the adjoining surfaces.
Next day the fillets are put in. What a job! Crouched over in this tight compartment surrounded by fuming epoxy. I had forgotten what an unpleasant task it is to deal with the stuff.
The freshly laid fillets still with peel ply on the motor mounts.
The battery trays are only sufficient for two batteries at the moment. Two more can be accommodated beside them, and two more in front astride the bearers. Here, the third tray is being introduced.
These lateral trays will be epoxy filleted to the hull, and after that the incomplete fillet seen here between the dash bulkhead and
the hull will also be rounded off neatly.
A discarded car battery serves to test the trays for size.
The final keel bolt is screwed
home, and the last battery tray is added. This one is big enough for
two batteries. If they prove to be too heavy in the motor compartment
they can be replaced into the driver's seat lockers after the trial
floating.
Each battery will have hold down
straps fitted like the one below right.
Everything is now removed so the
compartment can be epoxy coated, prior to painting.
I have had to place a pair of
larger stainless steel right angle brackets on back order, to replace
the enamelled ones which are currently supporting the motor baffle. The
forward battery tray will have to be grooved underneath to accommodate
the brackets, so until they arrive there is not much more I can do down
there.
The steering column too is coming in mid-month because of the Christmas break, so I cannot go any further with the helm mounting box. It seems like a good time to get into the fuel compartment and set it up for covering permanently with the deck, and that is a good reason to cut vents into the forward floatation compartment and ready it for the eventual attachment of the lower rubbing strake, as was done in the stern, with an internal batten of solid timber to screw into from the outside. Only, here there is a sharp bend to negotiate, unlike the stern.
The batten for the rubbing strake in the forward floatation compartment.
The steering column too is coming in mid-month because of the Christmas break, so I cannot go any further with the helm mounting box. It seems like a good time to get into the fuel compartment and set it up for covering permanently with the deck, and that is a good reason to cut vents into the forward floatation compartment and ready it for the eventual attachment of the lower rubbing strake, as was done in the stern, with an internal batten of solid timber to screw into from the outside. Only, here there is a sharp bend to negotiate, unlike the stern.
The batten for the rubbing strake in the forward floatation compartment.
The forward floatation
compartment is really a misnomer, because I am letting ventilation
holes into bulkhead A, so it is not watertight. With both rubbing
strake battens glued in and the compartment vented it is ready to be
sealed permanently by the deck. It does not really need to be epoxy
coated, as it will never be seen except by Houdini, and there is a strong argument for leaving marine ply
uncoated anyway (just as strong as for coating it).
Ventilation holes in bulkhead A.
The electrical wiring has to be
brought forward to the front compartment for the forward navigation
light. The conduit rubs up against the underneath of one of the deck
stringers, so a scallop is taken out of it to allow free passage.
It is also time to run the conduit through from the cockpit to the motor compartment, as the holes in the floor attached to the front of the dash bulkhead will soon become inaccessible beneath the battery trays. An elbow fitting redirects the conduit upwards when it reaches the end of the tray, and a second section will be fitted to it to bring it above the waterline.
Battery cables will be kept waterproof by conduit when passing beneath the waterline.
73. Backing Blocks - Foredeck
Just as in the rear deck, there
is a mooring cleat on the foredeck which requires backing. It will be
located immediately forward of the hatch, so the blocking will have to
pass from bulkhead B to bulkhead A, quite a long span. The other
hardware on the foredeck will be two fairleads, fender cleats,
spotlight, navigation lights, horn and clam shell vents on the hatch
covers. None of these requires heavy backing, as they are either
located over thick timbers already, or are lightweight and not
subject to stresses.
Foredeck backing blocks are first clamped into position,
and then bolted.
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Foredeck backing blocks are first clamped into position,
and then bolted.
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74. Laying the Forward Subdeck
There may be an easier way to do
this than the way I chose, and I would be glad to learn what it is,
because this is not easy. The deck running up to the windscreen is the
problem. Cutting the ply on a diagonal line to follow the screen is not
so bad, until you realise that it is not a straight line. The screen
bottom frame is straight, but because of the camber of the deck, the
cut line on the ply is a gentle curve convex towards the screen.
There is no room for theory and trigonometry here; you have to fit by trial and error. Taking a piece of scrap ply, and laying it between the apex of the central post of the windscreen and the forward end of the lateral post, gives you a gap between the scrap and the screen. The scrap can be scribed and cut to fill the gap, and the trailing edge can be undercut to fit flush up against the screen bottom frame. This can be done on port and starboard sides, and it is a lot easier to make the subdeck out of two halves meeting in the midline than try to make it out of one piece. Each half can conveniently span the area between the screen and bulkhead B, with a cutout to accommodate the hatch. That just leaves the area between bulkhead B and the stem to fill with another piece of ply, which is comparatively simple.
The scrap ply used to scribe for the foredeck is applied to the stringers and marked.
Most books will tell you of the importance of scuppers in the cockpit to allow free drainage of water, but they must be dealing with cockpits which can be drained overboard. The slipper launch cockpit sole is actually below the waterline, so simple scuppers will not work. The alternative is to allow any swamping water to run into the bilge and be dealt with by the bilge pump. There will be breathing holes in the sole anyway, to allow free air circulation, so any water can go down that route. The big unknown is how much water is likely to be taken on board, and how well the bilge pump will cope with it. I will only know that when it happens.
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Forward to February '08
There is no room for theory and trigonometry here; you have to fit by trial and error. Taking a piece of scrap ply, and laying it between the apex of the central post of the windscreen and the forward end of the lateral post, gives you a gap between the scrap and the screen. The scrap can be scribed and cut to fill the gap, and the trailing edge can be undercut to fit flush up against the screen bottom frame. This can be done on port and starboard sides, and it is a lot easier to make the subdeck out of two halves meeting in the midline than try to make it out of one piece. Each half can conveniently span the area between the screen and bulkhead B, with a cutout to accommodate the hatch. That just leaves the area between bulkhead B and the stem to fill with another piece of ply, which is comparatively simple.
The scrap ply used to scribe for the foredeck is applied to the stringers and marked.
I just use a block plane to
shape these parts. It is quieter and more accurate than a sabre saw,
and in no time at all a fairly good fit is achieved. Shooting from
underneath the windscreen a tiny bit of light shows through, but this
will easily be filled with the epoxy. Now, the shape has to be
transferred to the piece chosen for the subdeck, and some kind of butt
strap has to be fitted between the two short central stringers for the
two halves of the subdeck to sit on together.
The underneath shot
(right) shows a small slit of light where the scrap ply meets the
forward side of the bottom frame of the screen.
75. Draining the Drip
Channels - Fore and Aft
76. Scuppers in the Cockpit?
The disposal of an
old dishwasher made the selection of the drainage material easy. A
length of 10 mm. outside diameter copper pipe was cut off the machine
and segmented to be epoxied into four corners of the drip channels,
both forward ones in the forward hatch, and the two stern ones in the
aft. Plastic tubing is fixed to the
copper with small hose clamps, and it is plumbed to common points
whence it can be drained into reservoirs.
76. Scuppers in the Cockpit?
Most books will tell you of the importance of scuppers in the cockpit to allow free drainage of water, but they must be dealing with cockpits which can be drained overboard. The slipper launch cockpit sole is actually below the waterline, so simple scuppers will not work. The alternative is to allow any swamping water to run into the bilge and be dealt with by the bilge pump. There will be breathing holes in the sole anyway, to allow free air circulation, so any water can go down that route. The big unknown is how much water is likely to be taken on board, and how well the bilge pump will cope with it. I will only know that when it happens.
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Forward to February '08