Spars
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The drawing shows the design of the three spars of the scow:
| Mast | Gaff | Boom |
In order to avoid errors, the position of each fitting on a spar is given in degrees clockwise from a specified datum.
The mast is constructed from 66mm diameter aluminium tube, which is sleeved over the lower 800mm by a 70mm diameter tube and this is certainly strong enough. Both tubes have a wall thickness of around 2mm.
To make a mast from wood, use the mast shown in the plans of 'Tubby', the inspiration for this scow. This is solid wood (Sitka Spruce is specified) and tapers from 64mm (2.5in) diameter at the heel to (1.75in) at the top. Keep the full section from the heel for at least 300mm (12in) - you could keep a square section (or a square with the corners chamfered to clear the fillets inside the mast step) over this length to stop the mast rotating in the step.
If making the mast in aluminium, be sure to fit a plug to the bottom of the mast to prevent the tube from wearing out the mast step if it rotates. It is also useful to fit a light plug to the top, so that the mast doesn't fill with water if the scow is capsized.
To prevent an aluminium mast from rotating, a small plug could be fitted in one face of the mast, which would engage in a slot cut in one face of the mast step.
The 0 deg datum for the mast is the forward face, looking from the top. The fittings on the mast (from the top downwards) are:
|
Item |
Fitting |
Orientation |
Description |
|
Halyard block |
Cheek block to suit at least 6mm rope. |
225 deg |
Positioned at 225 deg (ie, 45 deg port of aft) to allow the halyard to drop onto the gaff to port of the mast. |
|
Halyard cleat |
Jam cleat to suit at least 6mm rope. |
180 deg |
Use a jam cleat with a fairlead or roller at the top, so that a knot in the end of the halyard will prevent the tail from disappearing up the mast when released! |
|
Tack line eye |
Eye |
270 deg |
Try to get an eye with a smooth profile, to avoid wear on the tack line. |
|
Safety line eye |
Eye |
0 deg |
This eye carries two 'strings': the strop for the kicking strap block; and the safety line (see 'Strings' page). |
(Note for pedants: As this spar projects forwards of the mast, it should correctly be called a 'yard', just like in a square-rigged ship. However that sounds a bit too much like something from a tea clipper to suit the scow, so I've stuck with the more widely-know 'gaff'.)
The gaff on my scow is made from a carbon-fibre moulding which was intended as the top section of the mast of a 49er racing dinghy, as we had some with damaged ends at work so mine were free! A practical alternative would be either 50mm (2in) diameter aluminium tube or 50mm (2in) diameter solid wood (which would be rather heavy), tapered to 40mm (1.5in) at the peak.
The 0 deg datum for the gaff is the forward/top face, looking from the top/aft end. The fittings on the gaff (from the top downwards) are:
|
Item |
Fitting |
Orientation |
Description |
|
Sail lashing eye (peak) |
Eye |
180 deg |
Could be replaced with a hole at 90/270 deg in a wooden gaff. |
|
Halyard eyes |
2 eyes |
180 deg |
Two alternative eyes are used to fix the end of the halyard to the gaff. The forward/lower one (ie, at 890mm) is normally used and the aft/upper one could be left off. |
|
Snotter eyes |
2 eyes |
270 deg |
The snotter is tied round the gaff - these eyes just stop the snotter from slipping along the gaff. |
|
Sail lashing eye (throat) |
Eye |
180 deg |
Could be replaced with a hole at 90/270 deg in a wooden gaff. |
The boom on my scow is made from the same carbon-fibre moulding as the gaff. A practical alternative would be either 40mm (1.5in) diameter aluminium tube or 40mm (1.5in) diameter solid wood (which would be rather heavy).
The 0 deg datum for the boom is the top face, looking from the aft end. The fittings on the boom (from aft forwards) are:
|
Item |
Fitting |
Orientation |
Description |
|
Mainsheet strop eye |
Eye |
0 deg |
Used to hold the strop for the mainsheet block in place. |
|
Sail lashing eye (clew) |
Eye |
0 deg |
Could be replaced with a hole at 90/270 deg in a wooden boom. |
|
Reefed lashing eye (clew) |
Eye |
0 deg |
A second lashing eye is required to reef the sail, as the length of the foot of the sail when reefed is much less than normal. |
|
Kicking strap eye |
Eye |
0 deg |
Required in wooden boom. |
|
Snotter eyes |
2 eyes |
270 deg |
The snotter is tied round the boom - these eyes just stop the snotter from slipping along the boom. |
|
Tack line eye |
Eye |
0 deg |
Required in wooden boom. |
|
Sail lashing eye (tack) |
Eye |
0 deg |
Could be replaced with a hole at 90/270 deg in a wooden boom. The eye, or hole, needs to be large enough to accommodate 3-4 turns of 3mm cord normally, and twice this amount when the sail is reefed, as well as the tack line - either use a large eye or fit a second eye aft of the first for the tack line. |
Some of the description above may be easier to understand if you look at a photo of the top of the mast and gaff
here, and a photo of the bottom of the mast and boom here. Back to design and construction