Thursday, June 05, 2008

An alternative solid mast for the OZ PDRacer

The OZ hollow mast is the bee's knees - the nicest mast we can come up with at this stage. A simple long box with the faces made of 4 planks 12mm (1/2") thick with a taper.

Easy enough to build about 35% to 40% lighter than a solid mast. Making the mast out of a number of pieces of timber also reduces the risk of a hidden defect in one of the mast planks bringing the lot tumbling down.

HOWEVER ... many people want to get the boat on the water quickly. I have drawn up a solid mast that will have the same bending characteristics as the OZ so will suit the sailshape that comes with the plans to give acceleration and nice handling when the wind gets strong.

The piece of timber you need will have to be a minimum of 60mm square (just under 2 1/2" square). This might mean that you have to glue it up out of two pieces to get the width. I would recommend the timber be free from knots or major grain deviations and be around the density of oregon (fir) or pine (Hoop or good Radiata (Monterey Pine in the US).

It does require that you consider some hull modifications with the mast partner - READ ALL OF THIS ARTICLE!!!

You can click on the image on the right to get a better view. You simply have to look at the mast design in your OZ plans and cross out the numbers on the drawing that match the measures in the first column and replace each one with the number in the second column.

This works for both the masts for both the lug and sprit rigs - just replace the numbers with the corresponding ones in the second column. The third column indicates how much extra weight you are putting up with at each height by choosing a solid mast. eg at the 62mm height you are carrying an extra 48% of weight. This is the downside.

The mast base can be made the same way as the plan - but make sure it finishes the same width as the original hollow mast at the tip using the same method.

The partner opens up a bit of a question:
1/ is it worth using the standard partner so that a hollow mast can be used later if desired or needed. This can be done by bulk up the sides of the mast by wrapping several layers of glass cloth (you will need around 4 to 6 layers and it only needs to be about 50mm wide (2"). Or you could use some thin bits of timber or some other method, or ...

2/ making the standard partner a couple of millimetres narrower widthways and fore and aft? You can leave the back block in the mast at the designed position but move the forward one back the 2mm needed.

I think I would choose the first option just for the flexibility of use it offers.

It is important not to just put the new narrower mast in the old partner thinking a couple of millimetres won't matter. It might as it is good if there is not too much slack. The risks are more water getting in the front tank - mast moving more when the boat is tacked with more wear and tear on the mast and partner.

If you are a big person and plan to sail the boat hard - with lots of leaning out to keep the boat flat then I would strongly consider adding a couple of extra millimetres to the bottom half of the solid mast.

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