Bosch battery-powered strimmer jam

Haven’t seen this before – something of a design problem with our 36V Li battery powered strimmer. It worked very well – sometimes lacking cutting ability on thicker stuff but overall much better than a petrol strimmer. Then it jammed solid.

Dismantling it found this:

Grass-packed bearing
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Initial thoughts on reinforcing the partners

The Wanderer, in common with most sailing boats, has a stayed mast. This means that there are wires that brace the mast to hold it vertical.

Note that this isn’t quite true. A careful look at the mast and wires shows that the section under the spreaders is unbraced by the wires. The shroud wires only really brace the top section above the spreaders. This suggests that the force of the sail in the lower section is actually taken by the mast pushing against the sides of the deck slot.

Larger boats normally have shrouds to triangulate the spreaders, then longer shrouds that pass through the spreader ends to the top of the mast. This boat, being a dinghy, doesn’t have this triangulation.

The deck seems fairly thick (around 6mm) presumably so it can take the weight of someone sitting on it.

All this implies that the existing deck slot is capable of taking substantial loads without modification. However, for peace-of-mind some strengthening is a good idea.

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Free standing aluminium mast – design tweaks and cutting the tubes

Right – time for a deep breath. This is where the designs become reality. First stage is check the design to get it as good as possible.

Mast head

The topmast tube needs to be extended a bit for the mast head fitting. The length for the stress calculations remains the same – 5.6m – but the overall length is a bit longer to allow for the masthead bung and the loops for the masthead blocks

Topmast

The main 2 3/4″ x 10swg tube is heavy when 5m long. To make the mast lighter where it counts I tweaked the topmast length and made it a bit longer and the main tube shorter. The trade off is that this reduces the strength at the base of the topmast. This should be ok as some of the load is distributed over the length of the mast plus as soon as the sail is reefed the load in the topmast decreases dramatically.

Mast foot

I’ve tweaked the design of the foot. The main tube (2 3/4″ x 10swg) will go all the way down to the heel fitting. The strengthener will stop a bit above the heel fitting. Theoretically the strengthener could stop at 395mm above the heel, but in practice I’m going to take it down to 100mm above the heel. The primary reason is that I want lots of strength at the mast pivot point.

Mast pivot

The Wanderer features a tabernacle with a pivot that makes putting the mast up much easier – a one-person job. Given that it will just be me on my own putting the mast up it makes sense to keep this feature. The similar GP14 without a pivot requires two people to put the mast up safely.

The issue is where to put the pivot on the mast tube. Drilling a hole through the tube is by far the simplest route but will weaken the mast in a highly stressed location. Ideally the pivot should be clamped to the mast but this raises other issues:

  • The mast might hit bits of boat as it goes up and down. Clearance in the hull was designed for a centre-pivot.
  • It is a lot more work unless I can think of a clever way to do it.

I’m going to reinforce the mast at the pivot point with some internal Scots Pine. This shouldn’t be too hard to do and means if I need to drill right through it should be strong enough. I need to have a play with a section of tube in the boat to find out how it will all fit together.

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Attaching the mast to the boat – thoughts

I’ve been giving some thought to how to attach the mast to the boat. There are a number of objectives:

  • Must be strong enough
  • Must be light
  • Should not change the boat too much – ideally the boat should appear to be standard
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Aluminium free-standing mast – some details

Now I’ve got two tubes that will fit together here are some thoughts about some of the mast details:

Overall design

Since I’ve got two tubes that fit together I’ll use them, rather than try to use wood.

Using the graphic method to find the lengths we get this:

Strength vs height for our aluminium mast
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Free-standing mast: Aluminium tubes delivered

Mast, yard and battens
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Wanderer aft cleats

One thing about sailing with small children is that you want to tie up to a jetty if at all possible. This means having usable cleats. It is possible to tie off to the traveller but having cleats is easier.

However, I don’t want the cleats to catch on the mainsheet or other ropes, This means they need to be somewhere out of the way. The best option appears to be to feed the rope through the handhold at the back of the side decks and forwards onto a cleat under the side deck.

I made up two plywood panels – one for each side – and attached the cleats using T-nuts. I’ve also added some spare T-nuts for other attachments, one spare cleat (it is always useful to be able to tie things on) and a clamcleat that will take string threaded through the scupper on the transom – this can be used for a foothold rope or for attaching the tent.

Reverse of the panel with T-nuts fitted. The T-nut recesses were sealed with thinned varnish.
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Graphical method for calculating mast section length

In previous posts I used a table for working out how long mast sections (main tube, strengthener, topmast) should be. Here’s a graphical way to do the same thing. It has the advantage of greater precision plus it is hopefully easier to see what is going on.

Mast strength vs height
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Wanderer centreboard replacement

There is definitely a trick to getting the centreboard in or out while the boat is on its trailer. So before I forget (and to remind me next time) this is how it went back in…

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Boat trailer suspension replacement – update

Following on from my previous post I’ve added angle brackets to the top of the clamps.

Angle brackets for suspension unit clamps

The plates were bending under clamping load, so these brackets allow much better and more reliable clamping. I’ve also painted the welds.

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