Blocks and stuff

Two years now since I first bought the plans and although I'm about £3000 poorer, I do have what is recognisably a boat in my back garden. I've also learned a great deal on the job and have an increased - and possibly ill founded - confidence in introducing modifications as work proceeds.

The centreplate has now been fitted and removed several times whilst I played about with various pulleys systems for operating it. The plate weights 50 kg and, due to the length of the lifting arm, requires a force of 63 kg to lift it: a couple of triple blocks reduces this to about 19 kg - manageable.

Now this pulley business turned out to be more complicated than I realised. First of all, the mechanical advantage of a pulley system (I think nautical people call it the 'fetch') can be seen from the number of lines leading to and from the moving block, so that a single moving block has two lines (fetch of 2 x) unless it has a becket when the mechanical advantage becomes 3 x. This, however, is only the theoretical advantage as each pulley rotates against friction: using a Barton single block I found by experimenting that about 8% of the force applied was used to overcome friction = 92% efficient. But, this friction goes up as the mass to be lifted increases and down once the pulley starts rolling. Also, ever wonder why some blocks have bigger pulleys than others? It's not to do so much with breaking strain but with the fact that the force needed to overcome pulley friction is inversely proportional to the diameter of the pulley.

With a couple of triple blocks (with the becket on the fixed block) I had a theoretical mechanical advantage of 6 x but with each of the six pulleys needing about 10% of the force applied to counter friction. The formula for calculating the force, F, needed to shift mass M therefore is given by:

 - where f = the % of force used to overcome friction per pulley (so 10% would be 0.1), n = the number of pulleys and m = the mechanical advantage

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It gets worse! Because the tension in each of the lines feeding to and from the pulleys is different (by the amount needed to overcome friction) one block will tend to twist in relation to the other. This results in the tension in each line being applied at a slight angle (reducing its effect a little) and can also lead to one or more of the lines fouling one or more of the others - particularly as the blocks approach close to each other. (Fiddler blocks avoid this tendancy to twist since the pulleys are not side by side.) To reduce this possibility, some thought needs to be given to how the lines are fed through the blocks (reeving), some examples from Harken and Barton being shown below.

A related consideration is the size of the line used; I'm not sure why but a 3 mm line seemed to require less force than a 4 mm which, in turn, seemed more efficient than a 6 mm - possibly the chances of one line fouling an adjacent one are reduced with smaller lines. Of course, it needs to be borne in mind that the likely tension in a line shouldn't exceed about 15% of the breaking strain (important for shrouds) or about 25% where tensions are less likely to vary a lot (like my centre plate).

It sounds just about impossible to arrive at an accurate estimate of line tensions in pulley systems - I mean you really have to admire those Vikings - but, somewhat to my surprise, the eventual force needed to raise my centreplate was just over 20 kg - near enough to the theoretical figure of 18.6 kg.

Costs

The current costs at the end of July (£2643) represent the expenditure on the basic coble hull in 9mm ply, together with seating, lockers, floor, and foredeck. It also includes varnish and paint, a battery (£60 - which many builders won't need), an auto bilge pump (£53), anchor (£35) and a 10mm steel centreplate (£150). In other words, a floatable hull can be built for about £2300 - not at all a bad cost considering the price being asked for boats these days. 

Looking ahead, a trailer will cost about £800, a motor £1000 and then the sails and spars will probably account for another £1000. With the addition of ancilliary items, a sum of about £6000 should see a well set up boat ready for sailing. Checking out the prices asked for new boats of comparable size this does seem to represent a considerable saving - perhap as much as a further £6000 - but this is all pretty speculative. I think that in idle moments, I try to persuade myself that it's all worth the time and effort whereas, in reality, it was never to do with saving cash but with the pleasure of construction.

Progress continues to be slow - locker lids are fiddly and coats of varnish need to dry. The centreplate is more massive than suggested on the plans and weighs in at 47kg but the reinforced structure of the centreboard case and the use of bronze bearings to help spread the load, should ensure that no undue strain is produced. To raise the plate a straight force of 53kg is required so will probably need a couple of triple blocks to reduce this pull down to about 9kg. Some experimenting needed here of course. The more immediate problem is dropping the plate into the case and then removing it several times over before final fitting; friends will help once but more than that is asking a bit much - possibly some sort of hoist will have to be built. To have the plate galvanised proves to be pretty expensive and I'm thinking that I'll use the same Hempel 2-part primer that I used on the outer hull - very hard finish and suitable for metal, wood and glass fibre.

The quoted weight for the boat using 6mm ply is 170 kg; using 9mm you'd expect about 255 kg and in fact the current weight - including centreplate is around 300 kg. Even with motor, mast, sails, etc. the final weight should be a bit under 400 kg. 

With the floors down the amount of space is quite impressive - certainly enough for sleeping under a boom tent. This was achieved by setting the side benches/lockers back a bit to follow the hull curve more. The floors rest on rubber strips since the vibration of a motor commonly causes constant irritating, sqeaking and rattling from floors that aren't screwed down.

Finally, having installed a battery to feed the auto bilge pump, it's as well to provide additional power outlets. Hence, the mini switch board.

To maintain my sanity and motivation, I enjoyed a couple of brief jollys recently; one to the trad. boat festival at Port Soy - which seems to have dropped in visitors compared to last year (in spite of the appearance of Iain Oughtred), and one to Lindisfarne where, of course, I spotted several working cobles.

Checking the order of things

Having decided to lengthen the foredeck, it now extends over the fore end of the side benches. In other words, I needed to complete the side benches first. Now I can sympathize with those who opt for simple open benches with perhaps bouyancy bags beneath, but a boat this size is capable of more ambitious journeys, which calls for more storage, which in turn requires lockers rather than simple benches. 

This turned out to be fiddly and slow going, particularly as construction needs to take account of what is likely to go in and through the lockers.  I've planned for an auto bilge pump so that a 19mm tube goes from the keel line up to the gunnel (through a locker), a battery to power the pump is housing in one of the lockers and electric cable has to pass to the pump and to other possible power outlets - for lighting or whatever. But most complicated of all, lockers need access hatches which are preferably water tight - which is, of course, impossible. My hatch seal design may or may not work - we'll see. (Voice from the future: it didn't work and the hatches needed to be replaced - hgh!)

 
The lockers that abut the transom are much larger and hatch covers may be a problem - I may be tempted by ready made ones - haven't decided yet.


Flooring comes next but first a couple of coats of bildge and locker paint should make the inner hull look a whole lot smarter. From the photos it may seem that I work in an organised and uncluttered fashion - huh! - far from it. Most of the time, every horizontal surface is covered with tools and materials, sawdust and shavings - I think of it as creative chaos!

Completed this year?? - no chance

There really hasn't been a lot of progress over this last winter - due partly to low temperatures and partly to equally low finances. Now at least it's getting warmer but a Robin’s nest has appeared on one of my tool boxes. Last year it was Swallows.

Reading other boat building blogs, I've only rarely seen mention of running out of cash, but the steady drain on financial resources needs to be anticipated by anyone planning this sort of project. I've increased my monthly expenditure from £150 to £200 but this is about at my limit. To make things worse, the costs will clearly increase as the boat nears completion: several steel parts need to be made up and galvanised, and then there will be paints and varnishes, rigging and tackle, sails, trailer, motor, and all that assorted sailing junk beloved of most sailors. I just try not to think about it . . . .

Recent weeks have been devoted to the centreboard case - strengthened to take a considerably heavier 12mm steel centreplate in place of the 9mm ply one shown in the plans. So 12mm sides are used with hardwood reinforcement about the pivot point. The pivot bolt is increased from 12 to 20mm and additionally rests in 28mm bronze bearings set into both the case sides and the centreplate. Before assembling the case, it's as well to make a 12mm ply pattern of the centreplate to ensure its fit and movement. This pattern can also be used to calculate the weight of the eventual steel plate. To do this, weigh the wooden pattern and then weigh, say, a quarter square metre of the same thickness ply: then divide the first weight by the second weight to derive the area of the plate and then its volume. Multiply by the density of mild steel (about 490lbs / cu. ft. or 7.85gms / cc) and, for this boat, you'll get something in the region of 35kg. When vertical (dropped) the plate will be relatively easy to move up through, say, the first 15 degrees or so, but, as it approaches the horizontal, the force on the lifting arm of the plate can be 2 - 3x the plate's weight, depending on the direction of pull. In other words, lifting gear must be used to reduce this about 4 times (two double blocks, one with a becket should do it but a drum winch might be ok). To prevent the raised plate from bouncing up and down a keeper bolt towards the aft end of the case is also advisable.

As can be seen the floor bearers are deep and substantial providing extra support to the case. Additionally, a length of 20 x 75mm floor will be bonded to both the bearers and the case on both sides of the case to form a rigid backbone at this point.

Elsewhere, the extension aft of the foredeck made it seem advisable to build in a stowage locker for anchor, chain, warps, etc. – might look quite nice or might just look silly . . . .

I remain totally absorbed and excited by this project and only dimly recall the days when I had a sort of social life . . . .