What could Dyneema do for you?

(download this article)

 When it comes to boats and rigging, we are surely no cleverer than our predecessors. What we have is better materials. Now if you subscribe to the view that classic boats result from a process of evolution (and also represent the leading edge of their day), then it makes sense to make best use of these materials. One obstacle is they are often introduced in such a fog of marketing hype, techno-babble and endorsements by handsomely paid celebrities that it can be hard to work out if they are actually any use. So here is my attempt to see how Dyneema, or Dyneema based rope might be used in traditional rigs. 

Rope makers seem to delight in confusing us with science and/or extraordinary polysyllabic names. Let me try to cut through the guff a little.

Using Wykeham-Martin Furling gears - an Unofficial Guide

Reefing versus Furling Gears

Buoyancy - a little guidance to keep you afloat

Let us embark on the debate between buoyancy bags and buoyancy tanks for open boats. I’m not sure how this will be affected by current EU legislation for small craft, if only because no-one seems clear about what that legislation means in practice.

Buoyancy bags have the following advantages:

  • they are lighter

  • it is easy to see if they are leaking

  • it is easier to maintain the hull structure

  • depending on the design, they are cheaper

  • the amount of buoyancy is easily adjusted.

On the other hand.....

  • you can’t stow anything in them

  • they are fiddly to install

  • they don’t fill all the space available for buoyancy because of their rounded corners

I think that is the major points covered. I prefer to see at least some bags because it allows you to adjust the amount of buoyancy. If you have a boat where a capsize is, if not commonplace, at least a foreseeable event, too much buoyancy can have undesirable consequences should the worst happen, for example:

  • if the wind is blowing on the bottom surface of the hull, it is driven onto the rig, itself angled downwards because the hull is floating too high. This increases your chances of turning turtle. As you watch the plate disappear back into its trunk, you can muse on the vicissitudes of life. Or decide who is to go under the boat to push the plate out again.

  • if the boat doesn’t turn turtle, then it tends to twiddle round such that the hull drifts to leeward. Right the boat and it could re-capsize onto you when the wind catches the sails.

  • at best, the centreplate will be high off the water. You are cold and tired and have the job of heaving yourself up to the plate before you start. Bad news.

At a first approximation, there should be buoyancy equal to about 2.5 times the weight of the boat and gear (not crew). Then select a nice warm, reasonably calm day to do some controlled tests to get your buoyancy right. Apart from the fact that you’ll probably be surprised how difficult it is to capsize the boat in the first place, you will confirm that you are able to right the craft, and be able to check that the plate is at or just above the water-line. When righted, it is helpful if the boat does not continue to fill through the centreplate case. Perhaps the most important benefit from this sort of experiment - and/or its ensuing adjustments - will be that you will be less worried about a real capsize. After all, you can’t love a boat you fear.

If you have or require tanks, then proprietary hatch covers are good for sealing them as long as the holes are big enough for the passage of articles for storage. If not, you’ll have to devise your own with bits of rubber and lips and catches and so on. Strive for simplicity here.

 (download this as a .pdf file)

 

A note on units

Like many in their prime of life (?), I think in feet and work in metres, having lived through the conversion from “British” to S.I. units. For those of you not obliged to convert, a few words of explanation might be a good idea, using round number conversions.

 

1 kg (mass) = 2.2 lb (mass). Mass is the amount of matter in a thing, whose weight is determined by whatever the acceleration happens to be at the time. Stood still, on Earth, the prevailing acceleration is due to gravity which is about 10 metres per second per second. So the force - or weight - of that 1 kg (mass) is 10 Newtons (N), or 1 kgf. Now kilogram force (kgf) isn’t these days a proper unit to use, but is more convenient to visualise than Newtons. So one refers to 1300kg breaking load, where one should properly say 1300kgf or 13000N. For those who can’t stand kg or N, it so happens that 1 ton - the 2240 lb type - is as near as makes no odds 1 tonne - the 1000 kg type - and the force aspect of things is usually implicit in commonly expressed weights, loads etc. By a neat irony an apple weighs about 1 Newton!

Pressures or stresses are forces applied over an area, such as tonf per square inch, Newtons per square metre (referred to as a Pascal (Pa)), or whatever. Because many of the things we are dealing with are millimetre sized - like wire and bolts and so on - N/mm2 is the handiest even though it’s not the pukkah unit. It also happens to be the same numerically as megapascals - MPa - which is what most material properties are specified in. So 20 MPa = 20 N/mm2. But use either, and you will need to divide by 10 to get to kgf, and a further 1000 to get to tonnes. The example here should confuse the issue suitably.

 

 

S.I.

“handy”

“British”

 

Max allowable stress

500 MPa

500 N/mm2

32 tsi

 

Size of plate

.025m x .008m

25mm x 8mm

1” x 5/16”

 

Area of plate

2 x 10-4 m2

200 mm2

5/16 in2

 

Max load

1 x 105 N

100,000 N or 10,000 kgf or 10 tonnes

10 tonf

 

A note on gauges etc.

There are moments when you wish Napoleon had been a bit luckier, and this is one of them.

Woodscrews are gauged by screw gauge - listed below - where the bigger the number, the bigger the screw

 

Wood Screw Data

Nominal DiameterClearancePilot Hole
GuageDecimal inchmmHoleHardwoodSoftwoodMetal

2

.082

2.08

3/32

1/16

-

-

3

.094

2.39

3/32

1/16

-

-

4

.108

2.74

7/64

1/16

-

5/64

5

.122

3.10

1/8

5/64

-

-

6

.136

3.45

9/64

3/32

1/16

3/32

7

.150

3.81

5/32

3/32

1/16

-

8

.164

4.17

11/64

3/32

5/64

7/64”or 1/8

9

.178

4.52

3/16

7/64

5/64

-

10

.192

4.88

13/64

7/64

5/64

9/64”or5/32

12

.220

5.59

15/64

1/8

3/32

11/64”or3/16

14

.248

6.30

1/4

9/64

7/64

13/64”or7/32

16

.276

7.01

9/32

5/32

1/8

-

18

.304

7.72

5/16

11/64

1/8

-

20

.332

8.43

11/32

3/16

9/64

-

Copper nails, Gripfast nails and metal sheet are dimensioned by wire gauge, where the bigger the number the thinner the nail or sheet.

 

Wire Gauges

16g 1.626mm     10g 3.251mm
14g 2.032mm     9g 3.658mm
13g 2.337mm      8g 4.064mm
12g 2.642mm     6g 4.877mm
11g 2.946mm     4g  5.893mm

 

Pilot Holes for nails

 GripfastBoat nails
Gauge HardwoodSoftwood

14

1/16

1/16

3/64

13

-

5/64

1/16

12

1/16

5/64

1/16

11

-

3/32

5/64

10

5/64

3/32

5/64

9

-

3/32

5/64

8

3/32

7/64

3/32

6

-

1/8

7/64

 

Thread Comparisons - Bolts and machine screws

MetricImperial
ISO metricTPIPitch (mm)UNCWhit.UNFInch dia

M3 (0.118”)

51

0.5

 

-

-

-

-

M4 (0.158”)

36.3

0.7

 

-

-

-

-

M5 (0.197”)

31.8

0.8

 

24

24

-

3/16”(0.187)

M6 (0.236”)

25.4

1.0

 

20

20

28

1/4”(0.250)

M8 (0.315”)

20.3

1.25

 

18

18

24

5/16”(0.312)

M10 (0.394”)

16.9

1.5

 

16

16

24

3/8”(0.375)

M12 (0.472”)

14.5

1.75

 

13

12

20

1/2”(0.500)

M16 (0.630”)

12.7

2.0

 

11

11

18

5/8”(0.625)