Technical Post - carbon fiber snatch block

How I made a carbon fiber snatch block because I wanted a snatch block but manufactured ones are heavy and expensive.

- make a mold shape with an angled step. The step height should be equal to the 1 washer thickness + 1/2 the sheave thickness.  (the sheave will have a s.s. washer on the inside between the two halves.)  You can see the angled step shape with the shadow on it.

- cover the mold with mylar packing tape - best mold release ever

- mark the mold with the size of finished piece - about 2x sheave diameter for the width and 4x sheave diameter for the length

- cut your pieces of carbon fiber cloth and unidirectional cloth. Use 1 layer of fiberglass on the inside layer to prevent corrosion where the s.s. washer would otherwise rub on the carbon fiber.  The small pieces of carbon fiber are to add thickness where the sheave axle goes through.  The twill is the top layer for looks

I'm not going to spell out the laminate schedule I used.  If you've used carbon fiber before you will know how it's a bit harder to wet out for an amateur and I don't want to give everybody the impression that they will make a suitable block the first time using carbon.  Composite properties depend very much on the skill of the laminator. If you are a so-so laminator the strength of the part might be as low as 1/3 the strength of a good laminator.  For such a safety related part, I don't feel like being responsible for the design for others to copy.

- wet out the fabric with epoxy resin, cover with clear plastic and squeegee hard.

- release the part from the mold (gentle tug will get it off)

- cut part into 2 halves, round the corners

- drill a hole for the axle and for the soft shackle. Soft shackle hole has beveled edges with a countersink bit to remove sharp corners.  Soft shackle is 5/32" Amsteel blue which should have a breaking strength > 4000 lbs.
- assemble and test

- total cost less than $15 (though I had the roller sheave).  If you don't have a suitable sheave a fixed plastic sheave with 1/2" bronze bushing would be suitable and costs about $15.  Bigger axle spreads the load better and reduces bearing stress on the carbon.

 

How it looks in use

On a budget and can't afford a marine snatch block? Check out a rock climbing rescue pulley.  You'll need a carbiner but quite cheap and strong.

http://www.amazon.com/Petzl-Rescue-Pulley-Red-Black/dp/B000BH956C

-Evan

Our Wild Life

I don’t know if it was the possum in the shower or the ducks at our door that made me realize it, but despite living on the edge of a big city we’re still firmly linked to the wild world around us.

 

we don't take cameras in the shower normally (though we may start) but this is a water dragon.

For the record, possums are not normally found in our shower. Nor are water dragons…
(Though enormous freaky spiders are…) But both creatures somehow showed up in the shower stalls in recent weeks and needed to be caught and released. The water dragon was first. Looking like a squirrel (except we don’t have squirrels…) in the dim corner of the shower it scurried away (and right into Maia) when I tried to get a closer look (actually, when I screeched…).

We chased it for a bit and realized there was no way it could get back out the vent, where it had most likely come in from, without assistance. So we decided to catch it and set it free. Water dragons bite—we’ve seen them tussle with the ibis in the park and the giant birds don’t win. But this one was little and it seemed very sad about being chased around the shower so after cornering him, and promising we were there to help, I grabbed him behind his shoulders and set him back out into the wild.

 

typically this is about all we see of a possum

Possums are much bigger than water dragons. And Maia found the possum just after getting over the water dragon, when she was finally willing to go to the showers alone again. She quickly came back out and told Saskia, who told Zack (another boater). So the trio decided that the possum would be happier if he wasn’t in the shower and successfully rescued the old guy and set him free. And Maia decided she shower some other day…

But between the creatures and the fact our shower is flood-prone, and often looks like a bio-hazard, Maia doesn’t really want to bathe anymore, ever. It brings back a memory of traveling down the US west coast. Expect with more creatures…

 

our handsome neighbour

Not all our interactions with the wildlife are unsettling though—we have a huge pelican for a neighbour, and the kookaburras to wake us, and Maia has a gaggle of ducks who have been visiting her since they were ducklings. Initially they’d wait patiently outside the boat for her. But then they learned to climb aboard the dinghy so they could quack through a window for her. Most recently they’ve been climbing aboard and waddling up to the door. We’re not sure if it’s because she’s been slow to respond to their visits or if they are tired of competing with the catfish for the food Maia gives them.

Yesterday when Maia fed her ducks the catfish rushed the surface, bit the duck's foot and held on. Okay, so maybe we’re not in the midst of an exotic sailing adventure but you’ve got to admit its all pretty wild.

Our resident flying foxes

It's Waterproof Right? Buyer Beware...

I was checking out some handheld VHFs recently and their specifications made me think of how waterproof they really were. When buying marine gear, one buyer beware specification is "waterproof".  That word can mean different things to different manufacturers (and their marketing department).  Others to watch out for are “submersible,” “splash-proof,” “drip-resistant,” “watertight”, and "not warranted against water damage".

Here's a quick guide to all those standards:

1)  waterproof - may withstand a heavy dew.  Meaningless without a standard to apply to it.


2)  IP ?? - the "Ingress Protection" rating.  IP Code  Often you will see these quoted as IP67 or something similar.  This is a solid standard if the manufacturer is quoting it, but make sure the rating is high enough for what you need.

The first digit is resistance to solid particles like fingers, marbles or dust getting in the equipment.  If the first digit is an X it means 'we didn't test for solid objects or we don't care'  Not very applicable to sailors. 

The second digit relates to how water resistant the item is. These include:

IP X5 - gently wetting it down with a garden hose.  Might be ok for something you keep inside but forget about real world water resistance in a cockpit.

You want IP X6 as a minimum.  This is "Water projected in powerful jets (12.5 mm nozzle) against the enclosure from any direction shall have no harmful effects. Test duration: at least 3 minutes

Water volume: 100 litres per minute.  Pressure: 100 kPa at distance of 3 m".  So basically blasting it with a powerful hose from 3m away.

IP X7  - immersion up to 1m for 30 minutes.  For a handheld VHF that might get dunked in the bottom of a dinghy this might be a good standard to look for

IP X8 - depths > 1m.  Usually specified by manufacturer.  Not very common in recreational marine equipment but quite good if you can get it.


3)  CFR - this usually is a reference to the US Code of Federal Regulation.  CFR 46 is a huge volume so just saying 'meets CFR 46' is very vague.  Raymarine is bad at using this as a specification though their newer equipment is starting to use IP X6 (displays and instruments).  Their current VHF say IPX7 (submersible) - anybody want to try?

46 CFR 110.15 gives a few definitions

Waterproof means watertight; except that, moisture within or leakage into the enclosure is allowed if it does not interfere with the operation of the equipment enclosed. In the case of a generator or motor enclosure, waterproof means watertight; except that, leakage around the shaft may occur if the leakage is prevented from entering the oil reservoir and the enclosure provides for automatic drainage.

Watertight means enclosed so that equipment meets at least a NEMA 250 Type 4 or 4X or an IEC 60529 IP 56 rating

So just saying CFR 46 doesn't really say anything unless you say 'waterproof to CFR 46' or similar language.


5)  JIS - A Japanese standard that ICOM uses frequently

JIS "4"	Splashing water from any direction shall have no harmful effect (Splash resistant)
JIS "5"	Direct jetting water from any direction shall have no harmful effect (Jet resistant)
JIS "6"	Direct jetting water from any direction shall not enter the enclosure (Water tight)
JIS "7"	Water shall not enter the enclosure when it is immersed in water under defined conditions (Immersion resistant)

 Again - JIS 4 is hopeless if you want to keep the water out.  JIS 5 is a bare minimum.

Here's a good object lesson:

Cobra Handheld VHF

Key Features

• 100% waterproof (JIS-4)
  
  But it's not waterproof as you or I understand it.  It's splash proof.  IIS-4 is a very low standard as you can see above.
  
  
   

The To-Do list

I stole this idea from Brian, project manager supremo on s/v Delos

Because we're a light weight catamaran we use smaller Post it notes however.  It should give a nice sense of accomplishment as stuff is removed from the list. 

- Evan