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lenm

Ocracoke 20 in OZ

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A disclaimer to everyone;  I am in no way suggesting there is any issue with the laminate designs from B&B for this or any of their models.  They know far more than I do about all of this.  I am merely suggesting that if you run in debris laden waters or tend to interact with the bottom of the waterway more than most and feel more comfortable with a bit more glass on the bottom that this may be one solution.

 

Lenm,

 

The drawing below is the most efficient way I have found to accomplish the laminating if you want two layers of glass on the bottom.  It eliminates all the steps in the glass except for the one on the topsides above the chine.  Fairing compound and a bit of sanding and that goes away.  On the smaller hulls there may be no need for laminate 6 and 7 depending on the skin girth at maximum beam and the width of the textile that you have chosen.  Even if the strip is needed, it's not the full length of the hull and can be tapered out to minimize fairing.  Glass scarfs should be at least 12 to 1, so .037 for 1208 or .444".  I am not comfortable with that and find it quite easy to extend the taper to 1.25 or more.

 

The biggest problem I have seen with glassing the bottom is how do you transition the glass strip from the straight keel to the forefoot.  As it (the textile) bends down the forefoot you begin to gather a lot of extra material at the edges.  In most cases people will solve it with darts, I suggest against this.  With any of the biax products that you'll be choosing for your build, you can work them down and flat with a little poking and squeegeeing.  Perhaps not with 30" of tail hanging over, but with a properly tailored piece, it's very doable.

 

Estimating the bottom size of the OK20 at 16' (water line length) x 3.5' , subtracting 6" overlap on keel and chine and ignoring the taper at the forefoot  yields a 16x2.5 area of the bottom (half) that would have added glass.  This is 4.44 yards at 20 oz per (1208) or 5.55 lbs of glass.  Add resin at a 60/40 ratio is 8.33 for a total of 13.88 lbs per side weight penalty for this change.  This is a conservative number because of the tapering bottom size forward that I ignored and hopefully you can squeegee your way to a better resin to glass ratio.  A lot of folks "in the know" will say that the biggest mistake a builder can make is second guessing the architect and beefing things up, so you'll need to evaluate the benefits of this modification for your own operating conditions.

 

Here's a visual.  The shape and size of the drawing is not in any way to scale.

 

 

Laminate diagram

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Many thanks smccormick for the diagram - that is most helpful to me!

Your method looks very well though out - some great ideas there.

 

Re 'overbuilding' - I agree with your comments.

I think it is human nature to want to beef things up a bit, however, in reality all the r&d has already been done by a good designer.

'Just follow the plans' has been my ethos for this project.

My impact tests confirm (for me personally) that a layer of 12oz fabric over the ply is ample and comparable to other composites/sandwiches which I am more familiar with

However (like you) I'd like a little more abrasion resistance on the bottom, and personally, something a little more subtle than a keel strip/guard.

Murphy's law - if there is a wife or buddy involved at the boat ramp (launching) they are bound to scrape the bottom of the boat on the rocks in an attempt to avoid getting their feet wet.

Over here in Australia, we have some rough 'corrugated' dirt roads that run for miles out to remote fishing spots. They test trailers and tyres and rattle suspension to death.  I have seen a boat 'rubbing' on its trailer, the bottom of it wearing right through its polyester gel coat where the support pads are.

I'll swallow the weight penalty and gain it back somewhere else on the boat.

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On 22 January 2018 at 12:42 AM, PAR said:

Do you have an approximate amount of force need to fully break each sample? Though the plywood was the thinnest, I suspect it took the most to bring to failure, with the H-80 a distant second?

Hi PAR,

My gut feeling is that it was around 30% stronger than the others samples to shear.

It's shear seemed a bit more dramatic than the other two samples. Once the fibres 'roll' it really lets go suddenly.

I think a piece of ply in a double diagonal configuration would be a further improvement.

The balsa was interesting. In particular the way the underside side skin looked 'punched' outwards, and delaminated after the initial impact.

This was 'end grain' balsa, perhaps the grain orientation was transferring more impact/load to the underside skin? i.e less energy absorption?

 

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Thanks again to those who offered advise re glassing - great help.

The final layer went down today and it all turned out superb!

I've glassed heaps of surfboards, windsurfers etc in my time but nothing ever this big!

Solo that is.

If anyone is proposing to undertake a 20 foot + boat, solo (and is inexperienced in glassing) I'd like to offer a few observations/reiterate earlier advise by others..

- Get the slowest harder you can. E.g 2hr pot life with an 8hr open time.

- Use a roller for the bottom. One with a long handle.  Makes it easy to lap glass over keel line.

- 17oz glass or lighter was easy to apply. 22oz is a lot of work to wet out unless you have help/someone working ahead of you.

- Mixing up batches of epoxy around 1kg quantity at a time was nice and manageable.

 

20180317_091923.jpg

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You are looking good there sir. I am right about there myself and starting to wonder what is the best glass for the hull. Allen did tell me but i failed to write it down( typical me) What did you go with. I seem to remember Allen saying Biax 1708 but for the life of me cannot remember. I like you also like to over build a little for some reason. 

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Thanks

Due to availability reasons (local supplier) went with 1700 and 2100 variants (no mat).

Need to be careful when rolling out though as the mat has an advantage of 'holding' the fabric together (as some people suggested earlier).

Make sure you meet the 30oz glass recommended for the chine joints.

 

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Yea, I'm liking the colour - Im thinking mid 60's Ford GT40 colours  - Awlgrip sky blue topsides, burnt orange antifoul and teak trimming/deck. 

le-mans-icon-ford-gt40.jpg

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