Jump to content

Recommended Posts

PF, That pic pretty much shows the same story - the cracks are where the hull is struggling (and now failing) to support the actual loadings. Shows Martin's analysis to be pretty damn good :-)

I'll leave it to some of the others with better layup design knowledge than me to recommend exactly which cloth, orientation, and how many layers, but it does not look like a difficult repair/upgrade to me.

Share this post


Link to post
Share on other sites

It is easy to impress by using computer generated graphics :-)

 

I think it does a good job of illustrating where potential problem areas are. Unfortunately it does not offer a way forward to determining proper laminates. The reason is I don't have tools for modelling composites - I have used homogenous, isotropic materials and composites are layered and orthotropic. What I miss is something like this:

 

http://www.ansys.com/Products/Simulation+Technology/Structural+Analysis/Structural+Technology+Leadership/Technology+Tips/Efficient+Workflow+for+the+Design+of+Thin+Composites+Structures

 

It is possibe to make do without this helper add-on by 'drawing' the layers and creating the local coordinate systems manuallly but it would be very time consuming.

 

But all hope is not lost. Back in 1986 I engneered both boards and hull reinforcements using pen, paper and pocket calculator(*). I actually found my notes the other day and have scanned them for your enjoyment.

Structural_engineering_of_daggerboards_in_1986.pdf

 

The engineering assumptions are crude but they worked. The boat is 30 years old and we have had no problems with boards or hulls. The original boards where all glass/epoxy and weighed 15 kg a piece. I built deeper and slightly thinner boards in 1995. Using carbon in the main spar of the boards the new ones weighs 13 kg a piece.

 

I still stick to the same basic engineering principles with few improvements related to better understanding of fatigue limits of composites. See http://hem.bredband.net/b262106/Boat/dagger.html and the documents linked to this page.

 

I will try to write an explaination to the hull exit reinforcement engineering of 1986 asap, maybe this weekend. It isn't exactly 'rocket science' so it shouldn't be too hard to grasp.

 

/Martin

*) Martyn Smith once told me he used those tools and no other to engineer the composite structure of Formule TAG.

Share this post


Link to post
Share on other sites

Here is the 1986 calculations explained in English. This is kind of brief but I hope it makes sense.

DaggerBoardLoads.pdf

I tried to attach the LibreOffice spreadsheet as well but that was not permitted.

 

It is my current understanding that real composite experts do not design reiforcements looking like this but it may still be the best option for paxfish. (The presence of the floor calls for a modification in the laminate scheme though.)

 

One thing not addressed here is how to avoid locally crunching the hull laminate.

 

/Martin

Share this post


Link to post
Share on other sites

So,  I'm a do-er. Armed with Martin's excellent analysis and a warm day, I forged ahead.

 

I removed the floor in that area, and any damaged glass.  On the inside, I layered uni, DB and Triax where it seemed appropriate (certainly Martin's presentation was in my head), I also wrapped 150 strands of heavy uni between the back of the case and the hull, wrapping them around fore and aft, and then let the strands splay like outstreched fingers in several directions. This will further support the board on the opposite tack (though there was no apparent problem there.

 

Another layer of glass, replaced the floor, a couple of layers over top.  There is no horizontal beam in addition to the floor as yet.  It doesn't "feel" like it is necessary (sole is sufficient now, with the heavy glass extending from there to the dagger board exit 2cm above it.) I added about 5 pounds of material overall, but removed about half that previously while grinding.  I removed 31 pounds elsewhere and my net will be negative!

 

Once everything cured, I ground out any remaining bad glass on the outside, and will soon re-glass there to clean everything up.   (need a warm day!)

 

Meantime, back at the house, I'm starting to tackle the dagger repair.   Eventually, I will build a mold and make new ones, but for now, I am repairing it.

Share this post


Link to post
Share on other sites

Photos or it didn't happen :-)

 

Here is one I created just before bicycling home today:

Wildfiresektion_med_sticksvärd__stressvektorer_minus_hull4.png

This is for the floor-less case with the board pushing on the left side of the hull opening. Red arrows show material in tension and blue arrows to the left of the hole shows the material is in compression.

 

The red 'ball' is just an indicator showing the centre of rotation when I was manipulating the view.

 

/Martin

Share this post


Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

Loading...

×
×
  • Create New...