Fiberglass and divinycel advice - How to make my dagger boards?

[Privilege]

I have a 44’ Privilege 435 cat with dagger boards...in fact, I have two halves of dagger boards as they are both broken. I need to make some more and would appreciate the advice of anyone with experience. They measure 11’ long, 24” from front to back and 4” thick. Because they are so narrow, it’s not feasible for me to laminate lengths of wood together as the main structure. I therefore intend to make them from divinycel foam and fiberglass.

My question is - should I make them as stiff as possible or build them to allow for a certain amount of flex when under load? I have enclosed a drawing below (please forgive the crudeness). As per the drawing, if I made the main backbone by laminating a few lengths of divinycel the entire length of the board, I could then made ribs from solid glass in a few places either side of the backbone which would presumably make a very stiff structure which I could then wrap in glass. The alternative would be to just laminate the foam for the full board, shave it to shape and then wrap it in a few layers of glass. Presumably this would be a little more flexible than the first method?

Privilege have not been able to offer and advice. I’d appreciate any input.

[Rucksta]

Foam core with solid glass leading & trailing edges (sail battens)
Biaxial and unidirectional lamination for the skins.

Go for stiffness.
A large part of the strength will come from the quality of adhesion between layers.
Primary bonds are preferred i.e. one resin mix.

This is similar to building a Malibu sized surfboard.

Consider engaging a board maker to resin infuse your fabricated and assembled components.

Finished product straight out of the bag.

[Spot]

Pictures of the broken board, thickness of current glassing, and details of the 'socket' on the board would help this discussion. Is there hope for an 'engineered repair' rather than a complete redo?

Too bad the builder is not supporting the repair. A shop drawing for the dagger board with dimensions and notes would be huge for this project.

You will need something stronger than a 'normal' surfboard but something not so beefy that it would cause collateral damage to the boat when things 'go sideways' so to speak.

A shop that is already using CNC for shaping, is familiar with foiling projects, and has glassed 'higher performance' items like racing SUP's or windsurfers (lots of carbon fiber, uni, and other inserts) would probably be better than someone doing things 'old school' with blown PU foam blanks and 1-2 layers of cloth with PE resin. It would be child's play for a composite airplane or wind turbine shop.

With a good design and lamination schedule this should be doable with hand or bagged lamination. All the best for the project.

[Chotu]

It’s not at all necessary to put in those extra pieces of glass running through the core.

Stiffness will be a function of the thickness of your board, backed up by the glass that makes up the outer skin. In other words, the thicker the board, the stiffer the board. Assuming the outer skin of the glass does not break. That’s just how it works.

Given that the dimensions of the board you have now were correct for the boat, you already have the right thickness and the right foil shape.

Go ahead and make up the section out of some H100 type foam. I think that was the weight that I used. 80 might be a little too mushy.

Get your foil shape perfected, and then laminate glass around it, then vertically. What glass are you using for this? You don’t want just biax. You are going to need fiber orientation running along the length of the board on the largest faces. That’s what makes it strong. Thickness makes it stiff.

Don’t worry about making the foreword and aft parts from solid glass in my experience. If anything, it will be good to have those soft as they will break apart if you smash these up again. They will break apart without causing any damage to the trunk.

Let me know if you need more specific instructions. I know that’s a general overview.

[Simi 60]

Originally Posted by Rucksta

Foam core with solid glass leading & trailing edges (sail battens)
Biaxial and unidirectional lamination for the skins.

Go for stiffness.
A large part of the strength will come from the quality of adhesion between layers.
Primary bonds are preferred i.e. one resin mix.

This is similar to building a Malibu sized surfboard.

Consider engaging a board maker to resin infuse your fabricated and assembled components.

Finished product straight out of the bag.

No it's not
The loads on dagger boards on a big boat can be immense

To the op
On boards made for the supershockwave 40 there was a fair amount of carbon uni about 150 wide running down the chord on both sides to stiffen them up

[foufou]

I have built at least a half dozen dagger boards and as many rudders mostly for small boats but one fourteen foot long rudder for a race boat. It was cored with fairly high density klegecel foam and the layup of carbon was not just "a few layers of glass". The layup was tapered so that at maximum load there were something like fourteen layers of carbon! A properly designed laminate involves finite element analysis though if you can get access to a comparable blade laminate schedule you can just copy that. The break-away feature is important and requires particular attention. Can you carefully grind the broken dagger boards to figure out how many layers of glass or carbon were used? Somewhere out there the layup schedule is known. Good luck in your search.

[Chotu]

Originally Posted by foufou

I have built at least a half dozen dagger boards and as many rudders mostly for small boats but one fourteen foot long rudder for a race boat. It was cored with fairly high density klegecel foam and the layup of carbon was not just "a few layers of glass". The layup was tapered so that at maximum load there were something like fourteen layers of carbon! A properly designed laminate involves finite element analysis though if you can get access to a comparable blade laminate schedule you can just copy that. The break-away feature is important and requires particular attention. Can you carefully grind the broken dagger boards to figure out how many layers of glass or carbon were used? Somewhere out there the layup schedule is known. Good luck in your search.

This is correct. And what I’m sharing is how to build a dagger board designed with finite element analysis with glass not carbon. In case the OP needs help.

You can save an awful lot of labor using triax instead of carbon tow or uni glass.

[Paul L]

One thing you could consider is build the boards with a sacrificial lower part, say a foot or foot and half at the bottom. That way if you just touch bottom you may save the majority of the board.
My carbon fiber rudder is made this way and I unfortunately tested it on a bar entrance.

[Simi 60]

Originally Posted by Chotu

You can save an awful lot of labor using triax instead of carbon tow or uni glass.

How is labour saved using glass when you have to cut, wet out and layup several X more glass than carbon?

[Spot]

If one need to design a foil or fin or wing, I recommend this website/application:https://finfoil.io/

It is the fastest/easiest way to get from point A to B designing foils without having to learn or use additional CAD software. The first picture I did in the thread was done in Fusion360 and would take a little more work to finish.

[Rucksta]

Originally Posted by Simi 60

No it's not
The loads on dagger boards on a big boat can be immense

To the op
On boards made for the supershockwave 40 there was a fair amount of carbon uni about 150 wide running down the chord on both sides to stiffen them up

Simi
The chord a line joining the leading & trailing edges.
Laying reinforcement down the chord is less effective than adding the same material to the skin.

Composite structure strength is derived from the separation distance of the skins.
The gain is exponential.

If the reinforcement was layed down the point of maximum thickness on each side (hope this is what your describing) then the gains will be realised.


To build a surf board take a foam blank, shape it, add reinforcement if required & hard points for attachments, lay up a suitable schedule of cloth & any artwork and infuse it with resin.

Yes the shape, loadings, core density, resin type and layup schedule are different yet the basic technique remains the same as building a rudder or daggerboard.

Another reason for the surfboard analogy is the blanks are usually one piece.
Glue bonds on foam are no stronger than the parent foam.
Usually the glue bond weakens the foam and failure occurs adjacent to the glue line.
Similar to a welded repair failing next to the weld as the parent material at the bond site has been altered
Use as few pieces of foam as possible to minimise glue bonds

The density of the foam and compression properties are important
OP stated Divinycel so H80 or H100 would be likely.
Divinycel has good skin adhesion properties and is available in 12' (3060mm)sheets so good choice.


Core selection, shaping and assembly are all important as is the layup schedule.
The most critical part of achieving the best results from the selected material is the application of the skins.
A single, multilayer primary bond with no voids or dry spots with a high reinforcement to resin ratio is best.
I can't see how a hand layup could work on a project of this size and complexity without time consuming and weak interlaminate secondary bonds.

Peel ply would help but it's still a secondary bond.

Hopefully you can see I wasn't proposing to cut the fin off a mal and stick iit in the slot

[Simi 60]

Originally Posted by Rucksta

Simi
The chord a line joining the leading & trailing edges.
Laying reinforcement down the chord is less effective than adding the same material to the skin.

I got my terminology wrong
What I meant was carbon on the thickest part of the board as per pic


Pic for demonstration purposes

Or a better pic again

[Chotu]

Originally Posted by Simi 60

How is labour saved using glass when you have to cut, wet out and layup several X more glass than carbon?

I said it is more convenient to use triax than uni or carbon tow.

In any case, they really should be using resin infusion techniques as well. As a minimum, they should have some help wetting things out and go wet on wet with slow Hardner and then bag it.

[Simi 60]

Originally Posted by Chotu

I said it is more convenient to use triax than uni or carbon tow.it.

Originally Posted by Chotu

.

You can save an awful lot of labor using triax instead of carbon tow or uni glass.

I must have missed that bit

In any case, they really should be using resin infusion techniques as well. As a minimum, they should have some help wetting things out and go wet on wet with slow Hardner and then bag it.

I wonder how we managed to do carbon layups back in the days before resin infusion and vac bagging?

[zstine]

why not use what's left of the old boards to make a female mold? The outer shape is not only important for the in-water performance, but also needs to match the trunk.