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Thursday, May 31, 2012

Busy Times

Progress continues on RM tooling preparation in the workshop.
In the office the A Cat design is approaching completion.
R10R and IOM developments are in the pipeline and consultancy work continues on aerospace components.






Saturday, May 26, 2012

Wax On... Wax Off

Progress on RM tooling: Plugs/patterns have been delivered, now preparations begin for taking female moulds.






Monday, May 21, 2012

The Importance of Attitude

Some revealing images from the A Cat RANSE modelling: Small changes in pitch have very interesting effects on wavemaking and, therefore, on drag.


Contrast the shape of the breaking bow wave in these two snapshots...


Taking that step forward or aft along the gunnel as conditions change is really important.

Wednesday, May 16, 2012

In the Metal

Katana appendage tooling machined directly as female moulds from solid metal...
The fin incorporates additional drought so it will be suitable for larger boats such as 10 Raters.
It can also be adapted for classes with restricted drought such as the IOM.
The lower Reynolds Numbers characteristic of IOM class boats makes it advantageous to use the top part of the fin mould, keeping the trunking design common.


Initial coarse passes shown. The machine will then return with progressively finer steps down to less than 0.1mm. The only final hand finishing required is a very light sand and polish.
For such small, shallow, rigid moulds that can be made directly as female tools, the investment in more expensive and slower to machine materials is warranted. 
The step of laminating a female mould from a pattern is eliminated and the final tooling will be capable of withstanding high mechanical pressures and elevated temperatures to produce very compact laminates.


Bulb mould also shown at coarse stage (below). The plate that will form the fin cavity is visible on the left. Pouring hole for the lead and vent holes fore-and-aft are visible on the right.


These shots also courtesy Alex Kryger, Aptec Composites.

Tuesday, May 15, 2012

A Class Catamaran Hull Shape Locked In

Well, the longer  than initially planned but more sophisticated hull CFD test schedule is nearing the end... Results have been very informative.


Work will now shift to deck shape and crossbeams while preparations continue for construction.
More will be unveiled soon.

Monday, May 14, 2012

Shiny

Progress continues at pace on Katana M hull and deck tooling...
Images courtesy Alex Kryger, Aptec Composites.






Sunday, May 13, 2012

Katana Deck Layout

As work continues on tooling, the new deck shape is now visible.
A good opportunity to share the thinking that led to the chosen layout.


Initially a dedicated RC tray moulding was designed with the aim of locating the mass of winch, batteries and servo as low in the boat as possible and close to the LCG to minimise pitching.

Discarded RC tray moulding design
This solution had drawbacks that seem to be accepted in most existing designs but we thought we could eliminate:
- Dedicated un-reinforced openings in the deck would be required for access, sapping stiffness (or requiring reinforcement in the form of lips or additional laminate that would add weight only to restore the stiffness lost by cutting the openings in the first place).
- The openings would have to be very large if the tray were to be glued into the boat after hull and deck were moulded together.
Even if the size of the openings were minimised by separating them into the three functions (winch, batteries and servo), the tray would not fit through any of the holes.
An option was to create three separate trays. However, because each would have to be larger than the hardware it must accommodate, opening size could not be optimised.
Being unable to drop the tray in through the access openings would have closed the door to one-piece moulding of hull and deck as a unitary laminate.


The lateral solution was to suspend the RC gear in dedicated wells moulded into the deck.
An important design constraint imposed when exploring this solution was to maintain the same low centre of gravity for winch, battery and servo.


The radius into the wells actually adds stiffness to the deck, complementing the inherent rigidity of the chambered shape.
Separate patches over each opening facilitate access and improve watertight integrity.


Another choice that will enhance ease of access is the re positioning of the sheet fairlead to the foredeck.
This solution also simplifies the sheeting run and has secondary advantages such as the elimination of a sheeting post, reduced likelihood of entanglement with other boats, and more direct transmission of sheet forces into forestay tension.


Friday, May 11, 2012

Katana Tooling Takes Shape

From virtual to reality.
The magic is being worked by Aptec Composites.
Images courtesy Alex Kryger.


Hull plug/pattern (above) and deck plug (below) machined ready for surface finishing.


Notice the bonding flanges for mast tubes and centreboard case, as well as all deck features, integrated in the tooling to ensure accuracy and repeatability.
The mould flanges and locating features to close the hull and deck mould together are also machined at this stage.


There is an interesting tradeoff between material cost and the expense of surface finishing.
In the case of larger simpler shapes we found it more economical to use an easy to machine stable but comparatively low cost material and go through the process of sealing and hand finishing to get the required surface finish.
Smaller and more detailed tooling will be machined from metal or plastic, requiring only a final polish after fine machining.
Where the parts must be made at high temperature, plastic plugs will be used to make female moulds in the same material as the eventual parts.

Monday, May 7, 2012

Katana Marblehead Tooling

Some images of the mould files ready to cut.


In the case of hull and deck, male patterns will be machined to replicate the finished outer surfaces. Composite moulds will then be taken from the patterns which also incorporate the mould mating flanges.


The finished moulds will be made such that they can be closed together to allow hull and deck to be cured giving a seamless product. The trick is a 'slip joint' where the deck laminate overlaps the hull, extending 12mm below the sheerline. An inflatable bladder made to exactly conform to the shape of the mould cavity applies pressure to the laminate during curing.


The deck incorporates recesses that form trays for the RC gear. The tooling is designed to allow them to be moulded as part of the deck. Alternatively the recesses can be left out of the deck, moulded separately and dropped in as required.


Locating features and bonding flanges for the new centreboard case and mast tube systems are also incorporated in the machined patterns to ensure accuracy.


Moulds for bulb, fin, rudder, transom and other small details are being machined directly from metal.

Sunday, May 6, 2012

Ways to Skin… a Hull

Previous posts on A Class Catamaran material choices hinted at the influences of core type on construction process.
Let’s look at the options in construction method and the unique requirements of each.

Foam Core
Hull panel lamination can take place in one or two steps, depending on whether a perforated core is used. 
A perforated core will involve a weight penalty because the holes used to allow entrapped air to evacuate will end up filled with resin. 
Assuming the resin content of the laminate on the mould side of a non perforated core is carefully controlled, a one step process will give a consistently more efficient panel. 
When considering labour cost, as well as the number of steps involved, the brittleness of foam must be taken into account. If plain foam flat sheet is used, it needs to be formed into the mould prior to lamination. This requires care and is usually done by gingerly heating the core material. 
It is possible to buy structural foam that comes ‘scored’ with cuts that allow it to conform to curved moulds. However the voids left by the cuts (that must splay open to allow the foam to deform) are also likely to trap resin, adding weight to the finished panel.

Honeycomb Core
As discussed in previous posts, our aim in the A Class is to maximize rigidity for the mandated minimum weight. 
We want to create a thick panel with as much fibre in the skins and as little resin content as possible. 
The lower density of honeycomb is more suited to our goal.
Since the bond between honeycomb core and skins relies on the thin edges of each cell being captured in just the right amount of resin, a very controlled process is called for.

Resin
The manageable dimensions, thin skins and simple shape of an A Cat hull are such that similar results can be obtained with prepreg and wet layup techniques. 
The challenge with wet layup is managing resin content with respect to de-bulking, evacuation of entrapped air, core bonding, and drainage from vertical surfaces into areas of the mould prone to pooling. 
More resin is safer in terms of interlaminar and core bonding, but it increases the risk of air entrapment, pooling, and filling of the honeycomb cells.
Resin has to be applied evenly and consistently, balancing the conflicting requirements and taking into account the effect on final laminate resin content of bleed-off into the vacuum stack.

On the other hand, prepreg, though guaranteeing a known and consistent resin content, involves additional steps in de-bulking and the application of glue film layers.

The contractors we are considering for production have facilities available for either prepreg or ‘wetpreg’, where 'room temperature' resin is applied in a controlled fashion before placing the fibres in the mould. 
We have decided to produce moulds capable of handling the temperature and pressure necessary to cure prepreg laminates in order to have the option of prepreg construction. 
Our plan is to experiment with both methods before committing to either. 
The final decision will be influenced by availability and delivery costs (prepregs need to be moved in an uninterrupted 'cold chain'), and how they relate to any performance differences evident in the two methods. 
Stay tuned for our findings!

Friday, May 4, 2012

Katana Marblehead Design – Foils

Marblehead foil design presents an interesting challenge because the maximum permitted draft is extremely generous.
In most monohull keelboat classes, maximum draft is well short of the crossover where additional righting moment gained becomes outweighed by hydrodynamic and structural considerations. 
For most classes, draft can therefore be regarded as fixed (always go to maximum).

In a rule such as the old International America’s Cup Class (IACC), where speed producing factors could be traded, it was even warranted in some conditions to exceed maximum draft and take a draft penalty (in the form of reduced sail area or measured length) because the increased righting moment was far more beneficial than the associated drawbacks. 

More often draft is taken as mandated by the rule maximum so it ceases to be a variable in the optimisation of appendages.


At this size, tall rigs are advantageous because the wind speed gradient (slowing near the surface) is very significant. 
Righting moment is therefore important if sail force is to be maintained. 
Again due to size (more precisely to Reynolds Number – the relationship between speed and length), wetted area is a dominant contributor to hull drag, so stability generated through hull beam is expensive in terms of drag. 

So far, maximising draft seems like a no-brainer because it gives more righting moment for a given heel angle and ballast mass. But what are the downsides?

This is an example of a complex design space where multiple variables are interrelated. 
Making the fin deeper increases bending moment for a given bulb weight. If this results in more fin deflection, it will reduce the gain in righting moment by allowing the bulb to move inboard more than it could if the fin flexed less. 
Note that there is the option to keep righting moment constant by increasing draft and reducing bulb mass (smaller force, longer lever). But for now we are assuming that optimum displacement must be maintained.
Assuming also that fin construction already maximizes stiffness, the only way to address the extra bending loss is to make the fin thicker. This in turn increases the thickness to chord ratio for a given fin chord. 
To compound this effect, the deeper fin actually needs to have less chord if area is to remain constant as draft increases… 
You now begin to see the fine tensions involved.

The low Reynolds number makes RC yacht foils particularly sensitive to laminar separation because the flow running along their surface is not very energetic. 
It therefore cannot be called upon to follow steep curves, especially after it has already traveled some length along the surface and has therefore lost some energy. Unfortunately this is also the point where we want the flow to follow the section in toward the trailing edge.

Increasing foil chord can help by making the Reynolds number for the whole foil higher. A longer chord also decreases thickness to chord ratio for a given fin stiffness. Decreasing thickness to chord ratio helps to keep the flow attached (by basically straightening the section lines the water has to follow), but also affects lift characteristics and possibly stall angle. 

The price for extra draft has to be paid in some combination of greater bending losses, increased thickness to chord ratio, and additional foil area.
Secondary considerations come into play for upwind sailing: Increased fin aspect ratio reduces lift induced drag. 

Moving the bulb down (and away from the CG) may increase pitching moment, a potential drawback when sailing in waves.

Without wading into the maths, we have identified the interrelated variables that make up this particular design space. 
The designer has to trade draught (good for righting moment) against fin bending losses, fin area, section thickness to chord ratio, Reynolds Number, aspect ratio and pitching moment.

Finally there are less constrained decisions such as chord distribution along the fin and rudder (basically the taper ratio), foil positioning, and lift sharing between fin, rudder and canoe body. Sharing of lift by the rudder can be controlled through rudder size, placement and angle. Hull lift is indirectly controlled by leeway angle and is therefore influential on fin section design which is usually done with reference to an optimum range of angles of attack.

Plugging in the numbers, the optimum balance usually comes out well short of maximum draught for a Marblehead that is to be competitive around a course in a range of conditions. 
Our previous work confirmed this consensus. 
The constraining factor was the ability of the section to retain efficiency with reduced chord and increased thickness to chord ratio. 

For Katana we paid a lot of attention to the section shape and were able to identify a small gain by basically smoothing out thickness distribution. This allowed us to push to a slightly shorter chord with similar drag characteristics to the previous generation. 

Improvements in construction allowed us to slightly reduce thickness for a given bending moment. However with the new section we could accept a slight increase in thickness to chord ratio (from 6.25% to 6.5%) which also makes the fin more forgiving in down speed situations. 

Taking advantage of the improved section and construction, we increased draught for the same foil area and deflection.

The bulb was revised, incorporating the improvements in foil section (that translate to bulb thickness distribution) and adding a beaver tail as successfully used on our IOM designs.

Elliptical chord distributions were chosen for both fin and rudder. This option was made practical and economical by the CAM technology being used to cut the moulds.

The new fin mould is machined over-length to permit experimentation with even deeper draughts and to allow use of the fin in bigger classes such as 10 Raters. 

The foils and bulb are also available separately so contact us to find out if they are suitable for your boat.

Wednesday, May 2, 2012

Core Issues

Picking up where the last A Cat post left off, we were contemplating the relative merits of foam core and Nomex style aramid paper honeycomb.

Honeycomb is a very efficient structural solution because it concentrates material in effective load paths between the skins. 
Each cell is braced at the interface with other cells, and there is a lot of empty space within the thickness of the material.

By contrast, foam contains some voids in the form of random bubbles but needs to be much denser to achieve a given global rigidity.

Foam does however have some secondary advantages: It has toughness when loaded in directions such that the skins cannot work effectively (for example blunt impacts), and the ability to keep working when deformed (such as in the ubiquitous bruises caused by knees and trapeze hooks). 
It also does not allow water to travel through it as each empty bubble is closed and separate from the others.

Honeycomb core uses inherently strong shapes
to keep the skins from moving relative to each-other.
Image from http://www.rocketmaterials.org/ 
As a simplified example, a honeycomb core with half the density of foam could be twice the thickness for the same weight. 
With purely global structural considerations in mind, honeycomb gives the option to build a thicker laminate for a given weight. 
Even if the mechanical properties of the honeycomb were slightly inferior to the foam at such a reduced density, the laminate would still be much stiffer because thickness improves stiffness in a non linear relationship – a small increase in thickness yields a large improvement in stiffness.

Resin fillets at the skin/core interface shown in blue.
Getting these fillets right without filling the cells or
starving some areas of resin is critical to the manufacturing process
With both core types, there is a resin-rich layer between each skin and the core. 
Core bonding in the case of honeycomb relies on little fillets of resin forming along the edges of each cell where it touches a skin. 
Foam cores have greater contact area with the skins. Bubbles that are open to the cut outer face of the core often trap resin because there is no path out of the bubble. This provides additional ‘keying’ and bonding area but adds weight to the finished laminate. In a well bonded foam sandwich panel the core usually fails before the skin-to-core bond.

An exaggerated representation of foam core/skin bond
showing surface cells filled with resin (again in blue)
As is often the case, the trade-offs have implications beyond the inherent structural merits discussed so far. Each solution has different requirements with respect to construction method. The choice must take into account the effect each option has on the build process and related constraints such as complexity and cost…