Working out a decent laminate for a rudder

I might have to replace an old transom hung rudder. The boat is 34' but only about 2.8T, pretty light in the scheme of things.

I'm thinking of shaping a core out of paulowina then laying up over that. I'm assuming that epoxy and a directional weave cloth would be most suitable, maybe mostly unidirectional with a couple of double bias layers. The current layup is chopstrand. From what I can see, there looks to be about 6mm of laminate on the current rudder. I think that works out to 10 layers of 600gms cloth. That seems like quite a lot!

I used an online calculator to work out the approximate weight of the laminate given the size of the rudder and a 6mm thickness, and it came in @ 7 odd kilos with about 4kg of core. I expect the rudder weight in kg will end up in the low teens. The class rudder (chop strand/foam build) is 15kg and the one on the boat is 22kg (heavy + water no doubt!)

The rudder will be a NACA0012 foil 183 cm tall (6'), 42cm (~1.4') wide and 5cm (2") thick.

Thoughts on the layup? Lighter/heavier? etc.

Thanks in advance.

 

I took the rectangular dims and ran them into Fusion. My result is 21kg with a light wood core (1/2 that of pine) and 6mm glassing.

I would think less glassing over a solid core would suffice, provided there are no pinholes and damages are repaired right away.

Not a professional, just an enthusiastic amateur.

 

Cool, thanks. I'm not so sure about the laminate calculator that I found online. 10 laminates of 600gsm to get to 6mm sounds like a lot. Ignoring the 6mm on the current rudder, I'd have guessed that 4-6 would have done the job, but I really don't know!

 

Just in case it's not a timber you use...

Paulownia


Paulownia (Paulownia tomentosa)

Common Name(s): Paulownia, Royal Paulownia, Princess Tree, Kiri

Scientific Name: Paulownia tomentosa

Distribution: Native to eastern Asia; also planted in eastern North America

Tree Size: 30-65 ft (10-20 m) tall, 2-4 ft (.6-1.2 m) trunk diameter

Average Dried Weight: 18 lbs/ft3 (280 kg/m3)

Specific Gravity (Basic, 12% MC): .25, .28

Janka Hardness: 300 lbf (1,330 N)

Modulus of Rupture: 5,480 lbf/in2 (37.8 MPa)

Elastic Modulus: 635,000 lbf/in2 (4.38 GPa)

Crushing Strength: 3,010 lbf/in2 (20.7 MPa)

Shrinkage: Radial: 2.4%, Tangential: 3.9%, Volumetric: 6.4%, T/R Ratio: 1.6

 

I don't think you are comparing apples to apples.It hasn't been clearly stated but I am guessing that the original rudder is a hollow assembly although-again unstated-it may have had foam poured in after assembly.Your wooden replacement may well be strong enough in itself and only rely on the glass sheathing for resisting impacts with debris.

 

Quite probably!

The original rudder is a transom hung, almost certainly female moulded and foam filled on assembly. So I expect that it is low density foam using the shell to provide almost all the strength.

That said the Paulownia is about twice the strength of Balsa, so I imagine that it will still need a decent laminate. Paulownia is stiffer than foam but about 30% denser than Divinycell H200.

I need to make an educated guess for the laminate.

Wet finger in the air I think that 4mm of well-designed laminate should probably be enough, but it would be great to hear from someone that has been down that path.

FYI about 500mm of the 1830mm height is above the water.

 

I think I'd use a very hard wood for the leading edge and trailing edge and sheathe the finished rudder with 2 plies of something like 280gsm cloth.The tough edges should resist knocks and the glass should be sufficient to cope with normal sailing loads-after all boats used wooden rudders for centuries and a glass sheathing will resist worms.

 

I dunno why you’d want a wooden core in a rudder. All the core is doing is making a form for the laminate. I’d use something that won’t puff up if a drip of water gets to it. If the rudder gets dinged. The leading and lower edges I’d extend with laminate if making with wood, or any soft core. Coosa would be my plan..otherwise use polystyrene and burn it out chemically after building up the laminate.

 

@Zoro , can you share a pic of existing or a link to the commercial replacement ...or are you sworn to secrecy?
Do you have access to CNC for making any of this or would you do the work by hand and regular shop tools?

 

1. It's a one-off male moulded project, so I need a core of some sort.
2. Paulownia doesn't absorb water.
3. Paulownia is stronger as a core than many foams.
4. Paulownia can be easily shaped using the equipment I have.
5. The core is a part of the structure in this instance, it isn't just a form, much like Coosa would be.
6. Hollow rudders end up full of water, heavy rudders are slow, this boat is fast, keeping weight out of the ends is a part of the motivation to replace the existing rudder.

I'm not sure how easily I can get Coosa around here, it looks like it would be a stronger core. I've never worked the stuff, I will have a closer look. Thanks for that.

 

The official replacement is ~$3300, I'm sure I can build one for a fraction of that.

The design (Adams 10) was penned in the early 70's, it was a pretty radical ULDB for the time. Alas I doubt there is digital anything for them! They have been one design racing them for 50 years and they still get to the front of most fleets.



I'm most probably going to be shaping the foil with a table saw and hand/power planes maybe a router with a jig. I don't know yet but pretty basic stuff in a backyard shop. A CNC router is on my shopping list but not yet!

I have wondered about 3d printing a core and building a laminate over that. Something like this guys does...



I'd be a bit concerned about it ending up full of water over time, but I guess you might be able to do a foam pour into the plastic shell.

Drawing it up in CAD would be pushing my CAD skills, but it's probably achievable.

 

I'd stick with original plan.Absolutely nothing wrong with a wooden rudder,boats used them for thousands of years and you are proposing to put a durable protective coating of glass over the entire thing.I wouldn't even think about alternatives as those pictures clearly show that you need a crush resistant upper section to deal with the compressive and shear loads from the bolts that attach the fittings and the rudder head.You should be able to take section templates from the existing rudder,or if it's beyond use a borrowed one.Then it just takes time and patience.I will repeat my recommendation for a hard species for a leading and trailing edge and you absolutely have to mark a clear and accurate centreline all round.

You are absolutely correct about hollow rudders winding up full of water and I have yet to see a rudder filled with pourable foam that wasn't wet by the end of it's second season afloat.The idea of a CNC machined core is superficially appealing and it isn't hard to create a program for the cutting once you have a clean CAD file.It takes seconds.What few of the advocates of the idea ever explain in detail is quite why you should imagine that the result will be symmetrical as many cores move as the internal strains are changed by having portions machined away.There is no question that the trajectory of the tool will be correct but no guarantee that the workpiece will wind up the desired shape on both sides or be symmetrical.They also skip over how you support and locate the blank when machining the second side.It can be done,but may not be a faster process for a one off than using traditional methods.

 

Here are some CAD musing from last night, not certified/warrantied by anyone nor suitable for distribution/production:



ITEM, QTY, PART NUMBER, MATERIAL , MASS
1, 1 , COMPONENT1 ,PAULOWNIA , 5.1 KG
2, 1 , COMPONENT2 , GFRP 6mm 15.8 KG
3, 1 , COMPONENT3 , GFRP 4mm ,12.9 KG
4, 1 ,COMPONENT4 , PU FOAM ,0.7 KG
5, 1 ,COMPONENT5 ,PAULOWNIA ,6.3 KG

To be around 14 kg, the paulownia wood core would only allow the weight from a 2mm GRP skin. With a foam core the skin could be closer to 4mm.
A smaller CNC or 3D printer could be used to make templates to check the foiled section. A carpenter's contour gauge or one of the drafting splines that can be bent and hold its own shape might be quicker if the existing rudder is handy.

If one wants to mess around with foil design without doing CAD, there is a nice editor here, the developer and I go back a ways: finFoil.io - Start designing fins today! https://finfoil.io/
The higher level outputs are for a fee but the messing around is free.

 

It would be important to specify the core densities. Foam cores can be purchased in a wide variety.

 

@fallguy True, true. Here are the numbers used, parentheses are online conversions from ouncemass.
Foam works out currently like 2 PCF pour foam, Foss Foam would use 20 PCF for this according to their website:
Rudder Construction https://www.newrudders.com/rudder-construction/
Which would be closer to the wood core weight...(edit) but it is not apples to apples because of the armature/rudder post.

Part Number Component3
Material Name GFRP
Mass 453.355 ouncemass
Volume 448.172 in^3
Density 1.012 ouncemass / in^3 (109.3 PCF) (1750 kg/cm^3)

Part Number Component4
Material Name Polyurethane Foam
Mass 25.09 ouncemass
Volume 1356.429 in^3
Density 0.018 ouncemass / in^3 (1.94 PCF) (31 kg/cm^3)

Part Number Component5
Material Name Paul(ownia)
Mass 223.811 ouncemass
Volume 1356.429 in^3
Density 0.165 ouncemass / in^3 (17.8 PCF) (285 kg/cm^3)