Jump to content
lateral

Rudder refoil

Recommended Posts

]Dunno whether anyones interested except Fish, but anyway it would be good to get some F/back.

Can always do things better next time. 

 

Old boat sailed like a pig upwind, old baggy main/#1 and really hard to bend a non-tapered mast &

flatten. In fact broke the backstay trying. must been weak. ....haha.

Couldn't get a balanced helm no matter what, and rudder used to stall out real easy.

Just seemed wrong.

So as we were fixing the boat (rebuilding)  I checked out a naca profile creating program on web.

Drew some out with right chord length for rudder & did a % change, I think because of better

reviews . Its an  great program.

I stuck these to 25mm mdf and cut them out to check over my rudder.

 

Old rudder

2015 OldStaller.jpg

Concave tail section

Concave WTF.jpg

 

Roughed up old rudder glassed & peelplyed ready for profile change.

2015-01-15 14.26.48.jpg

 

Epoxy 410 bog

2015-01-14 12.33.24.jpg

That gave me the profile to bog whole thing..

Boarded it, glassed, faired again, then paint.

 

Although I can't be objective because of whole new sail wardrobe, the difference was chalk & cheese.

Must satisfy my curiosity and use old sails sometime.

Reason I went for it was I had to look at it while I was fabbing up the lever for the AP.

It really looked an abomination.

 

2015-01-17 16.04.22.jpg

 

Naca profiles before setting out.

2015-01-13 18.54.22.jpg

Share this post


Link to post
Share on other sites

That is really interesting Lateral. Thanks for posting it.

I see in your top photo, the profile of your old shape didn't look great, a pronounced localised curve over the rudder stock, then a flat run aft (or as you show in another photo, a concave section).

So you just took your old rudder and re-faired it with naca profiled templates. We did the same for our keel, scaled off some naca profiles onto plywood, then bogged and faired the keel until it match the naca profile. I think we used epoxy with micro-beads.

 

Every now and then I hear stories about people replacing old rudders with modern low profile shapes and sometimes carbon stocks etc. Is that just around drag profile and total weight, i.e. for the die-hard racer?

Certainly modern rudders are completely different to the older style. I remember seeing Rikki's (I think), it was only a hand with wide in chord length, thin and very deep. I assume there are benefits in reducing total wetted surface etc etc.

 

Currently our boat goes like a demon upwind, and is hard to get to broach downwind, although it is possible. We have a tappered, pre-bent mast that we can crank the backstay on, and we can get her balanced nicely going upwind (easy to get her into the groove), so its sounds like there is no point in messing with my rudder.

Share this post


Link to post
Share on other sites

Ive done a lot of research on rudders/foils for us slow boats and generally speaking you cant go too far wrong with a NACA 00XX profiles for  rudders and keels, arguably there are better foils for certain conditions, but for all round use they are hard to beat. The biggest factors apart from the foil is area,  aspect ratio, plan section and how accurately the foil is made. Then its the actual execution of the job, the finishing detail and keeping it clean. You need to have smoothness less than the thickness of a human hair or #400 grit to optimise foil performance. Some foils perform better in theory,  i.e. in perfect conditions but the holy grail of laminar flow is not practical.

Share this post


Link to post
Share on other sites

Thanks for sharing this, I have exactly the same rudder with a twist, only one side concave, well one side deeper than the other so I have very interesting feeling on my tiller, so I have to do the same thing on my one. Just a practical question, have you bogged up between the guids, or used some sort of foam (closed cell PUfoam) to keep the weight low and glassed over the foam? That actually what I would/will do. I think some places I'm talking about 30mm to fill which is a lot... one more thing, which software you used to work out the new profil?

Share this post


Link to post
Share on other sites

 

 We have a tappered, pre-bent mast that we can crank the backstay on, and we can get her balanced nicely going upwind (easy to get her into the groove), so its sounds like there is no point in messing with my rudder.

 

Nice! Too spendy for me.

Got my main cut flat. Downside is head won't twist off easily. IMO anyway,  probably wrong.

So little sailing, so much to learn.

Going to have someone along soon who knows what they are doing. (On board)

Share this post


Link to post
Share on other sites

Thanks for sharing this, I have exactly the same rudder with a twist, only one side concave, well one side deeper than the other so I have very interesting feeling on my tiller, so I have to do the same thing on my one. Just a practical question, have you bogged up between the guids, or used some sort of foam (closed cell PUfoam) to keep the weight low and glassed over the foam? That actually what I would/will do. I think some places I'm talking about 30mm to fill which is a lot... one more thing, which software you used to work out the new profil?

nah. just bogged using the ribs as guide, with the same (410). It was ridiculously heavy anyway being solid SS shaft and SS sheet. Colin told me 410 was ok below WL, even tho west sayes it isn't.

 Its heavy!  :crazy: Probably 50kg.

Here is my NACA 0012. Obvious you have to table  your chords & print one out for each and grid your rudder with the appropriate chord...all good fun.

The website I used was:

http://www.windandwet.com/windturbine/airfoil_plotter/index.php

Or:

http://airfoiltools.com/airfoil/naca4digit

 

MyNaca0012-400.pdf

 

 

Just pick your co-ords from Univ. data base,or  paste in your own, and enter your parameters 

& your good. Be aware some browsers scale to fit.  I had to switch to Opera as Chrome

did that. (back then) Sculpin excel sheet has dissappeared, But you don't need it.

If you cant be bothered tiling them & aligning, send them online to local

print shop & get them on A1.

 

I'll give you the thread I started on SA cos some very helpful ppl there.

And, some stuff I don't remember doing. 

http://forums.sailinganarchy.com/index.php?/topic/162991-rudder-stalling-aoa/

http://www.ericwsponberg.com/wp-content/uploads/keel-and-rudder-design.pdf

 

 

Thanks for the memory jog native.

 

Edited- As meds interferring with brain/finger interface.

 

Tips for the uninitiated:

*If previous coat not peelplyed, coarse abraid, neat coat of 105 & wait for tack, then bog.

* Even can do with PPly'd surface.

* I didn't wet sand SS with 105 & coarse paper, but could be worth it. (Too messy.)

* hanging your rudder makes it easier...

2015-01-13 17.13.43.jpg

Share this post


Link to post
Share on other sites

We did a re-profile job on our keel a few years back to give it a little more lift upwind, pretty happy with it.

Used an Epler 836 Hydrofoil profile very similar to the Naca shapes with some subtle differences. We used glue under our plywood profile boards rather than bog so we didn't sand our shape down.

 

The surface needs to be spotless to work really well, very noticeable when the boat is getting a bit of growth. Probably not the best profile for a cruiser but good for a racer.

 
 
keel (2).jpg

Share this post


Link to post
Share on other sites

I used profilli when I was playing foil designer, it generated dwg files which i printed in freecad to full size pdfs. Then transferred to ply templates. The foil profile believe it or not is not that critical unless you have a specific purpose, most keels are a bit thicker than necessary  to carry the lead so thinning them down can make a big improvement especially downwind. Heres a great thread on SA on the subject. http://forums.sailinganarchy.com/index.php?/topic/2289-keel-profiles/

 

As willow mentioned unless you keep it immaculate youre not getting the benefit of a high tech fairing job.

Share this post


Link to post
Share on other sites

Sorry for stepping in so late. I have been traveling...

 

My view on foil sections is that choosing a proper foil section is not only for racing guys adding a fraction of a knot in speed but for everyone getting better, more reliable steering (talking rudders now). Pretty much any proper foil section is better than the foil section look-a-likes many boats come with. They are less prone to stall or suck down air. Preventing this is more important that minimising section drag. Control trumps low drag in real life and section drag is such a small part of the total drag anyway. Our aspect ratios on rudders and keels/dagger boards are not that good. Hence it pays off to go looking for sections designed with this in mind. Also, we are slow compared to air planes so looking at lower Reynold number designs is a good idea.

 

The Eppler E 836 mentioned is described as "This symmetrical hydrofoil represents a compromise between cavitation (sic) and boundary-layer requirements. It may be adequate for vertical surfaces like centerboards of small sailboats..."

 

'Reading between the lines' in later publications and while listening to talks given by racing yacht designers I get the impression that foil sections today are designed even more with control rather than theoretical drag in mind.

 

Finally an anecdote from 1995.

 

A brother of one of my colleagues had a small monohull. One of those with a rather wide stern and a flat hull-form designed with downhill planing in mind. It had a transom-hung rudder that was starting to show severe stress cracks. We figured out that my rudder molds could be used to produce a replacement rudder. The cord dimension was a little less than for the existing rudder but we could compensate by making it deeper. The section was NACA 63_2-015 (the exising rudder was one of those look-a-like sections).

 

I did a structural design based on UDWR glass and +/-45 degree glass with a timber spar down the middle. Epoxy laminating resin was used throughout. The new rudder came out at half the weight of the old one yet three times stiffer when applying a bending load.

 

The real treat though was the improvement in control. Broaching became a rare event while with the old rudder it was to be counted on when the wind was up.

 

/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...