Jump to content

Shape of new rudder


Wommasehn
 Share

Recommended Posts

Hi everybody,

 

Muckla needs a new rudder blade, the old one broke on my autumn-cruise (maybe I'll write down how and post it in the Boating and Cruising Stories). I bought marine ply and started building the new blade. Now that I've got to do that, I'd like to make a better one. This means a stronger onde above all - but at times, when she heeled, the old rudder lost its bite. I don't like to have a longer blade as this would give more leverage and thus a higher risk of braking again - but I think, it should have a greater area towards the lower end. 

As for the profile, I have setteled for NACA 0012 and even started ro route my half-blanks to that profile (see pic).

But I am unsure about the shape (sidewise-view). The old blade has this elliptical shape which is said to have little drag. Any ideas about how much more drag I might get when I make a fatter ellipse or even a straight downward side like on the Lasers appendages?

I recall having read a contribution of the late PAR about that, stating that the elliptical shape would'nt be that much better - but I can't find it any more. So who knows more??565808572_2022-03-0121_40_04.thumb.jpg.59ba98d147ffbbbef65c16dbbcdb6272.jpg

2022-03-06 18.27.04.jpg

Link to comment
Share on other sites


Hi

Plywood is generally not considered a great choice for rudders and centreboards as the grain is not running in the right direction on alternate laminations and hence provides no strength. On a high aspect ratio rudder like this it would be better to do the traditional plank/ rip into several lengths/ alternate them to minimise warp and glue back together. I consider ply mainly as a core and would use plenty of glass for strength if using ply.

I don't think having a squarer profile would make much difference to drag (although in theory the tip vortex is minimised with a narrow tip).

HTH

Cheers

Peter HK 

Link to comment
Share on other sites

Peter, thanks for your reply and thoughts.

As for the material, I had thought about that , but decided otherwise because building with solid staves is much more time consuming - and time is crucial for me ( I have to work a real lot). And then the old rudder blade had lasted for some 25 years, made of plywood and not glassed. So I hope that the new one  will have sufficient strength when I glass it over and make a more gradual transition from the rectangular section which is inside the rudder head to the profiled main part which gets immersed. Thus creating less of a weak spot (where the old blade has broken as you can see on the photo).

Cheers

Tobias

Link to comment
Share on other sites

  • 1 month later...

Slowly I'm getting on.......

As for my initial question, the lateral form, I decided to give it a moderate elliptical shape. The additional area is in the lower half at the trailing edge. To compensate for the larger area aft, I put the semi circle (which is inside the rudder-head and includes the pivot)  a little aft too. The result is that most of the rudder moves  a tad forward , thus providing a little more balance. I hope that the sailing characteristics will remain the same all in all - apart from improved rudder-bite when heeled.

Then I glued the two half-blanks together. When I took off the clamps, I found that the whole thing was bent.......

Having done the necessary cursing, I did a lot more of shaping the profile plus straightening the whole thing, mostly by hand-planing, until it looked/felt good enough. I used a negative template for the NACA-profile. For attaching  the haul up , I made a tag of copper and rivetet it to the trailing edge.

Here is the dry fit with the old blade for comparison.

(For whatever reason, I still can't fit a picture inside the text)

Meanwhile, I epoxied and glassed the whole thing and put on two coats of primer.

So its mostly ready.496022827_2022-04-1020_32_28.thumb.jpg.3fecf5bcc4d5184fac8668d41fb9f2dd.jpg

 

Link to comment
Share on other sites

  • 1 month later...
On 3/16/2022 at 6:41 PM, Wommasehn said:

... building with solid staves is much more time consuming 

Tobias:

I see that you are well along on your new rudder and doing a fine job. But I have to respond to your statement relative to future projects. I have built and used rudders and leeboards from both plywood and dimensional lumber. It seems to me that plywood is less work only if you cut out the profile, round off and taper the ends, and stop there. Once you start gluing bits of plywood, shaping, and fiberglassing, the labor increases significantly.

 

I agree with Peter HK on the superiority of dimensional lumber for construction of a rudder.  For a rudder as small as this, a single plank with good, straight grain would do the job. Worst case, you'd need to rip 3 or 4 staves and glue them up. This would require less work than the gluing of plywood that you completed. The blank can be shaped with a drawknife, spokeshave, and plane. No fiberglassing is required. And the rudder would be stronger than a plywood one.

 

I hope that Muckla is back on the water. Have fun! Andy

Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
 Share


Supporting Members

Supporting Members can create Clubs, photo Galleries, don't see ads and make messing-about.com possible! Become a Supporting Member - only $12 for the next year. Pay by PayPal or credit card.




×
×
  • Create New...

Important Information

By using this site, you agree to our Terms of Use.