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Hobie Cat 16 rudder profile? asymmetric?
http://www.hobie.com/au/en/forums/viewtopic.php?f=9&t=61313		 Page 1 of 1
Author:  FLOW67 [ Tue Aug 15, 2017 6:30 am ]
Post subject:  Hobie Cat 16 rudder profile? asymmetric?
I have a 2002 HC16 with plastic rudders and am trying to fix the hummmm.
I've inserted oversize nylon washers between the casting and the blades to take up some of the slack, and then tightened up the fixing bolts.
I have also sanded the trailing edge to remove the "bump" from the casting, although there was minimal "bump", but rather a bluff rounded trailing edge, so now its a smoother line with 2 mm Trailing edge.- all good ...

While doing this i noticed one side of the blade is curved in an aerofoil shape, this is on the outside on my boat. the other side of the rudder blade is pretty flat and is facing inside. i note that the rudder steering arms are both facing inwards which is correct. The rudders were fitted to the castings on the hulls when i bought the boat (second hand) and we have sailed now 3 times with terrible humming from very low speeds. It seems to me that since the outside of the hull is flat (the inside of the hull being a curve thus asymmetric) the rudder should also have the flat surface facing out and the aerodynamic profile facing inwards. Has the previous owner assembled the rudders on the wrong side? There are no markings on the rudders to say "in" or "out" that i can see. I also note that there appears to be a curve from tip to the end of the casting, ie. the rudder blades are not straight. the curve inwards.
I would much rather have new EPO3 blades, but the cost prohibits.
The rudders could easily be swapped from left to right to reverse this.
The blades are NOT the same shape on both sides, i checked and checked. :)

Can anyone comment on this aerodynamic/flat profile in or out ?
Cheers
Author:  mmiller [ Tue Aug 15, 2017 8:33 am ]
Post subject:  Re: Hobie Cat 16 rudder profile? asymmetric?
Asymmetric shape could cause weather helm on one tack and lee helm on the other... but not humm. Hum is from the trailing edges. Just needs some trimming.

Image
Author:  speed633 [ Tue Aug 15, 2017 9:57 am ]
Post subject:  Re: Hobie Cat 16 rudder profile? asymmetric?
hmmmm... humming can come from indecisive people, are you sure your rudders are asymmetrical?

But I agree with Matt, humming is a trailing edge effect.

For performance reasons if the rudders have significant asymmetry I would set them up such that the rudders produce lift towards the center of the boat. That way when you are sailing hard, the more submerged downwind rudder will produce lift that counters some of the weather-helm.

In theory, an asymmetric foil shaped for lift should be more efficient than a symmetric foil producing lift due only to angle of attack. In reality the asymmetry of your foils was probably not intentional, and whatever shape they are probably isn't the most efficient. All things considered you will probably never notice a performance difference, but you should be able to attain some piece and quiet from sanding the trailing edges.

~Joe
Author:  FLOW67 [ Wed Aug 16, 2017 2:00 am ]
Post subject:  Re: Hobie Cat 16 rudder profile? asymmetric?
Thanks for the replies

I have sanded the TE now. using a pair of scissors is not really effective on the blades. i used two different files and sand paper. so i will see. they are most definitely flatter on the inside than the outside. NOT symmetric which i don't believe is how the blades are today. Anyway they are original Hobie with the casting marks etc.

I will try them as they are with the TE smoother. If there is no improvement, i will swap the left to right etc, so the flat surface is out and get the lift to pull inwards as Joe mentioned. which also makes sense.

Just a shame the new EPO3 are so expensive. second had like hens teeth.

Also the blades were left out in the weather for i reckon about 10 years...! so they are a kinda yellow colour with black stains in these tiny grooves. sanding hardly touches them. i will try courser paper. its like the're etched in dirt !!

on the plus side, im really enjoying the sailing with my son, last H16 i had was sold in 1993 in Mallorca, Spain. "Harry the Hat" and I used to sail a lot together down there. He was a legend. the only person i met, about 65 at the time, who never had a proper job. Grew up at his parents hotel in Barbados and worked the beach at the hotel all his life until they sold up and moved to Spain. A great H16 sailor and friend.

Cheers
Author:  srm [ Wed Aug 16, 2017 9:05 am ]
Post subject:  Re: Hobie Cat 16 rudder profile? asymmetric?
There is a reason the EPO rudders are expensive - they work. Probably the single most significant improvement you can make to your boat.

You don't need EPO3 rudders. A set of original EPOs will provide a significant improvement over what you're using now and can be found for a fraction of the price of new EPO3's (there are several on ebay at the moment). Even if they're sun fried and faded, they can generally be restored to very good condition with a little elbow grease - sand them, apply a coating of epoxy, sand, paint, and wet sand to a super smooth finish. Well worth the effort and expense.

sm
Author:  abbman [ Wed Aug 16, 2017 11:57 am ]
Post subject:  Re: Hobie Cat 16 rudder profile? asymmetric?
+1 on EPO's. I found an original set several years ago to upgrade from the old lexan and they were worth every penny.

My lexan rudders would humm to.... I think they just forgot the lyrics.
Author:  dorienc [ Wed Aug 16, 2017 2:27 pm ]
Post subject:  Re: Hobie Cat 16 rudder profile? asymmetric?
Will the EPO rudders work on a Getaway? I had EPOs on my 17 and they still hummed?
Author:  mmiller [ Wed Aug 16, 2017 2:55 pm ]
Post subject:  Re: Hobie Cat 16 rudder profile? asymmetric?
EPOs will not work on EZ Loz rudder systems. They will work on older aluminum casting systems.

EPOs can also hummm. Same trailing edge issue. We trim rudders at every World sailing event where we provide the boats.
Author:  FLOW67 [ Thu Aug 17, 2017 5:06 am ]
Post subject:  Re: Hobie Cat 16 rudder profile? asymmetric?
Hi SRM
Thanks for your reply, I think your referring to ebay USA. Im in Denmark, and anything from USA is subject to import tax, even second hand stuff. so this bumps up the price by 30% on the landed charge.
plus often sellers in USA wont ship to Europe
I will look locally though, after trying my sanded efforts.
cheers.
Peter
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