Space between skeg/fairing pieces and rudder blade: water cross flow
While under water, I have noticed that some small parts of the grp skeg fairing pieces broke and let a wider space (1,5-2cm) with the rudder blade on an approx 10-15cm length above the bottom bearing.
I have not felt any noticeable increase of vibration in the rudder and was wondering (also because i do not intend to take the boat out before the winter) what can be the impact on the rudder, among others through turbulence or cross flow of water?
Anybody having some good information on the specifics of the water flow at this particular area of the skeg-rudder blade "reunion" and how close the fairing pieces need to be to optimize rudder performance?
As for when I'll take the boat out to repair the fairing pieces, has anyone good experience as for the material to chose (my grp fairing pieces might have just been too thin or simply not strong enough) and gluing method to avoid the screws?
Many thanks in advance,
Matt (Maje, Swan 41)

Dear Matt
The slots between rudder blade and skeg need to be as small as possible for best rudder performance. A bigger slot increases cross flow, and this reduces rudder force.
It is hardly possible to make zero slots unless the fairing pieces are flexible and chafing against the rudder blade. The suggested way is to make them fit closely when the rudder is in the hard over positions, and accept small slots when the rudder is on the centerline.
The slot between the top of the blade and the hull also needs to be small for the same reason.
It appears the GRP fairing pieces are a replacement, as the original ones were of stainless sheet. It would be preferable to stay with GRP pieces, as filler and paint sticks much better to their surface than to stainless.
If the fairing pieces are glued in place it would be necessary to force them off when access to the rudder stock is needed
Kind regards
Lars

Dear Matt The slots between rudder blade and skeg need to be as small as possible for best rudder performance. A bigger slot increases cross flow, and this reduces rudder force. It is hardly possible to make zero slots unless the fairing pieces are flexible and chafing against the rudder blade. The suggested way is to make them fit closely when the rudder is in the hard over positions, and accept small slots when the rudder is on the centerline. The slot between the top of the blade and the hull also needs to be small for the same reason. It appears the GRP fairing pieces are a replacement, as the original ones were of stainless sheet. It would be preferable to stay with GRP pieces, as filler and paint sticks much better to their surface than to stainless. If the fairing pieces are glued in place it would be necessary to force them off when access to the rudder stock is needed Kind regards Lars

Dear Professor. Thanks a lot.
Very clear.
Indeed the fairing pieces were a replacement as apparently (so my source) the original ones with screws had slowly let water in the skeg, the water soaking the foam (btw: were all the S&S skegs originally filled up with foam? why not polyester?) and letting tiny water infiltration through in the hull-skeg liaison area behind the rudder quadrant. The grp ones are a few years old and I'll have to replace them obviously at the end of the season - if they hold until then despite the small broken area and the cross flow -.
Thanks a lot for your answer,
All the best
Matt

Dear Matt
Thank you for the information.
If I understand you correctly there was a leak under the steering quadrant? This was probably not caused by the screws, but by a problem in the hull joint, or matting in of the rudder stock casing. The screws can easily be sealed with epoxy.
Skegs are filled with GRP, and solid.
Kind regards
Lars

Dear Matt Thank you for the information. If I understand you correctly there was a leak under the steering quadrant? This was probably not caused by the screws, but by a problem in the hull joint, or matting in of the rudder stock casing. The screws can easily be sealed with epoxy. Skegs are filled with GRP, and solid. Kind regards Lars

Dear Professor,
Apologies for getting back to you so late, I have been traveling the whole week.
Yes I guess you must be right on the origin of the leak (it was before my ownership, and there are no traces left now) , it must rather have come from the hull/skeg joint which makes totally sense. The previous owner might have wanted to avoid any risk by suppressing the screws, even though the water might not have come from there. For my own info and knowledge acquisition, would you have some details on that particular area of the hull where the hull and skeg join (size/thickness of the hull in the area above the skeg compared to other areas, as well as on the method used (i guess the mould was for the hull alone and the skeg was then only "polyestered"?)
forces etc… ?
Have a nice week-end,
Matt

Dear Matt
The rudder skeg is an integral part of the hull, and is laid up at the same time, the hull mould includes the skeg. The skeg laminate thickness is the same as in the centerline area of the hull, about 20 mm.
This would leave a hollow skeg, but it is then filled with glassfiber and resin, and the hull joint laid up over the skeg area inside.
The skeg has to be strong enough to take the rudder bending moment, the rudder stock is supposed to take the torque only.
Kind regards
Lars

Great. Thanks a lot Professor!
Kindest regards,
M