Rudder design.

[kindredspirit]

Anybody a whizz on rudder design?

I had a new rudder made in the winter after I had ran aground in Courtmacsherry Harbour (I ran aground whilst following both the chart and pilot book instructions, the two of which were out of date as the channel had moved 100 yards east and the markers were missing.)

Anyway, the marina boatyard boat designer and the local stainless steel fabricator both said that my rudder was designed wrongly and that only 10% of the rudder should be forward of the stock instead of 20 to 30% featured on the old rudder. So they made the new rudder with only 10% of it forward of the stock.

I only spotted this as my boat was being lifted into the water. So, I said I would run with this, although I wasn't too happy.

I know this has exacerbated my leaning problem (see "Lean to port" post) because I feel it gives "lift" to the boat as well as direction when it is designed this way. (The "lean" was there before the new rudder but not so pronounced at higher speeds. Naturally I will be going back to the old design (or better) when the boat is lifted out for the winter.

Does anyone have ideas on rudder design? I did a search on the internet but all I could come up with, was designs for saily boats. Is there such a thing as a correct formula or formulae for rudder design on power boats?

<hr width=100% size=1>Kev.

 

[Deleted User YDKXO]

The design of your rudder is the way it is because the naval architect designed it that way and I would be very loath to change it. I doubt that the changes made will generate any extra lift but they may affect the efficiency of the rudder at speed and, possibly, generate different loads in the rudder headstock to those that the designer intended with potentially damaging effect.

<hr width=100% size=1>

 

[oldsaltoz]

G'day Kevin,

Question; Is the total area of the new rudder more or less than the old one?

Also; does this craft have hard chines?

I ask the above because a few years back a friend had a similar problem in that the heel was not noticeable at very low speed, but increased as the boat got faster.

A partial fix was obtained by reshaping the rudder, but it was only when I measured the actual distance from the bottom of the 'V' at the transom, to the edge of the hard chine on each side that we found the cause; the boat was 50 mm wider on the port side.

The cure was a small wedge of microballoons to form a fixed tab on the outer end of the port hull to provide lift.

Might be worth checking next time she's out of the water.

I hope this helps.



<hr width=100% size=1> Old Salt Oz /forums/images/icons/cool.gif Growing old is unavoidable. However, growing up is still optional.

 

[kindredspirit]

Hi Brian,

New rudder is a smaller area than the old one by the amount of area reduced in front of the stock to agee with the equation that only 10% should be in front.

The chines are an integral part of the hull.

What's "a wedge of micro balloons" exactly?

Thank you for your interest. You may be able to come up with an answer.

Kevin.

P.S. If your friend's boat was wider on the port side I presume it leaned to starboard.

<hr width=100% size=1>Kev.