I am admittedly not.. a great sailor / tactician.. Nautical Engineer..
So? Is it the width from leading to trailing edge? The foil design? Both ?
Is it a, directly proportional calculation? Could I reduce length / depth and increase width ( sq.area) and have the same or more, directional stability? my boat will NOT be heading to the water this year.. as I wait to see if this house sells... Its $2000 Non Refundable for a season in a slip up here..
Find out what they charge to be a visitor. This year I will not be slipping my boat but the closest marina (5 minutes away) charges 6$ a foot for a mooring and 9$ a foot for a full services slip. at 6 bucks (156$ per week taxes included) a foot I will spend quite a few weeks (the sunny ones) at this marina: http://www.marinadelachaudiere.com/
Q1
We MAY??? do a temporary slip for a few weeks... North Point Marina has day week month and seasonal.. rates as well..
Getting ready to move for us is HUGE..
So is the issue ....sq. area? Florida has lots of skinny water.. at least where we intend to move to... IF we can sell This Place!!
I wouldn't worry about it too much.
I live in Sarasota and we have our share of "skinny water".
But rudders kick up and most other boats draw more than a Mac (even most powerboats of a similar size... at least when looking a "draft" that won't kick up!). So, you'll be able to sail where other boats are going.
If you get into dramatic rudder mods, (in my opinion) you're opening a can of worms. If you shorten the depth of the rudders: while heeling the "bite" of the rudder (singular, since when heeled, only one works anyway) you'll probably lose what little steerage you have. Also, the force on the rudder will probably increase (try putting one rudder "not all the way down" and you'll see what I mean). The stock rudders are balanced to keep these forces in check, if you modify them, you'll risk not having that balance. And, finally, the Mac rudders look a bit odd when raised now... they might start looking really goofy at twice the width.
I'm sure Roger's design team has looked at all aspects of the stock rudder system very carefully and allowed some room for extra forces to happen without anything breaking. That being said, the bets would be off if they are not secured in the fully down position as the moments of leverage would go through the roof both on the rudder pivot bolt area, and the transom bracket. The internal linkage forces would be much higher as well.
I haven't ever directly compared any of the after-market rudders, measurement for measurement, with the stock profile but I have read that the NACA profile is different (less drag) and the blade width ahead of the pintle pivot is greater. Greater width a/o depth at the same drag must equal better control.
I'm trying to modify my setup to achieve more performance with the existing hardware while staying within the design limits. Third party will definately get you there - I've never read a negative review - but you better have deep pockets! Shallower blades though, don't seem to be the way to go on a tender boat like the Macgregor.
I changed mine to IDA sailor rudders. As I recall, the stock Mac rudders pivot aft of the leading edge. That can cause the flow to separate and become turbulent when changing rudder positon since the leading edge will be moving in an arc instead of just pivoting. Having the pivot point be right at the leading edge maintains better flow.
Many rudder arrangements have the pivot point back prom the leading edge. The principle is the same as found on aircraft elevators and is done to reduce the effort required to position them. The arc described by the leading edge occurs only momentarily and the waterflow over the entire rudder is really the main force in play that can separate if angled too much.
The mac blades definately have more lead than the IDA and less trail as well. I can't tell from the pictures what the numbers come out at. Anyone?
FWIW - I measured the pivot center as 23% back from the leading edge. I looked up some design standards in my book by Steve Killing (Yacht Design Explained). He is a well-known IOR yacht designer. He says typical specs are:
- The center of the rudder foil's lift is typically 25% aft of the leading edge.
- The center of pivot is typically 15% aft of the leading edge.
This included vertical rudders like the Mac's. BTW, having the pivot center position at the same location as the center of lift creates a balanced rudder with no feedback, or feel. Some degree of feedback is desirable, and so the 15% - 25% rule.
So if all that means anything, it would seem that the Mac's rudder are a little too "balanced".
Cheers,
~Bob
P.S. FWIW - remember, when looking at the "swing-pivot" holes drilled in the rudders - that's not where the steering-pivot-center is. It's 3.5 inches forward of that.
One Edit: LOL! - I said "book by Jeff Skilling"....he's that corrupt Enron bozo that landed in prision. I corrected it to read Steve Killing.
Last edited by Currie on Mon May 23, 2011 6:49 am, edited 4 times in total.
As Crikey and others are discussing in another thread, small changes in the aft sweep of the rudders will change the location of the centre of pivot and the rudder feedback.
So as the rudders "wear in" , in time they will become more balanced! ('Cause I'm sure it's planned that way).
Given that my steering linkage bar continues to screech and etch an ever-bigger hole in the transom fiberglass - I'll bet you've nailed it.
Hahaha. Mine did that brand new and used to drive me nuts; I used to spray lub on it. That would help for a while but then as a more permanant solution I put a teflon sleeve from McMaster-Carr over it. That worked for a few years but then finally fell apart so I took a piece of 1/4" telflon, drilled a hole in it for the linkage, and mounted that over the linkage hole in the fiberglass. So far that's working.
Heim joints all around and the BWY engine linkage is the way to go. I forgot about the actual pivot point when looking at the rudder pics. It would be interesting if you could make rudder 'socks' to slip on and experiment with the lead and trail. My reason for hogging this subject so much is because of my mod-in-progress, which I'll post fairly soon. Besides hydraulics and heim joints I'm dropping the blades two inches lower and trying to work out a strong enough bungee hold down.
my boat will NOT be heading to the water this year.. as I wait to see if this house sells... 
, in time they will become more balanced! 
('Cause I'm sure it's planned that way). 
