CD36 rudder stops with Max Prop

[Troy Scott]

Folks,

I remember several years ago when I was sailing with Dave Perry in a Robinhood 36, I was serving as helmsman while he worked the sheets. Every time we would come about Dave suggested to me that I was moving the rudder too much. I was turning the wheel too much, trying to be quick in coming about in order and preserve boat speed. What I was doing actually slowed the boat. Moving the rudder so far past the point necessary for coming about increased drag. I'm wondering just when I might ever need so much rudder travel. This question comes up because I will need to reshape the rudder considerably to accommodate the Max Prop IF I MAINTAIN THE CURRENT RUDDER STOPS. However, if I don't really need so much travel, The rudder modification could be much more modest. Thoughts?

[Greg Kozlowski]

Stick with a regular fixed 3 blader and leave the rudder alone.

[Russell]

Personally I think it would be wonderful to have the max prop, just not worth the effort to me to do the rudder surgury. But since your willing, I disagree with Greg and say go for it. Max props are terrific. It is going to be a tough fit though, not only making sure it will fit, but also that it can be removed without dropping the rudder.

As for end stops, I would reduce them only as little as possible, keep in mind situations like moving at slow speeds in tight places, you want as much rudder as possible for doing things like turning 180 degrees on the boats own length.

But if you find you have to reduce a small amount to account for the max prop, its easy to do. I just did it a week ago. The endstop is a single bar that comes down in the middle of the quadrent covered with hose. I needed to reduce the travel distance by about 1/2" so the autopilot ram arm would not reach its ends when going hard over and risk damaging it. I simply took thick wall (1/4") hose and slide it over the existing hose and clamped it on.

[Bill Cochrane]

...Just like everything else on a boat.

Troy, this thread goes back a while but you may remember it.

I'm not so sure I'd accept less rudder travel. Tacking is only one of many situations. The slower the boat is going, the more rudder you need for adequate response (i.e., as Russell points out, close quarters maneuvering/docking). In reverse, you will wish for more rudder no matter what you do.

I don't have a basis of comparison as the prop was on the boat when I got her, but from talking to other skippers, Phoenix maneuvers a bit better in reverse than those with non-reversing props. And the fact that it feathers under sail keeps me agnostic in the locked vs. freewheeling thread.

[Troy Scott]

Russell, Bill and Greg,

How about a compromise that would only reduce the available rudder travel a few degrees in order to avoid major surgery on the rudder? I might still have to trim the rudder a little, but this seems smarter to me. You guys could do some experiments for me. In close quarters, find the minimum rudder deflection that will turn the boat in a tight situation. I'm assuming that deflection beyond that point would provide seriously diminished returns, but I would like to know what that point is before I start trimming. Can you really tell the difference in a full throw-over compared to one that stops a few degrees short of full travel?

[Oswego John]

You guys could do some experiments for me. In close quarters, find the minimum rudder deflection that will turn the boat in a tight situation. I'm assuming that deflection beyond that point would provide seriously diminished returns, but I would like to know what that point is before I start trimming. Can you really tell the difference in a full throw-over compared to one that stops a few degrees short of full travel?

Troy,

In regard to full rudder, I can't say for sure but the thought comes to mind that anything around or past 45° port or stb'd would act to slow the boat down, much like wing flaps do to an airplane.

After the rudder passes an arbitrary point, say 40°, the turning effect decreases and the braking effect increases.

Where are your full travel stops? Pretty much less than 40°?

What angle off of center line is your full throwover set at the present?
O J

[Russell]

Russell, Bill and Greg,

How about a compromise that would only reduce the available rudder travel a few degrees in order to avoid major surgery on the rudder? I might still have to trim the rudder a little, but this seems smarter to me. You guys could do some experiments for me. In close quarters, find the minimum rudder deflection that will turn the boat in a tight situation. I'm assuming that deflection beyond that point would provide seriously diminished returns, but I would like to know what that point is before I start trimming. Can you really tell the difference in a full throw-over compared to one that stops a few degrees short of full travel?

I really do not think a couple degrees will be a huge difference. But more then 5 I would not accept.

Its a hard thing to judge though. I can turn my boat on a dime with ease in winds less then 15 knots. But I can only do it in one direction and it depends entirely on prop walk, which your max prop, being properly pitched in reverse, wont provide you in the same way. In a way, I can see even more need for a max prop equipped boat to need maximum rudder then a traditional prop would.

You could always increase the length of the rudder aft to increase any surface area lost in making room for the max prop and then even more so to make up for lost angle. But honestly in my opinion such drastic things belong in the hands of experienced naval architects who really understand the affects this will have.

I envision your project requires carving out a small bit from the forward end of the rudder to make some room, and perhaps mildly decreasing rudder movement in order to minimize the amount of rudder surface area removed....

I do not think removing area from the forward end of the rudder around the prop is that big of a deal. Most of the power in your rudder is on the aft end of it, that forward area so close to the prop provides a tiny percentage of the turning power comparatively. But, how much do you really need to remove? I guess that is the question, at some point it becomes substantial. Likewise I do not think removing a couple degrees of rudder movement is a big deal, the CD36 rudder moves approximately 38 degrees by my calculation, I had to reduce it to 34 for the autopilot(max it handles is 35, I left room for error), thats 2 degrees on each side, I am okay with that. I would not however be comfortable with much more then that.

Less then 30 degrees I would find entirely unacceptable. It took me a week of thinking to even accept the total 4 degree reduction, I came very close to returning the linear drive and ordering the chain drive (and that would be an insanely complicated install). Ultimately I chose to accept a 4 degree reduction for two reasons: One, ease of install, chain drive would mean huge surgury on the entire steering system. Two, chain drive would not work as a back up if the steering cable or pedestal chain broke, linear drive is direct and independant to the shaft and can thus steer the boat even if the steering system fails (short of rudder falling off). That was enough for me to give up 4 degrees. So I guess what it comes down to, what is enough for you? How important is the max prop and how much do you really have to give up? Its not an easy question to answer, calculating each possibility (ie how much rudder to remove vs how much to reduce steerage) is not straitforward.

I think ideally you would not modify the rudder at all, but make the room by cutting into the deadwood instead. But having recently repowered, I know all too well how little space we have to work with between the tranny and the prop. Even putting in a modern and smaller engine it is so tight back there, you cannot even consider reducing the deadwood.

If you can make a maxprop work, I think thats great, and I hope you try, they are terrific. And I look forward to hearing about your solution. I do seem to recall hearing of at least one Robinhood 36 built with a max prop. It might be worth exploring how that was done. Give robinhood a call, I have often found them to be very open and helpful, but other times not so (cant blame them at all, they must get sick of CD owners calling them on long ago built yachts they really have no obligation to support).

[Troy Scott]

Russell,

Thanks for the real numbers and your thoughts! Right now I can't say (for OJ) exactly what the factory set maximum rudder deflection is, because my steering system is in pieces and on a shelf. But I recall experimenting with the Max Prop shortly after I took possession of this boat; That's when I realized I would probably have to enlarge the rudder cutout to accommodate the Max Prop. However, Dave Perry told me back then that they were able to install the Max Prop on an R36 without modifying the cutout. At the time I was baffled (I still am) because I just couldn't see how that was possible. We'll see. By the way, my boat has a linear drive Raymarine autopilot. I have removed the steering system to facilitate removing the wood core from all the areas of the cockpit sole where fasteners pass through. There was no rot where those fasteners passed through the cockpit sole, so my choices there were precautionary. However, there was compromised core around the removable plate that provides access for the emergency tiller. The "permanently bedded" receptacle for that plate was mounted flush, which meant that someone had routed out almost all of the upper layer of glass to inset it. This plate was set in caulk, but screwed into balsa! This was rot waiting to happen, and it did. I repaired that and created a solid mount that completely isolates the balsa core. Within the next month I should have the steering system back in. A few months later the engine/transmission/shaft will be in. THEN I can better judge the work that will be required to accommodate the Max Prop. If you get bored down there in the warm islands you would be welcome here helping me get this boat all fixed and back together

[Russell]

Troy,

I dont think CD36s varied through the years in their max rudder angles. So far as I can tell, even though the guts or the steering system did change a lot, the core of it did not. Likely the same sized quadrent was used and I am pretty sure the endstop is the same (a center endstop the the quadrent hits from the inside rather then external endstops). So I think my numbers are pretty accurate across CD36s. 38 degrees is about right give or take a degree. Changing this is as simple as increasing the diameter of the endstop.

I am sure some math wiz here can tell us how to calculate it exactly, given the measurment of the quadrent inside where the endstop hits, the measurement of it from center shaft to endstop location when centered and diameter of the central endstop.

Anyway, I too am perplexed by what Perry told you, I dont see it possible in any way that a CD/RH36 got a max prop without rudder modifications, unless there was substantial interior modifications to allow the engine to be mount furthur forward, thus longer shaft and allowing for deadwood modification. But that is a chain reaction modification that would mean the entire interior of the boat would be affected. If you can pry more info out of him I would love to know! I just cannot imagine it.

[winthrop fisher]

when i had a morgan 41,

i changed the prop size and found i went back to the first one,

i would tell you just leave it alone, but thats me.....

when your out in open water, you will not see any differents at all...

winthrop

[Bill Cochrane]

Troy,

Some thoughts...Phoenix is on the hard so all I can do in response to your query is speculate a bit. A few things to consider...

- Based on comments on the old thread and comparing photos, I don't think there was a lot of material removed from the rudder, and once you've taken up the grinder, the amount of effort involved is similar; that assumes that you're going to go through the laminate and need to re-glass, or not, in both instances. The delta between how my boat was modified and your proposal to remove less material, together with rudder travel limits, is probably not that large. There MAY be a small reduction in turning moment in close quarters maneuvering from water from the prop impacting on a smaller area of the rudder; that should also be partially or fully offset by the greater possible rudder angle.

- if you look at the photos in the old thread linked above, both my and the stock rudder are blunt on the forward surface of the rudder at the prop aperture. If I were attacking the area with a grinder I think I'd aim for a more rounded, streamlined profile. This MAY have the added effect of allowing you to remove less material.

- With no empirical evidence whatsoever to back it up, I'd tend to agree with Russell that a reduction of < 5 degrees overall (~2 degrees each side) would probably be acceptable. With the boat in the water, (yeah, I know, you're on the hard and in pieces) you could increase the stop diameter and experiment; just note that the effects would matter most at slow speed maneuvering. I did the calculations when changing autopilot electronics and IIRC (can't find the papers now) the travel is a bit over 35 degrees left and right, or ~70-75 total.

If it would help, next time I'm at the boat I could measure the aperture so you could do some direct comparisons with yours and see how much came off. I can come close scaling from photos, if that would help in the interim.

[Troy Scott]

Thanks Bill!

I do remember that thread. It's strange (embarrassing) to realize how long this project has been going on. The progress is in spurts, mostly my fault for being "picky".

I should know soon exactly how much work will be required to accommodate the Max Prop.

I

[wstringer]

We installed a Max prop on Eendracht, CD 36, 1981, without needing to modify the cutout or the rudder stops. As I recall, the shaft was shortened a bit.

Warren

[Troy Scott]

Thanks, Warren! That's great to know. Do you have a photo?

[wstringer]

I will have to talk to the boatyard tomorrow.

Warren