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Mizzen mast rake, weather helm


sanmi

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For a while now, I've noticed that my CS 20 doesn't point very well compared to other boats (including other Core Sound boats). It could be they way I sail her, but I suspect it has a lot to do with excessive weather helm. To get a balanced helm, I often raise the centerboard a bit and luff the mizzen. On a hunch, I took a level to the main and mizzen while the boat was moored in a quiet marina to see whether the mizzen is raked too far back. I discovered that the rake of the mizzen is considerably more than the main. 

 

I calculate that a three to five degree difference would move the center of effort back 6" to 10", but I'm uncertain how much of a difference that would make in weather helm.  

 

I'm not looking forward to rebuilding the mizzen mast step, but if I decide to do it, I'd like to feel optimistic that it will make a difference. Has anyone observed this in their Core Sound boats?

 

Thanks,

Frank San Miguel

San Jose, CA

CS 20 Mk 2 Wren

 

 

Mainmast rake:

main.thumb.png.7d31e715fc3a304085628e8983da325e.png

 

Mizzen rake:

mizzen.thumb.png.6436ba504cfcab22b3b4c9a84b0171e4.png

 

Calculations:

522316169_ScreenShot2020-07-06at9_09_58AM.thumb.png.a3d5dae8dee8c24040d6a793ea9da182.png

 

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More aft rake in the mizzen would give you more, not less, weather helm.  (I think?)  I rely on a filled mizz to help move the boat.  If you bring the centerboard up on a beat, the nose will fall off and make your beat less efficient.  Don't dismiss the need to keep crew weight forward and keep the boat only slightly heeled when going to wind.  Also make sure sails are snugged down relatively flat upwind, with both downhaul/halyard and snotter/sprit tension pulled in. 

After that, we await word from Graham. 

"Relatively flat" = heeled enough to keep the chine in, but not a lot more.

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@Paul356 thanks for the tips. 

 

@Thrillsbe I haven't given much thought to how I would correct the mizzen rake, but you are probably right that it is easier to hack up the thwart than the cockpit sole.

 

From the photos above, it looks like the mizzen is raked at about 10 or 12 degrees and the mainmast seems to be raked at about 2 degrees. I was standing on the boat when I took those photos, so my weight will have influenced the fore-aft trim when I was near the bow so maybe the main mast is actually raked 3 degrees. I don't have the CS 20 mk2 drawings, but I am fairly certain the mizzen should not be raked that much (the CS 15 mizzen is raked at 3 degrees). 

 

So, assuming:

* the designed calls for 3 degrees (my 10 degree mizzen is raked an extra 7 degrees from where it is supposed to be).

* the center of effort on the mizzen sail is 10' above the cockpit sole.

Then:

* the center of effort of the mizzen sail is 1.2' farther aft than the design calls for. ( sin(7) * 10' = 1.2' ). 

 

Graham has suggested raising the board a bit which moves the center of lateral resistance farther aft. This does reduce the weather helm to some extent.

 

So now to validate my assumptions:

1. What is the correct mizzen rake.

2. How much does the mizzen rake affect the steering.

 

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Frank, 

I modeled 10 deg of mizzen rake (see below) and that seems like a LOT. as you said measurement error possibly. Do you have a current profile picture of the boat sitting in the water with masts up. If you get me a decent side picture from  a ways off (like 10 boat lenghts or so then we can measure the rake in the photo very easily in Rhino. 10 deg would certainly cause weather helm. 

I am also suspicious of the rudder as this could cause what feels like weather helm if it's not going down all the way. Maybe try putting it down while the boat is on the trailer and send us a side picture of that as well. Or just make sure it's going down fully. The mizzen will easily overpower the rudder in any breeze and if the rudder is raked back in the cheeks just the slightest bit then the pressure from the mizzen will be amplified until the point that you'll be holding the tiller with all your might with both hands. I've had to sail through parts of florida bay like that before. 

 

385395232_chickoverlay.thumb.jpg.e8357091d37ac26d85949d9e39344ebc.jpgprofile.thumb.jpg.a23a96d0b9d3a56ad2f79e5b6a0c8efd.jpg

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Alan,

 

Thanks for that analysis! You are so right - 10 degrees is just a measurement error. I just went outside and measured the masts on the trailer. First I attempted to level the trailer using the mizzen mast partner and the companionway thwart (picture 1). Then I used a 4' level to check the rake of both masts (picture 2,3)

 

* main : 2.8 degrees (1.375" over 48")

* mizzen : 4.7 degrees (2.25" over 48")

 

These measurements are much more precise and also more in line with your expectations. The difference is 1.9 degrees, not the 7 degrees that I speculated earlier. A rough calculation shows that this would move the mizzen's center of effort back about 3 inches. 

 

I wondered how much the extra slop in the mast step contributes to the rake (on this boat, the mizzen mast step is shallower than others), so I stuck a shim in the aft edge (picture 4) and measured again.  It reduced the rake by half a degree (2" vs 2.25" over 48")

 

While the boat was "leveled" I mounted the rudder (picture 5). It seems ok, but I'm not sure what it is supposed to look like).

 

 

picture 1. leveling the boat on the trailer:

239205257_ScreenShot2020-07-07at7_47_20AM.thumb.png.b6d64b1a0360ee636e1745c1bc94ebbb.png

 

 

Picture 2 - mizzen rake:

1467816728_ScreenShot2020-07-07at7_49_13AM.thumb.png.cc2df64531d4e923f146d0a16ec8744d.png

 

 

Picture 3 - main rake:

1710399726_ScreenShot2020-07-07at7_48_52AM.thumb.png.b6c2e00ecc1ea6f750d8ad9241a641c6.png

 

 

Picture 4 - rudder:

1021244072_ScreenShot2020-07-07at8_14_47AM.thumb.png.2289274709cab053e413a82ea8896732.png

 

 

 

Picture 5 - shim on mizzen step:

1404581182_ScreenShot2020-07-07at8_15_14AM.thumb.png.6d419350912dfb3d81248c7624a6c13f.png

 

 

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Rudder looks ok to me. Hmm. On a beat the mizzen acts even more in the mains backwash which is why it sometimes feels on the verge of luffing. Sheeting it in harder is the natural reaction but this imbalances the forces. Causing weather helm. I wonder if this explains your helm? On the EC in our CS-20 i often experimented with the best trim on a beat. I found that keeping the boat perfectly balanced when trying to point (i.e. not sheeting in the mizzen more even when i could see it on the verge of luffing) was ever so slightly slower in light air. So i would sheet in the mizzen just a touch and have a bit of helm as you should have when pointing. Over do it and the extra rudder needed slows you back down so there is a balance. Staring at your VMG on a gps is the only way to know what is best, fastest.

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Ok, I think that answers my question about the mast rake - it is not the likely culprit. I'll experiment with sail trim, centerboard position and ballast / no-ballast. 

 

I love the idea of using GPS traces to validate different approaches. I do this kind of data analysis for my bike rides all the time but it never occurred to me to do it for the boat (it has been such a long time since I raced dinghies).


Our next outing will be to the sheltered waters of Tomales Bay so I should be able to get some good data.

 

Thanks for your help!

 

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