Mac25 Upwind

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fritz3000g
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Mac25 Upwind

Post by fritz3000g » Sat Jul 07, 2018 5:57 pm

Hi there,

I've been sailing my Mac25 this past week on Green Bay, in Wisconsin. Couple weeks ago I was in Lake Winnebago (very large lake). In both cases I had newbie guests along and because of weather forecast miscues had to do significant upwind beating into waves. This was uncomfortable and scary for some of them. Also it took forever.

I noticed that the Mac25
  • 1. gives a lot of leeway (low keel surface area)
    2. has intense weather helm under main only (raising keel halfway helps some)
    3. and isn't very sea kindly (very light boat)
I'd like to improve its upwind performance and comfort, and wonder if anyone has ideas?

I already plan to increase the rudder surface area which should help with 1 and 2. I know I could add a boom vang and traveller too, but wouldn't expect dramatic improvement from this.

I can't think of what I could do for $3 besides fill the bilge with water.

Any other ideas? Thanks!

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sailboatmike
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Re: Mac25 Upwind

Post by sailboatmike » Sat Jul 07, 2018 7:17 pm

yes she will be slow, uncomfortable, round up (weather helm) and give a lot of leeway but I think you have answered your own question when you put "Under main alone".

Raise the foresail (normally jib in heavy conditions) will counter all of these.

1) When beating (sailing up wind) the jib provides the real power for the boat, the jib and the main act together and if you get it just right you get what you call the "slot effect" which is much like a biplane where the wind going over the jib accelerates and increases the lift the main gives, but in general sailing upwind with out a jib is always going to be difficult and SLOW.

2) Because you are not using a foresail the boat is totally unbalanced, think as the sails much the same as a weigh scale with the mast as the pivot point, when you push down on one side (wind on the main) unless you have a counter force on the other side (wind on the jib) the bow will be pushed into the wind and hence round up the boat, so you give more rudder input to try and keep it straight, creating more drag and slowing the boat still further.

3) She wont be sea kindly because of the above, she will be yawing around her pivot point as the main turns her into the wind and the wind and swells turn her back the other way. Nothing to do with the weight of the boat its about sail balance and keeping the boat balanced.

4) Because she wasn't balanced you lifted the keel giving her less tendency to round up (weather helm) due to having greater surface area to counter the lateral movement, HOWEVER she will never sail upwind correctly with the keel 1/2 up because it stops providing lift up into the wind causing you lose MORE SPEED.

The answer in three words is RAISE THE JIB when sailing into the wind

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Freedom77
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Re: Mac25 Upwind

Post by Freedom77 » Sat Jul 07, 2018 7:28 pm

Sailboatmike is correct. The Mac 25 is a little more tender than the V25. Both have large mains. Freedom is masthead rigged which allows for bigger headsails. I sail on Lake Mead Nevada which has some nasty winds. I've been sailing for almost 50 years. My advice, for what it's worth, is when closed hauled up wind in heavy wind ease the main and spill some of the wind. This will help with rounding up as the main wants to push the boat to leeward and it pivots on the keel. Also, not a good idea to sail with the keel in any position except fully down. As far as weather helm, easing the main will help some. I have a "spade" rudder which helps balance it. I also installed mid boom main sheet which help immensely with main sail shape and adjustment. Keep us posted. Fair Winds and Full Sails...Old Salt. 8)

Sisu
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Re: Mac25 Upwind

Post by Sisu » Mon Jul 09, 2018 12:36 am

Hi Fritz, I have a Mac25 on Winnebago - we keep it in Neenah and race there Tuesday nights. Upwind is not the boat's strong suit but we manage to make it work. The important thing is to keep the boat under control and FLAT. It's a light, flat-bottomed boat, which is great for trailering and downwind sailing but horrible for stability in waves or heavy air. The secret to good upwind sailing in breeze is sail control, which is why a vang is really helpful for depowering the main (reef points are great for this too). As said here before, never sail without a jib. The rig is too far back as it is. You'd actually have better luck if you have a genoa hoisting that with no main in breeze rather than sailing with main only.

Now, if you've gone completely around the bend like me, you can spend a winter attempting to attain the ultimate upwind performance: take your keel off, strip it, seal it, and fair those slab sides that don't produce any lift into a proper 13% NACA foil. Took me almost 2 gallons of West System, but the boat can actually go to windward this season! I'll put my build log up someday when I have the time. I also got a Ruddercraft HDPE rudder this winter and that helped controllability a lot.

If you want to come down to Neenah sometime I'll show you how our boat is set up.

fritz3000g
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Re: Mac25 Upwind

Post by fritz3000g » Mon Jul 09, 2018 9:33 am

Thanks everyone!

So the basic lesson is that I need the jib for beating and to get to that rudder upgrade.

I didn't hear any mention of the stock around-the-boom furling that comes with the Mac25. In a blow, if I've got the jib up I'd have to heavily reduce the main, or gusts would dunk the rail and scare people (or I'd have to let the sails flog a lot). In a close haul, the end-boom main sheet keeps the boom pretty low so that seems like it would negate the boom vang unless I'm missing something. Stock furling seems like the answer, especially if the main isn't important anyhow in that scenario. Is that right?

I don't typically get much yawing in close hauled (I do when running or in broad reach) even with just the main. It's the bow crashing over the backs of waves that bugs people. Becomes a problem when the waves hit about 30".

Sisu, I'm not sure what this means: "fair those slab sides that don't produce any lift into a proper 13% NACA foil"
Last edited by fritz3000g on Mon Jul 09, 2018 1:49 pm, edited 1 time in total.

Sisu
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Re: Mac25 Upwind

Post by Sisu » Mon Jul 09, 2018 9:57 am

In my experience the around-the-boom furling for the main leaves something to be desired, but it's better than nothing. Hard to do it without the boom starting to droop. There's a reason that MacGregor dropped it on later models in favor of a "traditional" reefing system.

The point of the vang is to keep the boom at a constant angle when the sheet is eased. Without it, easing the sheet makes the boom go out, but it also makes the boom go UP. This dumps most of the power from the sail in one shot. With a vang you can better control how much the main is depowering when you ease the sheet. It also does wonders for your downwind speed, reducing twist in the main.

OK, I was being a little nerdy there, but the basic issue with the Mac/Venture keel is that aside from the bulb at the bottom, the sides of the keel are flat. A keel reduces sideslip by generating lift, just like an airplane wing. So in order to enhance the keel's lift, I built the midsection of my keel up with thickened epoxy to conform to a NACA 0013 section, a wing profile that is generally considered a good all-purpose sailing foil. The cost is a little more downwind drag, but the boat goes upwind very well now, no more leeway than any other boat I've sailed. The "Template Guide Method" of this PDF (starting page 9, bottom) describes the method I used: https://www.westsystem.com/wp-content/u ... ishing.pdf

fritz3000g
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Re: Mac25 Upwind

Post by fritz3000g » Mon Jul 09, 2018 11:37 am

Interesting Sisu. If you get the boat out of the water at some point I'd love to see a pic. Thanks!

Sisu
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Re: Mac25 Upwind

Post by Sisu » Mon Jul 09, 2018 2:02 pm

Will do. Been meaning to upload the build log for awhile now.

As far as waves go, a flat-bottomed boat will never handle waves as comfortably as a deep, round hull. As long as you're not trying to cross the big lake in an October gale there's nothing unsafe about the Mac, but the hull shape is just inherently less comfortable in waves.

fritz3000g
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Re: Mac25 Upwind

Post by fritz3000g » Thu Jul 12, 2018 10:06 am

Sisu, I've been trying to figure out why making your keel into an airplane wing shape would help with leeway. An airplane wing and a sail both provide lift, but in one direction. A keel would provide lift in both directions (both port and starboard), cancelling each-other out, unless somehow you could rotate the keel.

Anything you can share on why this helps would be really interesting to me. Thanks!

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sailboatmike
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Re: Mac25 Upwind

Post by sailboatmike » Thu Jul 12, 2018 3:41 pm

fritz3000g wrote: I've been trying to figure out why making your keel into an airplane wing shape would help with leeway. An airplane wing and a sail both provide lift, but in one direction. A keel would provide lift in both directions (both port and starboard), cancelling each-other out, unless somehow you could rotate the keel.

Anything you can share on why this helps would be really interesting to me. Thanks!
Ok good question, if you think of a aircraft wing it really does not provide anything much without speed and importantantly "Angle of attack" you will notice that aircraft lift their noses high when going slow, this increases lift as more air hits the bottom of the wing. (I know is a REALLY basic explanation) but that is the basics of it, you can YouTube it and there are some good videos that show flow over the keel

Sisu
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Re: Mac25 Upwind

Post by Sisu » Thu Jul 12, 2018 5:26 pm

Mike got it right. To answer your "lift in one direction" question, here's my best shot - bear in mind I'm not a physics (or any kind of science) major although I am a pilot:

The typical explanation of lift hinges on Bernoulli and the "sky sucks theory," but that's an overly simplified way of looking at it. If that were the only thing going on, this aerobatic airplane wouldn't be able to fly with a symmetrical airfoil:

Image

A better way to think about it is that the wing needs to push down more air than the plane weighs. If you only want lift in one direction (i.e. you don't plan to fly upside down too often) an asymmetric airfoil helps this by curving the airflow over it so that it exits the wing on a downward trajectory, but if the plane flies at enough of an angle of attack it can accomplish the same thing (essentially, it's presenting a downward-angled ramp to the air). The aerobatic airplane can fly just about as well upside-down as rightside-up.

Since we only have one keel and it needs to generate lift on either tack, it needs to be symmetrical.* When you load up the boat going upwind, you're actually propelling the boat sideways against the water and inducing that same angle of attack that you would have on an airplane wing. The resulting lift vector gives you (mostly) forward motion upwind. You can also stall the keel, just like an airplane wing. If you just point the boat upwind from a standstill without sufficient forward motion, you'll just go sideways as if you don't even have the keel down. That's why you usually have to gather speed on a reach before hardening up to closehauled.

So, if you want to reduce leeway and increase lift, you can sculpt your flat keel into more of a foil (teardrop) shape like the "bulb" at the bottom already has. This is what I did and it seems to be working. I essentially added chord to the shape. There's no free lunch in physics so the penalty is a bit more drag, particularly downwind, but our upwind performance is much better this season to my estimation.

*Some high performance boats actually do have asymmetrical keels - they either use a moveable trim tab on the back of the keel or they use twin daggerboards that they switch on each tack. If you want to see something really nuts look up the "tandem keel" that New Zealand used in the 1992 America's Cup.

fritz3000g
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Re: Mac25 Upwind

Post by fritz3000g » Fri Jul 13, 2018 8:36 pm

Ok. So the forward motion and the fact that it is slipping is what provides the lift. Like this? (force strength indicated by arrow size)

Image

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sailboatmike
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Re: Mac25 Upwind

Post by sailboatmike » Fri Jul 13, 2018 9:08 pm

fritz3000g wrote:Ok. So the forward motion and the fact that it is slipping is what provides the lift. Like this? (force strength indicated by arrow size)

Image
Well almost, as the boat gains headway (moves forward) it moves the wind from true (the direction it is actually blowing from) to apparent (which is the wind taking into account the wind the boat is making by moving forward), apparent wind increases as the boat gains speed, which would bring the apparent wind forward to the true wind direction. You can feel this effect when your in the car, the true wind might be 5mph at 45 degrees to the direction you are heading but when you speed up the car the wind comes forward and the faster you go the more the wind comes forward until its almost a dead head wind.

So this means as you gain speed you have to come off the true wind to keep the lift happening and the speed up

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Freedom77
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Re: Mac25 Upwind

Post by Freedom77 » Sat Jul 14, 2018 10:17 am

The roller reefing on Macs is useless. By the time you need to reef it's usually pretty dicey. Find a sailmaker and have a double set of reef point put in. You will be glad you did. Fair Winds and Full Sails...Old Salt :D

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sailboatmike
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Re: Mac25 Upwind

Post by sailboatmike » Sat Jul 14, 2018 4:36 pm

Freedom77 wrote:The roller reefing on Macs is useless. By the time you need to reef it's usually pretty dicey. Find a sailmaker and have a double set of reef point put in. You will be glad you did. Fair Winds and Full Sails...Old Salt :D
The problem I have with roller reefing is that you have to take the vang off, in general if its so windy we have to reef we want to have as many options to depower as possible, as soon as you remove the vang you are losing one of your depowering tweeks, if your sailing down wind its probably worse because you can't tension the boom with the main sheet and hence the boom will ride up causing the main sail to belly out which as we all know actually increases power.

Add to this having to be up on the cabin roof to wind the handle, slab reefing with single line and halyards back to the cockpit is the safest in my book, I mean who wants to be on the cabin roof as the boat is bucking around.

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