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Just a beginners question--rules are don't motor above 6 knots with rudders and board down because it will become unstable, but I read about people clocking greater than 6 knots sailing with all boards down.
Why isn't it an issue when under sail, with the boards acting like wings and trying to lift out of the water, same as when motoring?

Thanks!

I am not an expert, but I remember reading on here that it is the motor and the pressures created by it towards the rudders that are the reason for not going over 6 kts.

Gary

It actually is an issue Greg. The 6 kt limit is a conservative number - I have motored in the 8-10kt range with one rudder down for many seasons without a problem. When I do this, I dont make drastic helm movements and ease the rudder into the turn rather than slamming it over and putting a lot of force on the rudder, brackets, and steering gear.

A classic way to break rudders on a Mac is, in heavy seas, sliding down a wave at >10kts (motor or sail), hitting the bottom with the boat at some angle to the direction of travel, and then having a rudder or board trying to bite into the water to bring you back on course...

SNAP!!!

My own view on this is

0-6 kts - fins ok...
6-10 kts fins need to be handled prudently
>10 kts fins up (I almost never am in this mode btw)

My seems to be at its best handling , on motor, with the CB down slightly , about 4" on the control line .
I dove under it and this is about 6" (rough estimate) of CB showing. I run with at least one rudder down .

I agree with the 6 knots = 7.2 mph , I don't run my boat much faster than this , except speed trials ,
so this is about right for me.

I am a beginner , and all this is coming from experimentation.

As a side note ,,,, I have learned I can hit my slip on one try if I have some CB down . If its all the way up , I can not make the tight horseshoe turn that it takes to get in it. This , of course , is at idle speed .

To get to my slip , I must make a left turn , then my slip is the first one on the left , thus a horseshoe turn.

The choice for Rudders and Centerboard down are actually based on two different things.

I always have at least one rudder down at low speed around docks as well as the centerboard full down. This is the maximum control configuration. You get the tightest turning circle and the most responsive helm control. You also get control when coasting in neutral. The motor leg it self doesn't have the needed surface area to turn quickly when the prop is not pushing or pulling.

Interestingly even with all boards full down at low speeds my boat responds much quicker turning left than it does to the right. I find I can easily approach a dock on the starboard side at an angle and make a quick last minute left to pull along side when coasting in neutral. When the dock is on the port side and I need a last minute right I can put the helm hard over and the boat still goes straight for quite a ways before slowly changing heading. It's not a prop walk thing as the motor is in neutral. Lately I have changed my tactics for the port side approach. Rather than a hard right at the last minute I put the helm hard to port (left) and give the motor a bit of reverse. This not only stops the forward motion but pulls the stern over against the dock.

At what can be thought of as sailing speeds, below or at hull speed (6.5 knots, 7.4 mph) it is perfectly fine to leave all the fins down. There is no more stress on them under power than there is sailing.

Above this speed is when you have to make some choices. The only real issue of going faster with the rudders down is you could break them or their brackets. Even at speeds up to 10 knots they can be useful in the right conditions. Having a rudder down will greatly improve the boats tracking under power.

Having the centerboard full down above hull speed is where you can get in trouble. The board will generate lift at higher speeds under power. If the boat is perfectly balanced this causes no problem. But as soon as the boat rolls to one side or the other the lift from the board becomes unbalanced. This will cause the board to climb toward the surface and can start an unexpected and uncontrollable roll.

Hence the over conservative factory statement not to motor faster than 6 knots with anything down. They play it as safe as possible.

Tracking at higher speeds will actually be much better with just a bit of centerboard down. The boat will wander quite a bit less. On our X this means 2" to 4" of line down (I marked lines on my line every 2" with a sharpie so I can easily tell how much is out). This just lets the tip of the board peak out of the trunk but keeps most of it's surface area inside. It will not roll the boat at any speed in this configuration as it does not present enough area to develop lift. It simply acts as a skeg that increased directional control. Likewise I'm sure a small amount of board on a M will have a similar effect.

With my smaller 50 hp motor I used to always keep a rudder down as well as the board well into the 7 knot range. Without the rudder the motor could not balance the tracking force of the board and would wander. Now with my 90 I find that the rudder is not needed. The larger motor leg and the bigger 14" prop are able to balance the boat nicely and it tracks well without a rudder down.

Our standard slow cruise is now ~3,000 rpm giving us 7-8 knots with the board full down and the rudders up. The boat handles well in this configuration, the bow is still low for visibility and the motor is quiet. This is a consistent configuration with ballast empty or full.

Our standard fast cruise is now ~5,000 rpm giving us 13-14 knots with the board 4" (of line) down and the rudders up. This is also a stable point and takes advantage of the hull form stability the boat gains above 12 knots. The boat tracks well at this speed and ceases the wandering that happens in the 9-11 knot speed range. This is a ballast empty configuration.

I have no issues leaving the board slightly down (2" of line) all the way up to our WOT speed of 20+ knots.

By the way, under sail it's usually all boards full down however down wind pulling up the board and one rudder will increase your speed while still giving you plenty of steering control.

Spot on Summary in my book Duane... although I can not vouch for any thing over 15Kts with my Yam 50!!

Very interesting write up , Duane !

You confirmed a lot of my theorys . I suspected the 50hp motor wasn't enough motor , with enough "foot" to control a 26' boat . Thats why it needs the rudders to assist in the control.
A 90hp has a big enough foot and prop to control the boat.

A few years ago , when the southeast was in severe drought , our marina got so shallow we couldn't run the gas motor to get out . I mounted an electric trolling motor on the swim platform of our Bayliner .
This motor ( 28 lb thrust ) would push the 6,000 lb boat at about 3 mph , but steering was next to impossible , constantly over steering , Much like a Mac at 3mph with all fins up.

A Mercury mechanic friend of mine ( many years ago ) wanted to mount twin 50s on the back of his houseboat to replace the 90hp on there , same situation , He said He was afaird the 50s wouldn't provide enough control . Didn't understand until becoming a Mac owner.

Not that I am unhappy , contrare ! I am not ! I am very satisfied with our

The Power Thruster will definatly give you better slow speed docking control.

Bob

I sometimes motor at 7 knots with all boards down(centerboard down to clear bottom) but have a quick Release Cam on rudders incase I hit a floating object or shallow waters, also good if to much effort force is applied to rudders. In the Gulf of Mexico I don't use them and cleat the lines to Stationary cleats.
I could only find These units at Duck Works and max. line is 1/4 inch,adjustable to 50-500 lb force. You will know when the Cam is released as the rudders will kick up to your amout of slack in line and steering is to much effort.
It's a nice feature to have in thin waters or areas that have floating debree and when rudders are over stressed.
I still have the original Aluminum brackets on my 97 MacX with to fatiuge or stress problems.

Dave

teethlaw wrote:Just a beginners question--rules are don't motor above 6 knots with rudders and board down because it will become unstable, but I read about people clocking greater than 6 knots sailing with all boards down.
Why isn't it an issue when under sail, with the boards acting like wings and trying to lift out of the water, same as when motoring?

Thanks!

I tried searching for length of line for depth of CB on an , couldn't find it. Anybody have that post/link? Rob

Tahoe jack did these calculations back a few years ago and they are very close to what I experance.
Quote
(Centerboard vs line on 26X
The line that retracts/deploys the board is on the right or starboard side of the forward side of the cockpit. The full throw of the line is 23.25 inches. This allows the board to be in the full down position - 63 inches below the bottom of the hull. The geometry of the line vs. the pivot results in a non-linear relationship between adjustments to the line and levels of keel deployment. For motoring stability in rough water, about 8-9" of board will tend to behave as a skeg or stabilizer, without significant adverse wing-lift effect.
A boat hook can be marked with critical depths as measured from the aft gunnel. If a depth finder is used, verify the accuracy vs. dip measurement. Except at docking-speed, the rudders should be secured down up to 8knots, then fully up. Partially deployed rudders are easily damaged when exceeding 4 knots. Dependant on loading, the Mac26X normally has a true water line about eight inches or 0.7' below the stripe.

Keel board line-Hull to deepest board extension-.Stripe to deepest extension

2.5"---------------9"or0.75'----------27"or2.25'
3.75"--------------12"or1.0'----------30"or2.5'
5"-----------------21"or1.75'---------39"or3.25'
6.75"--------------24"or2.0'----------42"or3.5'
11.6" (half-way)--41"or3.4'----------59"or4.9'
23.25"(max)------63"or5.25'----------81"or6.75'

-.From the boot stripe to the bottom of hull is about 18" or 1.5' at midship.
-.From boot stripe to bottom of Suzuki 50HP skeg is 24" or 2.0'.
-.From boot stripe to rudders full down is 39" or 3.25'.
-.From the cockpit gunnel-top to the boot stripe is 41" (3.5') at the aft stanchion.

Minimum water depth below the boot stripe is 1.5' (contact midship), no motor down, no rudder down, no keel board down. For a one-foot margin, 2.5' below boot strip or 5.9' below aft gunnel. From true water line 0.7' below the stripe, the minimum water depth to hull-bottom is 0.8'. For a one-foot margin, water should be at least 1.8' deep.

Absolute Minimum Depth below boot stripe for motor down, no rudders, matching keel, is 24" or 2.0' with a 2" keel line set. This is drag-motor/keel board zero margin. A soft bottom will churn up major mud and risk damage to the cooling system. The suggested low speed minimum depth below boot stripe for full motor down is 3' below stripe, or 6.4' below aft gunnel, and keel line setting of 2". (This allows a one-foot margin for the motor and keel).
This is about 2.3' of true water depth.

For full rudder down slow maneuvering, motor full down, and keel board matching rudders at keel line setting 5", drag-bottom at 3.25 below stripe, or for a one-foot margin, 4.25' below stripe or 7.7' below gunnel. True water depth about 3.5'.

Full rudder down, motor down, keel board full down, one-foot margin - .requires 91" or 7.7' below stripe or 11.2' below aft gunnel.

Simply put - 2" keel line for motor down match-6.5'gunnel; 5" keel line for rudders down match-7.7'below gunnel; all down, 11.2'below gunnel. Rough-water motoring skeg-2"keel line-no rudders above 7 knots. High speed sharp turns - no skeg. Tahoe Jack)

Dave

SkiDeep2001 wrote:I tried searching for length of line for depth of CB on an , couldn't find it. Anybody have that post/link? Rob

Thanks Dave Rob

Duane Dunn, Allegro wrote:Interestingly even with all boards full down at low speeds my boat responds much quicker turning left than it does to the right.

I've noticed this phenomenon too. Murphy's Law, it's especially prevalent when docking in a crowded area or dealing with current/wind/dock...

Slight asymmetry in the rudders or perhaps slight imbalance in load?

I once motored up the Chesapeake at night, from Baltimore to Elk Neck, steering by just shifting my weight in the cockpit at about 13 kts.

An interesting note: Your speed through the water can be deceptive.

I use my GPS for speed and do not have a water wheel (no log line either!). While motoring at what I thought was under 6kts, we experienced the thrill of heeling due to the daggerboard being down. The combination of the current and my speed combined to create the necessary speed. I also suspect that the current makes it much worse as when it gets broadside to the daggerboard, it gets more leverage and has more of an impact on steering. Boards & Rudders up solved the problem.

I have gotten up to 7.5kts (reaching) thus far sailing and have seen some posts where folks have achieved 10+kts. I would suspect that this would invoke the same joy ride if the daggerboard were down. In fact, i would suspect it would be worse as the heeling of the boat would assist the daggerboard in flying.

My suspicion is that when achieving these speeds the boat was on a reach with the daggerboard partially retracted negating the effect. It would be interesting to here from those that have pushed their boat to these speeds. How fast has anyone gone close hauled with all the boards down?

With all boards down, docking is ridiculously easy and the boat can be easily managed at the slowest speeds. I compare putting it on the trailer with the boards up to playing the old "Lunar Lander" game.

Jim