MacGregorSailors.com

HI Im interested in wat{ Bill at BOATS 4 SAIL} said re a a partically filled ballast tank ,could may be cause of capsize .I thought I read that its advised when motoring that a partically filled BALLAST tank saved fuel and gave some stability.as im new to mac 26x and 65 and a wife that nags alot about the boat rocking . In light of bills comments Ill keep it full .



regs JOHNNY {SYD AUST }

johnny....

Full or empty only.........The waterballast tank is not designed for partial operation ( i.e. where there may be some heel in the boat during operation)

Take Bills advice - The ballast tank must be either completely full or empty - you should check it before every sail ....

Yes, empty or full, clearly stated in the manual.

The problem with a partialy full ballast is the water will swish around, causing a lot of unstability, way worse than having no ballast at all.

If you want a greater understanding of why the tank either needs to be clear full or completely empty, try googling "Free Surface Effect" (or look it up in a book on Naval Architecture).

Have you ever seen one of those oil-and-water things with the two colored liquids in a long rectangular clear box which slowly rocks back and forth, and "huge" waves wind up crashing back and forth within the box?

- Andy

Try to carry the largest cake or turkey pan you can find, full of water with no lid . Do this outside BTW. Walk 50 feet across uneven terrain. Did most or much of it end up on the ground? That shows an un-baffled partial filled ballast. The water moves around gaining velocity and momentum. Car fuel tanks are baffled and Most large Motor Cycle tanks are to some extent as well, as they straddle the main frame support.
Bottom line as others have said , its either full or empty. That said, Mine is always full.

Yes, the dangers of partial tanks cannot be overstated.

Raises a question however. I recall someone in this place once mentioning that the original boats (later ones too?) actually have baffles. Has anyone ever been able to confirm that?

The brochure drawings don't have any detail like that, but it would be very interesting to see a section drawing of the inside of the main tank.

The real reason the partial ballast tank is dangerous is that the ballast will not be where you need it as the boat heels. The ballast will flow to the lower parts of the tank, relative to gravity, and the side channel of the ballast tank that is lifted high, will have (weightless) air instead of ballast. Whatever ballast water is missing from a full tank, will be missing from where it is needed most, when the boat is heeled on either tack. The remaining water in the tank cannot help the boat right itself.
Sloshing is also not good, but the real problem is lack of effective ballast.
Baffles would help with sloshing, but not with the lack of high-side ballast. Two separate tanks would be some help, if kept equal, but the PITA factor would go way up then. There is enough to keep track of, the way the boat is designed.

The only times I've ever experienced stability issues with my Mac have been when the tank is partially full (while draining under way). An empty tank is always preferable to a partially full one. We motor empty 95% of the time. Almost 1500 nautical miles under power this way with no stability problems even in rough seas. All with a crew of 5 on board.

I would second motoring over 10 knots with empty ballast. When people comment that the Mac is a wet boat I'm thinking they must motor with full ballast.

I sort of agree with ALX. Afterall, it is not just a big tank, there are channels and sub tanks which make up the ballast system on my X, so in a way, I would consider that to be somewhat baffled since the water can only slosh in the sub tanks and not throughout the whole ballast system. When you think of a heeled boat, the ballast water which is above the water line is what provides the righting force, if you have half or less in the tank, then there is no such force. Seems to make sense that ballast water below the waterline would have the opposite effect and actually cause more heeling. With movable ballast, it all needs to go on the high side (above the waterline).

Absolutely either or. Full or empty. The X sails and motors just fine with or without ballast in light air conditions. Anthing more than 10mph, get that ballast in fast.
With or without ballast, my X is a wet boat at any speed above hull speed when the surface chop is more than a foot high.

Actually my standard motoring operating mode is the opposite. Any distance of more than a couple miles or any speed faster than hull speed, get the ballast OUT.

The only conditions I feel merit motoring with ballast in, is larger, short period seas (4'+). In these conditions even at 5 knots the boat tends to launch off every second or third wave and slam back down if the tank is empty. It's a much more comfortable ride in heavy seas with the ballast in so the boat cuts through the waves instead of rising over them.

For small surface chop, 1' - 3', empty ballast and a speed of 8 - 12 knots will give a smooth quite dry ride. The boat also is easier to steer at speeds of 11-12 knots with all boards up and an empty tank than at slower speeds with no fins. It get's more directional stability and wanders less requiring less input from the helmsman or autopilot.

Duane, I agree with you that conditions warrant a different approach and experience teaches you which works best. The main issue I saw in this question was safety.
Wet depends a lot on wind direction and speed. At 15 knots wind, and 1 foot chop, my boat is dry going downwind, wet going more than 6 mph any other direction. With or without ballast directional control is much improved at speeds above 10, where there are no fins down.

Don't mistake the water in the "ballast" tank of a Mac for ballast. When you are on an even keel, what it mostly does is add inertial mass to the boat to slow down the rate of heel. There is some lowering of the center of mass, but it is not very effective, because the "ballast" is the same density as the water that you are displacing. It is not until this "ballast" is lifted above the waterline that the weight of the water becomes true ballast. It is at this point that the boat will stiffen up.

Rich