Stability with Water Ballast

A potential builder of the Didi 950 asked me a question about stability with water ballast. He could not find an explanation on the Internet describing the effects of water ballast on a boat when capsized, so here it is.

After looking at the stability curve, he was concerned that the stability curve with water ballast to windward, the normal position for sailing in strong winds, has a very large area of negative stability. He wanted to know how that affects the time that the boat will take to right itself if capsized. This is a natural question following the amount of discussion that has been happening after our recent capsize in the Didi 38 "Black Cat" and the very rapid manner in which she returned to upright.

Shown below is the stability graph of the Didi 950 in fully loaded condition; click on the diagram to enlarge it. This is the condition of lowest stability due to the inclusion of crew, stores, liquids and many other weights that are above the centre of gravity (CG) of the boat. There are three curves shown. When looking at the graph, consider that the area enclosed by each curve above the horizontal 0 line is a measure of the energy that is required to take the boat from upright to the point of vanishing stability (AVS) where the curve crosses the 0 line. Until the AVS is reached, the boat will return to upright if no additional heeling force is applied to it.  Beyond the AVS the boat will continue to full capsize unless there is another force being applied that will return it to the positive side of the AVS.

The green curve is with ballast tanks empty, so akin to sailing a boat that has no water ballast. This curve is very similar in form to that of "Black Cat", with the area enclosed by the curve above the 0 line many times greater than the area enclosed by the curve below the 0 line. She would right herself very quickly with no water ballast. The red curve is with the windward ballast tanks filled, good for powering to windward or power-reaching in strong conditions. The blue curve is with the leeward ballast tanks filled. One would not sail her like this but it is a situation that could result from an accidental gybe in strong winds.

Didi 950 Stability Graph. Click to enlarge.

With no wind or waves and the ballast tanks on one side filled, the boat will not rest upright. It will heel over until it stabilises at a heel angle that places the CG vertically in line with the centre of buoyancy (CB). That will be the nearest crossing of the curve with the 0 line, which is at 5 degrees in this case, seen on the blue curve. Add some wind to bring the boat to 0 degrees heel and the righting moment that is working is the point where the red curve hits the left edge of the graph. Without water ballast the boat must heel to 6 degrees to reach the same righting moment. That is where the power benefit is coming from with water ballast, the boat will sail more upright than with empty tanks, in the same wind strength.

Note that all three curves are closely bunched when the boat is heeled 90 degrees. This is a knock-down situation, probably from losing control when driving hard downwind under spinnaker. The mast is horizontal but not in the water. This bunching of the curves at 90 degrees is because of the position of the ballast tanks in this design, low in the boat fairly close to the vertical CG. There would be a bigger spread if the tanks were located high up under the deck.

The red curve shows the benefit of increased righting moment when the windward tank is filled. There is considerably greater gain in stability shown by the red curve than lost stability, shown by the blue curve, when ballast is on the wrong side.

All three curves show that the wind alone can't capsize the boat. When the mast hits the water there is still considerable righting moment available for all three situations. If the boat is in large waves and hit by a big one while knocked flat, the added energy from the wave can capsize the boat in all three situations. 

It seems counter-intuitive but the condition most likely to invert the boat under wave action after a knock-down is with the water ballast to windward (red), i.e. the condition in which the boat will be sailed in strong winds. This is because after the water ballast passes beyond the point where it is vertically above the overall CG of the boat that extra weight is on the wrong side of the CG and is helping to capsize the boat rather than to bring it back to upright. It pulls the red curve below the green curve and reduces the AVS from 133 degrees to 122 degrees. 

Overall it takes more energy to capsize the boat from upright with water ballast than without, evaluated by comparing the area enclosed by the red curve with the area enclosed by the green curve. When the area enclosed by the blue curve is compared with the green curve, there is very little difference. It will take a similar amount of energy to capsize the boat without water ballast and with water ballast on the wrong side, when going from upright. Ironically, the wrong side has the greatest amount of reserve stability after a knock-down and has the greatest angle of AVS, so it is the condition least likely to capsize after a knock-down.

Back to our capsizing boat. Once past 122 degrees it is into a big range of negative stability that shows as the area enclosed by the red curve below the 0 line, taking it all the way to 180 degrees, i.e. totally upside-down. But see that the curve does not return to 0 at 180 degrees, which means that it is unstable at that angle. Same as happens when the boat is upright, the water ballast off to one side prevents the boat from resting at the 180 degree position. It has to rotate to where the CG is vertically aligned with the inverted CB. That is at the point where the curve crosses the 0 line. If the red curve is extended to the zero line it will be to the same angle that the blue curve crosses,  i.e. 160 degrees.
 

There is no windward or leeward when the boat is upside-down, the sails are under water. The boat is stable in the 160 degree position, so leaning 20 degrees to one side of upside-down. It needs to get past the nearest zero crossing to come back to upright. The boat doesn't care which way it goes. It needs a lot of energy to go back the way that it came along the red curve but very little energy to get to the 140 degree AVS crossing of the blue curve. With the motion from just a small wave it will continue past that 140 degree point. Once that point is passed, the righting moment of the blue curve takes control and will return her to upright. If the rig is still standing then the sails will fill and she will be back into the stability situation shown by the red curve. She has capsized along the red curve and righted herself along the blue curve.

In essence, it will take a lot less energy for the boat to right itself with water ballast than without, so she should right herself more quickly with the water ballast. The difference is that without water ballast she can go either way from inverted to upright but with water ballast she has to go full circle.

To visit our website, go to http://dixdesign.com/

Composite Construction for one of our Plywood Favourites

Here is some teaser information.

One of our popular plywood designs that has been around for a few years is about to be elevated in status. Tooling is soon to start in Australia for it to become a production boat. I will tell more about it in the next few weeks. For the moment, suffice to say that it is one of our most popular plywood designs and it will be available both as complete boats and as hull/deck/bulkhead kits for owner completion.

It is to be produced by a boatbuilder who has a depth of experience and knowledge spanning many years that will ensure quality products, strong and well-finished.

Watch this blog for more information in coming weeks. The design is being built in plywood by many amateur boatbuilders around the world, so I am sure that it will be popular as a GRP boat as well. Two boats are on order before the mould construction starts.

See our full range of designs for professional and amateur boatbuilders at http://dixdesign.com/.

Didi 950 Build - Stringers going on

Earlier this month I posted about the start of construction of the Didi 950 of Micheal Vermeersch in Ohio. Construction is progressing nicely.

The backbone components have all been fitted and glued. They have slotted connections, egg-crate fashion, with all of the bulkheads. Further into the project all of those junctions will be strengthened with structural epoxy fillets.

The sheer clamps have been scarphed into lengths sufficient to span the full length of the boat, from available shorter lengths. Before that they were also bevelled along both edges to remove excess material to save time later planing them to shape.  The sheer clamps are the major strength members along the deck edge. Normally they would sit flat against the inside of the hull skin but with these designs I turn them diagonally across the corner for a cleaner and lighter detail that also has a better balance of gluing area between the components that are being joined.

Now Michael is fitting the hull stringers. With this method of construction the stringers are arranged to suit the different regions of the hull skin. In the photo below you can see the sheer clamp, spanning the bottom corners of the bulkheads. Just above that is a topside stringer. There will be another similar stringer close to the chine that will accurately align the upper side panel.

The other full-length stringer that is in place is the tangent stringer that defines the edge of the curved portion of the hull that is laminated from thinner plywood. On top of the hull there is a stringer partially slotted in, defining the other edge of the curved panel.

Below the tangent stringer is a short stringer at the right in the photo. This is an additional stringer needed to fill in the space where the side panel increases in width due to the tightening of the radius toward the bow.

In the foreground are stringer lengths that will be scarphed together into the long lengths required. The angles have been planed and are ready for gluing.

 

 

 

 

 

Didi 950 with hull stringers being fitted.

To see more of this and our other designs, go to http://dixdesign.com/

Dave Immelman - Professional Sailor looking for a Boat

Those who followed our abortive Cape to Rio Race on the Didi 38 "Black Cat" and my recent blog posts about it will know that we went through situations that were demanding of quality seamanship, a cool head and total commitment to the well-being of boat and the other crew members. Those qualities come from thousands of miles sailed in all conditions, good and bad, sometimes under much pressure.

Our navigator on "Black Cat" was professional sailor David "Wavy" Immelman and he proved to have all of those qualities and more to spare. At any time that there was tough work to be done on the boat, Dave was there to do it or to assist. We had never sailed together before but Dave proved to be very capable and a sailor with whom I will be very happy to sail in the future.

Dave had prepared the boat for the race and onward voyaging. He was to take over as skipper after my departure in Rio and was to take her to the Caribbean for cruising and racing. He had committed to the owner of "Black Cat" for long term voyaging and racing. Unfortunately those long-term plans disappeared into the blue with our rudder, so Dave is looking for another boat.

Dave Immelman is RYA Ocean certified, with experience that includes Volvo Ocean Race, Americas Cup and Solent Racing. He is also tough, having rowed single-handed across the North Atlantic.

If you know of a racing, cruising or charter boat that may have a suitable opening, please message me so that I can put you in contact with Dave to supply his CV.

Our 2014 Calendar Stocks Available

A few people have told me that they were disappointed that we had sold out of our 2014 calendars before they were able to order and have asked if we don't have a spare copy that they can buy. Sorry, all were sold and we have none left except for one hanging on our own wall.

All is not lost though. I have set up a link where anyone who wants can order direct from the publisher at the same price that we were charging. You may even get a discount from them if you time it nicely when they have a special offer running.

Cover of 2014 Calendar

To order, please Click here. From this page you can also order my book "Shaped by Wind & Wave" if you don't already have that, either in hard copy or ebook format.

Our website is at http://dixdesign.com/, where you can see all of our designs.

DH550 "Wild Vanilla" Gets a New Owner

Professional boatbuilder Phil Harvey asked me to draw a 55ft cat for him using my radius chine plywood methods. I had too much work already, so Phil and I shared the design work to produce the new boat. That is the origin of the DH550 name.

Their boat, "Wild Vanilla", has been sold and will change hands in a few weeks. Her new owner took the photo below and sent it to Phil and Laura. I can't resist showing this gorgeous photo of a very pretty boat.

"Wild Vanilla" is the pot of gold at the end of the rainbow.

To see our full range of designs please go to http://dixdesign.com/

Didi 950 Construction Starts

Michael Vermeersch has started building his Didi 950 prototype in Ravenna, Ohio. This is the prototype of a very new design. Micheal is building from a pre-cut kit that we supplied and reports that the quality and fit have been excellent thus far.

Didi 950 Kit, ready for unpacking

 Yesterday Michael and a friend set up the bulkheads on the building stocks and he is now preparing to fit the backbone components to tie the bulkheads together.

Didi 950 Bulkheads set up and waiting for backbone.

This design has been drawn to comply with the Class 950 Class Rules. Watch this space for progress reports on this boat and the other Didi 950s being built in Australia, Latvia and Greece.

Please note that precut kits for our plywood designs supplied in USA must be ordered from Dudley Dix Yacht Design and not from the company that cuts them, Chesapeake Light Craft. For more info and pricing go to our plywood kits page.

To see our full range of designs please visit http://dixdesign.com/ .