Boat Plans Pdf | Stability with Water Ballast

Boat Plans Pdf

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 cant 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 doesnt 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.

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Boat Plans Stitch And Glue | Our 2014 Calendar Stocks Available

Boat Plans Stitch And Glue

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 dont 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 dont 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.

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Dinghy Boat Plans | Website Hackers

Dinghy Boat Plans

We had a hacker problem a few days ago, someone hacked our main website and implanted malicious code into all of our pages on Sunday. Two of our alert supporters were very prompt in notifying me about the problem so that I could fix it. Thank you Vincent Houle and Luca Lafranchi for helping us, instead of deleting our website from your bookmarks.

I replaced all of the bad files about 1pm Eastern time but I dont have a record of the time that the hacker posted on my site on Sunday. I also replaced my password with a much more complicated one for added security. The old one was quite complicated and way out of the ordinary, so I dont know how the hacker figured it out. I guess that it is possible but not likely that someone at the hosting company stole it. In an attempt to cover all bases I am moving my website to a server that I consider to have a better level of ethics and better security controls.

I hope that the transition will go smoothly, probably later today or tomorrow. If you do notice a break in our web and/or email service, please bear with us and try again in a few hours. If you continue to have problems then please post a comment below this post.

A major reason for my decision to change service providers is that many supporters in Turkey have not been able to view our website for a year or more. It is being blocked for some reason in Turkey and I have not been able to get this corrected. I hope that the change of server will reconnect us with our Turkish supporters.

Everyone should have good security software on their computers that have access to the Internet. Your security software should have caught the malicious code if you visited our website while the code was active. If you visited Sunday or Monday and your software didnt alert you to a virus then please do a virus check on your computer for the JS/Kryptic ANQ trojan, which can compromise your computer system.

Thanks for your support and for bearing with us through the change of servers.

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Boat Plans Pdf | Beautiful Shearwater 45 Available

Boat Plans Pdf

The Shearwaters are boats of which I feel proud. They are very pretty, they are capable in all weather conditions and they are fast. The Shearwater 39 has an enviable reputation as one of the most seaworthy designs ever to come out of South Africa, a country that is known to have some of the most severe sailing waters in the world. These boats turn heads wherever they go, partly because they are so pretty and partly because they sail past many boats that look faster.

Shearwater 39 "Shoestring III" on launch day.

The Shearwater 45 grew naturally out of the reputation that was earned by the 39 and was developed out of the moulds of the 39. It has more length with no freeboard increase and with very little extra beam. The resulting boat is very sleek and gracious to look at and it gained a surprising boost of speed that allows it to outsail most boats of equal size and many that are up to 25% longer. In gaining that speed it has lost nothing of the seaworthy characteristics of the smaller sister. What it did gain is greater responsiveness and fingertip steering due to the change from a semi-balanced transom-hung rudder to a balanced spade rudder.

Shearwater 45 sailing on Chesapeake Bay.

We have good examples of both of these designs available on our brokerage pages. The most recent to come available is the gorgeous 45 "Maggy May". She is lying in the Caribbean and is fully equipped, maintained in top condition and ready to cruise to anywhere that you care to go. She is only being sold due to serious illness of the owner. To view "Maggy May" and the other Shearwaters that we have listed, go to http://dixdesign.com/brokerdd.htm.

Please visit our website at http://dixdesign.com/.

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Pontoon Boat Plans | Online Security

Pontoon Boat Plans

I know that this is a subject that has no more to do with boats than it has to do with any other aspect of our lives. Nevertheless, I am writing about it. This is a natural extension of my recent posts about hackers and my steps to circumvent the problems.

Thee are many, many people and organisations in the modern world that are trying at all times to separate you from your money or to harm you in whatever way they can. These organisations are illegal but they are big business. So we have to all take whatever precautions we can to foil them in both personal and business dealings.

Today I read a white paper titled "Ten Current Security Threats for Individuals". Some of what it says is simple common sense, much of which is ignored by most people. "It will never happen to me" works for everyone until it does "happen to me", or someone close. Other parts of it contain important information that we can act on, particularly how we set up and use our smartphones and how we use open WiFi connections at restaurants, hotels,  airports etc. We tend to brush off or disregard the dangers of these services because they can be so useful.

Take the time to set up the proper security features on whatever devices you use, to protect your own information from those who are out to harm you. They are not specifically targeting you or me, or the kid next door. They are taking advantage of whatever avenue is open to them to steal whatever they can. The victim just happens to be the one who left the door or window open to give them access, or didnt lock it securely enough. Take the time to read the white paper, it will help you to understand the dangers that we all have to guard against.

To get back to my hacker problem, I remedied the situation very quickly, thanks to my vigilant supporters who alerted me. The problem was fixed before Google or Nortons knew about it. The site scanner that was set to scan my site hadnt even picked it up yet but found it when I queried it. Since then I have investigated further and found that the scanning service that I had signed for only scanned 500 files and my site has nearly 700 pages and more than 3,000 files. It gave me an "all clear" report that morning but had obviously looked at the wrong files. The service that I have on my new server scans all files every day.

I will continue to do all that I can to keep my website safe for visitors. It is, after all, my main connection to my supporters and builders, present and future, in faraway places. For your part, please do whatever you can to keep yourself, your electronic devices and your finances safe from the baddies of the digital world.

To see our boat designs, please visit dixdesign.com/.



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Boat Plans For A Chesapeake Deadrise | Didi Cruise Mini available in UK

Boat Plans For A Chesapeake Deadrise

The Sail 4 Cancer charity in UK has a Didi Cruise-Mini that has been donated to their cause and is available for sale. This is a cruising version of the Didi Mini Mk2, with a lifting keel and reduced rig, as well as more headroom. It is a small performance cruiser in the image of the Mini 650 trans-Atlantic racers.

Didi Cruise-Mini, a sleek little performance cruiser.

I sailed one of these boats recently on Puget Sound when I was there for the Wooden Boat Festival. She proved to be a speedy little cruiser with delightful manners. She handled beautifully, carries her rig very well and has comfortable accommodation for weekending or short holidays afloat. We sailed past everything under sail that we saw on the water, sailing higher and faster. This was despite towing a clinker dinghy that was almost as long as she was. We had no desire to tow the dinghy but it was also destined for the Wooden Boat Festival and dragging along behind us was the simplest way to get there.

Twin rudders and lifting keel give access to shallow anchorages.

This boat that is available is in the Hamble yard of Boatshed, in the charge of Peter Delbridge. Anyone interested in it can contact Peter at the email address (peter at boatshedhamble dot com). It was built as a rehabilitation project by a man who was recovering from a serious illness. He completed the boat, sailed it once then donated it to Sail 4 Cancer when he moved abroad and could not take the boat with him.

Lifting bulb keel, excellent stability with shallow draught.

It is a complete boat, with all hardware and sails, ready for coastal cruising. The lifting bulb keel and twin rudders will get her into shallow anchorages that are inaccessible to larger or deeper-draught boats.

Swept spreader rig for easy handling, less complication than a Mini 650 rig.

100% of the selling price will be donated to Sail 4 Cancer (less yard fees etc), a UK charity that provides water based respite services for families affected by cancer. This is an excellent opportunity to buy a boat at a very affordable price and benefit a worthwhile charity at the same time.

Efficient cockpit with cuddy for shelter when on watch.

She is priced to sell, at £6,000 and well worth a look to see if she may suit you.

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

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Boat Plans Skiff | Didi 950 Build Stringers going on

Boat Plans Skiff

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/

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Boat Plans Skiff | DS15 Didi Sport 15 Build in South Africa

Boat Plans Skiff

Until now I have only shown the DS15 prototype, being built in Virginia Beach, USA, by Aussie Hunter Gall. Hunters project is currently stalled due to circumstances but one of the the other builds is steaming ahead apace. This one is being built by Jim Foot in Port Elizabeth, South Africa. This weekend Jim applied the last coat of epoxy to the outside of his hull and is preparing to paint it.

This series of photos shows the basics of Jims project up to this stage. This is a radius chine plywood design, with hard chine along the topsides. It is the same family of designs as the Didi Mini Mk3 and Didi 950.

Frames set up on building stocks, with stringers going on.

Stringers all done and bottom panels fitted.

Same stage, showing stringers running into forefoot.

Side panels fitted and first layer of radius progressing.

Stitch-&-glue detail of topside chine.

Second layer of radius going over first layer.

Completed radius chine, running smoothly into flat panels.

Glass taping centreline seam. Jigsaw joints of bottom panel can be seen.

Epoxy coatings started.

Nice finish starting to show.

Clean stern and nice finish.

Final coat of epoxy, ready for sanding and painting.

I will show more of Jim Foots project as it progresses toward launch date. I tend to show the projects for which I receive the best photos, so please send me your best pics if you want to see your project on this blog or the Dudley Dix Yacht Design main website.

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