Water Ballast?

[kbowen]

Considering a centerboard boat in the 20-25' range like the heavier "shallop" types, what are the pros & cons of water ballast?

[David Cockey]

Pros:
- Can be simple to remove for trailering though it may take a while to empty and/or refill
- Neutrally buoyant if the boat swamps.
- Water is free.

Cons:
- Center of gravity of water ballast will be higher than rocks/concrete/metal. Boat will not be as stiff as for the same weight of rocks/concrete/metal assuming they are lower in the boat.
- Larger volume than rocks/concrete. Much larger volume than metal. I.e. more space required for the ballast.
- Plumbing and tanks are not free.
- Ballast system needs to be emptied for winter if there is a chance of below freezing temperatures.
- Under way each tank should be either full or empty to avoid free surface effects in the tank.

[Craic]

Another pro for waterballast would be that you can empty it and re-fill it any time you're under way to modify the character of the boat in accordance with the weather requirements. In some boats you can switch from uncapsizable displacer to lightweight planing dinghy, and back. Two different boats in one, no other ballast can do that.

As to the time and effort it takes to let the ballastwater in or get it out again, that is an issue of clever design, not of principle.

[David Cockey]

Time to get water in while floating is mostly a matter of inlet/outlet size.

Time to get it out while floating depends on size of the pumps (if any) and amount of power (human, mechanical, electrical, etc) available to operate the pumps.

Time to get water out while on a trailer, etc. also depends on the inlet/outlet size.

We had a Sea Pearl with water ballast which worked quite well. As Craic points out the character of the boat could be changed. The factory system filling and emptying by unscrewing deckplates and reaching down in the tanks and removing/installing a plug. I added inlets/outlets controlled by a valve and vents so the deckplates didn't have to opened.

[kbowen]

Do you advise putting tank(s) at the center of gravity of the boat or distributed?

[Craic]

The further you spread the ballast out from the pivoting point of the hull movement the more you dampen the dynamic movement through inertia. Which may be welcome or unwelcome, depends what the designer aims to achieve. I like the dampening effect of a 'horizontal' spreading.
What I would avoid is to have several independent tanks, too much fuss to manage them. Keep it ingeniously simple.

[wtarzia]

How about metal/stone ballast to the extent that you can still trailer the boat with your choice of vehicle, or attain some minimum hull dynamic you seek, and then make up the difference with water? -- Wade

[Craic]

Forgot to mention three more Pro issues of waterballast:

- Hull rigidity. The ballast tank as a closed box structure adds rigidity. Same effect as when you put a lid on an open shoe-box.

- Safety. With a waterballast tank, what you get in practice is a double hull underwater.

- Better recovery from inversion. While you must avoid a free surface inside the ballast tank when the boat sails upright, you can induce, control and use a free surface effect inside the tank to aid recovery if -ever- the boat should be inverted.

[wayne nicol]

craig,
please elaborate on the recovery from inversion, and how effective would this be with part water part solid ballast, as per few post back. very hypothetical question, i know. too many variables- but would love to hear more.

[Rational Root]

I'm not associated with Swallow Boats other than I've been on a Bay Raider for a 10 minute sail at Beale Park. Nice boat, nice people.



Since they do water ballast, have a look at their comments
http://www.swallowboats.co.uk/content/view/110/59/

and their videos
http://www.swallowboats.co.uk/content/view/127/

They have some sort of asymmetry in the tanks if you invert while the tanks are empty, one will fill, the other will not, see the pics down the page.

[phiil]

Ecological concerns: Tanks must be thoroughly emptied and/or cleaned when traveling between different bodies of water to avoid transfer of unwanted organisms. I dump clorox in mine.

[Venchka]

Con: Loss of storage space.
Pro: Large supply of fresh water in an emergency?

[David Cockey]

How about metal/stone ballast to the extent that you can still trailer the boat with your choice of vehicle, or attain some minimum hull dynamic you seek, and then make up the difference with water? -- Wade

No reason not to have both solid ballast and water ballast.

[wtarzia]

Would someone explain the "free surface effect" for me? Thanks. -- Wade

[Old Dryfoot]

I think in this case it is in reference to the sloshing movement created in a partially filled tank. It allows the ballast to move and shift within the boat.

Of course I could be wrong here so please someone correct as needed.

[Venchka]

Not sure what it is called, but water ballast must stay in one place. Just like solid ballast stays put. Unless you want a sandbagger.

[wtarzia]

I understand the sloshing part, but I have heard of the free surface effect in its relation to an inverted boat and the problems associated with righting (I think in the book, "Single Handed Sailing"). I couldn't quite follow those discussions as they assumed more knowledge than I had. -- Wade

[Old Dryfoot]

I understand the sloshing part, but I have heard of the free surface effect in its relation to an inverted boat and the problems associated with righting (I think in the book, "Single Handed Sailing"). I couldn't quite follow those discussions as they assumed more knowledge than I had. -- Wade

In my understanding the free surface effect is as an undesirable shift in the boats center of gravity which counters it's natural righting moment. Not having the context of the book I can only assume that the free surface effect in relation to an inverted situation is this undesired change in CG. The loose water trapped in the hull acts as movable ballast and tends to stabilize the boat when inverted. The effect is not confined to water alone and can be generated by any material that is able to act in a fluid like motion.

Hope this helps

Here is a visual as I understand it. The black section is fixed ballast, the green is a liquid; the third circle demonstrates the free surface effect countering the boats natural righting moment.

[ahp]

To have hobby horsing minimized, pitching oscillations die out quickly, concentrate the ballast amidships. In any mass-spring-damper system, if mass, or moment of inertia is increased, as you would by moving the ballast to the ends, an nothing else is changed, the damping ratio is reduced and oscillations go through more cycles before dying out.

[wtarzia]

Thanks for the circles! They explain much better than the stuff I had read. -- Wade

[openboater]

The lake I live on is zebra mussel free. If you were to put a waterballasted boat in here and the word got around, there would be a lynch mob at the boat launch waiting for your return. Just something to think about. My neighbors are very active enviromentalists and I know many lakes residents in other areas are also.

Just something to consider.

[Craic]

The lake I live on is zebra mussel free. If you were to put a waterballasted boat in here and the word got around, there would be a lynch mob at the boat launch waiting for your return. ...

You wish to imply that sailboats with a waterballast tank were / are / will be responsible for the global spreading of the Zebra Mussel?
Or are you just warning us about the somewhat peculiar mindset of your neighbors?

Hey, why not tell them the Zebra Mussel just love to nest inside the primary -seawater- cooling circuit of powerboats? Or that most sailing dinghies have deep centreboard wells which are ideal for carrying mussel seed around?

Only joking. Better not tell them.

[kbowen]

Thanks for the note about the Swallow Boats , this is increasing my interest in water ballast. I am considering a Mackinaw, which was originally a gill-net fishing boat, and I wonder if the weight distribution of a couple tubs of lake trout and nets might not be similar to a confined load of water?

[kbowen]

Considering the sloshing, I saw a wonderful gizmo int the Maritime museum in Greenwich UK which was intended to help a freighter captain calculate his stability: It had a balancing wooden board with a profile of the ship on it and little boxes for each storage compartment where you could put weights that approximated the tonnage in each hold. But for the oil bunkers it had little weights that hinged up (making the whole things less stable) for when the tank was partially full, but flipped down for when the bunker was either full OR empty. Apparently the stability was more compromised when the tank was half full than it was when the tank was either full or empty! Interesting....

[Craic]

...
Here is a visual as I understand it. The black section is fixed ballast, the green is a liquid; the third circle demonstrates the free surface effect countering the boats natural righting moment.

Interesting visual. Shows clearly what happens inside an inverted submarine if the naval architect has forgotten to contain the water ballast inside a separate tank.

[blisspacket]

Wakeboard boats carry heavyduty flexible tanks of various sizes to boost their displacement and wake creation. The tanks are an inexpensive way to play with water ballast. Building water ballast into a wooden hull would require some clever glassing/sealing to keep rot from setting in.

[kbowen]

Cool, do you have names / websites for any manufacturers of the tanks?

[Peter Belenky]

Since (fixed) water ballast is unlikely to contribute much to initial stability, it is important to adopt a hull form that will stand up, even without ballast. A flat-bottomed sharpie can be relatively stable under reduced sail when light or slightly deeper, heavier, and more powerful with water ballast. Only at substantial angles of heel will it be less stable than the same hull with fixed ballast. A narrow boat with a steeply rising floor will lie over without ballast, and its inferior stability with water ballast (compared with fixed ballast) will become apparent at much smaller angles of heel.

[tomlarkin]

Cool, do you have names / websites for any manufacturers of the tanks?

Here's a Google search - lots of hits.

[kbowen]

Since (fixed) water ballast is unlikely to contribute much to initial stability, it is important to adopt a hull form that will stand up, even without ballast. A flat-bottomed sharpie can be relatively stable under reduced sail when light or slightly deeper, heavier, and more powerful with water ballast. Only at substantial angles of heel will it be less stable than the same hull with fixed ballast. A narrow boat with a steeply rising floor will lie over without ballast, and its inferior stability with water ballast (compared with fixed ballast) will become apparent at much smaller angles of heel.

Thank you for the reply, this is getting good: I haven't settled on a design among the Mackinaw types, but the Lake Erie, and Collinswood skiff types seem to have flatter floors, while the Isle Royale and Chapelle's "Western Lakes Mackinaw" have rising floors. The Chapelle is 24'7" x 7'6", (In American Small Sailing Craft). Are you voting against water ballast in such a boat?

K

[Peter Belenky]

Are you voting against water ballast in such a boat?

K

Actually, I am. A traditional boat will be more work and investment than a modern hull, and both its value and its performance will be diminished by the modification. Water ballast has (one of) two purposes: shifting for added stability in a high-tech racing boat; ease of stowing and dumping in a light, trailer boat that is able to look after a relaxed crew. In either case, it is best to have a modern hull designed for home construction. Monocoque plywood is standard.

Phil Bolger has thought this through systematically:

http://www.ace.net.au/schooner/jochems.htm

http://www.sailingtexas.com/sbolger26100.html
http://picasaweb.google.com/Pemaquid...edByPhilBolger#

There's also this:


http://www.cmdboats.com/trailersailer24.htm

For the high-tech version, you can look at Dudley Dix's work:
http://www.dixdesign.com/didiminiMk3.htm

[kbowen]

[QUOTE=Peter Belenky;2645475]Actually, I am. A traditional boat will be more work and investment than a modern hull, and both its value and its performance will be diminished by the modification. Water ballast has (one of) two purposes: shifting for added stability in a high-tech racing boat; ease of stowing and dumping in a light, trailer boat that is able to look after a relaxed crew. In either case, it is best to have a modern hull designed for home construction. Monocoque plywood is standard.


How is contained water ballast different than several large tubs of fish and gill-nets which would have been the standard cargo for a historic Mackinaw boat? I am not after "performance" so much as I am trying to understand how and why the original hull-type developed. I am still waiting for more drawings from the Smithsonian and so haven't done any displacement calculations yet, but my hope is that a couple hundred pounds of batteries (for a short-range auxiliary) will equate to the beach-stones the old guys carried in the bilge, and that another couple hundred pounds of water will act like the variable loads the originals carried. Do you see any chance this could work out?
fwiw, I am imagining some combination of strip-planking and FRP, w/ or w/o diagonal veneers, but I need to find some similar, successful modern designs to pull scantlings from. ...maybe I really need to bite-the-bullet and find a N.A. ....

Thanks again for your time,

K

[Peter Belenky]

..my hope is that a couple hundred pounds of batteries (for a short-range auxiliary) will equate to the beach-stones the old guys carried in the bilge, and that another couple hundred pounds of water will act like the variable loads the originals carried. Do you see any chance this could work out?
K

There was a discussion here:
http://forum.woodenboat.com/showthre...Boats&p=658609

A 29' Mackinaw boat was described as having 1,500 lb. of lead pig ballast. That translates into about half that weight for a 24-footer. Historic Mackinaw boats also used shifting sandbags. Underestimating the amount of ballast a boat will need is likely to produce a surprising lack of stability.

Please note: All of my comments are derived from study and speculation in the complete absence of first-hand experience.

[kbowen]

A 29' Mackinaw boat was described as having 1,500 lb. of lead pig ballast. That translates into about half that weight for a 24-footer. Historic Mackinaw boats also used shifting sandbags. Underestimating the amount of ballast a boat will need is likely to produce a surprising lack of stability.

Please note: All of my comments are derived from study and speculation in the complete absence of first-hand experience.

Thanks for bearing with me. The next step seems to be to wait for my drawings to be delivered and calculate displacement. I did see a photo of a Mackinaw at the museum in South Haven Mich. which was obviously floating much higher than Chapelle's drawings. However, the other designs I have already received from the Smithsonian (WPA work) show the lines measured up from the keel, but no load waterline, so I don't know how deep they loaded them or at what trim. I would like to sail the Zimmer version some day (reportedly 600# lead), but I am not going to build anything unless I can figure enough flotation / closed storage to survive a knockdown. I'm going to shut up now until I can get drawings and a rough idea of displacement.
Thanks again,

K

[Craic]

Kbowen,
I myself am totally sold on the Swallowboats waterballast system. I have had long experience with a long line of different fixballasted and unballasted boats, but the waterballasted boats I have now from them are the best boats I ever had - for what I do with them.

But having said that, and being really a true waterballast fan so, I would be very sceptical about retrofitting waterballast to an old hull which was not designed for it in the first place.