Water Ballast Longitudinal Position

I’m in the early phase of designing a trailer sailer, mostly in my head so far.. It will be for camp cruising around the shallow coastal and estuarine waters nearby. Likely ~5.5m LWL, ~2.4m beam. Freestanding cat rig. Not a racer.

I want to use water ballast either side of a pivoting centreboard. Water ballast down low that will be dumped at the boat ramp to reduce towing weight. I’ll probably also set it up so I can pump the ballast tanks out on the water to reduce draft/weight if I need to pull it off a sandbar.. I’m not entirely sure how to position the water ballast longitudinally relative to say LCB..

I cant find anything online about the effects of positioning water ballast fore or aft.. The articles I’ve found discuss lateral position wrt RM effects. I can see benefit in maximising the length and width of the tank(s) to reduce pitching and rolling (increase inertia). Apologies if this is a dumb question. Perhaps it’s as simple as positioning the water ballast CoG at the same longitudinal position as LCB to maintain the design waterline when the tank(s) are flooded.. I don’t want to overcomplicate things but I do want to ensure I get the ballast in the correct longitudinal position. Perhaps there is potential benefit (albeit with additional complexity) in having another tank(s) fore or aft to improve stability at different points of sail? This last question is more for interest, improved understanding. Where I sail usually requires a lot of tacking so I’m not planning on transferring water from side to side every tack.

 

Add the water ballast where you would add "rail meat", people, ballast.
Often, sailboats want weight aft by the time water ballast is being employed.
Depends largely on the design of the boat and it's sailing characteristics.

Custom design means no characteristics known, so no way to know.
You could use portable water bags to sea trial differing positions before framing them in permanently.

Have you considered building to a proven design/plan that uses water ballast?

BB

 

Thanks BB. Yes, I’ve been looking for a proven design, plans for a couple of years now without finding the mix of features and appearance I’m interested in. The closest I’ve come is this design: BLACKTIP https://www.woodenboat.com/blacktip but regrettably no plans. I quite like the Swallow Bayraider but want a simpler freestanding cat rig.

 

I spent ten years exploring the Florida Bay and the Everglades in little 16' boats. The sort of system you have in mind isn't really practical, and isn't needed. For a beachable craft, you are looking at about 600 pounds of hull, rig, battery, and kicker. Weight for trailer transport would be around 700 pounds with fuel and kit. You can unload and unrig the boat on the beach, getting the weight down to 450 pounds or so, then it can usually be manhandled as needed. Fixed ballast tanks are not helpful. A couple of simple collapsible 5 gallon plastic jugs are plenty.

Once you get over about 16 feet, is gets quite difficult to built a durable camp cruiser that is beachable. These little boats need to be stout, and the weight adds up fast. I used a 21' DM375 mast on mine (tricycle stays) and a boom off a Hobie 16. My spreaders were the same section as the mast on a Rebel 16. But I could cartwheel up the beach in a surf and not hurt anything.

I do have one small design that uses a couple lengths of 4" lay-flat discharge hose as moveable ballast tanks. It's basically a sailing sea kayak intended for the 700 mile Florida challenge race, which includes long coastal legs in trade-wind conditions as well as a 40 mile portage - not a normal design brief. The hoses lay in bilge troughs either side of the centerboard housing. The rest of the floor is enclose flotation.

The physics of how water ballast works eludes most people. Ballast in general wants to be in one lump near the cg so as to minimize inertial moments. The mast and rig will provide be more than enough inertia to the craft. Water ballast should not be placed below the waterline as a normal operating condition. I usually look at the the 10-degree heeled condition and try to accommodate shiftable ballast wide, near but above the heeled waterline, and near the longitudinal cg. If you want to put ballast below the waterline, it needs to be lead.

 

You're welcome.

Maybe there's a reason you can't find plans for what you're looking for...?
I don't mean to rain on your parade, but I agree with philSweet, it's just not necessary.

 

Thanks Philsweet. That’s great feedback. I looked seriously at the 15’ Bedard RoG for a while but realised sailing is a social thing for me as much as anything. I want to take a friend or three along sometimes and I’m happy to compromise on beachability for that.

I’m still really interested to learn about any science around the longitudinal positioning of water ballast if anyone can help.

 

There's your movable ballast right there!
300 - 700+ pounds! (Including you)
Crew love to be useful.

My science says put them slightly aft of CoG.

 

That’s definitely an excellent solution for movable ballast when on they’re board BB Which will hopefully be most of the time.

I’ve found a few articles online about water ballast including this one which is quite good from my perspective: The Physics of Water Ballast | Swallow Yachts Association https://www.swallowyachtsassociation.org/?page_id=1183

I’m keen on learning more about effects related to it’s longitudinal position if anyone knows of any articles that discuss this in more detail.

 

Good article.

There are a few boat designers in this Forum that will fight you over these words

"Note the increased stability does not require the water ballast to be raised above the water surface, as sometimes believed. As long as the CoB moves out more than the CoG the boat will remain stable, and the heavier the full ballast tank, the more the stability."

 

Thanks rwatson. I’m not about to argue on behalf of the author of that article. I was just hoping to find an article that talks about about positioning the water ballast longitudinally to further my understanding.

 

Its OK, I am on the side of water ballast users.
Like the others have said, your boat is too small to even bother with permanent water ballast,
You will be more concerned with arranging bodies in the boat, than bothering with other weights.
In fact, with such a small boat, keeping weight OUT of the hull will be a bigger issue.

 

And faster without the added weight of two pumps and/or valve set-ups, hoses, wires, tanks, switches, enlarged battery capacity, and most importantly; the weight of the water itself.
Just think of your rail meat... oops, I mean, passengers as contained, mobile water bags (up to 700 pounds!).

Your interest in the "science" of it all can be studied under the heading "Ballast".
There is a lot published on the topic.

 

Thanks rwatson and BB. Yes, “adding lightness” will be near the top of my priorities. I’ve a strong interest and a bit of hands on experience with infused composites so that’s a given.

 

Do you want the overall trim to change between the 'light' and 'heavy' configuration? If not you need the CoG of the tanks to coincide with the centre of flotation (the centre of the waterplane area).

To improve handling in waves you generally want to minimise weight in the ends (i.e. minimise moment of inertia), so short, fat tanks close to the longitudinal centre would be preferred.

 

Thanks tlouth7. Agree, for my purpose, the trim shouldn’t change by adding water ballast. I need to read more to better understand the difference between LCF and LCB.. Per earlier comments above trim can be manipulated by moving organic ballast when needed.

I’ve been thinking the opposite about moment of inertia (Maximise moment of inertia to resist pitching and rolling and motion sickness). Is there greater benefit to have low moment of inertia so the hull can pitch and roll (respond) rapidly to ride over the waves and chop rather than minimising pitching and rolling to “push through”? I think I want to minimise pitching and rolling. Which is probably the primary reason I like the idea of water ballast.