Water Ballast Longitudinal Position

Increased moment of inertia means that a boat is slower to respond to a wave by pitching up, but also slower to stop pitching as the wave passes. This tends to result in more exaggerated motion, though I appreciate it is a counterintuitive result. Perhaps the best way to think of it is like this: the worst hobby-horsing will occur when wave encounter frequency corresponds to the natural pitching frequency of the boat. Minimising moment of inertia maximises natural frequency, hopefully moving it above typical wave encounter frequency.

 

Hi Roo2,

Here a small article about ballast, in general. MODERN SAILBOAT DESIGN: Ballast Stability - Wave Train https://wavetrain.net/2013/04/09/modern-sailboat-design-ballast-stability/
Also not very specific to your question about their longitudinal position, the most important aspects of ballasting is reviewed in this article. As always on a boat, everything is a question of compromise. Seeking a greater stability given a fixed beam has a cost , different if you choose between bringing more people -or more movable weight- on board, having a canting keel or a liquid ballast. This cost does not only include money, but also complexity, maintenance and practicability.
Longitudinal ballasting of sailing craft is often use to increase the downwind sail area on ocean racer, controlling the boat trim at high speed. It is used in conjunction with wide stern. On a 18 footer catamaran, you do the exercice when standing aft of the aftbeam on a hot run. On big offshore downwind machines, where the crew weight is only a very small fraction of the total weight, you may have a good profit in having aft water ballasts instead.

Beware of a too small floatation plan, because having a so called #rapid response to wave# also implies a porpoising tendencie that you do not want. To minimize pitching and rolling, I prefer playing with the translation of the LCB at angles. If a small change of angle results in a big translation of you LCB, you will then obtain the effect that you desire.

 

Centre of Buoyancy is the centre of the overall submerged volume. This is necessarily directly in line with the Centre of Gravity when the boat is in equilibrium.

Now imagine you add a small weight somewhere. The boat will move downward slightly in order to submerge more volume and so displace more water. But trim may also change to bring the CoB under the new CoG. Here is the clever bit - trim will not change if the new mass is positioned directly in line with the CoG of that new slice of water that has been displaced. For a small downwards movement we can approximate this slice of water as being a prism with the planform of the old waterplane, multiplied by the change of height. So the place to put extra weight and avoid a change of trim is centred on the CoF. NB the amount a boat will move down for a given added weight is also related to this - the bigger the waterplane area the smaller the height that slice of water needs to have in order to have the relevant volume and so displace the correct amount.

We can do something similar when thinking about pitching. For a small change in pitch (bow down) we can think about a wedge of water behind the CoF being lifted out of the water, and a wedge in front of the CoF being submerged. Hence waterplane coefficient tells us something about how much pitching moment is generated by passing waves, or how much the bow must pitch down in order to counteract a pitching moment generated by the sails.

https://www.boatdesign.net/attachments/waterplane-coefficient-jpg.39386/

 

Thanks so much tlouth7! That’s an excellent explanation. I think a sentence in the article linked above by Alan is also along the same lines. “when the sea gets rough a tender boat normally has a smoother motion than a stiff one.”

Thanks for the article link and your explanations Alan. I had found that article earlier this week but reading it again has improved my understanding some more. That Blacktip design linked earlier has a good flotation plan by the look of it. A bit like the mini transat designs..

So.. Concentrating the ballast around LCF (both longitudinally and transverse) is good while underway. Is this also good for stationary stability at anchor? Damping roll and pitch a bit while sleeping. Given the boat will be slightly too big to easily pull on and off the beach..

 

As a committed trailer-yachter, I like your concept. Water ballast greatly increases the range of vehicles the boat can be towed with, making a larger boat practical for more people. I also like the free-standing cat-boat idea. I've thought about that myself and I think it could result in a boat that's much quicker to rig. That's a very important consideration for me because I rig every time I sail. My boat has a Bermuda rig with furling head sail, lazy jacks, and other features designed to make cruising better, but all of that complicates pre-trip rigging. Since I rarely day-sail that's not a problem for me, but anything you can do to simplify rigging makes shorter trips more feasible.

Regarding positioning of the water ballast it seems to me that, unless you really need a moveable C of M to achieve your desired sailing performance, it's not worth the effort. You're only going to sail the boat with full water ballast anyway, so, from a design point of view that's the same as any other fixed ballast.

 

Freestanding cat rigs are the least handy of all rig designs once you get beyond tiny boats. The masts have to be very tall and very heavy. The length is the real issue. Most 22' day sailors can be stepped in the parking lot by one person with relative ease - but not the Nonsuch 22. It has a 41' spar, and it weighs a blue ton. For trailer-sailors, the deck-stepped tricycle rig is the easiest to manage overall at the ramp. More complicated staying arrangements require some fiddling to tension the rig properly.

 

@philSweet ,

You're right, of course, but it's worth looking at a cat rig with the assumption that some kind of built-in mast raising system will be included. I'm envisioning a mast with tabernacle where the pivot point is as high as is practical while keeping the stowed height below trailering limits. That way, the mast simply pivots down into a crutch at the stern using a block and tackle arrangement, or possibly even a lead screw. It might be necessary for the mast height and sail aspect ratio to be compromised, but this is a trailer sailer. Trailering and rigging considerations are as important to the success of the design as sailing qualities.

 

Hi Tedd and Phil.

My primary design consideration above all others is to simplify, minimise rigging time at the ramp (this is a digression from the subject matter = water ballast). I have a Weta trimaran that I love most of the time.. I live in a relatively hot climate and hate the sunburn / sweat during the 45+ minute rigging and then de-rigging times. It deters me from taking it out a lot.

Tedd - I agree with the concept of a simple single water ballast tank in a trailer sailer for all the reasons you state. I’m also thinking about a heavy pivoting centreboard with a lead tip but this adds complexity that I’m having trouble getting past. A complex heavily loaded block and tackle or expensive, heavy and likely unreliable hydraulic cylinder to control it.

Phil - Thanks for pointing me to the Nonsuch 22. I hadn’t seen that one. My thoughts on the cat rig are just preliminary but with the hope of reduced rigging time. Probably not more than 10m long mast, carbon fibre, thin wall following Sponberg’s guidelines. I dont think it would be too heavy but haven’t done the calc. I’d considered a two piece mast (like my Weta) but the load conditions are completely different for freestanding so not sure the joint would be easy to do like it is in the Weta (which is mostly under compression, not so much bending like a freestanding rig).

Instead of a tabernacle I’m thinking about the mast sliding into a deep slot in the bow. The mast foot seats in the forward most end of the slot. The sides of the slot provide transverse stability while pushing the mast forward and up. The mast is retained in the front of the slot in it’s most upright position…somehow.. Perhaps a strut gets pivoted into the back of the slot to hold the mast forward and upright. Some ideas but no detail yet

 

@Roo2 ,

Your slot idea is interesting. If you used a lifting keel you might be able to have the trunk for the lifting keel also be the slot for the mast. That would mean that you'd only be able to step the mast with the keel down, which would be a limitation, but it might be worth it to simplify the system. You'll lose a lot of interior space, though.

My thinking with the tabernacle is that, since the mast position is so far forward on a cat boat, if the tabernacle pivot was high enough then the mast would simply pivot down into the towing position--no need to reposition the mast after dropping it, as is necessary on most trailerables. But I haven't looked into the geometry in enough detail to see if that's actually feasible. The mast might still overhang the stern by too much. Remember that you've got 12 feet of trailering height (in most of the U.S. and Canada) and the top of your deck is probably only going to be 6 feet or so above the road, so your tabernacle could be as much as six feet high above the deck. That will probably get the foot of your mast out ahead of the prow when it's down.

Regarding the centerboard idea, you might take a page out of Macgregor's book. His swing centerboards are quite light, requiring only a simple cable and roller arrangement with no real mechanical advantage. That means you're relying almost completely on the water ballast but it does keep the centerboard arrangement simple. (I know there's a lot of hate for Macgregor and his boats but his designs are very trailerable. Credit where it's due.)

 

Thanks Ted. Perhaps we should kick off a new thread covering the future evolution of tabernacles and swinging centreboards. Surely there’s a Netflix series in that..

My mast step slot idea is right near the bow so would eat a bit of space in the v berth foot area and possibly split an anchor well in half.

During trailering I think I’d need to have the mast foot over the back of the tow vehicle to minimise rear overhang to be legal here in oz. Not sure because I haven’t seen it.

I think you’re right about keeping the centreboard light. The local waterways are sandbar infested so I definitely want a pivoting board.

 

You cannot really change damping by changing the position of weight in the boat. Damping (i.e. energy dissipation) is a property of the interaction of the hull and appendages with the water. Bilge keels add damping. Slapping a wide stern in and out of the water adds damping (but is frustrating if you are trying to sleep). A flopper stopper adds damping.

By the way if you are looking at heavy masts, stepped far forward, and easy to raise and lower then have a look at how classic gaff rigged yachts on the Norfolk Broads do it: a high tabernackle, a long section of mast below the pivot with a load of lead on the end to counterweight the mast, and a coffin lid on the foredeck to allow that counterweight to swing upwards. These aren't freestanding rigs, and indeed you use the forestay to help raise and lower, but they are trivially easy to drop in order to "shoot" bridges without stopping.

 

Thanks tlouth7. I guess I should have said increasing the rotational inertia (which increases as a function of the radius (of the water ballast mass) squared. So spreading the water ballast out across the bottom of the hull along the pitch axis will increase the roll axis moment of inertia. This should reduce rolling at anchor a bit. But.. Is there benefit for the hull to also have low roll inertia when under sail?

 

I know someone with one of these: Quickflow® Water Ballast Hull | Bar Crusher https://www.barcrusher.com.au/technology/quickflow-water-ballast-technology and the difference in comfort when stationary is very noticable. It’s crossed my mind that a similar flooding tube(s) might be good on a trailer sailer except I’m not sure how easy it would be to empty it without the big outboard..

 

Not "science" exactly (no equations), but this discusses some longitudinal positioning effects of movable ballast on Roxanne, which seems relevent
Movable Ballast | Roxane & Romilly https://www.roxane-romilly.co.uk/discussion/movable-ballast
Lead ballast, but I suppose a kg is a kg, in context.

Re the lay-flat hose design mentioned above, Iong time ago I saw a review of a commercial windward rail ballast bag for keelboats which could be pumped full of water using a towed turbine pump on the end of a hose. I THINK the leeward rail bag could be pumped full of air (not certain of that), and both of them also would act as fenders. Struck me as a nifty idea, so it probably disappeared without trace.