Water tank capacity/yacht stability.

[Mike Banks]

I would leave the tankage as it is and keep them sequentially emptied and filled with your water-maker. Any old sailing ship man would tell you the one store of which one can not have in excess is fresh drinking water.

Another thing about stability is the motion of the vessel. Too much weight in the keel will certainly make the masts stand up, but instead of a gentle lifting and roll to the waves, there is also an up-and down motion. Test it for yourself using a bottle and adding more and more sand to it, and watch how it behaves in the wavelets as more sand is added.

To get more stability, without generating too much of the bobbing up and down cyclical deck-wetting one needs to have the design ballast LOWER DOWN which means a greater draft--not good if one wishes to close-shelter in the lee of an island.

Cruising boats ideally need to have as shallow a draft as possible commensurate with stability and comfort in a seaway. The designers of such sailing and even powered vessels vessels know that for any monohull, and many multihulls too, this will involve some carefully considered compromises.

When one makes changes to these designs, very seldom do the results turn out well overall. Sometimes they compromise the vessel, and some vessels have been so badly affected they have foundered.

As for having your chain locker and anchor stored lower than the waterline, I can see problems in that arrangement too. I have mine just above the waterline so my chain locker can drain in a similar way to the aft cockpit, so that it can be washed as it comes aboard, and unless in really rough water, remains dry. I would keep my anchors on deck. Their total weight is probably the equivalent of ONE passenger.

But that is just me. Others may entirely disagree.

[Fore and Aft]

Coopec 43 despite the maths stand back and look at your Roberts 43 they are a tank and I am sure Bruce would chuckle at the thought of worrying about a 40kg anchor on the deck. Is he still around? If so why not call him and get the correct opinion from the actual designer?
I would also be thinking about resale value, a big yacht with only a small water tank capacity might put some people off. Not everyone wants to run a watermaker everyday.
Cheers

[Paul L]

Different than other posters I've found my watermakers to be very reliable while cruising. I think with a WM you should always have at least 2 tanks. A failure of the membrane or seals could end up dumping gallons of saltwater into your tank before you realize it. If you only have one tank all your water is contaminated.

I'm not familiar with your boat and whether it suffers from a lack of stability. Rather than removing tanks/adding lead I'd start with careful loading of the boat, keeping heavy items out of the ends and down low where practical and reduce weight in the rig if practical.

[coopec43]

Originally Posted by MartinR

Keep the tanks. Increasing stability is often not what you want. Increased stability means an increase in performance, but also a decrease in motion comfort.

For a good cruising yacht there has to be a balance between these, and I am sure the designer of the yacht has accounted for this.

Also, an increase in stability will increase rigging loads, probably not significantly in your case.

The Mauritius 43 is not a production boat so you can't say "I am sure the designer of the yacht has accounted for this". The designer didn't specify the weight/section of the mast, was vague when it came to engine options/weight and left the option of type of construction (sandwich or solid glass) up to the builder.

[billknny]

We have 1000 liters of tank capacity in our keel, and a watermaker. We keep the tanks close to full all the time, because if our watermaker ever crapped out that gives the two of us 6 to 8 weeks of time to address the issue, and we ALWAYS have enough water to finish even the longest passage no matter the status of the watermaker.

We also never put city water in the tank, that way we avoid issues with Cl2 when we flush the membranes. In the rare case we have an extended stay at the dock, we have a connection directly to shore water.

Having the weight down deep certainly helps us to windward, but the extra weight slows us down when running. The difference for us is not enough one way or the other in a total weight of 38,000lbs.

[Midnight Son]

MJH and MartinR summed it up.

[coopec43]

Originally Posted by TrentePieds

Quote: "...the yacht would be more stable... "

Precisely why are you worried about stability? I think that few of us are qualified to second guess Bruce Roberts in regard to the stability of his designs. You can make this 13 ton boat STIFFER if you chose, but that is NOT synonymous with making it more "stable".

There way be merit in making stowage space for hardware, such as ground tackle, where you presently have storage space for water, but doing so is not likely to have a significant impact of the boats "stability".

TrentePieds

As I said to Martin, the Mauritius 43 is not a production boat. The designer was vague about engine size, left the draft dimensions open to the constructor and offered alternatives for building material.

The Yacht rigger (who supplied the mast) suggested as it was a fairly heavy section I should use lead instead of iron for ballast.

Thanks anyway for taking the time to comment.

[coopec43]

Originally Posted by Fore and Aft

Coopec 43 despite the maths stand back and look at your Roberts 43 they are a tank and I am sure Bruce would chuckle at the thought of worrying about a 40kg anchor on the deck. Is he still around? If so why not call him and get the correct opinion from the actual designer?
I would also be thinking about resale value, a big yacht with only a small water tank capacity might put some people off. Not everyone wants to run a watermaker everyday.
Cheers

Yes Bruce is still around. Your comment regarding re-sale value is valid but it is the last thing on my mind!

You may not worry about the 40kg anchors on deck but when you add an outboard, a couple of jerry cans of fuel, hard dodger, gas bottles, dinghy, liferaft, solar cells, radar, guard railing (built to survey) it become a worry.

[coopec43]

Originally Posted by Paul L

Different than other posters I've found my watermakers to be very reliable while cruising. I think with a WM you should always have at least 2 tanks. A failure of the membrane or seals could end up dumping gallons of saltwater into your tank before you realize it. If you only have one tank all your water is contaminated.

I'm not familiar with your boat and whether it suffers from a lack of stability. Rather than removing tanks/adding lead I'd start with careful loading of the boat, keeping heavy items out of the ends and down low where practical and reduce weight in the rig if practical.

I think you've got it!!

I was going to keep at least one tank and use my bottled emergency water in the event of a catastrophic failure of the WM contaminating tank water.

Now I've decided that initially I will keep all three tanks and then remove one to make space for heavy item stowage if I later find that the way to go.

Thanks.....

[misssherry]

In the 90s, we cruised with a small watermaker and 2 40 gallon water tanks. While in Trinidad, Venezuela, Columbia, and some anchorages in Mexico, I didn't run the watermaker because of the polluted or silty water. Very nice to have the tankage available so we didn't HAVE to make water.

[Fore and Aft]

I agree with your rigger about using lead instead of iron for ballast. Just doing that alone will lower your centre of gravity.
Is yours the ketch or sloop version? Surely if yours is the sloop then you have already reduced weight and windage aloft by removing one mast?
There must be Hundreds of Mauritius designed yachts afloat and I have never heard of one being tender?
Cheers

[TrentePieds]

Coopec:

I understand your concern, but coming at your "problem" the way you do is not the best way. What you really need to do is an "inclining experiment". That is what competent designers do with their "construction #1" of any given design, but it can obviously also be done with aged vessels. This is the ONLY real way to address the multivariate concept of "stability"

The objective of the inclining experiment is to determine EMPIRICALLY at what Angle of Heel your Megacentric Height goes negative so the vessel will turn turtle. This Angle of Heel is called the "Limit of Positive Stability or "LPS". Competent designers can make a "good guess" about this from their "schedule of weights and moment arms" but it is enormously tedious and normally not necessary in yacht design. Given that your boat is a Bruce Roberts design of utter conventionality, I would think that she needs to roll to AT LEAST 120º before your GM goes negative. Many boats of this type will, provided they are adequately battened down so they don't take on water in a major roll, roll right to 180º, i.e. turn keel uppermost, and then continue to roll till they are upright again because as their CF (Centre of Flotation) moves around in the roll, their GM NEVER goes negative. Many lifeboats are designed to take advantage of this trait.

I think it's fair to prognosticate that you would chicken out and take the pressure off 'er long before you reach LPS. I know I would :-)!

A parameter you should probably pay more attention to is the "period of roll". A vessel that heels easily and has a long period of roll is characterized as "tender", and a vessel that requires a lot of force to heel 'er and has a short period of roll is characterized as "crank". The latter kind of vessel is seriously sick-making when at sea. the former kind tends to induce anxiety in the uninitiated. So your best bet is to aim for something in between that suits you and your crew.

You can lengthen a vessels period of roll by stowing heavy objects outboard far from the centreline and shorten the PofR by stowing heavy objects inboard close to the centreline. Just so, you can alleviate a tendency to "bury" by stowing heavy objects midships, and alleviate a tendency to "hobby horse" by stowing heavy objects near the ends.

So unless you have something more substantial than a sailmaker's off the cuff pronunciamento that your mast section is heavy, don't assume that your boat's stability is inadequate and in need of correction. A brief enquiry to BR by letter or e-mail will elicit, I'm sure, information about your boats LPS, and other cruising peoples' opinion as to whether she is tender or crank when normally laden for cruising.

One more thing you might want to do, if you are fond of math, is calculate the "number of pounds to immerse an inch". You are 13 tons light ship (i.e. with half full tanks) I believe. Cruising supplies for four might, if you are normally frugal, amount to two or two and a half tons. Calculate for yourself how taking two'n'an'alf tons aboard will affect your draft.

If you don't already know how to do that, I'll be glad to teach you :-)

All the best!

TrentePieds

[Fore and Aft]

Now that last paragraph TrentePieds explains exactly why a lot of smaller multihulls are just not that fast.
Cheers

[RaymondR]

It's about thirty years since I had to do any stability calculations but I think there are a couple of errors in your reasonimng.

What defines righting moment is Gravity to Metacentric not metacentric height.

The angle of vanishing stability is generally achieved when the outermost bouyancy goes underwater.

In a ballasted keel boat you would probably have to take all the ballast out to render it unstable, different matter in a trailer sailor or non ballasted boat where transverse form is exploited for stability.

In a Roberts Mauritius you could stack hundreds of pounds on the deck without any serious effect on stability.

[coopec43]

THANK YOU ALL FOR YOUR HELP!

Having read all the posts I reckon I now know what to do. Time to move on!!
Cheers
Clive