Holding Tank Size, Placement and Hobby Horsing

[serah]

Our boat is a relatively heavy displacement cruising Douglas 32'; a full keel, with a forefoot cutout. As we work on our rebuild, increasing tankage is a top priority for us. For water, we currently only have a 36G fibreglass tank, glassed into the keel. There is a 27G poly holding tank under the v-berth. We are considering removing this tank, and using the space for an additional 30 - 40G water tank in that space. If we were to do so, we would add a smaller holding tank ahead of the water tank under the v-berth.

I have read concerns about having too much weight forward in the boat which will cause hobby-horsing, though I also understand that in a boat with our build characteristics, it may less of a concern.

Two questions:

What is a reasonable size of holding tank for a boat that we are designing to be both a local liveaboard in the PNW for two people, (we will have regular and easy access to pump out facilities) and a cruiser, heading for the S. Pacific and possibly, circumnavigation? We're thinking in the 10-15G range, hoping for the low end.

Will having the water tank that far forward affect performance/comfort, e.g. cause bucking in rough weather? Obviously if we are in open seas in rough weather the holding tank will have been emptied, so that will not add to the problem.

[zeehag]

the holding tank on a boat i sailed for a year had been placed in bow with a macerator unit to use to empty when out in deep sea=== long passages-and a pump out feature as well. the tank was big--more than 15 gal--- did not create any problems at all--and that was a light (ha) performance cruiser--- even full, there was no problem. are you able to place the tank at the area abaft the forefoot instead of in the bow??

[serah]

HI zeehag,

Unfortunately, we are rather limited in extra water tankage possibilities. We've considered custom tanks in the 8" or so of space below the cabin sole, but that would require completely cutting out the floor for installation/access. A flexible tank would be a consideration (for water at least, I can't bring myself to being comfortable with a flexible tank for black water!) but the only section with enough space is still quite far forward. We may add that at a future date, but also, the fibreglass in that area is rather uneven, which would also not work for a flexible tank. We can work that out though. However, to achieve the volume we want, the v-berth seems like the best candidate. We have a manual pump out by thruhull from the tank, and also a deck pumpout on the foredeck.

We do have an obscene amount of cockpit storage, so we will likely have both diesel and water in gerry cans stored there... but purely as a supplement.

[gdavids]

I am a big fan of placing the holding tank under the v-berth. It is generally easy to pump out before heading out of the marina for a sail, so zero weight up forward. It gets heavy when you are at the dock or at anchor - i.e. when it does not really matter. If you are leaving an anchorage loaded up, you probably don't have too far to go until you can pump it out.

If you put a water tank under the v-berth, you will have a lot of weight up forward all the time you are sailing.

[zeehag]

after you load your boat for sailing , row round it and look to see your load is level--that the boat isnt squatting, or low in the bow , either. make sure most of the weight is midships--i keep mine between masts to keep the load trim such that hobby horsing is minimal.. granted i have way more space than yours does-- but there are ways to keep the malignant motions minimal.. mine has a sprit that pierces seas-- they are notorious for that-- does yours have that tendency?? if so, is more important to keep that motion small as possible. if bow is too light , it can pound, especially when weight is placed in stern.. if too heavy, will plow.
another consideration is the anchor tackle--chain and anchor are heavy and usually stored in bow. presents a challenge for perfecting weight trim from load placement.

[serah]

I just can't see a way to add the volume required low and centered. I think what we'll do before we start any changes is to flush the holding tank, and fill it with salt water and see how she does. It'll give a pretty close semblance of the water weight. She's nearly empty right now, though I'll certainly keep an eye on her fore/aft trim, especially as we'll hopefully be adding a large Bruce to go with our 35# CQR on the bow. We carry 220' of 5/16" chain as well up forward. We do not have a sprit, but she's relatively narrow (9'^6"), and we haven't noticed a tendency to plow. She seems to want to cut through quite well, actually.

The somewhat shoddy plan is to have the tank gravity fed to the main tank in the keel, via a valve that we open to drain at intervals. As the voyage progresses, less weight will be up forward; however, might be rather awful if stuck in a blow 4 days out from port on a 4 week leg.

She has a medium length overhang up forward, and a long one out back, if that makes a difference...

[RainDog]

Originally Posted by serah

Our boat is a relatively heavy displacement cruising Douglas 32'; a full keel, with a forefoot cutout.

My boat is 34', 13,200 lbs. I have a 16 gallon holding tank and a water tank I would guess is 30 gallons under my v-berth. I do not notice much of a different with the tank full vs. empty. I would have no qualms about locating the tanks there. Note that I currently only carry 50' of chain, so if you have all chain plus the tanks you would have more weight up front than I.

I get my 16 gallon holding tank emptied once a month (1 person). I use the boat head about half the time and the bath house the other half. I have no idea how full it is when they empty it, but it has never completely filled.

I would think you could get away with a 10 gallon tank if you emptied it once a week and used off boat bathrooms at least some of the time. Away from a marina, I would think holdiing tanks size is a non-issue. You can empty it pretty easily. However it seems like almost all small boats come with a ~15 gallong tank, so I would probably bow to the wisdom of the builders and go for 15 if you can fit it.

[LakeSuperior]

Holding tanks on the order of 30 to 40 gallons are barely enough for a week of cruising. With a small tank pay careful attention to how it will overflow.

[Kefaa]

"30 to 40 gallons are barely enough for a week of cruising."
How many people are on your boat - 40 gallons? You may want to switch your diet...

[LakeSuperior]

Originally Posted by Kefaa

"30 to 40 gallons are barely enough for a week of cruising."
How many people are on your boat - 40 gallons? You may want to switch your diet...

RainDog according to his post uses slightly less than 15 gallons per month tankage and uses the public bathroom half time. So his personal use might be 30 gallons per month or call it about 8 gallons per week.

So if RainDog had 4 people (two couples for example) on his boat and they were using the head 100% of the time then he would need 32 gallons in one week.

If he didn't want black water coming out of his tank vent then he would want a 40 gallon tank.

I don't know what RainDog eats but he and I may have similar diets because our holding tank usage is about the same. My guess is he likes a beer now and then. Cheers

[donradcliffe]

Our holding tank is a bladder up in the forepeak--it holds about 25 gallons and lasts 6-7 days for the two of us, but we reserve its use for #2.