fresh water ballast

omeron, i think your suggestion is probably very practical and allows for as many different ballasting solutions as you have the energy to create im giving it serious thought..

eric, thankyou for your response. ive trawled the internet looking for pictures to see where these round the world racers put there tanks. as for loading the rig i think i will be ok - to an extent. the boat was designed to sail fully crewed with lots of bodies on the rail and i was only thinking to replicate this effect - maybe 1200kg is a slight over estimate on crew weight though! ive been in touch with german frers and he has kindly sent some more info through to me on righting moment as well as the lines plan for the aft end of the boat, thinking about how this could be improved for off the wind. its all seriously complicated stuff so i may need someone to help make sense of it when the time comes.

for the moment the project proceeds slowly, here is a link to a website ive made to track my progress:

http://web.mac.com/wkd928/iWeb/Site 3/Welcome.html

 

Thanks drnick.
I seriously believe all drinking water should be carried in 5lt clear bottles
sealed as fresh from the source. Unless exposed to sunlight they keep
fresh for a long time. (months)
Natural spring water is ofcourse much better than processed water.
There are many available/unused space on a boat that you can squeeze
in a bottle, that, allocating precious space for a tank is really a waste.
The risk of contamination is always there. And you rarely get to the bottom
of the tank before you refill. So there is always a percentage of your tank
containing stale water.
All other benefits mentioned earlier withstanding.
Another benefit is, you can buy different brands with differing mineral
contents. If your source is always the same, you end up drinking the
same water composition. Depending on whether that is the correct composition for your body or not (which you never know) you increase
the risk of kidney stone formation. If you drink different waters, you
spread the probability and decrease this risk.

 

i have learned that the righting moment is 380 kg/m via email.. what does this mean and more specificaly can i use it to figure out how much weight at max beam will cause 10 degrees of list?

RM = GZ x displacement
380kg/m = GZ x 10500 kg

GZ = 0.36/m or RM = 36% of displacement for every meter away from the CG??

 

Just one small correction: fresh water weighs 1 kg/ltr, or 2.2 lbs/ltr. One of the benefits of metric measurement is that, for fresh water, volume and weight will always have the same numeric value. 300 ltr weighs 300 kg, about 660 lbs.

 

This last means GZ = 0.36 m for that displacement condition and an specific (10º?) heel angle, not RM = 36% of displacement for every meter away from the CG.

If 380 kgm is the righting moment we want to counteract, water ballast moment has to be equal and contrary to get equilibrium. Taking moments to COG and asuming some simplifications (small angles), we can roughly say:

ballast weight = 380 / d

being d: = 0.5 *( B - b)

and 'b' the beam of the sided water tank.

Example:
If beam is 5 m and width of tank is 0.6 m, then (roughly):

ballast weight = 380 / (0.5*(5-0.6)) = +/- 173 kg


Cheers.

 

guillermo, thanks for the reply which has clarified some things and made others less clear - to me at least. i see that RM is specific to a particular situation and changes depending on the angle of heel ie it starts at zero when the boat is upright.

looking at the number supplied, 380 kg/m and the displacement of 10500 kg this gives GZ as 0.36 per metre. the number seems likey to be at 10 degrees looking at common GZ values for boats in this size range although i dont get why it is 'per meter' - i had thought it was an actual distance ie that the centre of buoyancy had moved 0.36m lateraly from the centre of gravity.

no doubt you guys who look at these numbers all day long understand what this all means but i cant reconcile the idea of 173 kg on the rail tipping this boat over by 10 degrees!

 

drnick,
GZ is not xx (whatever) per metre. GZ is a distance and is expressed in length units (metres in this case). The RM is a moment and is expresed in kgm (kilogrametres), not in kg/m (kilograms per meter)
May I suggest you the reading of some educational pages?
These are great: http://web.nps.navy.mil/~me/tsse/NavArchWeb/1/toc.htm

Cheers.

 

thanks for clearing up my basic misunderstanding and for replacing it with a more complex one!! the web link looks promising and i might read it, but im off sailing for now do you honestly think 173 kg will heel my 50' boat 10 degrees?

 

You should post the GZ or RM curves (specifying load condition), for us to know what are we talking about.
Cheers.

 

ok, ive read most of the stuff in the web link guillermo posted - its taken a few days for it to sink in but i think ive got a grasp of the basics.. here is the information i have for the boat:

displacement 23000 lb
ballast 9500 lb
RM 379.7kgm
LCB 6.5
KB 0.38
KG 0.383
crew 15

unfortunately im not sure if these last measurements are in meters, feet or inches or if the number given for LCB refers to the station on the arrangement plan. either way it indicates that the centre of buoyancy and gravity are at the same height in the hull. thanks to guillermo i understand now that GZ can be increased for a given displacement by increasing the horizontal distance between the CG and CB (centre of buoyancy) ie canting the keel or shifting crew weight.

looking at GZ curves for boats with comparable displacement, 0.36m seems likely to be the righting arm at 10 degrees heel - and as guillermo illustrates this indicates that 173 kg at the rail (max beam is 14'11") will heel the boat by this amount. what happens after this point, i dont have any more information for but im guessing its behaviour will be typical for an IOR design. the weight of 15 crew on the rail at aprox 75kg each (1125kg) is significant and will have been factored into the boats design ie she is designed with aprox a ton of moveable ballast.

in an attempt to reduce draft the previous owner replaced the original keel with a stainless fin and lead bulb of his own design, as far as i can tell the fin takes the same plan form as the original with probably a section not far off a simple naca foil. as you can see from the pic this has resulted in a change in the for/aft trim and i think a reduction in displacement as well. the rudder was also shortened with a single horizontal cut which has reduced draft but has not improved the feel through the helm.

im told the boat dosent go to windward like it used to but that downwind performance has improved, this is about all the information i have so far. as for my contribution to the boats ongoing story, somewhat naively i have bought her with the intention to make a cruiser, sail back to new zealand to live happily ever after and teach my kids how to sail - one day at least thats the two to ten year plan! as im finding out this is amore ambitious plan than i had realised.. fortunately im also finding that the boat has a lot of strengths as i understand more about the design.

back to issues of ballast - to correct a long list of deffered maintenance the boat has been hauled out and is sitting on the hard, during which i had intended to simply refitt the old keel. keen to explore any possibility for increased performance ive been looking at options for ballast, hence this thread.

so now im thinking of perhaps remodelling the fin bulb keel instead of water ballasting. so far ive thought that i could lower the bulb to increase the fins span and at the same time lower CG and increase RM. at this stage its completely an intellectual exercise as there are a bunch of other important things to do firstly like repowering and sweeping spreaders etc.

 

some more calculations: correcting previous; RM of 379.7 gives GZ of 0.036m so this is unlikely to be at 10 degrees of heel - missed a zero in last series of calculations.

assuming crew weight of 1275kg acting at a point 227cm horizontaly to weather from CG gives a new 'shifted' CG aprox 24.5cm in the direction of the added weight ie this can increase GZ by a maximum of this amount.

as the boat heels to leeward the new combined CG rotates upward till at 90 degrees the 'shift' of 24.5 cm is now vertical and no longer contributes to RM. i take the point that in the real world the additional ballast will also raise the CG and reduce the AVS.

is it a gross oversimplification to then assume the CG could be lowered by this amount? probably! hopefully someone will chime in here and offer some heuristics on calculating a prudent number to lower the CG by such that the rig is not overstressed - the shrouds terminate at the deck with -48 rod from the verticals and -30 from the diagonals. aside from the hull and keel themselves im guessing that its probably the rig that would mainly be stressed by this type of alteration.