Ballast/Displacement/Draft

[BlueSkyNick]

Boat A has a ballast/displacement ratio of 40%, a standard fin keel, Draft 1.8m.

Boat B has similar displacement but less ballast so a ratio nearer 30%, however it has a deep fin keel, say 2.4m draft, with a bulb at the bottom.

In general terms, does the righting moment of Boat B's keel compensate for its lighter ballast, given that the COG is lower in the water?

Hence, is there much difference in the seakindliness of the two boats?

Both boats are 40ft LOA.

 

[alant]

Boat A has a ballast/displacement ratio of 40%, a standard fin keel, Draft 1.8m.

Boat B has similar displacement but less ballast so a ratio nearer 30%, however it has a deep fin keel, say 2.4m draft, with a bulb at the bottom.

In general terms, does the righting moment of Boat B's keel compensate for its lighter ballast, given that the COG is lower in the water?

Hence, is there much difference in the seakindliness of the two boats?

Both boats are 40ft LOA.

These might help

http://www.sailingusa.info/design_winds.htm
http://www.sailingusa.info/cal__capsize.htm

 

[wotayottie]

I'm not sure there is a definition of seakindlyness that is calculable as opposed to subjective.

The difference the factors you mention would make to stiffness is simple. The keel is rather like the weight on a pendulum . Make it longer and for a given weight the leverage is greater and hence the resistance to heeling. Make it longer and you also increase the rotational moment of inertia and so alter the natural period of roll. Make it longer and all other things being equal, you increase hydrodynamic lift and so pointing ability.

However, there are lots of other factors affecting each of these three things such as hull shape for example. But if you are talking AWBs, the real world difference for iron keels is not going to be earth shattering.

 

[doug748]

As wotayottie has said, a subjective measure so a fruitful area for inconclusive argument.
An attempt here:
http://www.sailingusa.info/motion_comfort.htm
(You have to enable popups)
You will also have to make a guess at the shape of the different virtual boats.

 

[Twister_Ken]

You'd need to some easy but tedious calculations to calculate the effective depth of the mass of each keel and then arrive at a righting moment for each. Bear in mind that the weight of the rig and any gubbins attached is equivalent to 'negative ballast'. Alternatively get a bunch of big lads to stand on the rail of both boats and see how much they heel. Doesn't work if the boats are in cradles unfortunately.

 

[BlueSkyNick]

These might help

http://www.sailingusa.info/design_winds.htm
http://www.sailingusa.info/cal__capsize.htm

Thanks Alan, they help a little bit, as I hadnt seen the Motion Comfort Calculator before (initially I thought it was something to do with using the heads). It doesnt take draft into account, interestingly, whereas AVS does.

More playing with numbers to do...

 

[Tranona]

Boat A has a ballast/displacement ratio of 40%, a standard fin keel, Draft 1.8m.

Boat B has similar displacement but less ballast so a ratio nearer 30%, however it has a deep fin keel, say 2.4m draft, with a bulb at the bottom.

In general terms, does the righting moment of Boat B's keel compensate for its lighter ballast, given that the COG is lower in the water?

Hence, is there much difference in the seakindliness of the two boats?

Both boats are 40ft LOA.

You have neatly captured the debate about the value of weight in a boat. The trend towards using less of the weight for dead ballast recognises that it is not the only way of achieving the required stability. There is a viciuos circle. The more weight you have in ballast the less available for the rest of the boat, particularly the bit you live in, the more sail area you require to get it moving and then the more weight you need to balance the extra weight of the rig. The only advatntage of weight is that it can lead to a slower more gentle mtion at sea which some people value highly.

The "measures" that try to capture "seaworthiness" are generally based on the assumption that comfort and motion in heavy seas is the objective so tend to favour heavy, high ballast ratio designs. However the vast majority of boats are never used in these situations so designer have tended towards less ballast, more space for living and greater performance, particularly off the wind and in lighter airs. Less ballast, but placed lower down, together with different hull forms maintain stability and abilty to carry sail.

 

[BlueSkyNick]

You'd need to some easy but tedious calculations to calculate the effective depth of the mass of each keel and then arrive at a righting moment for each. Bear in mind that the weight of the rig and any gubbins attached is equivalent to 'negative ballast'. Alternatively get a bunch of big lads to stand on the rail of both boats and see how much they heel. Doesn't work if the boats are in cradles unfortunately.

TBH, i was looking for a subjective view from those in the know, rather than having to work it out with arithmetic. Basically how does a lighter, long skinny keel with a bulb on the end compare to a more conventional cruising boat's keel. Wotayottie's explanation of the pendulum effect is a good one, esp. about the natural period of the roll which hadnt occured to me.

I do recall a certain boat with 27 of us stood on one rail without getting the gunwale wet !

 

[BlueSkyNick]

Sorry Tranona, our posts crossed in the wires - thanks.

 

[BobPrell]

Comfort Ratio is quoted in lots of places, it was invented by Ted Brewer.

Here's wot he said on his site.

"COMFORT RATIO (CR): This is a ratio that I dreamed up, tongue-in-cheek, as a measure of motion comfort but it has been widely accepted and, indeed, does provide a reasonable comparison between yachts of similar type."


see http://www.tedbrewer.com/yachtdesign.html

 

[timbartlett]

Bear in mind that if you make the keel deeper you also increase the heeling lever. So a lot of the advantage of putting the weight lower will be lost.

It's probably easier to demonstrate than to prove: go out in a dinghy in a reasonable breeze, and sail close-hauled but with the centreboard most of the way up. Yes, you'llmake a lot of leeway, but you won't heel very much. Then put the centreboard down and do the same thing. You'll stop sliding sideways -- but you will heel over more.