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post #1 of 38 Old 01-20-2014 Thread Starter
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displacement/ballast ratio

Hi.

How significant is the displacement/ballast ratio? For example, a 36% vs 44% ratio; would that make a huge and noticeable difference in the boat's cruising capabilities?

Thanks.
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post #2 of 38 Old 01-20-2014
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Re: displacement/ballast ratio

In and of itself, the displacement to ballast ratio only gives you one small portion of the story. It really is not all that useful without knowing the vertical center of gravity of that ballast, and the beam of the boat. For example, a deep draft boat with a smaller D/Bal say, around 33% but which employed a high density in a bulb, would inherrently be more stable than a shallow draft boat with a much higher ballast ratio but in the form of a low density ballast carried in the bilge or an encapsulation envelope.

Similarly, a very narrow boat with a very high ballast ratio carried very low, might not have as much stability as a more moderate beam boat with the same ballast ratio or even with a lower ballast ratio.

In other words, boats act as a system. While a boat's statistics and ratios may provide helpful information, they cannot provide a comprehensive picture of the behavior of the boat as a whole. For that reason, modern designs which are often lighter and have smaller ballast rations than more traditional designs may in fact have significantly higher stability (relative to their displacement and drag) across a broad range of heel angles.

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post #3 of 38 Old 01-20-2014
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Re: displacement/ballast ratio

This is one number out of many. Is everything else completely identical between the two boats? That is pretty unlikely.

If the additional weight is at the very end of the keel, perhaps in the form of a bulb, it could make the 44% boat stiffer. If it is evenly distributed throughout the keel it could be less noticeable.

Chances are that the lower B/D boat has a wider beam and a hull form that provides more form stability, so the builder was able to get away with less ballast.

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post #4 of 38 Old 01-20-2014
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Re: displacement/ballast ratio

Ballast displacement ratio is an important indicator regarding boat stability but one that is pretty meaningless if you don't see the whole picture. It is an indicator regarding a way of lowering the CG of a boat but the CG can be also lowered by a superior draft, or a more efficient keel, a bulbed one, being the most efficient a torpedo keel. it can also b lowered even more if the ballast is lead instead of iron.

if you compare for instance American production cruising boats with European ones you will see that American ones will tend to have a superior BD ratio but that does not mean that the CG is lower since European boats have normally a bigger draft and more efficient keels, including torpedo ones.

Lowering the CG is also a way of increasing the stiffness of the boat (power) but not the only one. That can also be made through increasing beam. Normally narrower boats need a bigger BD ratio regarding beamier boats to have the same positive stability and power. However, regarding beamy boats you have always to keep a CG lower enough to allow the boat a good stability at 90 degrees and a good AVS (reserve stability) even if that low center of gravity is not needed to have the power to sail the boat.


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Last edited by PCP; 01-22-2014 at 11:38 AM.
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post #5 of 38 Old 01-20-2014 Thread Starter
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Re: displacement/ballast ratio

Thanks for the responses; a bit advanced than my current understanding of boats, but I am learning as I go.
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Re: displacement/ballast ratio

Here are the two boats I had in mind:

SONIC 23 sailboat specifications and details on sailboatdata.com

EDEL 6 (665/660) sailboat specifications and details on sailboatdata.com

Does the data point to one boat being a better, more comfortable cruiser?
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post #7 of 38 Old 01-20-2014
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Re: displacement/ballast ratio

Neither appear to be particularly good designs. Both appear to be somewhat dated, light duty, coastal cruisers. Neither would fall into a category of a comfortable cruiser. Unless they were extremely inexpensive for some reason, there are much nicer boats out there in their general size and normal price range.


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Re: displacement/ballast ratio

These are in the 3.5-5K range.

There is a J24 for sale - but different purpose boat, is it not?
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post #9 of 38 Old 01-20-2014
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Re: displacement/ballast ratio

What will be of interest to you also is DWL displacement to water line ratio .
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Re: displacement/ballast ratio

Quote:
Originally Posted by Markwesti View Post
What will be of interest to you also is DWL displacement to water line ratio .
But by itself, even that only tells you a small piece of the story. Take two equal length, beam, and weight boats but one with a short waterline and the other with a longer waterline. The D/L of the long waterline boat would perhaps suggest it is less suitable as a distance cruiser, yet that would not at all be the case.

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