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Old 09-10-2013, 12:17 PM   #1
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Adding ballast...

There has been some discussion about adding ballast to improve vessel stability. By adding ballast the vessel must ride lower in the water with more wetted area leading to a higher fuel burn. Am I correct in that assumption? Would adding ballast to a semi-displacement hull be self-defeating?
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Old 09-10-2013, 02:30 PM   #2
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ancora,
Here'swhat Atkin said re his 26' SD overnight cruiser Tang.

"
Tang is somewhat narrower than recent practice shows in small cruising boats and will therefore be faster with less power. A good four-cylinder motor of approximately 140 cubic inch cylinder displacement and pulling about 35 h.p. at 2,200 r.p.m. will be ample for a speed of 15 mile an hour. I would put in about 800 pounds of inside ballast. Most motor boats are better for a little ballast."

This is dated but from a very learned source.

And of course performance will suffer. On a FD hull it would just be from wetted surface but w a SD hull w a significantly submerged transom the penalty would be considerably higher.

Have a party w the portable ballast included and see what happens. Since boats are pointy at the fwd end favor a bit aft for the ballast. Aft ballast will degrade speed more but fwd ballast will affect handling to a much greater degree. Negatively.

On a planing hull ballast will degrade performance so much (w much more submerged transom) that it would probably never be recommended. With all considerations given to the expression "never say never".

Sometimes ballast is employed near the gunwale and just under the outside deck. The idea being that roll will be dampened better as the ballast will be farther from the CG. Stability on most boats will be negatively affected to some degree .. usually small.

Most trawlers are too heavy as is so ballast will probably be a negative unless you have special considerations or a light boat .. not likely.
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Old 09-10-2013, 06:55 PM   #3
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Quote:
Originally Posted by ancora View Post
There has been some discussion about adding ballast to improve vessel stability. By adding ballast the vessel must ride lower in the water with more wetted area leading to a higher fuel burn. Am I correct in that assumption? Would adding ballast to a semi-displacement hull be self-defeating?
yes and yes...buy probably miniscule in both arenas unless you go crazy.

moving weight around in a vessel can help...a lot of people think more weight in the center and low improves the ride...they are almost always incorrect.

as recommended...add some weight, move it around and split it up to put weight outboard and fore and aft to see what happens...or hire an NA to do the calculations...you would be surprised what and where ballast actually improves the ride.
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Old 09-10-2013, 06:59 PM   #4
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Quote:
Originally Posted by ancora View Post
There has been some discussion about adding ballast to improve vessel stability. By adding ballast the vessel must ride lower in the water with more wetted area leading to a higher fuel burn. Am I correct in that assumption?
Yes, adding ballast increases the total weight (displacement) of what you are trying to push through the water. If the hull form remains the same; almost the same length, beam, entrance angle, and transom immersion, the fuel consumption will increase. The tricky part is that you could increase displacement a lot by cutting the boat in half and adding 10'. Then your fuel consumption will go down. But transverse stability will stay almost the same.


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Would adding ballast to a semi-displacement hull be self-defeating?
Probably. It depends what exactly you are trying to fix. Adding ballast (down low) will not appreciably change the low angle stability (that from say 0 to 15 degrees of heel) because low angle stability is mostly dependent on your waterplane area(a cut through the hull at the waterline). So adding sponsons will be more effective than adding ballast.

High-angle stability becomes more center of gravity and displacement dependent so the added ballast will help at 25-45-65 degrees of heel, but modern powerboats rarely heel that far. And when they do there is big trouble.
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Old 09-10-2013, 09:20 PM   #5
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About that Ballast

I have to ask, as I have had a few rolls that rather shook people up and scatter things about the boat. From what I have been reading here and a couple of other posts, I could look to reduce some of the "sharper" rolls by having a bit more "ballast" if it was distributed more to the outer edges and a bit above the waterline.

Over the last couple of years I have reduced the average fuel loading because I really do not use that much in 1 year, and do not want to be carrying 250 gallons of diesel around just getting old and collecting moisture.(especially FlyWright's kind)

I also do not recall having as much roll when I brought the boat north after purchase in Alameda; probably about 2/3 full at that time.

As you can see from the picture, my tanks on a 34' are about centered on the waterline and 150 gal. each side.
Am I trading fresher fuel for a more "rolly" boat?
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Old 09-10-2013, 09:32 PM   #6
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Fuel tanks that are all the way out against the hull do make weight in the area that resists rolling...but how much is the real question...a hundred gallons a side may make a big difference...I just dropped 150 gallons a side and I'm, gonna find out this winter's trip whether I need more weight spread out.
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Old 09-11-2013, 06:46 PM   #7
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Originally Posted by Delta_JimS View Post
I have to ask, as I have had a few rolls that rather shook people up and scatter things about the boat. From what I have been reading here and a couple of other posts, I could look to reduce some of the "sharper" rolls by having a bit more "ballast" if it was distributed more to the outer edges and a bit above the waterline.

Over the last couple of years I have reduced the average fuel loading because I really do not use that much in 1 year, and do not want to be carrying 250 gallons of diesel around just getting old and collecting moisture.(especially FlyWright's kind)

I also do not recall having as much roll when I brought the boat north after purchase in Alameda; probably about 2/3 full at that time.

As you can see from the picture, my tanks on a 34' are about centered on the waterline and 150 gal. each side.
Am I trading fresher fuel for a more "rolly" boat?
Rolling has two components, the speed (time) of the roll, and the angle (ultimate heel before returning upright). Most inexperienced people are comforted by quick rolling, when the boat snaps back upright from any small heel angle. More experienced people are happier with a slower and deeper (angle) roll. If things are flying around you may be suffering from too much stability and raising the center of gravity could slow that roll down. One way to try this is put four friends on the flying bridge and time your roll, then try it with them down stairs or sitting on the floor. Those tapered tanks are biggest at the top, so filling them is raising the center of gravity.
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