Actually, the empty polyethylene tanks are very light when empty (~15 pounds for 15 Gal tank). A bladder tank of similar capacity is about 6 pounds. Just look at the shipping weights in a Marine catalog for the ballpark comparison. The weight is in the stuff that fills it up! That's comparable to water or ~ 8.4 pounds per gallon.
I don't thing this is a weight issue from a class rules standpoint, but Larry will have to weigh in (pun intended...)
For my money, I want my stuff in a rigid walled polyethylene tank, not some bladder.
Ahhh, but the weight differential increases as the tanks get bigger. A 25 gal polyethylene tank weighs in at 44 pounds (at least according to a Defender Industries listing) while a 25 gal bladder type is still about 6 pounds. I have no idea how big those holding tanks are, but I'm pretty sure they are more than 15 gallons. 36 extra pounds carried well forward is some very serious weight-recall the debate over the stove which is carried much closer to midship and weighs in at only ~12 pounds.
If replacement was permitted, I know I would consider swapping out for the lighter tank setup-I race a bunch of J-29's in PHRF, and weight in the ends is deadly.
Steve - the holding tank is about 14 gallons - you can measure the dimensions and calculate the volume. Think of your 25 gallon analogy above. Cut a 55 gallon drum in half (take the smaller half) and that would be about the size of a tank you describe. No way that sucker fits in the v-berth!
We're talking about a 10 pound differential here on dry weight between a bladder tank and a poly tank. If you store a gallon of fresh water (or beer) that's pretty close to the weight differential. You'll get more mileage out of keeping the tank pumped dry.
No way you'll get me to store a couple of gallons of water forward of the head area! And I don't think I agree with your estimate of the size of the tank-I figure the tank is (at the top) about 3' wide aft, 1' wide forward, and about 3' long. That would be a top surface area of a little over 5 square feet. So conservatively, figure 5 cubic feet of volume (the tank is deeper than one foot, but tapers quite a bit). A gallon of water occupies .1337 cu. ft, so I would estimate 5/.1337=37 gallons.
I can't believe I just wasted so much time on...waste.