Tank Size Effect on Performance

[kscheben]

I am looking for opinions on the right tank size to replace the fiberglass tank on my 28 Bertram Bahia Mar. I use the boat mainly for offshore fishing and need all the range I can get. I was just told today by the tank fabricator that he could squeeze in a 315 gal tank to replace the stock 190 gal one. The boat also has a 74 gal front tank that I has hoping to drain and retire due to the cost of replacement (requires engine removal). We are estimating the weight of the new tank and fuel to be roughly 3000 lbs when full. This would be nearly 1300 lbs heavier than the current tank at full capacity.

My question is how badly would the trim of the boat be effected by this change? Has anyone else increased their tank size this much previously and could share their results with me? I am leaning towards keeping the new tank down to 250 gallons in order to minimize the risk of the performance change. Thanks in advance for your help with this!

Ken
Babylon, NY

[Rocket]

Ken, you don't have to fill it all the way...You could establish for yourself what % full the boat likes. There is no substitute for capacity, and more is better.

[mike ohlstein]

I'm not sure that I agree. I'm guessing that we're talking gas....

Partially filled tank = condensation = water = ethanol phase seperation issues...

If you need 200 gallons to spend a day in the canyon, put in a 240 galon tank. Maybe 250 if you're thinking about adding a gen set. Just try to keep about 10% more fuel than you need.

JMHO

[In Memory Walter K]

I agree with Mike given your fiberglass replacement is because of the Ethanol problem. The maintenance and storage rules have changed from what we're used to because of it. You know what you usually would want for your longest trips and assume some more because of the added weight. Walter

[lobsta1]

I believe you are over estimating the total weight. (Unless your tank is made from concrete) Gas weighs 6.25 lbs per gallon. That makes the weight of your tank over 1000 lbs.

I know you want the max fuel, but you might consider reducing tank dimensions by 1" in all directions. Nothing is straight on a boat & this will give you a little wiggle room.

[Carl]

Not sure if this is feasible for you, but could you add a fuel compartment to the new tank for long trips. Basicaly just a baffle without holes. It could be closed off and cleaned out when not being used. Make your main tank compartment very similier to where the old once is/was so trim stays the same. In addition now you don't have to think about all the empty space to create condensation.

It was something I was considering when building mine, but then I only added a few inches to the back where I had an aux tank anyway.

[Hyena Love]

Assuming you run gassers, the addition of 100 gallons of gas placed on the deck of my B28 (pillow tank) basically over the fishing chair plate knocked 1.8 nmph off my cruise in flat water. But, thats running with a burned up tranny that was slipping a bit and fully loaded for a week of offshore fishing.

Boat actually ran better with more weight, except for the speed issue.

[kscheben]

Thanks for all the replies everyone. Hyena, I'm curious if you had to use trim tabs or anything else to "retrim" the boats attitude at cruising speed. I'm hoping that you did not notice that big a change in the boat's handling with the extra ~600 lbs in the aft.

[Hyena Love]

No tabs on the Hyena.

The weight was distributed pretty evenly cause we had so much gear, food/drinks/ice, spare parts and whatnot in the cabin. Plus, no bodies in the cockpit cause of the pillow tank.

Like I tell Uncle Vic, its not that she is dragging her ass, it that she is fixing to mount you.

[Matt29]

Ken,
I was wondering who quoted you on building the 315 gal tank on LI? And if you got it how the boat ran. Any info at all with your tank replacement would be helpful as I plan on doing the same this winter.

Thanks,
Matt

[dougl33]

I'll second Al's comment on the weight of the tank. We pulled his 250 gallon fiberglass tank from his 1978 33 a couple of weeks ago. While it was a little akward to move about, it couldn't have weighed more than 200 pounds.

I'd say you're looking at around 2200lbs tops.

[Jack]

Ken - My Bahia Mar has the same configuration as yours. 180 main and 74 aux. When I was still gas, I put an extra 100 gal on the deck for canyon runs. With the extra weight, she dropped at least a couple of knots on the cruise and ran pretty bow high. Even now, with diesels, I prefer the handling with a full aux tank. I don't think that 315 gal in the stern would work very well without the weight of the aux tank to balance it off. I suggest you do some arithmetic and test her before making any final decisions.