Fuel Tank Placement

[dcrienks]

I am restoring a 25' 1978 Starcraft Chieftain. It had twin I/Os. I took them out and I am installing a transome bracket with a 26" offset swimplatform to hold a single 250HP Yamaha four stroke outboard. The existing aluminum fuel tank is 60 gallons or so and is about 20" wide x 9" tall x 70" long and sits perpandicular to the centerline of the boat. I have decided to take it out and install a new plastic tank by Moeller. I plan to creat some storage where the I/Os used to be.

So here are the questions,
1. By moving the weight of the engine back behind the transom, is there a placement of the tank that will help in boat performance? Or do I just install the tank as far forward as I can?

2. I was going to put in a 58 gallon belly tank to get the weight as low in the hull as possible, is this important?

Thanks, Red

[marshmat]

Hi Red,

I'm not sure what your original I/Os were, but I suspect the single outboard is a fair bit lighter than the two sterndrive engines were.

Most likely, you want the boat's centre of gravity to be in roughly the same spot as it was before. At about 170 kg (370 lb), your fuel tank will have a pretty substantial effect on the CG. Depending on how the weight (and moment) of the new engine compares to the old ones, it may not be necessary to move the tank, or it may prove to be beneficial to move it forward. (Moving the tank too far forward, though, can cause trim issues when the tank is near empty- they're usually kept near the CG so that they don't change the boat's balance too much as they drain.)

Have you done a weights-and-moments calculation yet? If you haven't, it's a worthwhile exercise to go through before finalizing the locations of too many heavy things.

[dcrienks]

Thanks!

The twin 4cyl I/Os and drives were about 1400 lbs. total

The transom plates (to cover I/O holes), swim platform/motor mount, stringer supports, and the Yamaha is about 850 lbs total. Signficant weight reduction.

Red

[Submarine Tom]

Red,

I used to drive a commercial, 12 passenger I/O with Volvo-Penta's.

They were removed, a two foot hull extension added and two, four-stroke

outboards (Merc's) installed. The boat ran great BUT, in a sea that caused

significant roll, the boat would "hang" when it rolled appreciably. It was

most unsettling and disconcerting. I should add that no survey was done,

no tank changes were made and no ballast added. I expressed my

concerns to the owner and nothing was done. I left the company shortly

there after. The boats (two) have both now been sold. My point is, I would

heed the advice given thus far and exercise caution.

Best of luck, Tom.