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View Full Version : Relocating Tank for Weight Distribution, Opinions?



FlatRacer, aka BarrelBack
05-12-2003, 07:13 PM
Sometime in the next year I hope to be buying A Donzi from a friend. It's a late '60s barrel-back 18' with a V-drive and a small block. As it rides now, the bottom's a bit wet at speed, not so bad that it hooks or anything, but it does lose some speed because of it. I don't mind this so much because top speed is not my goal for this boat (I still have my racer if I want to go out for a fast ride).
If and when I do get this Donzi (I've been promised I'm first in line), I plan to someday restore it. The one thing I would most want to change is the location of the gas tank. I believe this would be the best way to free up the hull. First choice for location would be in the stern, two tanks on either side of the engine, outboard of the stringers. Second choice would be under the cockpit floor, again, two tanks outboard of the stringers.
I never liked the idea of the tank in the bow, for weight distribution reasons. Obviously, it's convenient for the manufacturer, but in my opinion not optimum for performance. The trim changes as the fuel level drops, and the hull wetness is exacerbated in the V-drive because the drive isn't trimable. Cavitation plates of course are of no help in bringing the bow up. Rocker plates might help a little, but probably not near as much as moving the tanks back.
Am I on the right track, or am I "all wet"?
Thoughts, opinions???

FR :p

GEOO
05-12-2003, 08:30 PM
I think the best place for a fuel tank is centered at the CG. This way the fuel level will have little effect on the CG. I have never owned a fast V-Drive, but I have read about them. The trick is to have a well balanced boat with the proper CG (approx. at 1/3 the length??); however what ultimately controls the boat from running too wet or to high is the location of the V-drive. If the V- drive is located to far foward the boat will run bow high. If the V-drive is located behind the CG the boat will run to wet.. The shaft entering the hull has all the lift of the prop and thrust pushing at that point.
I would try the boat out first. If the boat is to wet add a sand bag or two to the stern. Does the boat like the CG moved back? or does in porpose at slow speeds but run better at high speeeds? If this is the case keep the CG and move the v-drive. Good Luck.

How do you stop a screaming 14 day old boy??? eek! The Donzi video's didn't work!!!

MOP
05-12-2003, 10:02 PM
What about props, that may help to do what you want.

FlatRacer, aka BarrelBack
05-12-2003, 11:07 PM
GEOO:
what ultimately controls the boat from running too wet or to high is the location of the V-drive. If the V- drive is located to far foward the boat will run bow high. If the V-drive is located behind the CG the boat will run to wet.. The shaft entering the hull has all the lift of the prop and thrust pushing at that point.
Ah, not true. You would think so at first, but when you examine the whole system, the location of the gearbox itself (aside from the standpoint of, again, weight distribution), does not matter much. There was a HUGE raging debate about this over at the V-drive forum on the HotBoat site a few months ago. I'll try to look up the link for you.
I like the sand bag idea. Maybe an ultra light fuel load and a back seat full of those water bladders the wakeboarders use in their ski tugs would fairly approximate how it would ride with two tanks under the floor. Hmm......

M.O.P. is right, there are some things you can do with the rake of the prop blades to fine tune the bow lift and stern lift, but only to a point.

I'm thinking if I had the deck off anyway, and the tank needed to be replaced anyway, could I put two tanks under the floor, or in the engine compartment?

FR :p

GEOO
05-13-2003, 04:04 PM
FR,
Why doesn't that theory make sense??
I liked it!!
The thrust from the prop is tranfered to the shaft to where it meets the hull.
Much like taking a stick and pushing the boat, from the bottom, at the location of the shaft entering the boat.

FlatRacer, aka BarrelBack
05-13-2003, 06:05 PM
GEOO:

Pop open a beer and get comfortable before you digest this one:

http://forums.hotboat.net/cgi-bin/ultimatebb.cgi?ubb=get_topic;f=8;t=001258

You've got to read the whole thing. The light didn't really dawn for me 'til halfway through the second page. Enjoy!

FR :D