Fuel tank location

I’ve got an 18’ tinner that had a 15 gallon fuel tank come with it. It mounts beneath the flooring and I’m wondering where the best spot to mount it will be. Planning to put a 115 2-stroke or if I can find a 90 4-stroke on the back. Is the tank placement forward smart to balance the weight?

 

All things being equal,the centre of buoyancy is the optimum place as the trim of the boat won't change with varying fuel load.

 

where would the centre of buoyancy be?

 

About 1/3rd of the way from the stern, so the poll is not too good.

Also, there is a sure misalignment between a 15 gallon tank and a 115hp engine. Given no stringers at all; the engine size you are choosing gives me great pause.

I suggest you offer pictures of the transom and thickness and prior hp. The decision to increase hp is less than arbitrary.

 

Yup, that'll do it...

 

At rest this is correct but the tank should be on or behind the center of lift at normal planing speed, as this is a planing hull. This is a little harder to determine

I think that the width of the splashwell is way to wide. A wave coming over the back will add a lot of water into the splashwell.
While our boats were wider at the chine and a bit longer, we would stick 70 gallons of fuel against the transom with the addition of a big block and pump that weighed in around 1000 pounds
and never had the issue of keeping the bow up. We have modified about a dozen boats with outboards that had fuel tanks in the front, ( I realize that you are not putting the tanks up front)
that caused the bow to drop at higher speeds.

The best way to get to a solution is to find a similar hull, (deadrise, chine width and length ) with the hp that you want at a dealership and put the tank at that location.

 

you think he can find that boat somewhere?

and yes, that transom and a 90hp motor is in question until we see/hear more

 

A quick google search will reveal many riveted boats with the length, beam and similar deadrise.
Lund being one, Alumacraft, I think perhaps Crestliner .

 

Have you replaced the transom, or is this still the original transom?
It appears to be riveted on, and I don't see any holes for securing bolts for a previous outboard motor.
Have you filled the holes - or did it previously just have a very small 'clamp on' (rather than 'bolt on') motor?

A photo from above showing the structure would be useful.

 

And lets see ( ie picture ) this tank, preferably 1/3 fwrd of transom in the boat.

Thanks

 

To my mind,the most pressing question by far is whether the structure of the transom can cope with the loads imposed by a motor of the size mentioned.We can't see what reinforcement is on the inboard face of the transom or how it connects to the bottom or sides.I can't recall how often I have heard of a phone call to a dealer or builder along the lines of "Will your XYZ Sport be OK with a !!!Hp motor?My local outboard shop has one on special offer".The answer was usually no,because while the hull shape ought to be fine,the structural aspects of the hull had been calculated for a motor that didn't exceed the power on the maker's plate in the hull.

Just pause and consider how the thrust from the propellor beneath the hull will be transmitted via the transom and the forces involved.The general principles for locating the tank are already here.

 

None of those boat brands would be devoid of longitudinals afaik. Not trying to pick a fight; just not sure what he has here is all.

 

I think we are focused two different items here
I agree that the boat should have longitudinals
I am skeptical with putting the tanks at the CB at rest or to far away from the transom.
The boat is light. At higher speeds the Center of Lift will be quite far back. If a heavy load is in front of the center of lift, it will push the front of the boat down. It should be taken into consideration that there will be at least one or two people, driver and passenger, already, ahead of the center of lift.
(for clarification- I am using the center of lift being the location the of effective combination of displacement and dynamic vertical forces summing moments around the bottom of the
hull at the transom. Ignoring thrust, skin drag, wind drag etc. AND running at 30 mph)

So many assumptions have to be made here as the information on this hull is incomplete.

So with this in mind some observations

The OP should provide:
1) the hp rating on the hull, with the idea of not exceeding it
2) the chine width, transom deadrise
3) the weight of the boat
4) the size hence weight of the fuel tank that he is considering and the shape, to determine the location of the full tanks center of gravity

WRT to the longitudinal stringers
It appears that the floor stiffeners are riveted. If there is mastic between the stiffeners and the hull, it would be difficult to weld in longitudinals.
There are other riveted boats in the marketplace. A trip to a dealer to poke around would/could reveal how longs are attached to riveted cross frames.
Not hard to do, just look at the rivet pattern from the bottom.
I would expect that their are at least 4 longs, riveted directly to the bottom of the floor and then cross "frames?" stiffeners.

In order to transfer the thrust of the motor to the hull bottom and the moment (torque from the thrust wrt to the transom) to the hull sides there needs to be cross structure to carry
these forces to the hull sides and transom. I believe that the OP is planning a heavy plywood tansom (from a previous thread) instead of structural aluminum stiffeners on the transom face.

We have repaired quite a few transom to gunwale cracks (maybe half a dozen) due to inadequate strength in this area.

 

Sadly, the OP doesn't seem to care about any of this.
He/she just wants our advice on where to place the fuel tank.
As has been said, center-of-mass 6' forward of the transom
should ought to do it.

 

A 15 gallon tank would be enough to run about 70 minutes. A 115 HP outboard will burn approximately 11 GPH if it is a somewhat modern engine.