Fuel tank location

I think it’ll do a little better, but not if it bends the transom at WOT.

Say 5gph; it’ll run maybe 3 hours with a 90hp 4 stroke

maybe you mean a 2 stroke 115; gets us closer

neither are good plans

The sole of the original boat probably added to the structure somehow.

 

Fuel consumption calculator - MerCruiser - Volvo Penta - Crusader - Mercury - Evinrude - Honda Marine - Yamaha Marine - Suzuki Marine - Tohatsu - Johnson - Outboards | GPH MPG LPH KPL https://www.boat-fuel-economy.com/fuel-consumption-calculator
The old rule of thumb was 10gph for every 100 hp output of the engine, so at WOT, this is close, but with 4 stroke technology that has been improved
It is unlikely that you would run a boat at WOT so the fuel consumption falls off significantly with lower rpm

Our previous 9500 pound Thunderjet 29, with 500 hp, twin 250 Mercs, gave us around 2 mpg at 28 mph which equates to a 14 gph burn.

Edit
Using the formula of 10gph = 100 hp used would show that the 500 hp combination would use 50 GPH. Perhaps at WOT fuel consumption could get up towards 50gph

 

His plan is for a two stroke.

 

Have you actually done a calculation for the HP rating? There is more to it than just saying, "I want to put a 115 HP O/B on it". Some have talked about the strength of the transom? Is this going to be tiller steered or have a wheel (remote steering, I hope so, with that size engine.) What is the transom height? Do the math before you decide on the engine size. For a boat of that type with remote steering and a 20 inch transom height, the formula is 2 X (Boat Centerline length times Transom width ) - 90 = HP (you can round it to the nearest multiple of five. EX if it's 93.6 round to 95) Use decimal feet. example 18 feet 2 " is 18. 17 Ft.

example 2 X (18.17 X 7.25) - 90 = 173; round to 175. However, there are different formulas for no remote steering, less than 20 inch transom, which greatly reduces the HP.

In Canada the rules are found in TP 1332 at https://tc.canada.ca/sites/default/files/migrated/tp1332e.pdf starts on page 35. Of course they are in metric and Kilowatts (rather that HP) but the principle is the same. Actually Canada gives you a nice chart to use as an alternative way to calculate Kilowatts

 

Thank you to the helpful and not to helpful replies, yes the transom has a 1-1/2” wooden core. The aluminum has no motor mount holes drilled through it as the previous transom had multiple little holes through it. I epoxied and solid riveted an aluminum skin to the outside of the transom to patch all the preexisting holes. The inside will have a knee brace connected the transom to the floor. Then two additional knees connecting the transom to the stringers. There is also an inner aluminum sheet I have yet to fully finish that has tabs connecting to the sides of the hull and to the gunnels. This piece will have the brackets that the knees from the stringers will connect to.

 

The transom looks a bit better from this angle.

Almost all the comments have been contributors.

In my Lund aluminum skiff; there is a web above all those stiffeners that a plywood sole sits on. Did you remove that for repairs?

I do think 15 gallons will be pretty light on fuel for 90hp.

 

Are you able to send me a picture of the stiffeners youre referring to?
I understand 15 gallons may be low, with my current 16' fibreglass having a 100hp 2 stroke ive got two 6 gallon tanks, it works well for me fishing as i usually dont need to venture too far from the ramp, I plan on looking into larger tanks for more fuel storage/ peace of mind.

 

I've also got all the internals such as stringers and multiple other structural pieces. They were all removed so i could get the old pour in foam out. I understand my pictures are lacking lots but the boat simply doesnt have them installed at the moment.

 

We get all sorts of boat pictures and projects here. The highest duty we have is to make sure we don’t give you advice that can result in loss of life. A 115hp motor without the longitudinals and transverse and a stiff sole well connected could result in a boat that fails in a heavy chop miles offshore.

People want to help.

 

Here is a picture for reference. This is not a recommendation to weld..rebuild as original

 

It would be fairly cheap to have a backup tank with 6 gallons…or even 3. The easiest way is to make a disconnect at the engine because the 15 gallon will probably have anti-siphon valve that uses engine pump vac to flow fuel; so you can’t just use a tee..

 

You can have a two way valve. However, a quick disconnect is easier.

 

thank you! I’ve got the stringers here after I initially removed them and those braces along the sides connecting the floor and sides. Also the braces running across at the bow. I’m going to add additional braces for a rear deck so I can have hatches as well as a bow casting deck so to say that will have many braces running both longitudinally and front to back

 

I don't really understand the need for 100Hp plus on a small open boat that isn't going far from a ramp,but then I would be paying twice as much for the fuel.I remain convinced that adding the weight of all that fuel as well as a big engine at the stern isn't the best weight distribution and my experience in outboard hulls that plane is that having the tank beneath a seat moulded integrally with a centre console causes no problems with fuel delivery,so a thwart in about that location would seem to be the logical location to keep the c of g sensible and avoid the bow aiming for the sky.

 

I have little knowledge of planing boat design, but it seems dangerous to put such a powerful and heavy motor on the back of a light boat. As the fuel is burned or the passenger placement and load changes it will change the weight distribution. If the front is too light at high speed it could just flip over. It seems an analysis of the aerodynamic characteristics vs the weigh distribution is in order.