Twin I/O to Twin Outboard Conversion Project

A bit further 'down the road' I'll need to run new power and fuel lines to the transom and into the bracket. I've been looking for some standards on fuel lines, in terms of splicing, connectors etc. T.I.A. for any references/resources on fuel lines. The existing fuel lines are/were 5/8" ID, which seem quite large, but they were feeding 6.2L engines. I'm not sure what would be more typical for V6 & small V8 outboards.

My thoughts are to run 2 new fuel lines from the tank all the way out to the outboard transom area (as shown below) with enough slack, so the re-power shop can terminate them as needed.
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⅝ is almost comically larger than necessary. The sierra fuel filter/ separator before my 250HO has ⅜" fittings and that is plenty. At 10% over power, or 275 hp, it burns about 27USG an hour, so less than a half gallon a minute.

 

Yes, perhaps the original design was for a jet engine . . . I'm thinking 1/2" or 3/8" . . . I'll let the re-power shop provide some guidance on that.

 

Yes. An engine that needs more than 2 quarts or 2 liters a minute needs rethinking. And fuel flowing freely through a ⅜ line would still be far more than that.

 

On the fuel lines, the existing 5/8 ones can not do harm if they are in good condition, and could be run to filters, with 3/8 hose on the output side to the engine(s). Restrictions are generally in fittings like hose tails and filters.

 

I have the design review report from the Naval Architect. Nothing earth shattering . . . overall in good shape with some tweaks. I'll be going through the report and incorporating the recommendations accordingly.

 

Yes, the existing 5/8" ID lines are much shorter than I would like. So, I'll probably run new 1/2" lines to the where the fuel/water separators will be located and let the re-power shop run what they prefer from there.

 

Was digging around my garage but couldnt find my scrap from my build. Local place had some that was an inner layers similar to automotive hose and essential shields 315 on the outside, that uses a fitting like a reusable hydraulic hose. It was about double the cost of shields 315 per foot and the fittings were pretty stout as well. No fuel smells at all even on hot days, not sure if it was worth the premium over a double wall shields product. Gonna be running with outboards capable of 30gph each consumption, over a longer route. Dont think 5/8 is all that crazy, although a clean run and under 15 feet would think 1/2 inch flows well enough. That shields 315 is good stuff for a double wall rubber hose, locally its around 10$ a foot.... the only downside.

 

Keep in mind TP that inside diameter is disproportionate to area...
5/8" is huge for your outboards IMHO.

 

Yes,



About 0.31 sq. in. in this case (5/8" ID). The 1/2" ID would be only 0.2 sq. in. (35% less)

 

Correct.

And, more to my point, 5/8" ID would be over 50% bigger than 1/2" ID.

Cheers

 

Without checking specs, I'd imagine 5/8" is overkill for the newish 600 Hp V12 Mercury outboards. Carburettor fed 2 strokes up to 300 Hp were fine on 3/8" hose.

 

After some discussion with the N.A. , I'll be making some changes to the outboard transom on the bracket. While Coosa BW26 is generally fine for transoms, in higher HP outboard applications (power & weight), compressive strength and duty-cycles over time become a concern. Having Coosa at the core, surrounded by the amount of fiberglass needed for overall strength, results in a much softer core than the surrounding laminate. The solution being to have the area of the outboard mounts a more solid core than Coosa.

The N. A. suggested G-10 material or perhaps making my own core. My plan will be to make my own core, given the price of G-10 and potential lamination challenges of G-10 vs. the VE resin. Here is the updated plan for the transom . . .
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I'll be making the cut-outs in 1" Coosa and filling the voids with VE resin/glass . . . probably will use some vacuum assist to get a decent level of glass density. Then the whole transom will get encapsulated in more glass, and even more glass during installation. My force & torque calculations put the transom glass thickness at around 0.4" each side, and I'll be going to a little further than that to get the overall transom thickness to around 2.00".

 

Thats a fair whack of glass layers to get 1 inch. Will you taper the edges?

 

It sure is a hunk of glass . . . Each of the glass inserts is about 270 cu. in. I'm hoping to be about 2/3 glass and 1/3 resin . . . So, 0.8 gallon of resin and 8 yards of cloth to make both pieces ~ $170 in material. G-10 plates would cost $650-700 total. . . and then 'fingers crossed' on the lamination integrity. Other fiberglass plate products such as GPO-3, Aqua-Steel, Whalelite would be about 1/2 the cost of G-10, but more resin friendly.

Based on the N.A.'s input and my calculations the transom is going to have 2x the amount of fiberglass than I had originally planned.