Converting a Mono Hull single to twin Outboard

I have a 21.5' Aluminium boat, Which was designed for around the 30 knot mark with 115 HP with a light center console. The original brief was to build it from 4 mm keel sheets 3 mm sides and deck.
When I assembled it I overbuilt using 6 mm bottom, 5 mm sides, 4 mm decks 10 mm transom and a heap more bracing and stiffeners. I configured it to a center cab. I then fitted it out with a 140 HP Suzuki 4 stroke. The boat was originally designed with a bracket style transom with stepped down of an inch from the keel sheets. I have since plated that section in due to leakage issues with the design of the longitudinal girders terminating on the outside of the inner transom sheet. long story short, full fishing load, three large men with a 140 HP Suzuki this boat is a dog, struggling to get 26 knots at WOT. I sold off the outboard and have been looking at my options.
Here lies the question, I currently have 2*90 HP Etecs, 20 inch legs, based on the pics below I am thinking of cutting out the old transom and building it to suit the twins with 20 inch legs, what problems do you all see? and what advise for transom redesign?

 

The 140 Suzuki 4S isn't an overly-strong power plant by some reports. The 90's are not going to give you much more speed, imo. You certainly won't find the performance "electrifying". A bigger single would have been the best way to get that. But I'd say your boat's comfortable cruise offshore would be around 20 knots anyway, what with it being a moderate vee and all. And the 90's will do the job for that.

 

I would be happy with a 24-25 knot cruise at 4800- 5200 RPM, I have a feeling the Twin Etec might not make it. The new 175 Yamaha I priced was 22k, its a lot of money into a boat that's not worth much in the current economic climate!
I plan on taking the cab of a putting a smaller center console back in, as well as some other weight reduction ideas. I have a Dolphin Superskiff that generates the second hand 90 Etecs (they dont sell for much used). It would be convenient to have both boats running the same engines as well.

 

Where are you using the boat, over a bar etc ? 4800-5200 RPM is way too high for any satisfactory fuel use figures, you should be able to cruise in the low 4000's at worst with the 90's, if not, that boat must be excessively heavy. If so, then if you are reduced to one engine for any reason, and need to re-enter a bar crossing, it will be unable to hold the back of the wave, in all likelihood.

 

To be honest I don't know know the most efficient operating range is for the 90 Etec I am currently running on my skiff, its probably not my biggest concern for the distances I am covering. Its more comfort noise wise for passengers versus good operating speed. The boat I was thinking of adding twins to runs well at around the 24 knots. The problem was with the low output of the 140 if i tabbed the nose down to use the bow to cut slop and small waves it washed the speed right back to 14 knots. I don't think the boat is really that heavy, it was just severely under powered with the Suzuki, especially in the mid range RPM's. If I could achieve that 24 knots at low 4000's id be pretty rapt. I do currently cross bars with it and have a go around option if I need to.

 

The 90's should be enough, imo, fitted properly, and with props that allow them to rev out to at least the middle of the recommended rpm range.

 

Also of note my dealer told me I can't get a counter rotating gearbox for the 90, should I have a bigger separation gap based on that? When I build the new transom how do I position the cav plate height based on the transom angle?

 

Perhaps a single jack plate for the two motors?

 

You don't need counter-rotating props, there is not enough torque at work to make them necessary. You already have tabs to correct any lean, anyway. And spacing the engines wider would achieve what, more response to differential engine trim ? It could create ventilation issues if spaced widely.