River pontoon passenger boat

I have started with some project for water taxi across the river. It will carry about 40 passengers. My plan is to install inboard diesel engine (more economic than outboard). This one boat will be aluminium pontoon boat (appx. 12x5m) or steel catamaran.
I need some recommendation for hull design, what is better, aluminium pontoon or steel catamaran, and also what is cheaper. If someone have solution for construction diesel engine to be something like inboard on pontoon boat (single one), I would be appreciated to him.
Also every other recommendations on design (especially initial design)would be useful for me.

Thanks in advance!

 

Assuming you do not want the boat to plane, just make the hulls as long and slim and smooth as possible for the best fuel efficiency.

 

Thanks for reply,

No, it would be full displacement boat, and because purpose is to float across the river, speed will be about 8kn. One inboard diesel on boat with 3-4 pontoons can be problem, but 1,2 outboard will be to expensive and it is not for permanent service of about 16h daily.
For hull lines important is to be cheaer in mean of construction, and as I need small speed good resistance performance is not necessary. For aluminium can be used some tube design for aluminium pontoons, maybe in fore part with some chamfer for wave piercing.
For steel design it can be much more prismatic pontoon design.
On deck will be seats for passengers and one thing what can reduce stability performance is that boat need some glasses (possibility to use in winter time) and interior will be heated/cooled. It can be some plastic, but for clear view I think glass is better, but much more weight will be added on higher level above deck. It is one more thing on what I try to find best solution.

 

Normally the engine is in a 3rd. pod, or short pontoon, in the center aft. Usually they are inboard/outboard propulsion systems here in USA. Mercury is now making a 4 cyl. diesel IO package for recreational pontoons, and other type boats as well. Yanmar may have a IO package as well. I know Gibson houseboats are installing Yanmar diesels.
I would want several water tight bulkheads in my pontoons. I'd want a wider bottom than a higher depth in shape.