i have a new design project as some of the longer term members here know, I've built some welded aluminum boats and have a bunch of aluminum available. I also live in the Seattle which has great inland waterway access almost all the way up to Alaska.
What I envision is a four seat craft with a lot of additional cargo capacity, and if possible making this cargo space useful for sleeping two people inside the plane while it's not moving. The cargo/sleeping space could be a partial 'trade' with the two rear seats.
It also needs to have the removable wing tips that can be switched with longer wings in order to get better range. I want to ultimately have a 3000 mile range so I can use it to go to Hawaii, for example.
Longer wings will allow flying within ground effect at a higher distance above the surface.
I want it to have enough wing area so that it will fly slowly enough to be able to follow the contours of ocean swells and remain in ground effect. I think 50mph is a good ballpark figure.
So far I envision a reverse delta with twin engines on pylons like the Russian BE-103 uses: http://cdn-www.airliners.net/aviatio.../7/1111773.jpg
I realize that with longer wings it will be able to fly out of ground effect and so will need to be registered as an aircraft, but that's for later.
I hope to initially use two 40 hp industrial (lawn mower) engines like the Kohler Command V twin. And initially use them with direct drive.
Later, for long range, I might need to switch to larger water cooled engines, and of course, diesels would be ideal for the long range. I've done some preliminary spreadsheet calculations using rough assumed ballpark figures. With a gross weight of 3000lb and a span of 40 ft it needs to have 150+ hp to get a marginal climb rate, and marginal is OK for it when it is overloaded with fuel. This is assuming 1000lb of fuel, which will give a 3000+ mile range.
However, those figures are for flying out of ground effect, and flying in ground effect can increase fuel efficiency up to 250% but for now I'm going to figure it as if there is no ground effect, which will provide a large fudge factor.
There are two types of ground effect, span dominated ground effect, and chord dominated ground effect, and the chord dominated ground effect will provide better efficiency but requires flying lower (unless AR is less than 1, and this won't be)
Chord dominated ground effect creates an unstable condition, which can be partly compensated for with use of a reverse delta with negative dihedral and a large horizontal tail: http://www.aerospaceweb.org/question/aerodynamics/ground-effect/flarecraft-l325.jpg
On these types of WIGs the horizontal tail is normally a lifting surface because it helps to automatically prevent the unstable pitching up the occurs when the plane increases it's height above the surface. At low heights the center of lift moves back to about 50% chord so the tail lifts very little, but when higher the center of lift moves to around the 25% point, at which the tail must lift more.
This site has some discussion of the stability issues and different airfoils used to help with that: aero