hey fellow model boat fans, im new to this forum and kind of new to model boats. at the moment i have a airstreak .15 monohull boat, but i traded a O.S .46LA for a fibreglass Hydroplane. I got no idea how you set these up and i do not know what size outboard motor to put in. Please help me as tried in another forum but nobody responded.

PS. It is 790mm(31.6inchs i think)which is the length of the pontoons, and it is 340mm wide (13.6inchs i think)

It sounds like it is an outboard tunnel boat, right? For that length, you'd use a 3.5cc engine.

sorry my fault, tunnel hull. I have had a look at some other boats similar to it and it is a bit bigger than most 3.5cc class boats. So if I was to put to a 3.5cc would it be underpowered because i want to race, what would you recomend as a good fast, realible motor?

The newer tunnel hulls are getting larger, some of the 3.5's are getting up around 34" as the RPM and HP increase. Some of the first ones were only 24" when the first K&B 3.5 came out.
Right now there are only 3 of them to choose from, the K&B, O.S. and the Thunder Tiger is coming back out again. There will be one more this year, hopefully in a few months, the Irwin. There are others like the MAC, Nova Rossi and Rossi engines that fit the K&B lower units.

i know i can get the O.S and the thunder tigers in Australia, but i cannot find K&B outboards anywhere. So if it comes down to a O.S or Thunder tiger which one has the more power and realibility?

I'd go with the O.S. as it has been redone since they pulled it out of the U.S. market and redid the motor but when they re-introduced it the price tag went up a bunch. The Thunder Tiger has a fickle carb (hard to set). One thing that lead to some it the carbs problems were people who started messing with the low-speed needle.

I think that boat can be set up for either a 3.5 or 7.5. All depends on your budget. The engine trimming is the most important part to get it to run proper no matter what engine. Ron is right on the OS for the better choice if you can get your hands on a K&B. But K&B's are very popular & I have even seen new ones in the box for sale on this site in the marketplace. Ebay also has a good choice most of the time for new in the box engines.