boat motor size

[Thunderchild]

I wish to put a motor on a boat. not the type that dip into the water as it is an inflatable boat. i wish to use a propeller like a plane does. what power motor do i need. I would pressume that the faster it runs the smaller the propeller can be. so what power/speed/prop. diameter do I need for four people/220 Kgs /440 lbs
thanks

[samz90]

You need to be a bit more specific. Whats the: Boat size, make , model. Is it and outboard you want?

[Ralph]

It will need to be big! For instance, look at the airboats that they use in the Florida Everglades. These have props about 1.5 to 2 metres in diameter. They are also completely flat bottomed & hard bottomed.

The prop needs to generate more thrust than the drag of the boat. This isn't going to be easy to measure. If you have access to an outboard, then you could find out something like 3/4 throttle gives you the cruising speed you want for your airboat. You could then tie the boat to a bollard with a heavy duty spring balance and find out what sort of thrust the engine actually produces at 3/4 throttle.

Then you need to design the air prop to produce a similar amount of thrust. This is waaaay out of the scope of an AB reply, plus I can't remember off the top of my head!

As a complete guess, I would use a 500 to 800cc high revving motorbike engine coupled to the best copy I could make of an Everglades airboat prop. I would also be prepared to concede that I may need a bigger engine since an inflatable hull bottom will produce a lot more drag than a flat rigid hull bottom.

[Thunderchild]

OK i get the picture: I am wasting my time as all that gear is too big and will just sink it. the problem is that being only a small rubber boat it cannot take much equipment. i will just end up sitting on the beach remote controlling it and thats not what i had in mind so i'll just stick to oars

[Ralph]

How about something like a 2hp Seagull outboard? They're very small and portable and there's loads on eBay. They won't give sparkling performance, but they'll get you out messing about in your inflatable and there's not much that's better than that!