ORIGINAL: Neverdone
I guess I dont understand I have been talking with other people around here about using one of these motors in this boat.... The boat weighs less than my HPI Savage (a 1/8 scale monster truck) which is where these motors came from and they have enough power to make it flip over backwards so I figured that it would be enough power for this boat. The biggest thing that they are worried about is cavitation since these are high RPM motors. O.S. recently took one of their truck motors as put a marine head on it and they claim that it is a rocket. I was hoping that maybe this might work for my boat. You seem to be very knowledgeable about boats so what kind of prop might I need or should I try with this boat and motor. If I cant find a marine motor in my price range these are what I will be using. I really appriciate your help.. Thanks sorry if I am annoying
You're not being annoying, asking questions is the way one learns. Also, thinking outside the box is the way new things are discovered...
I know a bit about boats, as do others here. The RPM range you cited for the engines isn't going to be a problem. To give you an example, it's been estimated by people who know way more about boats than I do that the .45 MAC engine in my Sport 40 hydro is turning close to 30,000 RPMs when the boat is at the end of a straight. Anyone who has seen this boat run knows that cavitation is not a problem. Boats typically use high RPM engines and are propped accordingly.
It takes significantly more power to propel a boat on water than it takes to run something on land, or even in the air. For example, the Rolls Royce Merlin engine that has been used in unlimited hydros could get them somewhere in the 200 MPH range, give or take some. In a P-51, the same engine could propel the plane at over 400 MPH. Who knows what the engine would do in a properly designed land vehicle?
Adding to the problem is that a hydro has to not only run fast, but be able to get up to speed. There's a balancing act to find a prop that will work at speed, but that's not too big for the engine while getting the boat up on plane. Prop it too big or too small and it won't be able to get up on step. I've seen plenty of hydros plow through the water struggling to get on plane only to end up failing. This is where choosing an appropriate engine is important. If the engine doesn't have adequate torque to spin that big prop while trying to get on step, or it's lacking the RPMs necessary to spin the prop fast enough once the boat is planing, the boat isn't going to run well.
As Ron said, 1/8 scale hydros usually use .67 size engines. Even with those, people I know who run them say the boats tend to be underpowered and under weight. I know if you keep an eye out that you'll see a decent engine in this size range for $200 or less. This year alone I've picked up a CMB .67 with tuned pipe for $215 and a brand new OS .81 for around $200. They're out there...