Old Prop on New Engine?

[tboltmike]

I just replaced the original engine on my 18' trihull, 1100 lbs dry.
The prop is a 3 blade 13 x 18 pitch that came with old 1975 115 Johnnie. It pushed the boat 39 mph at 5200

On the new 130 ETEC, it turns 5800 she goes 42 with 4 souls, bimini up and a stiff breeze and 46 at 5800 with 2 on board and top down and not much different with just me. Interesting the the revs are the same. This was my first time out with it and was too in awe to take much data...maybe next time!

Should I keep the old prop and have it tweeked? The dealer says the Viper would be a definite improvement.

Would a larger diameter be in order? If the dia is increased, does pitch need to be lowered?
Thanks, Mike

 

[SparkieBoat]

Re: Old Prop on New Engine?

5800 RPMs is about the exact right RPM for your motor. unless you are having some type of major prop slip, I do not see where a new prop would make much difference. but if the dealer is convinced, tell him to let you do a test run with it, if it does not show noticeable improvement you get your money back.

 

[Texasmark]

Re: Old Prop on New Engine?

Agree

 

[jimmbo]

Re: Old Prop on New Engine?

If you want to experiment a bit try a Raker. Slightly different setup than yours but with the 19 Aluminum prop the boat went right around 50mph. With the 20 in. Raker I was able to lift the engine 1 1/2 inches higher on the transom and was still able to trim out further without the engine ventilating on turns. Speed went up to 55 mph

 

[Texasmark]

Re: Old Prop on New Engine?

20 in. Raker I was able to lift the engine 1 1/2 inches higher on the transom and was still able to trim out further without the engine ventilating on turns.

I can attest to the importance of raising the engine if you can. Did it on 3 boats and the results were stellar.....and if you break/blow out in turns or rough water get yourself a cupped prop which has a much higher pitch on the trailing 1/4" of the blades and it bites better than a smooth (non cupped) prop.

 

[SparkieBoat]

Re: Old Prop on New Engine?

it seems to me that the number one biggest cause for under performing outboards is that they are mounted too low, every single boat I have ever purchased, I have raised the motor at least 2 holes, some I got a jack plate and went even higher. I am not sure why most boaters do not understand the simple fact that raising your motor can reduce drag and improve hole shot and MPG and MPH. I would bet you that 9 out of 10 boats at your local ramp could go up at lest 1 hole.