Prop selection 3.0 cobra -140 Hp

[Randy stoner]

So given we are in the winter months, i have time to plan the new boating season once the weather breaks in the Spring up here in Ontario Canada. Last year i had some really bad luck, first blowing the outdrive on my 1991 sunbird 175 corsica then the motors block cracked. And yes i fixed them both. Rebuilt outdrive (OMC) and a brand NEW 3.0 4 cyl. Cobra 140 HP. Prior to all this bad luck, i did have a chance to get out a few times with the old engine. Revs were 4700 rpm with a 17p 3 blade Steel prop and a highfive 17p Stainless five blade prop. Both gave me a solid 35 mph by gps with myself and four grand kids. Three in the front (bowrider) and my grand daughter 17, in the back. Im 220 lbs so i think it went well. The original motor was a 3.0 Cobra 130 hp. This new motor has another 10 hp and i dont know if that would be much of a difference. I also have an 18p four blade prop and short of trying them all out on the new engine in the spring, has anyone had any experience with a 3, 4 and 5 blade prop on a 140hp i/o? I will most likely try all three but am especially wondering about the 4 blade how it performs as its made out of Aluminum and the other two are steel and stainless steel respectively. Its Sunday Dec 20 and I'm board and looking at possibly another lockdown so i'm on here. BTW, i know the boat was not worth what i did but im getting older and just dont want to buy another fixer upper someone elses' troubles. At least i know what i have now and it should serve me well for a lot of years if i take care of it.

Thanks

Randy

 

[alldodge]

Revs were 4700 rpm with a 17p 3 blade Steel prop and a highfive 17p Stainless five blade prop. Both gave me a solid 35 mph by gps

This isn't right, the both are 17P, a 3 blade and a 5 blade. The 5 blade would be at least like a 19 pitch (and could be a tad higher), but the RPMs stayed the same?

More blades gives more lift but also takes more HP to turn. I would continue with the 17P 3 blade

 

[Scott Danforth]

140hp is at the crank
130hp at the prop

when I had my 3.0, I had on board a 15p for water sports and trolling, a 17p for general use, and a 19p for cruising all 3-blades at 4600 RPM would be about 41mph in the boat with only 2 people on board

you dont have enough motor to spin the 5-blade

 

[Randy stoner]

You would think so, however the 130 I had reved 4750 rpm with the five blade according to the tach that is. Im going to try out the four blade. With a brand new 140 hp 3.0, im hoping for 40 mph.

 

[alldodge]

Need to check your tach

 

[Randy stoner]

The marina that put the motor in tested the motor in the water at wot with a mechanics tach. 4650 with the high five. but didnt have the speedo peto tube in the water and no gps.

 

[alldodge]

The marina that put the motor in tested the motor in the water at wot with a mechanics tach. 4650 with the high five. but didnt have the speedo peto tube in the water and no gps.

There will be more then 100 rpm difference between a 3 and 5 blade prop. Someone has defective equipment, is trimming up to high, doesn't know what their doing, or is lying

 

[Randy stoner]

Well quite frankly, i know that the speed was via GPS with the 130 on it and it was 35 Mph. with the HighFive. That is a fact. So at the very least I will obtain that and with a more powerful motor i will at least get that again. Hull design too has a lot to do with speed as well and with your own admission you could get 41 mph out of yours. I wont getting into a pissing match on here about who is right or wrong, but i know the marina im dealing with and they are the BEST around in my area and everything they have told me so far holds out, so they are NOT lying or using faulting equipment especially when the GPS verifies it. Enjoy your boating.