prop results help please

[drem312]

I have a 25ft wa boat weighs about 3850 dry it has a 350 5.7 Gm motor with the cobra outdrive.I have a 3 blade ss 15x 17 prop on there now. The specs for the motor I believe are 4400-4600 wot. I currently can reach 5100 with a speed of 32.4 other numbers are 4150@ 26.3 4000@ 25.00. 3900@24.1 mph on the gps should I go to a 19 pitch I usually have 4 onboard the testing was done with just me and a 1/4 tank of gas .what is a crusing speed for the boat

 

[steelespike]

Re: prop results help please

Near as I can figure the numbers don't add up. I think your tach is high.
You need to varify your tach.

 

[Bifflefan]

Re: prop results help please

I agree with Steelespike. Those numbers are not correct.
Either the tach is off or the prop hub is shot.

 

[drem312]

Re: prop results help please

Any idea with this setup. The size of boat etc what I should see in numbers with speed and rpm.? So if my wot should not exceed 4600 then about what speed for this boat should I see? I will check tach tonight

 

[chriscraft254]

Re: prop results help please

Drem, got a chance to dig the old prop out of the garage. I was running a aluminum 15X17 prop before. I now am running a 14 3/8 X15 SS Apollo XHS prop. Seems to put me right where I need to be.

 

[Maclin]

Re: prop results help please

I do not see how any advice can be given without knowing the drive ratio. The numbers from the original post work if the drive ratio is around 2.0 and the slip is 15 to 20%.

 

[steelespike]

Re: prop results help please

Any idea with this setup. The size of boat etc what I should see in numbers with speed and rpm.? So if my wot should not exceed 4600 then about what speed for this boat should I see? I will check tach tonight

with an accurate tach it will be a lot easier to figure where your at.
Dissregarding your relatively heavy boat useing 15% slip and a 17" prop your rpm is probably around 3836.
Calculators however can be missleading so your actual rpm and speed will be more accurate.

 

[drem312]

Re: prop results help please

Youre right about the out drive ratio I should of added that the drive says 5.0 on it and I have a 5.7 set up I did call and speak to a tech who said the two drives have a diff ratio but would work with a diff prop set up to play around with diff props. When I had the timing light on rpm it matched that tach so I will double check at a higher rpm tm thank you all for the advice

 

[steelespike]

Re: prop results help please

Assuming a 1.59 gear ratio based on the 5.0 outdrive,5100 rpm at 15% slip would make over 43 mph.your boat may not make 43 but slip would have to be something like 30% to even come close to 32.pretty much unheard of.

 

[drem312]

Re: prop results help please

Ok took the boat out tonight and hooked up a timing light that displays rpm so when I opened her up the tach on the dash said 5000 and the speed was 32.4 but the timing light had a reading around 4200-4300 are those rpm numbers better? I think the tach like you said is junk

 

[steelespike]

Re: prop results help please

Yes, those numbers seem more reasonable.Speed by gps would help pin point your exact
exact setup.