Prop Recommendation

[pwrplay]

Ok,

Well I got the boat out this morning on a fairly calm lake after a tune-up (plugs, wires, cap, rotor), and opened her up to WOT.

It looks like I may be a little over or under proped (not sure which one) . I was at 4200 RPM's and 33MPH (as per GPS) with my 15X16 4 blade aluminum prop.

My Merc owners manual says it should be 4200-4600 RPM @ WOT.

I can't confirm, but I do believe I should be getting close to 40MPH as well at WOT and thats with the stock 3 blade prop.

One thing though, I did have about 50-60 Gallons of fuel on-board as well as around 8 gallons of water. Its a 25 ft Maxum 2400SCR 8'6" beam and 5000lbs dry.

"EDIT" Its a 5.7L Mercruiser dual carb with an Alpha 1 drive.


So what do you guys think? Should I swap props to get a bit higher RPM's ? If so what would you recommend, keep in mind it is an Express Cruiser and I am not looking for top end, just good getting on plane and cruising with the family on-board.

 

[steelespike]

Re: Prop Recommendation

Did I miss something?You did a good job with the info but I didn't see any indication of what motor you are running.Your 4 blade would give up some top end but 7 mph seems extreme.
You do have about 540 pounds of liquid in there.Speed is probably with a light load.But again 7 mph seems high.

 

[pwrplay]

Re: Prop Recommendation

Oops ...hehe..engine size would help eh? Its a 5.7L Mercruiser dual carb with an Alpha 1 drive.

Thanks

 

[steelespike]

Re: Prop Recommendation

Keep in mind i'm not an expert but I think a 3 blade 15" prop would put you at about 4,400 to 4,500. 5,000lbs. thats a bunch of boat.Be sure throttle opens all the way check timing,is flash supresser clean.and maybe check compression.Bottom clean?good prop?

 

[BAYLINER185]

Re: Prop Recommendation

Well you do have a 4 blade prop and they do add more drag then a 3 blade.

You are right at the low end of the RPM range for the motor. You can either try a 3 balde prop same pitch as the 4 blade and see if that wil lbring RPM up a little since its less drag adding more speed or drop maby about 1 " Im almost thinking 2" of pitch but that may be too much in this case. You would have to try it and see what happens.

I have to ask a dumb quesiton but were you sure the Bow was trimmed as much as it could have been to get that bad boy out of the water? I know with the extra weight though youd be supprised.\

On my cruiser by my self I can do 45 MPH. I had my girl and her 2 daughters last week on it for the 1st time and it was hard to get it to 40. Extra weight or bodies makes a big differance.

Whats the boat normally do without all that stuff is what you have to ask your self?

 

[walleyehed]

Re: Prop Recommendation

Call any prop guru you want and you'll get the same answer...NEVER prop a boat for empty conditions....ALWAYS test with a normal load.
On this size of boat, a 4-bld is not creating performance robbing drag...the stern-lift will over-come what a 3-bld can't....with this size/power.
If you want the best over-all lift (less wetted area and drag) something like a BayPro series SS 4-bld will make a believer out of ya.
Sometimes, aluminum is just not the answer....this is a case where I would recommend the jump up to SS.

 

[pwrplay]

Re: Prop Recommendation

Thanks for all the help guys.

Thinking about this a bit more, I do recall last year I did WOT test without any additional weight (just me in the boat and about 1/4 tank of gas and was able to get about 35 or 36 MPH on a bit of chop. So I probably am in the right range asfor propping , but as walleyehed suggested a SS prop would probably be the best route to go for a bit more RPM and speed.

I really think the 50-60 gallon of fuel and water and waste add up to around 500 lbs of additional weight plus the rest of the gear on the boat as well. So that probably accounts for the low top end speed.

Thanks again everyone.