?'s about prop size and top/cruising speeds

[ne7800]

Ok i hope i have all the info i need. Boat is a 1980 Four Winns 180 Marquise with a mercruiser 470 170 hp (I know I know lol)and a alpha 1. Weight of boat from what i can find is around 2500 lbs most of the time it is just me (235 lbs) the wife (115 lbs don't tell her i told) and our daughter (120 lbs). we use it mostly for fishing and just plain old cruising. current prop is a 14.5x19 and "if" you cand belive my speedo it will cruis around 30-33 mph and top out at just below 40 but "if" you can belive my tach at crusing speed i am at the top end of the recomended wot of 3800-4200 at when i run it to real wot it is closer to 5000 rpm (no i dont take it that high, just the once to see what it was). so any way, where should i be at? any help would be awsome.

thanks in advance
Neil

 

[hwsiii]

Re: ?'s about prop size and top/cruising speeds

Neil, the information I have on your boat and motor shows a base boat weight of 2,500 pounds at 18', and the information I have for your motor shows a 1.7:1 gear ratio and WOT recommended RPM of 4,000 to 4,600 RPM. I know it doesn't agree with your information so please check it for me, especially the gear ratio and RPM that is very important. If you have to get the serial numbers off the motor and outdrive and call your local mercury dealer and verify it. When I have that information I can run some numbers for you. How many people were in the boat when you did your test and how fast was it going at 5,000 RPM, it is not clear in your post.

H

 

[ne7800]

Re: ?'s about prop size and top/cruising speeds

It was all 3 of us in the boat when i did the test, the 3800-4200 is what is printed on the valve cover, at 5000 rpm was almost 40 (again if you can belive my speedo) i'll have to check on the gear raito

 

[hwsiii]

Re: ?'s about prop size and top/cruising speeds

Neil, if your tach is right then the gear ratio I have for your outdrive is wrong. Please get me the correct gear ratio and i will give you some numbers.

H

 

[ne7800]

Re: ?'s about prop size and top/cruising speeds

ok the info i have says the gear ratio is 1.85:1 that is according to sterndrives.com and my serial number

 

[hwsiii]

Re: ?'s about prop size and top/cruising speeds

Neil, here are the numbers for your prop. You will have slower hole shot and less acceleration but it will bring your RPM down to where they should be and you should have better gas mileage. You need a 22 pitch prop.

Boat and Motor

Prop Picker

H

 

[ne7800]

Re: ?'s about prop size and top/cruising speeds

Thanks H the 41mph would that be top end or cruising speed?

 

[hwsiii]

Re: ?'s about prop size and top/cruising speeds

That would be at WOT not cruising, use a GPS for exact speed ,anything else is a crap shoot. LOL


H

 

[ne7800]

Re: ?'s about prop size and top/cruising speeds

Ok thanks alot H, i appreciate the help

 

[hwsiii]

Re: ?'s about prop size and top/cruising speeds

No problems at all, I am glad I could help.


H