Still confused about what prop to use?????

[chadelliott]

I have a 1983 Sea Ray seville
3.8V6 OMC 800 Stringer

Factory WOT rpm's are 4200 to 4400, The current prop that is on it is a 15x17 and I can only get 4000 at WOT with a speed of what the speedo reads at 38mph(i think i am going a bit faster than that though) I put the extra prop on which is a 14x19 and could only get 3500 rpms @WOT and a speed of 35 mph.

Confused on which way to go.....should I get a 15x15 or a 14x???? to achieve maximum rpm's @WOT????????

 

[steelespike]

Re: Still confused about what prop to use?????

Please read the stickies at trhe top of the page.
What size,type is a Seville?Unless its a monster your performing under par.
to keep it simple;increase pitch (Higher number) lowers rpm.Decrease pitch
increases rpm. a 1" change will net ABOUT a 200 rpm difference.Motor needs to operate near the top of its rpm range with a typical load.
You probably have something else going on besides prop selection.

 

[chadelliott]

Re: Still confused about what prop to use?????

it's an 19ft open bow. Runs like a champ, no engine issues.

 

[hwsiii]

Re: Still confused about what prop to use?????

Chad, if you will give me the horsepower and gear ratio on your boat I will be glad to help you.

H

 

[chadelliott]

Re: Still confused about what prop to use?????

Chad, if you will give me the horsepower and gear ratio on your boat I will be glad to help you.

H

HP is 120 according to the owners manual ,gear ratio I am not sure of at all. This is my second boat after a 12 yrs. with out one. And that was an outboard 2-stroke.

 

[hwsiii]

Re: Still confused about what prop to use?????

Chad, I have looked for the the gear ratio and RPM for your motor and I do not have any information for your setup. My information shows the 3.8L for 1983 as 170HP with the stringer 800 drive and not 120, and when I look up 120HP I find it for 1981 and has a stringer 400 drive not the 800 drive, I can't run my numbers without knowing the correct HP and gear ratio.

H

 

[steelespike]

Re: Still confused about what prop to use?????

Chad, I have looked for the the gear ratio and RPM for your motor and I do not have any information for your setup. My information shows the 3.8L for 1983 as 170HP with the stringer 800 drive and not 120, and when I look up 120HP I find it for 1981 and has a stringer 400 drive not the 800 drive, I can't run my numbers without knowing the correct HP and gear ratio.

H

H ,As you probably know the stock hp is 140 and I think your right on with the 3.8 at 170.Boat is 2,000lbs spit balling Don't you think he should be at about 40 and closer to his max rpm?with a 17.assuming a healthy motor and hull. Just seems to me a 15 is extreme as an all around prop.
Did a little research found reference to a 1:86 ratio I believe upper is 21/20.

 

[hwsiii]

Re: Still confused about what prop to use?????

Steele, I can't tell you how much time I have spent looking for the gear ratio on this &&&& rig. I know it isn't a 1.86:1 ratio as he is showing a negative slip for both props if that was the case. I also found references to a 1.71:1 and a 1.77:1 but that is not right either as he shows a negative slip with those ratios as well. What appears to work I won't say because I don't want to influence the outcome, but the prop slip comes to 11% on the 17"and 17% on the 19" with his present props and that should be about right. But yes steele I think he ought to be doing about 40 MPH and he is real close now.

Chad, you need to get the serial numbers for the motor and the outdrive and go to an OMC dealer and have him look up the information for the motor HP and the gear ratio and then get back with us. I also need the model of the boat, as I believe it is a SRV 190 but I am not sure.

H

 

[chadelliott]

Re: Still confused about what prop to use?????

Yes you guys are right about the HP, it is 170. I looked at the 4 cyl. version. And it is an SRV 190. My 15x17 prop has seen better days and a closer look at the hull, it is quite filthy. I am going to try a new 15x17 prop, clean and polish the hull. Then we will see what she does. Thanks for all the diligent research you guys do.

 

[hwsiii]

Re: Still confused about what prop to use?????

Chad, I ran the numbers and in my opinion you have a 1.5:1 gear ratio and you should be running a 15" pitch prop and that will solve all of your problems. All of my analysis show a 1.5:1 gear ratio and a 170 HP motor and you should not lose any speed but gain your RPM up to maximum RPM and your hole shot will improve a lot.

H

 

[chadelliott]

Re: Still confused about what prop to use?????

Chad, I ran the numbers and in my opinion you have a 1.5:1 gear ratio and you should be running a 15" pitch prop and that will solve all of your problems. All of my analysis show a 1.5:1 gear ratio and a 170 HP motor and you should not lose any speed but gain your RPM up to maximum RPM and your hole shot will improve a lot.

H

Sounds good, I will try a 15" pitch prop. How much does the diameter affect rpm and speed etc......

 

[hwsiii]

Re: Still confused about what prop to use?????

Just buy a 15" aluminum prop in the same make and model you are using now and it will be fine.

H