Prop change

[Monkaroo]

Good morning guys ,
Wondering what you guys think about a change in props . I'm running a 4 blade stainless 22 pitch prop . If I go to a 17 pitch to get my rpm's up from 4500 WOT to around 5500 WOT , what will it do as far as top end speed ? I'm running a 175 Evinrude on a 18 ft. procraft fish and ski . Right now I'm topping out @ about 52 mph @ 4500 . Thanks , Monk

 

[Monkaroo]

Re: Prop change

I think I might have answered my question with this link . My friend Emdsapmgr helped me out on the pitch selection yesterday . According to this link I should get a decent increase in top end speed , even though the 17pitch theoredically travels 17 inches per revolution , compared to the 22 pitch traveling 22 inches per rev , an added 1000 rpm's should give me some better top end . Thought I would post it in case any of you might be interested . I hate being a big dummy on some of this stuff . I'll be the first to admitt I'VE GOT A LOT TO LEARN ! lol


http://www.caravelleboats.com/propeller info.htm

 

[Dhadley]

Re: Prop change

Are you sure the tach is right? Speed? 0% slip is right at 50 mph with a 22.

 

[Monkaroo]

Re: Prop change

Sorry Dhadley , I haven't been on in a few days . I'm not sure about the tach being right or the speedometer really . They are both the original as far as I can tell . Do you know a way that I can check the accuracy of the tach ? I have never put much confidence in the OEM speedometers .

 

[Silvertip]

Re: Prop change

Your friend did not have the RPM/pitch/speed thing right either. The reason you get 1000 more rpm is because you dropped the pitch 5 inches. Therefore the engine must run faster to maintain the same speed as the other prop. Hole shot will be much better however. So theoretically the speed will be the same but the engine will rev in the proper range. The prop is the one-speed transmission for your boat. Dropping pitch is like downshifting in a car -- more rpm to maintain the same speed. I suspect also that the tach, speedo or both are incorrect. Most likely the speedometer is the culprit. A timing light with a built-in tach (or another tach) can be used to verify its accuracy. I would also not drop pitch to 17 inches if the instruments are reasonably accurate. I would check engine height first, then ensure it's developing full power (compression check) and tune up. Then try a 19P prop. Sometimes getting the engine up on the HP curve is all that's needed to give the extra RPM that's needed.

 

[Monkaroo]

Re: Prop change

Thanks Silvertip . I hate to be a pest man , but I would like to run something by you and the other guys . OK when I bought this boat (1984 Procraft fish & ski with 175 Evinrude ) , it had a 19 pitch 3 blade aluminum prop on it . OK I bought a 22 pitch 4 blade and tried it . During the prop swapping I was having some real problem with the engine dying coming out of the hole . Since then I have replaced all three carbs . The motor is running perfect now . It cranks and idles and jumps out of the hole now with no problem . BUT :redface: the throttle cam follower ( roller ) that opens the flaps on the carbs would not adjust to the mark on the throttle cam without the engine idling @ 1800 rpm's . Once I finished setting the idle , the follower with the flaps closed is probably 1/4 inch below the mark on the cam now .
OK after getting it to run good and comparing the two props . This was the results . The 22 pitch 4 blade stainless shows the boat running about 50 mph at top trim and 4500 rpm's . The 19 pitch 3 blade aluminum shows it running 45 mph @ 5000 rpm's . The engine running 4500 rpm's is in no way lugging or anything . It's really smooth and seems to be all good , but a 175 engine on an 18 foot boat should have more to play with . The boat has no problem with hole shot with either of the two props .

 

[Monkaroo]

Re: Prop change

I'm looking to buy a timing light with a tach too . I'm gonna make sure I'm getting the right readings like you said . Does anyone know what kind is the best and where to get it ? I'm also looking at some fishfinders with the speed wheel . Or maybe even a GPS . I know the OEM speedometers are not accurate . I've never trusted them . I plan to raise the engine up a little bit with the jackplate too . I'm not getting the bow up as much as I would like .

 

[Silvertip]

Re: Prop change

A GPS is the most accurate way to indicate speed. Since you have a jack plate I suggest adjusting that upward in 1-inch increments until you optimize the performance with the 19P prop. At 4500 RPM you are indeed lugging the motor. That engine wants to spin at 5500. The 19P numbers when run through the calculator show about 6% slip which is an excellent number but I don't believe it since the numbers for the 4-blade show an almost perfect 0% slip which in a liquid is not possible. If you have throttle issues, set the throttle lever wide open (engine off of course) and see if the throttle plates are perfectly horizontal in the throttle bores. If not, you have a link&sync issue to deal with. That process is way too detailed to go into here. Miss one step in that procedure and you do everything all over again. And if you are doing all of this adjusting with the throttle cable attached, that's wrong also. Throttle cable adjustment is the last step in the process.

 

[Monkaroo]

Re: Prop change

A GPS is the most accurate way to indicate speed. Since you have a jack plate I suggest adjusting that upward in 1-inch increments until you optimize the performance with the 19P prop. At 4500 RPM you are indeed lugging the motor. That engine wants to spin at 5500. The 19P numbers when run through the calculator show about 6% slip which is an excellent number but I don't believe it since the numbers for the 4-blade show an almost perfect 0% slip which in a liquid is not possible. If you have throttle issues, set the throttle lever wide open (engine off of course) and see if the throttle plates are perfectly horizontal in the throttle bores. If not, you have a link&sync issue to deal with. That process is way too detailed to go into here. Miss one step in that procedure and you do everything all over again. And if you are doing all of this adjusting with the throttle cable attached, that's wrong also. Throttle cable adjustment is the last step in the process.

OK , I have allready did the adjustments like you said . My throttle plates are perfectly horizontal at WOT . I also did my adjusting before connecting the cable , and then connected the cable accordingly . I'm kind of surprised @ that being technically the right way since I was just really playing it by ear . lol . I really don't have any throttle issues except for the cam follower not being centered with the mark on the throttle cam . It's all working great . The throttle plates are closed at idle and are all in sync ( full horizontal ) @ WOT . I have just recently installed the 6" jack plate with 4 " rise . My first out with it was set at center of the height adjustment which was too high . So I moved it all the way down ,which is great as far as hole shot , but I'm not getting the bow up at full tilt like it needs to be . I'm going to do what you said and raise the plate 1" and see what happens , and go from there . I guess I wasn't thinking that 1" would be enough to make a difference . Just adding the plate , even with it adjusted all the way down gave me probably 2 or 3 inches more rise . I really appreciate your help man . Thanks a million !

 

[Silvertip]

Re: Prop change

If you look at the spacing of the mounting holes on the transom bracket on most engines, you will see they are spaced about an inch apart. Yes -- one inch can make a difference. Less lower unit in the water, the less drag. Too high you lose water pressure and experience blow out. Perfect is just before that happens.

 

[Monkaroo]

Re: Prop change

Ok , maybe the one inch will be my perfect setting . That's about what I've got that I can deal with . On the 4" rise plate that I have , like I told ya earlier the center was too high . It was doing just what you said , blowing out , and not coming out of the hole right . When I slowed the boat down , even with the engine trim all the way down , it was trying to hop in the front a little bit like it would do with the trim still up . It makes sense to me now that 1" can change it , after you telling me that . I have changed it allready , but it will be next weekend before I can run it . I can't wait to try it out . I have a feeling that is gonna be exactly what I need . Man I can't thank you enough on that one . If you hadn't got me straight on the adjustment , I would probably never have moved it back up . Thanks again !