Wrong pitch

[wbrannies]

I have a 1966 18" deep V Fiberform, w/ a Mercruiser 120 inline 4 and a Mercruiser # 1 I/O.<br />I had to change the lower unit, because it was not there. After i finished it i lanched it.<br />PROBLEM... I jumped on it and it was like i was starting out in 4 gear, no rpms and felt like a 50 horse power moter, and it topped out at about 25mph, at very low rpms.. Someone finily asked me the pitch of the prop was and i checked it and it wss a 21 pitch, Is this my problem!!!!

 

[jdcook]

Re: Wrong pitch

Check your manuf. recommended rpm range first. It is probably between 4000-4400.<br />but i would suggest that you go to a 17 or a 19 pitch prop. this should increase you hole shot without sacrifcing any top end speed. Changing props can be a easy soulution. some prop shops will let you try two or three props and then buy the one that fits you needs the best.

 

[MrBill]

Re: Wrong pitch

I have never seen a small in-line 4 Mercruiser engine with a 21 pitch prop. I would suggest that you should be running a 17" pitch. Also, it sounds like you switched out the outdrive, if you changed the upper unit portion of the outdrive, you could also have the wrong gear ration (should be 1.98:1). As JDCOOK mentioned rpm's s/b around 4400.

 

[GregCC167Concept]

Re: Wrong pitch

Just to add to what they told you, I agree, make sure you have the right gear set first. Then check your recommended rpm range against what you are actually turning. If your gear ratio is what it should be and you are low on the rpm's, you sould be able to drop the pitch on the prop and pick it up a bit. I believe the rule of thumb is about 200 rpm's for each drop in pitch - right guys?<br /><br />When I got my OMC 4 cyl Cobra, it had a 21 pitch prop on it and I was low on RPM's and my hole shot was awful. I droped to 19 and I'm good. And see if you can get away with what JD suggested. See if they will let you try a couple different props and bring back the one(s) that don't work out. The place I bought my prop offered to do this for me as long as I brought it back in the same condition it left in. Turned out I didn't need to do it but it was nice to have the option.

 

[wbrannies]

Re: Wrong pitch

What should the diamoter be w/ that 17 pich.<br /><br />Thank you very much

 

[Bondo]

Re: Wrong pitch

Don't Worry about the Diameter.... <br />You pick the Pitch......<br />The Engineers pick the Diameter.....<br />It Depends on Many things in the basic design of the props.....

 

[horndog]

Re: Wrong pitch

i have a 13.5" diameter x 19 pitch prop on a 16' with a merc 120. I probably should drop it down to 17 pitch, but I don't want to invest too much into this boat

 

[wbrannies]

Re: Wrong pitch

Well anybody got a extra 17 to trade for a 21