?? Prop questions ??

[ajkolar]

I have an ’88 OMC Cobra drive (on a 21’ VIP cuddy, ~ 3000 lbs). Will any 15 spline prop fit this drive, or are they manufacturer specific. I guess what I am asking is will a 15 spline 14 ½ x 23 mercury (mercrusier) prop fit my OMC cobra and vise versa?<br /><br />Last year I dropped a new (rebuilt) 5.7 SBC 4 bolt main into my boat. When there is only 2 or 3 people in the boat if I opened it up to WOT it would run right past 4600 rpm and I don’t know where it would stop (feels like there is still 400 or 500 rpms left in her), but I don’t ever run it past 4600. My current prop is a 14 ½ x 21 stainless steel prop. I am considering swapping to a 14 ½ x 23 stainless but a little concerned my hole shot afterward. Should I consider a 4 blade prop? If so what dimensions, I am not real familiar with how the extra blade is going to affect the rpm’s (I know the 200 rpm per pitch, but only as it applies to a 3 blade prop of the same diameter)<br /><br />I also considered buying like a cheap 14 ½ X 19 aluminum prop for tubing / skiing purposes only. This is a fairly heavy boat and I would like to pop out of the hole for these types of activities. Thanks in advance for all your help and suggestions.

 

[Dhadley]

Re: ?? Prop questions ??

The Merc prop will fit and work IF you shorten the splines in the hub. Any good prop shop can easily do that.<br /><br />There are 4 blade props made specifically for stern drives too. We have to take into account that, unlike outboards, we cannot (easily) raise the motor. We have had good success using the slightly smaller diameter 4 blade props with I/O's. Usually in the 13 3/4" diameter.<br /><br />We're assuming you still run the exhaust thru the drive, right? Or did you install transom tips when you changed motors?

 

[ajkolar]

Re: ?? Prop questions ??

Yes, I actually do have thru hull exhaust. So, are you saying that if I switch from a 3 blade to a 4 blade, I should reduce the diameter size of the prop? What about pitch, from 3 to 4 if you downsize in diameter, do you need to adjust pitch or would a 3 blade 14 1/2 x 21 be equivalent to a 4 blade 13 3/4 x 21. If this is true, I assume I get a better hole shot, but what about top end? I have heard that top end doesn't suffer so much with newer four blade props...<br /><br />I assume you mean shorten the splines on my prop shaft? or do you mean some kind of alteration to the prop?

 

[Dhadley]

Re: ?? Prop questions ??

To use a Mercury prop on that drive, you have to shorten the slines in the hub of the prop. Any good prop shop can do that. No big deal.<br /><br />If you're over rpm's with the 21 3 blade I'd start by trying a 23 4 blade. You'll need to test it anyway to see the difference on the holeshot. And top rpm.

 

[ajkolar]

Re: ?? Prop questions ??

Ok, thanks for the info. You think a 23 4 blade of the same diameter then (14 1/2)? Thanks again!

 

[Dhadley]

Re: ?? Prop questions ??

It just depends on how much rpm you need to pull out. I'd try whatever I could find, either 14 1/4" or 13 3/4". The more you try the better decision we can make.

 

[gatorred]

Re: ?? Prop questions ??

corvette 5.7 will turn to 5500 but cam in off road use will not even turn to 5000 see if you can get a rpm of tork and hp on the engine if it was pro built set the prop up to turn it in that range the cam spec sheet will tell you on cam set i was on a 36 ft cabin boat with two 5.7 it turned to 4500 and that all it run that where the tork fell.

 

[ajkolar]

Re: ?? Prop questions ??

This is a marine engine with a WOT RPM range of 4400-4600 RPM. So 4600 is where I would like to fall at WOT. <br /><br />I know I am going to get try a different prop to try and bring down RPM's. My real dilema is whether to try a 4 blade or a 3 blade.....

 

[Dhadley]

Re: ?? Prop questions ??

I'll fall back on what I said earlier -- try everything you can get your hands on. You'll be amazed at the different traits on "similar" props.

 

[gatorred]

Re: ?? Prop questions ??

ok just don't over turn it on 19 pitch 3 blade's or 4 blade's three sean to run more on top end then 4 blade comprop on 80 hp merk but but 4 blade was easyer on gas now with jack plate 3 blade 21 pitch alum can't be moved up at all it will blow out, but 4 blade stainless steal 25 pitch can be jacked up and run's more top end to<br />goes by use of prop and boat to my 80 hp wouldn't run 21 pitch at all to much prop,19 pitch on my bass boat would part blow out then blow out all way,like you have a ho 302 in bronco with 4x4 not working in middle of worst snow storm or ice storm it didn't work. so a prop,what it made off, use and type of boat is fact not over looked when re proping I have a 19\14.5 pitch for merk inboard, to my omc prop was 19\14.5 same as that what it did was a shock left me scraching my head why