Many prop questions wrapped into one

[fvshawn]

Hey guys, need to put some myths to rest, and answer some questions I have about selecting a prop. I have a 1991 Chris Craft Concept 207 , so it is 20' 7" and weighs in just over 3200 lbs dry. The boat has an OMC Cobra 5.0 H.O.Ford Block with 300 original hours on it, somewhere around the 225hp mark I guess. Right now I have an Alumanum Solas four blade 14 1/4 19p. I usually pack around a couple hundred pounds of cargo and fuel,and four or five friends and this gives me about 45 MPH at the suggested max RPM of 5000. Cruising by myself I can easily get 48-49 MPH and well into the 5300 RPM range.I would like a better hole shot, but don't really want any loss on top end speed( I know, pick just one!)My questions are as follows:
I want to change to a five blade, high five with PVS perhaps, with the five blade should I go to say a 21 or 20 p and expect the same numbers as for top speed and WOT RPM?
Has anyone tried the PVS prop, and if so can you really get better hole shot with a taller pitch?
If I find a used prop with the same diameter,the pitch I want, but not sure about the hub fitting my leg, can a prop shop just press out the hub and put in the hub I need or is that a tall tail?There seems to be a few used ones out there that were on mercs that have 15 splines like mine, will they match up?
I removed my aftermarket fins off my leg that the previous owner put on, in an attempt to get over the 50 MPH mark,but have not had a chance to get the boat out yet, am I dreaming of that being a quick fix?
I have done a bunch of looking on this site, and know that stainless runs a bit quicker all around as it makes thinner blades, but is that going to be enough of a gain if I stick with a four blade the same pitch and expect a better hole shot and top speed?
Any well tested advice would be helpful before I purchase as I can find an exchange dealer here,seems to be a foriegn word.

 

[180shabah]

Re: Many prop questions wrapped into one

weighs in just over 3200 lbs dry.

so 35-3600 + people

There is a reason 5000 is the max, if you have to spin 5300+ to get 48, then you need to accept the reality of a 45mph boat.

next - none of your numbers quite jive - you need to verify everything. Tach, gear ratio in drive and GPS speed.

Post back with solid numbers and we'll get you straightened out.

 

[fvshawn]

Re: Many prop questions wrapped into one

Hey again, I had the boat out and got some numbers to work with. I ran these numbers with full load of fuel, most of my gear on board that I usually have, but just me in the boat. I got up to plane in just under five seconds and ran it up to the WOT RPM of 4400, not 5000 as I had said before. Trimmed perfectly, I acheaved 45 MPH at 4400 RPM but still had pleny of throttle left. The prop and pitch numbers are 14 1/4 four blade 19 p and I believe the gear ratio of this 91 5.0 HO OMC Cobra is 21/17 . I really don't think I want to suffer a lower cruising speed to get more hole shot, so I guess my real question is what can I expect from a High Five prop, or either a High Five or four blade with the PVS (Performance Vent System ) . What about dropping to a 18 Pitch if I do have to drop to get better hole shot, is it just going to be a big search to find that odd pitch.

 

[180shabah]

Re: Many prop questions wrapped into one

OK, those numbers sound a little better,

If you are onlyable to get 4400 with a 19" prop and the high end of your rating is 5000. Then the fix is easy, get the same prop you have now, just drop the pitch to 17".

Hole shot will improve, RPM for a given cruise speed will be higher, but with less load on the engine, top speed MIGHT even increase a hair. There is not really anything to gain by proping for the bottom of your WOT range.

 

[Bondo]

Re: Many prop questions wrapped into one

I got up to plane in just under five seconds and ran it up to the WOT RPM of 4400, not 5000 as I had said before. Trimmed perfectly, I acheaved 45 MPH at 4400 RPM but still had pleny of throttle left.

Huh,..??..??..??

WOT is Wide Open Throttle.....

How can you be at WOT,+ still have Throttle left,..??..??..??

Btw,... An OMC hubbed prop will fit a Mercruiser, but a Merc. hubbed prop will Not fit on an OMC...

 

[180shabah]

Re: Many prop questions wrapped into one

but still had pleny of throttle left.

OK - missed that part, scratch everything I said above until we get some clarification.

An OMC hubbed prop will fit a Mercruiser, but a Merc. hubbed prop will Not fit on an OMC...

Never dealt with an OMC. What is the difference? Is the merc shaft longer?

 

[fvshawn]

Re: Many prop questions wrapped into one

I refered to WOT as what the manufacturer recomrnds as the highest RPM, and it recomends no more than 4400 RPM, but at that RPM I have plenty of throttle left. If I hammer down I can quickly get into the 4800 RPM range so I think I really am already underproped so I shouldn't drop a pitch size for a new prop, but what about these PVS props,could that be my answer? If the hubs are different between Mercs and Cobras, can I get it pressed out and putr the corrrect one in for a Cobra, and is it fairly cheap to do this?

 

[Bondo]

Re: Many prop questions wrapped into one

Quote:
An OMC hubbed prop will fit a Mercruiser, but a Merc. hubbed prop will Not fit on an OMC...

Never dealt with an OMC. What is the difference? Is the merc shaft longer?

Ayuh,... The Length of the Splines is different....

The OMC prop on a Merc, the longer splines are over smooth shaft...

The Merc. prop going onto an OMC shaft, the splines run into the smooth shaft, not allowing it to seat properly...

 

[180shabah]

Re: Many prop questions wrapped into one

I refered to WOT as what the manufacturer recomrnds as the highest RPM

That would be the WOT spec and it is a range - in your case 4000-4400

YOUR WOT is still undetermined but according to you is north of 4800, so you are correct, dropping pitch is the wrong direction.

You need to increase pitch, but by how much? At least 2" and maybe 3 or 4.
A vented hub can make up the difference in your hole shot when you add pitch. It basically lets the prop "slip" in the water and come up to speed faster.

FWIW - '91 5.0 is 200php

 

[180shabah]

Re: Many prop questions wrapped into one

Also your gear ratio should be 1.59:1
21:18 upper
14:26 lower

There is no 21:17

 

[180shabah]

Re: Many prop questions wrapped into one

Ayuh,... The Length of the Splines is different....

The OMC prop on a Merc, the longer splines are over smooth shaft...

The Merc. prop going onto an OMC shaft, the splines run into the smooth shaft, not allowing it to seat properly...

Thanks for the clarification.

 

[fvshawn]

Re: Many prop questions wrapped into one

Ok, great food for thought so far, but I am still looking for a few more answers. If I go from a four blade to a five, with no pvs do you generally stick with the same pitch , or does it vary. Next question still open is if I find a used prop from a Merc, will getting it re-hubbed work for my Cobra, or is it just going to add expense that should be avoided? Love this web site for the knowledge base found here!

 

[Bondo]

Re: Many prop questions wrapped into one

If I go from a four blade to a five, with no pvs do you generally stick with the same pitch ,

Yes...

Next question still open is if I find a used prop from a Merc, will getting it re-hubbed work for my Cobra, or is it just going to add expense that should be avoided?

Generally, Yes, rehubbing ain't Cheap...