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View Full Version : 3 or 4 blade OMC prop????



gjohns6896
08-06-2002, 09:52 AM
I am trying to find a replacement for my stock Viper 14.25 x 21" OMC prop. However, all I can find on eBay, or anywhere, are smaller diamter ones or really HUGE pitched one. So, is it a possibility to get a 13.75 FOUR blade prop with a 24" pitch? I am thinking that with a smaller diameter, the bigger pitch would make up for that. Am I wrong? I need to send my prop in to get fixed, but am NOT going propless for two or more weeks.

Basically, are the measurements for the four-blade props the same as the three-blade props, or should I go smaller in one area of the other <pitch/diam> so it will give me more speed and less RPMs?

According to my gauges, I am running 56-58 MPH @ 4000 RPM @ WOT with the 350 OMC King Cobra.

Thanks in advance,
George

MOP
08-06-2002, 10:25 AM
Are you sure your tach is reading correct? I would try to get a few more top RPM's 44-4800. The 23 will only drop your rpm lower. What size is your boat 16-18?

gjohns6896
08-06-2002, 05:12 PM
I have already been through all of this...it is an 18 <1992>.

I KNOW that going UP in pitch will only DECREASE my RPMs, but I don't have any other choice for speed! But, if I go DOWN in diameter, won't that offset some of the increased pitch, and in turn keep my RPMs about the same, +/-? I just want this 4-blade I am looking at to work.

Would it? it is 13.75 x 24 , mine is 14.25 x 21

George

Sagbay32
08-06-2002, 06:17 PM
George that prop will kill your speed demon ideas.

Did you clean out the screen in the carb and replace all filters? How about a compression check while you're at it?

gjohns6896
08-06-2002, 07:24 PM
Mike, why do you think the "smaller" prop with more pitch will slow me down?

I was under the impression that ANY increase in pitch will add more speed. But with such a LARGE increase in pitch, I went down in diameter to compensate for pitch. So, you still think I am barking up the wrong tree? Or am I not even in the right neighborhood!
<not to be a smart-ass> wink
Cheers,
George

MOP
08-06-2002, 07:35 PM
Increasing pitch if you have the power to keep your rpm where it belongs can give you more speed. But you are falling short now and will possibly do damage lugging the engine down further. You may well gain some speed by lessening the pitch or perhaps you have something not right with the engine. Without mods the 18 guys are saying high 50's.

gjohns6896
08-07-2002, 08:59 AM
I JUST got my boat back from the mechanic Monday, and I have driven it twice since then...it runs better, and is more consistent, but not faster. He adjusted the carb, replaced that crappy kinked fuel air hose thingie, checked all the fluids and drive stuff, and everything is in GREAT shape!

The only thing left is the prop. It does have some nicks in all the blades, right in the same spot. That is why I am looking to get a different prop, but that is just the beginning.

I also noticed my Tach is not working 100%. The other day I went to start it, right before I took it out of the water Saturday to take it to the mechanic, and the needle was all the way over at the other side of the tach...past 6000 and buried there. But once I got it started, it calmed itself down and went back to "normal." My mechanic said that these gauges are nothing but starting points, and should not be used to drive your boat, only to give you an idea of what is going on. So, maybe I AM running 4600 RPMs. It sure does seem like my motor is winding out all the way!

Nonetheless, should I just get ANOTHER prop the SAME size as my stock one, <and what brand/model?> or should I just accept what I have and drive my Donzi around the lake at 58 MPH, all day long?!?!?!

Cheers,
George

MLerner
08-07-2002, 12:37 PM
George, for what it's worth, I have a '93 18, Cobra OMC and turning the same prop you have. GPS last weekend was in the mid-50s mph. Tailwind suggested a newer Viper, which he says boosted his speed by more than a few mph. The only difference is that the newer ones appartently are 14.75x21 instead of 14.25x21. Here's the address he forwarded to me: propfixer@gkpropeller.com
http://www.gkpropeller.com
Hope this helps.

gjohns6896
08-07-2002, 07:01 PM
THANKS, MLerner!
I will check it out!

Also, I found a 13.5 x 22 Raker that looks promising! It was only used twice, and I can get it for $200. Only problem is I don't know if going DOWN in diameter is what I want to do. But, going UP in pitch will not "hurt" me in any way. Any other comments on this one?

I think I am staying away from the four-blade jobs. Too much drag, I think.

Cheers,
George

tailwind
08-07-2002, 08:39 PM
Hi George!

It is always good to hear from our "OMC Brothers" here! Although we are sometimes looked down upon, I do think the longevity of our King Cobra drives outway the Merc concerns, and am proud to have the privilege of owning my 1991 OMC 18 Classic 18 2+3!

As a newbie to the DONZI prior to purchase some 10 months ago, I too wished to realize a published speed of 60 to 65 mph. Since purchase, I now have approx. 150 hours on the Donzi and have learned that gps speeds will vary from day to day & month to month by perhaps a factor of 4 to 5 mph easilly, different fuels, temps etc? Additionally, like most autos, aircraft, boats, etc, in order to realize max speed, it does require a small matter of finnese in trimming etc.

I am not trying to add anything here, but honestly am still learning myself! I found that trimming the outdrive up, I was able to achieve almost 8 mph on the gps! (forget the trim tabs here) Trim your nose way high till you slow down etc.

We should however observe our WOT limitations, according to our owners manual, I guess for a reasonable life? Having said this, and other considerations, posts, etc. The 14.25x21 prop was indeed the best prop from the factory during these years. Since then, many advances have been made in both prop construction and also rated HP. (Prop shaft hp)

I recently replaced my Viper 14.25x21 for the new 14.75x21. More cup, swept blades, lost 4 to 500 rpm indicated rpm - from 55 mph @ 4800rpm, to 62.5 mph @ 4200 rpm!
Gotta get my tach checked! Bottom line here: 50-55 mph is way cool with a sbc.

Are you trimming your drive for the extra 5 to 7 mph? If not satisfied, I will send my old OMC prop for you to determine a baseline

Let me know,
Randy

gjohns6896
08-07-2002, 11:09 PM
Randy, I just e-mailed you.

Also, my tach is WAY OFF, so I won't even consider doing ANYTHING according to that thing. Like my mechanic said, it is only to HELP you as a starting point, nothing more. Hell, even my gas gauge is always fluttering back and forth...my needle would have fallen off by now!

:rolleyes: wink eek!

My tach did bounce to 4600 one day...but then bounced back to 4000...and I can't get it to do any mroe than that. Even at 58 MPH...so it MUST be the crappy tach!

Nonetheless, I am still in search for more speed.

Anyone have a USED/new 14.75x21 Viper for sale?

Cheers,
George

gjohns6896
08-09-2002, 02:06 PM
Well, I cannot find what I want, and since I have to have it RIGHT NOW, I just bought one off of eBay. I got the 4-blade 13.75 x 24.

I KNOW the pitch is way huge, but I am sure the diameter will bring it down a bit...to "normal" speed. Anyhow, I want to try out a 4-blade just to see how it works, sicne they are becoming sooooo popular. :rolleyes: I will keep you all posted, since no one else cares about me and my Donzi!
:(
Cheers,
George

tailwind
08-09-2002, 10:28 PM
Hi Madpoodle:

Seems like this will be an everlasting topic! Knowing that our tachs are indeed inaccurate, can we assume an overall indication when making incremental prop changes ? Having observed previous posts here, you do have it together when it comes to dialing these DONZI's in. Should we invest in a dwell tach? Make etc? Accuracy?

Knowing that all boats are perhaps different, please give the registry your opinion on both tach calibration, and also prop selection. Having the multitude of experience, and also props for sale, perhaps you will be the best one to fill us all in!

Best regards,
Randy

RickR
08-10-2002, 09:33 AM
gjohn and Tailwind
If you have Faria gauges check the blue gauge housing in the back for cracks (from the hold down screws being too tight)
If there are cracks, remove the gauge, hold the crack shut while taping it (black tape).
When reinstalling, tighten the screws just enough to hold the gauge in place.

If that doesn't work, Faria has a lifetime replacement policy. :D

BTW The King Cobra has a 1.43/1 gear ratio.

gjohns6896
08-10-2002, 01:21 PM
RickR...my gauges have "DONZI" stamped right on them. I have not looked anywhere else for another name, but they are stock, as far as I can tell. I think only the speedo and tach have the Donzi name, and the others say something else, I will check it out. If they ARE bad, how do I go about getting new ones?

And like I said before...two times my tach DID swing up to 4600 RPMs, but it was only there for about 2 minutes each time. It never did go faster <MPH>, just displayed more RPMs. So, I am guessing my tach is bad.

MADPOODLE...my gas gauge does flutter, but I am certain it does that because of the stupid float they put in the tank...since the fuel is sloshing about in the tank all the time, it is surely that, and not a bad gauge.

Just for educational purposes, let's say I am running at 4600 RPM @ WOT. I move UP from a 21 to a 24 pitch, but down in diameter from 14.25 to 13.75. What will this accomplish? I'll even go you one more...now I will be running a 4- blade prop.

I already purchased the prop, so now it is a waiting game to see if it will actually work, and my SBC will turn the bloody thing. Maybe I will go down to 4000 RPM <displayed correctly>, and maybe not. Speed might even go UP here, too!

Thanks for all the thoughts, Gents!

George

gjohns6896
08-10-2002, 01:29 PM
Sorry, I just thought of one more scenario...

People always compare the pitches of three-blade props. It is my understanding that comparing the 3- or 4- blade pitch to one another will not yeild the same results. Meaning, a 24" pitch in a 3-blade is NOT the same as a 24" pitch in a 4-blade. Meaning the blade pitch is not the same for those two props. The 4-blade one is more "flat," and has less cup to the blades...hence the one-extra blade. Soudns to me like I am better off running a 4-blade as opposed to a 3-blade.

I am confused now...but still thinking this stuff over. A 4-blade HAS GOT TO BE BETTER than a 3-blade...in ANY situation. Less cup to the blade will mean it spins freely in the water, and add one more blade and you get that extra top-end speed, and that low-end grunt they are so well-known for.

Maybe I am all wrong, but you CANNOT compare 3- and 4- blade props to each other unless you keep these theories and numbers in mind.

More confused... :confused: :rolleyes:
George

RickR
08-10-2002, 05:49 PM
gjohn
See if the other matching gauges say Faria?
Contact http://www.faria-instruments.com/ for return info.
Reducing prop diameter should not increase RPMs in the cruising and top end ranges. High performance props slip around 10%.
Reducing 1" of pitch increases RPMs around 200.

gjohns6896
08-10-2002, 10:15 PM
I am trying not to be a smart-a*s here, but I THINK I disagree with you Madpoddle. Not entirely, just about the comparison of 3- and 4- blade props. Sure, the pitch is the same, AGREE!

HOWEVER, it seems to me that turning 4 blades will yeild MORE water turning. That leads to FASTER driving. Sure, the engine will drop a bit, as there is more friction there, but the engine is working just as hard as it was before...if not harder, so it HAS to go AT LEAST as fast as a 3-blade prop turning LESS water.

Is this a moot point? Am I wasting my time trying to explain my thoughts?

I WILL run the 4-blade. And yes, I intend to be very careful and run it at first with just me in the boat. Who knows, it might have enough GRIP to turn the darn boat OVER! That would suck!

Cheers,
George

http://www.donzi.net/photos/props.jpg

RedDog
08-16-2002, 01:54 PM
George - Did you ever test the 4 blade prop - inquiring minds want to know the results?

gjohns6896
08-16-2002, 02:28 PM
I JUST got the prop in from FedEx today, and the weather here is not giving me a break. It just started raining and will NOT quit until Monday. So, I guess I will get ahead on work so I have all the time I need to test the new prop.

Will let you all know how it comes out.

Thanks for the "support."

P.S. GIVE ME MORE STARS! I WANT ALL FIVE!!!!
:p :D wink
George

Sagbay32
08-16-2002, 06:14 PM
Geeez George,
Ya candyass, put on a raincoat and goggles. We need some results!

gjohns6896
08-16-2002, 11:15 PM
At any rate, the prop does NOT fit my shaft. <Story of my life>
I e-mailed the guy I bought it from, but other than that, I don't know WHAT to do.

I am going tomorrow AM to visit my mechanic to see what HE has to say.

This is the problem...there is a HUGE gap <read, about one inch> between the lower unit housing and the prop. I even tried removing the washer between the prop and the unit but that did not help any. The splines on the new prop start further down inside the shaft of the prop and therefor the prop does not slide down as far on the shaft itself...causing a gap between the unit and the prop. And there are no splines left to put on the washer under the nut either. It just seems like the hub on the new prop is about an inch shorter than the old one.

I got the letter about this being an "OMC" prop with the item, so why would it NOT fit my OMC drive?

Confused, but not mad!
:confused: :confused: eek!
George

gjohns6896
08-17-2002, 11:38 AM
I just got back from visiting with my mechanic, and the theory we came up with is that this prop I bought, made by OMC, does not have an OMC-matching hub. He took out a Merc shaft and slid the prop on it, and it fit perfectly...but not with the OMC shaft.

Now I am on to the guy who sold it to me...since it is NOT what I bought. :mad:

George

HyperDonzi
08-17-2002, 12:14 PM
Be careful! your boat might get wet!!! :D

gjohns6896
08-17-2002, 03:54 PM
MP...thanks, but after seeing the peop ON my shaft, and talking with my mechanic, and knowing that this prop is NOT for an I/O, I would simply want my money back or the equivilent of that.

I am now waiting on the seller to respond to my e-mailS!

Hyper...speaking of wet...how is puberty treating you? Too bad your first LOVE is a BOAT!!!

:D wink

George

tailwind
08-18-2002, 09:23 PM
Sorry to hear of you prop misfortune! Let me know if you would like to try my cobbled up OMC Viper 14.25 X 21.

Best regards,
Randy

tailwind
08-18-2002, 09:31 PM
PS , let us know early this week, as we are about to stretch our Bonanza wings- Sedona, Grand Canyon, Napa Valley or wherever, we will be on our way later this week. Will be back in approx. 5 to 15K miles or 2 to 3 weeks! Alaska, here we come!

Have fun!
Randy

gjohns6896
08-18-2002, 09:39 PM
Randy, thanks for the offer, BUT I found another prop for sale. It is a Viper but sized 14.75 x 21, not 14.25 x 21, so I will check that one out. I hear they add something to the top end. More prop spinning the water = more speed.

Have a fun trip! FLYING HIGH....

George

Cuda
08-18-2002, 10:18 PM
Unless I'm missing something also (another distinct possibilty), I have to fully agree with Madpoodle. A prop is a screw that screws through water instead of wood (or fiberglass :) ). Take a coarse thread screw and turn it one revolution. Do the same with a finer thread screw. Which one will go deeper in the wood? The guage of the thread being the same as pitch. It is the same principle no matter what the medium. I also think that with the same pitch and diameter props, the three blade will turn more rpms with the same power due to less drag, or conversly, it will take more power to turn the four blade the same rpm. Just my opinion, I could be wrong. :)