PDA

View Full Version : prop dia. question



vanstee
03-22-2005, 06:12 PM
When I had the outdrive of my ragazza redone they put one from a 6-8 cyl on which has a different diameter hub. By using the bigger hub, I had to go from a 13-15 to a 15.25-15. When I tried it last year it dropped my wot to around 3000 and top end WAY down. My question is what do you normally see by changing the diameter? I'm familiar with changing the pitch (1" pitch = 800rpm approx.) but not a diameter calc.
Thanks!

gcarter
03-22-2005, 06:22 PM
Could it be the ratio also changed?

vanstee
03-22-2005, 06:30 PM
Could it be the ratio also changed?
The mechanic said the ratio is changed in the top end and I only had the bottom end redone.
I'm clueless and he's frusterated as he has had it more than me. Can't find anyone in N.C. Ohio to work on OMC.

MOP
03-22-2005, 07:10 PM
What pitch are you running now and what other changes were possibly made? Lets clear that up first, sounds like the ratio is different in the new drive. This is not a bad thing just have to find the correct prop.

Phil

vanstee
03-22-2005, 07:15 PM
What pitch are you running now and what other changes were possibly made? Lets clear that up first, sounds like the ratio is different in the new drive. This is not a bad thing just have to find the correct prop.

Phil
Ok here goes.
They replaced the lower unit on mine with a refurb OMC lower unit from a 6/8 cyl setup. Had this done at Kunkel's marine in Ontario, Ohio (by mansfield). I'm not sure of the specifics but he says that the gear ratios in the lower end are identical to the 4 cyl setup and the only ratio that can be changed is in the upper part of the outdrive and that wasn't messed with. Had to have it rebuilt when the magnet came loose in the lower unit and decided to have my gears for supper.

MOP
03-22-2005, 07:38 PM
Your guy don't know these drives.
OK from the book!
V8 lower set 17:21 upper set 24:29
4.3 V6 lower set 14:26 upper set 21:19

As you can see OMC changed ratios in both the upper and lower. Back to what prop is on it now, it seems to be way over geared for that particular prop. But don't get up set unless you can't find a low enough pitched prop to spin to the right RPM. Actually running higher gears makes many boats feel and handle better as the lower pitched props do not walk to one side nearly as bad as high pitched props. Most boats can reduce or eliminate chine walking and rolling up on their chine in turns by upping the gearing and decreasing the pitch, a high pitch prop tends to paddle wheel a lot more one of the causes of chine walk.

Phil

vanstee
03-22-2005, 09:37 PM
Your guy don't know these drives.
OK from the book!
V8 lower set 17:21 upper set 24:29
4.3 V6 lower set 14:26 upper set 21:19

As you can see OMC changed ratios in both the upper and lower. Back to what prop is on it now, it seems to be way over geared for that particular prop. But don't get up set unless you can't find a low enough pitched prop to spin to the right RPM. Actually running higher gears makes many boats feel and handle better as the lower pitched props do not walk to one side nearly as bad as high pitched props. Most boats can reduce or eliminate chine walking and rolling up on their chine in turns by upping the gearing and decreasing the pitch, a high pitch prop tends to paddle wheel a lot more one of the causes of chine walk.

Phil
What does it say for the 4 cyl?

vanstee
03-22-2005, 09:57 PM
What does it say for the 4 cyl?
Also, the prop they put on is a merc prop number 78116A45 and is 15.25 x 15 and using a Flo-TorqII model number 834899. They want me to bring the prop in and have the diameter shaved to a 13 or so. This a good idea?

MOP
03-23-2005, 09:19 AM
Also, the prop they put on is a merc prop number 78116A45 and is 15.25 x 15 and using a Flo-TorqII model number 834899. They want me to bring the prop in and have the diameter shaved to a 13 or so. This a good idea?

Final ratios for the 2.5 and 3.0 4's were both 1.71 they changed the 3.0 to 1.86 in 1989 to help them carry a bit more load. Try this if the boat is out of the water, disconnect the coil wire. Make a chalk mark on the engine balancer pulley and on the side of the prop, put in forward tap the engine over slowly until it has made one revolution, though not precise it will give you a general idea of the ratio. The correct ratio will turn the prop about ½ turn per engine revolution, I have hunch yours will turn + or - 3/4 of a turn. Back to the 15 being cut, being as they SCREWED UP they are trying to get it to run by chopping the prop down that sucks. They are trying to induce as much slip as possible to correct the problem they caused by their ignorance. The drive should be changed out to one that will not destroy the engine, 4 bangers do not last long if not allowed to spin up. My feeling is get rid of that drive and pickup a 4 cyl drive, I truly doubt that you will get near acceptable results cutting the prop diameter down. I sure hope you have not been paying them to fix their screw up!

Phil

vanstee
03-23-2005, 10:04 AM
Is there a way that you can either scan or fax me a copy of the ratios so I can say here, this is why it's wrong?

MOP
03-23-2005, 01:28 PM
Is there a way that you can either scan or fax me a copy of the ratios so I can say here, this is why it's wrong?

I will try to hook up my flat bed scanner later thats gathering dust.

MOP
03-23-2005, 07:14 PM
Hope this comes out nice and clear.

Phil

vanstee
03-23-2005, 10:21 PM
Hope this comes out nice and clear.

Phil
Yes it did. I'll be paying them a visit tommorrow. I'll advise on the results. and, Yes, I have been paying them all along.

vanstee
03-26-2005, 04:02 PM
Yes it did. I'll be paying them a visit tommorrow. I'll advise on the results. and, Yes, I have been paying them all along.

He actually tried arguing with me and telling me that they still are the same when it's clear they are not. the lower unit is 13:xx instead of 14:xx. Would that make much difference? He said so what do you want me to do and I said fix it right. He said he already lost his butt on it so I said if he would have done it right in the first place we wouldn't be in this spot.
Still arguing.. will advise.

MOP
03-26-2005, 05:16 PM
It is simple it is over geared, he has the wrong gear set in there and should change it. You should be able to run the same/correct for the baot prop you did before the so called fix.

Phil