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Subject Topic: Transmission ratio Post ReplyPost New Topic
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Joined: October 15 2006
Posts: 165
Posted: June 04 2008 at 08:21 | IP Logged Quote suwanneered

Hi Mates: Back again with another question??? As you know the original tranny that was n the 280 was a 1.52 to 1 series 71C behind a 305 Chevy Mercrusier.
The tranny was toast as it wasn't pickled after going down in a cane in Fla and sat for a long time with sea water in Motor and Tranny which I have replaced both. I put in a new 350   260HP and bought a tranny off e bay and had it rebuilt however it is not the same ratio as the old one. The new one is a 72C series and the ratio is 1.91 to 1 and the tranny is 3/4 inch longer than the old one  and it did fit in and hook up. I am still worrying about it. I had never had the boat in the water as it is on stands in my yard so I could rebuild it.Think it will work OK?As I read the tranny I installed is rated for gas engines over 300 HP.Thanks,Bill
The boat is a CC 280  1984 model
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Nauti Cat

Joined: September 14 2007
Posts: 1049
Posted: June 04 2008 at 10:40 | IP Logged Quote Nauti Cat

Hi Suwanneered,

You may have this info already but check out this link:   http://www.perfprotech.com/store/App_Themes/PPT/images/tech- specs/VD_Inline.pdf

It describes the 71 vs. 72.

The only thing I cannot locate are the implications of changing the ratio.  I would guess that it is similar to a car's differential/rear end.  If that is the case regarding automotive, tire size would come into play (larger or smaller diameter tire would change the final drive ratio) so would the props pitch (which is equivalent to distance traveled) may need to be reviewed.

I wrote to one of the marine tranny shops on line asking them on the CC application what the differences are between the 71 and 72 and the different gear ratios.   I'll post their reply as soon as received.

How close s the prop to the rudder?  I know on mine it is pretty close and 3/4" may not make the two meet, but it would be too close for my comfort as mine are about 1.5" apart.



Nauti Cat in NY
Formally owned by a
1985 Catalina 280

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Joined: January 02 2007
Posts: 845
Posted: June 04 2008 at 12:10 | IP Logged Quote diveryates


Your right on the implications for the propeller pitch. The 1.9 to 1 ration means for every 1.9 turn on the engine, there's one turn on the prop. This means more torque with higher RPMs across the board . Adding pitch to compensate is really the only variable. Props on 280s are pretty close to the hull, so be sure there's no cavitation at the highter RPMs that your setup might experience- loud bubbly/popping sounds at speed usually means cavitation. We had this problem and used a smaller diameter prop so it wasn't as close to the hull. We increased pitch to compensate. The complication is that your using a transmission out of spec, so trial and error to get the right prop setup may be called for. Driveline alignment is critical. If out of the water for some time, wooden boats were allowed to sit in the water so the hull could settle B4 alignment. Fiberglass is more rigid but doing the algnment in the water is always a good idea, especially if the drivetrain is newly installed- I would recommend a pro do this. 

On the singles at least, the rudder is located so the prop can easily be removed. You will note the rudder flange is slightly off center.

The diffenence between the 71&72 seriesis horsepower rating. The 72s being rated for higher Hp. The 71s handle the 350. From my research, the newer 71 models (10-17-006) also a higher pressure transmission fluid pump, resulting in more efficient power flow.

Enough of my gab...hope this helps...



Roy & Laurie, S.F. Bay Area
'83 280, hardtop, single 305
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Joined: October 15 2006
Posts: 165
Posted: June 04 2008 at 22:05 | IP Logged Quote suwanneered

HI Tom: Thanks for the reply and also Diver Yates! I talked with a prop shop and they said according to the computer I needed a 17 inch prop whic will not fit on the CC280. I have a 16 inch prop and mounted it and it clears the bottom about 1 1/4 inches and clears the rudder by 3/4 to 1 inch.I know that is close but O don't want to cut the shaft off until I see if the 72 series Tranny will work or not. It is 3/4 longer than the 71 series the difference is in the rear flange part and it fit the foot print of the old tranny just like the 71 but I had the lower the rear of the engine and tranny to hook up to the shaft If I cut the shaft off before I know  if the 72 will work it will be to short for a 71 series if the tranny I have installed don't work for some reason.
Since I hooked it up it spins in the new cutless bearing fairly easy. So maybe it will work.The tranny deal had been a thorn ever since I started trying to replace it. I will let you know how it works out when I get it in the water in about a month or so.Thanks a lot,Bill
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