prop size for swapped outdrive

[the_cod_father]

Hey guys, new to the forums,
i got a new outdrive from a friend as my current one is beyond roached, the problem is the gear ratios are different, what im wondering is can i use the new outdrive with a different prop and if so what size prop would i need?

my boat is a 1988 citation 2100cc cuddy cabin
its got the 3.7L 165 hp motor in it (everything upgraded)
the current gear ratio on it is 1.84
the length is 20 feet
the net weight is 2600 lbs

the gear ratio on the new out drive is 1.5 and came off of a mercruiser 350 V8

any and all advice would be greatly appreciated as this boat has become a "project"

 

[alldodge]

Don't what pitch prop was, but figuring you will need to reduce 4 inches of pitch

21P with 1.84 = 17P with 1.5

 

[QBhoy]

Hi there...I wouldn’t bet my salary on it but I think you may just be out with the realms of easily finding a suitably low enough pitch prop that’s off the shelf suitable for the outdrive you have, assuming it’s an early alpha or it’s predecessor.
My reasoning is that I have an 18ft boat that weighs in at an above average 3000lbs dry, with twice the HP that you have...with a 1.47 ratio alpha...and it runs a 21” prop sensibly...a 23” badly apart from top end and a 19” if I want hole shot for water sports. That might suggest to me that you would likely need a pitch down as low as 13” or so at a rough guess...may be wrong but I’m not sure that’s a common thing when it comes to props easily acquired for such a fitting. May be wrong though.

 

[the_cod_father]

ill give 13 a shot, my original goal was to steal the gears out of the upper of my old outdrive, clean them up and throw them in the new upper as the new outdrive is in much nicer shape then the old one. ill upload pics of both outdrives tomorrow to give you an idea

 

[alldodge]

Gear sets may or may not not be compatible with a different case. In any case shims will need to be measured prior to install

 

[Bondo]

ill give 13 a shot, my original goal was to steal the gears out of the upper of my old outdrive, clean them up and throw them in the new upper as the new outdrive is in much nicer shape then the old one. ill upload pics of both outdrives tomorrow to give you an idea

Ayuh,...... Welcome Aboard,........ Yer gonna need to regear the drive,......

15" is the smallest pitch available that fits our drives,.....

 

[matt167]

Stick the smallest prop you can find for it and see if it will do. You can get more power out of the 3.7L should you need it

The 3.7L was a lightweight powerhouse for what it was. It's a shame that Mercruiser canceled the production rather than perfecting the engine..

 

[QBhoy]

Ayuh,...... Welcome Aboard,........ Yer gonna need to regear the drive,......

15" is the smallest pitch available that fits our drives,.....

Exactly Bondo...this was my very point. Agree.

 

[QBhoy]

Stick the smallest prop you can find for it and see if it will do. You can get more power out of the 3.7L should you need it

The 3.7L was a lightweight powerhouse for what it was. It's a shame that Mercruiser canceled the production rather than perfecting the engine..

Incredible power from them...when they managed to stay running.

 

[matt167]

Incredible power from them...when they managed to stay running.

yup, the later engines in '88-'89 were almost there, but the 4.3L filled the gap without all the R&D so it was bye bye 3.7L

My dad had a "488" in an Imperial VC230 and it was a great engine, until the head gasket blew

 

[QBhoy]

yup, the later engines in '88-'89 were almost there, but the 4.3L filled the gap without all the R&D so it was bye bye 3.7L

My dad had a "488" in an Imperial VC230 and it was a great engine, until the head gasket blew

A friend of mine had a 88 23ft daragh cabin cruiser with one in it. Endless problems, countless days out ruined, tows home (ruining my day too), 2 engines and a rebuild of the second one later...he eventually gave in and put a 3.0 tks in it. Never a bother since.

His suffered water pump shaft seal failure, overheating and head warping with the different metals along with the usual other issues. Spent a fortune on both engines and even put an alternator on one of them. Hell of a thing...but when it did run well...so powerful and pushed that hull along at 35 mph without bother.

 

[Faztbullet]

With a lil balancing,cam regrind,head work,sheet metal intake,replumb and 1050 Holley they will make a lil over 500HP on alky.. Go a buddy that runs one in unlimited dirt...smokes the V-8's....

 

[WesNewell]

1.5/1.84=0.815217391 times current prop pitch = pitch needed. IOW if you had a 19P, 15.489130435.