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View Full Version : What length shorty on an 18



tamburello
03-21-2009, 04:05 PM
Do the 18s run best with a 2" or a 3" Imco shorty?

- Jeff

BUIZILLA
03-21-2009, 04:09 PM
are you converting from an Alpha?

Mr X
03-21-2009, 04:27 PM
The -3 " shorty will only work with the complete Imco drive. It will not work with the Merc upper, as the prop will hit it.

tamburello
03-21-2009, 04:32 PM
Yeah, I'll be converting from an alpha. on a 94 classic.

- Jeff

BlownCrewCab
03-21-2009, 06:01 PM
A 3" high X dim is too high. I did mine with a 3" X and ended up with a 1" spacer. If your going for all out top speed and don't mind cavitating in the turns then 3" is for you, if you want to be able to do slow speed stuff then 2" is as high as you can go.

osur866
03-21-2009, 07:58 PM
I'm running a Imco -2 on an 18 and would also say 2" shorter is as far up as I'd like to go, even with the -2 and a 3 blade prop you will still cavitate in a tight turn FYI, but all in all I gained about 3 mph with the -2 and the handeling at higher speeds improved also, be prepared to go up at least 1 prop size maybe a little more. Your midrange slip #'s will suffer a bit as well. Steve

tamburello
03-21-2009, 08:48 PM
Thanks for the responses. That's surprising that the 3 in. shorty doesn't work well considering that the Alpha SS works so well on the 18. If I'm not mistaken the SS is three inches shorter than stock.

- Jeff

Choprpilot
03-21-2009, 10:42 PM
I believe the SS drive is 2.5"s shorter..

Lenny
03-21-2009, 11:33 PM
SS is 2 3/4" shorter. I have one on my LE. (jim's ;) ) Anyways, I would go two inches so that you can still generate some bow lift with a good prop. :yes:

BUIZILLA
03-22-2009, 08:20 AM
2" is the ticket, any shorter and passengers ride on the engine hatch... :lifeprese:

BigGrizzly
03-22-2009, 08:43 AM
I would go with the 2 inch on that boat, like B cab said if speed is what you want go for the 3, like ted said with the merc bravo you can only turn a max of 15 1/8 diameter( on some props less on others) prop and the cav plate bolts needs to be recessed and it is still too close to call. Been here.

Mr X
03-22-2009, 09:54 AM
When I installed my 3" shorty on the Merc XR upper, there was only 7 inches clearance. So even my 14" Mirage + was hitting the upper and would not clear.

BigGrizzly
03-22-2009, 05:24 PM
Ted is the XR upper different then the std bravo? The reason is this is a 3 inch shorty. and will only take 15 inch props, bet they are real close.

osur866
03-22-2009, 07:06 PM
Grizz, you sure there isn't a spacer in there? Sure looks like there's a spacer in between the lower and that upper. When I ordered mine I was told a -3 would only work with the Imco upper because the prop would hit the cav. plate. Imco's upper coupled to their -3 has a stepped up cav. plate in order to acompany the prop diam. Just what Imco told me. Steve

BigGrizzly
03-22-2009, 07:10 PM
Yes there is a spacer but it is above the cavitation plate, so it doesn't matter. Notice the aluminum splash plate I recessed the bolts and did allen heads

Mr X
03-22-2009, 07:52 PM
Griz, the XR is the same as the Bravo.
The spacer would make a dfference as to the prop hitting on a -3. Are you sure the picture you posted is a -3 Griz....
it looks like a -2 to me.

BigGrizzly
03-23-2009, 08:17 AM
The reason I was wondering is we used the Bravo1 then the Hydro befor we put the spacer on. the boat was 2 mph faster above 120, but took forever to plane and was ill handling forget about cavitation it was always there in some form. Prop height is in my opinion is still too high but it runs well the way it is. Thanks Ted, just don't know why. Fitting props to this boat because of the small props was a long process

tamburello
03-23-2009, 11:11 AM
I moved to the Los Angeles area at the end of last year and I just looked up where IMCO is located, turns out they are about a four minute drive away.

Here's a pic of the stepped up cav. plate on the 3" shorty IMCO mid section.

It looks like I'll go with a 2 inch shorty when I get the boat moved out here from Michigan and do the Bravo conversion.

Thanks for the input!

- Jeff