All OMC boats were originall built with LONG-LEG drives. The era of short-legs came into popularity in the early seventies when OMC began rocking the engine (mainly Fords and inline GM's) as a way of providing thrust angle trim. The marketing people at OMC thought this was necessary to compete with MerCrusier and Volvo and named it SelecTrim.
Kudos to Scott for giving this short-leg approach a try.
You need to count prop rotations to engine rotations (slowly by hand) to determine your 120's OVERALL gear ratio. From the sounds of your description (36 mph at how many RPM - 4400-4500 ? ) you are running a 16:20 upper gear set, instead of a 19:20. The former yields a 1.92:1.00 overall and the latter a 1.61:1.00. The stock 155 used a 1.54:1.00.
Since you are already at the taller end of the prop spectrum (you could search for a pitch of 19", 20" or 22" (but they are hard to find) you probably need to consider re-gearing the upper set (most likely from a 165 inline 6 OMC) If you are going to reprop, get an SST vs trying to modify an Aluminum prop. DO NOT PROP DOWN (shallower, shorter less pitch), that will only make matters worse. Read my prop pages for more background info and prop part numbers: http://hhscott.com/evinrude/choosing_a_prop.htm
If it were mine, I'd try to find a 19" SST OMC Part Number 172493. It should drop your RPM's slightly and possibly increase top end by about 2-3 mph.
As far as the blowout in turns, you should add an XP Stingray. These OMC hulls always have a tendenancy to cavitate at WOT. Add trim tabs if you want to control that or simply slow down or find some rougher water to breakup the porpoising rythm. I'm not sure why you think the thrust angle has changed, but a 68 Sportsman should have all floor mounts which you could adjust the front engine mounts to compensate the thrust angle.
Be happy! You should have excellent hole-shots and skier-popping abilities. It should be jumping out of the hole. Sounds like the best cure is just slow down and always take more friends (weight) on the boat!
Lee
----- Original Message -----
From: Scott Veazie
To: omc-boats@...
Sent: Monday, June 22, 2009 9:30 PM
Subject: [OMC-Boats] 68 Evinrude Sportsman 155 with short shaft OMC stringerdrive
Ok guys, here's the deal. I got my 68 Sportsman from someone on the list, and it runs great. The bad news was that the outdrive's upper gearcase was toast due to no oil. I looked and looked and decided to try a 120hp OMC low profile drive (electric shift) on the boat since it would bolt right up. Now the differences between a low and high profile are basically 4" in length. With the old drive, the cavitation plate was about 2.5" below the keel (too low IMO). The new drive is about 1 3/4" above the keel (which I've read is ok depending on prop, etc).
I took the boat out today for a real test, and while it worked, I did not like it's performance. I decided to run the old beat up and chipped 14x17" prop from the old drive on the 4 cylinder 120hp short drive, and here's what it was like:
1. Rpm's came up too fast
2. Excessive blowout in turns, and some cavitation
3. Too quick to redline
Now, I did get her up on plane at WOT but it was in the redline. Made about 35 mph which is not bad I think. Now I did have my wife take the helm and run her up on power while I hung over the stern and watched the drive, specifically the cavitation (anti-ventilation) plate, and it was in the water fine, not riding too high, if anything too low. I think my drive depth is fine since I'm seeing the plate still in the water, but the angle of propulsion has changed, I'm working with a different gear ratio, and my prop might not be any good. Should I get a shallower or steeper prop? Have the prop cupped?
I would like to make this combo work correctly, it's a nice super clean drive with good shifting and a nice ball gear, and I like the idea of a shorter drive because I can lower it down in my driveway and most the way down trailering, as well as having less draft needed to operate. Additionally, the short drives are easier to come by. Sorry for the story, but any help would be great!
~Scott
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Received on Tuesday, 23 June 2009
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