DP Class

[Smokin' Joe]

This class was long dominated by the Merc 500, the first production outboard with tuned exhaust (the Mark 20H 'conversion'
was designed in 1958 by Edgar Rose and produced and run then, but this was an out and out racing motor). The Merc 500 first came out in
1961 parallel to the first fiberglass Allison, a flat bottom with rounded chines. The 13' Allison ran about 50 mph with the Merc 500, rolled
up on its side in the turns, and was a dream of a rig. By 1975 DP boats were typically 13' Allison or Delta pad-Vs. The Delta was an early attempt to copy the Allison bottom but the chine deadrise angle was too flat and the Delta misbehaved in the turns. The Critchfield was later molded off the Delta. The 1976-79 45 ci OMC 55 competed in DP and SD. The OMC was at a disadvantage due to the 12:29 gearing. Carlton Callahan still has his 1978 55. The 1980-83 OMC 60 hp shortshaft had the same low gearing but managed to compete. The gearcase housing and propshaft are identical with that of the 3 cyl. 75, the E motor. Carlton set two records with the 60 which are shown below, as is a photo of his Delta. The stock 1980-83 60 is the original un-blueprinted SST45 powerhead.

The last photo shows a pieced-together D motor on Hans' 14' Allison, a foot longer than a proper 'D' boat.. The exhaust housing is from a 1978 55, the gearcase is from a later model 75. The powerhead is our spare SST45 powerhead, probably 2 mph faster than stock. Now for the interesting part. The 60 exhaust housing is very different from the 75 one. In the latter the gearcase is part of the tuning, it forms a convergent pipe that perhaps acts as the convergent part of an expansion chamber. The same gearcase on the 55-60 does not, for the following reason: there is a flat plate restriction with small opening at the lower end of the 55-60 housing that acts as a closed pipe megaphone. I have cut out that restriction so that the flow pattern in my 60 exhaust housing
should be pretty close to that in the 75. Because I have not calculated the right pipe lengths for tuning the 60, the experiment is uncontrolled. I'll report later on the outcome.

The last two photos (Dec. 2013) show Carlton in front of Callahan Marine in Lufkin (note the McCulloch Scott sign!) and with Darrell Beaulier in Carlton's extremely well-
equipped prop shop. Darrell and I also had a very good time talking with Sue Anne, who was behind the parts counter, and Corky, who was sitting on his stool.

History note added: at the 1980 Waco kilos I set the EP record at 70.56 mph with an XR-14 Allison. Jay Cox set a record in GP with a Merc 850 XS on the sister boat. We had time on our hands
afterward and decided to try to compete with Carlton for the DP record. Lloyd Ezell let us borrow his Merc 500 and prop, we rigged it on Jay's boat. Both our XR-14s
had about an additional 14" 'set-back' that day so we figured we had a chance even with 10 hp less. However, we couldn't jack up more than split bullet due to the
water pickup (that's what limited my E record). Jay, who was a 'big boy' was too greedy and insisted on driving. Lloyd Ezell was a 'little boy' and would have been faster. We came close to Carlton but it wasn't enough. Afterward, Corky really paid us a nice compliment: He walked up to the rig, looked at it, and said something like 'You boys borrow a motor, borrow a prop, and nearly set a record, never seen anything like it'. I'm glad Carlton got the record, in retrospect.