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Thread: DP Class

  1. #1
    Team Member Smokin' Joe's Avatar
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    Default DP Class

    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.
    Attached Images Attached Images       

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    Wow, you just brought up something I hadn't thought of in years.

    At some point around that time DP fell into a probationary status with APBA over I believe boat count. There were several guys running the class in Florida, but when the (I think) divisionals in Lakeland came up, only two boats showed. Not running the class was going to be a black mark with Detroit over the probation. So we did the "build a boat" ordeal you talked about.

    They scheduled EP and SD first in the lineup, and DP near the end. Danny Critchfield and I ran EP, Tony Patrinostro ran SD, and then we built a DP for me to drive out of Danny's boat and Tony's motor. Since all I was doing was making a start we didn't worry about pulling weight from the E boat, the prop was whatever was closest to the door in Jeff Titus's van, and the thrust-block was set wherever Danny had it in the morning. Pushed it off the trailer after the five minute gun had already fired and the battery was too low to crank it - a bunch of guys jumped in and picked me up to fire it stock-outboard style.

    I ran across the course and down the back, and caught the other two headed for the clock. Boat actually felt good. Went into the first turn and drove around them. (!) No one more surprised than me! Started backing off and making it look good, and on the last lap drove wide into the weeds and let them both past and took third . . . . .

    Reason: The last thing Jeff yelled into my helmet was "whatever you do, don't win! - the midsection is drilled for the power trim on the SD boat, and if its inspected you'll be thrown out". Bummer. Would have been a much better story if I could have won! I think this was 1979.

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    That was a good read, thanks Smokin' Joe for the history lesson.
    I've always had a special appreciation for small boats with moderate power. Have a round-tuit Delta waiting for good weather and attention.
    Question about the Delta in the 3rd pic... Is that pad widened and/or notched into the Vee in the last 18" or so of running surface?

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    It was nice to see this thread. We run the UIM [T750] version of this class in Canada. The class is growing here and most racers are running Allisons, Deltas and Critchfields. More set back, improved props and a wider selection of motors have made this a very cost effective class for people entering the sport.

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    Team Member Smokin' Joe's Avatar
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    The 7" pad of the 14' Allison is optimal. I once widened the pad of a new 13' to 8", the pad was too thick,
    transom ran deeper, drug water. On the next 13' I widened it to 7" and that was my fastest boat of all.

    Quote Originally Posted by Tom M View Post
    That was a good read, thanks Smokin' Joe for the history lesson.
    I've always had a special appreciation for small boats with moderate power. Have a round-tuit Delta waiting for good weather and attention.
    Question about the Delta in the 3rd pic... Is that pad widened and/or notched into the Vee in the last 18" or so of running surface?

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    Quote Originally Posted by Hounddog View Post
    It was nice to see this thread. We run the UIM [T750] version of this class in Canada. The class is growing here and most racers are running Allisons, Deltas and Critchfields. More set back, improved props and a wider selection of motors have made this a very cost effective class for people entering the sport.
    What's the name of the UIM Class, which motors run besides OMC, and what are the rules, please? I can build props for that motor. Same casting as SST60
    that I'm now building. I recently sent a new 'EP' prop to Canada.

    My experience is that you do not want a lot of set-back for closed course. I ran a 2x4 offset on the Allisons, two 2x4s was too much, handling suffered. The Critchfields that were
    fast in NOA (Rusty Campbell) an no set-back but did run props that I built for them.

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    Here's a link to the 2013 Rules, see PDF shown in the first post.
    Some amendments have been discussed also noted later in the thread.
    http://hpbc.ca/viewtopic.php?f=24&t=1007
    Hounddog may have more to say on the subject

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    As per the previous e-mail. The UIM class is T750. The Canadian Rules are a little different. We allow a much wider variety of motors.
    In short here is the basics.
    - Any 2 cylinder two stroke.... So this group includes OMC, Mariner, Nissan/Tohatsu, Suzuki
    - 4 stroke motors up to 60 hp..includes the new race Mercury
    -ETEC motors up to 60 hp
    - 3 cylinder 43 cubic inch Yamaha, Tohatsu/Nissan
    - 3 cylinder 49 cu. in. twin carb Mercury

    So far the quickest boat in the class has been a 60 hp 2 cylinder OMC, on a Delta, with 6 inches of set back. Most run approximately 4 inches of set back. Our class weight is 600 lbs. It is a 55 to 60 mph class on the oval.

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