fasteddy
Web Wheeler
Reged: 01/30/01
Posts: 12703
Loc: Flat Creek, GA
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Just couldn't wait any longer.
We put the Cobalt 18DV in the water yesterday for the trial run and engine break-in. We had it running on the hose for about 2 hours already, leak checking and setting up the carb and timing and thermostat opening.
It fired right up, idles ok (little fast), warms up fine and quick.
We took it up to 3000 onto plane and did a little up and down with the throttle to feel it out, then did 10 idle to 4000 runs to seat the rings.
This thing is scary fast. It jumps onto plane with two people on board and 36 gallons of fuel, minus about 200lbs of seats, in under 2sec, easy. The speedo won't register above 30, but phone gps says 4400 rpm is 54mph, and with a 13 3/4" 19pitch prop, it wants to go past 4800rpms and keep climbing. The acceleration out of the hole is so strong you have to hang on tight. We have a 21pitch prop to put on it, and it's gonna need it. I now predict well over 60 top speed.
The engine is a 350ci 84 vintage stock 260hp Mercruiser, with the heads shaved some and a Comp Cams XP256H grind cam and lifters and springs. This cam is ground with 4* of advance built in, 256* duration, and it has a mild to noticeable idle at 750rpms. Compression is about 9.25:1, and it's running on regular. Ambient temps were around 105*, so I expect even better performance in cooler, denser air.
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"If you can't be a good influence, don't worry, you can still be a horrid example."
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FrankR
Web Wheeler
Reged: 02/01/04
Posts: 10224
Loc: Columbia, SC
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You may also want to try a larger diameter prop.
Smaller diameter props let the engine spin up faster, but only because of cavitation and slippage. Sometimes a larger diameter bites better, slows down the rpm and uses less fuel for a given speed. If it works, you'll fly out of the hole.
That's a lot of torque for an 18 footer - don't waste it.
Frank
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'89 G-Raider Supercharged 3.0L, MegaSquirt 2, lockup A/T, 2.5" exhaust, 155k, Cibie H4s/Oscar SCs, Hella Micro DE fogs, Cobra CB, Superwinch hubs, LSD rear/Aussie Locker front, Bilsteins, 165A alt., Lifeline AGM, Rust-Oleum
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fasteddy
Web Wheeler
Reged: 01/30/01
Posts: 12703
Loc: Flat Creek, GA
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The 21 pitch came with the boat. Not a lot of room on an Alpha1 for more diameter. A 4 blade is an option. I'm a little leery of putting a lot more strain on the outdrive. Alpha1's are not all that strong. I wish I could find a Searay Alpha1 that they put behind a 454 - stronger upper gears is what I hear. We will be careful on the launch with the 21pitch - it's actually uncomfortable it pulls so hard with the 19.
I think I can get a 14" prop on there and still have some clearance
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"If you can't be a good influence, don't worry, you can still be a horrid example."
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FrankR
Web Wheeler
Reged: 02/01/04
Posts: 10224
Loc: Columbia, SC
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I once had a 20' Bertram that was a very heavy boat after I rebuilt it with a full enclosure and large fuel capacity - probably 4,500 lbs of very deep-V. It originally came with a 250cid L-6 Chevrolet engine coupled to an old Mercruiser drive. I replaced the engine with a long stroke 292cid L-6 (truck and bus engine) and modified the mounting and made a custom scatter shield and exhaust to fit it to a Gen1 Alpha I. I have forgotten the diameter prop I used, but after getting too much slippage with a 13.75" I replaced it with at least a 14" diameter (I think it was a 16" diameter) and it made a world of difference in the ability to use the huge amount of torque in that engine. Top end was slightly sacrificed, but the boat would cruise offshore in a heavy sea almost effortlessly at whatever rpm I wanted and with zero bog in following seas. The Alpha I is tougher than you might think. I had 1.65 gears in that one and always wanted to change it to 1.50 to see if it would run faster with even more prop pitch and still have the same guts - I think it would have been perfect because I never thought I was using all of the torque, but I never got around to making the gear change. You might want to look into your gear ratio, too.
Frank
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fasteddy
Web Wheeler
Reged: 01/30/01
Posts: 12703
Loc: Flat Creek, GA
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I've got a 1.47 in it now. Of course, it only weights 2800 dry.
One of the first hod rods I ever saw had a Jimmy 292, with 3 Stromberg 2bbls on a log manifold. High boy 32 Ford Vicky. It was a monster. It firmly addicted me to acceleration at the age of 3, in 1952. It used to beat flat head fords like whipping a stepchild. The owner was inspired by a book by a writer named named William Campbell Gault to paint the thing conch shell pearl pink with copper plated wheels and trim. Not the first lick of chrome on it.
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"If you can't be a good influence, don't worry, you can still be a horrid example."
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FrankR
Web Wheeler
Reged: 02/01/04
Posts: 10224
Loc: Columbia, SC
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292 was a torque monster. A lot of dirt racers still use them, I think.
At only 2800 dry weight, you may have enough blade area and not need the larger diameter. Figure your slippage using theoretical/actual top speed. Some say 15% slippage is desirable, but I always liked it below 10% if I could get it. It helps to do your prop trials fully loaded - or at least half-loaded. The speed you can make with a stripped boat is almost never real world and can lead to serious inefficiencies when loaded.
Frank
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'89 G-Raider Supercharged 3.0L, MegaSquirt 2, lockup A/T, 2.5" exhaust, 155k, Cibie H4s/Oscar SCs, Hella Micro DE fogs, Cobra CB, Superwinch hubs, LSD rear/Aussie Locker front, Bilsteins, 165A alt., Lifeline AGM, Rust-Oleum
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fasteddy
Web Wheeler
Reged: 01/30/01
Posts: 12703
Loc: Flat Creek, GA
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Some light research leads me to believe that 14 5/8" is the max diameter prop that will fit. Lake Lanier is a little sticky/loggy, and if I go max diameter on the prop, I won't be able to spit out as much if the prop gets into floating debris. I'm going to stick with the 21 pitch we have unless we over rev with that like we do with the 19.
I'm going to do some temp tests on the outdrive housing when we get some running time on the boat. I have one of those nifty I/R temp guns to do so. If I see alarming upper gearcase temps, I'll rig a transom shower for it.
On our test run, we had a guy in a Cobalt 360 cruiser pull up and ask us if we had a 454 in it. He was running twin 8.1's. That is one helluva nice boat - he bought his used for a buck and a half, and ships it between Lake Lanier and Mexico Beach, FL, twice a year. That's about $6k one way shipping cost, plus shrink wrap and wrap stripping fees, plus removal and reinstall of the radar arch each time. Mu$t be nice...
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"If you can't be a good influence, don't worry, you can still be a horrid example."
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