The Alpha Dilemma......

[Riley]

I had a long talk this evening with a marine mechanic friend of mine regarding the Alpha...and now I have more questions than when I started! First of all, common thinking has the Alpha designed for a maximum of 300 h.p., and that the outdrive can survive 330-350 hp with a drive shower, frequent lube changes, and a cautious hand on the throttle. Here is my question: Is it high torque or high rpm that kills the Alpha?
Just about anybody can build a 350 that will put out between 330 and 350 hp. Heck, the GM crate motors are in this range. So would an Alpha survive longer with a 350 hp engine spinning a large prop at a lower rpm, or a small prop at higher rpm? For the sake of discussion, let's assume the Alpha in question is a newer unit running the 1.47 gears.

I look forward to any input you all may have!

[Formula Jr]

Is it high torque or high rpm that kills the Alpha?

Marine mechanics tell me it is torque. There's no way "around" torque. Running 350hp, is running 350hp. It requries the same torque to push an X lbs, V boat to 60+ regardless how you get there. Horse power is really a silly kinda number that applys to outdated mining practices. What actually heats up in a stressed drive? Is it friction, or is it the slight bending and twisting of shafts? There isn't much friction since the metal parts never really touch eachother. Thats what the gear lube is for. Now take a metal coat hanger and start bending it in one place. That spot at the bend is getting pretty hot isn't it. Soon it will be too hot to touch and it will snap. If you put the whole shebang under water and twist the coat hanger, it will also snap, even though you are cooling it. It takes the same number of bends to snap the hanger if you do it slowly or if you do it fast. Now try the same experiment with a piece of 3/8ths rebar...... Not much bending or heat getting generated there, ah? Torque and torsion. Thats what its all about.
The Alpha is a good drive designed for the most common power packages placed in 16 to 20 foot boats. 350 hp is just out side of its design parameters. Its amazing any of these drives work as well as they do given all the 90 degree turns.


Who would race with a co-pilot named "Chubby"?
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[This message has been edited by Formula Jr (edited 06-19-2001).]

[RickSE]

You mentioned big props. It's my understanding that two things happen in regards to running big props.

1) The Good News. Larger pitch props are more efficient. A lot of people change their drive ratio in order to run a larger pitch prop and take advantage of the increased efficiency.

2) The Bad News. Going to far on the ratio can hurt the drive, ex. the 1.86? upper gear set. Although this ratio lowers RPM's in the lower unit it greatly increases thrust load on the prop shaft. Moral of the story, running large pitches will decrease the lower unit speed but will increase the thrust load on the drive.

[Greg]

You guys are starting to spook me. I have an Alpha SS that I was going to hook up to Nate's old 388. Now, after reading the posts over the last few days, I am starting to question the sanity of doing this.

[PaulO]

One other problem with the big slow prop versus small fast prop. Props generate force in all sorts of directions besides straight back. The torque steer you feel with unassisted power steering is the result of some of that. Bigger props do more of those "bad things" like torque steer and prop walk, and leaning to one side than small diameter props do. On boats like ours where you don't have a large mass to dampen it (big hull) and you don't have twin props to counteract each other, it is worse. I just had a propshop "make" a custom prop for my 20 cigarette. He determined that stock offerings were too big on diameter and too small on pitch. He cut down the diameter of a 25 pitch mirage prop and I got the best of both worlds, minimal torque steer, minimal list to starboard (LH drive), and minimal porpoising but great speed.
PaulO

[RickSE]

PaulO,
Who did your prop work? I'm in a similar situation and feel I may need to take some of the diameter out of my 21P Mirage Plus. I'd prefer to send the prop to someone back East who knows what their doing.

[Murphy]

We all agree the Alpha is not a true performance drive. But I've been looking into converting and it is not an easy decision. The consensus of the guys at several Merc sites I've contacted is that your parts bill for drive, transom mount, gimble, shifter and steering hardware will be about $7,000. If you can;t install it yourself, look at another couple grand. I'd love to do it, but for that kind of money I can have the Alpha rebuilt every few years for the next 15-20 years. Handling, maintenance, and prop selection are major factors in any situation. The general consensus again is that horsepower, no matter how you calculate it is a common way of deciding. 350 is the point where you have the power to do some damage. As far as the rpm to prop size is concerned my advice is put the engine first. Find out the mfg. WOT rating. Change your prop pitch until you get it and you'll be in the zone.

Good luck,

Murph

[Varyak]

Murph,
The cost of regular rebuilds may be less for a while, but what is the cost of discovering in the middle of nowhere that you (oops) exceeded the outdrive's capacity? This can particularly happen when you are around other Donzi boats/owners and the testosterone flow exceeds the gear lube drive flow and cooling.
Mark

[BigGrizzly]

PaulO and Ricks, YUou could have saved your self alot of trouble. Turbo Props are a 1/4 smaller in diameter thinner blades and nore effecient than the Marage plus. I just got done with a marage plus and a Turbo of the same pitch. The turbo was 1.8 mph faster then the Marage Plus on the GPS testing both within a hours time. The turbo had less torque list then the Marage plus. I tested them on my Criterion. A Turbo also performed better on my son's boat than the BravoI did.
Back to the First question: It is the torque load that takes the alpha apart. In order to produce HP you have to increse torque. Some high performance boats change gear ratios to avoide turning to steep a blade angle, Think about, When does a prop stop being a prop and becomes a paddle wheel? Too steep a blade angle and acceleration decreases and loads taken up by the outdrive increases dramatically. Plus Prop exhaust increases heat of the outdrve, even with silent choice.

Randy