Torque/ Alpha 1 problems

[Pessemist]

I am tearing up prop hubs and Alpha drives.

In 2 seasons I twisted the vertical drive shaft of an Alpha 1, I twisted off 3 prop hubs and now I have destroyed something inside the new Alpha unit unit.

The upstart is I have never hit anything. Aluminum prop is pristine.

After consulting with several marine mechanics and several posts on these forums it seems that the torque of my engine is too much for the Alpha stern drive/prop combination.

I need several questions answered:

1) Will a Dolphin Fin on the sterndrive unit make things harder on the drive? Mine came with one that had a drive shower mounted on it. I am thinking of taking it off and tossing it.

2) Does increasing the diameter of a prop (say 1") add significantly to the problem of too much torque? Does it significantly add to stress in the stern drive gearing system?

3) What effect does increasing/decreasing pitch have on stressing the sterndrive?

I know all about how plowing off plane stresses the stern drive. I was driving this boat like it was made out of porcelain glass and it still broke again!

I am tired of getting towed in so unless I can dial this problem in I am selling this thing.

The boat originally called for a 15.25x15" prop with a 1.32 gear set. I had to settle on a 1.47 gearset and have been running a 16x16" aluminum prop. I like the performance. Could probably go to 16x17" as it right now can hit 4600 at WOT when 4500 is the top limit.

I am wondering if a 15x16" prop might take some stress off the sterndrive (torque stress is what I am trying to minimize).

I am sure a 15.25x15 prop (original spec) with the 1.47 gearing would allow too much over-rev of the engine at WOT. (probably hit 4800+)

Any comments would be appreciated.

 

[steelespike]

Re: Torque/ Alpha 1 problems

So what is the motor?Displacement and hp?I don't have a clue but isn't the alpha 1 only rated for just so much power?
Were these items destroyed all new?

 

[theBrownskull]

Re: Torque/ Alpha 1 problems

The alpha is rated for 300 hp max. We need info. like steelespike said.

 

[jestor68]

Re: Torque/ Alpha 1 problems

If the Alpha can't take the power/torque, replace it with a Bravo.

The Alpha should not require a drive shower.

Prop diameter is determined by shaft speed. With a 1.47 ratio, you probably shouldn't be running over 14-14.5 diameter props.

 

[Pessemist]

Re: Torque/ Alpha 1 problems

If the Alpha can't take the power/torque, replace it with a Bravo.

The Alpha should not require a drive shower.

Prop diameter is determined by shaft speed. With a 1.47 ratio, you probably shouldn't be running over 14-14.5 diameter props.

7.4 L Mercruiser.

Original prop spec was 15.25x15 with a 1.32

Thinking of going with a 15x16 with the 1.47 gearing?

was running 16x16 and blew gears. driveshaft and prop hubs

all props aluminum

 

[jestor68]

Re: Torque/ Alpha 1 problems

7.4 L Mercruiser.

I'm not aware of any 7.4L Mercruiser motor that does not exceed the torque limits of the Alpha 1 drive. Whoever mated that engine to that drive made a mistake.

You need a Bravo 1 drive. A few more repairs to your present drive might well pay for a Bravo 1 drive.

Reducing the diameter will take some of the strain off the drive by allowing the engine to accelerate quicker, but the only lasting remedy is to upgrade the drive.

 

[jestor68]

Re: Torque/ Alpha 1 problems

deleted

 

[QC]

Re: Torque/ Alpha 1 problems

Bravos were invented to solve big blocks and Alphas

 

[H20Rat]

Re: Torque/ Alpha 1 problems

7.4 L Mercruiser.

Yeah, don't need to post any more than that... The ONLY fix you can do without upgrading to a bravo is to pull out 2, maybe 3 spark plugs! Otherwise you will continue to destroy random expensive internal parts.

 

[craze1cars]

Re: Torque/ Alpha 1 problems

Changing props from one diameter to another will change nothing with respect to the amount of torque sustained by the drive, assuming the rpms are still in the proper range. The only way to reduce torque forces on a drive is to go to a lower pitch and substantially raise rpms. But then you will be tearing up drives due to excessive RPMs instead of excessive torque.

If a 16" diameter prop gives you 4500 rpm at a given speed, and a 14" diameter prop gives you the same 4500 rpm at the same speed, the torque forces on the drive and the prop hub are identical.

If you want to prevent spinning hubs, get a solid hub. But this won't prevent you from tearing up drives...in fact it might promote more drive damage. Mercruiser put Alphas behind big blocks for about 2 years back in the 80's sometime I think? Presumably yours is one of them. Even Merc acknowledged this was a mistake by ceasing the practice real quickly...surely they got eaten alive by warranty claims and pulled the plug.

If I were you I'd treat prop hubs as disposable in an effort to protect the drive and just carry spares, or I'd sell the boat. Given the age it's likely not worth the expense of a Bravo upgrade.

 

[agallant80]

Re: Torque/ Alpha 1 problems

I'll trade you my 5.0 Merc for your 7.4

 

[Tail_Gunner]

Re: Torque/ Alpha 1 problems

You have a not so good combo...26' boat read heavy and a engine with low hp very high tourqe. About the only thing that can be done is bennet trim tabs loose the whale tail under prop 400 rpm...use a four blade big eared pusher and easy on that throttle. Or find a bravo on a old boat that is junk and switch them...I am living with the exact same scenario......One drive down already and prudently working on the next...

 

[Wind dog]

Re: Torque/ Alpha 1 problems

What is a drive shower?

 

[Pessemist]

Re: Torque/ Alpha 1 problems

You have a not so good combo...26' boat read heavy and a engine with low hp very high tourqe. About the only thing that can be done is bennet trim tabs loose the whale tail under prop 400 rpm...use a four blade big eared pusher and easy on that throttle. Or find a bravo on a old boat that is junk and switch them...I am living with the exact same scenario......One drive down already and prudently working on the next...

The boat has great trim tabs on it. I am already so easy on the throttle that if I were any more easy I'd be sitting at the docks.

The drive or prop hub always seems to blow when I am mushing through the water starting back on plane. On plane she seems to fly.

So if I want to underprop 400rpm I could go to the original spec of 15.25x15? With the new stern drive gearing I'll probably be at around 4900-5000rpm at WOT. (4500rpm is limit for this engine).

Obviously I'll never run at WOT with it this way but I'll be happy to run at all.

What do you guys think?

* For the guy that asked, a drive shower is a system that forces water up tubes and sprays them on the top of the sterndrive. The theory is most failures are the top gears and the shower will help keep their temperature in range. Usually they are mounted on a Dolphin fin which attaches to the sterndrive. But not always. Mine does so I am dumping both.

 

[Pessemist]

Re: Torque/ Alpha 1 problems

...use a four blade big eared pusher and easy on that throttle. ...

If I go with a four blade does that involve any pitch change considerations? I mean if I was going for a standard 3 blade 15.25x15" Would I just step up to a 4-Blade 15.25x15?

My concern is the failures while plowing so I want to get on step as fast as possible without tearing out the hub or drive

 

[Wind dog]

Re: Torque/ Alpha 1 problems

Thanks for answering my question. ;-)
Hope you get her straightened out.

 

[Tail_Gunner]

Re: Torque/ Alpha 1 problems

If I go with a four blade does that involve any pitch change considerations? I mean if I was going for a standard 3 blade 15.25x15" Would I just step up to a 4-Blade 15.25x15?

My concern is the failures while plowing so I want to get on step as fast as possible without tearing out the hub or drive

Maybe a 13...the ideal is to lower the gearing making the prop easier to swing which should lower the stress...the four blade mximizes the thrust and in theroy that should help...However it still takes a certian amount of thrust to get you up and away and it seems that is what is destroying your drives and props...You sure your not hitting something from time to time.

 

[Pessemist]

Re: Torque/ Alpha 1 problems

Maybe a 13...the ideal is to lower the gearing making the prop easier to swing which should lower the stress...the four blade mximizes the thrust and in theroy that should help...However it still takes a certian amount of thrust to get you up and away and it seems that is what is destroying your drives and props...You sure your not hitting something from time to time.

That's what I thought at first but last couple times was in a 30' dredged shipping channel.

Always when boat is plowing towards plane and always with at least 4 people on board. Never happened with just 2 peopple on board. I'm pretty sure it just too much torque.

 

[theBrownskull]

Re: Torque/ Alpha 1 problems

What is a drive shower?

This is a drive shower. Used to cool the lower unit.

 

[500dollar744ti]

Re: Torque/ Alpha 1 problems

I'm not aware of any 7.4L Mercruiser motor that does not exceed the torque limits of the Alpha 1 drive. Whoever mated that engine to that drive made a mistake.

Mercruiser did it, still wasn't a good idea.