Problems adding horsepower/torque to sterndrive??

Hello Group,

I'm looking as possibly getting an I/O bow riding boat in the future. I currently have a straight inboard ski boat and an outboard ski boat. The ski boats are great on power and speed, but the comfort isn't quite there and my wife and I are having a little one soon so we are looking for something more "family" oriented....

Anyway, Most of the Sea Rays or Bayliners that I ride in a nice, but kinda slow and doggish. I have pleanty of experence with automotive engines and thought that I might be able to up the power some and change the prop a little to add some power. I'm worried though about the outdrive itself. How much can they handle? The Alpha one drive seems popular on the boats I see. Are some outdrives better than others for power? I would expect that I get a V6 boat or better...

Thanks.

 

Hi TA69, welcome aboard

When you talk about the average Sea Ray or Bayliner bowrider being "slow and doggish", I suspect you're noticing that many such boats can take a while to climb on plane.

This is, of course, quite different from your ski boats, which are designed for rapid acceleration- often at the expense of ride comfort in less-than-ideal weather.

The thing is, most bowriders are fitted with more power than they really need. A 4.3 L V6, pretty much the de facto standard for 18-20' sterndrive boats, produces betweeen 190 and 225 hp, depending on which version you get. Few people ever use all this power- many boats in this class can do 45-50 mph, at which point only the driver is having any fun. Only when you're heavily loaded with gear and crew, or have a medical emergency, do you really use full power in such a boat.

Getting a deep-V bowrider to climb to plane quickly is not a matter of tossing more power at it. Bowriders are designed to carry a lot of weight forward; to do so, they have to be able to easily lift the bow. Unfortunately, when you don't have two or three crew up front, the centre of gravity ends up too far aft- the bow lifts but the stern doesn't, putting the boat in a high-drag situation where the engine can't rev high enough to use its full power. The climb to plane can thus be quite slow. Your ski boat, with its midships-mounted engine, has the CG farther forward and probably tends to lift all at once.

Solutions include larger diameter / shallower pitch props, which can improve acceleration but tend to reduce top-end speed. Trim tabs are also an option. "Whale tail" hydrofoils mounted to the drive are sometimes used to improve time-to-plane, but some people see these as an indication that the hull wasn't designed properly to start with.

In the engine tweaking department, generally speaking, more top-end power is useless in an 18' bowrider. To get any benefit from engine mods, they should be directed at improving torque around the 1000-2500 rpm band. Extra torque at low rpm is just about the only engine improvement that will noticeably improve the performance of a small bowrider in average use, hence why boats with the 4.3 V6 tend to be preferred over their sisterships with the 3.0 I4 even though both tend to cruise at the same speeds and neither owner is ever likely to use full power or full rpm.

The Merc Alpha is a cheap, popular low-end drive with some well known design flaws. It's commonly used with engines up to the 5.0 V8. Personally, I have a strong preference for the slightly more expensive, substantially more reliable Volvo SX for 4- and 6-cylinder engines, even though its spare parts can be rather pricey at times. For engines of more than 250 hp, Mercury fans tend to switch to the Bravo 1 or 3, while Volvo fans go with the Duoprop drives. A few hardcore types will buy an industrial Konrad or Sternpower leg.

 

Thanks for the info mat.

You are correct, I am not worried about top speed at all. My main concern is towing power. You are right in the fact that I precieve the time to get on plane is too long.

My hope was that I could beef up the torque in the engine and possible add a few HP. Then, do as you say and add a larger, lower pitch prop and possibly trim tabs. Thus, I could build up more speed at the motor and get the boat out of the hole better. If I wanted more top speed I would just let the motor wind up higher (if the stern drive can handle it)(I would balance the motor for high revs)

I'm not looking for a rocket ship in a bow rider... I just want a decent good ski machine in a nice riding bow rider format... (but not the 50K new Mastercrafts)

Since the Alpha one is what I see the most, what is it's biggest down falls??

Thanks.

 

I once owned a Glastron Carlson CV-23 powered by a 280 hp V-8 and the Mercruiser drive now known as Alpha-1. In the 2nd year the engine blew up and was replaced by a 320 hp block the import company had in stock.
After 20 hours run in and subsequent oil change I made a full throttle departure that lasted maybe 3 seconds because the vertical shaft in the stern drive snapped.

I guess you now know where the limits are.

(The new engine was a warranty case and to me was 100% free of charge, but the stern leg wasn't.)

 

Well, and then there was the "family oriented" point......................................?

 

For a short while before the bravo 1 drive came out they coupled big block 454 330 hp engines with alpha 1 drives on lightweight boats. The consensus was that you had to take it really easy on the throttles getting on plane which is exactly what you are not liking now, so... There are a lot of boats with 5.7 350s at 250-270 hp running alpha drives (I've had two myself and ran for over 10 years with these) but I'd leave about that amount of room below the 300 hp spec for the drive to stay out of trouble. Relatively speaking, even though they're 30 year old technology, the parts are half the cost of the next better option around here which is why I still run alpha drives.

 

Does anyone know if the Alpha one drives are all the same? ie. Do they use the same parts in the ones that back up the 4 cyl engines as they do the ones that back up the 8 cyl engines?? I looked at Mercurisers web site and it shows a bunch of different engines that can be mated to the Alpha one, but it doesn't show different versions of the drive for different engines...

 

I could be wrong, but I believe other than the main gears in the upper unit that change the gear ratio (1.32, 1.47, 1.62, 1.81, or 1.94) I'm not aware of other differences.

The lower unit is the same.

The transom assembly is the same.

 

As far as I know, Jeff's assessment is correct- the gear ratio is the only thing that is changed when mating the drive to a different engine.
It should be noted that older versions of the Alpha (Gen. I) are slightly different from newer ones (Gen. 2) and many parts are not interchangeable between the old and new drives.

 

In fact it looks as if they changed suppliers!
The gimbal and transom housings are identical, but the leg is different in every detail: water pump, bellows, even the way the lower gear housing is attached is different. Not all of these changes are improvements, some only were made to reduce production cost.

 

The transom assemblies aren't 100% identical - I actually am just about to make the change from generation 1 to generation 2 drives on a boat because one of the 1990 transom assemblies was shot and I couldn't find a source for the gen1 transom assemblies (new) anymore and was putting new outdrives on anyway so I figured it was a good time to go from 30-year old to 19-year old tech

There are two extra studs on the top of the genII transom assembly that go through the transom. The bolts on the bottom of the old went through the outside transom assemblies where caps or acorn zincs were screwed on - the genII has studs on bottom two like the rest. The trim cylinders are different and are parallel on GenII whereas on Gen1 they mount wide on the transom assembly but narrow on the drive itself using the thick rubber bushings to absorb the angle of the cylinders on the straight mounting shafts. The bottom of the bell housing and shift shaft are slightly different. There's a gear lube passage and speedometer water passage in the new transom assembly. The gen1 won't mount up to the gen2 transom assembly or the other way without modification, though I've heard a few people have done it to mount the old alphaSS drives (which are ~ 2" shorter) to genII transom assemblies with a couple or a few hours of labor.

I haven't found a good side-by-side comparison, I presume because when the design change was made in 1991 it was before the Internet was here, and probably more fun to write about high end gear vs. the entry level. Before deciding to make the switch I was curious if there were some quantifiable real-world differences, positive or negative from the small design changes.

 

You are correct Jeff, I forgot about these details. The gaskets are also different between Gen-1 and 2 because of oil/speedo passages. And the cylinders are mounted cheaper, i.e. without nuts and washers. But the changes are minor compared to those of the drive itself.
That looks only identical from a distance.