Help Please with Carbed Merc 350/Alpha One Gen II Gear Ratio

[rotaryenginepete]

Hello all, I am new to this forum.
I've taken on a HUGE project for a first time boater (I've been around boats but never owned one), but I am trying to do everything right the first time. I'm from northwest Florida, on the coast. My project is a complete restoration/overhaul of a 26' 1974 Reinell Cabin Cruiser. The original set up was a carbuerated 350 mercruiser with an alpha one/MR outdrive. When I picked up the boat (for free, the guy just wanted it out of his yard) it had been previously swamped, and the engine/outdrive rusted beyond belief. Anyways, I will get into the rebuild itself in the Boat Restoration Forum.

My issue right now is I am trying to figure out the best drive ratio and prop to start with when I do get this thing in the water. I acquired a 1981 350 that came out of a Firebird. I have completely torn down the engine and am rebuilding it to a Mercruiser spec engine, with all merc parts, gaskets, seals, etc. I expect the HP output will be in the 275-300hp range, with the torque close behind. I am looking to order a complete new gimbal/outdrive for the engine. I decided I am upgrading to the Gen II for a couple reasons. One is the exhaust system, I am upgrading to 4" risers and on down for the exhaust system so I need the 4" y-pipe the Gen II has. Also reliability, its newer and more reliable, AND everything mounts up the same with the transom. The boat should end up weighing in around the neighborhood of 6-7K lbs, including the 55gal tank and 5 passengers. My question is what is the best ratio to start with? I don't know the original ratio. I tried the mercury site, but it didnt help much. I'm looking/asking for anyone with a similar set up or someone who knows what they are talking about to help me out on this. I am mechanically inclined, and I have most of this project covered, I just need some help in selecting the ratio because I am not familiar with exactly how they effect a boat of my size and power. All I want is decent performance and efficiency out of the boat. Nothing special in any one category, just overall good performance. Much thanks in advance for anyone's input!!

 

[achris]

Re: Help Please with Carbed Merc 350/Alpha One Gen II Gear Ratio

The original ratio on that engine would have been 1.50:1, but on that set up I would opt for the 1.62:1 and be able to swing a slightly larger (pitch) propeller. The higher the pitch the more efficient the prop. That is a lot of boat for one engine. Don't expect it to be any quicker than about 26-28 knots. As a comparison a Bertram 25' (about the same weight) use to run twin 165 or twin 470 engines (about 330-340hp total) and had a top speed of around 30 knots, burning around 12gph.

Chris........

 

[Sprky]

Re: Help Please with Carbed Merc 350/Alpha One Gen II Gear Ratio

The original ratio on that engine would have been 1.50:1, but on that set up I would opt for the 1.62:1 and be able to swing a slightly larger (pitch) propeller. The higher the pitch the more efficient the prop.Chris........

Wow! someone on this board that can actually think..................................Must be the name.

Make the torque with the gear not the motor.

We have also used the 1.81 ratio in similar situations and had good results.

 

[rotaryenginepete]

Re: Help Please with Carbed Merc 350/Alpha One Gen II Gear Ratio

Thanks so much for the help guys. Guess I will probably end up getting the 1.62:1 ratio so I have a little more room to play with when picking out a good performance prop. And on the weight thing, I am completely rebuilding the interior structure of the boat so hopefully I can make the thing a bit lighter than it was originally to help increase performance and efficiency. Again thanks for the input!

 

[John_S]

Re: Help Please with Carbed Merc 350/Alpha One Gen II Gear Ratio

Your engine, assuming standard heads for that vintage and 4brl carb, would be about 250HP@prop. If you can find a current boat of similiar size and weight, running the 5.7L alpha (250HP@prop), compare the ratios and the stock props. With such a heavy boat you don't want the standard 1.50/1.47 ratio. You might find enough data that says the 1.81 ratio is a better starting point than 1.62.

 

[Sprky]

Re: Help Please with Carbed Merc 350/Alpha One Gen II Gear Ratio

+1....The 1.81 ratio will actually give you a wider selection of props to choose from.

 

[achris]

Re: Help Please with Carbed Merc 350/Alpha One Gen II Gear Ratio

.... You might find enough data that says the 1.81 ratio is a better starting point than 1.62.

I do agree with this. The only reason I didn't suggest it is that with the 1.81 is that it also spins the water pump impeller slower and at low engine speeds you might be borderline with pumped volume for that engine. If that IS the case, you may need to put an engine driven pickup pump on. I don't have any practical experience with that, so this is just theory....

Chris.....

 

[Hydro Head]

Re: Help Please with Carbed Merc 350/Alpha One Gen II Gear Ratio

They do make a 2.0 in the Gen 2, but it was used primarily on the 4 cylinder models. Not sure if there is any issues with accommodating the torque. The higher gear ratio the better with such a large boat and a single drive. Say 32 to 34 mph at 4800 to 5000 with a 2.0 gear and a 17" prop, and a 24mph cruise at 3500. ?? If you go with a taller gear I think your starting point is a 15" prop, which is getting down there in terms of selection. There is very little information for this boat.

 

[rotaryenginepete]

Re: Help Please with Carbed Merc 350/Alpha One Gen II Gear Ratio

I was looking at a 2.0, but every time I did a prop calculator it was off the charts. The engine will be around 300hp, and I know thats the upper limit for Alpha drives. Some of the work I have done on the boat has been structural, and it is looking like the weight will be considerably less than it was when I got it. I cut out the entire cabin wall, bulkheads, the head, engine mounts, and floor. After 4 trips to the dump, all the interior debris weighed in at about 1200lbs. I will be rebuilding all of that and refiberglassing the entire inside of the hull, but with less material and stronger wood and won't be including a full head in the redesign of the interior. I am thinking now that it will be closer to 5000lbs empty, maybe 6500lbs loaded (but around 800lbs lighter than it originally was..) However, from what I have read it looks like going with a 1.62 or 1.81 will give me a better selection of props. Once again I am looking for a good medium between performance and economy. Thanks again for all the input. It has definitely helped, and this is a great forum too!

 

[rotaryenginepete]

Re: Help Please with Carbed Merc 350/Alpha One Gen II Gear Ratio

They do make a 2.0 in the Gen 2, but it was used primarily on the 4 cylinder models. Not sure if there is any issues with accommodating the torque. The higher gear ratio the better with such a large boat and a single drive. Say 32 to 34 mph at 4800 to 5000 with a 2.0 gear and a 17" prop, and a 24mph cruise at 3500. ?? If you go with a taller gear I think your starting point is a 15" prop, which is getting down there in terms of selection. There is very little information for this boat.

Excellent point, since it is driven by the gears. I will have to look into the math for what the rate of flow would be as opposed to the OEM set up.

 

[achris]

Re: Help Please with Carbed Merc 350/Alpha One Gen II Gear Ratio

Excellent point, since it is driven by the gears. I will have to look into the math for what the rate of flow would be as opposed to the OEM set up.

This should help you.... (these are at 1000 engine rpm)....