Mercruiser drive ratio; high horsepower in small boat

[region4]

Hypothetically, if I upgraded from a 165HP inline 6, to a 350 V8, would the 1.5 (V8) alpha one drive ratio be adequate, or too high? I ask because this is a high horsepower motor, pushing a lightweight boat.

Currently I run 51mph @ 4100rpm (WOT) with laser II 25P, in a 1971 jolly roger (18 footer, 18 degrees deadrive V hull). The hull rides flat, stays glued to the water (there is zero aerodynamic hull lift), and is rated for a maximum of 260hp. If I went to a 350 V8, and Alpha drive with 1.5 ratio, the prop spins 10% faster. But I suspect that the engine would still need a higher pitched prop. Am I on track here?

Mercruiser 233 had a 1.32 ratio, back in the mid 1970's. Is the 1.32 ratio worth considering? Is this a better ratio for my application? Or will the ubiquitious 1.5 suffice?

Finally, I've read the forum threads comparing the relative merits of varying prop shaft speed versus propeller pitch. In the end, it seemed inconclusive which was a better combination. It did seem to indicate that the availability of higher pitched props was limited, thus increasing prop shaft speeds was necessary.

Bottom Line: Current setup runs well (thanks to all the help in this forum), but the feel/sound of a V8 would be my upgrade choice, if I have a failure down the road.

 

[Scott Danforth]

Re: Mercruiser drive ratio; high horsepower in small boat

stick with the 1.47:1 commonly available ratios and you will be fine.

 

[haulnazz15]

Re: Mercruiser drive ratio; high horsepower in small boat

Just use the 1.47 ratio (1.50) for the standard alpha or Bravo I drives. The 1.32 is a good drive, but the parts are NLA and it doesn't really do anything that a prop pitch change can't do. I really have no idea why Merc used the 1.32 drive ratio with the 351w over the 1.50 anyway.

 

[Bondo]

Re: Mercruiser drive ratio; high horsepower in small boat

Just use the 1.47 ratio (1.50) for the standard alpha or Bravo I drives. The 1.32 is a good drive, but the parts are NLA and it doesn't really do anything that a prop pitch change can't do. I really have no idea why Merc used the 1.32 drive ratio with the 351w over the 1.50 anyway.

Ayuh,.... I agree, No reason it was needed with the 351s,....

It was also used with the 454 Chevy, but as you note, it's outa production anyways,...
So the 1.5:1 is yer only choice for a V8 motor,...

 

[Walt T]

Re: Mercruiser drive ratio; high horsepower in small boat

A 1.47 ratio the engine is running a little slower but with the V-8 it should be able to run the 25. Obviously you're running a 1.65 now. The 1.47 will get you 58 mph at 12% slip and that is only at 4100 rpm which you state is your current wot rpm. At 5000 it would be nearly 70 all things being equal. That ain't gonna happen. Drag starts taking it's toll on the drive and hull and to reach 70 will take much more top end HP than the stock 350 will generate. I remember those Jolly Rogers. The bottom was a deeper V than most of the V hulls, which made em a little better rough water boat than others of their size range. That is a rounded pad V, not a flat pad. Those are dangerous if they get up on the center of that back pad. That will create that deadly rocking at speed. Keep in mind that back then a lot of manufacturers used a standard Glastron hull and put their own decks on it including Jolly Roger. Jolly Roger was only around a short time I think. However they were pretty good river boats out on the River in Bullhead / Laughlin and I remember one had a 454 and it was an inboard. I know the owner, I don't think he has it anymore since he's all obsessed with drag boats now. I think most of those were outfitted with outboards. At any rate with the V-8 and 1.47 I would try the 25P you already have first and go to wot very gradually. If you start to feel that chine walking or rocking you've reached the speed limit of the hull. I really don't think you will and it'll be fine and you'll be happy. Thanks for the trip down memory lane.

 

[region4]

Re: Mercruiser drive ratio; high horsepower in small boat

Since the boat is rated for 260HP max, the 350 EFi vintage 2001 (which I believe delivers 260hp at the prop) would seem a logical choice. I have seen these in donor boats.

Increasing weight in the stern is also a consideration. The beam is 84", so it has some buyoancy in the stern, but is still a pretty small boat. Is the v8 considerably heavier than the inline 6?

Should I consider a V6 MPI? Is this a significant bump over the current inline 6, or is that just apples to apples, and thus not worth the hassle of a swap? I always hear that displacement means torque, and torque is what boats need.

The sound/feel and slow turning V8 is a nice package...in the end.

 

[Bondo]

Re: Mercruiser drive ratio; high horsepower in small boat

Since the boat is rated for 260HP max, the 350 EFi vintage 2001 (which I believe delivers 260hp at the prop) would seem a logical choice. I have seen these in donor boats.

Increasing weight in the stern is also a consideration. The beam is 84", so it has some buyoancy in the stern, but is still a pretty small boat. Is the v8 considerably heavier than the inline 6?

Should I consider a V6 MPI? Is this a significant bump over the current inline 6, or is that just apples to apples, and thus not worth the hassle of a swap? I always hear that displacement means torque, and torque is what boats need.

The sound/feel and slow turning V8 is a nice package...in the end.

Ayuh,.... The V8 is heavier than the I6, 'n the V6 as well,....

Donno what the I6 weighs, 'n I believe the V8 is 'bout 200lbs heavier than the V6,...

The Mpi V6 is a Serious power package,...
More power than the 305 2 bbl. carb, 'n I believe abit more than the 5.0l 4bbl. carb motors,...

But,... That said, the 350 Mpi would be My choice for the direction yer goin',...

Myself,... I'd be movin' towards a 383, Vortec heads, 'n a 4bbl. carb,...
The Alpha wouldn't "Like" it, but it'd be a Blast as long as it lasted,......

Ya keep sayin' the boat's rated for 260hp,...
I've never seen an I/O hull, horsepower rated,...

 

[haulnazz15]

Re: Mercruiser drive ratio; high horsepower in small boat

Ya keep sayin' the boat's rated for 260hp,...
I've never seen an I/O hull, horsepower rated,...

I was under the impression that the maximum HP was defined by USCG for boats less than 20' in length? I'm sure there is some leeway as to the computation, but why would it matter what kind of monohull it was (I/O, O/B, Inboard, etc)?

 

[Scott Danforth]

Re: Mercruiser drive ratio; high horsepower in small boat

unless it has an I/O. then anything you can float works. ther is NO HP limit on I/O's

so Im with Bondo. put in a 450hp 383. watch your hole shots and have a blast.

engine weight less exhaust manifolds:
the I6 is 440# the V8 is 534#

 

[region4]

Re: Mercruiser drive ratio; high horsepower in small boat

The nameplate in the boat does not say MAX HP; rather it says Maximum recommended HP = 270 (not the 260 I mentioned earlier in the thread).

Further, I read the CG rating system. I agree that it seems to apply only to outboards, and specifically excludes inboards and i/os (which it considers inboards) from its jurisdiction.

 

[haulnazz15]

Re: Mercruiser drive ratio; high horsepower in small boat

The HP capacity plate you are reading is likely the Manufacturer's recommended max HP. Doesn't make it illegal to have more HP, but I'm not sure why you would bother dropping big money to put a screamer into it. A good small block V8 will be more than enough bower in an 18' runabout. Heck, the 302/305 engine was generally the "premium" engine in most 18' boats. You'll probably get 60+mph out of that boat if you dare to push it that high. As long as the transom is solid and the boat stays afloat with the extra weight in the back you'll be fine.

 

[GaJeff]

Re: Mercruiser drive ratio; high horsepower in small boat

Oh how I would love to put a V8 in my boat. Just got done with deck transom and stringer. I beefed up everything under the deck and made my bilge to accommodate a small block. After the build I went for a test run and the engine didn't sound right so I tore it down and took it to the machine shop. I was supposed to last me until at least next year but it didnt. So I just rebuilt it. It's only a 4cyl but I was getting 32-34 mph from a tired motor so I'm sure I'll get more power from a fresh built one. Maybe in a year or so I'll get a small block.

Mine is a 19.6 so if your doing that we'll in an 18 maybe I need to consider an I6. Although 50+ seems a little to fast for me. My son like to do do-nuts, he calls them Reeses (4yr old). I could do some awesome do-nuts with a boat capable of 50+.

Jeff

 

[Walt T]

Re: Mercruiser drive ratio; high horsepower in small boat

You could also do awesome time in the ER

 

[Bondo]

Re: Mercruiser drive ratio; high horsepower in small boat

You could also do awesome time in the ER

Ayuh,.... That's always a potential side-effect for those of Us who chase the sensation known as the Adrenalin Buzz,....

 

[NHGuy]

Re: Mercruiser drive ratio; high horsepower in small boat

I guess what's meant is circles aren't too smart at high speed. But I can tell you that above 45 or 50 MPH in a little boat is a hoot. The main thing is you have to be careful not to hit any waves too big for your given speed and angle of attack. It's a learned thing, every boat acts differently, you will learn yours.
You also said the hull stays on the water and rides flat. You might try some trim to raise the bow and reduce wetted area, that maximizes your speed. But if you get it too high it will start to porpoise or the prop will slip or both. Just be careful not to over rev the engine. Getting out there in flat-ish water and trimming for top end is one my favorite things. Good fun!
You know what else is great about a dinky boat and plenty of motor? You can out holeshot everybody but the best ski boats, and you"ll have more top speed, so they can be reeled in after they beat you off the start.

 

[Walt T]

Re: Mercruiser drive ratio; high horsepower in small boat

Yep, NH hits it on the head. It's really fun to watch their smiles turn to astonishment when a big *** 22 foot cuddy passes them easily

 

[region4]

Re: Mercruiser drive ratio; high horsepower in small boat

I guess what's meant is circles aren't too smart at high speed. But I can tell you that above 45 or 50 MPH in a little boat is a hoot. The main thing is you have to be careful not to hit any waves too big for your given speed and angle of attack. It's a learned thing, every boat acts differently, you will learn yours.
You also said the hull stays on the water and rides flat. You might try some trim to raise the bow and reduce wetted area, that maximizes your speed. But if you get it too high it will start to porpoise or the prop will slip or both. Just be careful not to over rev the engine. Getting out there in flat-ish water and trimming for top end is one my favorite things. Good fun!
You know what else is great about a dinky boat and plenty of motor? You can out holeshot everybody but the best ski boats, and you"ll have more top speed, so they can be reeled in after they beat you off the start.

Going in circles in a HUGE concern, especially with respect to prop walk.

I notice in the current setup (RideGuide rack and pinion/cable) that there is a constant torque on the wheel, which if left unchecked by the driver, will rotate the outdrive to port, and progressively slam the boat into a severe turn to port. This is always a concern, but at higher speeds, it is a serious concern, especiallly for a unsuspecting passenger who has no hand holds.

Is hydraulic steering the best way to address this? If so, do 5.7 mercruisers all have hydraulic steering, as standard? Is there a better steering system?

 

[Bondo]

Re: Mercruiser drive ratio; high horsepower in small boat

Going in circles in a HUGE concern, especially with respect to prop walk.

I notice in the current setup (RideGuide rack and pinion/cable) that there is a constant torque on the wheel, which if left unchecked by the driver, will rotate the outdrive to port, and progressively slam the boat into a severe turn to port. This is always a concern, but at higher speeds, it is a serious concern, especiallly for a unsuspecting passenger who has no hand holds.

Is hydraulic steering the best way to address this? If so, do 5.7 mercruisers all have hydraulic steering, as standard? Is there a better steering system?

Ayuh,.... Just addin' Power Steerin' takes care of that, 'n most V8s have P/S,...

 

[dan02gt]

Re: Mercruiser drive ratio; high horsepower in small boat

My concern would be how adding a few hundred pounds to the stern of a already stern heavy boat. It my have issue getting out of the hole due to all the weight in the stern making it harder for the bow to break over. It may also be more prone to porpoising at lower speeds. Trim tabs can cure all that, but if it was mine I would put a 4.3L MPI in it.

 

[Yankeyspeed]

Re: Mercruiser drive ratio; high horsepower in small boat

Have you thought about "lump head" kit for the I6? I have read its a 10% gain.