Prop Question

[404NotFound]

Hello everyone. I have a 1993 Webbcraft 20' ski boat with the 4.3LX Mercruiser and an Alpha 1. My prop is a stainless 19 pitch, and off the top of my head I don't remember the diameter. So on the motor cover it says that the max rpm for the 4.3LX is 4400 RPM, but I regularly hit 5200 RPM at WOT. This makes me think I should go higher in pitch, but I do worry about getting it to plane.

Does anyone have any advice on a size I should go to?

 

[alldodge]

Howdy
You tachometer is off, if you did hit 5200 you would be asking where to get a new motor

Pickup a handheld tach or see if it can be rented at the local autoparts place

 

[jimmbo]

You are under Propped if the engine is hitting 5200. You are sure about the 4400 rpm? Does it also show the recommended WOT RPM range? My chart shows 4400 - 4800 rpm for a 4.3 LX.
A 4.3 is pretty small for a 20 ft boat, especially one for skiing, so under propping might be needed. Just don't run it above 4800.

 

[404NotFound]

Howdy
You tachometer is off, if you did hit 5200 you would be asking where to get a new motor

Pickup a handheld tach or see if it can be rented at the local autoparts place

I will do that. I didn't even know such a thing existed.

 

[404NotFound]

You are under Propped if the engine is hitting 5200. You are sure about the 4400 rpm? Does it also show the recommended WOT RPM range? My chart shows 4400 - 4800 rpm for a 4.3 LX.
A 4.3 is pretty small for a 20 ft boat, especially one for skiing, so under propping might be needed. Just don't run it above 4800.

Yessir. On the engine cover it shows the recommended WOT RPM is 4400, but everything I have seen stated just as you said, 4400-4800.

Off topic, but does anyone know how to locate a speed sensor?

 

[alldodge]

speed sensor

It this is an actual ski boat with an inboard motor, then this would be a wheel on the transom of the boat which feeds the speedometer

If this is a runabout with an I/O then please clarify

 

[jimmbo]

He said it has an Alpha 1 outdrive

 

[404NotFound]

It this is an actual ski boat with an inboard motor, then this would be a wheel on the transom of the boat which feeds the speedometer

If this is a runabout with an I/O then please clarify

I guess it is technically a bow rider.

 

[achris]

...A 4.3 is pretty small for a 20 ft boat, ....

No, it's not. It's probably the best engine for a 20 footer. Mine's heavy, and the 4.3, even when it was the 1994 4.3LX, was more than man enough for the job. A V8 would be too heavy in a 20 footer...

..., but does anyone know how to locate a speed sensor?

Download any one of many free GPS apps available onto your phone. Way more accurate than any boat based speed sensor...

Yessir. On the engine cover it shows the recommended WOT RPM is 4400, but everything I have seen stated just as you said, 4400-4800.

Then someone has scratched off the '-4800'. Those engine (and I had one) have their maximum power at 4600rpm. And Merc recommend 200rpm either side of that. I ran my 4.3LX one size under (propped) and it was great. I would get a proper tuning tacho, as it's uncommon for the dash tacho to be accurate enough for setting up props.

Even with the MPI, I still run under propped, by about 400rpm. At full throttle I'm just banging on the rev limiter. Even if you're getting to 5200, that not going to damage your engine.You're just over the peak power, and losing more than you're gaining.

If you're using it for skiing or towing stuff, then being underpropped by 2" is the right thing. It's what Merc recommend.

Chris.....

 

[jimmbo]

No, it's not. It's probably the best engine for a 20 footer. Mine's heavy, and the 4.3, even when it was the 1994 4.3LX, was more than man enough for the job. A V8 would be too heavy in a 20 footer...


Chris.....

My 18 1/2 ft came from the factory with a 5.7, It feels about Perfect., And it can out accelerate most Outboards

 

[achris]

My 18 1/2 ft came from the factory with a 5.7, It feels about Perfect., And it can out accelerate most Outboards

I'm not surprised! In fact, I'd be shocked if it couldn't.

And in YOUR boat that engine may be perfect (since it appears you have 'outboard envy'), but in most boats that size, that's WAY more engine than they need.

My old 4.3LX got dropped into a 23' Caribbean Crusader, and it became a 36 knot boat. That was after pulling out a 5 litre V8 that on its best day was 30 knots.

Most of the time I'm traveling at between 15 and 25 knots (as are most boats in our size class), and the V6 is doing it with ease. Would a V8 do it easier? Yes, of course, but I don't need the extra 50kg in the ***-end, or to dump fuel into 2 cylinders I don't need, and would rarely use.

If you want a boat that can leap from wave top to wave top (and miss 3 or 4 waves in between) then a V8 in 18 feet of boat is the boat for you, but most of us don't have spines that can handle that for very long (I did in my youth), and so a 4.3L V6 in 20 feet is plenty good enough. (I've had this discussion with you before, a few times, and I'm over it)....

Chris........

 

[jimmbo]

(I've had this discussion with you before, a few times, and I'm over it)....

Chris........

I don't recall any discussion with you about it

 

[404NotFound]

Gentlemen, we all have our tastes when it comes to power plants and I respect that, there is no need to argue. The GM 4.3L was used to power the 4850lb Blazer, and although it had 4 gears to assist in that, it was enough to get it around. More power out of one is obtainable as is shown by the turbocharged GMC Syclone and Typhoon. Would I love a V8 in my 20', 2800lb boat for the sound alone, most definitely. But, when I go out with my family, I have noticed that the younger/lighter members choose the bow, and the older/heavier choose the stern and I am afraid the added weight of a V8 would throw off the balance entirely. Also I have been pleasantly surprised with the fuel efficiency of the little 4-barrel V6.

 

[Starcraft5834]

I had a 20ft cuddy with a 3.0! that was underpowered.. until I put a 15p 4 blade on it... jumped up on plane quick and got 30+ out of her... right prop makes world of difference

 

[404NotFound]

I had a 20ft cuddy with a 3.0! that was underpowered.. until I put a 15p 4 blade on it... jumped up on plane quick and got 30+ out of her... right prop makes world of difference

That is genuinely impressive. How does one determine the best prop without going the "trial and error" route?

 

[Scott06]

That is genuinely impressive. How does one determine the best prop without going the "trial and error" route?

Best bet unless you really know props is to find a local prop shop or marina who will let you try before you buy. I was able to try several by giving them my credit card and under the agreement of you break it you bought it... Best way to learn a little as trials are required

 

[QBhoy]

My 18 1/2 ft came from the factory with a 5.7, It feels about Perfect., And it can out accelerate most Outboards

Agree. V8 in an 18ft is a lovely thing

 

[QBhoy]

I don’t think you will ever see 5200 out a 4.3lx. They usually pop and hold back on the limiter at about 4900/4950 any I’ve seen or had.
A 4.3 is plenty for a 20ft. I know of many that do just fine. I had that same engine in a 2255 23ft cruiser and a sealine 218 23 ft cruiser at nearly 2t. All be it, it had Duoprops. Both did about 35 mph. Plenty. The 4 barrel 4.3 carbs are near as damn it as powerful as the old 5.0 from the late 80’s.

 

[achris]

I don’t think you will ever see 5200 out a 4.3lx. They usually pop and hold back on the limiter at about 4900/4950 any I’ve seen or had.....

Injected, yes. The carbed engines don't have a rev limiter. It will run as fast as the prop allows it.

Chris......

 

[QBhoy]

Injected, yes. The carbed engines don't have a rev limiter. It will run as fast as the prop allows it.

Chris......

Are you sure on this one Chris ? It’s the carb I’m referring to.