Only 4400 rpms on V6 4.3 2005 19P 3blades

[Kubaat3lover]

Question to You, especially @achris ,
i have got 19ft bowrider and it runs around 4400rpms (what tacho says) at WOT and it gives me around 50mph (GPS says 80-85km/h),
90% of time Im crusiing 2500-3000rpms and its fine, gives me around 18-19mph (16,5 kts)
Im running regular 3 blade prop from Quicksilver 19P, but with boat from previous owner 21P but i think its too much. I dont have any issues with holeshot etc.

 

[achris]

Although you have the engine running on the recommended spec, you're at the very bottom end. In my opinion (and that of the Merc service rep for WA at the time) that's too low. The V6 doesn't have the nice fat torque curve of a V8, and on the mid range can feel 'wanting'. Having personally owned 2 of theses engines, and installed many more for customers, I always prop to the very top, even over is ok. The benefits of light loading has many aspects. The previous V6 (4.3LX) would run to about 5000, and the current one will run to 4950 (the rev limiter). I'm running a 4 blade 17" 'Vensura'. I did try a 19", but only made 4600, and that was without much in the boat... So the 17" gets the green light.

Chris...

 

[Kubaat3lover]

I was wondering about 4 blades, calculator says 18/19P, what do You recommend?
3,4blades? 17//18P? take care

 

[alldodge]

Don't know what the drive ratio is but if its 1.47 then don't change a thing, your at 7% slip

 

[achris]

3 blade/4 blade will depend on the boat (hull shape) and how you load it and what performance criteria you are looking for. If you're looking for top speed, 3 blade every time.

I have covered the advantages (to me) of a 4 blade in quite a few threads, but here they are again. Remember, this is my hull, your hull will perform differently..
Better stern lift, meaning the stern of the boat doesn't 'plough' on acceleration.
Onto and hold plane at a lower speed.
Allows the hull to track the water (rather than forcing the bow up and causing 'slamming' when gravity overcomes the bow lift).
Better response to the drive trim.
I can comfortably cruise at a higher speed with less bouncing.

As I said, that's for my boat. The only way you'll know how different props perform on your boat is to try them. You might not like how a 4 blade performs (although I haven't heard from many people who didn't like it). As you have a bow rider, you may have people who like to ride up front, and in this case you may want/need a prop that does produce bow lift (a high rake 3 blade is ideal for this)...

Chris...

 

[achris]

Don't know what the drive ratio is but if its 1.47 then don't change a thing, your at 7% slip

The V6 MPI is supplied with a 1.62 drive. I think the carbed engine was supplied with the same, and maybe 1.81 for high altitude... Definitely never a 1.47...

I also almost never use slip as an indicator. I wrote a program that calculates the propeller Angle of attack. It's a much better metric...

Chris...

 

[Lou C]

I have a 15.5x15 on mine (1.62:1 drive ratio) and it will pull 5,000 or a bit more even WOT. While this definitely lowers top speed the more important result is that it reduces the strain in the engine every time it has to pull the boat up on plane. I’m willing to sacrifice top speed for better acceleration and longer engine life…

 

[Kubaat3lover]

I dont know what ratio I have, I can provide You S/N, but engine is from 2005. I dont care about top speed, i like cruising and tubing/sking

 

[alldodge]

The V6 MPI is supplied with a 1.62 drive. I think the carbed engine was supplied with the same, and maybe 1.81 for high altitude... Definitely never a 1.47...

I also almost never use slip as an indicator. I wrote a program that calculates the propeller Angle of attack. It's a much better metric...

Chris...

Then we need more accurate info because using 1.62 or 1.81 puts it into negative numbers, and I'm sure you know that is not possible

 

[achris]

I have a 15.5x15 on mine (1.62:1 drive ratio) and it will pull 5,000 or a bit more even WOT. While this definitely lowers top speed the more important result is that it reduces the strain in the engine every time it has to pull the boat up on plane. I’m willing to sacrifice top speed for better acceleration and longer engine life…

And a nicer boat to drive. More responsive to throttle, and doesn't overrun/die off when running up and down swells... As much as I love my V6 in this hull, I'm aware of it's shortcomings in the low to midrange if it's over propped....

Chris...

 

[achris]

I dont know what ratio I have, I can provide You S/N, but engine is from 2005. I dont care about top speed, i like cruising and tubing/sking

Drive serial number will give us the ratio... (Not engine or transom serail number)...

 

[Lou C]

One benefit of the V6 in boats this size is fuel economy; I have found that with the Rochester Quadrajet 4bbl 90% of the time I’m running on the small primaries which give good low speed response and excellent fuel economy. But you still have the top end benefit of the huge secondaries; and the fabled Quadrajet howl at WOT!

 

[achris]

One benefit of the V6 in boats this size is fuel economy; I have found that with the Rochester Quadrajet 4bbl 90% of the time I’m running on the small primaries which give good low speed response and excellent fuel economy. But you still have the top end benefit of the huge secondaries; and the fabled Quadrajet howl at WOT!

My 4.3LX had the Weber 9600 (aka Carter AFB/Edlebrock 1409). Similar in performance to the Rochie, small primaries, big secondaries and yeah, a real roar when they pop open, and a proper kick in the backside. I have a friend who was always impressed with the acceleration from about 2800. I believe that's all down to the prop.

 

[achris]

i have got 19ft bowrider and it runs around 4400rpms (what tacho says) at WOT and it gives me around 50mph (GPS says 80-85km/h),

That's 43.5 knts. (I prefer to use nautical terms when dealing with boats. )

Plugged those numbers into my calculator (assumed a 1.47 ratio)... and somethings doesn't look right... (I suspect you're using the dash tacho. )



As a contrast, here's what my numbers run...


Chris....

 

[alldodge]

Unicorn, slip below 5%

 

[achris]

Unicorn, slip below 5%

That's what I said, something isn't correct...

Here is one of the best documents on propellers. I recognise it, it's a direct copy of the Merc propeller booklet they used to give away in the 90s. Still about the most definitive article I've ever read on props, (and the basis for writing my program).

Basics About Propellers

In determining boat performance, propellers are second in importance only to the power available from the engine itself.

www.killcaremarina.com.au

 

[Kubaat3lover]

Drive serial number will give us the ratio... (Not engine or transom serail number)...

Its

1A268871

 

[achris]

Its

1A268871

Well, I stand corrected, that's a 1.47 drive... I've rerun your numbers using 1.47...



Better, but not where I'd like to see them. I still think your tacho is reading low. (they all do)... You should get a digital tacho on the engine and compare the readings.

 

[Kubaat3lover]

I have IR digital laser tachometer but its quite difficult to measure cam @ WOT with 1 hand

 

[alldodge]

Never a 1.47