Yamaha performance bulletin 40/30 vs 60/40

[cerevisiaephilus]

This has been bothering me for a while, bored at work now, so I thought I'd ask. The Yamaha performance bulletin for the 40/30 jet and the 60/40 jet are very similar. Same total boat weight, more or less, same top speed, same acceleration, if anything, the 40/30 does better.

Does the 60/40 outperform the 40/30 only when you increase the total weight of the boat? Can anyone explain the physics behind this? I guess I'm confused by all the talk about different sized jet lower units affect performance. Seems like if they both have the same RPM and the same impeller size, how does horsepower matter? Exactly what is producing the resistance on the impeller? The physics of a propeller make a lot more sense to me. And I understand the chart on the Jet Outboards website, just not the actual mechanics of how a bigger power head on the same size pump can make a difference.

https://www.yamahaoutboards.com/sites/default/files/bulletins/bulletin_4stroke_midthrustjetport_jet-drive_g3b_1656ccjdlx_f40jea_2011-06-13_alm.pdf

https://www.yamahaoutboards.com/sites/default/files/bulletins/bulletin_4stroke_midthrustjetport_jet-drive_g3b_1656ccj_f60tjr_2010-05-18_alm.pdf

 

[Ranchero50]

A quick theory on jet pumps is mass x velocity = thrust. Mass of water, how fast it leaves the nozzle = how much thrust the pump delivers.

RPM is a function of mass, more water = more power needed to maintain RPM.

In hot rod mentality, mass it torque, velocity is horse power.

You need mass to move a load, velocity to move fast.

Kind of a garden hose vs. pressure washer deal.

Per the bulletins, there's only 10hp difference in the pumps, the ambient conditions nullify the comparison (temp, wind, weights).

In reference to different pumps, the pump needs to be matched to the engine or neither will perform correctly. Power curves and RPM range determine which pump setup is recommended.

Cornfused yet?

 

[cerevisiaephilus]

Definitely confused! But thanks for the response because I think you do a good job of bringing out where I'm getting confused.

If the difference the extra horsepower makes is the ability to maintain the same RPM with more mass, i.e. more water, the difference between 40/30 and 60/40 is nothing. Because...

...they have the same RPM and same pump size, so they only have so much water they can move. They're limited by pump size and RPM, not horsepower. I can see a difference if the pump size were different: at the extreme, for example, make it so big that the 40/30 just couldn't spin the impeller to max RPM, then there would be a difference. But these motors have the same pump size and same RPM, so they're drawing the same amount of water!

As for the small difference in the pumps, it's technically 12hp, I suppose. I work in a manufacturing environment where we have lots of liquid pumps. The difference between a 1.5hp pump and a 10hp pump is dramatic. Of course everything is different, impeller size, line size, not just power head.

 

[RiverBottomOutdoors]

Wind velocity is different on the two tests. Wind really takes a toll on my jet drive. I gain .5 mph just by sitting down when I'm driving.

 

[hotshotinn]

I know my 40/30 Yamaha four stroker is a good performer on a light 1648.Its not scary fast like some peoples boats that are on here buts its a good performer.Im glad I didnt buy a 60/40 now knowing they dont go any faster than the 40/30

 

[Djknyork]

About a 90lb weight difference so something doesn't jive. Possibly trim/set-up? The 60/40 should easily pull way from the 40/28 at least IMO.

 

[dhoganjr]

I don't see how those 2 tests are even close to being accurate, for one the test dates are 13 months apart. That could mean as much as 2 model years difference, the test in 2010 could have a 10 model year and the test in 6/2011 could possibly have a 12 model year. The boat is 20 lbs heavier on the 40 test and is a ccj deluxe model. Wind speed is higher on the 60 test, plus altitude and temp is higher. 0-20 speeds were taken with an aftermarket speedometer, usually not that accurate. There are way to many variables to compare and make a decision from these two reports.

IMO you should be able to feel the difference in a 60/40 vs a 40/30. The 60 should pull away, neither will be incredibly fast, but you should get at least 2-3 mph faster. Plus a better holeshot and be able to carry more weight. The 60 won't work as hard to keep it on plane.

Just my .02 worth from those 2 reports.

(Disclaimer) Don't take it to mean that I'm putting down the 40/30, that is not what I'm getting at, they are good engines. If you are deciding between these 2 try and get a ride in both and decide from there. But I think you'll notice more than the reports say.

 

[hotshotinn]

It not adding up to me either but thats what it says.From running the river around me the guys seem to have heavy loaded up boats on the river and the 60/40 Mercurys are popular,my light 40/30 yamaha on my 1648 will eat them up but i did up my boat light with aluminum flooring and one battery to run everything and i have my motor mounted as high as possable.I think the marinas that the guys bought their boats at dont have a clue as far as motor setup here in Michigan.Sure they can mount it but when its mounted too low speeds will suffer.I bet if i were to setup one of their boats they would do better

from what ive heard the 60/40 Mercury would be the better choice over the 60/40 Yamaha anyway

 

[dhoganjr]

I would think for a true comparison test run the same boat back to back with the engines swapped. Height and everything set exactly the same. Every boat will perform different, even the same make and model.

Set-up has a drastic effect on performance, a 1/4 inch too low can lose several mph. People think you can just bolt them on and go, and they will run, but a little fine tuning will make a world of difference.

 

[LarryMc]

hotshotinn » 03 Jul 2014, 06:21[/url]"]

from what ive heard the 60/40 Mercury would be the better choice over the 60/40 Yamaha anyway

You've heard right. The 60/40 Merc has a large series jet lower with a 6 7/8" impeller. The 60/40 Yamaha 4 stroke has a medium series lower with a 6 1/8 impeller. At one time I had a 50/35 2 stroke Yamaha with the medium pump and 6 1/8" impeller on a 1750 Duracraft. It would barely get on plane and the top speed with two people was less than 20 MPH at 5200 RPM. I swapped the Yamaha out for a 60/40 Merc and there was a world of difference, the Duracraft got on plane a lot quicker and the top speed with two people increased to 27 MPH at 5200 PM. Granted the 60/40 4 stroke Yamaha might do a little better than my old 50/35 2 stroke, but it would require a big jump in RPM to do so and I doubt that it would ever come close to a 60/40 Mercury with the larger pump and impeller.