[SPLASHED] I/O (Sterndrive) Conversion to Outboards

[Lou C]

Looks great, I'm still getting used to yellow "ground" wires lol.

 

[Scott Danforth]

Looks great, I'm still getting used to yellow "ground" wires lol.

 

[tpenfield]

Here is an interesting chart. I compared the speed vs RPM of the 'standard' CY338 Outboard (Bracket) with my boat CY338 with extension bracket . . . I used the data from BoatTest done back in 2019 when the OB version first came out. The stock OB had 350 HP Verado engines w/16x18 props. My boat has the 350 HP Honda w/ 16x19 props.
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This chart really demonstrates the benefit of the extension, as can be seen in the separation of the performance lines in the 2,000 - 4,000 RPM range.

There is still some testing and tweaking to go in order to get the Honda up closer to 6,000 RPM. I'll be interested to see how it all translates to mpg's

 

[flashback]

would i be correct in thinking that your setup is planing out sooner according to what i see in the graph?

 

[tpenfield]

would i be correct in thinking that your setup is planing out sooner according to what i see in the graph?

Yes, my boat planes out at about 18 mph . . . the 'stock' 338 OB has to get up around 23 mph.

 

[flashback]

Yes, my boat planes out at about 18 mph . . . the 'stock' 338 OB has to get up around 23 mph.

I like that, for me it would be perfect getting up that slow.

 

[Pmt133]

Two questions.

Do you think the the apparent (I say that only because of the lack of top end RPM in current configuration and any number of things can help correct it) slight over proping of your current setup is also helping your low speed planing? I've seen it go both ways with that.

Do you think if you end up adjusting height for RPM that you'd lose some of the low speed planing. Again... seen it go both ways.

Outside of some really trick setups or setups with a lot of tab, there arent too many vessels I've run or seen that can both get on and hold a plane at 18 mph. It's quite impressive. I can do it on the 4winns but you need to constantly adjust throttle because it's the edge. That's also about 1/6th the boat you have.

I suppose a third question but this isn't really related to your vessel... do you think the factory OB model could be improved with a different prop selection? I've got to assume they're running the enertia on that. On my hull I needed about 4 extra (22 mph) to get on plane with the enertia. While I don't think in this case it would help, you can't help but wonder.

 

[Scott Danforth]

Wonder what your curve will look like with 18p props

 

[tpenfield]

Wonder what your curve will look like with 18p props

Mike said it would raise the WOT RPM, but the speed may be about the same. I'll know in a few days, BUT . . . the lower units might be too deep. I plan on getting some video of the ventilation plates soon.

The vent-plates are about 1" above the planing surface, but it may need more. It may be as simple as trimming out more. Not sure at this point.

 

[tpenfield]

Two questions.

Do you think the the apparent (I say that only because of the lack of top end RPM in current configuration and any number of things can help correct it) slight over proping of your current setup is also helping your low speed planing? I've seen it go both ways with that.

Do you think if you end up adjusting height for RPM that you'd lose some of the low speed planing. Again... seen it go both ways.

Outside of some really trick setups or setups with a lot of tab, there arent too many vessels I've run or seen that can both get on and hold a plane at 18 mph. It's quite impressive. I can do it on the 4winns but you need to constantly adjust throttle because it's the edge. That's also about 1/6th the boat you have.

I suppose a third question but this isn't really related to your vessel... do you think the factory OB model could be improved with a different prop selection? I've got to assume they're running the enertia on that. On my hull I needed about 4 extra (22 mph) to get on plane with the enertia. While I don't think in this case it would help, you can't help but wonder.

You guys ask a lot of questions . . . some are above my pay grade and it may be a matter of trying things and see what works. My main concern is that the engine be properly loaded so as not to have adverse long-tern effects.

18 mph is the 'hump' speed and I can feel the bow come down once I get to that speed and the boat quickly speeds up to around 22 mph. I have not run the boat enough to see when it falls off plane, but the numbers are fairly low vs. the old configuration.

 

[tpenfield]

Since the Mercury engines from the factory were spinning 16x18 props, that might be the right number.

 

[redneck joe]

whats another $10k at this point??

808K views · 5.1K reactions | This video demonstrates two identical boats with the same fuel and weight onboard. The only difference is the props! Sharrow Propellers achieve their maximum advance rate and efficiency much earlier on in the RPM and sp

This video demonstrates two identical boats with the same fuel and weight onboard. The only difference is the props! Sharrow Propellers achieve their maximum advance rate and efficiency much earlier...

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[nola mike]

1. Do you have that graph from your old setup?
2. I'd like to see an MPH x Time graph...wonder if they'd still overlap

EDIT: Oooh... Space the dots out every x seconds, and keep the rpm v. mph on the axes.

 

[Pmt133]

In my defense, my role at work has been transitioning more and more to asking questions... so I'm just in a questioning mood lately.

I also sat in a meeting all day playing with salt line modeling software so I've not been properly stimulated and was quite bored lol. (Note I'm like a toddler where I need plenty of play time.)

I agree... just trying stuff is going to be the way to go for some of it. The important thing is you're in the WOT range and it's performing up to par.

 

[tpenfield]

I'm not sure I ever recorded numbers for my Sterndrive configuration, except the top speed was 45 mph @ 5000 RPM (+/-)

 

[Scott Danforth]

boat test should have the numbers for a 2016 with I/O's

Edit

I did find the VP powered boat. https://boattest.com/boats/cruisers-yachts/338-bow-rider-2015

 

[tpenfield]

boat test should have the numbers for a 2016 with I/O's

Edit

I did find the VP powered boat. https://boattest.com/boats/cruisers-yachts/338-bow-rider-2015

Yes, I was going to pull those numbers. The boats they tested back then had the V-P engines, but pretty much the same. One thing the re-power guy (Mike) mentioned is that bottom paint will take a few mph off vs. a new/clean hull. With my Mercruisers, I also painted the propellers, so that may be another few mph.

As best I can tell, here is the difference in the boat: Sterndrive vs. Outboard w/ extension.
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The biggest difference is the time-to-plane and the planing speed (hump speed).

 

[nola mike]

Yes, I was going to pull those numbers. The boats they tested back then had the V-P engines, but pretty much the same. One thing the re-power guy (Mike) mentioned is that bottom paint will take a few mph off vs. a new/clean hull. With my Mercruisers, I also painted the propellers, so that may be another few mph.

As best I can tell, here is the difference in the boat: Sterndrive vs. Outboard w/ extension.
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View attachment 418460
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The biggest difference is the time-to-plane and the planing speed (hump speed).

Where did the mercruiser #'s come from? scott's link looks like the VPs are closer to your current setup than the stock OB setup. Anybody have the skill to overlay all 4 graphs? Ted, do you have a GPH graph of your new setup yet?
Also interesting on the VP test that there isn't too big of a MPG hit from going from 3500-->4500 rpm/ 30 --> 42 MPH

 

[Scott Danforth]

the volvo pentas are 5.3 Gen V LT VVT motors putting out a rated 355 hp at the crank (320hp at the prop) The Mercruisers were the 6.2 liter home grown version of the Gen 3 SBC

 

[Pmt133]

The divergence on the dual prop is about what I'd expect. I don't know the exact speed where the second prop starts to really add drag but I'd say it looks like around 45 mph. Guess I could calculate it but I don't feel like it.

Those merc engines are interesting. They basically look like a gen 1 and gen 3 had a one night stand. I've driven the 5.3 volvos and they are soft on the bottom but otherwise run very well. The 6.2s are animals... then again I'm never one to complain about too much power.