Prop pitch and RPM at WOT

[jamesdgreen2016]

Hello!

I have a brand new 20 foot bowrider with a Mercruiser 6cyl 4.5l 200hp engine in it. The prop I have is a 4 blade 19" that the tech in the shop I bought the boat at says is good for my boat. It was a prop I bought before the boat. The prop that came with the boat, which is not currently attached to it, is a 3 blade 19". Now, when I read the manual for the engine I have it says the 250HP 6cyl merc engine is to be targeting 5200RPM at WOT and the 200hp 6cyl variant which I have is to be targeted at 4800RPM at WOT. With the 4 blade 19" prop I have (listed below) the RPMs at WOT with the boat are about 5200 with two people on the boat. Im assuming because the other prop that came with it is also 19" it would be around the same RPM at WOT? I called the store that sold me the boat to speak to one of their techs and ask if the fact that its hitting 5200RPM at WOT and the manual suggests a target of 4800RPM at WOT is going to be a problem. He said that as long as I wasnt running it at WOT for long periods at a time, like hours, it wouldnt be. Do you guys agree? Or what do you think I do here? [h=1]Turning Point Hustler #21501930 Aluminum Boat Propeller

http://ww2.boatpropellers.iboats.co...501930_Propeller/765/35668/?cart_id=412279958[/h]

 

[Scott Danforth]

add more people and you will be spinning at a lower WOT.

pretty sure you will be fine at 5200, however keep in mind, you dont have to hold the throttle against the stops

 

[jamesdgreen2016]

Thanks! Yeah when I filled the boat up with people and went to WOT it dropped to more like 5000RPM. Still a bit higher than the 4800 range but I generally dont run it at WOT, more like 3/4 speed, and wont be running WOT for more than a few minutes at a time when ever I do, so if Im cool to leave things as they are thats what Id like to do - the prop is giving excellent pick up at the low end and good steady performance with lots of people on board which are my main objectives.

 

[Sea Rider]

What's achieved when an engine over revvs :

-Carry more load, passengers.
-Better hole shot with less throttle if lightly or medium loaded.
-Faster plane.
-At cruise speed need to throttle less, better fuel consumption.

What you can do with actual 4 blade prop as a performance limit is keep adding passengers till engine revs at middle wot rpm range, don't add any more to maintain good hole shot and good end speed.

If with a 4 blade prop19 pitch engine over revs, with a 3 blade same pitch will over rev more due to less blade water resistance.

Happy Boating

 

[Cortes100]

I have the same engine package, 4.5L, 200hp version. You're not going to hurt anything under propped by a couple hundred rpm, just back the throttle off. When you have a full load, it might be perfect.

FYI, the 200hp version has a rev limiter set at 4950. So not sure how you are getting a 5200 reading??
I was using a Merc Spitfire 19P 4-blade and it would easily tag the limit switch. I now moved to the 21P Spitfire and it's pretty much right on. Still pulls great on the bottom end, cruises way nicer and still hitting 48-4900.
I have an Alpha 1:81 at 3000' if that matters.

 

[jamesdgreen2016]

I know, the mystery deepens! I read that in the manual as well about the rev limiter and thought how can it be going past 5K when its supposed to be limited before that. But not knowing much I decided I didnt know enough to know if that was odd. It seems odd. Here is a picture at WOT. You can see its a bit past the 5K around 5200rpm and its running a little over 40mph. And yes its the same engine the 4.5L MPI 6cyl 200hp merc boat is 2017 model.

 

[jimmbo]

Have the dealer verify that the tach is accurate. Two reasons. 1) You say the engine is running higher than the rev limiter, 2) The boat was supplied(supposedly by factory) with a 19 in 3 blade, you are using a 19 in 4 blade, which should lower the revs compared to a 3 blade

 

[Cortes100]

ooohhh.....what if........... it has the wrong engine cover and you actually have the 250 version under a 200 badge.......

If the dealer is close by, they can easily plug in their machine to verify your rpm's.

 

[porscheguy]

I imagine the difference between the 200 and 250 boils down to mapping and where the rev limiter is set.

 

[Scott Danforth]

I imagine the difference between the 200 and 250 boils down to mapping and where the rev limiter is set.

And what they charge because the 250hp has a higher waranty accrual

 

[porscheguy]

Translation: more warranty claims on the 250?

 

[jimmbo]

I imagine the difference between the 200 and 250 boils down to mapping and where the rev limiter is set.

I doubt it is just mapping. To get a 25% increase in power requires more than just mixture and spark timing. Deeper breathing could mean different valve timing, better induction and exhaust, higher compression. 400 rpms isn’t going to do it either

 

[jamesdgreen2016]

Yeah when I read the manual (which is for both the 200 and 250 engines) and it said the limiter for the 250HP version capped out at what mine is capping out at, and the 200HP version capped out at less than mine is capping out at, it did make me wonder if they put the wrong one in there. I also had the boat loaded to max capacity or maybe a little over max capacity weight wise and at WOT was able to get it to about 40mph and fairly quickly, and was still achieving higher than max WOT for the 200hp version when I did that, which is to say it performed a lot better on both take off and max speed than I was expecting it to under that much load. Anyway if you all, and the shop tech also, thinks running this engine at 5200 RPM max at WOT with this prop is fine then Im just going to be happy it performs well and not worry about why.

 

[porscheguy]

I doubt it is just mapping. To get a 25% increase in power requires more than just mixture and spark timing. Deeper breathing could mean different valve timing, better induction and exhaust, higher compression. 400 rpms isn’t going to do it either

I don’t know about that. Keep in mind, the 4.3mpi is rated higher H.P. than the 4.5 200. It’s likely the 4.5 200 has a very conservative tune, and the 250 has turned up the wick to maximum and gained an extra 400rpm. Mercury found an extra 50hp with their 400r verados by taking a 350 and giving it an extra 800rpm.

as long as the breathing can keep up, upper end H.P. can go up quickly with rpm.

 

[jimmbo]

The key is the breathing, which could mean, different valve timing, larger valves, more valves per cylinder. When I hear ‘it just a different mapping’ it sounds just like ‘ different jets in the carb will get another 30 hp’. Until I see an actual parts list showing the engines are identical mechanically, including induction, I will stand by my previous post

 

[jamesdgreen2016]

It's an interesting question I guess what the differences are between the 200 and 250 versions. A quick google search shows we're not the only people talking about it. These are two threads from other boat forums on the same topic.

https://www.baylinerownersclub.org/f...5l-gctid804882


By the way, does anybody know if the boat has a rev limiter on this engine set to 4900rpm and I get the boat tested and it actually is going up to 5200 rpm, which seems more likely than a gauge malfunction, what are the possible causes of this? If one possibility is that the 200 and 250 engines are identical and mine is just programmed as a 250 by mistake, what are the other potential causes if this isnt the case?

Suppose there are other differences between the 200 and 250 engines aside from just the way they are programmed, should I be concerned that it is going to 5200rpm?

 

[jamesdgreen2016]

https://www.baylinerownersclub.org/...d-on-the-real-difference-opinions-gctid808227