Prop for Merc 4.5L

[Cortes100]

I'm perplexed.............looking at a boat that has the new Merc 4.5L engine. These are 2016/17 bowriders in the 2400-2500# range. Both the 200hp and 250hp versions come from the factory with a 15 x 17P prop. The drive is an Alpha but don't know gear ratio.
17P sounds pretty low to me. These style boats running the 4.3L use 21P props.
Unless props have changed a whole bunch in the last year, that just doesn't make sense. Am I missing something?

 

[Sea Rider]

Factory delivered props doesn't mean that will work OK moving your combo as deck weight affects performance. Need to test current prop as usually loaded at wot on flat water, check achieved max wot range, compare thar reading against min-max wot rpm range factory stated. Will know if in the lugging or over revving side of the equation.

Ideal is OB revvving at t least middle to max wot rpm range. What's current wot rpm achieved as loaded, if having a tach to check that out..

Happy Boating

 

[Cortes100]

Yes I understand how props work and how you test them. Forgot this is an aluminum 3 blade 15 x 17P.
Same boat with a 4.3L running the stock 3B 21P will be in the 50 mph range. So why would these come with a 17P from the factory. Only diff is the engine which has the same or more HP than the 4.3
Unless the gears are way different on these 4.5's, I can see this 17P revving out real quick.

I'm asking these dumb questions, cause it's real hard to find any info on these new engines, it's like everything is a secret.

 

[Sea Rider]

Useless comparing both. For your peace of mind, check if both engines comes with different gear ratio, if so, that's accounts for different delivered pitch props.

Need to dial a prop for combp to run at max wot revs as usually loaded. If don't count with a tach, need to buy one ASAP., without one OB is lost in space.. Middle wot rpm will give slight more top end speed, max wot, slight less. Max MPH has a limit, can't go beyond that.

Happy Boating

 

[Maclin]

"Am I missing something?"

Yes, and you already mentioned it, the gear ratio. Just ask the dealer that is selling them. Or look closer, it is on the left side of the drive I think, as you look at it from the rear. You are a lot closer to it than we are :eyebrows:

If the ratio(s) are the same as what would be in the "older" 4.3 or a 5.0 then the mystery continues.

 

[QBhoy]

Maybe worth noting that some 5.0 bowriders came with a 1.47 ratio Maclin....perhaps some of the newer 4.5's came with the same ?? Just supposing ?

 

[Cortes100]

This is long needed for an update. Still looking for the optimum prop.
After the break in period, found the original Black max 17P was pretty much useless. The dealer doesn't have a lot of info on these 4.5's but they are giving me what ever I want when it comes to the prop.
I'm now running a Nemesis 4-blade 19P which runs nice. Pulls hard through out the range, but it will still over rev by at least 300 rpm. Cannot get a real read since I don't hold it there. This 4 blade works real nice for water sports, just need to watch the WOT. Not like I run the engine at the max rpm, but I would like to prop so it hits that target. I was thinking a nemesis 21P, which will get me down 300 rpm to my target, but wondering what the holeshot will be? I could even live with the 19P and then swap out to a 3 blade 21P for the days we're just cruising.

The engine is making good power down low, so it's very hard to compare against 4.3 props. Any ideas on the 4 blade 21P ?? If I can easily over rev the 19P would the 21 work?
The dealer is letting me try them, but they are 2 hours away, and this is starting to be a pain.
Any help is appreciated.

 

[Scott06]

17" seems way off I hOpe they didn't make u buy it?

Vensura or Revolution 4 would be good choices in 4 blade, in aluminum a Michigan vortex probably 20" wou,d also do . I have a 21 ft bowrider 3200 lbs dry, I swapped out a 4.3 for a 5.0 but kept the 1.82 V6 drive ratio. I use a vensura 21" tops out at 44 mph but can pull me up skiing with 7 people in the boat.

They have vent plugs for more whole shot. Rev 4 is available in inch increments so maybe try a 20 and a 21 pitch. Normally increasing 2" would drop WOT 400 or so, but with a big prop like these for more bite might drop more than typical. Since it's two hours away maybe you can take a couple props to try?

 

[Cortes100]

No I haven't had to pay for the props. I do know a little about props, what pitch does, # of blades, rpm's at WOT, etc. But things like cupping, blade design between different brands is way beyond me. The dealer is no prop expert, but is willing to help figure this out by letting me try diff props. I'm just trying to get some expert advise so I can make one more swap and end this experiment. The 2-4 hour drive is what I'm tired of.

 

[porscheguy]

Are you restricting yourself to $100 aluminum props on your new boat which I assume cost over $30K. Now I'm going to assume your dealer has a steady stock of aluminum props because none of them want to carry stainless.

I'm not going to knock aluminum because there are situations where it's use is ideal. And most of them certainly aren't bad or unreliable. It's just that in terms of performance, the only time you may see an advantage with aluminum is holeshot. But for top end, lift, handling, etc, stainless will always win.

I think you're wise to consider a two prop solution since no single prop is the best one for all boating conditions.

You can figure out your gear ratio if you know your engine HP, top speed, and rpm at top speed. Knowing this would also help us to make more informed recommendations.

 

[Scott Danforth]

I would call Mercury they make both props and the 4.5/Alpha that you have

https://www.mercurymarine.com/en/us/contact

I know for a fact that they have a group of test engineers that do nothing but test drive combinations and propellers and you can get some guidelines direct from the source

 

[QBhoy]

You think the Rev 4 would suit this combo ? I thought you needed pretty big horsepower to best suit it ? I do know that they make a horrible clunk going into gear because of the shear size and weight of them. They also apparently lower the rpms considerably in comparison to same pitch 3 or average 4 blade. Big huge diameter on them.

 

[porscheguy]

It's possible a rev4 would work well in his application. And if he's over revving the engine, it's obvious that he needs something to bring the RPMs down. Hopefully OP will tell us his HP, top speed, and rpm at top speed which could help us figure out his gear ratio.

 

[QBhoy]

Fair comment Porscheguy...I'd imagine the gear ratio is one of two.
I always thought the Rev 4 was for high power boats...or at least high power to weight ratio.
For example, the most famous power boat race in the world recently saw a sunus 31 racing boat with twin Merc 400's come an amazing second overall (against much bigger and more powerful boats in a higher class) running with the rev4 props. Think their average speed worked out at around 85mph. Incredible, considering their record breaking boats competition.

 

[Cortes100]

Power pkg; 4.5L MPI 200hp, Alpha with 1:81
Here are a few numbers recorded running the Nemesis 4 blade 19P @ 3000' elevation via GPS
3200 rpm is 25 mph
3500 rpm is 29.2 mph
4000 rpm is 35 mph
4800 rpm ??; not recorded on my paper. (If I remember correctly, it was around 43-44). I don't hold the rpm at 4800 very long so don't get a proper mph read.

I did use the prop calculator from the Mercruiser website and also gave them a call. Got the same answer recommending the same prop but in 21P. So that will be my next test (when ever I can go pickup another prop). Also found the rev limit switch is set at 4950 which I have heard kick in.

My last boat with less HP, I could put the throttle down, and use the trim to get that last 100 rpm to reach the max. This new boat is totally different. Acceleration and throttle response is very good, no matter what speed I'm going. Maybe that's normal for an MPI? Right now it pulls real hard out of the hole so hopefully it can pull the 21P, and also lower my WOT rpm.

 

[porscheguy]

On a modern fuel injected engine in a brand new boat, instant throttle response should be expected. It sounds like a 21p prop is going to provide the best all around performance. You can probably turn a 23, but acceleration will suffer. With a 21, you may barely touch 50mph.

Don't be nervous about running it at full throttle.

 

[QBhoy]

Given the gear ratio you have...it seems crazy to have a 17" prop standard.
Similar boats with the older 4.3 carb with roughly the same hp and ratio usually turn 21" easily and occasionally a 23".
That said...did the dealer prop the boat for altitude and have a guess at it ?
At sea level a 17" is way out for that....the only slight counter argument is that the new 4.5 Merc revs a fair bit higher than the older carb and mpi. I think it can Rev to 5200 rpm ? I think...
The above said...I'd imagine somewhere between a 19-21" would suit depending on what you are doing. Remember if it does Rev to 5200...you are a fair bit away from the older equivelant carb and mpi at 4800 (from memory)