More HP / Changing Pitch

[300sflyer]

I recently upgraded our new to us twin screw 31' Chaparral, with two 325 HP Merc 5.7s from the original 260 hp engines.

http://www.mercurymarine.com/_media/pdfs/productbrochures/899883260_357MagBrvo4V_SS.pdf

I only have about 3 hours on the new engines, and have not gone anywhere near WOT yet, as they are still breaking in. The drives are Bravo 2's, with a 2.2 gear ratio. The props are aluminum 17-1/2 x 21. I can tell by the way the boat excellerates, that WOT will be well above the limit of 5000 RPM. At 4000 RPM I am at about 30 MPH, and there is still a fair bit of movement left in the throttle levers to WOT.

I have the option of changing the gear ratio in the two drives, but that is quite expensive to have done, and the drives run just fine, so I would rather not tear them apart.

Would going to SS 17-1/4 x 25 be the way to go? That is the highest pitch available on Bravo 2 props. Also, can anyone comment [good or bad] on the SS Titan Solo props vs Merc OEM ones?

Thanks!

 

[Barnacle_Bill]

Re: More HP / Changing Pitch

Your questions can be more accurately answered once you find out what your RPMs are at WOT.

 

[300sflyer]

Re: More HP / Changing Pitch

The engines have a rev limiter from the factory, so the true WOT RPM will likely not be know, until I change the props.

 

[hwsiii]

Re: More HP / Changing Pitch

Flyer, if you will fill out this form I can tell you what your max WOT speed should be with the new motors.

You might try reading this for a better understanding of changing props for better performance, and for the major differences between aluminum props and stainless steel.

http://forums.iboats.com/showthread.php?t=369057


If you would prefer to not go to this much trouble there are many other people who will come along and gladly help you.

1. Year, make and model of boat
2. Length, width and base weight of boat, look for boat decal on back of boat
2a.What is the maximum recommended HP for your boat
3. Number of people and gallons of gas normally on boat and what do you use the boat for
4. Year, make. manufacturer and model of motor and recommended Wide Open Throttle (WOT) range
5. HP and gear ratio of motor IMPORTANT
If you don?t know the ratio, you need to pull the plugs out and put a piece of tape across the prop and the lower unit and then cut it between the prop and the housing then do the same thing on the flywheel and turn the motor until the tape lines up with each other on the prop, it is easier if two people do this, so one can watch the prop while the other counts the revolutions of the motor
6. Anti-ventilation Plate height above keel of boat if it is an outboard in inches, use a straight edge under the keel and sticking out to the anti ventilation plate for a reference
Motor Transom Height

7. WOT RPM and speed from your current prop and how much gas and how many people were in the boat for the test data and is the speed by GPS. Make sure you trim the prop up until it starts ventilating and then just trim in until it quits ventilating.
RPM _________ Speed ______ GPS ______ No. of people ______ Gal. Gas ________
8. Make, model, diameter, pitch, number of blades and whether SS or aluminum prop and does it have a hydrafoil, dolefin or trim tabs
9. Has your motor been tuned up lately and have you checked that the carburetor butterfly is opening all the way by just pushing the throttle at the helm and not at the carburetor, checked compression, and looked at the plugs and checked spark as well as timing and advance, all of the foregoing could be the reason your prop is not attaining full RPM
10. What problems are you trying to cure or what are you looking for the boat to do that it is not doing the way you think it should or to your expectations and does the prop show any damage that you can see

REMEMBER, The numbers I give you will be NO better than the information you give me. I use a computer program I designed to find the correct pitch and then knowledge to pick a Better prop design for your boat.

The only thing I ask of you is to come back and give me a report of WOT RPM and speed for my database.



H

 

[300sflyer]

Re: More HP / Changing Pitch

Thanks for your help!

Here is the info:

1- 1995 Chaparral Signature 31
2- 31' [plus bow pulpit] 10'-8" beam, and 9750 lbs dry.
2A- Unknown
3- 4 adults, tank size is 150 gallons. General cruising, looking for best mileage.
4- 2009 Merc 357 S/B 4600-5000 RPM.
5- 325 HP each, with Bravo II 2.2 gear ratio.
6- Unknown
7- At 4000 RPM, [not WOT] GPS speed was 30 MPH. 2 people, 140 gallons gas on board, and full fresh water tank.
8- Merc 17-3/4x21 3 bladed aluminum props, with trim tabs on boat.
9- Engines are new and function as they should.

Thanks,

 

[hwsiii]

Re: More HP / Changing Pitch

I am looking for the speed you had with the old motors, Not the new ones. What was the old WOT RPM and speed.



H

 

[300sflyer]

Re: More HP / Changing Pitch

I did not test it myself, but according to the previous owner, it was 31 knots, at 4500 RPM.

Thanks,

 

[hwsiii]

Re: More HP / Changing Pitch

Flyer, if those numbers are correct then you should be able to do right at 41 MPH at WOT, with an effective prop pitch of about 18.6".



H

 

[300sflyer]

Re: More HP / Changing Pitch

Thanks for the info.

If I recall, 40 MPH is about 35 knots. I assume this is with the new engines? Are the current 21" props effectively 18.6"? Where would I end up at with 25" of pitch in SS?

Thanks,

 

[hwsiii]

Re: More HP / Changing Pitch

Flyer,

If I recall, 40 MPH is about 35 knots. I assume this is with the new engines? Are the current 21" props effectively 18.6"? Where would I end up at with 25" of pitch in SS?

Yes, the 41 MPH is with the new engines.

The 21" pitch are currently 18.6" effective pitch. I can't think of any possible SS prop in a 25" pitch, that will fit a Bravo II drive, that will get anywhere near the correct RPM with that pitch.

But, I will have to say I am NEVER wrong, UNLESS I am breathing.

Don't buy anything until we see how the new motors do with the old props at WOT.



H

 

[300sflyer]

Re: More HP / Changing Pitch

I was considering these props.

http://www.mercurypartsexpress.com/us/index.php?target=products&product_id=193692

I would like to run the boat at about 30 MPH at about 3000 RPM for better fuel economy. I'm not really concerned about the whole shot. Do you think this is achievable?

Thanks,

 

[hwsiii]

Re: More HP / Changing Pitch

Mike, No I do not believe that is a viable option. Because I believe that when you lug the motor to the point that I think these props will lug it your gas mileage goes way down. Just because you are turning less RPM with more speed does not mean that you are getting better gas mileage, because you will never be able to attain the WOT RPM that you should be able to at 5,000 RPM with this prop, and thus the motor is being strained at all RPM, and will use more gas at the lower RPM settings. You will also need a set of fuel flow gauges to know how well you are doing with your MPG, to verify the advantages of doing this.

BUT, there is another very knowledgeable gentleman on here that disagrees with me on this theory, and I recommend that you talk to him about this and the theoretical associated advantages of this. He might very well agree with you and what you want to do.

His name is 45Auto, and you can send him a PM or just change your Title and add his name to it, and I am sure he will see it and gladly try to help you.



H

 

[300sflyer]

Re: More HP / Changing Pitch

Thank you very much for all the information! I will contact him to get his opinion as well.

When I spoke to the folks at Mercury Marine, [I know the Service Manager here in Canada] he stated that going from 21 to 25 pitch would drop the WOT RPM by 5-600. Given that I have gone from 260 to 325 HP on each engine, I figured they would turn the 25" props at about 4600-4800 RPM range at WOT.

I guess there is only one way to really find out.

 

[hwsiii]

Re: More HP / Changing Pitch

Mike, I would appreciate it very much if you come back and tell me the results that you get.



H

 

[300sflyer]

Re: More HP / Changing Pitch

I will indeed.

Bare in mind it may be a month or so before I can run the new engines at WOT. The weather here is not very good yet, [ it's suppose to be 5C / 40F tomorrow afternoon with possible snow flurries] and I need some more time on the engines first.

Thanks,

 

[hwsiii]

Re: More HP / Changing Pitch

Mike, I understand and I will be waiting.



H

 

[QC]

Re: More HP / Changing Pitch

I don't think that 45 Auto disagrees about lugging or propping that will not allow you to reach recommended WOT RPM range . . . What he points out, and laws of physics and data confirm, that in fact lugging (as used in this thread) is in fact more fuel efficient. BUT DON"T DO IT!

With that said, if these are factory 357 Bravo 4V from Merc reman they are rated at the crankshaft and should be considered a 300 at the prop.

FWIW, there were no 5.7 260s in '94 or '95. They were rated 250 at the Propshaft. The point is, I believe you have gone from 250s to 300s if we are comparing apples to apples. That is extremely important if we are trying to make prop recommendations through a paper exercise . . .

Personally I am thinking 22 or 23 inch. No data, no nuttin', just gut feel going from the existing 21s.

 

[300sflyer]

Re: More HP / Changing Pitch

QC,

You are correct, at least partly anyway.

I consulted my OEM Merc Service Manual, [#17] as well as calling Merc to confirm my findings below.

The original 5.7 4 barrel carbed engines, were rated at 235 HP at the prop. I would assume that would translate to about 260 HP at the crankshaft.

The new 357s would indeed be 300 HP at the prop shaft. That means an extra 65 HP at the prop, for each engine. My "gut feel" tells me that going from 21" to 23" would still not be enough prop at WOT. This is why I'm leaning towards 25".

 

[TilliamWe]

Re: More HP / Changing Pitch

...

The original 5.7 4 barrel carbed engines, were rated at 235 HP at the prop. I would assume that would translate to about 260 HP at the crankshaft.

The new 357s would indeed be 300 HP at the prop shaft. That means an extra 65 HP at the prop, for each engine... This is why I'm leaning towards 25".

Maybe a cupped 23, if it's not quite enough, but I doubt 25s will be necessary.

Of course, we'll only know when you are able to test the boat with its current set up. Waiting anxiously for that.

 

[300sflyer]

Re: More HP / Changing Pitch

I have 6 hours on the engines now. Not much I know, but the weather has not been great. Hope to put some more time on them this coming weekend.

I hit 4400 rpm just briefly for a second or two this past weekend. Did not stay there long enough to get to max speed at that RPM. Still lots of movement left on the throttle levers at 4400. At a steady 3700 rpm, the speed was 30 MPH on the GPS.