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correct prop

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wizard43844

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i have a mercruiser 120 with alpha outdrive on a 17.5 foot glasstron boat, it labors to come up on plane, i have a 15 inch prop with a 17 pitch, do i need more pitch or less to get on plane quicker, i'm not interested in speed, just getting on top as soon as i can
 
It sounds like you are over-propped.You should be able to reach max operating rpm plus (200 rpm more) if you are running the correct prop.If you are not getting max rpm ,and I doubt you are,the generally accepted approach to finding the correct pitch is:200 rpm per inch of pitch.I run 15X15 on my late model 130's.With that in mind,find your current wot rpm and subtract that from manufacturers listed wot rpm and reduce your pitch by 1 inch for every 200 rpm you are not attaining.This is not an exact science.Hull condition and weight distribution can have quite an affect on performance.
Good luck and I hope this helps.
 
kimcrwbr1 said:
How well is the engine tuned up, If you have access to different pitch props trial and error is the best way to size the prop. How you load the boat how clean is the hull and is it trimmed properly are all factors. Less pitch equals faster hole shot and higher WOT rpms. Confirm the engine is in top condition, do a compression test and spark gap test, check the progressive advance curve on the distributor. Did you check/change the gear oil and buff the hull and is the prop in good condition.
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Man, Kim....... you took most of the words from my mouth. :D
Ditto Kim...... you cannot perform an accurate WOT RPM test, unless all these ducks are in a row. Align these ducks, then begin to try propellers.

As for the 200 rpm per inch of pitch....... as a very general rule, this may work out to be close, or even close enough.
More technically presented, this is somewhat inaccurate without factoring final drive ratio, prop diameter -vs- pitch, etc.

.
 
Tell me the boat's loaded weight and I'll insert it into a spreadsheet to get you in the ballpark. A 4 blade prop will greatly improve the hole shot but slow down a little on the top end. As the guys stated the engine should be in tip top running condition before proceeding w/prop swaps. I have a few props I'll never use and can't sell...alum and SS...for just the shipping fee and a few bucks. How many splines on the prop shaft?
 
Kim a perfect reply.

Welcome aboard !

Please post back the compression numbers performed on a warm engine with all the plugs out and the throttle fully down.

Keep track of what plug went in what hole and write down the numbers so you have a base to compare down the road.

Use a screw in type gauge seen one real cheap at harbor freight.
 
guyjg said:
Tell me the boat's loaded weight and I'll insert it into a spreadsheet to get you in the ballpark. A 4 blade prop will greatly improve the hole shot but slow down a little on the top end. As the guys stated the engine should be in tip top running condition before proceeding w/prop swaps. I have a few props I'll never use and can't sell...alum and SS...for just the shipping fee and a few bucks. How many splines on the prop shaft?
Click to expand...

don't know boats weight, but i have a 15X17 prop that has a 1 inch hole with 15 splines on mercruiser outdrive......thanks for reply
 
Wizard, are you certain that this propeller is correct for the Merc drive?
Some props will interchange with regard to the shaft splines.
Your Merc drive requires thru-prop-hub exhaust.... is the propeller correct for this?


.
 
RicardoMarine said:
Wizard, are you certain that this propeller is correct for the Merc drive?
Some props will interchange with regard to the shaft splines.
Your Merc drive requires thru-prop-hub exhaust.... is the propeller correct for this?


.
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am i sure it's correct prop for the outdrive ??? no !!!! but it has the exhaust through the prop...and i ran it all last summer, i just think it needs something to get it up on top quicker, as it labors a bit...the motor does have a high rpm miss on occasion, don't have a tach to know where but i can live with that if it will just come up on top quicker...i just didn't know if i was under proped or over proped,...i think from the replies, i need to drop back to something like a 15X15
 
kimcrwbr1 said:
check the progressive advance curve on the distributor.
Click to expand...
No gasoline Marine engine will perform well if the progressive advance and/or the total advance is not correct.
With the assumption that your ignition system is mechanical advance, often the flyweights and/or spring return system becomes rusty or corroded.
If the flyweights are restricted in their travel, and/or if the return springs have become compromised, the system will not offer the correct advance.
If the system can't throw the correct spark lead to each cylinder, the engine will not produce any useful power at the higher RPM range.

Throw your timing light on it.... check BASE advance.
Next, increase RPM, jot down the advance at an array of RPM, including the RPM where the advance is "Full In", and see if it meets the OEM specs.

It's basically that simple.


If it proves to NOT be ignition related, then you have at least eliminated this from your P of E list.

It's a win for you either way.



.
 
I doubt the advance is wrong, What may be wrong is the advance is not working.

the 120 distibutor is not one that would see much tweaking............

maybe the springs are bad or the counter wieghts may not be moving if it is a timing issue at all.


A lot more info is needed here

1. year of boat

2. Aproximatly how may hours if that can be obtained??

3. As suggested the condition of the tune (plugs [what type and part number and gap] points, what dwell or gap, timing setting base and at max rpms.

4. is it possible a incorrect outdrive may have been installed (or upper gear case) that changed the gear ratio? Can the ratio be determined if you look?


5. get a tach for a 4 cyclinder and see what rpms the boat is spinning. It is the ONLY way to determine what prop is needed if one is needed.

6. Of course a compression check also may help in determining the motor health............to low and it will be a slug.........(below 140 psi +/- 10% would be of concern)


Nothing wrong with a 15 pitch prop bieng used, Also what condition is the prop he has now? Is it a rebiult or new, or is it a used one with damage?

jmho
 
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kghost said:
I doubt the advance is wrong, What may be wrong is the advance is not working.
Click to expand...
Wouldn't that be one in the same? :D :D

kimcrwbr1 said:
and work up a timing curve write down the base timing and idle rpm, every 500 rpm write down the timing up until the timing is all in and the rpm.
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I was suggesting this earlier and may not have been clear.... so yes, I fully agree.

Not all Marine engine curves are available in graph form. I just looked for a 120 curve graph...... no dice!
So if he plots this out on paper, he may not be able to find a curve graph to compare his numbers to.
However, he should be able to find the OEM specs, of which will hopefully offer not only BASE advance, but the total advance as well. These numbers can be compared.

The important part will be in seeing if the curve progresses in a rather linear fashion, and like you guys have also said, the "full in" amount, and "full in" rpm.

The TA and TA rpm will be the more important of the two.
If BASE needs to be adjusted to accommodate TA, then within reason, BASE can become sacraficed some.

If way off, and if this can't be adjusted out, then take it to a shop and have them run it on an old school Sun or Allen machine and make the corrections to it.

.
 
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Has anyone suggested to check the drive ratio? 15x17 on a 2.5Ltr in a 17' boat does not sound like too much wheel. If the drive ratio is correct, the engine checks out and the drive will trim all the way down, maybe you need to check the weight of that boat to see if it is water logged.
 
No problem..........so few to read these days who really is paying attention........it took me about 30 minutes to finish my post.....had to think and it is not so easy this time of year for me anyway............
 
Engine Won't Reach Operating RPM. Check

1. Fuel condition. Type and Octane possibly old fuel
2. Propeller pitch or diameter, damaged blades
3. Restricted fuel pickup tube or anti siphon valve Fuel System Test
4. Crankcase oil volume, high oil level can cause aerated oil and lifter collapse
5. Marine growth on hull and outdrive
6. Wrong gear ratio in outdrive
7. Restricted carburetor air intake (clogged flame arrestor)
8. Restricted exhaust system (broken exhaust shutters/flappers) in engine transom shield or drive
9. Poor cylinder compression Compression Test
10. Carburetor defective, or wrong type.
11. Fuel pump pressure and vacuum
12. Boat overloaded, improperly loaded, or improperly trimmed.
13. Engine Overheating
14. Engine timing and ignition system operation
15. Remote control cables and linkage for proper travel to open throttle plates fully.

Above copied and pasted from another website Don s is the author.

Added by me:
16-Valves need adjusting.
 
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