Dumb Question About Swapping Props

[jayhanig]

How do you normally go about homing in on the proper prop? I know my 1997 GM 3.0 liter Mercruiser ought to go to 4600-4800 RPM at WOT. I also know it actually will turn faster than that if I were to allow it. I am leery of overrevving the engine and as a result don't really know how far out of whack my present selection is.

Right now I'm running a 3 bladed SS Solas 14.25" X 19.

What happens when you buy a prop and it turns out it's not the correct pitch? Will a dealer normally take it back? Do you just start a collection of $350 props out in the garage? And how far over desired redline is acceptable for a short burst to determine my real WOT?

 

[tpenfield]

Re: Dumb Question About Swapping Props

. . . Right now I'm running a 3 bladed SS Solas 14.25" X 19.

What happens when you buy a prop and it turns out it's not the correct pitch? Will a dealer normally take it back? Do you just start a collection of $350 props out in the garage? ?

they go on eBay . . .

 

[emilsr]

Re: Dumb Question About Swapping Props

Some dealers offer a demo program; try before you buy for a small fee. I've seen this with both Mercury dealers and local prop shops. It's worth asking about.

I wouldn't recommend turning your engine faster than it's rated (4800 rpm) for any length of time. That said, don't rely upon the accuracy of the tach on the dash.

 

[jestor68]

Re: Dumb Question About Swapping Props

4800 rpm is the max continuous rpm. I doubt that your 3.0L will go much over 5K rpm with the 19 pitch.

Go ahead and run it up for a few seconds to see where it tops out. I wouldn't go past 5500-5600 rpm; but I doubt that it will go that far anyway. Your target rpm range is 4400-4800 rpm.

We all rely on the onboard tach, unless we have reason to think that it's not accurate.

If it went to 5K rpm with a light load, I'm not sure that I would change anything. That way you'll have a little in reserve for when you want to carry more folks in there.

 

[Silvertip]

Re: Dumb Question About Swapping Props

A prop with too little pitch (as you apparently have) is not necessarily a bad thing. You have a throttle so you can use it to limit revs ate 4800 RPM. If you tow skiers or tubers then the extra grunt provided at holeshot is welcomed. However, If you don't do water sports, then the next step is to see just exactly how under propped the rig is. You do that by loading the boat with how it will be operated the majority of the time. Now make some WOT runs -- however brief they are to avoid scattering the engine. 5K for short blasts won't hurt is. If you get 5000 with that load, I personally would leave it alone. If it bothers you, increase pitch by one inch. Don't worry about diameter -- just make sure you are buying a prop for your engine. Just because it fits the shaft does not mean it is right. The hub must match the hub on the engine or exhaust goes around the prop rather than through it. Or there may be interference at the hub. Don't worry about diameter. That takes care of itself if you use an application chart. As pitch goes up, diameter comes down a tad and vice-versa. Remember, if you under prop a bit you can always throttle back. If you over prop you cannot throttle up because the engine is lugging and it simply cannot rev any higher. Besides, lugging an engine is worse for an engine than a couple hundred excess revs used carefully.

 

[jayhanig]

Re: Dumb Question About Swapping Props

Thank you, everybody. Your points are taken to heart.

What's frustrating right now is that it's started freezing here overnight so I've had the boat winterized (it lives on a trailer out in the driveway). I've already got three props... the original 14.25 X 17 aluminum; a 14.25 X 17 SS; and the current 14.25 X19 SS. I'll wait until spring to try to sell the 17" SS.

Managing overspeed with the throttle isn't alien to me. I used to fly turbocharged airplanes that could easily be overboosted on takeoff if I didn't pay attention during the roll.

 

[jestor68]

Re: Dumb Question About Swapping Props

Managing overspeed with the throttle isn't alien to me. I used to fly turbocharged airplanes that could easily be overboosted on takeoff if I didn't pay attention during the roll.

There ya go. As an experienced airman, you know the value of having some power in reserve. :joyous:

 

[eavega]

Re: Dumb Question About Swapping Props

I'll wait until spring to try to sell the 17" SS.

Maybe we should start an iBoats "Prop Swap" program. I recently sold my boat with the 70 HP Jonson on the back, and have been trying to sell my 17" P SS, and 19" P SS for that motor, while at the same time trying to acquire a 17P-19P prop for the Alpha 1 outdrive on the new boat (Currently runs a 20 P SS which is adequate for skiing or tubing, but wanted a better hole shot for wakeboarding). No dice. It would be nice to try different props on my new rig until I get what I want without having to worry about selling the rejected candidates!

Rgds

 

[emilsr]

Re: Dumb Question About Swapping Props

We all rely on the onboard tach, unless we have reason to think that it's not accurate.

Then let me be the first to tell you that you have a reason to doubt it's accuracy. They're more likely wrong than right...even brand new...and typically vary by 100-200 rpm at WOT.

Check it with an accurate "shop tach" and you might be surprised what you find.

 

[Home Cookin']

Re: Dumb Question About Swapping Props

But even if the tach is wrong, it's a consistent error as to the boat, so as long as it's just a hundred or two, not problem.

Around here we have several prop shops that sell refurbished props. They have allowed me to test props (although I kinda knew them) but i htink they'd do the same with a deposit or a refundable purchase.

A boat dealer/repair shop was having a "yard sale" and let me test about 6 props until I found one I liked. So look for clearances like that. Luckily for us we can still run the boats in the off season due to weather and that we use outboards (ahem); you wouldn't want to be asking for favors in the high season.

As for extras: we typically keep the aluminum prop that came with the motor for a spare when we switch to SS. Old SS props can be traded in to the prop shop. So when I junked a Johnson I kept the prop, and traded it towards refurbishing the prop on the Yamaha replacement motor.

when trying different props remember that SS and aluminum props of the same specs do not behave the same, primarily due to weight I understand. ALso, in testing my props, I found the diameter to be an important variable.

For the typical pleasure boater, don't try to over-engineer it. Even though I tried to keep things constant, I know that there were variables in each test run such as wind and current. Each test run lightened the load some by burning gas and I probably refueled durnig the process. Can't do much about water temps and salinity variables either.

Get what feels good and sounds good. Some people care about hole shot, some about top speed, I was looking for good performance at my normal cruising speed. For me, it's more important what it does at 30 mph. One prop was terrible at low speed. So I made a performance chart and noted speed at steps in RPM, as well as lowest planing point, as well as speed/RPM at WOT. But speed/RPM at WOT is the benchmark.

 

[emilsr]

Re: Dumb Question About Swapping Props

But even if the tach is wrong, it's a consistent error as to the boat, so as long as it's just a hundred or two, not problem.

Not exactly....

You need to turn the engine in a particular rpm range. Let's say that's 4600-5000 and you're reading 4700 so you think you're okay....but the tach is reading 200 rpm high so you're actually turning at 4500 which isn't good for the engine. I use those numbers as that's what the Livorsi tach on my panel reads; have seen similar inaccuracies with other test subjects. Some better, most worse.

Easy to verify with either a scan tool (MEFI engines) or a "shop tach".

I'd check it, but that's just me....

 

[jestor68]

Re: Dumb Question About Swapping Props

Then let me be the first to tell you that you have a reason to doubt it's accuracy. They're more likely wrong than right...even brand new...and typically vary by 100-200 rpm at WOT.

Check it with an accurate "shop tach" and you might be surprised what you find.

Ok; I'll bite. On what do you base your statement that the majority(?) of installed tachometers are inaccurate?

In 23 years in the business, I've found the vast majority of installed tachometers to be quite accurate.

 

[emilsr]

Re: Dumb Question About Swapping Props

Ok; I'll bite. On what do you base your statement that the majority(?) of installed tachometers are inaccurate?

In 23 years in the business, I've found the vast majority of installed tachometers to be quite accurate.

All I can tell you is the ones I've tested were rarely within 100rpm of actual, and the older they get the more inaccurate they (sometimes) become. I'm not "in the business" (whatever that means), just a guy who has owned, rigged and even raced a few boats over the past 30+ years. I'm not a mechanic and don't claim to be, but verifying the accuracy of a tach isn't exactly rocket science.

If you say the ones you've tested are mostly accurate that's great, I just haven't found that to be true, at least at higher rpms. It certainly does no harm to check. As they say; "your mileage may vary".

How are you testing them? It's pretty easy on the MEFI boats to compare the scan tool to the tach but you have to do this while running; they seem to be pretty accurate at idle.