what rpms are normal

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87merc150

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My boat runs really well and top speed is about 55mph but when i look at my tac at that speed it is burried at the 6000 rpm. I'm scared that is going to blow the engen so i have only ran it like that a few times.
I have an 87 18ft skeeter bass boat with a 150 merc on it. It also has a whale tale. Do I need a different pitch prop or should I be ok. Thanks
 
j_martin

j_martin

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Re: what rpms are normal

ClassyGlassy said:
Pitch up to get RPMs down..........
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Which 150. The V6 engines like to run about there. It's starting to back off electrically if the advance module is still on it.

I run my XR4 at 6200 with a light load, and it settles down when I take on water in the live wells. It's happy at 6200, however.

What prop are you running. If you run aventilated prop, you can ditch the wale tail. Trimmed in, with a high five ventilated prop, when I hit the throttle it spools up to about 3000 rpm and catapults out of the hole.

hope it helps
John
 
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87merc150

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Re: what rpms are normal

it is a v6. I don't know what a aventilated prop is but I looked at my prop and the numbers are 43 89856A 4 22p. It is stainless steel but other than that I don't know a lot about it.
 
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sschefer

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Re: what rpms are normal

Like John say's, it all depends on which 150 you have. We all get confused with Merc's max RPM methodology. The bottom line for me is that the stock upper main bearing is rated for 6000 RPM. But, that's just what I use as a guidline for my motor. The XR series motors might have a better bearing, I don't know for sure. Merc say's my 91 carb'd stock motor should run between 5200-5800 and I'd feel safe recommending that to you also.

I prop mine to 5400 RPM heavily loaded with gear so I have some wiggle room if I decide to unload it and play. It's a heavy 18' tinny and runs at about 45 WOT.
 
j_martin

j_martin

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Re: what rpms are normal

87merc150 said:
it is a v6. I don't know what a aventilated prop is but I looked at my prop and the numbers are 43 89856A 4 22p. It is stainless steel but other than that I don't know a lot about it.
Click to expand...

A ventilated prop has holes in the barrel in front of the blades that let exhaust spoil the bite at slow speeds. As the prop speed increases forward through the water, the slip stream effectively closes the holes. Effect is almost like a high rpm centrifugal clutch.

I suspect you have a bone stock XR2 or XR4, and in that case propping up 2 inches in pitch should bring your rpm down. Your peak power would be around 5700 or 5800 rpm. At 6000 rpm yer losing torque fast.

In any case, the V6 merc don't like to be lugged. Keep your worst case WOT rpm above the minimum. That's more important than the maximum.

That boat should run 60 or so, unless it's rigged heavy or waterlogged. There's no way there should be a wale tail on it. That's an indication of poor rigging or operation.

The first thing that's likely to wear out on mine would be the trim motor. With the engine tucked in tight, it launches like a sling shot, and trimmed out to where the bow is slightly flying, and the prop is surfacing, just before it loses water pressure, it does low 60's with an 18' Tracker/Nitro hull.

hope it helps
john
 
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87merc150

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Re: what rpms are normal

Thanks for the help guys. So if I want to keep my max rpms somewhere around 5600-5800 I should bump up to a 24 pitch prop and get rid of the wale tale. Is it better to get the ventilated prop also.
 
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sschefer

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Re: what rpms are normal

87merc150 said:
Thanks for the help guys. So if I want to keep my max rpms somewhere around 5600-5800 I should bump up to a 24 pitch prop and get rid of the wale tale. Is it better to get the ventilated prop also.
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Actually, the first thing you want to do is make sure your tach is right. That might sound silly but I've seen a ton of them setup wrong. I check mine against my timing light that has a built in tach.

With your current prop you're burying the Tach so you really don't know where it is. Propping for your boat should be about 23p with a good biting performance prop. If you run a recreational multi-purpose prop you can probably jump up an inch and if you go 4 blade then drop down an inch.

Like John say's get rid of the Wally World whale tale, it might help add down force and assist with trailer control on the freeway but it's hurting you on the water. A well setup Skeeter is one of the faster Bass Boats on the water. I'm looking at a 2000 18.5 Team ZX as a home for my go fast 200 that I'm building.
 
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87merc150

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Re: what rpms are normal

I have taken the wale tail off. Now I just need to wait for it to stop snowing here in Idaho so I can get it back in the water and test it out. I am going to keep the prop I have now but if I do upgrade should I go with the 3 blade prop or the 4 blade. I was looking at some of the props here on iboats and I don't see them saying there is a good biting prop or a recreational prop so how can I tell the difference.

Thanks for the timing light idea I will look into that.
 
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sschefer

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Re: what rpms are normal

87merc150 said:
I have taken the wale tail off. Now I just need to wait for it to stop snowing here in Idaho so I can get it back in the water and test it out. I am going to keep the prop I have now but if I do upgrade should I go with the 3 blade prop or the 4 blade. I was looking at some of the props here on iboats and I don't see them saying there is a good biting prop or a recreational prop so how can I tell the difference.

Thanks for the timing light idea I will look into that.
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Understanding props is somewhat of a black art. There is a book put out by Merc that explains it very well. If you go to Mercs website there is a prop guide that you can use. Merc sells some of the best props around but not always at the best prices. Find the prop for your boat with the merc prop guide and then compare it's spec's to other manufacturers to get the best bang for your buck. Don't be hasty with your decision, props are not cheap.

Typically you would use a 3 blade prop with your hull. A four blade is an option and the newer designs are smoother running and nearly equal in performance.
 
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87merc150

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Re: what rpms are normal

That's what I was thinking. You don't want to buy two or three props and find out the one that fits your boat the best. I'm guessing my prop is stock so is there a better option out there for my boat. The entire boat is still stock so I would guess that the prop on it should be ok. But if I could gain a few miles per hour and not run the motor to hard I would be interested. As J_Martin said my boat should run about 60mph. I can go about 55mph so what am I missing to run my boat safe and get up to its potential. I am using a gps to see how fast I'm going.
 
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sschefer

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Re: what rpms are normal

87merc150 said:
That's what I was thinking. You don't want to buy two or three props and find out the one that fits your boat the best. I'm guessing my prop is stock so is there a better option out there for my boat. The entire boat is still stock so I would guess that the prop on it should be ok. But if I could gain a few miles per hour and not run the motor to hard I would be interested. As J_Martin said my boat should run about 60mph. I can go about 55mph so what am I missing to run my boat safe and get up to its potential. I am using a gps to see how fast I'm going.
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With your boat bone stock and setup to please a wide audience you probably would be well served in tweaking the setup. Things like Jack Plate height and offset can be experimented with as well as trim, prop and even lower unit styles. These all cost money although you may already have an adjustable jack plate that you can raise or lower to adjust. There is a qausi rule of thumb for setting engine height.

Set the cav plate on the engine level with the bottom of the hull then raise it 1/4" for every 1" of jack plate setback. You still must never exceed the point where you water pickups are not in the water. This is where lower units with low water pickups come into play. Additionally, you never want the prop to break the surface unless it is what is known as a surfacing style prop.

Proper setup may improve the top end speed. Nothing is guaranteed. You have to be careful because all performance comes at some other cost. If you run with your stock lower unit too high for too long you can end up with excessive heat build up and dramatically shorten it's life.

Have fun, just know the caveats. If it doesn't seem like a good idea it probably isn't.
 
ClassyGlassy

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Re: what rpms are normal

sschefer said:
Like John say's, it all depends on which 150 you have. We all get confused with Merc's max RPM methodology. The bottom line for me is that the stock upper main bearing is rated for 6000 RPM. But, that's just what I use as a guidline for my motor. The XR series motors might have a better bearing, I don't know for sure. Merc say's my 91 carb'd stock motor should run between 5200-5800 and I'd feel safe recommending that to you also.

I prop mine to 5400 RPM heavily loaded with gear so I have some wiggle room if I decide to unload it and play. It's a heavy 18' tinny and runs at about 45 WOT.
Click to expand...

My '76 1500, 155hp, says to run it at 5800-6300 RPM. It tells me to make sure I try to get close to the top end empty capacity so that when I'm full capacity I have enough prop to run around the lower RPM range. The manual states that running under the appropriate RPM range can damage the motor as well as cause problems with performance. It also states that 1p difference can cause as much as a 500 RPM change either direction.
 
ClassyGlassy

ClassyGlassy

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Re: what rpms are normal

87merc150 said:
As J_Martin said my boat should run about 60mph. I can go about 55mph so what am I missing to run my boat safe and get up to its potential. I am using a gps to see how fast I'm going.
Click to expand...

I already stated it:

Pitch up to lower your RPM and go faster!
 
ClassyGlassy

ClassyGlassy

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Re: what rpms are normal

sschefer said:
Typically you would use a 3 blade prop with your hull. A four blade is an option and the newer designs are smoother running and nearly equal in performance.
Click to expand...

4 blade prop gives you more punch out of the hole to get onto plane faster and it will allow for a much smoother top end ;)
 
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hempfarmer

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Re: what rpms are normal

87merc150 said:
My boat runs really well and top speed is about 55mph but when i look at my tac at that speed it is burried at the 6000 rpm. I'm scared that is going to blow the engen so i have only ran it like that a few times.
I have an 87 18ft skeeter bass boat with a 150 merc on it. It also has a whale tale. Do I need a different pitch prop or should I be ok. Thanks
Click to expand...

Your engine will last much longer if you can avoid running it at 6000.
 
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sschefer

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Re: what rpms are normal

ClassyGlassy said:
4 blade prop gives you more punch out of the hole to get onto plane faster and it will allow for a much smoother top end ;)
Click to expand...

And therin in lies the black art.. I've learned to be very careful about making recommendations for props.

I run a 3 blade Merc Silverado (general purpose) 14.25x19p on my 18' Tinny f/s. It's heavily loaded with batteries, fishing gear and my fat rear. I'm up on plane in under 3 seconds and my top speed is 45mph. Of course I'm also 40 hp over max rated for the boat and I've got trim tabs and a hydraulic jack plate. I have the 19 for the flat lands and a 17 for high altitude. That's 1000.00 worth of props for one boat and that's enough. If I spent a a couple of more thousand I might squeak a little more peformance out of it but why.. I'd rather pay a weeks rent on a cool cabin in the mountains some where and fish until my arm fell off.
 
Faztbullet

Faztbullet

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Re: what rpms are normal

The prop calculater shows you are very close to best set-up( 2.1 gear ratio,22pitch,10% slip, 55mph=6160 rpm) if you go up a pitch and lose 500 rpm the speed drops to 52.5 mph, to get 60mph you will need to turn 6400rpm with a 23p
 
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walleyehed

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Re: what rpms are normal

Many opinions here...I'll add mine. The engine does not know the difference in 5600 vs. 6000rpm. It may be going down the back side on torque but if less wetted surface is a result of the correct prop, less torque is needed. Also, you want to be close to the top of your rpm range WITH a normal load so you are never lugging it. We've proven this time and again testing props with cylinder head temp and exhaust gas temp.
Generally if set-up for top range the cylinder head and EGT are both lower. Keep in mind these are carbed engines with manuals dated near their manufacture date. Our fuel today is nothing like it was 10-15 yrs ago. We've found in about 99.9% of the 2 strokes tested, the top recommended part of the range is "target" rpm on todays fuel...a lot less problems. We also found the CHT went up quite a bit when set up for max rpm in the middle...anything less shows a much higher EGT and CHT.
 
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sschefer

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Re: what rpms are normal

walleyehed said:
Many opinions here...I'll add mine. The engine does not know the difference in 5600 vs. 6000rpm. It may be going down the back side on torque but if less wetted surface is a result of the correct prop, less torque is needed. Also, you want to be close to the top of your rpm range WITH a normal load so you are never lugging it. We've proven this time and again testing props with cylinder head temp and exhaust gas temp.
Generally if set-up for top range the cylinder head and EGT are both lower. Keep in mind these are carbed engines with manuals dated near their manufacture date. Our fuel today is nothing like it was 10-15 yrs ago. We've found in about 99.9% of the 2 strokes tested, the top recommended part of the range is "target" rpm on todays fuel...a lot less problems. We also found the CHT went up quite a bit when set up for max rpm in the middle...anything less shows a much higher EGT and CHT.
Click to expand...

Absolutely Agree 100%... gotta find the sweet spot.. Takes a lot of trial and error. You may still be able to find a dealer with a try a prop program although they are few and far between these days.
 
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