Mercruiser WOT?

tank1949

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So what is a marine WOT definition, now?
I recently purchased an older SR with (2) 383 carbureted stroker motors and Bravo 1 drives. Boat is a 270 Amberjack. During sea Trial with three adults and a full fuel tank of 150+ gal of gas, owner got motors to 6300 RPMs. The MCs motors/throttle levers would NOT go any faster/forward, but GPS indicated 48 MPH. My years of boating experience indicates that the props (SS Mirage) are not pitched correctly and should be re-pitched so that boats runs about 5000 RPMs (max), if boat?s throttle arms are pushed as far forward as possible.
Back in the day, each inch of pitched added or reduced from prop, usually meant an increase or decrease of 200 RPMS on motors.
Older STD 350 (5.7) Chevy carbureted MCs were generally propped to 4200-4400 RPMs with a load on them/boat. But, few boaters ever ran their motors at those RPMS.
A buddy disagrees. He indicated that WOT is the maximum SUSTAINED RPMs (5000) but boat can do very much faster, and in this case 6300 RPMS.
To me WOT means that the motors/boat/weight combination will not go faster than a prescribed RPM when throttle levers are all the way forward.
Perhaps terminology has changed or I was incorrect in my earlier assumptions. Or, he and I are both using incorrect terminology?

Props are counter rotating and cupped. Do I need to add pitch in props and how much?
 

shaw520

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Your thinking is correct,... I would not allow these motors to rev above 5k rpm,...so it seems you could more pitch to achieve a 'wide open throttle' (WOT) rpm of 4700 -5000. Some guy will purposely 'under pitch' their props to gain 'out of the hole' performance,.. but will not allow to rev past 4800-4900 by not going to WOT.
 

bman440440

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IMO yes you need more pitch in your props... that is if they where built to standard specs... now being that they are 383's that have been built by and engine shop more than likely it is possible that they where built for higher RPM's but not likely... now for the WOT question... for boats it is the MAX rpm you can get out of them not sustained RPM's... so under normal load in your boat IMO I would prop them for 5000 rpm's... now for the wrench in the works... if they where built for hi RPM's you need to find out what they where built for and prop accordingly. that's my 2c and remember you get what you pay for :laugh:
 

Bondo

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My years of boating experience indicates that the props (SS Mirage) are not pitched correctly and should be re-pitched so that boats runs about 5000 RPMs (max), if boat?s throttle arms are pushed as far forward as possible.

Ayuh,.... Yer right, nothin's changed in marine motors,... prop for 4800/ 5000 rpms at Wot, average loadin',....

Unless the PO put ridiculous crazy cams in those motors, 'n had a full-on balance job done,.....
I'd think 6300 rpms is way past design limits,...
 

tank1949

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Ayuh,.... Yer right, nothin's changed in marine motors,... prop for 4800/ 5000 rpms at Wot, average loadin',....

Unless the PO put ridiculous crazy cams in those motors, 'n had a full-on balance job done,.....
I'd think 6300 rpms is way past design limits,...

He added 12-210-2 Comp cam kit 218/218-.454/.454 and fully balanced crank/ flywheel/rods/pistons. He provided me with build information on motors! Still, 6300 RPMs is way too high.
 

harleyman1975

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Your getting good advice here! I would be leery of a boat that revs that high. Are we sure the tachs are accurate (set correctly)? 6300 is really screaming.
 

Bondo

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Ayuh,.... I'm guessin' the torque falls off long before 6300,...
 

shaw520

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A stroker cranked SBC meets its max torque curve at aprox 4400rpm and begins to fall off after that.
 

Scott Danforth

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WOT is wide open throttle.

If the 383's were built with the right parts, they would turn 6300 RPM all day long.

Depends on cam, intake, and balance that was used for the build.

383 strokers were not factory fair in the SR amberjack. So the motors were most likely built

Don't confuse stock cast crank motors with stock cast conn rods and cast pistons with a crappy factory balance limiting RPM to a motor built With forged internals, blue printed builds and a fully balanced 3rd rotating assembly

The limiting RPM may be the bearings in the drive which may only be 6000 rpm. Depends on what was done to the drives

However 6300 RPM may be a tad high, for the drive, but not the motor. Depends on the build.
 

tank1949

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WOT is wide open throttle.

If the 383's were built with the right parts, they would turn 6300 RPM all day long.

Depends on cam, intake, and balance that was used for the build.

383 strokers were not factory fair in the SR amberjack. So the motors were most likely built

Don't confuse stock cast crank motors with stock cast conn rods and cast pistons with a crappy factory balance limiting RPM to a motor built With forged internals, blue printed builds and a fully balanced 3rd rotating assembly

The limiting RPM may be the bearings in the drive which may only be 6000 rpm. Depends on what was done to the drives

However 6300 RPM may be a tad high, for the drive, but not the motor. Depends on the build.

According to invoice on work and parts put into these motors, excluding performance Edelbrock high-rise and 750 cfm carbs, these motors were completely balanced all the way through (per invoice), so they should not easily come apart. But, I am only used to or at least understanding of standard MC motors. This performance stuff is out of my league.

I am no 100% sure on accuracy of tacks. RPMs are displayed via a Scan Flow 9000 device. After I do some cosmetics, I plan on attaching a multi-meter to coil and see how those rpms compare to Scan Flow's. If they are the same, I will see if local prop shop can add 5" to each of the props. Scan Flow implies that their device can be calibrated, but I have not found a way to do that if my multi-meter tacks indicate different RPMs.
 

Bondo

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I will see if local prop shop can add 5" to each of the props.

Ayuh,..... Ya might Try some props 1st, a prop shop can only alter a prop a couple inches bigger or smaller,.... Max,...

For a 5" difference, the props start out as different castin's,....
'n 5" is a HUGE change,....
 

shaw520

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5" is a drastic change,.. best to go one at a time until desired WOT rpm is achieved,.. In most cases you are better off slightly under propped than you are over propped,... you can hold back rpm's manually with the sticks,.. but over propping (depending on HP) can lead to over torquing the outdrive....costing you an outdrive
 

Bondo

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but over propping (depending on HP) can lead to over torquing the outdrive....costing you an outdrive

Ayuh,.... The Bravos could probably handle it, but Detonation could quickly burn down the motors,....
 

Scott Danforth

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I wouldn't go 5". I would go plus 1 on the number of blades and 2" max. Bring max RPM down to 5500 - 5800

From the brief description, the motors were made to run.

Then use common sense with the throttle
 

shaw520

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^^^ Good advice ^^^,.... my 230 SR had a very strong running 5.7, it would over rev if I went full throttle,... I just never did,... sure made 0-5000k rpm a real pleasure and much easier on the old Alpha 1,... plenty of power out of the hole for sure. I personally prefer it that way than the other way.
 

HT32BSX115

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So what is a marine WOT definition, now?
I recently purchased an older SR with (2) 383 carbureted stroker motors and Bravo 1 drives. Boat is a 270 Amberjack. During sea Trial with three adults and a full fuel tank of 150+ gal of gas, owner got motors to 6300 RPMs.

Most marine manufacturers specify a range of "WOT" RPM. They also suggest selecting props (propping) to achieve the UPPER RPM when at full throttle(WOT) when the boat is loaded typically (where you'll usually load it) , operating at the pressure altitude where you boat, to get best over all performance, but under no circumstances at an RPM LESS than the lower RPM in the range.

If you see an RPM substantially higher, (and there is NO RPM liming circuitry present) Check tachometer(s) FIRST. they are frequently in error and the error is usually MORE at higher RPM.

Once you determine accurate tach(s) ensure that you know the drive ratio and actual boat speed(GPS) do not trust pitot type speedometers (unless you verify!)

When you know all the above info, (and If running too SLOW, you know the boat is NOT over loaded or the hull is covered with marine growth, the trailer is still under the boat, or you're dragging the anchor!! .............) ONLY THEN should you start messing with props!

Cheers,

Rick
 

tank1949

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Most marine manufacturers specify a range of "WOT" RPM. They also suggest selecting props (propping) to achieve the UPPER RPM when at full throttle(WOT) when the boat is loaded typically (where you'll usually load it) , operating at the pressure altitude where you boat, to get best over all performance, but under no circumstances at an RPM LESS than the lower RPM in the range.

If you see an RPM substantially higher, (and there is NO RPM liming circuitry present) Check tachometer(s) FIRST. they are frequently in error and the error is usually MORE at higher RPM.

Once you determine accurate tach(s) ensure that you know the drive ratio and actual boat speed(GPS) do not trust pitot type speedometers (unless you verify!)

When you know all the above info, (and If running too SLOW, you know the boat is NOT over loaded or the hull is covered with marine growth, the trailer is still under the boat, or you're dragging the anchor!! .............) ONLY THEN should you start messing with props!

Cheers,

Rick

Boat would do 48 MPH per GPS and tacks read 6300 with throttles fully forward. I plan on testing tacks FIRST! Thx!
 

Scott Danforth

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Tank. Talk to the PO to find out exactly what went into the build of your 383's..

Nothing Mercruiser applies as the motors were custom built.

You need to know if your tachs are accurate.

You need to know which cam is in each motor, and if it's setup straight up or 4 degrees advanced or 4 degrees retarded.

It would also be helpful to know which crank is in there. A cast stroker cranks is ok, a forged is better.

Armed with all that info, you can then pick a prop. Unfortunately, since your motors aren't stock, most of the guidelines are no longer valid
 

harleyman1975

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That thing must get up and go with that set up! I'm coming around (to not walking away so fast) now that I am hearing more of the story.
 
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