Over prop? How to know?

tywiesz

Cadet
Joined
Dec 28, 2022
Messages
23
Hi,
situation is;
bought a boat with 3200rpm at WOT. Idle 450 so thought must be the gauge, replaced it and now idle 1200 rpm and at WOT 4800 rpm.s Is this boat over proped? coz it does not go to at least 5600 rpm.s? Or as it is a new boat to me (boat 10y old maybe less, previous owner not available to ask questions) how do I know if the engine actually can still provide/go 6000 rpm's? so, is it over prop or engine? Maybe both, I thought to this:
go full throttle and start trimming up a bit by bit till I see something,
so while trimming the outboard up my rpm's were increasing and increasing till total cavitation and big lose of a speed at 5600 rpm's . Is this the max what the motor can do? if this is 400 rpms short of 6000 rpms than that would have been 4800 +400= 5200 at WOT for current condition, is this still over prop?
What do you guys think? I noticed the prop got "lips" dunno how to call it, the prop blades outer ages a bit profiled to combat this boat easy cavitation. Should I un-profile the edges back to normal, than I would lose the water grip and maybe cavitate at 3000. There is hydrofoil plate installed to help the cavitation already. Grrrr, that is what happens with used boats, full of problems and mysteries, isn't?
cheers
T
 

racerone

Supreme Mariner
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Dec 28, 2013
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32,702
Can you read the name / make of the boat ?-----Can you post the make / power of the motor ?
 

tywiesz

Cadet
Joined
Dec 28, 2022
Messages
23
Can you read the name / make of the boat ?-----Can you post the make / power of the motor ?
Custom built 7.1 metre aluminium twin hull pontoon boat with side console remote steering. The boat was
completed around 2011 and identified with the hull serial # HANPCS01101 with honda bf90a outboard motor
 

QBhoy

Admiral
Joined
Mar 10, 2016
Messages
7,322
I think your gauge is wrong…or hope it is. If you keep shifting into gear at an idle speed of 1200 rpm…it won’t end well. I’d imagine it isn’t actually idling that high. I think you’d know all about it, by the noise it might make shifting into gear
 

tywiesz

Cadet
Joined
Dec 28, 2022
Messages
23
Hi,
thanx for bringing that up, old gauge was showing around 500 rpms is this what supposed to be for 90hs 4 cyl.4 stroke outboard? No noise when shifting. Old meter idle 500 WOT 3200, new meter idle 1200 and WOT 4800 motor trimmed full down and if trimmed up to moment of cavitation the motor goes to 5600 rpms, so maybe the meter is good but my idle too high than? I can adjust the idle of course if needed.
cheers
T
 

QBhoy

Admiral
Joined
Mar 10, 2016
Messages
7,322
Hi,
thanx for bringing that up, old gauge was showing around 500 rpms is this what supposed to be for 90hs 4 cyl.4 stroke outboard? No noise when shifting. Old meter idle 500 WOT 3200, new meter idle 1200 and WOT 4800 motor trimmed full down and if trimmed up to moment of cavitation the motor goes to 5600 rpms, so maybe the meter is good but my idle too high than? I can adjust the idle of course if needed.
cheers
T
I think maybe if it was actually 1200 rpm at idle…it wouldn’t be too long before the gearbox ends up in bits. Is it not audibly uncomfortable shifting into gear, if you think it is actually 1200 ?
Anyway. If I was you…I’d find out the gear ratio on it…then run some numbers into a prop slip calculator. Ratio, gps speed and rpms at full throttle required and also the prop pitch. The number given as a result of this, will tell you all you need to know.
 

tywiesz

Cadet
Joined
Dec 28, 2022
Messages
23
I think maybe if it was actually 1200 rpm at idle…it wouldn’t be too long before the gearbox ends up in bits. Is it not audibly uncomfortable shifting into gear, if you think it is actually 1200 ?
Anyway. If I was you…I’d find out the gear ratio on it…then run some numbers into a prop slip calculator. Ratio, gps speed and rpms at full throttle required and also the prop pitch. The number given as a result of this, will tell you all you need to know.
Hi,
wow, I didn't even know such thing existed, thnx. It came out 49.47 for 14" prop and 45.59 for 13" prop, not sure what is mine, will have to double check it. Can you interpret the results?
according to ppl from internet idle should be 950, so I'm too high.
cheers
T
 

jlh3rd

Petty Officer 2nd Class
Joined
Jul 10, 2017
Messages
196
how do you know the replacement tach is right? And just assuming the original gauge was wrong isn't the best way to troubleshoot a suspect rpm.
Did you know that tach gauges have to be set to the charging impulse of a particular motor? There is a selector switch on the back of the gauge.......
 

tywiesz

Cadet
Joined
Dec 28, 2022
Messages
23
how do you know the replacement tach is right? And just assuming the original gauge was wrong isn't the best way to troubleshoot a suspect rpm.
Did you know that tach gauges have to be set to the charging impulse of a particular motor? There is a selector switch on the back of the gauge.......
Hi,
very good question, I actually don't know. Setting is set to no.1 for four poles as my motor honda bf90a is.
Do I need another independent tacho meter?
cheers
T
 

jlh3rd

Petty Officer 2nd Class
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Jul 10, 2017
Messages
196
yep...at least get that rpm verified....idle on my new 115 merc and the original 60 hp merc is around...680 ?... trying to remember....definitely not 500 or 1200......
you need that rpm to be accurate before solving other problems...
 

tywiesz

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Joined
Dec 28, 2022
Messages
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good point, I got what paid for hahhah. To my ear idle is set too high gona lower it and verify my current meter with another one. I may just coil a bit of wire on h.volt of one spark plug and connect frequency meter than multiply x4 as got 4 cyl.motor, right?
cheers
T
 

dingbat

Supreme Mariner
Joined
Nov 20, 2001
Messages
14,474
Looks like your idle speed is 750 plus/minus 50 rpm per the manual

Bear in mind that setting the idle speed requires the boat to be in the water, in gear. Will idle much higher out of the water.
 
Last edited:

jlh3rd

Petty Officer 2nd Class
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Jul 10, 2017
Messages
196
I idle in neutral when checking idle speed (like a car)...I understand if in gear idle speed is lower when in water...but not when in neutral, right?...
 

tywiesz

Cadet
Joined
Dec 28, 2022
Messages
23
Hi,
good point but total confusion for me, idle speed as per the manual is it in gear or neutral? Of course in gear and neutral rpm's will vary as Dingbat says. If I go by my ear and adjust it I got it at 950 in neutral, will check that in gear latter.
cheers
T
 

jlh3rd

Petty Officer 2nd Class
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Jul 10, 2017
Messages
196
actually, I was responding to dingbat's post....as kindof a question...
 

dingbat

Supreme Mariner
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Nov 20, 2001
Messages
14,474
I understand if in gear idle speed is lower when in water...but not when in neutral, right?...
Nope.....the water creates back pressure in the cylinders.
Search the forum for how many times people say the motor runs "great" (on a hose) until they put them in the water...;)

Different motor, but mine idles in neutral at spec, ~800-850 in the water and 1200-1250 on a hose. Dropping it in gear reduces another 100 RPM or so

Read the specifics of your motor in the manual.
 

jlh3rd

Petty Officer 2nd Class
Joined
Jul 10, 2017
Messages
196
Nope.....the water creates back pressure in the cylinders.
Search the forum for how many times people say the motor runs "great" (on a hose) until they put them in the water...;)

Different motor, but mine idles in neutral at spec, ~800-850 in the water and 1200-1250 on a hose. Dropping it in gear reduces another 100 RPM or so

Read the specifics of your motor in the manual.
not doubting you, just ask'n......I fully understand water exhaust creating back pressure at the exhaust manifold possibly affecting idle....
My motor is a 2021, .computer controlled idle......I have the all the manuals and paperwork that came with the motor....Curious, so I'll see....
I have no idea about his honda.....
 
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