Faster

[Fieldsken1]

I got an 87 Malibu 19 ft with a 175v6 engine I think it's a 87-92 with a 14.8x17 prop and an alpha 1 outdrive
It tachs 4500 out of water but in water top is 3800 rpm @ 40 mph It planes fast and runs great. I just want to go faster
Also why won't it get up to 4500rpm?? Some say it's bad to run at 3800. I figured the motor is not working so hard better on motor. Less rpm = longer life. Just like overdrive in a car
Question 1
Is it bad to go 3800 instead of 4500??
Question 2
How can I get more top end without losing the fast plane?
Question 3
Should I leave it alone and be happy???

 

[southkogs]

Is 3800 wide open throttle on the water? I've got a 3.0, and at 3800 RPM I'm cruising at about 33MPH (roughly). WOT for me is closer to 5000 RPM, and I'll get a little over 40MPH with that.

I'm thinking you should be getting a few more RPM and MPH ... but I would think somewhere around 50 is what that boat's designed for on the top end.

Welcome aboard.

 

[achris]

I got an 87 Malibu 19 ft with a 175v6 engine I think it's a 87-92 with a 14.8x17 prop and an alpha 1 outdrive
It tachs 4500 out of water but in water top is 3800 rpm @ 40 mph It planes fast and runs great. I just want to go faster
Also why won't it get up to 4500rpm?? Some say it's bad to run at 3800. I figured the motor is not working so hard better on motor. Less rpm = longer life. Just like overdrive in a car
Question 1
Is it bad to go 3800 instead of 4500??
Question 2
How can I get more top end without losing the fast plane?
Question 3
Should I leave it alone and be happy???

1. You should not be revving the engine to 4500 out of the water... Quickest way to destroying the water pump impeller, followed shortly thereafter by the engine overheating and destroying itself.
2. You need to find out the drive ratio. If it's still the stock drive supplied with the engine it'll be a 1.84:1... I don't know the Malibu hull, but my Bertram is HEAVY and with that engine I'd be swinging a 17" prop with ease. I'd be doing a few checks on the engine first. Check compressions, check timing and timing advance, change fuel filter, and check the throttle cable is actually pulling the throttle all the way open....

Ok, on to busting some myths!
Overpropping a boat engine is in no way related to an overdrive in a motor vehicle. With a motor vehicle you have the ability to select different gears (ratios) depending on road speed, not so in a boat. You have one 'gear' and you are stuck with it, throughout the engines' rev range. If the engine is overloaded at the top end (not producing enough revs at WOT), then it's overloaded RIGHT THROUGH THE RANGE, from idle up! This will significantly SHORTEN the engine life. The very best way to get the longest life from your boat engine is two-fold. 1 DO THE MAINTENANCE, 2 DO NOT overprop, if anything, slightly underpropping will help it.

So, direct answers to direct questions.

Your question 1. Very bad. Check your engine, get those revs UP....
Your question 2. Get the engine working properly (the reason I suspect something amiss with the engine is because you say it pops up onto the plane quickly. If it was truly overpropped, then plane out would take forever.)
Your question 3. NO... something is not right. Fix it or risk the engine being killed.

Chris......

 

[Fieldsken1]

First of all I am using the muffs when out of water. Not stupid
Motor has tune up new dizzy new everything. Just won't get up to RPMs when under a load
Throttle cable does go to WOT

 

[Durango318]

Yeah, revving an engine without load tells you nothing and just hurts things. And yes if it is over prop-ed it's bad too. Best guess there's something wrong with the engine. Lately I've been seeing a LOT bad fuel problems. The potency of the fuel could be gone if it's old and just wont give full power. If the fuel "phased" it wont idle for beans and you could seriously damage the engine if you manage to get it under load. Stop, check it!

 

[Durango318]

"Throttle cable does go to WOT " ?
Elaborate please.

 

[Fieldsken1]

Idles fine. Sounds good. No mess I can't figure it out

 

[achris]

First of all I am using the muffs when out of water. Not stupid...

I didn't say without water.... I was referring to the fact that on the muffs, a hose can't supply enough water for the impeller at much above idle... Even on muffs, revving to 4500 is a ticking time bomb for the impeller.

Straight out of the Owners Handbook.....

 

[achris]

ok.. just plugged the numbers into my prop calculator, and something's off.



Are you using a properly calibrated and checked tacho? are you using GPS for speed check?

Now those same numbers with the revs set at 4800... (still too low)



It only comes good when I use a drive ratio of 1.65:1 and an engine speed of 4800.....



I think you need to do more testing, with calibrated equipement and find the drive ratio...

Chris......

 

[achris]

Here's another calculation.... 3800 RPM on a tacho connected to a 6 cylinder engine but set to 8 will be....5066rpm.... And if the plane out time is good, I'd be checking to make sure the tacho switch is set right... Just one more possibility....

And those numbers look even better!



Mr Fieldsken1, nobody is implying you're stupid. We are all just offering advise. We can't see or hear the engine or boat. We rely solely on your eyes and ears to give us input. Things we suggest or ask you to check may seem, trivial, but we don't have the benefit of looking at it ourselves. I once spent 4 days working with a guy in Norway to find a very interesting problem on an outboartd engine. It turned out to be a missing ground wire. Had I been there to look at the engine I would have spotted it in a few seconds, but I wasn't, he was my eyes. We went thorugh everything with a multimeter and eventually found the problem... That went well, and we got the problem fixed. Hopefully we can with your problem too. Please be patient and work with us... It will take time and effort on your part. The alternative it to take it to a shop, and pay for it to be done. I'm guess you'd rather save the money for fuel, that's why you're here. Am I right?....

Thanks.....

 

[Scott Danforth]

check the speed with a GPS, not the gauge on the dash. same goes with RPM, check the RPM with a diagnostic tach, never trust the cheap gauge on the dash.

Now on to your motor

As Chris pointed out, your boat has one gear, and it is loaded like a truck, loaded with bricks while pulling a trailer while going up-hill in sand. your car going down a flat highway is maybe putting out 15-20hp to maintain speed. your boat motor is putting out 125-150hp to maintain speed. it is always better to under-prop than over-prop. over-proping leads to over-loading which causes over-heating and detonation which is your pistons eating themselves

verify your RPM. this is critical. the tach supplied with your boat has a variety of different settings on it. it could simply be off a bit on the switch, or it could be bad.

verify your speed. the pitot tube speedometers are notoriously off.

 

[Fieldsken1]

I checked tach it's good. Done by shop
I checked with gps. Matches speedo.
It's at the shop again. Checking gearing

 

[Fieldsken1]

Where can I find that calculator

 

[achris]

Where can I find that calculator

Only on my computer, I wrote it.

 

[wahlejim]

achris I am amazed at your patience. You give great advice and videos for free that most people should be paying for. Every time that someone replies to one of your comments with ignorance, your patience shines. For example, the comment about muffs and being stupid. My response would have been something along the lines of: "Even with muffs on, I wouldn't rev it to 4500, but this is only what I do for a living. What do I know?" Instead, you come back and throw a ton of knowledge and help their way. You are a much bigger man than I am. And now a software engineer to boot!

 

[Fieldsken1]

I did not mean to attack anyone. I have been on other forums and they just want to insult people. Sorry. I need help and am thankful for the advise
I took it as you thought I ran it without muffs. I took it wrong. Sorry.

 

[flipbro]

Do a Google surch. There's a few good ones. But as mentioned you need to verify rpm and speed.

 

[Fieldsken1]

The RPM is correct saids the shop
Speedo is right saids my GPS
it had a 3.0 when new someone put a v6/175 in it
If the did not change the gearing couldthat be the answer??
From what I read it should of had a 1.91 with the 3.0

 

[achris]

Using a 1.94:1 drive with the same prop would make the revs higher, not lower. If the drive really is 1.94, then the scenario is even worse. I'm sorry to have to disagree with the shop, but you need to have a calibrated tacho on the engine while you are doing the run, not have the dash tacho 'verified'... I have an 'advance timing light' with a built-in tacho, and I also have the compter software for the MPI engines, which has a tacho simulator, and my tacho checks out as 'close' on the computer, but I still do all my prop tests and 'baseline data' tests with the timing light tacho, and it's much better than the dash tacho...

Chris.........

 

[achris]

Do a Google surch. There's a few good ones. But as mentioned you need to verify rpm and speed.

Be careful using the 'on-line' calculators. I have checked a few and none of them allow for cupped blades. When calculating for cupped blades, you MUST add 1" to the pitch for the numbers to be right... (I have written that into my calculator )

Chris........