Lugging under WOT

[SimpleM]

Ok, I am an old salt when it comes to OWNING a boat, but I was still a bit of a young pup, (and did not have access to "iboats" and the legions of those that know far more than I) when I owned my last one in '94, so self maintenance and proper boat awareness is something I am in the process of reacquiring. Thanks to all for providing your knowledge. Now, please help me understand something about proper propping... ugh... pun... Anyway, as I understand it there is a rpm range my boat should fall into under WOT. I understand pretty much why this is important as well. Here is what I am running into. I have a 1987 19' Sea Ray Seville bowrider, 3.0 Alpha 1, 175 hours. Good compression, I think it may be running a bit rich, but then it only has 1.25 hours on it since burning off the winter "fog" so I'll give it a couple more before I pronounce it rich. Everything seems to be working fine, and the boat is not waterlogged or unusually heavy. However, @ WOT I am turning only about 4100 RPM fully loaded at 32mph indicated, and about 4175 at 33mph indicated with light load. I am turning a Michigan aluminum 14.5X17. According to everything I have read on the prop sites, this should be the best all around prop for this boat, but I feel it is at the rock bottom of the acceptable WOT RPM range, almost like a speed prop. My PRIMARY use for this boat will be pulling the kiddies around in tubes, and on wakeboards. Should I get a 16X16 or go on down to a 15.25X15. I am tempted to just do the 16 route, but I am not sure it will put my revs up into the 42-4300 range if loaded. Top end is not any issue, as I rarely run over 30 for any reason, but I would rather undershoot the 4600 top than overshoot. I can only afford one prop this season (gotta buy the toys for the kiddies) so do you think the 16 will be enough, or go 15?

 

[Silvertip]

Re: Lugging under WOT

If 4100 is the best you can get fully loaded, you are seriously over propped, the engine is not making full power, or the throttle is not opening fully. And the size of your boat, the power plant, and a 17P prop should allow that engine to easily reach 4600 RPM. So before spending money on a different prop, make darned sure you have an engine that is functioning properly (or as a minimum, verify that the tach is accurate). Why do you think it is running rich? Is the choke not opening fully? Now then -- why would you even think about a 16 inch diameter prop when a 14.5 is called for. Going 16 x 16 would not likely change anything. 15 x 15 would raise raise rpm a little. But since you need to raise RPM by about 400, that means with a 14.5 inch diameter prop, pitch would need to drop to 15P (roughly 150 - 200 RPM per inch of pitch. Going down in pitch can have a rather interesting effect in that a small change downward can give the engine the extra bit of help it needs to "get over the hump" so to speak. This is especially true on a seriously overpropped boat. You need to do some serious investigation regarding engine condition and state of tune. I feel a prop change is not going to solve the performance issue because there is something else wrong or this rig is extremely overweight (either with people or gear or both).

 

[Bondo]

Re: Lugging under WOT

Ayuh,.....

Getting it to run Right is the 1st thing to do.....
Then start playing with the Prop......
Is there the remote possiblity that the outdrive Ratio has been changed,..??
You should have 1.98:1.....

Btw,..... That 16x16" prop sounds like it's for a Houseboat or some other Barge.....

 

[SimpleM]

Re: Lugging under WOT

You know, now that I think about it, lack of a fully opening throttle is possible. I will check this first. I'm pretty sure it's not a weight issue, but it could be. I will go weigh it on the tractor trailer scales nearby and repost findings.

 

[Silvertip]

Re: Lugging under WOT

When you check for throttle opening, remember that the top plate in the carb is the choke plate. The throttle plates are way down inside. If the choke plate is not standing perfectly vertical on a warm engine, you need to fix that as it would be the cause (at least one of them) for running rich.

 

[CharlieB]

Re: Lugging under WOT

19 ft bowrider with a 3.0 and an Alpha drive, I would start with a new set of spark plugs and try a standard 14 X 19 SS prop, it should be pretty close.

 

[SimpleM]

Re: Lugging under WOT

19 ft bowrider with a 3.0 and an Alpha drive, I would start with a new set of spark plugs and try a standard 14 X 19 SS prop, it should be pretty close.

Plugs and wires new. If it's lugging with a 14.5X17 wouldn't a 14X19 ss be worse? I am thinking more possibly I'm not reaching WOT as suggested by Silvertip. The rich run issue is based on the fact that I am smelling rich exhaust, or at least an exhaust smell that I did not smell on my last 3.0, but I will investigate the choke issue.

 

[SimpleM]

Re: Lugging under WOT

Thanks to all for replying to this, turns out I had 2 problems. #1= Loose distributor cap... That'll make one not run out. The second thing is it looks like I have a bad tachometer. Once I got my distributor tightened up it will hit 38mph indicated, compared to (31 or 32 before with same load) and the power curve is very smooth now.

However, now the tach only indicates 3200 rpm @WOT and that can not be right if the speedo is even close. I know this is the prop forum, but I originally thought it was a prop issue. (thanks to you guys for advising me to check the engine before re-propping as the 14.5X17 seems perfect when it is running right)

Since your advice has been so good so far, if you will indulge me, is there anything to look for like possibly some kind of voltage regulator that powers all of my 12 volt instruments. Trim gauge is not very calibrated, but everything else seems fine. Oil pressure is fine, temp and volts seem fine. Should I just get a new tach or is there something besides the coil wire to look for. Also is there a way to calibrate the trim gauge?

1987 Merc. 3.0 Alpha 1, Faria gauges.

I may also have another carb related question I will post in the Merc. Engine forum. (if the new plugs do not fix the issue) If you guys that originally helped me would be on the lookout for my post there I would appreciate it.

Thanks TONS!

M...

 

[Maclin]

Re: Lugging under WOT

I do not have anything for WHY the tach would have changed since you fixed the engine......but if the tach has a 4-6-8 cylinder switch it could be on the 6 cyl setting. A 4 cyl engine will only read 2/3 of what it is really running if the tach is set for a 6 cyl (The engine is actually at 4800 when the tach reads 3200). On the 8 cyl setting it would read half so I do not think it is on the 8 cyl setting. Again, I do not know if your tach is like that and I do not know how the setting (if it has one) could have changed.

I'm just sayin'.....

 

[TilliamWe]

Re: Lugging under WOT

What Maclin is saying seems very likely to me too.

 

[SimpleM]

Re: Lugging under WOT

Hmmmm. It is an in-dash tach so I will have to research where such switch may be. Sounds likely tho. I know on my old hotrods most aftermarket tachs had said switch, but It never crossed my mind that my boat tach might have one too.

 

[RCSConstruction]

Re: Lugging under WOT

I HIGHLY doubt it's a "switch" In my experience it's the color of the wire that is hooked up that determines what motor/rpm's. And how would tightening your distributer change the switch???? Impossible...
Any way you jiggled/grounded/loosened anything at the coil? Wires all tight?
That's where your RPM gauge is connected to.

 

[SimpleM]

Re: Lugging under WOT

Yes RCS, I checked the wires to the coil; tight as a tick. I assumed it was just like my old car tachs, but thanks for the confirmation. I am finding many boating issues are slightly different than automotive.

I found out about the distributor by accident. I hit a wake rather hard and immediately the motor just lost power. It would just load up and drop power as you gave it throttle, almost like the choke was engaging. However, I noticed it pulled the rpms down as I ran the trim pump or even turned on the blower so I figured it was more likely an electrical issue.

I limped it back to the dock, and as I checked the motor and all the wires, the coil was the first place I looked. There just happened to be a guy that worked at the marina that noticed me with the doghouse off, and he came by, asked a couple of questions, stepped into the boat, had me rev it up, pushed down on the distributor cap and said "give me a screwdriver". Once he tightened the distributor, it was as if I had a new boat. EVERYTHING got better. Idle, power curve, top end. Evidently this was an issue that had been present a while and was worsening, the wake slam just jarred it to a new "level" or lack there of.

As for the jump in the tach, it is possible that I was mis-reading the tach in the beginning, (when I wrote the initial post) and 41-4300 I thought I was seeing was actually 31-3300. I have mild dyslexia, and my brain often will associate what I think I am SUPPOSED to be seeing in place to what is actually there. (my speel chekcer alwyas gets a workuot ) Therefore, if I was fixated with the speedo not going as high as I presumed, then I may have initially transposed the 4 for the 3 subconsciously thinking that it HAD to be 4100 to be running 30mph+. However, yesterday I paid attention closely to the tach and it was clear it never broke 3300 rpm.

I will use a portable gps to check true speed, and a portable engine diagnostic computer to check the rpms at a plug wire before I do anything else like purchasing a new tach. Still, 19 foot boat, 3.0, 17 pitch and 30+ mph HAS to be more than 3300 rpm. Even if the gear ratio was changed, a 3.0 would not make enough power at 3300 to push it over 30. Would it?

Again, thanks for the replies, I will keep you posted.

M...