4400RPM@WOT?

[Merc90HPnewbie]

Hi all. As a boating newbie I just wanted to check to see if this was appropriate. I have a 1985 Mercury 90, straight 6 cyl. Pushing the throttle all the way forward gets me 4400 RPM on the gauge in the dash. I believe this gauge is run to the throttle box and gets its info through the factory wiring harness somehow (what I am trying to say is I don't think it is an aftermarket tachometer). At that maximum RPM, the boat is going somewhere between 35-42MPH. The speedo on the boat that uses a small paddlewheel says 35 and the speedo in the fishfinder (how does this work?) says 42. Does this sound reasonable? Should RPMs be higher/speed be faster with this configuration? It's a 16 ft 1985 Glasston Bowrider. I confess I dont quite have the experience to set the trim quite right but it is definitely up on plane. I dont offhand know the pitch of the prop but I can find out if it might help your diagnosis. The prop is kind of rough but serviceable for now.<br /><br />Thanks!!

 

[Kenny Bush]

Re: 4400RPM@WOT?

The paddle wheel runs the graph speedo, and your dash speedo has a peeto tube on the transom or in the lower foot of the motor. I have found that the graph paddlewheels are much more accurate(unless you are using GPS). In regards to you WOT RPMs, you should be running about 5200-5600 depending on prop selection. You need to make sure your motor is running at its designated WOT RPM in order to keep it clean. I would first check RPMs with another tach just to make sure your dash tach is correct.

 

[mellowyellow]

Re: 4400RPM@WOT?

what prop r u runnin? any damage to prop? <br />should be pushing around 5500rpm.

 

[Chinewalker]

Re: 4400RPM@WOT?

If you can confirm that...<br />A: the tachometer is accurate.<br />B: you're getting full throttle at the engine end of the controls<br /><br />...then you need to do some prop work (going down in pitch) to get the revs up to around 5500. Not doing so will result in a blown motor as the motor will work harder to maintain 4400 than it will at 5500. Kinda like driving uphill, towing a trailer in 5th gear all the time...<br />- Scott

 

[Dhadley]

Re: 4400RPM@WOT?

Are you using any trim at speed?

 

[Merc90HPnewbie]

Re: 4400RPM@WOT?

Thanks for all the replies. Is there a clamp on tach or something simple I could use to verify the RPMs? If I remember correctly the dash tach needle rests at almost 1000 RPM even when the engine is off, so I may be putting too much trust in a flaky gauge.<br />I dont know the pitch of the prop offhand but I will check it out. There is some damage, there's a small chunk missing (I would say it's a half moon about 1/16" deep by 1/4" long?) from one blade. I noticed another blade was bent in slightly but I hammered it out. I will try to get a picture and post it soon. I have a spare new prop, the splines are a little corroded from sitting in the boat but the prop itself is brand new. Maybe I'll try to clean up the hub and try out the new prop?<br /><br />As for trim I do try to move the engine around a bit - again I'm no expert so I'm just trying different things to see what works - so far nothing has really affected the RPMs drastically. I dont have the engine trimmed all the way down causing a lot of drag from pushing the whole hull through the water if that's what you are thinking DHadley <br /><br />Thanks again to all

 

[tmac58star]

Re: 4400RPM@WOT?

Pull that hammered out, chunky prop off and put the spare on, noting any numbers you may find on it (dia. and pitch). Then make a run and note rpms off the most reliable tach you have. You may need to drop pitch to raise the rpms into the ideal WOT range.

 

[corm]

Re: 4400RPM@WOT?

Well guys the factory RPM range on this motor is 4500-5000 RPM. My personal experience is that they worked best proped to run at 4850. The port timing is real low on these motors compared to the 115HP or 150Hp L6. The powerhead will turn 5500RPM with out hurting it, but it will be a bit over it's powerband. It makes no difference where the tach needle sits when the motor is off.<br />My guess is that your tach is ok if you have a 21" P prop on your boat. Prop no.= 48-******x-21 the no.21 is the pitch of your prop, it may say 19 or 23 anyway that is the pitch of your prop.<br />If you have power trim you can gain alot of RPM by triming the motor out to a neutral steering position. If you don't have power trim set the pin so that you have neutral steering or 1 pin hole out from that position for better speed, but you will experience some tourqe on the steering wheel.

 

[Merc90HPnewbie]

Re: 4400RPM@WOT?

Thanks for the tips. I took a look at the prop (before I read the above posts) and I believe it said 19P which I thought might be 19 Pitch? The numbers are a little hard to read. Is there a way I could measure it to determine the pitch?<br /><br />I will reread corm's post until I understand it. Trim and motor angle are things I dont quite get yet <br /><br />Thanks again to all the helpful people here!

 

[Merc90HPnewbie]

Re: 4400RPM@WOT?

PS I am the 3rd owner of this boat. The last owner only had it a year before moving up to a bigger boat. As for maintenance/repair I think all he did was change the LU gear oil. So basically the boat is as the first owner had it. He had a 'happy fisherman' troller plate installed and had some rod holders in the boat so obviously he was in to fishing. I took this stuff off cuz I am not interested in fishing (yet?) but might it be a clue that he changed to an incorrectly-pitched prop intentionally? Although if he wanted to go slower at a given RPM he went the wrong way in pitch right?<br /><br />Can someone explain the basic rules of prop pitch?<br /><br />I understand it like this; If you want the motor to spin faster you decrease the pitch (go from a 21 to a 20, 20 to a 19, etc). And your top end speed will stay the same, but the RPMs will increase. ???

 

[mellowyellow]

Re: 4400RPM@WOT?

better bite on lower pitch, less top end....

 

[Kenny Bush]

Re: 4400RPM@WOT?

Pitch is the "distance" moved (Based on zero friction) through the water with 1 revolution or 360 degrees rotation. So a 19 pitch prop when turned 1 revolution will move 19". A 21 pitch would move 21" ect.... With this being said, the lower the pitch, the slower the speed. The lower the pitch, the more power out of the hole you have. The trick is to go as high in pitch as possible and still come out of the hole in at least 3 seconds. If you are skiing more, you probably want to drop down a pitch so that you will have extra power out of the hole.

 

[Merc90HPnewbie]

Re: 4400RPM@WOT?

For skiing, suppose you go down in pitch to get more power out of the hole, I assume you lose top end speed (speed of the boat relative to the water, in KPH or MPH) how would it affect the max RPMs of the motor (at WOT?)<br /><br />Thanks<br /><br />PS any way to look at a prop and measure it's pitch assuming its markings are worn off?

 

[Kenny Bush]

Re: 4400RPM@WOT?

Yes you want to keep that motor running at its factory WOT range.... So you will reach a point where you are pitched up so high that your RPMs begin to fall below the recommended range. This is not good. A 19p should be good for you unless your boat is heavy and you ski all the time, in which a 17 would be better. Also, I like using the "vented props" because they spin out of the hole easier(you could maybe use a 19p vented to ski with and have the high speed we all want). Talk to your local prop shop as there is much more Custom prop information that I have not talked about, and some of that technology could help in your application.

 

[Kenny Bush]

Re: 4400RPM@WOT?

Also the prop shop will have a jig to measure pitch. If you are good with math, you could calculate it based off the angle.