Logo
FULL SITE MENU

Half the speed

E

ejbommer

New member
I have a 1988 2 cycle 15 hp Mercury outboard attracted to a 16 ft Modified Vee Jon boat. From what I understand I should be able to hit a max speed of 20 mph(According to Mercury prop selector and the Marina I purchased the boat from) but I am lucky to get 8 mph. A couple observation I have made is:
  1. The motor starts and run fine. Under heavy loads The engine sounds strong.
  2. In the forward direction the throttle only goes 1/2 throttle, but full throttle in reverse. If I manually move the throttle blade to full while running down the lake I get about 10 mph. It seem like its backwards, but I see no way to adjust the linkage to reverse it.
  3. According to the Mercury web site for prop selection I should use a 9.5 pitch prop but it looks like I have a 15 pitch prop. Not sure if my prop is over loading the motor.
  4. I see timing marks, but I would assume if I am running 1/2 speed and its a timing issue I would here it miss fire at high loads.

Any advice in what else I should look at is greatly appreciated. Not really expecting much, if I could get it to go 12 to 15 when I am by myself and no gear I would be ecstatic.
 
ejbommer said:
I have a 1988 2 cycle 15 hp Mercury outboard attracted to a 16 ft Modified Vee Jon boat. From what I understand I should be able to hit a max speed of 20 mph(According to Mercury prop selector and the Marina I purchased the boat from) but I am lucky to get 8 mph. A couple observation I have made is:
  1. The motor starts and run fine. Under heavy loads The engine sounds strong.
  2. In the forward direction the throttle only goes 1/2 throttle, but full throttle in reverse. If I manually move the throttle blade to full while running down the lake I get about 10 mph. It seem like its backwards, but I see no way to adjust the linkage to reverse it.
  3. According to the Mercury web site for prop selection I should use a 9.5 pitch prop but it looks like I have a 15 pitch prop. Not sure if my prop is over loading the motor.
  4. I see timing marks, but I would assume if I am running 1/2 speed and its a timing issue I would here it miss fire at high loads.

Any advice in what else I should look at is greatly appreciated. Not really expecting much, if I could get it to go 12 to 15 when I am by myself and no gear I would be ecstatic.
Click to expand...

I was wrong the prop is a 9" 10.5 pitch. Part number 48-17874-10.5
 
I run a 1990, 15 horse on my 14' deep V utility (dry weight is 250 lbs).

When I'm running "alone" I can maybe hit 20 mph on perfectly flat water and no wind.

Throw a second person on board and it will maybe make 15 mph under near perfect conditions.

You can fiddle a bit with the pitch on these but it is not going to make a significant difference in speed unless you are really bogging the motor.

On your boat, depending how you load it an 8 or 9 pitch might be a little easier on the motor but it wouldn't do much for the speed "assuming" the motor is properly tuned and running properly.

You can rig it with a "tiny tach" (brand name - wire just kinda wraps around the top spark plug lead wire and it gives you the rpms on a little digital display) - that would confirm you are getting up in the 5000-6000 range that you are looking for.

(given the wot range - about 1000 rpms - and the limited selection of props 7 thru 11 pitch - just about all of them will get you in the range unless you are running really heavy (not enough motor))

What's the history on the motor? last spark plug change? carbs ever cleaned/rebuilt? gas is fresh and stabilized?

PS - forget the "predictions" for a prop selector - voodoo at best - was that 20 mph going over a waterfall with a 75 lb 12 foot boat? There is no way to effectively predict how you will use/load your boat. The "selector" can give you an idea of where to start, but the only sure way to determine performance is fastening the prop and giving her a try - then adjusting from there.


Last note - without seeing your set-up, I would think it "should be" capable of 12 mph minimally, but perhaps not a whole lot more. These are "little motors" and 16' of boat is quite a bit for a 15 to push.
 
I purchased the motor used a couple of years ago. I was told when I purchased the motor it was just serviced professionally(I could tell some part had just been replaced, so I gave him the benefit of the doubt). In the time I have owned it, its always ran well but slow, but I was fishing small lakes so speed didn't matter. I bought the new Jon boat this year and now I want to do some river fishing and I am worried about running a 7 to 8 mph boat on a 5 to 7 mph current.

The gas is new, no carb work, Spark plugs are new.

I guess my next step is to get a tach reading and see where I am at for RPM. I guess its all direct drive so if I am getting the max RPM I must be getting max speed. The tiny tach may be a nice tool to have.

Any ideas on the throttle linkage seem weird I am getting only a portion of throttle movement forward that I see in reverse.

And maybe the professional messed it all up, during the last service.
 
There must be a "link & synch." procedure on your motor. That is to setup the spark advance to match the throttle opening. It sounds to me that there maybe a problem with that, and it's also affecting your throttle opening. Look at the linkage closely, and see if there's a link going up to a plate under the flywheel that should be able to rotate some for spark advance, that might be stuck for some reason (linkage rigged backwards by mistake?) or something?
 
Thanks I will look up on how to adjust the linkage.

i did also see some linkage under the fly wheel. Seems to move between 1\2and full throttle. When I first seen it I was thinking is was a governer.
 
Back
Top