Slow 9.9 Mercury

[Quallsj]

Hey, guys. I have a 2005 Mercury 9.9 that is running extremely slow. Like 5 or 6 mph on my 14' flat bottom. I know it should be doing around 20 mph. I don't think the prop is spun, but rather I think it just has the wrong-sized prop on it. It has a 8.5x8 prop, and the edges are fairly rough. I've done tons of research on all this, but still can't really ascertain which specific prop I would need. Diameter? Pitch? The motor just came out of the shop. It runs well...just SLOW. Can anyone share some expertise? Thanks!

 

[WesNewell]

Start here.

Worldwide Leader in Marine Propulsion

Mercury Marine is the world's leading manufacturer of recreational marine propulsion engines. Get sales, service and parts info, and find a local dealer near you.

www.mercurymarine.com

 

[racerone]

Sounds like it is running on one cylinder.----Have shop test it again.

 

[Quallsj]

Sounds like it is running on one cylinder.----Have shop test it again.

Thanks! Will do.

 

[Sea Rider]

Welcome to Iboats,

Some issues:
Is this motor a 2 or 4 stroke one ?
Is it a first or second hand motor powering any other application before ?
How loaded was the boat when achieved just 5-6 MPH ?
Was the motor running at a much higher speed before ?
Why was taken to the shop, the shop did, fixed what ?

No way a 9.9 will run 20 MPH pushing a 14 footer not even in dreams. If that's the prop factory delivered with it's a 16 year oldie. Make a prop slip test to rule out if that prop it's already shot to go from there.

With a healthy motor and prop need to install an induction tach go for a wot run as loaded and check max wot rpm's achieved to know how much prop pitch to drop for the motor to run at least middle to max wot rpm range factory stated. Installing blindly any other prop will be a wild guess and could turn being counter productive as well...

Happy Boating​

 

[Quallsj]

Welcome to Iboats,

Some issues:
Is this motor a 2 or 4 stroke one ?
Is it a first or second hand motor powering any other application before ?
How loaded was the boat when achieved just 5-6 MPH ?
Was the motor running at a much higher speed before ?
Why was taken to the shop, the shop did, fixed what ?

No way a 9.9 will run 20 MPH pushing a 14 footer not even in dreams. If that's the prop factory delivered with it's a 16 year oldie. Make a prop slip test to rule out if that prop it's already shot to go from there.

With a healthy motor and prop need to install an induction tach go for a wot run as loaded and check max wot rpm's achieved to know how much prop pitch to drop for the motor to run at least middle to max wot rpm range factory stated. Installing blindly any other prop will be a wild guess and could turn being counter productive as well...

Happy Boating​

 

[Quallsj]

Thanks for the info. This is a second-hand 4-stroke motor. When I first got the motor, it wouldn't start very well at all. It would turn over but never actually start. I took it to shop for that. Turns out carb was pretty gummed up. He cleaned the carb and installed a new fuel line. The motor starts up perfectly now...just runs extremely slow in the water. The boat is fairly loaded as it has me (310 lbs.), marine battery, small battery for fish finder, 3 gallons of fuel, rod and reels, a couple of small anchors, and a couple of small tackle boxes. Unfortunately, everything except the anchors and tackle boxes have to be in the back with me, so it's definitely heavy in the back. I don't really know how to correct that unless I add a sack of concrete or something to the front, but then it seems I'm just adding additional weight for the motor to push. When I bought this motor, the guy had just put a "new" prop on it. However, it's smaller that then OEM factory prop. I am wanting to go back to the factory-sized prop. Wouldn't that be a good idea? Or should I put the current smaller prop back on ( I have it off now) and take it out and test the RPM's with it? Thanks for the help.

 

[Sea Rider]

You're welcome,

Issues : Is it possible to move some weight up bow, how about whatever is boxed ? What described is for solo boating right ? what about sitting a boating mate up front for better load distirbution.

Motors factory delivered with an installed props are usually medium pitched, good for break in periods with new motors out of he box, after that need to prop right the motor for current application usually to less prop pitches, the issue is how much less ?

Would advise to order an induction Hardline hour/tach model HR-8061-2 install it and go for a wot run as loaded. Need to know the current wot rpm the motor is revving at when at full throttle grip. If revs high and the speed is low, current prop is shot, kaput. Check Amazon for them, best investment ever.



You can also test both props and check which ones achieves a higher wot rpm, but will need the tach for that. Motors must run indide their wot rpm range factory stated, although medium to max wot rpm is preferred. Which numbers are stamped on the factory prop ?

Happy Boating​

 

[WesNewell]

No way a 9.9 will run 20 MPH pushing a 14 footer not even in dreams. ​

roflmao
Assumes 400# boat and 400# person and gear.

Your setup​

Boat type: Aluminum - Tiller
Usage: Overall good performance/0 ft
Engine Manufacturer:
Mercury
Engine Type: Outboard
Engine Year: 2005
Number of Engines: One
Engine Family: FourStroke
Engine: 9.9 2cyl (209cc)
Drive / Gear Case: 8-9.9 HP 2 cyl, 2.08 Ratio

Details​

Calculated Pitch: 10.72
Calculated Weight with engine(s): 901.10 lbs
Calculated Engine RPM: 5,500
Calculated Boat Speed: 23.06 mph

 

[JimS123]

Is the motor running slow, or the boat? If its the former it's probably running on 1 cylinder. If its the latter, you have the wrong prop.

An 8 incher is probably too low, thus the motor is probably revving high. At any rate, a little flat bottom tinny should easily run over 20 (miles, not kilos.....LOL)

Adding a tach is a good idea, but using mercury's prop selector will provide instant data without any expense.

 

[Quallsj]

You're welcome,

Issues : Is it possible to move some weight up bow, how about whatever is boxed ? What described is for solo boating right ? what about sitting a boating mate up front for better load distirbution.

Motors factory delivered with an installed props are usually medium pitched, good for break in periods with new motors out of he box, after that need to prop right the motor for current application usually to less prop pitches, the issue is how much less ?

Would advise to order an induction Hardline hour/tach model HR-8061-2 install it and go for a wot run as loaded. Need to know the current wot rpm the motor is revving at when at full throttle grip. If revs high and the speed is low, current prop is shot, kaput. Check Amazon for them, best investment ever.

View attachment 338242

You can also test both props and check which ones achieves a higher wot rpm, but will need the tach for that. Motors must run indide their wot rpm range factory stated, although medium to max wot rpm is preferred. Which numbers are stamped on the factory prop ?

Happy Boating​

Thanks for all the info. Greatly appreciated! The current prop is an 8.5 x 8, but that's not factory. Factory is supposed to be 8.9 x 9.5. Not sure what Mercury means by "8.9." 9 inches maybe. I just measured and my anti- ventilation plate is 1 3/8" BELOW bottom of the boat. Would that be enough to decrease my speed by half or so?

 

[Quallsj]

Is the motor running slow, or the boat? If its the former it's probably running on 1 cylinder. If its the latter, you have the wrong prop.

An 8 incher is probably too low, thus the motor is probably revving high. At any rate, a little flat bottom tinny should easily run over 20 (miles, not kilos.....LOL)

Adding a tach is a good idea, but using mercury's prop selector will provide instant data without any expense.

Thanks for the info. The boat is running slow. I'm thinking I may need to raise the motor as the anti-ventilation plate is 1 3/8" below bottom of boat as well as get a new prop.

 

[racerone]

Have the shop test this motor.------Perhaps shop for a 20 HP motor.

 

[Sea Rider]

Answering several posts in a row :

-One thing is what the Prop Selector tells, a different scenario once combo runs on water. As an example, my 18 HP motor max propped runs 42 KMH solo boating, max combo weight around 450 kilos (990 Lbs)

-An good induction tach brand as suggested only costs around $35.00 but you guys preffer to buy many cold ones instead and increase your own waterfun, which don't have any issues with...

--About the motor height, if the water flow at speed bangs over the small anti splash plate will produce water back or over transom splashes and slow the combo real down. As long the water flow skims right under the upper splash plate or over the anticavitaion plate no issues depending on water condition and geographical location and boating area.

For comparison, whats the weight of that barebone 14 footer ? Tech question, would anyone dare say that a flat 14 footer boat has less hull drag than a deep V keel hull with equal, lenght, weight and same 9.9 motor ? For me a 9.9 HP it's a very underpowered motor to power a large 14 footer boat and have fun with, worse if being a 4 stroke slow fast response one.

Before playing with motor heights and props have that motor checked by a pro, something is way wrong with it....

Happy Boating​

 

[JimS123]

Thanks for the info. The boat is running slow. I'm thinking I may need to raise the motor as the anti-ventilation plate is 1 3/8" below bottom of boat as well as get a new prop.

That certainly isn't the optimum position, but it's not what's causing your slow speed. You're not running a little slow, you're running a lot slow.

OTOH, the difference between an 8 and 9 pitch probably won't triple your speed either.

Buy what the prop selector tells you, or look at a Michigan catalog and see what their recommendation is based on boat size. You need a new prop anyway. Then, verify that both cylinders are running.

 

[Sea Rider]

A Tohatsu 9.9 HP motor well transom height matched, with deck load well distributed, trim set to 90 deg powering a flat bottom 14 footer should achieve max 400 wot revs per one pitch decrease while running on flat calm water cond. How can you check that, wih an induction tach install.

The OP's is losing too much water fun, check if at least both spark plugs are firing strong to begin with with a spark tester, check Lisle 20610 Spark Tester at Amazon...



Can the OP upload a nice pic of said boat and motor to have a close look ?​

Happy Boating

 

[Faztbullet]

You want this one... The neon ones will lie to ya.

 

[Sea Rider]

You want this one... The neon ones will lie to ya.
View attachment 338412

The neon ones will lie to you depending on your vision state, it's a matter of watching the spark jump intensity and color at a dark place as a garage, put a large towel on yourself and motor and have someone else crank if for you to check the spark. If it's a intense bluish color is good to go, if yellowish or redish the ignition coil output is low and if there's no spark seen it's kaput. The OP has 2 nice cheap options to go for whichever one likes best...

Happy Boating​

 

[racerone]

Well---2 pieces of wire glued on carboard with a 3/8" gap would be cheaper yet.

 

[Quallsj]

Hi. I have a 2005 Mercury 9.9 that seems to run great EXCEPT the boat is extremely slow relative to what that motor should do. It's running maybe five or six mph loaded. I noticed the anti-ventilation plate is 1 3/16" below the bottom of the boat. Could that alone cause such a drastic drop in speed? Thanks!