# Mercury 9.9 four stroke rpm questions



## Azoutdoorsman (Jul 16, 2018)

I had posted about my motor height a few weeks back. Finally got around to building a jackplate to get my motor up to the correct height. It is a 2013 4 stroke 9.9ml on a Lowe 1467wt. Boat weights roughly 750lbs fully loaded with 2 people. I think i can bring the motor up 1 more inch judging by my test run yesterday. I had installed a hardline tach and went to the lake. At wide open throttle I'm running 7-7.8 mph and only turning 4100rpms. My prop is a 8.9x8.5 pitch prop. According to mercury prop selector it is telling me to drop down to a 6.5 pitch prop but I still feel like the motor will not get up to the 5-6k spec. Has anyone else had this issue with these motors? I feel like my boat is comparable weight to some of the fully decked and built boats I've seen with 9.9 four strokes that run 15-18mph with 2 people. Thanks guys


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## Weldorthemagnificent (Jul 16, 2018)

Weight distribution is huge. I have had 14’ boats that would not plane with a 20 hp. Sometimes you can get away with moving fuel tank and battery forward. Sometimes it needs a little help and that’s where a hydrofoil comes in giving the transom lift needed. Once the boat comes up on top, the revs should climb on up there. Have you tried the boat with just you in it?


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## TheLastCall (Jul 16, 2018)

Is it getting up on plane? Once up on plane your motor will have less resistance and will climb in rpm.

Hydrofoils are an option if you can get it to plane.

My personal experience is with a Princecraft Yukon 15 and a mercury 4stroke 15hp. It would plane without hydrofoils if I had minimal weight in the boat it would take a while for the bow to come back down. Now with hydrofoils, same conditions ( minimal weigh ) I get close no bow rise and it jumps on plane in 1 boat length. Once on plane I can run at half throttle and keep it on plane with minimal decrease in speed.


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## Azoutdoorsman (Jul 16, 2018)

It feels like it is just about to plane out but will not get the rest of the way. I had shifted weight around quite a bit yesterday and also tried it alone sitting from the back of the boat up to the front seat. It did climb rpms but I couldn't see my tach from the front seat


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## GYPSY400 (Jul 16, 2018)

Try with the boat as empty as possible .. that's a good sized boat for a 9.9, which should have a 25

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## Azoutdoorsman (Jul 16, 2018)

I will try to run it with bare minimum this week if I get a chance. I have not thought about using a hydrofoil but that could possibly help me.


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## ppine (Jul 17, 2018)

A kicker motor on a fishing boat has to work pretty hard. Reduce the pitch to be easier on it. 
You should definitely be getting more turns than you are showing.


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## Pappy (Jul 17, 2018)

How bout we start with some basics before spending the man's money?
What trim pin location is the engine in? 
Where is your weight located? What stays in the boat and where is it located?


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## Azoutdoorsman (Jul 18, 2018)

I habe tried the trim in each position and I have moved weight around throughout the boat. The only thing I have not done is run it with bare minimum gear, extinguisher, pfd paddle and throwable. I'm starting to believe that the rpms are low because it won't get out of the hole and onto plane. If I get time this week I was gonna raise the jackplate 1 more inch and see if i can gain a few rpms without losing water pressure and get onto plane. If not possibly try a hydrofoil and then a prop


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## GYPSY400 (Jul 18, 2018)

Azoutdoorsman said:


> I habe tried the trim in each position and I have moved weight around throughout the boat. The only thing I have not done is run it with bare minimum gear, extinguisher, pfd paddle and throwable. I'm starting to believe that the rpms are low because it won't get out of the hole and onto plane. If I get time this week I was gonna raise the jackplate 1 more inch and see if i can gain a few rpms without losing water pressure and get onto plane. If not possibly try a hydrofoil and then a prop


Take us a picture of your motor and boat out of the water..(the stern obviously )
why do you need a jack plate?

A 9.9 on a 14ft boat with basic safety gear and running necessities should plane out with one person aboard with the trim on the lowest setting.. don't worry about what your tach says.

I own a 14ft STEEL flat bottom boat and I can get it to plane out when it's not hauling an atv






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## Azoutdoorsman (Jul 18, 2018)

I have a jackplate because my transom is slightly longer than a short shaft and slightly shorter than a long shaft. This is what the stern looks like with fuel tank me and fire extinguisher loaded at the rear of the boat. All other weight is loaded between middle and front seats


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## GYPSY400 (Jul 18, 2018)

Where does the cavitation plate sit compared to the bottom of the hull? Without the transom saver attached
And the motor on the lowest trim setting
And do you have full floors installed? The added weight might be too much for a 9.9
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## Azoutdoorsman (Jul 18, 2018)

On the lowest trim setting it sits about 1 Inch low but when the motor is trimmed out to be horizontal to the bottom of the hull, the anti cavitation plate is level with the hull. The boat does have a floor installed with thin marine vinyl over the top of 5/16 or so ply wood.


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## Pappy (Jul 18, 2018)

In order to get the boat on plane the trim position should be closest to the boat with the engine...or fully trimmed in. This will help lift the transom and push the bow over and on to plane. 
Shiftable weight should be as far forward as possible at least for your "testing" purposes. 
Jacking the engine any farther than you already have it will not make the difference you are looking for. 
At this point I would rather see the cavitation plate about 1" under the bottom of the boat to make sure the propeller is not ventilating and stays hooked up. 
Give this a shot and keep us in the loop.


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## Azoutdoorsman (Jul 18, 2018)

Thanks pappy. I may not have time to get the little boat this week due to taking the big boat camping this weekend. I'll definitely empty the boat and load bare minimum and try again next week and test with everything else loaded in the front.


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## thedude (Jul 18, 2018)

As others mentioned trim closest to the transom to start. If it were me, I would want a 2 stroke in a motor that small.


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## Azoutdoorsman (Jul 18, 2018)

Ya I've tried all positions and right now I'm wishing I would've gone 2 stroke but the reliability of this 4 stroke is what I wanted since mainly this boat is only used for restricted HP lakes. This year has been a real bad year for rain and snow run off so some of our local lakes are beginning to get extremely low and that is why I've started using this again for local lakes.


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## Pappy (Jul 19, 2018)

Reliability compared to a 2-stroke? 
I routinely run 2-stroke engines from the 30's 40's and 50's on 100 mile runs! 
Just teasing you (not about the runs) but a cared for 2-stroke can be handed down to your grand kids the same as a 4-stroke.


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## Azoutdoorsman (Jul 19, 2018)

Ya I know pappy. I'm saying I wish I would've bought an older 2 stroke or a 15hp 4 stroke and re decal to push boat a little better


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## johnbt (Jul 23, 2018)

I really think you should consider replacing your 8.5 prop with a 6.5.

I did, and my pig of a heavily loaded 1648 went from 5.5 mph @ 4500 rpm to 10.7 mph @ 5850 rpm. 

www.tinboats.net/forum/viewtopic.php?f=4&t=43098

All of the power is in the upper end of the rpm range. (It will do 13 or 14 with one person. More than I need for the reservoirs.)

Good luck. John


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## Azoutdoorsman (Jul 24, 2018)

Thanks for the heads up John. I'll try to change it up in the next couple weeks.


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## thedude (Jul 24, 2018)

Pappy said:


> Reliability compared to a 2-stroke?
> I routinely run 2-stroke engines from the 30's 40's and 50's on 100 mile runs!
> Just teasing you (not about the runs) but a cared for 2-stroke can be handed down to your grand kids the same as a 4-stroke.


[emoji3]


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## Azoutdoorsman (Aug 31, 2018)

johnbt said:


> I really think you should consider replacing your 8.5 prop with a 6.5.
> 
> I did, and my pig of a heavily loaded 1648 went from 5.5 mph @ 4500 rpm to 10.7 mph @ 5850 rpm.
> 
> ...



Thank you so much for the suggestions. Finally ordered a 6.5 pitch prop and took it out for a run this evening. Boat went from 4100rpms 7 mph to 5700 rpms and 11mph with 2 people and ran 13.2 by myself. Still may try a hydrofoil to help lift the rear a bit but night and day difference between the props.


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## johnbt (Sep 6, 2018)

That's great, I'm happy it worked out that well. You picked up more rpms than I would have predicted.


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