Prop Help! for Heavy 10ft Jonboat with 9.9 hp 2 stroke Merc

[yugom70]

Hi guys and gals, been searching all day on this site trying to figure out what I need. First off boat: Wooden Jonboat 9.9 feet long, 48 inches wide. Heavy, 400 lbs or so, lots of freeboard 24". I have a 2002 mercury 9.9 mariner 2 stroke with factory prop 9x9p. With another 400(motor and gear and me) to 600lbs(another passenger), I should be able to get up to 12 to 24mph according to service manual, I am getting under 8mph (gps) and not planing with just me in the boat! I have removed the prop and am stripping the paint which is failing in places, (will be spare prop when I find my replacement). Also when I trim it up it porpuses. Motor is set to factory specs on boat 1 inch below bottom of boat. Also plan on getting Boyesen Carbon Reeds, so any advice on that would be helpful as well. No I have no RPMS, the cost of a tach is what a prop costs. Looking at a 4 blade prop since its a little heavy, what do you guys think???

HELP!

 

[pootnic]

Re: Prop Help! for Heavy 10ft Jonboat with 9.9 hp 2 stroke Merc

Might not be set up right.I know you say "factory specs"?? but every boat will be different,especially small ones.
I'd try to get it as light as possible and play with the motor settings(height,trim angle,weight shifting),see if you can get it to atleast plane.
Sounds like the bow might be plowing,put some weight in the stern and give the motor some more angle,might help.
600lbs in a 400lbs boat is pretty heavy in a 10' span,your going to have to play around abit to get it to plane.
Those tiny tachs are around $50 and to tell you a specific pitch would only be a wild guess for me.
I do have an old wood/fiberglass that my grandfather made(weight unknown),I think it's around 11'.With a 9.9hp it will flat out fly but I have to sit almost on the stern or it will start to plow,same pitch as your current prop.

 

[yugom70]

Re: Prop Help! for Heavy 10ft Jonboat with 9.9 hp 2 stroke Merc

Might not be set up right.I know you say "factory specs"?? but every boat will be different,especially small ones.
I'd try to get it as light as possible and play with the motor settings(height,trim angle,weight shifting),see if you can get it to atleast plane.
Sounds like the bow might be plowing,put some weight in the stern and give the motor some more angle,might help.
600lbs in a 400lbs boat is pretty heavy in a 10' span,your going to have to play around abit to get it to plane.
Those tiny tachs are around $50 and to tell you a specific pitch would only be a wild guess for me.
I do have an old wood/fiberglass that my grandfather made(weight unknown),I think it's around 11'.With a 9.9hp it will flat out fly but I have to sit almost on the stern or it will start to plow,same pitch as your current prop.

Its not plowing its porpusing actually, I also shifted the trolling battery to forward position, I'm almost thinking I need a 4 blade prop, with trim tabs for this tank. I will get some pics up tonight so you can get an idea what I am looking at. BTW I built this thing myself, haha.

 

[steelespike]

Re: Prop Help! for Heavy 10ft Jonboat with 9.9 hp 2 stroke Merc

I'm confused how does a boat porpoise when it doesn't plane.As suggested that little boat doesn't have enough of a foot print to get 800 to 1000 lbs up on the water.
You need to lighten the load to just you and the motor ,paddle and gas and see if it will plane.We need to know your wot rpm and gps speed. A tinytach will get your rpm. some cells have a gps app.

 

[yugom70]

Re: Prop Help! for Heavy 10ft Jonboat with 9.9 hp 2 stroke Merc

When I apply power the front end pops a wheelie but doesn't achieve plane. I am going out fishing tommorow, will just take motor and gas and see if that works for me. Also going to adjust angle of trim position on outboard. I hope you are wrong about the footprint, I really would like to plane .

 

[steelespike]

Re: Prop Help! for Heavy 10ft Jonboat with 9.9 hp 2 stroke Merc

There are a 8" and 7" pitch props available I think your 9" is the highest pitch so surely not appropriate for that load.
If you can get it to plane we can tell which prop might get you to the correct rpm.
Would be really good if you can get the rpm at some point. gps speed will help a little.
Normally you would trim down (prop closer to the boat) to try to plane.
If you could rig a tiller extension it might help get it planed.
Even if you could just lean way forward with finger tip control.

 

[yugom70]

Re: Prop Help! for Heavy 10ft Jonboat with 9.9 hp 2 stroke Merc

My gps speed was 8mph, feels just short of plane. I ordered a 4 blade 8"pitch prop. Also I am going to try and angle the motor a little in towards boat since my design was a straight up and down transom, trying to get an angle of 15 degrees on outboard. Also wondering how an outboard bracket might help, having the motor say 8 inches off the transom how that might effect things. I am going to pick up some kind of a tachometer as well so I can get into the 5 to 6000 rpm range (once I get it to plane). Sheesh!

 

[steelespike]

Re: Prop Help! for Heavy 10ft Jonboat with 9.9 hp 2 stroke Merc

if you built the boat you could build a jackplate and build in the angle.Also you could cut some wedges to put between the mount and the transom. If you do a search on the net for home built jackplate.

 

[yugom70]

Re: Prop Help! for Heavy 10ft Jonboat with 9.9 hp 2 stroke Merc

Could not make it out today, next monday after work will be out. New prop will be here tommorow, installed boyesen power reeds, and buying timing light to dial in motor. So after timing, and prop, and possible .044 main jet (suggested by boyesen) will be ready to go Monday. Transom was adjusted to 15 degrees the other night, motor looks good.

 

[Star]

Re: Prop Help! for Heavy 10ft Jonboat with 9.9 hp 2 stroke Merc

That 9.9 should make a log plane and the 9 x 9 should do it!!! I don't think you motor is running right. Is the boat plywood? Pics would be nice.

 

[yugom70]

Re: Prop Help! for Heavy 10ft Jonboat with 9.9 hp 2 stroke Merc

That 9.9 should make a log plane and the 9 x 9 should do it!!! I don't think you motor is running right. Is the boat plywood? Pics would be nice.

I got this motor dialed in last night, and new prop arrived and installed. After closer examination on older prop the shaft sleeve of the prop seemed a out of balance possibly causing the cavitation that I was seeing, looks like a spare now after I get it tuned up. I think its a trim issue more than anything, so the adjustment of the transom should help quite a bit since motor is now parallel with boat. Find out on Monday. Wish it were sooner . Ok pics up so you can see the tank of a small boat. Still tweaking some things on trailer and boat itself later today.

 

[yugom70]

Re: Prop Help! for Heavy 10ft Jonboat with 9.9 hp 2 stroke Merc

OK been out fishing with the new prop, power reeds, installed tach (still not reading rpms right). My engine runs alot better at idle with the reeds and prop for sure, and when I open it up the *** end digs in deeper than ever and forms a large boat wake. My next thing to try is to make some custom wedges and bring the prop up a bit just to see what it does. Can't afford a 110 bucks for the trim tabs at the moment. Any other ideas? Sheesh! Oh yeah speed has decreased to 6.7 mph ( from 8 gps) but I had a kid fishing with me, but no battery or trolling motor so almost the same weight really.

 

[steelespike]

Re: Prop Help! for Heavy 10ft Jonboat with 9.9 hp 2 stroke Merc

The transom needs to be at about 15 degrees,either by wedges or a jack plate.
When you get the transom right you need to strip out all the weight and see if it will plane. Without a tach save yourself the time and just throw your money in the waste basket.
Until you strip out the weight and get it to plane we can't tell anything except that your original prop did better than the new one.
When a boat doesn't plane the motor can't get into its rpm range its pretty much impossible to tell what prop or what it might do.
If it won't plane with the weight out check that it is running on both cylinders. Pull one plug wire at a time and ground it and see if it will start on one cylinder at a time.You might try running it and carefully feel each plug if one is dead it will be hot but not like the one that is firing.


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[yugom70]

Re: Prop Help! for Heavy 10ft Jonboat with 9.9 hp 2 stroke Merc

The motor is running great, and I have adjusted my transom to 15%, and will be out fishing hopefully sunday as light as I can get. Digital tach needs to be set to a different setting and my directions were left at home. Other than that still have not figured this out, I am hoping raising the motor will do somthing, we'll find out.

 

[steelespike]

Re: Prop Help! for Heavy 10ft Jonboat with 9.9 hp 2 stroke Merc

You may be up against it A typical 10' al jon probably weighs 65 lbs dry.
With the boat light, if you go out by yourself a tiller extension to get your weight forward
may help.
I used to plane our old 15' plywood boat with 8 or 10" of water in it.
It was 18 hp and I would go up forward and get my 150 lbs partially on the front deck.
Once planed I could go back and open the drain. Of course I had twice the hp and my planing surface was probably about 78 square feet if your lucky yours is probably 28 sq ft. you can't count that which isn't in the water.

 

[Texasmark]

Re: Prop Help! for Heavy 10ft Jonboat with 9.9 hp 2 stroke Merc

My 2c is and his later comments reflect it that his bow is way up in the air and due to his straight transom and all with him, battery, engine, trolling motor and all, on a short planing surface, he can't get the bow down. Just too much "stuff" aft. Course if he had a 25 hp engine he could power himself through the ditch and with that rig fly once up and running.

I'm for the 15 degree transom mod too.

I ran a 7.5 Merc on a 10' Lowe Lake John, high sides, wide beam for a john that size, and all my gear, sometimes with another person and had no problems getting on plane or hauling arse for years. It was just me and the engine back there. Everything else was up forward but I did have a factory transom so I had the tilt we are talking about. Engine was out of the crate. Stock.

Mark

 

[yugom70]

Re: Prop Help! for Heavy 10ft Jonboat with 9.9 hp 2 stroke Merc

My 2c is and his later comments reflect it that his bow is way up in the air and due to his straight transom and all with him, battery, engine, trolling motor and all, on a short planing surface, he can't get the bow down. Just too much "stuff" aft. Course if he had a 25 hp engine he could power himself through the ditch and with that rig fly once up and running.

I'm for the 15 degree transom mod too.

I ran a 7.5 Merc on a 10' Lowe Lake John, high sides, wide beam for a john that size, and all my gear, sometimes with another person and had no problems getting on plane or hauling arse for years. It was just me and the engine back there. Everything else was up forward but I did have a factory transom so I had the tilt we are talking about. Engine was out of the crate. Stock.

Mark

I did the 15% transom mod allready everyone. The only other thing I can do without trim tabs if raise motor and transom up, so tommorow weather permitting taking 1 1/2" wedge to raise motor and prop up to see if anything changes in the dynamics. From research I have done where the prop is located in the water in relation to your boat can make a big difference, think racing boats. Pain in the *** making an original design type of boat. I have also tried moving my weight forward with only me in the boat, still slugging. I need an expert here in the Seattle Tacoma area lol!

 

[yugom70]

Re: Prop Help! for Heavy 10ft Jonboat with 9.9 hp 2 stroke Merc

I tried to edit last post, the only other thing I can think of doing is trim tabs other than raise the transom and add trim tabs I what I meant to say. I am stumped! Hopefully I can figure out how to get the tach to work proper too, but it seems like its ripping up the rpms at WOT. There are alot of bubbles but the exhaust exits out the prop too so hope its not cavitating.

 

[steelespike]

Re: Prop Help! for Heavy 10ft Jonboat with 9.9 hp 2 stroke Merc

Your really up against it with your weight.Picture a 9 ft dingy about 100 lbs with a 9.9 and one person it would really scoot but add two more adults and it will struggle.Your way beyond that.
In the old days we called the stick you put on top of the transom to hold the motor uo "rev sticks"
Your obviously handy you might make some "after planes"A wooden structure to extend the bottom beyond the transom
6 or 8 inches.It would add some planing surface where you need it most. these would be full width except for and area open for the motor.These aren't to handy for backing up. Perhaps you could fabricate large simple trim tabs that would be simpler than an after plane
yet could be adjusted slightly for best affect.

 

[yugom70]

Re: Prop Help! for Heavy 10ft Jonboat with 9.9 hp 2 stroke Merc

http://www.veradoclub.com/smf/index.php?topic=137.0
This what I found that I am going to try, cause after I get in the back of my boat my ventilation plate is a solid 6 inches in the water, creating a huge drag, So will try and raise the transom up and another few inches and add a block to raise it up another 1 1/2" or so to see if that improves anything.