12.5' with 9.8 running slow!

[saabsaviorlee]

Hey guys I graduated from my sevylor fish hunter 280 into a 12.5' wood floored dinghy from newport vessels. Took her on the maiden voyage to lake pleasant in az with an 82 mercury 9.8 it was myself, a buddy, cooler, 6 gallons of gas, 1 marine deep cycle battery, 30#minn kota, and assorted fishing gear. GPS clocked us at 6.2 mph does this speed sound normal? I had the motor serviced before the trip with new plugs fluids and overall bill of good health. Should I be getting faster speeds? Does octane affect speed in this 2 stroke motor? I'm not sure what pitch the prop is and I haven't had a chance to run it solo or with less gear. Any advice would be appreciated!

 

[Lyle29464]

Re: 12.5' with 9.8 running slow!

A lot of weight. You must know the RPM's. Then prop accordingly. If you have too much prop you can ruin the engine. Check out "Tiny-Tack .

 

[Drowned Rat]

Re: 12.5' with 9.8 running slow!

Agreed, that's a lot of weight for a 9.8hp on a boat that size. 6-8 mph is right in the ballpark.

 

[saabsaviorlee]

Re: 12.5' with 9.8 running slow!

Thanks for the replies guys. I'll look into shaving weight where I can and find out what I'm at in the rpm band.

 

[saabsaviorlee]

Re: 12.5' with 9.8 running slow!

I found out the pitch is 7" and the prop reads 7p a1 should I try reducing pitch?

 

[Sea Rider]

Re: 12.5' with 9.8 running slow!

Too poor engine, that sib must be used with at least a 15 HP, although 18, 20 woul be ideal for optimun performance and carrying more passengers Check with you solo if it's possible to plane the sib.

Happy Boating

 

[saabsaviorlee]

Re: 12.5' with 9.8 running slow!

I would love to rock a 20hp and I plan to move up eventually but the best fishing near me is in 10 horse restricted lakes so I'm trying to get the most oomph i can outta my 9.8 my bow will rise and I think it settles onto plane as its setup now it just cruises so slow haha. Should I have the tilt all the way down against the transom or at an angle my motor has pins to adjust the angle any thoughts?

 

[Sea Rider]

Re: 12.5' with 9.8 running slow!

Inflate well all tubes and keel to factory working pressures (3.0/3.5 psi) Place rod at hole so to have a perpendicular engine with respect to sea level once sib is floating, move your weight towards bow and full throttle, bow should rise up and then drop down, try solo and check. There's a great CC difference between a 9.8 and 9.9/10 HP engine.

Happy Boating

 

[fbpooler]

Re: 12.5' with 9.8 running slow!

Tach it as a 7 Inch pitch is a low pitch prop for that motor. We used a 5 inch on a 20 ft deep vee which is the barge prop, and most modern lower units have a 2:1 +/_ ratio. Too low a pitch can damage the engine by allowing it to over rev.

Don't know your elevation, but that can have an effect also. I also agree that is quite a load for the setup but am surprised by the low speed you found. Go to the Mercury Outboard site and run through the prop selection program.

 

[Sea Rider]

Re: 12.5' with 9.8 running slow!

You must install an induction tach to read actual max rpm at wot, readings will determine if under or over propped, then check prop charts to match the one that will give best performance at near max rpm factory stated for your engine. Buy a Tiny Tach...

Happy Boating

 

[saabsaviorlee]

Re: 12.5' with 9.8 running slow!

Thanks for all your tips. I ordered a tach from eBay and a 2 blade 9x10" prop. Currently I have a 3 blade prop, will going to a 2 blade help keep the rpm up with the higher pitch? Also prop selection was incredibly limited is there anything available to change spline numbers or fit the newer 9.9 hp mercury max props on my 82 9.8?

 

[Sea Rider]

Re: 12.5' with 9.8 running slow!

Install tach and go for a wot spin, get max rpm factory parameters for that engine and check achieved rpm at wot, change prop and check which one achieves the nearest max rpm for that engine. Post results.

Happy Boating

 

[fbpooler]

Re: 12.5' with 9.8 running slow!

I cannot find specs on that motor, so I will make wild guesses. Assume 5000 to 6000 RPM range, .5 lower gear ratio, 7" pitch prop. We will use 5500 RPM for calculation.

Formula is RPM x Gear ratio x Prop pitch / 1056 = MPH therotical sped

5500x 0.5 x 7 / 1056 = !8.23 MPH
Assume 10% prop slip = 16.4 MPH
20% slip = 14.6 MPH

PLease keep in mind that my calculations are a wild guess.

I would agree the motor is not delivering the speed for some reason. It could be too much weight for the rig, improper loading and/or inflation or some other reason. Have you done a compression test? My guess is that the motor is not developing the RPM it needs to provide the design horsepower.

Please let us know what it tachs out in service, Sea Rider is giving good advice.

Note that the formula does not involve horsepower but does address engine RPM. The engine must have enough power to run in the operating range, so adding pitch to the prop you have will result in straining the engine even more than it is presently being strained. That could destroy your engine. The tach readings are very important in your case.

Going to lesser blade area by going to a two blade prop at the same pitch would unload the engine somewhat, but only with a tach can you determine the effect.

 

[Lyle29464]

Re: 12.5' with 9.8 running slow!

I would love to rock a 20hp and I plan to move up eventually but the best fishing near me is in 10 horse restricted lakes so I'm trying to get the most oomph i can outta my 9.8 my bow will rise and I think it settles onto plane as its setup now it just cruises so slow haha. Should I have the tilt all the way down against the transom or at an angle my motor has pins to adjust the angle any thoughts?

10 HP decals are cheap.

 

[saabsaviorlee]

Re: 12.5' with 9.8 running slow!

fbpooler, that's what speed I was thinking I should be seeing. Given weight and everything I would think at least 10mph. As for as weight I mean i guess I could stand to lose a few pounds but aside from a trolling motor battery I don't see anything that would be drastically over weight. I'm going to experiment with where I put the weight as I believe the rear was overloaded and thinking back I think I was squatting the rear end. Hopefully If i can level out the weight it will lower the nose onto plane and Ill be better off. Also how much impact could having it in the 1st trim position via the removable pin vs the bar built into the bracket cause? (best guess maybe 10-15 degrees) I'm hoping I'm just an idiot and unintentionally had it trimmed too high and all this stress is for naught. My Ebay tach is en route and Ill be testing her out again this labor day weekend. Thanks for all the advice and Ill let ya know whats up!

 

[saabsaviorlee]

Re: 12.5' with 9.8 running slow!

10 HP decals are cheap.

Unfortunately 20 hp motors are not, but I like your problem solving approach!

 

[fbpooler]

Re: 12.5' with 9.8 running slow!

We have a 10 ft Avon Rover with a 9.8 Nisson. With two people and fishing gear, it planes readilly at a comfortable speed. We had four people in it several years ago, and the speed was similar to your situation.

You really cannot make good comparisons, but with the battery and trolling motor, you effectively had three aboard. Now a 12 ft boat will plane easier than a 10 ft but obviously you had too much weight, wrong trim setting, wrong pitch prop, improper loading or some other reason to keep the boat from planing. Until a boat planes, the speed is low and the engine is heavilly loaded. You sometimes can overcome that with a prop of lesser pitch or smaller diameter, but then you want to watch that the engine does not over rev at speed. That tach will be your friend in working through this.

You asked about octane. If there is any question that the gasoline was not fresh, replace it.

 

[saabsaviorlee]

Re: 12.5' with 9.8 running slow!

Tach arrived and installed! Updates to follow this weekend. Stay safe this holiday weekend!

 

[saabsaviorlee]

Re: 12.5' with 9.8 running slow!

Alright fellas I have more information. With the tach installed it revs to 2300 rpm WOT fairly empty boat. Just the 9.8, tank, myself, girlfriend, cooler. When she sat on the cooler at the bow it leveled out and seemed to plane and speed up. The wake was less pronounced as if the rear end wasn't getting dragged. Should my motor be producing more rpms?

 

[fbpooler]

Re: 12.5' with 9.8 running slow!

Make sure the tach is adjusted for your motor. If so, have a compression test run and also the crankcase pressure tested. A leaking crankcase seal will rob horsepower. If it is a reed valve engine, you could have a bad reed valve set, bent or broken leaf which does not seat properly. You could also see problems from bad gas partially plugging one or both high speed jets in the carb with gum. This gas problem is very common if gas is left in carbs after using a motor. I would check it first, as it could very well be the problem and is easy to fix.

Bottom line is that the engine is not performing as it should. Forget about trying to do anything with a prop until the engine is in good condition.