Prop for 115 2S on SF 20

[oldbluesplayer]

FWIW - I've been running an old Johnson V4 - 115 on my Seafari 20' - the prop is a 13-3/4 by 15, and it spins it readily. I think my voltage regulator is blown, my fishfinder reads input voltage and shows 16+V, and consequently I believe my tach is way off, WOT it's showing close to 8000 rpm, can't be right, the motor would blow...

make a long story short, I'm picking up a 13-1/4 x 17 to give a try.... and gonna fix the voltage regulator. One thing I hear over and over is to Not lug a 2S motor, you need to let them spin up where they want to be, so proppage and tach reading is important.

Bill

[muddywater]

Oldbluesplayer... 8000 rpm... I certainly hope your tach is off
I am eyeballing a 16 pitch Mercury Vengeance right now and wondering if the diameter is too small (13 1/8 I think). Let me know what happens if you happen to get that 13 1/4x17.

I am getting a little ahead of myself here. I just pulled a wiring harness out that crumbled in my fingers, so I have some work to do before I can do any testing.

-Muddy

[eggsuckindog]

I change my answer, I thought it was an 18', your right should be a 15 on a 20 I would guess, sorry.

[Fr. Frank]

The inline-six motors are all Mercury, no Yammie stuff there. They were nicknamed "Tower of Power", and Mercury made them in everything from 70 to 165 hp. They are very good motors. The 115 Mariner should be more than adequate to push a 20'. I agree with going to a smaller pitch prop to keep the rpm's up. PowerTech will custom pitch any of their props for a very small fee.

Here is a very good prop for what you describe as your needs. PowerTech prop

One caveat: the inline 6 motors are naturally balanced (no balancer) and they ALL will self-destruct if run for more than a few minutes at 6600 to 6800 rpm. For some reason, this rpm sets up a harmonic vibration that causes connecting rod failure. Fortunately, if you are not racing, you have no reason to need more than 6200.

In my much younger days I got to oversee the destruction of 2 XS1500 motors from harmonics while racing.
It would be an understatement to say I love the "Tower of Power". I still have 2 inline six motors. The '73 XS1500-EL produced a genuine 165 hp and weighed only 274 lbs. (Mine were on the back of a 14' boat. Yeehaa )

[muddywater]

Fr. Frank I was hoping you would respond. Thanks for the info and the warning.

When I was a kid, I had a 1960something Merc on a 12 foot boat that I sank 3 times. You would think I would have learned after the first time not to make hard turns when only the prop is in the water. That motor was still running when I gave it away a few years back. You have got me beat though. I have seen some of the other crazy cajuns around here with 115's and 150's on 14 foot aluminum flats. It is fun to watch. The guy with the 150 had to stack a bunch of people in front to keep the boat on the water. There is even a Seacraft 20 for sale with a 250 on it over in Baton Rouge.

[Fr. Frank]

My dad got me started racing in spade-hulled "kneeboats", similar to class G when I was 8. My 1st boat was 6 1/2 feet long with an 18hp Merc racing engine.

My fastest "Tower of Power" was an XS1500 on a 14' Cacci Craft, which threw a rod one day at about 82-83 mph at, you guessed it, 6750 rpm. The connecting rod just missed me.

The fastest I got on the water personally was when I was managing a marina and Alan Green, the founder of Quicksilver Surf products, brought me a new 21' Allison with a 225 SuperSix Suzuki, complaining that it would only go 74 mph. The dealer told him he couldn't make it go any faster. I told him for $6K plus expenses I would get him 20% more speed, 90 mph. After blueprinting the hull and changing him over to a Merc 225 EFI, adding a jackplate, nosecone, etc., etc., we got the boat to radar a true 112 mph at 6700 rpm. Only cost him $19K including the motor.

The fastest boat I ever rigged was in Ocean City, MD, when I rigged a homemade 25' plywood and glass Chincoteague Scow with twin 300hp 3.4L big block Mercury motors for . Chincoteague Scow racing is a phenomenon at the Eastern Shore. This boat ran a reported 118 mph, although I never ran it over about 85 when testing it.

BTW, I quit going real fast on the water after I stuffed a 30' Velocity at 65-70 mph off of Ft. Lauderdale during a regatta in 1985. That one hurt. I had bruises in the shape of the throttles for a couple weeks.

[muddywater]

Great stories Frank!! My first boat was a pirogue when I was 5 or 6. You would have smoked me

I might be needing my prop sooner than I thought. I replaced the crumbley wiring and the stator (since the wiring was completely gone on it). I rigged up a new ignition and refilled and bled the trim/tilt which was bone dry. We took the boat to the ramp today. It was it's first time in the water in 17 years. The motor had not been started in 17 years and it is a 27 year old motor. Keep in mind that I did not rebuild the fuel pump or carbs or anything else except wiring and splicing some new stuff onto the idle stabilizer. I did take the advice of some people on THT and sprayed Marvel Mystery oil in the cylinders and stuck some new plugs in it. It started in under half a minute. Another guy who was bringing in his boat came over to look and we noticed that it had a couple dry-rotted internal gas lines broken in half squirting gas all over the place but the motor was running even with the broken lines!!!! It probably would have been easier to start but every time I would pump the bulb on the gas line, the gas would just squirt out one of the broken hoses (which is a really bad thing if you are on the water and trying not to kill fish). Sooo, I will need new gas lines before I do anything else to it. I would not have gotten very far anyway though. The starboard deck drain through-hull has a severe leak and would have sunk the old girl after an hour or so especially since there was no bilge pump. When I got the boat, there was enough mulch in the bilge to plant a garden. You could not even see the bilge pump.

-Muddy

[Bushwacker]

I ran a 115 on a Seafari for years with a 133/4x15 SST prop, motor raised 1 hole (cav plate +1" above keel). Was best all around prop for this boat. Would run 5400-5700 rpm, 32-35 mph depending on load.

[muddywater]

Thanks for the input Bushwacker. Anybody have any comments on a higher diameter/lower pitch prop vs smaller diameter/higher pitch and it's effects on Seacraft speed and handling? What diameter is too small?

For example Mercury Vengeance 13-1/8 x 16" vs 13-3/4 x 15" SS Powertech