What should I expect for RPM at WOT? It’s on a scout 172. The prop is a 14x19 and is currently turning 4500 at WOT. This seems on the low side to me, but since I just got the boat I’m not sure if its too much prop or maybe something else is going on. Thanks
Most 2 strokes that I’m familiar with should be propped to run between 5,500 and 6,000 with a medium size normal load. 4,500 is way too low. It either has too much pitch in the prop, or the engine isn’t right. 19" is a lot of pitch for that rig and HP. A 15 or 16 pitch will put you a lot closer to where it needs to be, assuming the engine isn’t the problem. Can you borrow a different prop to try? I think that is the problem.
Capt. Larry Teuton
Swamp Worshiper
4500-5500 full throttle operating range per operating guide. I have a 2004 and run WOT about 4500
17 MAVERICK HPX-V
What age, model, etc. Is your motor?
I have a '99 Johnson 90 (60 degree v4) and my wot is ~5300 all trimmed out. Boat is a 99 Sea Hunt 172. That’s all I got at the moment, don’t know what prop is on there but I can look this weekend, maybe tomorrow.
You need a 17 pitch prop or either some cleaner water from a jack plate. I run 5200 RPM WOT with my 19 pitch, but I have a jack plate, and trim tabs to allow me to get maximum trim without the boat porpoising. That boat will run better with a lower pitch. Good luck.
“Wailord”
1979 17’ Montauk
90 Johnson
Wilderness Ride 115
ideally you will be at 5500rpms WOT trimmed up with average load.
is that rpm trimmed up or down
a 19 pitch should be about right for that boat and motor combo
if the motor is running correctly a 19 pitch prop turning 5500rpms with a 2:1 gear ratio (which is what the Johnson 90 2 stroke has) and 13% slip (normal) will be about 42 mph, and that’s about right for that set up
I would think you have a motor issue or its not trimmed properly
how fast are you going at WOT?
The motor is a 1990 V4. I didn’t think to take a look at the top speed while I was running. It will probably be a couple weeks before I get back out on the water, so I’ll have to see at that time. It felt like I had to keep the motor trimmed down lower than I have on other boats I’ve owned. With trimming the motor up much at all the bow would rise up quite a bit.
Was able to sneak off to the lake yesturday afternoon. With me, another adult, and two small kids the boat would run 34 mph (GPS) and turn about 4800 WOT. The whole shot is about ok unless we hook up the tube with the kids, its pretty crummy with the tube. Thoughts?
I think you will find very few 17’ hull setups with a 90 Johnson that can turn 5500 rpm. Short of a bass boat or lightweight skiff. You are going to have to settle for the 5000-5200 range with a typical load, and you are only going to get there with a good SS 17 pitch prop.
The reality is, in that setup with a light load you will porpoise before hitting 5500.
“Wailord”
1979 17’ Montauk
90 Johnson
Wilderness Ride 115
something isn’t right, and I don’t think its your prop
with those numbers you have almost 22% slip
here is a performance bulletin with a very similar setup
http://yamahaoutboards.com/sites/default/files/bulletins/SCT_175Sportfish_F90LA_2013-09-27_OCC.pdf
13 x 19 prop
its a 4 stroke so its turning higher rpms, but it has a lower gear ratio, so its pretty equal
at the speed and rpm listed, its 11% slip, which is where you need to be
I still think you have a motor or an operator (trim position)issue
Chris,
Would my prop being a 14 not a 13 have any effect? Looking at that bulletin my speed and RPM match pretty closely up to when I quite gaining RPM.
You are getting slip. Comparing your rpms and speed to this report is futile. You have different gear ratios and max rpms. At 4500 he has 11% slip, you have 23%. That’s wasted fuel blowing out of the prop. You should be running about 36 mph at 4500 RPM, and a very comfortable cruising speed at 3000-3500. Not barely getting off plane like this boat is doing.
Something isn’t right in your setup. Too much weight, weight not distributed properly, motor mounted too far down, mounted too far up, peer performing prop design for that hull, too high pitch, etc. You need to get the slip down to gauge performance. Either make changes to suit that prop, or start trying a different prop.
If you have the SST painted stainless prop, or running an aluminum prop your performance will suffer. You need a good SS prop like a Viper, or Stiletto on that motor.
“Wailord”
1979 17’ Montauk
90 Johnson
Wilderness Ride 115
yes, a 14 is going to be harder to turn, its larger in diameter
as for the performance bulletin, that was to show your boat with a 90hp motor and the speed with a 19 pitch prop
you will not be able to compare mid range numbers
quote:
I have a '99 Johnson 90 (60 degree v4) and my wot is ~5300 all trimmed out. Boat is a 99 Sea Hunt 172. That's all I got at the moment, don't know what prop is on there but I can look this weekend, maybe tomorrow.
this would be exactly what you would expect to see from your setup with a 19 pitch prop.
5200-5300rpms
a 17 pitch will get you 5500-5600
going from a 19 to a 17 isn’t going to gain 1000 rpms and 8mph
Thanks for all the input. I went out and gave the motor a once over. The guy I got the boat from said his mechanic had disconnected the VRO so he just premixed. This isn’t an issue for me however what they called disconnecting the VRO was just unplugging the electrical plug. The oil line was still hooked up and ran down to the place where it exits the cowling. I’m pretty sure this line needs to be plugged or it would suck air where it would normaly suck oil. Is this correct and could it cause what I’m seeing? Thanks
I would suggest capping the line, but that shouldn’t cause the low rpms
As an FYI, I run a 2001 Keywest 1720. Prop is a 17 3/4 X 17 SS. I was also concerned about my RPM range thinking it was too low. I went to Skip’s propeller service and talked with him and he advised that I had the correct prop. 4500-5500 was the range. He advised not changing to a lower size unless I started experiencing planing or power issues. I wasn’t convinced, but took his advice. Next time I was out by myself, I nailed and trimmed and hit 5000 RPMS. Mid-range. My boat runs good, planes fast and handles well. Like I said. FYI
Thanks again. I ran a compression check today had 130, 129, 129, and 125 not sure where else to go from here.
Not sure if it helps, but I am running a 19 pitch stiletto that really runs like a 20 pitch.
I run about 5200 rpm wot under most any load/conditions. My Whaler is a little similar in weight, length, and hull shape.
I have hit 5500 rpm one time. That is with trim tabs, a jack plate trimmed up about 4", 4 gallons of fuel, by myself, in a 2’ chop with wind to my back. It hit 47 mph which is about 3% slip. Without trim tabs and a jack plate it was hard to get any efficiency out of a 19 pitch prop on this V4 Johnson. The boat would porpoise or start to blow out prior to any great WOT. That is why I suggested trying a lower pitch prop. You will likely get more bite, and also pick up some rpm from the lower pitch. Combined, you might be able to get to 5300-5400 under most any load, have great hole shot, and run 40-42 mph.
Another option is to check out your prop and make sure it isn’t damaged, or have Skip’s add some cup for extra bite. Then you can play with the motor height and you might be able to make the prop you have work. Props are tough to figure out, and even tougher to diagnose over the internet. You really need someone to ride with you, take down some numbers, and talk to a prop shop. Not a dealer, most boat dealers don’t know schnit about prop performance. With the exception to Chris.
“Wailord”
1979 17’ Montauk
90 Johnson
Wilderness Ride 115
good compression is a start, but you could have a coil failing, a dirty carb, a fouled spark plug, the motor mounted to low, the hull water logged, any number of issues.