Pontoon Boat & Deck Boat Forum

Well tonight I finally got a chance to install the new prop.
It is a 14 x 10 with extra cup, (Black Max).

The good news is that my speed increased from 14 to a little over 17 mph, (gps), but the bad news is that the rpm's are way too low.
At WOT I'm only hitting about 3600. That was with only me and my wife on board. When I ran the prop calculator I figured myself, my wife, my three adult age kids and about an extra 100 lbs. in gear, (coolers, fishing gear, etc.). Tonight we were probably running about 560 lbs. less than I entered into the calculator. I have no idea what kind of rpm reading I'll get when we have a full boat tomorrow night.

I'm sure that my motor is mounted too low since I cannot see the cavitation plate while at WOT. In fact it appears to be at least two to three inches below the water surface. I have rooster tails shooting up to the top of the motor which is surely a sign of a lot of "drag". Is that possibly what is causing my low rpm readings?

I plan on raising the motor the next time I have it out of the water and in the meantime I simply plan on running low rpms out to a fishing spot and using the trolling motor.
Also, what can I expect once I add the extra 550+ lbs. to the boat tomorrow night?

I don't want to hurt my motor but I also don't want to take it out of service until the end of the season. At that point I'm going to have a local mechanic do a full tune up on the motor and will also have him make sure that I'm really at WOT when I think I'm at WOT. And like I said before I'll be utilizing a cherry picker to raise the motor at least a hole if not two.

I just hate to spend yet another $150.00 - $200.00 on the chance that a different prop will work. If raising the motor and giving the motor a tune up will work, that's the route I'd like to take. Why does this have to be so damn difficult?

OK... So first off I have to question the accuracy of your RPM readings. I'm quite skeptical that you were doing over 17 MPH at 3,600 RPM...

Why you say? My 18 foot boat with a 60 HP motor struggled to break 18 or so MPH and rarely did turning well over 5 grand with various 14" props ranging from a 10 to 13 pitch... So the simple fact that you went over 17 MPH on a 20-foot boat with an older 50 HP motor tells me your pretty much right in the ballpark of what is to be expected...

Okay, Teecro......you've got me thinking now....

I did make sure that I had the china tach set to the proper setting. Just in case someone else uses one of these, the proper setting for a four stroke is two strokes per cylinder. This is the one setting that I hadn't tried yesterday.

The way this simple tach reads is a pickup wire is wrapped around a spark plug wire three or four times and then zip-tied off, to itself. If I remember correctly I wrapped the wire around either the number two or three cylinder and I think I gave it four wraps.

I think I'll try the following tomorrow:

Wrap the pickup wire around the number one cylinder plug wire. I think I'll also give it a few extra wraps to make sure it's getting a good reading, (maybe 5 or 6 wraps as compared to three or four).
I also think I'll wrap the wires with electric tape instead of using a zip tie.....the zip tie seemed to allow the pickup wire to be less than tight on the plug wire. That might have affected the reading.

I was very happy with how smooth the motor was at idle, (no vibration at all), and that the motor was running about 800 rpm at idle. I was also very impressed at how quickly it got onto plane and that the bow was MUCH lower than it has been previously. In fact when I first throttled up my wife was squatted down, getting the gps out of our carry on bag. As soon as I pushed the throttle forward she ended up on her butt because of how quickly the boat accelerated.

You might be onto something Teecro......Let me do a little more testing and I'll get back to you after the weekend.

Honestly, I really appreciate your help and input. You've been great giving me differing things to try and consider.

Dave

This comes from the real "Tiny Tach" website... "Some engines, such as multi-cylinder outboards generate considerable electronic "noise" and may require a light filter to assure a clear RPM reading." So I can't help but wonder if your knock off is very sensitive to noise...

Teecro.....

I would have to say that you called it exactly right. The "chach" is totally off of it's rocker.

Tonight we went back out and tried adjusting the pickup wire. I moved it closer to the plug and made sure that it had good contact with the plug wire. When we left the dock I felt really confident that I would be getting more accurate readings.

Well I had two of our adult children on board along with my wife and a full cooler so I expected the speed and r.p.m.'s to drop some. We were carrying approx. 360 extra pounds and also had about a 5 - 7 mph wind that wasn't present last night. Like I said, I fully expected some drop in performance.

If you will remember my wife and I hit a little over 17 mph last night on a perfectly calm lake. Tonight with all of the added weight and the wind we still briefly hit 16 mph going out, (wind coming from the side).
At that point we checked the tach and it was showing WOT at 2360. Yeah.....as if that was right.

Even coming back in we were hitting 15 mph, (with a head wind), and dropping back to 3/4 throttle we were cruising along at 14 mph. I didn't even bother checking the tach at that point.

So it appears to me as if we are propped up really well. I get up on plane FAST and I have to trim down a little to keep the bow from rising too high. If I trim all the way up it sound as if the motor starts to rev a little and the bow comes up pretty high. With just a bit of down trim the bow levels out and the motor sounds great.

Now having said all of that I am a little OCD when it comes to making sure everything is right. I think I'm going to go ahead and buy an original tiny tach with extra long lead and filter. That way I can run the lead up to the console and keep an eye on the rpm. I just want to make sure that I really am spinning this prop at the right speed. I don't want to ruin a motor to save $50.00 or so on a tach.

Again, I really appreciate your help. The information you posted from the Tiny Tach site really made me start to question the accuracy of this knock off. I think I'll just take it off of the boat and put it on my backup lawnmower as a way to monitor service hours. It might also come in handy to adjust idle and full throttle on a chainsaw or generator. I'm sure I can find some sort of use for it, other than the boat motor.

Have a good weekend.

Dave

Glad you day went well...

Keep in mind that for 70 to 90 dollars you can get a real boat tach for the likes of www.overtons.com

http://www.overtons.com/Boating-Marine/ ... _refined=1

Point taken, Teecro.

Once I lift the boat for the season I'm going to take it to a local mechanic. I'm planning on having him replace the impeller, add a tach and volt meter and do a general tune up on the motor. He said that the impeller and complete fluid changes, (power unit and lower unit), would be $100.00. I figure if he's that cheap on the fluid changes I won't be out much to have him add two gauges that I think are important and also change plugs, wires, etc. I simply want the boat ready to go when spring comes around. We had no idea we were "boat people" until we made this purchase. Now we can't wait to hit the lake at every opportunity. If only life would stop getting in the way of our relaxation!

Likesspace wrote:... We had no idea we were "boat people" until we made this purchase. Now we can't wait to hit the lake at every opportunity. If only life would stop getting in the way of our relaxation!

Even though I grew up on the bayous of LA with my Father three years ago I would have never so much as entertained the idea of purchasing a boat much less 2 of them... I've worked on boats (550-ft) for closest to 30 years and wanted nothing to do with water while I was off from work. We moved to a lake view home in TN and today I can not wait to get home to the jadeite green waters, crack open a bottle of beer and go get me some marina food...