Max speed?

[BBQSteve]

Having read most of the related posts about props, batteries, ESCs and even motors, I am now very puzzled. Anyone have any idea of max speed of the stock Mystic 29?

There is a reason for my question. I have been playing around with mine and various props including an X642 which does very well. Apparently too well. The picture tells the story.

IMAG0005.jpg

No I did not take my GPS for a ride in the car nor any other funny stuff. Previous run with stock prop was 65.4 mph. I think this has to be a mistake, especially with my setup, but this GPS has never given me problems before. The GPS trip data was reset before each run including max speed.

Any ideas are certainly welcome. A video of a previous day test is here:

http://www.youtube.com/watch?v=Gv6BWbvlf0o

[john722010]

Steve,
I have just been playing with my Mystic for a couple of weeks. I have the stock motor and prop with Seaking 180 esc and 2X2S 5000 mAmp. My GPS readings are all between 34-35mph and I have been running on really flat water for long straight shots. I think I would check the GPS. If it is accurate then please tell me your tuning secret. I have been playing with battery placement and trying to get a very small amout of porpoising at full throttle out of the boat. I just got a 642 prop which I will add this week and see what it does.

Good luck
John722010

[TheShaughnessy]

im gonna say gps error. Probably happened in one of the corners or something. Physically impossible to go that fast with the given rpms and overall pitch of the prop. and it isn't doing 70 in the video so...

[BBQSteve]

John, I think my speeds are what you are recording also. One thing I thought about was that the GPS was sandwhiched between shaft bulkhead and clear cover, vice secured by tie wrap in a more fixed mounting. Wondering if vibration or as Shaughnessy suggested in the corner moving the GPS.

The fact that it recorded the 65.4 on the first run with stock prop and 73.2 with the X642 seems to indicate at least accurate differential between the two props. I'll have to find a better location or mounting method for next time.

My boat is running fairly reasonably flat depending on which direction it is running. You may have noticed in the video that left to right was almost porpoise free while right to left was a little more bouncy. Left to right was into a strong wind and right to left with the wind. Personally I like the way it rides with the balance I have and corners without much loss of speed, but it is not where they recommend. The rear of my batteries are 7/8" back from the propshaft bulkhead measuring from the back of the bulkhead and 10 3/8" from the indented portion of the hull transom, not where the rudder mounts and not from the underside of the hull. Hope this is clear.

Some other things are, I run hardshell batteries (another discussion on this board elsewhere) vice softshell so you may need to move your batteries back, if they are softshell, more to get the same balance. My shaft centerline is right at the lowest portion of the hull giving the prop a surface profile, half in, half out, at full throttle. And is perfectly parrallel with the bottom tunnel. Again, this is my setup. Some others may agree, disagree or share theirs later.

Shaughnessy, I agree with the physical improbability of going that fast with a 1.4 pitch at about 24000 rpm's (14.4v X 1800 = 26640 allow for load approximately 24000 rpm). At that rpm and using 15% slip may speed with a X642 should be about 50 mph and with stock prop 48 mph. All of this is just theory however.

I was hoping Darin would jump in since he has probably the most experience with this hull and setup.

Anyone know where you can buy an inexpensive radar gun?

BBQSteve

[TheShaughnessy]

Interesting for sure. Maybe try a run with no high g force turns and see what the gps says. Hard to argue the gps but it is also saying u just broke the namba record which, it doesn't look like you are in the vid. Maybe you acheived 0 slip?

[TheShaughnessy]

I had to put in 29000 rpm with that prop and 0 slip to get speed calc up to 72. Has to be an error (sorry) have u tried the gps against the speedo in your car?

[BBQSteve]

I have not had any reason to doubt the GPS till now and really still don't, but I will take it for a ride tomorrow. Hmmm... 29000, that's about 16 volts.

BBQSteve

[HondaHead]

I just ran 36 mph on stock motor and esc with 5s(2 and3 cell)
On a Grim 42x55 prop and started melting solder joints

[john722010]

Been very windy, so I haven't been able to make some additional runs.
I changed the timing and need to see if this improves speed, then I will try a X642 to see what improvement I get.

I run constant 34-35 mph stock with seaking esc and everything stays cool to the touch except the batteries that get upto 115 degrees. Not sure what the loads you were drawing with that prop.
JohnD

[BBQSteve]

Here is an update to my speed. Same setup as before only this time the GPS was mounted fore-aft vice port to starboard. 38.8 mph consistently. I did not change props during this session, but intend to raise the strut 3/8" and try different props soon as this weather clears off again.

Guess the GPS does not like to be mounted as previously stated. Don't know why as the GPS is supposed to work on time and distance, not dependent on direction of device to movement. Anyway I feel better that I can record more accurate data. Look forward to more discussion on speeds.

BTW, this is on 2-2S batteries. If you look, I had a discussion earlier where Darin said the X642 was too much prop for this setup. I stated that my setup was running warm, but not hot (I have not measured temp yet because it is on the low side of warm.) I think you will enjoy the increase in speed and if you have high-C batteries will have the same results. Make sure the prop is sharp and balanced well. I do have a problem with the stock ESC going WOT sometimes when I do not want it too. Luckily I have regained steering in time to avoid some pretty nasty crashes, like into a neighbors boat. Only reason I have not changed out the ESC yet is because I will have to change out connectors to the next size, another point of contention in another thread. I have not had stock connector problems yet either. Total time on water 4 hrs.

BBQSteve

[john722010]

Wind finally calmed down so I was able to get in a few runs with the X642 s/b prop. My speeds with the standard prop averaged 34 mpg and with the new prop I was at 38.1 mph. I will adjust the strut over the next few runs to see what works best. At full throttle the boat has almost lost all porpoising. I would like to put some negative angle on the prop but the stuffing tube is so long that it mates to the strut tight and does not allow for any bending. I figure I can live with this for now as there is very minimal porpoising. I know that I need to blueprint the ride pads as they are not perfectly flat but I plan to do that in the winter as a major project. For now I will tweak and have fun, shooting to hit 40 mph with the 4S setup.
John722010

[BBQSteve]

When bending the stuffing tube, I place a dowel about the same diameter as the distance from the hull to stuffing tube at the exit from the hull to make sure the tube does not bend more at that area. Place another brass tube over the stuffing tube about an inch and bend negative with the strut removed or disconnected. If there is too much length, just file off the stuffing tube just enough to mate at the angle you want.

I find it interesting I am running faster on 4s than you on 5s.

BBQSteve

[john722010]

Steve
Thanks for the hints.
I am only running 4S, I think Hondahead has the 5S setup.
I will try working on the stuffing tube and strut again.
John722010

[john722010]

I just finished filing down my stuffing tube and was able to put a small amount of negative pitch and I set the strut so the shaft is flush with the ride pads. The porpoising has been eliminated and speeed jumped to 40.2mph on the GPS on 2 X 2s cells 5000mAmp. Probably can tweak things more but I am happy with 40 on 2 cells and the X642. The moto,r batteries, and ESC all were between 90-110 degrees.
John722010

[BBQSteve]

Sounds great John, glad you got the shaft where it needs to be. I have been playing with my RC car setup while the wind blows, but will get back to the boat soon. Hmm, 40.2 got me by 2 mph, but will have to put my X642 on and give her a try. Also glad the X642 did not give you any heating problems either. Didn't think it would.