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Gary
10-22-2011, 03:44 PM
If you have a x645 prop that is 1.6 + 1.7+2.832 dia. and you take a x648 that is 1.6 +1.90 +3.040 dia. and cut it down to the x645, the 648 pitch/prop will be the same correct? Basically the same prop now as the 645 correct.... or no. They are both 1.6 pitch but different diameters.

Darin Jordan
10-22-2011, 03:48 PM
No... there is ramp-up, cup, etc... ALL KINDS of factors involved. That's why you can't just directly compare...

For example... the 42x55 and the 45x55 are listed at exactly the same pitch... Which is actually faster??

There is so much more to it than just diameter and pitch ratio... it'd take a whole forum to try to explain it all... You really need a good pitch gauge to get a full understanding of how each prop works.

LarrysDrifter
10-22-2011, 03:52 PM
Pitch stays the same. You are only decreasing diameter. Im running an X450/3 cut down to a 47mm diameter with good results. Boat picked up more speed, but heat too as there is more pitch as compared to a stock X447/3.

Darin Jordan
10-22-2011, 03:54 PM
Guys... it's going to depend on the pitch progression of the two props... It's not as simple as simply making them the same diameter...

Gary
10-22-2011, 08:33 PM
Well I will try and answer my own question tomorrow. I cut down the 648 to the 646 that I normally run so will see if there is a speed difference. Could be a waiste of a prop and time LOL.

Boaterguy
10-22-2011, 10:29 PM
this is how I think of it, I don't know if it is right or not but it's what I think is right.
you cannot compare a prop to a screw, because the object it is screwing into is not fluid.
on the other hand, a prop with a larger diameter will move more water seeing as the water surrounding the section that it is moving is not tied to it.
so if you take that "fluid screw" and cut it down, if the pitch was perfect enough that the distance from the hub (and therefore speed) was to keep the exact same pitch- speed proportion, then cutting it down would move proportionally less water. (ie. the graph would be linear)
of course we can't depend on that because on cupping, changes in pitch, and exact prop RPM

Fluid
10-23-2011, 10:12 AM
Well I will try and answer my own question tomorrow. I cut down the 648 to the 646 that I normally run so will see if there is a speed difference. Cutting down the x648 to 46mm diameter could result in a small speed increase, because the effective pitch of the modded x648 is a bit more than the x646. You have in effect made an x746. Reducing the diameter reduces the load - but increasing the pitch increases the load. If the rpm remains the same with the x648, you should go a little faster.

It can depend on just how you reduced the diameter. If all you did was cut a little off the tips, then there will be little change in prop efficiency. If you removed much from the leading edge of the entire blade to reduce the blade area, you may have removed some of the leading edge cupping. This would cause the blades to throw off more water, increasing lift and reducing efficiency. I doubt you ruined the prop, but you have to cut and bend props to learn how to go really fast. None of the props on my record holding cats were "stock", all had some blade modifications.

The prop below holds the Q-Offshore record. It was already de-tongued and pitched up at the tips, I modified it at the race to gain a little speed and I got 2 mph. I am not a fan of the "Barr" cut shown, but in this case with limited tools it was the best I could do. Not pretty but it worked....

http://i54.tinypic.com/2qx6mhu.jpg

.

Gary
10-23-2011, 07:32 PM
Welp, did some back to back testing and the cut down 648 managed 93mph at the same rpm as the 645. The best 645 pass was 92 in exact water conditions and battery temps wich is where it always runs. Next i will try not cutting it down or mabe add some cup.

Fluid
10-23-2011, 08:12 PM
Testing on two different days is not an accurate method of comparison if the difference is so small. The number of satellites can change, effecting GPS accuracy. At those speeds air density can have an effect too. A very slight difference in water conditions can change speeds by several mph, nothing else changed.

I think it is time for you to try different props. What is the prop rpm, amp draw and voltage depression with the x645? What is the model size and weight? What motor and packs? You could try a Prather S230 if the motor will pull it, cut it down if not. Another choice could be an ABC1816 or 1818, either could give more speed. These all will probably need different strut settings than what you use now. At this point you will have to experiment, the "easy" testing is probably over.



.

Gary
10-23-2011, 10:11 PM
It is a PT supersport 45, 12.5 lbs , Neu 1527.5y and it pulls down to 18.50 volts on average at 42,000 rpm. I run 10s 2p, 18.5 volt 65-130c thunder Power. I do not know the amp draw but can make 4- long passes and end up putting 2,000 -2,300mah back into each battery. Thanks for the help Jay.

Fluid
10-23-2011, 10:47 PM
That x646 is laying down speeds that are right on the money by my calculations, so it seems to have enough blade area for the hull. You are pulling a lot of amps though, probably over 300 based on what I logged on my SAW boat last week. I'd try these ABC props, sharpened and balanced:

1718 - smaller diameter, more pitch, may load close to what you have. If it loads too much, detongue.
1816 - same diameter, same pitch. Will probably load more than x646. Will probably need to be detongued.
1818 - same diameter, more pitch. Detongue, may still be too much. Consider back cutting the blades - zero at tips, 0.1" at hub.

I have just begun experimenting with the ABC cleaver props. (I have quite a bit of experience with the ABC H-series, but have seen the cleavers go faster.) The latter have more lift than the Octura X-props, so you will probably have to flatten your strut and/or raise it slightly. They can also bite harder, so be prepared to feather the throttle as the boat accelerates. Don't give up, they can go very fast. You have the equipment to go 100 mph, just prop and final setup is needed.

What timing advance are you using? I was running 20* with my Q-Offshore record holder and the 1515/1Y motor liked it. I gained ~3 mph from a timing change of 15* to 20*.


.

Gary
10-24-2011, 10:28 AM
I run 20*timing. I will order all three props and try them out.I also will try a 648 at the same size just to make sure I coverd all my options with these props before I move to another brand. I always pay attention to the loaded prop rpm and felt that the higher I could keep it, the faster the boat could go and there would be less load on the ESC at the same time. .

Darin Jordan
10-24-2011, 10:34 AM
Gary... 94mph! That's awesome! Looking forward to seeing more results... You have one of the best in the business helping you out. What a blessing! Can't wait to see where you get with this. Have FUN!!

Gary
10-24-2011, 01:48 PM
Thanks Darin. Im hopeing to hit 100 without killing the boat. I have been at 93+ mph for far to long and getting no where so I will keep you updated. Now that I look at the video it looks to be way to much prop spray,but have nothing to compare it too. Lots of sun in vid but you can still see the water spray. http://www.youtube.com/watch?v=mvi9KIth-kg

Gary
10-26-2011, 08:08 PM
Well I just came back from lake for the 3rd time and tried the x648 and 1816 S/B, Barr cut and Tung removed like I normally do. Conditions where perfect with NO ONE around. RPM was reduced -3,000 rpm and speed slowed to around 88-89 mph. I think that a higher pitch at this point will do no good only because I believe the motor just doesnt have the grunt/torque to turn more pitch, wich explains the slower speed. Probably my best bet is a 1527 - 2,300 kv on 25 volts at this point but im running out of steam with this chase. Think I need a break.

lohring
02-29-2012, 06:52 PM
I like to think about propellers like airfoils. Slip is another way of measuring angle of attack. The leading edge pitch shouldn't have too much slip or the prop will become inefficient. Conversely, the boat won't go much faster than zero slip on the leading edge pitch because the angle of attack starts getting negative. The trailing edge will have greater pitch to generate thrust. This can take the form of gradual progression or more sudden cup. The lower the thrust requirement, the lower the progression. Thus riggers usually run props with small amounts of progression and monos like lots of cup. The "average" pitch is what prop ratings show.

We've bent so much pitch into propellers that only the outer area was running a positive angle of attack. There was over 50% trailing edge slip. That happens when rpm is limited but you still want to go faster.

Lohring Miller

carlcisneros
02-29-2012, 08:42 PM
Gary, look at your video once more.
do you see your rooster tail on the boat?
THAT is the TYPE of water you are looking for.
ONE SOLID WALL OF WATER. no pulsing done at all except when you ran over your own wakes.
THAT is what you are looking for. SOLID WATER

Also, you might try the ABC 1816/3 blade cleaver. (they can be a pain to get hooked up) once they ARE hooked up
(the 3 blade cleaver props) watch out!!!!!
The blade area on them is actually less that that of the 2 bladed cleavers that Jim makes.

there is one more little thing on the ABC props, they have what Jim Schaefer refers to as a hidden pitch on the trailing edge.
It is from anywhere from 4.32 to and up to 4.78" right on the trailing edge. ( and that is in addition to the props actual pitch)
I have done alot of testing on the smaller props for Jim a couple years ago for the 20 nitro riggers (actually the CMB Valvola 21) and man, can he make props.
I did testing on the 1500, 1600 and 1700 series for him.

Gary
04-07-2012, 04:14 PM
Just ran some 96's so almost to my goal. Carlcisneros, that rooster tail dosent seem to be to much? I would think minimal rooster means more push and less spray.