Computating prop slippage. Reasonable #s

[175sf]

:confused:What would be a reasonable # to shoot for without blowing alot of money? I have a bassboat w/200 promax.,lwp w/bobs. 187.1, 10 inch setback, 2.5 inches below pad, load with 3 batts, 24 gal gas, etc. ran today with 28 big ear chopper 5700 rpm/1.87/28pitch/71.5 gps= 0.115 Does that sound right? I then ran (I think its a stalleto) looks like chopper with some cut, it was also a 28 pitch. Hole shot was alot better. Its #s was 6400 rpms/1.87/28pitch/68.5gps= 0.245 slippage. The big eared ran effortless. You think these numbers are within reason for me to send this prop for an offshore cut and raise the motor up to bring the r's w/big ear up to about 6500? Feels like I'm lugging this engine. Thanks for any advice. Jeff

[staylor]

..and the Stilleto is not really considered as being a prop that runs very well on an engine thats raised up- so its just not hooking up. The chopper numbers seem reasonable for the rig as listed. Why don't you try raising the chopper up in its current form to see if you gain rpm? When the prop is cut you've reduced blade area which tends to increase slip, which is not considered as being the ideal way to gain rpm compared to simply trying a prop with a bit less pitch. In addition, with the slip you list for the chopper, the normal approach would be to have a prop shop add some tip cup to reduce slip- but this also adds some pitch which will drop your RPM further.You might look around to see if you can try out a Trophy Plus or a Tempest Plus in the 26 inch range and see how one of these runs. Another option is to try a chopper with less pitch. I never like to spend money to mod a prop unless I've exhausted all other options and have decided that its worth the risk to mod a prop thats running pretty good to begin with.
Doug

[staylor]

I had a rig running similar props some years back. The boat was a 21 Checkmate, vee bottom with 7.75 inch pad, 1700 lb hull but with light rigging, running a fairly hot 225 Evinrude. First prop I tried was a blueprinted 22 Raker, which was over-revving. Next prop was a 24 Turbo II, a prop with less blade area than the Raker, which had been the best prop on my older 18 ft Baja with 175 Evinrude. This prop had been blueprinted with tip cup added and ran 67 mph at 5900-6000 rpm. I did my slip calcs with 25 pitch because of the added tip cup, and decided it was slipping too much. Next prop was a stock chopper, an OMC SRX in 27 pitch. The prop still ran at 5900, but speed was only 68, indicating that I needed either less pitch or less slip. I assumed this chopper had so much pitch for this rig that it just plain wasn't hooking up. Last prop tried was a 24 Raker, which had much more blade area than the 24 Turbo II, again blueprinted with added tip cup. This prop ran at 5950-6000 rpm with a top end of 71-72. Slip was down in the 5% range and the rig just plain felt great. Set-up was done.
Doug

[175sf]

Thanks Doug, My boat is about 1100 lbs. and lifts with little effort. I borrowed a 25 trophy plus today, it will probably turn it out of sight but will give me some idea what to do. I do more fishing than riding, however I want it to be the best it can be. I had a pro-v 150 on this boat years ago with a pro-series yama prop and it would pick the boat up and set it on the pad and run 67. This engine has twice the power and no reason it should not perform better. Just in set up I guess. Thanks for info. Jeff

[Jimboat]

9% slip at WOT is a good number to shoot for.

[DS77]

estimating prop slip, kinda like estimating how many gals. of water flow down a creek on any given day its bot impossible and there is one thing sure no... two props are the same maybe close but not the same.

[pyro]

The man is right. "Pitch" is just another figure for math numbers, it's difficult to measure on some designs, and doesn't necesarily run like it's marked to run, in terms of pitch.

In other words, a 26p chopper might run 13% slip, and a 26p Trophy on the same boat might show 9% slip. It doesn't necessarily mean that one prop is slipping more than the other, but may instead demonstrate that a 26 isn't always a 26.

Slip is a "baseline" figure when you're trying different pitch props of the same model, where the slip is likely to stay fairly constant as you move up or down a pitch, allowing you to do some hypothetical calculations: http://www.rbbi.com/folders/prop/propcalc.htm

Your test numbers show, at least, that the cut prop doesn't have enough blade to carry your boat with a load. On a much lighter boat, they might run similar slip numbers, and the cut prop might even be faster.

Don't count out the Trophy and Tempest thru-hubs, they run good up high, have lots of bow lift, and they carry a load well, often running "taller" than the ptch they're marked...

[Wizard]

Agree with all above. More important than slip is running your motor in the proper rpm range. The 28 Chopper is way too much if it's only turning 5700. Your Promax should turn 6500 minimum at WOT. I'm pretty sure it's limited to 6750. Lugging that motor will kill it quickly.

The Promax's are not very good motors for heavy bassboats. The bass hulls carry alot of weight forward which makes them run only by using alot of trim. The Promax's are RPM motors and don't have much grunt down low. The 3L motors work better because they have the torque to turn bigger props to moderate RPMS.

I agree the 25 Trophy is a good bassboat prop. Ran them for years on all my bass hulls. If that Trophy runs at the top of your rpm range then you'll be good. I'd use the Stilletto for a doorstop. :D

What hull by the way?

[175sf]

Its a 175 starfire Skeeter. Bout 1100 #s plus all my junk and 3 batts. I ran today with 25 trophy plus. Ran good, motors about 2.5 or 3 in. below pad. The trophy ran 68.3 gps/6500 r's. I think that comes out to 17%. I had another boat like this with a Stock 175 merc and ran a tempest and it ran as good as this set-up. Guess I hate that I'm wasteing this engine. Its a super good running engine. (200 promax .15 over with alum. flywheel, unlimited ECU, offshore mid, Brucato SVS, Diamond marine fuel system.) etc. Boats up on the pad w/bout a foot of white water way back. Think I'm go n to stick a 27 tempest and come up with the engine 1 inch below the pad to keep my r's up. If that don't work, Would it be possible to get a allison hull w/bullet fish layout. You think this engine will perform on one of them before I break the bank.
Thanks Jeff

[staylor]

.....as the slip seems high- and if you're carrying a full fishing load you may need to drop the motor down so the prop shaft is about 3.5 inches below pad. This will make the Trophy bite a little better and give you more ability to carry the bow. The other alternative is the Tempest which has a bit more diameter and should bite better at the present height.
Doug

[JWTjr.]

Keep in mind that slip is a necessary part of the equation. The boat won't go if the propeller can't slip enough. Don't make the common mistake of thinking that slip is a bad thing, and therefore go after the elusive "lowest possible slip %". When you do that, you'll undoubtedly cast aside perfectly good propellers that slip more than the one you feel is best (due to it's lower calculated slip %) but actually doesn't perform as well. A prop that slips 12% but performs just as good as another that slips 9% is not necessarily a poor choice.

Also, remember that most folks don't own a pitch and rake measurement tool, and so when calculating propeller slip or top speed, are really only guessing when using the pitch as stamped on the prop as the pitch constant in the equation. Remember in any calculation the fewer the variables, the more accurate the outcome/answer. In this particular calculation, the gear ratio is a reliable constant. If the tach reading has been verified as accurate, that is another good constant. If you're using a GPS or radar gun as a speed measuring device, that's a pretty good constant. That leaves pitch and slip as the variables. If you can accurately measure the prop pitch, that leaves only slip to factor for, and you'll get a pretty good result. But not really knowing pitch may leave you hunting for "excessive" slip that is really not excessive at all.

In most outboard performance craft, slip ranging from 5% to 12% is typically acceptable. Actual slip below 10% is not very common; below 8% is downright rare.

John

[175sf]

Hey Doug, This boat needs very little positive trim. Bout blew it over today w/trophy. I agree the tempest will sho nuff stick it up. Thats the reason I thought I would lift the engine up to turn the 27 tempest bout 65-6700 rpm because it needs very little trim to lift the bow. I guess what I'm saying is the motor is definitely controlling the boat. All these test are fully loaded so 72-75 may not be to bad. I guess if remove 2 105 amp batts and ran 1/2 tank of gas it would make a big differ. Boats comes out of hole good w/all props. Its a beautiful boat, good fisher, not to heavy, just should perform. This engine is really to racey for this boat. Would prob. be justice on a bullet or ally. Think I'll just buy a bullet and be done with it. Jeff:cheers:

[175sf]

Keep in mind that slip is a necessary part of the equation. The boat won't go if the propeller can't slip enough. Don't make the common mistake of thinking that slip is a bad thing, and therefore go after the elusive "lowest possible slip %". When you do that, you'll undoubtedly cast aside perfectly good propellers that slip more than the one you feel is best (due to it's lower calculated slip %) but actually doesn't perform as well. A prop that slips 12% but performs just as good as another that slips 9% is not necessarily a poor choice.

Also, remember that most folks don't own a pitch and rake measurement tool, and so when calculating propeller slip or top speed, are really only guessing when using the pitch as stamped on the prop as the pitch constant in the equation. Remember in any calculation the fewer the variables, the more accurate the outcome/answer. In this particular calculation, the gear ratio is a reliable constant. If the tach reading has been verified as accurate, that is another good constant. If you're using a GPS or radar gun as a speed measuring device, that's a pretty good constant. That leaves pitch and slip as the variables. If you can accurately measure the prop pitch, that leaves only slip to factor for, and you'll get a pretty good result. But not really knowing pitch may leave you hunting for "excessive" slip that is really not excessive at all.

In most outboard performance craft, slip ranging from 5% to 12% is typically acceptable. Actual slip below 10% is not very common; below 8% is downright rare.

John :thumbsup:Thanks John, I see what your saying. Its the unknowns (pitch) that may give a false reading. Its a hit and miss, I'm relying on gps recalls and my tach which I know it fact. Was a merc tech. for bout 20 years before they come with all this high tech stuff. Don't know why I'm suddenly starving for knowledge. Enjoyed your post, certainly points to ponder. Merry Christmas, Jeff

[Wizard]

I've driven both an Allison XB2003 and a Bullet 20XD with a 200PM. Both were excellent performers with this motor. Actually I preferred the Bullet as it certainly fishes better and is easier to drive. I got about 84mph out of the 200PM 20ft Bullet. Very nice ride.

I think your at a wall with that motor on that hull. A 225 3L would be a different story on the Skeeter. Nows a great time to buy a used Bullet. Try the For Sale section on www.fastbass.com. :cheers:

[staylor]

...and if its lugging the motor nothing says you can't try running it higher if you've still got enough water pressure. Just going on the horsepower increase alone, if you ran 67 with 150 hp then going to a 200 hp with the same rig weight should get you up to 74-75 mph if the prop is hooking up. Since you are getting max speed with the Trophy without a lot of positive trim indicates that the boat should have no troubles going quicker if you can get the prop slippage down to a lower value. I notice on the Skeeter site that most of the set-ups they list run props in the 14.5 to 15.1 diameter range, and the Tempest is within this range, the Trophy is smaller. After you try the Tempest,if you've got more props you can borrow, see if anyone runs the Rev 4 in 25 pitch. Some of the guys running Rangers like these better than the Tempests. These have more blade area than the Tempest and about the same large diameter. I don't think they have as much rake angle on the blades as a Tempest, but if you're carrying the bow OK with neutral trim this won't be an issue.

Life sure was easier doing prop selection back in the '60s- you'd go from a stock aluminum to a Michigan AJC (Aqua-Jet-Cupped), and add more cup on a trailer hitch ball till it hooked up right.
Doug