Woody
Well-Known Member
What effect does increasing prop cup have on rpm? Is there a good rule of thumb? My props currently have a .4 cup, what rpm drop or other performance effects might be associated with increasing to .5, .6, and so on.
Prop Cup Question
- Thread starter Woody
- Start date
- Oct 6, 2006
- 12,153
- Boat Info
- 1996 450DA
- Engines
- 3116 Caterpillars
Increasing the cup effectively increases pitch, up to a point. At some point, where depends upon the specific boat/engines/prop diameter, etc. the cup gets so great that the prop will ventilate and lose bite in turns.
This is one that quickly gets into the trial and error territory and is best left to the prop professionals.
This is one that quickly gets into the trial and error territory and is best left to the prop professionals.
KnotEasy
Member
"Is there a good rule of thumb?" Simple answer, increase in RPM and some speed.
Woody
Well-Known Member
The reason I ask is I'd like to drop my WOT rpm by about 40. I have seen general comments that one step in cup = 'X' rpm(of course I can't find that now). I was thinking to tweek for a few rpm that cup would be a simpler way vs repitch and it might come with benefits.
I did come across a little information on cup location that I had never seen before, cup on just the leading edge or cup on leading edge around including the tip. While cup increases pitch, that on the tip may also increase bow lift for example. I'll be going over this with the prop shop for sure.
- Oct 6, 2006
- 12,153
- Boat Info
- 1996 450DA
- Engines
- 3116 Caterpillars
Honestly, I would not spend prop money to reduce rpms by 40. It you can turn slightly over rated rpm's the chances are very good that as the boat ages and you put more "stuff" on board (as we all seem to do) you will quickly end up propped correctly. Your condition is slightly under-loading your engines and is a better situation for engine longevity, as long as you don't run over rated rpms.....and who runs these boats at WOT for longer than a minute os so anyway.
Woody
Well-Known Member
I should have included this at the start...I'm turning 2,725rpm WOT. In your opinion too high or not?
Woody,
I agree with Frank. Don't worry about being at 2725 rpm. I have these same engines (mechanical 450, aren't they?) and have gone through the propeller tweaking several times with two companies in the Detroit area, both excellent services.
When we bought our boat in 2007 it was five years old and it appeared that we were not pulling full WOT but it was only after installing accurate tachometers from Aetna (another Michigan company) that I realized we were only 2550 and 2560 at WOT. So I had the pitch reduced by 2" to 24" and we now spun up to 2710 both engines but lost mid range speed. Rated WOT is 2600 for these mechanical engines so 100 over is perfect according to Boat Diesel. Even Cummins in our area says you can run these at 90% all day with an occasional burst to maximum.
However my cruise dropped from 22 knots to 18 so I obtained a second set of props from a 2003 460DA with the 480CE motors. The pitch was higher at 27". So I tried a different propscan shop and they took a different route on the two sets, more pitch, less cup. One set went to 26", the other to 25.5". That produced a better cruise, and kept near the WOT, but still I wanted a little more mid range.
All this happened year after year at winter haul out so I had a bit more cup placed in and that seems to be the sweet spot for my boat. A 2675 WOT and a 21 knot cruise at just over 2300 rpm. I am sure that your boat with my props would be a couple of knots faster.
So if you are willing to experiment (see if you can get a second set of props) you could have a good prop shop do a little further tweaking but I wouldn't do it for 40 rpm. I had to start the process because we were short of WOT by a bigger margin, not over. Also another proof of the succeess here is that my aftercoolers were a toast color when I bought the boat. After repainting them five years ago, they haven't changed at all so I look at this as some sign that the motors are more relaxed and running cooler. I'm certainly no expert but it's just my observations.
James
I agree with Frank. Don't worry about being at 2725 rpm. I have these same engines (mechanical 450, aren't they?) and have gone through the propeller tweaking several times with two companies in the Detroit area, both excellent services.
When we bought our boat in 2007 it was five years old and it appeared that we were not pulling full WOT but it was only after installing accurate tachometers from Aetna (another Michigan company) that I realized we were only 2550 and 2560 at WOT. So I had the pitch reduced by 2" to 24" and we now spun up to 2710 both engines but lost mid range speed. Rated WOT is 2600 for these mechanical engines so 100 over is perfect according to Boat Diesel. Even Cummins in our area says you can run these at 90% all day with an occasional burst to maximum.
However my cruise dropped from 22 knots to 18 so I obtained a second set of props from a 2003 460DA with the 480CE motors. The pitch was higher at 27". So I tried a different propscan shop and they took a different route on the two sets, more pitch, less cup. One set went to 26", the other to 25.5". That produced a better cruise, and kept near the WOT, but still I wanted a little more mid range.
All this happened year after year at winter haul out so I had a bit more cup placed in and that seems to be the sweet spot for my boat. A 2675 WOT and a 21 knot cruise at just over 2300 rpm. I am sure that your boat with my props would be a couple of knots faster.
So if you are willing to experiment (see if you can get a second set of props) you could have a good prop shop do a little further tweaking but I wouldn't do it for 40 rpm. I had to start the process because we were short of WOT by a bigger margin, not over. Also another proof of the succeess here is that my aftercoolers were a toast color when I bought the boat. After repainting them five years ago, they haven't changed at all so I look at this as some sign that the motors are more relaxed and running cooler. I'm certainly no expert but it's just my observations.
James
- Oct 6, 2006
- 12,153
- Boat Info
- 1996 450DA
- Engines
- 3116 Caterpillars
If you have 450hp 6CTA's, then the rated RPM at WOT is 2600, so you are turning 125 rpm over the rated speed. As long as you keep the cruise rpms at Cummins' recommendation or below, you aren't going to hurt the engines. You may, however, gain a little speed by repitching or adding a little bit of cup, however, 100 would amount to less than 1/2" of pitch and the speed gain would be very little.
So, I am back to the beginning, this about 100 rpm and isn't very much of an RPM goal to shoot for. If you choose to change things, I would take the props to a reputable prop shop and tell him you need to reduce your rpms buy 100. Stay away from giving the pro precise instructions like "add 1/2" of pitch or increase the cup". Let him ask you the boat specifics and let him decide what to do. This way, the prop shop "owns" the change and, if he is reputable, he will adjust the props for you if he misses the mark. Keep in mind that being able to run 25-50 rpm over rated WOT is a lot better than running under rated WOT, and it isn't something to lose sleep over. Just keep your cruise speeds at or below Cummins' recommendation and you will not hurt the engines by overloading them.
So, I am back to the beginning, this about 100 rpm and isn't very much of an RPM goal to shoot for. If you choose to change things, I would take the props to a reputable prop shop and tell him you need to reduce your rpms buy 100. Stay away from giving the pro precise instructions like "add 1/2" of pitch or increase the cup". Let him ask you the boat specifics and let him decide what to do. This way, the prop shop "owns" the change and, if he is reputable, he will adjust the props for you if he misses the mark. Keep in mind that being able to run 25-50 rpm over rated WOT is a lot better than running under rated WOT, and it isn't something to lose sleep over. Just keep your cruise speeds at or below Cummins' recommendation and you will not hurt the engines by overloading them.
Woody
Well-Known Member
I have the mechanical 450 Diamond. I used a photo tach to check engine rpm. Man it's noisy down there when those things are cranked up. My spare set of props needs repairs anyway due to me locating some rocks with them last summer. As long as they are going in to the shop I thought I'd do some adjustments ...maybe. I've been a bit uncertain on what the acceptable WOT range should be, thanks to all for the input.
fr8dog
Well-Known Member
Tony Athens from SBMAR, who many consider the expert on the Cummins line, recommends 2700 at WOT. He says the same for the 480CE engine, although that engine has a ecm controlled limit of 2680. Sounds like you are just about perfect.