Up-Propping

[doc g]

Well, I had the boat hauled for normal bottom job, stab seals etc and decided to have the props redone to add pitch ,since we do not run fast anymore . Anyone that doesn't understand should refer to Brian DeGulis' thread at http://www.samsmarine.com/forums/showthread.php?t=8373&highlight=bigger+props .Specifically I wanted to lower my cruise rpm about 200 from 1325 to 1125 which is 10.5 KN. Long story short , I have FloScans and had #s (rpm/speed /fuel rate) from when the boat was last hauled . ... There is no difference after adding the 2" of pitch. Oh , and I already accused the prop shop shop so, let's not go there. I will of course be diving the boat and looking very closely at the "new stamp of dia x pitch". What do you guys think ???...... Pat

 

[MikeP]

Since it takes the same HP to move the boat at the same speed, I can't see any way that changing a few hundred RPM that this occurs will make any difference. If it takes X HP at 1325 RPM to move the boat at 10.5 knots, it still takes X HP to move it 10.5 knots at 1125 RPM.

I can understand a MAJOR change in RPM might result in some improvement due to reduced frictional losses. But otherwise, it seems to me that expecting an improvement when the engine has to produce the same power violates the laws of physics.

 

[doc g]

The idea is to more fully load the engine at a given rpm,increase combustion temp , turbo boost ,air box pressure and hopefully gain some efficiency in the process. As Brian did ,to the tune of 4gals /hr more efficient. Read the referenced posts.... My question is how can the prop shop have added 2" of pitch and I see no difference ??

 

[MikeP]

I read the post you referenced back when it was first posted and commented extensively at the time.

I think I misunderstood your post - sorry. Are you saying there is no difference in the speed/RPM? I guess I thought that you were saying there was no difference in fuel use at the same speed. But if there was a prop change and no change in speed per RPM, then I'd have to assume that either there was insufficient change to make any difference at that RPM or, the prop was not properly revised.

I certainly agree that a change of 2" of pitch should show SOME difference in RPM/speed. It'll be interesting to hear what you find.

 

[Brian Degulis]

There should be a very big diffrence with 2" of pitch. If you made 10 kts at say 1500 RPM you should make that same 10 kts at around 1200 RPM (very rough estimate) but certainly a noticable diffrence.

The fuel savings comes from lowering the losses encountered when an engine is very lightly loaded. If you compare HP to gallon ratios at 80% load to 15% load you will see there is a very big diffrence as much as 40% in some cases.

This is why fuel consumption should never be used to determine to determine HP produced on an engine driving a fixed pitch prop. That method works on trucks because they have transmissions to change ratios and adjust loads but not on our boats.

Brian

 

[doc g]

Yeah , I think Mr prop -Man is going to be paying to haul me . Is there any way to measure pitch on a prop underwater. How will I know if I'm looking at a 30x31 vs. a 30x33 prop?? The resident mechanics are taking about trans issues,slippage etc. Bottomline ,2" more of pitch ,same rpm = faster speed thru the water and it aint happening. Rpm /speed and fuel use are the same ...............Pat

 

[SKYCHENEY]

I don't know. I took 2 1/2" of pitch out of my props so that I could reach rated max loaded RPM. The top end seems different now, but at 1300rpms, I seem to go the same speed. I know it doesn't sound right, but that's what happened on mine. FWIW, I went from 28x31 down to 28x28.5, but I still run about 9.5 kts at 1300rpms.

 

[REBrueckner]

"....There is no difference after adding the 2" of pitch..."

What does that mean? no difference in speed over ground at the same RPM? Then the prop guys did nothing.

Or do you cruise 200 RPM slower, about the same speed over ground, but your fuel consumption is the same as it was??? Then the prop guys did do something, but you got no improvement in efficiency.

At your old RPM, your cruise speed should have picked up with more pitch unless the bottom is dirty and you can't go any faster....

How do you measure speed over ground?? GPS in slack current....a two way run test is better....

 

[doc g]

Same rpm,same speed and same fuel consumption as before the 2" inches of pitch was added. I am using rpm /speed/fuel data from a previous haulout to compare the new data to . I have data from 800-1400 every 100 rpm .....Pat

 

[Brian Degulis]

I don't know. I took 2 1/2" of pitch out of my props so that I could reach rated max loaded RPM. The top end seems different now, but at 1300rpms, I seem to go the same speed. I know it doesn't sound right, but that's what happened on mine. FWIW, I went from 28x31 down to 28x28.5, but I still run about 9.5 kts at 1300rpms.

I don't know Sky? Removing 2.5" of pitch means that the theoretical distance the boat travels per RPM is reduced by 2.5". So with 31" pitch at 1300 RPM assuming 2:1 gear 16.5 kts theoretical @ 28.5" pitch 15.25 kts. So a 1.25 kt decrease. The slip and other real world factors should not have changed much so you should have lost around 1.25kts.

I don't know the water line lengh of your boat but if 9.5 kts is around the point you start exceeding hull speed I guess that could decrease the effect because you start needing alot more power per kt beyond hull speed. Still I think you should have been able to clearly see a diffrence.

My first attempt at over proping was adding 2" of pitch to the existing 32x33 props. That dropped the 10.5 kt cruise RPM from 1600 to 1300.

Brian

 

[SKYCHENEY]

The top end sounds about right, Brian. I see about 15kts at fast cruise which is probably about a knot slower than before. My LWL is about 47' which puts my hull speed at 9.2kts. If I lost a couple of tenths at that speed, I doubt that I would notice as it varies that much from day to day depending on loading, wind, current, etc. My point was just that Docg may not notice a big difference at slow speeds and that he should run them up to see if those props were actually pitched properly.

 

[Brian Degulis]

Yeah I guess running them up would be a good idea if he's got good #s to compare. He's saying that acording to his flowscans the fuel consumption has not changed at all. Forgeting the speed for a moment it's just not possible to add 2" of pitch and not see an increase in fuel consumtion at any given RPM. Sounds more like the prop guy forgot about the pitch change or he's got another customer wondering why he can't get full RPM after his prop job.

Brian

 

[bobk]

I dropped my props at Miller Island last week to add 1.5" pitch on the 48MY. This will put it 0.5" above my Hatt manual. I'll report the speed change when I get them on.

While at the shop I asked for an explanation to what my son experienced with his 1987 Silverton, twin large gas. His boat was way over propped when he bought it. He reduced pitch and got MORE speed at his cruise rpm. Miller Island speculated there could have been some non-obvious issue with the old props that caused cavitation and that reduced the speed from where it should hae been.

Bob

 

[doc g]

If he was not hitting rated rpm his engines were not developing rated hp,therefore sometimes when propping for max speed (planing boats) you need to downsize the props . Bigger wheels will make the boat go faster,providing the horsepower is there to swing them..... This is only pertinent to planing high speed boats though...... Pat

 

[the kuz]

Recently had props tuned to ISO class I and .75 -.80 pitch taken (now both are 22x23.0, 3 blade Michigan Nibrals) on repowered 1977 36C (330Bs with 1.5 ZFs) . Boat had gotten fat over the last 3 years, and was turning about 50 turns under 2800 rated RPM @ WOT (now turns about 2840 WOT). The 36C is very sensitive to weight changes. Speed vs RPM changes were noticable even for this little adjustment. Normal slow running was 8.3 -8.4 knts @ 1100 RPM and to get 8.3 now requires about another 50 turns (1150 RPM). Cruise was 23 - 24 knts @ 2360 RPM, now need about 2430 to get 24 KNTS. WOT is about 1 knt less (27.5 vs 28.5) than sea trial 3 yreas ago which turned about 2850 WOT (with one prop 22x23.7 and the other 22x23.8 and the boat 1200 -1500 lbs lighter).

Still working on fuel consumption changes. Hoping that HP is HP and that the loss will not be proportional. Also noticed that the boat is quicker out of the hole, and single engine trolling speed at idle is .1 -.2 knts slower (a good thing for rockfish!). I would expect with a 2 inch pitch change you should notice some speed changes throughout the RPM range, even though they may be less on your larger/heavier boat. Let us know when you get definative info.

Best Regards, Bob K

 

[34Hatt]

The one thing I have notice here is the question of what the prop shop did???
Now you notice Bob "Kuz" seems to have exact #s which sounds like he had them done on Prop scan or MRI. When I have mine done I get a print out of the wheel dia and pitch of every blade over different points. I will not waste my money with any prop shop that does not do this. Been there done that No more.

 

[doc g]

The scans of the props were supposed to be delivered to my boat on Tuesday, We'll see !!!.................Pat