Black art?

[scotch&water]

One more note, this style of boat have a reputation for being tail draggers - too much weight in the back. I added perhaps 200Lbs with my install.

 

[baeckmo]

One more note, this style of boat have a reputation for being tail draggers - too much weight in the back. I added perhaps 200Lbs with my install.

This is putting extra load on the propeller blading. On top of that, you replaced one 20 inch diameter propeller with two 16 inch (??) dia;s in tandem. The problem is that the 16" set must produce the same thrust as the 20" prop, but with only 64 % of the disc area, which leads to a much higher sensitivity for cavitation and ventilation.

If you are lucky, you may get away with adding a coarse cup to the 20 inch pitch screws, but I suggest you try with spacers to increase the working depth of the propellers. That said, the best drive for your setup is probably still the Bravo II, because it will produce higher thrust in the critical speed range, given the cavitation/ventilation limitations. Good luck, Happy New Year, and never mix water into the scotch!

 

[Mr Efficiency]

The drive is running just 16" diameter props, do you say ? This is the wrong drive for the boat, if that is the case.

 

[baeckmo]

The drive is running just 16" diameter props, do you say ? This is the wrong drive for the boat, if that is the case.

The Bravo I and III drives swing maximum 16", and the Bravo II takes 20" propellers. Standard Mercruiser B III series start with 20" pitch, haveing 16" (fwd) + 14 3/8" (aft). The 26" pitch set is 15 1/4" + 14" diameters. The pitch/dia ratio for the Hills(?) propellers used here is unknown to me, but should follow the same pattern as the originals. Possible difference is SMALLER diameters due to four blades instead of three, so even worse for the operation.

 

[Mr Efficiency]

Duoprop is obviously a different kettle of fish, maybe he should redline this thing for a short time and see if it will plane, and at what revs, and how fast, at least that way he will have some figures to work with.

 

[gonzo]

Increasing the ratio for an engine that generates max power at lower RPM is the opposite of what you want, since the propeller diameter can't be increased.

 

[scotch&water]

Hi all, the Bravo III is a different set up, Volvo was the first on the block for Duoprop so Mercury had to have their own setup; that is what I have, advantages are supposed to be better hole shot, don't see that better slow speed control yes improved, backing in to the slip good. So information I have for changing ratios 220:1 at 3600 RPM = prop 1636 RPM So what would the increase of RPM at accomplish? The 220: is what I have now. Water in Scotch just a dash.
181:1 at 3600 RPM = Prop 1988 RPM

 

[gonzo]

The increase in propeller RPM would increase your speed. It would also load the engine more and prevent it from over-revving.

 

[baeckmo]

The increase in propeller RPM would increase your speed. It would also load the engine more and prevent it from over-revving.

Would you please explain to us how the rpm increase you suggest would improve cavitation performance?

The problem in this case is not top speed, but never coming over hump speed due to cavitation/aeration!!! The propellers are simply loosing their grip. The issue with tandem, contra-rotating ("TCR") propellers is that as soon as the fwd prop is cavitating (or ventilating) it is "throwing" bursts of vapour/air into the aft prop, which leads to a sudden thrust collapse.

Cavitation on single propeller is often more gradual, spreading from tip to root with increased thrust load. The alleged "hole-shot" capacity of the TCR in a "tail-dragging" hull is seriously impaired by even small amounts of air pulled down from the rolling wave along the transom. Like Mr E said, its a different kettle of fish.

So, S&W, if you decide to keep the B III unit, you have to adress the following issues:

• Reduce the aeration by adding antiventilating fences between transom and drive leg (feel free to contact me for advice).
• Try to relocate some onboard weight from aft to fwd, and of course carry all the unnecessary paraphernalia found onboard back to terra firma.
  
• Check antiventilation plate height versus keel/bottom as proposed previously (operational height NOT equal for BII and BIII units in different hulls). If necessary adjust with spacers.
  
• Talk to your propeller supplier about additional cupping of the 20" pitch propeller set.
• Don't listen to nonsense about increasing prop shaft rpms, that would require low pitch propellers that are not available.

 

[scotch&water]

Thanks Ganszo, that is the direction I need to go, now to find the correct RPM that will work the work on the pitch of the props. One interesting bit that I picked up is Mercury makes a combo set of props for the Bravo III in that the back propeller is 4 blade and the front is 3 blade, has any one run across that?

 

[gonzo]

Baekmo:
The length of the Bravo II and Bravo II are the same. Therefore, if it didn't ventilate before, it should not now. As far as cavitation, dual props will have less loading. If it did not cavitate before, there is no reason to suspect it does now with lower blade loading.
Increasing RPMs will increase speed without any need for low pitch propellers. To maintain the same propeller pitch as before, the shaft needs to turn at the same speed as before. Therefore, the gear ratio for an engine that generates maximum power at lower RPMs needs to be reduced not increased, to maintain the same propeller RPM.

 

[scotch&water]

Answer to beackmo, Don't believe I have a cavitation problem, I did note a surge of RPM with one set of props, anti ventilating fence? Have moved a lot, fuel tank ahead 9" all that I can, will check height of plate, thank you.

 

[baeckmo]

Baekmo:
"The length of the Bravo II and Bravo II are the same. Therefore, if it didn't ventilate before, it should not now."

That is wrong, since the inflow into the 20" propeller disc is completely different to the inflow into the 16" (or less) disc diameter of the BIII. You can not claim a similarity here, which is why there are adjustment parts available from the original part supplier.

"As far as cavitation, dual props will have less loading. If it did not cavitate before, there is no reason to suspect it does now with lower blade loading."

Again wrong, since we are comparing the loading of the 20" diameter disc and the 16" (or less) diameter of the forward prop in the BIII setup.

"Increasing RPMs will increase speed without any need for low pitch propellers."

Only if the propeller is free from cavitation/ventilation, and you have reached planing speed, which is not the case here!

"To maintain the same propeller pitch as before, the shaft needs to turn at the same speed as before. Therefore, the gear ratio for an engine that generates maximum power at lower RPMs needs to be reduced not increased, to maintain the same propeller RPM."

Irrelevant statement as long as there are cavities (gas or vapour) in the flow field, making the system non-linear!

 

[gonzo]

I can't discuss "propeller inflow", since I don't understand what you mean. I usually use that term for nozzles and inlets
The Bravo III has two propellers. Calculate the total BAR and you will see the Bravo III has a larger area.
There is no reason to suspect cavitation or ventilation if it didn't happen before. Nothing changed radically.
You final statement is a claim that there are "cavities" in the flow field. Maybe it is a translation issue. However, you first have to prove they exist. Also, all propellers have a non-linear response, whether the cavitate or not.

 

[baeckmo]

In previous notes I have described the Bravo III propeller sets as "16 inch fwd + 14 3/8 aft" for the 20" pitch set . That was meant to indicate to the awake reader that I am aware of the fact that it is a tandem setup, did this escape you Gonzo?

Since there are obvious misunderstandings regarding the propeller working principles here, I'll be back later with a short "propeller basics", comparing some critical numbers to show what is going on. Gimme some time and a scotch without water......

One issue I think you are missing is that we are talking about a diesel engine here, speeding to the rpm limit (with all "normal" pitches) at the 12 knot hump speed. The propeller set is not producing enough thrust to accelerate the hull to normal planing speeds. In this rpm range the engine is operating on the governor limit, meaning that the fuel injection is restricted and the power output likewise. So even when the props are spinning at max rpms, they are not absorbing full power, nor producing full thrust. The basic reason is disturbance in the water flowing into the propeller disc. The typical disturbance causing a loss of thrust like seen here is cavitation. Whether it is gas cavitation ("ventilation") or vapour cavitation is not quite clear. The static pressure in front of the B III propeller is lower than that in front of the B II propeller, but the peripheral speed is considerably lower (~35 m/s vs ~61 m/s). My guess is thus that the main reason is ventilation.

A rough cavitation check shows that the forward prop (16"x 20") has more than 30% of its blade suction surface covered with vapour cavities at 1640 rpm and 12 knot forward speed, leading to a major loss of thrust. But with the notion that this hull is a "tail dragger" I suspect that there is a serious downpull of aerated water from the water "walzing" at the transom. This is caused by the acceleration of the water in front of the propeller, and this acceleration is higher with the smaller tandem props than with the original, single Bravo II propeller. Increased fluid acceleration means lower pressure in the blade leading edge region (more intense "suction").