Engine Backpressure?

[Ken Juul]

I reproped a couple years ago, old prop would easily run at 3000 rpm.  For the last 2 seasons unless the prop was fouled, the most I can get is 2600.  I'm thinking I need to reduce pitch.  I got the following back from the prop manufacturer.  The new prop is a Campbell Sailor 3 blade. What do you all think?

"The prop we supplied you is a 14' diameter x 8" pitch. This is the size we have been installing on the catalina 34 with the 25XP. We feel this is a good match for the boat and engine. It is possible you have a obstruction in the exhaust riser which has been a common problem with the Universal engines. There is a flap valve in the riser which comes apart and increases the back pressure which causes the engine to not get up to full RPM. Please check this out and also have the tach calibrated as this could possibly be out. Otherwise if everything is okay we can have it repitched down. "

Idon't think it is a tach problem.  Will go to 3000 in idle and stop despite more throttle.  First I've heard of a flapper valve in the exhaust.  This one has me stumped.

[Mike Vaccaro]

Ken,

You might want to review all of the material on the site about props--lots of good discussion over the years.  

The "stock" fixed three-blade propeller for the Cat 34 equipped with a Universal M-25 series engine is a 15 x 9.  Several folks have indicated that they've had good luck re-pitching to  10 or 11" to increase thrust.

"Propping" a boat or aircraft engine is an interesting engineering exercise for which there is no one correct answer.  The real issue is torque and thrust (which will determine how well the boat performs under power) and then running the engine at a reasonable power band.  If an engine is optimized to produce a desired torque at X RPM and you want to "optimize" the installation, then the prop should be pitched to match that "sweet" spot.  At all other power settings, the prop is a compromise.

In boats like ours, the engine is an auxiliary and the primary means of propulsion is sail.  It's only been in the last decade or so that manufacturers have intentionally "over sized" engines to produce desirable, trawler-like powering characteristics, bowing to the fact that many sailors don't much of the time!

If your engine/prop combination provides adequate maneuvering for docking and reasonable cruising characteristics (4.5 to 5.0 knots or so) at 1900-2000 RPM, you're in pretty good shape as far a sailboats go.  If properly sized, the engine should power the boat to hull speed in calm water and low wind conditions.  A very efficient prop may even power the boat at hull speed at a reasonably low torque (RPM setting).  Any "excess" thrust will help when powering against waves and wind.  When conditions become more extreme, there may only be enough thrust to assist with steering (i.e., moving water past the rudder).  In most conditions (even docking!), some combination of power and sail will optimize performance.  

Fixed pitch sailboat propellers are generally designed to produce maximum forward thrust (which they are good at).  They aren't quite so good at producing reverse thrust, however.  If you have a fixed prop, you may notice the way it cavitates when you put it in reverse and throttle up.  This is do the fact that the blade twist (angle of attack) is optimized for forward motion through the water.  They are the most inexpensive propellers with a minimum of maintenance requirements (they usually need to be cleaned occasionally, but that's about it.  Anti-fouling paint can extend time between cleanings--be sure to put an epoxy barrier coat on the bronze to avoid any chemistry problems with the anti-fouling paint).  

Folding props are adequate for forward and work in reverse, but some torque (RPM) is required to get the blades to deploy.  Their primary advantage is drag reduction under sail at some increased cost and maintenance.  

Feathering props are able to optimize forward and reverse performance, but at considerable cost with an increase in maintenance requirements.  High end propellers can actually allow you to manipulate pitch with torque which essentially provides you with a vice-less propeller (or you can even go so far as to adjust the prop walk to assist with docking and undocking!).

If the family budget allows, a feathering prop is the optimum solution.  The folding prop is a good compromise and a fixed prop works just fine.  Each one of these requires some degree of familiarity with handling characteristics that can only be obtained with practice.  Who knows, in 10 years, bow thrusters may be commonplace on boats like ours!

Does that mean you want to re-pitch your prop?  Only if you've had problems.  There is no valve in the Universal M-25 exhaust riser in the Catalina 34.

Cheers,

Mike

[Ken Juul]

I am quite happy with the way the prop performs both forward and reverse.  I am concerned however about the engine.  From what I have read, cruising RPM should be in the 2200-2500 range, saving full throttle for emergency/bad weather situations.  With the current set up, I don't have much of an emergency cushion.  Haven't had any overheat or smoke problems when I have had to fight the waves at full throttle, but still not sure if the current set up is ok.

[Ron Hill]

Ken : Mike is correct.  Your boat (and ALL C34's) came with a 15" diameter 2 or 3 bladed prop!!!  Why you decreased that diameter in unknown???
The pitch of the 3 bladed came as 9" and many others and I recommend that you repitch your sailor prop up to 10".  I then repitched mine further to 11".  I''ve recommend that  most people with a sailor 15" prop (M25XP engine) repitch up to 10.5".
You need to check you engine tac reading (signal taken from the alternator) to the actual RPM at the drive shaft.  You may have changed your alternator/belt size/pulley size - all of which will change the "RPM" reading from the alternator.  What you want is to cruise at about 80% engine power and NOT have black smoke coming out of the exhaust (regardless of the rpm reading).  
Maybe you need to rebuild your injection pump if you aren't getting  enough engine power - see my article in the next Mainsheet issue.  

[Ray & Sandy Erps]

Ken,

As to part two of your question, high exhaust back pressure can cause a decrease in power.  I had my exhaust riser off this fall when I replaced the hump hose and didn't see a flapper valve but I am aware that the inside of the exhaust pipe can plug up a bit.  

If you're a diver, might be easy enough to put back on the original prop and see if your higher RPM's return.  If they don't, a plugged exhaust might be something to check on as well as a plugged fuel filter.  Once those have been ruled out, then it's time to start testing the expensive stuff.

[Ken Juul]

Thanks all for the inputs.

Ron to answer your question about the prop size, the Campbell Sailor is more like a wing than a flat blade.  It has a narrow proflile to minimize drag when sailing, but the camber in the blade faces gives it a tremendous bite.  In flat water it will push the boat at 6+ knots at 2500.  Because of the narrow design, prop walk in reverse is also minimized.

I've lived with it for 2 years, I guess I can wait another.  The manufacturer suggests it is the engine not the prop, so I think I will try all your suggestions to make sure the engine is putting out sufficient power at the correct RPM.  If that doesn't correct it, I'll do the prop next year when I pull to paint.

Ron, not sure on the Mainsheet schedule.  Plan on recommissioning the boat in March.  If publication date is later than that can I get an advance copy of the injection pump rebuild?

[Ron Hill]

Ken : The Mainsheet should be out by the end of this week.  

BTW, I forgot to mention that there is NO flapper valve in the exhaust riser.  In some of the early C34 owners manuals, Catalina erroneously mentions a "flapper valve".  It was a hold over from the C30 which had a flapper valve over the outside exhaust port - so a following sea couldn't backup into the exhaust system.  On our C34s the exhaust hose is high and doesn't need a valve.  
All engines need "some" back pressure.  The muffler usually supplies that correct amount - that's why I have always recommend that everyone always replace their muffler with one from Catalina.  

[John Langford]

Hi Folks,
Ron made the useful point that all engines need some back pressure and then advocated buying only a Catalina replacement muffler. I agree in principle, but the Catalina replacement is expensive, delivery times are long and if you are in the middle of a cruise waiting for a new muffler would be a nuisance.

Assuming you could not repair a leaking muffler, I would like to know if one of the widely available and inexpensive Vetus mufflers might work. I could begin to figure that out if I knew how much back pressure the stock muffler (for the Mark 11) provides and the volume of water it holds. Does anyone have any ideas?

[Stu Jackson]

John

Good to hear from you.

I saw a Vetus in a C34 a number of years ago.  I've also looked into them in the WMP catalog, because the lips on our Catalina mufflers are subject to lots of stress.

The major issue I see with them is the size of the inlet and outlet ports.  Our hoses are 1 5/8 inch and I haven't seen that size on a Vetus.  As far as I know it worked on that C34, because the owner was happy with it.  I don't think back-pressure and water volume is such an issue, since the size indicates that it has pretty much the same volume.  Their choice of inlet and outlet locations are also a consideration. The support and base would also have to be considered and I don't remember how it was done on the one I saw.  I should have taken a picture of it, but it was before the digital age and I didn't carry a camera around like I usually do today.  

I haven't gone any further because I'm still reinforcing the lips on our original muffler!  The rest of it is working just fine.

[Ron Hill]

John : As Stu has pointed out MOUNTING and HOSE SIZE are just two of the factors that you'll have to contend with in a muffler other than Catalina OEM.  The geometry of the shape may also be another factor, as your muffler on a MKII has side entry and exit ports.  

The muffler is made of fiberglass so you ought to be able to repair it.  You may not be able to put the nice white gel coat finish on it, but who care about that.  
If I had to do mine over again I would have repaired rather than replaced it.  A few thoughts.  

[Andrew_Smith]

Ron,

I have been exploring this same issue recently because we are only getting about 5.6 knots in relatively calm waters, and that is running the engine at around 2300 RPM.  At full throttle the max RPM is just above 2500 RPM.  The engine runs like a champ, but every year I have to clean black soot deposits from the fiberglass near the exhaust.  No changes have been made to any of the pulleys on the engine, however I have not checked the RPM's.  I tried using the fluorescent light technique without much success.

The propeller on the boat is a 15" diameter with a 10.5" pitch.  I still have the original propeller that was 15" diameter with a 9" pitch.   I read most of the propeller book by David Gerr, and went through the BP-delta calculations for the M25XP.  The result that I came up with was a 15" diameter and 9.5" propeller for a 3-bladed fixed propeller (standard dimensions).  The book listed two indications of an overpitched engine, low RPM and black smoke in the exhaust.  He recommended that the engine be able to reach full RPM's under full throttle, and suggested that running the engine with too much torque reduces the total horsepower and can eventually reduced the maximum achievable speed and place additional strain on the engine.

My questions are as follows: how fast are boats with the M25XP able to motor with the original 15"x9" propeller.  If people are using the higher pitch propellers are they seeing signs of black smoke in the exhaust, or is it just me?  I am considering going back to the original propeller, after inspecting the propellers of several other C34 is the same boatyard.

Andrew

[Ron Hill]

Andrew : You diagnosis is correct - smoke and low rpm usually indicate an over pitched prop.  
Couple of things to consider.  Beside checking how correct your rpm is, don't forget to check how correct your actual pitch is.  I wrote an article on "Prop Scan" in the C34 Mainsheet tech notes - you need to read it.

My first suspicion with your replacement 15" 3 bladed prop is that it's not a "sailor" prop like the one that came with the boat.  It may have a larger blade cord and surface area (per blade) than a sailor.  This would act in the same way as over pitching.  Think you need to do some checking.

My 3 bladed 15" prop is pitched to 11" and I have no problem. Can easily cruise at 6.7 kts - no wind/clean bottom in a boat overloaded with spare parts!