Fuel Consumption Puzzle

[barnakiel]

Originally Posted by David M

Mark is exactly right. If you re-prop the boat and it cannot reach its maximum governed RPM with a clean hull and a healthy engine, then you have over propped the boat...

Nailed on the head!

But an overpitched prop may be the owners choice - it is only with variable pitch props that we may strive to attain the best proping per any given rpm. Thus any fixed pitch prop is a compromise - and very often one optimised for lowest fuel consumption at cruising speed.

Still, much as it will not reach the top rpms with all consequences as mentioned by David and Mark, if you motor-sail a lot, or if you are willing to go a bit slower, then you may sometimes opt to stick to the overpitched prop.

Oh, ah, urghhh - variable pitch props, where are you beauties ??? ;-)))

b.

[Ziggy]

At first glance, it would seem that the prop is overpitched. But a quick calculation, using the pleasure duty rating for Perkins 4-108 (45 SHP @ 3600 rpm), yields 18x11 prop to get to 6.8 kts. If we use the commercial duty rating for 4-108 (37 SHP@3000 RPM), we get 18x15 3-blade prop to get to 6.4 kts. So I don't think 18x11 prop is overpitched at all. I would suspect a problem with the engine leading to loss of power. May be poor compression or incorrect timing?

[Highlander40]

Some motor sailers use a variable pitch prop so that when the engine is sharing the load with the sailing rig, you increase the pitch.
The only true way to tell what is going on is by measuring the exaust temperature.
This tool can be used to evaluate a fixed prop setup too.

[barnakiel]

Originally Posted by Highlander40

... Some motor sailers use a variable pitch prop so that when the engine is sharing the load with the sailing rig, you increase the pitch...

Interestingly, as long ago as in the 60'ies all Albin Viggen were supplied with fully controlable pitch props! And well maintained these units still work on some boats I have seen. A dream to have but somehow left behind by designers. Perhaps an idea to commercialize!

b.

[Chief Engineer]

I have worked on those.....when no one else would

[David M]

Originally Posted by barnakiel

Nailed on the head!

But an overpitched prop may be the owners choice - it is only with variable pitch props that we may strive to attain the best proping per any given rpm. Thus any fixed pitch prop is a compromise - and very often one optimised for lowest fuel consumption at cruising speed.

Still, much as it will not reach the top rpms with all consequences as mentioned by David and Mark, if you motor-sail a lot, or if you are willing to go a bit slower, then you may sometimes opt to stick to the overpitched prop.

Oh, ah, urghhh - variable pitch props, where are you beauties ??? ;-)))

b.

One minor point. A variable pitch propeller changes its angle of attack between the hub and the tip. Almost all props are variable pitch.

A controllable pitch props pitch can change be changed remotely. I think you meant to say controllable pitch.

[osirissail]

- - I agree you are most likely over-propped. I have the original Perkins books and manuals and they list the 4.108(M) as having a max engine rpm of 3,600 and a max continuous operating rpm of 3,000. These are measured with a mechanical rpm meter. Alternator (electronic) rpm guages are notoriously wrong. You can borrow or buy a strobe rpm meter and see what is exactly happening.
- - Theoretically your prop size should allow the engine (while the boat is moving) to reach max engine RPM's (near 3.600). It is dangerous test as if your engine is too old and worn you might blow something. But I normally "stab" the throttle when at normal max boat speed through the water and see if the rpm's get close but does not look like they are going to get there too quickly. If you can get there too quickly then you are under-propped.
- - But normally high fuel consumption along with not being able to achieve engine max rpm's is caused by over-propping. But also check the distance from the tip of your propeller blade to the nearest surface of the hull. Your 18" prop may be too large in diameter and if it is closer than 3.6" to the hull surface you are getting hull interference which will degrade performance.
- - Basically for the size/weight boat you are in the economy range of HP and you cannot expect great speeds. More HP will get you better speeds but you have to match the HP to the Propeller and the Propeller dimensions to the hull configuration. With a prop in an "aperature" your restrictions may make getting a bigger engine a bad investment. To make this even more restrictive the hull/keel shape forward of the propeller and greatly affect the speed of the boat. If the propeller is "hidden" by the keel/hull shape water cannot get to it easily and you loose efficiency. Boats are designed originally for what is thought by the designers to be the best compromise for average owners. Trying to change things sometimes just doesn't work.

[donradcliffe]

I had a 4108 with a 3-bladed Maxprop, and tried 3 pitch settings for it.

The rated RPM on the 4108 is a marketing ploy. As you have already observed, the engine is really not happy above 2800 rpm. The oil leaks and noise increase significantly when you run at higher rpms, and I suspect the service life will also shorten dramatically.

The fuel consumption at 6 knots was about 0.6-0.7 gallons per hour, and the highest pitch setting seemed to have slightly better fuel economy--nothing like the dramatic change you measured.

I think you are on the right track if you repitch and drop the max rpm to 2800.

The overheating issue may not be related to the tranny/prop change, and physics says that the engine may run a bit hotter if you develop the same hp at a lower rpm,as the cooling pumps are running slower. However, it also says that an engine which is burning 1 gal/hr is putting out 50% more waste heat than an engine which is only burning 0.67 gal/hr. Try the repitch and see.

[barnakiel]

Originally Posted by David M

One minor point. A variable pitch propeller changes its angle of attack between the hub and the tip. Almost all props are variable pitch.

A controllable pitch props pitch can change be changed remotely. I think you meant to say controllable pitch.

I meant to say what I said. In everyday conversations among sailors the term used is 'variable' meaning 'adjustable' meaning 'controllable'.

I believe you are 100% correct stating the technical term is controllable pitch - I mean units with external (from the cockpit) control over the angle of attack of the blades.

Interestingly, due to varied angle of attack vs. varied speed through the medium (and some other hydrodynamic factors too) the propeller with "twisted" blades is in fact .... fixed pitch, while the one with "straight" blades is variable pitch .... (or nearly so).

Anyway, there is at least one self-controllable pitch propeller around but does any maker has a small dia externally contrallable pitch prop (say something to go with a 40 footer?

Cheers,
barnie

[Cheechako]

I think you are right on with the 2400 cruising 2800 max scenario for that engine....

[Wainui]

The 4108/107 Max continous RPM is 3,000.
My Cruising RPM is around 2,400.
My boat is 25,000lbs with a 4108
3 blade 17'' MaxVP is pitched at 6.8''
I get 6.5 knts @2,400 and 7.2 @2,800

[Cheechako]

The old scheme of propping an engine for a little less than maximum RPM is not as clear as one would think. It is interesting how diesels are often rated. I was looking at larger diesels for a commercial/govt client at the Commercial Boat Expo a couple of years ago. One thing is obvious, depending on the use of the engine it's max RPM and rating changes. For instance, a 300 hp diesel engine can be rated at 2500 rpm for Commercial use or 3500 rpm and 350 HP for recreational use. If it is to be used 24/7, (like a passenger ferry etc) then it may be derated even more. So if you are thinking there is only one correct RPM to prop your engine at... well... think again... It may depend on how much HP you need and how you use it.

[Highlander40]

One thing that is not clear. Are the engines the same or do the have different injectors, pumps and timing?
From what I have read, over propping is the best way to shorten the life of a diesel. This may be why so many engines get replace at a fraction of the hours that they are capable of running.
Of course sailboats only have toy diesels anyway.

[Cheechako]

The universal impression I got from the people at the show was that they are the same engines. The placards displayed on the engines actually showed differnt horsepowers under each different Rating Spec. (There are different rating specs for different uses ) The engines had to be purchased according to the rating spec in order to qualify for warranty. What I gleaned is that essentially the engines are derated RPM and HP to meet a continuous duty, 24/7 application. This is for large engines, no idea for like a 4-108, but I suspect it's the same based on the fact that most I've seen have been propped for well below 3600 rpm. The 4-108 in my Tanton 44 cutter drove the boat at 8 knot hull speed at 2450 rpm in flat water. (38+ ft waterline) 17" max prop, hurth gear unknown ratio (1.6?)

[Bowman Sailor]

Firstly, my apologies to anyone who was following this thread and waiting for an answer. I spent the last four years removing Aurelia's teak deck and refinishing it with LP paint.

Aurelia was hauled last August for bottom paint and new zincs. Unfortunately, the epoxy bottom job done in 1994 finally failed. So, the bottom paint and old barrier coat were blasted away and she sat on the hard for eight months drying out. The new barrier coat was finished a couple of weeks ago and Aurelia was launched on Saturday with - and here, finally is the tie to the old propeller thread - a re pitched prop.

I had a prop scan shop re pitch the 18 inch three blade prop from 11 inches to 13 inches. The difference is amazing. I motored the 3 hours from the boatyard to Aurelia's home slip and did so at 7.5 knots at 2,600 RPM. This compares for maybe 6.5 knots at 2,700 - 2,800 RPM before. I can't comment on the BSFC, but with this performance increase, I don't care.

Cheers,

Steve