Over Propping Questions

I´m considering purchasing a Carver 530 with 475 hp Volvo TAMD 74 engines. Their max unloaded RPM is 2650 but both engines only reach 2200 under power

Hull is clean, props not damaged and Volvo engine surveyor says engines are in very good condition so i surmise the props have too much pitch.

I will only be travelling at 10 knots and about 1500 rpm for 95% of the time.

Question

1-Will the current props cause undue strain to the engine if I ran it at 2100 rpm for an hour a few times in the future?

2- Would fuel economy at 10 knots be better the way it is, or better with the props repitched to achieve 2650 rpm under power?

You have a planing hull, and since you are already talking about running it up for an hour or so, you will be far better off to go ahead and reprop to your top speed, and not worry about a few rpms at off plane cruising speeds.
10 knots is likely getting into hump speed, not a great scenario for an overpropped vessel.
You have a big investment here, best to talk to a professional about such a critical issue.

Before running off to get your prop's re pitched have you checked the actual RPM with a laser tach or similar (Depending on tach model and how it's driven 10% error is not out of the question) couple of following items to note as well,

1) Do you notice any black smoke when at full RPM?

2) Have you checked your no load RPM ?

3) Have you checked your throttle linkage is going to full throttle on the injector pumps?

4) As a general guide dropping the pitch by an inch usually increases engine RPM by around 200 RPM.(plus or minus)

5) If running at your present 100 RPM below max throttle and no sign of over loading (black smoke) then no real harm should ensue, your temperature gauge will be a prime item to watch in these cases as any real increase in temp will mean possible over loading as does the possible black smoke,so then yes look at a reduction in pitch,

Cheers Steve

To add to Capt Steve.
Exhaust temp gauges would be a good addition. Find out what temp range Volvo wants to see
Find out what max RPM Volvo wants you to obtain when the boat is fully loaded and in gear.

A simple rule of thumb is to pull back 10% from observed WOT .

For most that would be 200RPM , an EGT is not expensive and is the only way you can fully load the engine without harm

Any operation at almost WOT is usually very expensive in fuel burn.

For a displacement boat matching the prop to the intended cruise speed is not over proping , it is good engine management.

All boats up on the plane are a compromise , so the prop decision should be chosen by where the boat operates most of the time.

Sport fish operate on the pin much of the time and accept short engine life as normal.

When I bought my boat the manufacturer suggested we run at WOT for 10 minutes during the survey. This will show if the boat is over- or under-propped.


If it reaches its WOT speed but doesn't reach the WOT rpm it likely is over propped.


If it reaches the WOT rpm but doesn't reach the WOT speed it's under propped.


People have been known to over prop their boats thinking it will improve their fuel efficiency/economy. It might but the difference might not even be noticeable.


Find out from Carver what the WOT speed is for that boat with those engines. That likely will give you more clues about the current propping.

If those engines are that seriously over propped on a planning hull I would walk. I bought a boat some years back with 250 hours on engine there was a few hundred rpm of over propping. Less than 50 hours latter the engine blew. Maybe you want to be a big time gambler? Some over propped engines are damaged some not why gamble when you can buy a boat without this potential problem. There is a reason a sea trial with WOT full load testing is done and it is not to entertain the surveyor. It is easy for others to tell you not to worry it will not be their money or inconvenience it engine blows. I speak from experience.

You'll get better mileage with the current props and the lower rpm will wear the engine less. The turbo won't be running flat out so your exhaust gas temperatures won't be eroding your cylinder parts.
The paperwork I have on that engine (maybe a slightly different model) says max hp may be used 4 hours in 12. Remember Volvo is only interested in the engine lasting thru warranty.
Most boats I've owned were over propped. Better mileage, lower rpms in the hands of a careful operator.

stanfromhell said:

Question

1-Will the current props cause undue strain to the engine if I ran it at 2100 rpm for an hour a few times in the future?

2- Would fuel economy at 10 knots be better the way it is, or better with the props repitched to achieve 2650 rpm under power?

Click to expand...

1 The usual evidence of overpropping is black smoke at the top end of rpm that you can reach. Keep yours below the point of putting out black smoke for peace of mind.

2 Yours are likely set up to run 10 knots at lower rpm, so don't waste the effort put in by a PO to set up for better economy at that speed.

My mechanic, when suggesting a more powerful pair of engines, recommended overpropping to bring operational rpm down. I have an extra 4" pitch added, cruise at 8 knots, 2000 rpm, and am very happy with the reduction in engine wear, improved economy, less noise, in engines that could be run at 2700 rpm for the same 8 knots.

Here is advice to me from a marine engineer with a few pics that he shared about overpropping and reducing engine wear.....

"Half truth at best ... and it appears a poor understanding of turbocharger operation.

Look at the attached pics. They show what reduced turbocharger output does to an engine running at the load needed to obtain a certain speed through the water..... (snip)

A turbocharged engine uses valve overlap to scavenge exhaust gas from the cylinder, both intake and exhaust valves are open for a short period and the flow of relatively cool scavenge air cools the valves and seats. If there is not enough scavenge air flow - as would be the case when turbo is not delivering the mass of air required for a given load - what you see in the photos is the result."

Egt

The pics above are a great reason to monitor exhaust gas temp. Probably the most important gauge on a turbo Diesel engine.

All too often Carvers of that era left the factory propped to achieve full RPM lightly loaded. I personally know of several. Then reality hit with added people, water, fuel, frying pans, enclosures, dinghy and kayaks.

As PSN and Ed note there is no upside especially on a planing vessel for over propping. Urban myths about the advantages of over propping should not be believed for one second on this vessel. And this vessel is SEVERELY over propped if it truly can only reach 2200 RPM.

If the boat owner is (1) willing to prop the vessel correctly so it can archieve full rated+ 50 RPM during sea trial (verified by photo tach), (2) discount the price for major rebuilds and (3) you're willing to gamble - then you may have a deal.

I'd call Carver and ask them for the standard prop specs for those engines in that hull. I know that as of ten years ago they spent a great deal of time dialing in new models. I stored our boat at the marina where it was done and spoke with the test crews. Also ask them what sort of loading was used during that evolution (per the comment above). They had a great engineering staff prior to the late-2000's down turn and virtual shutdown of the production line. I'd bet the core of that brain trust is still around.

Yes on #1

No on #2

!0 knots will be a considerable load so best to be propped t 2700.
I wouldn't call Carver. I'd call Volvo.

I would not buy the boat unless you are getting it at a serious discount. Overpropping is never good on a planing hull. ANd like someone said, 10 knots is a very poor speed for efficiency and engine wear.

Just passing along what a marine engineer passed to me.

Engines arent my strong point, just operating them.

psneeld said:

Just passing along what a marine engineer passed to me.

Engines arent my strong point, just operating them.

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If you notice, I edited my statement. You were correct. There is valve overlap, The reason a diesel can get away with it is the timing of the fuel charge. The overlap occurs when there is no fuel charge being introduced. When the fuel is injected, the overlap is gone so the unburned fuel does not go through. It is done for better efficiency. The 2 strokes cannot get away with it since the fuel charge has to be introduced earlier while the exhaust valves are still open...and some fuel does escape unburned.

I am still reading and trying to get a better understanding....

Baker said:

If you notice, I edited my statement. You were correct. There is valve overlap, The reason a diesel can get away with it is the timing of the fuel charge. The overlap occurs when there is no fuel charge being introduced. When the fuel is injected, the overlap is gone so the unburned fuel does not go through. It is done for better efficiency. The 2 strokes cannot get away with it since the fuel charge has to be introduced earlier while the exhaust valves are still open...and some fuel does escape unburned.

I am still reading and trying to get a better understanding....

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Extra rum ration in Ft Pierce if you make it

Again, not my words of wisdom, but I am fortunate to have plenty of angels on my shoulders.

I think anybody who tells OP to over-prop a planning boat is blowing smoke and cares not about your exhaust valves. Since 10K is well above hull speed of the boat in question their would be considerable load on engine. The engine would be overloaded at all speeds the damage greatest at higher RPM but still cumulative. Because a few people have crossed oceans on a raft and got away with it does not mean it is safe or the best way to go. Over propping increases the risk of engine disaster and is a good reason to pass on a boat for sale. Also consider there are future buyers who will not consider your boat if you keep it over-propped. If you buy a boat that has been over-propped for some time you really have no way of knowing how it was used and how much latent damage there is. If you buy and correct you are still making an unnecessary gamble since you have the option of buying a different boat that has been properly set up run and maintained. A boat so poorly propped would raise all kinds of questions about the previous owners knowledge and care.

eyschulman said:

I think anybody who tells OP to over-prop a planning boat is blowing smoke and cares not about your exhaust valves. Since 10K is well above hull speed of the boat in question their would be considerable load on engine. The engine would be overloaded at all speeds the damage greatest at higher RPM but still cumulative. Because a few people have crossed oceans on a raft and got away with it does not mean it is safe or the best way to go. Over propping increases the risk of engine disaster and is a good reason to pass on a boat for sale. Also consider there are future buyers who will not consider your boat if you keep it over-propped.

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Fully agree!!!

just fyi valve timing overlap doesn't have anything to do with forced induction. many normally aspirated high performance engines have loads of overlap.

kev_rm said:

just fyi valve timing overlap doesn't have anything to do with forced induction. many normally aspirated high performance engines have loads of overlap.

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And an open valve or port may be far enough from an injector nozzel so as to not let fuel escape. Also flame travel and air movement inside the cylinder probably plays a part. If air travel was clockwise the injector may be placed so the unburned fuel didn't escape the combustion chamber.

So a fact that a port or valve was open may not mean fuel will escape.

The key to understanding the "overpropping" discussion is weather the boat is a planning boat or a displacement boat.

A planning boat should get rated RPM at WOT to be safe when operating pulled back a bit.

The displacement boat is matching cruise power and RPM to properly load the engine at a more efficient fuel flow and noise level.

The usual mistake is calling a properly done cruiser "overpropped" when it is just using knowledge and engineering to aid the vessel efficiency.

FF said:

The key to understanding the "overpropping" discussion is weather the boat is a planning boat or a displacement boat.

A planning boat should get rated RPM at WOT to be safe when operating pulled back a bit.

The displacement boat is matching cruise power and RPM to properly load the engine at a more efficient fuel flow and noise level.

The usual mistake is calling a properly done cruiser "overpropped" when it is just using knowledge and engineering to aid the vessel efficiency.

Click to expand...

understood, but the boat in question is a planning boat with planning power and the OP has no idea how the boat was used by previous owners. He can know how it is presently propped with a proper sea trial. And if it is seriously overloaded good cause to walk or get a discount for engine rebuilds. gambling on rebuild can be a PITA particularly if engine goes in middle of a trip in the middle of nowhere as happened to me in 2007. Best to use caution and good judgment in this instance. As to the theoretical argument for over-propping a FD boat it holds little water in real life where the fuel saved is on most boats such a small fraction of upkeep that its purpose is very questionable as is its efficacy regarding possible engine damage. Engine manufacturers usually specify how to prop a boat those who believe they know better right or wrong are playing with voiding their warranty. My power boat has a 5 year extended warranty which was not granted before a full day of testing by a JD tech including on the water load measurements at every 100 RPM. Actually on high power boats I am a proponent of under propping. This allows for better manners in marinas and docking and is a safety margin for dirty bottom or prop. An under-propped motor can rev up spread more good juices to the engine with less likelihood of overheating or damage. On common rail engines under-propping is like down rating to a lower M rating without paying for the expensive chip to do the same thing.

Very well put eyeshulman,
I like to be propped about 100rpm under so my 3000rpm engine turns 3100 at WOT. On a long grade a truck driver will, upon finding he can maintain speed in 7th gear at WOT, downshift to 6th gear and let the engine sing a bit and back off on the throttle some .. at the same speed.

But FF is right a small amount of fuel will be saved and the engine will make a bit less noise. Some noise comming from torsional vibration may be greater on an overpropped boat whereas valve, chain and belt noise will be higher on the underpropped boat. But generally speaking an overpropped boat will make less noise and burn less fuel. No engine manufacturer recomends it though.

One measure of long term engine wear is piston miles.

A boat cruising lightly loaded at 2000 RPM will create considerably more piston miles than a boat happy with the same power at 1500 RPM.

Yes a 1/2 or 1 GPH lower fuel burn is minor at a few hundred hours a year ,

and with a modern engine the high RPM vibration might similar , or not too bad ,

the noise level should be way lower on the low RPM boat.

The mfg. want every engine to get past the warranty period and propping a displacement boat like a sport fish works for them.

Weather its best for a displacement boat ?????????????/

FF,
I haven't forgotten my promise to you but I get pulled in.
Piston strokes are one thing but probably more important is piston to cyl wall load .. or side load. The combustion pushes the piston down but it's connected to the crankshaft off center so it has a strong side load depending on combustion pressure that is dependant on engine load. The more the load the more the side load on the piston.

So yes fewer strokes overpropped but higher piston to cyl wall pressure. Of course I don't know what causes the greatest wear but I'm betting on lower rpm and greater piston to cyl wall force or pressure. That's why they say downshift going up steep hills. And "don't lug the engine" is a well established saying older that us.

Setting an engine to run lower RPM in the BMEP sweet spot should hardly create excessive side wall loads , its where gen sets and big pumps operate to get the biggest service life.

The flip side , a high operating speed with very low load may not create enough combustion pressure to pressurize the cylinder well.

The rings only seat because of combustion pressure behind them , if its too low cylinder wall burnishing occurs , lots of blow by , short oil life , oil hone marks departs and the engine gets really old rapidly.

The cylinder operating temperature will be lower causing carbon deposits to collect behind rings , and probably on valve stems

The longest engine lives seem to come from a well engineered system approach.

Simply screaming the engine with a water ski boat propeller , and pulling back to cruise may not be the best long term setup.

Volvo has noticed this and most of their sailboat engines are "overproped" when using the factory tables.

“Volvo has noticed this and most of their sailboat engines are "overproped" when using the factory tables.”

By the same token Yanmar sent out a bulletin, pertaining to certain engines, years ago that they wanted the loaded WOT RPM to be increased to 3450+ not 3300 RPM as originally published. That is with a no load RPM of 3700. Best to get the info from each individual mfg as they should have data based on original specifications and long term field findings for their products.

General statements about all diesel engines are often loaded with errors as the engines can and do vary greatly.
if you can secure a chart that shows your engines hp produced v s fuel use you may be surprised that it uses less fuel to produce a unit of hp at various rpm.
Knowing that relationship and that all engines are not the same in that area can also help choose the best rpm to operate within.
If I had a fixed speed that I would not want to exceed (as in over propped) It would be prudent to 'pin' the throttle so that the new 'safe' WOT rpm (fuel load) would not be exceeded.
I have witnessed many folks who say they are not going to travel at more than XXX knots get tied up in a current , time constraint or weather issue only to push the throttle(s) much further then their claimed upper limit.