WOT rpm lower vs engine damage?

[gtfireftr]

Changed my props to 4 blade, now I can get to 4100-4200 WOT as opposed to 4600-4800
I can get on plane much faster, with no help from tabs, my cruising speed is higher at a lower rpm...
My question is....... is this going to be damaging to my engines?
Am I better off spinning at higher rpm, or lower rpm......we are talking 500 rpm.... can this hurt?
I am a car guy, and cannot grasp how a lower rpm will hurt the engines.......
Of course there is torque etc.........
Just a little advice please.......

Thanks

 

[480ces]

In a gas application I don't see any issues, personally I'd have the props tweaked to give you another 300 RPM or so.

In a diesel application you'd be overloading the engine.

 

[scoflaw]

4 blade is a nice upgrade but you still want to be in your suggested rpm range. Your engine is now working too hard, and will put more load on the lower end of your motor

 

[fc3]

You must select a propeller that gets the engine into the specified range for WOT. The engine is overloaded if it will not turn rated WOT RPM.

 

[gtfireftr]

Sea Ray states that my WOT should be 4400-4800.... My WOT is 4100-4200....
So this will cause an issue with my engines...... The fact that I can achieve a cruising speed at a lower rpm is not advantageous?? Just want to understand.

 

[SeaAyeOwe]

Sea Ray states that my WOT should be 4400-4800.... My WOT is 4100-4200....
So this will cause an issue with my engines...... The fact that I can achieve a cruising speed at a lower rpm is not advantageous?? Just want to understand.

It may be an advantage to get on plane better - the point is that the reason the engine will now not reach wot is that the power required to turn the new blades through the water at those higher revs is more than the engine can put out now. Therefore you are overloading the engine - as someone has stated this is not good for the lower end.

In all honesty I am not sure how material the overload is, but I would get the props tuned so you are within the designed rpm range.

 

[keokie]

The guy in the slip next to me just had his boat pulled to get new engines. We had been telling him he was over-propped for a season and a half. He finally tulliped a few valves and put a hole in a piston. The problem with overloading an engine is it greatly INCREASES the temperature in the combustion chamber. Get valves, pistons, head gaskets etc. too hot for too long and they fail much earlier.

Also had a friend years ago that blew three head gaskets on two motors within a few minutes of each other after crushing for a couple hours at about 3800 rpm, but just below WOT. Impellers and cooling system were good and the boat was not overheating at the time, only running slightly warmer than normal according to the gauges.

 

[CliffA]

plus your fuel economy will suffer by overloading the engine......

cliff

 

[gtfireftr]

So spinning these 4.3's at 4k rpm all day is okay?

 

[fc3]

So spinning these 4.3's at 4k rpm all day is okay?

Yes, if short engine life is acceptable. Otherwise, the rule of thumb is 80% of your WOT RPM so somewhere around 3500 - 3800.

 

[CliffA]

its your boat, your props and your money....do what you want....you asked for advise and received several replies....yet you still question the replies from others more experienced than you on this subject.....

it is pretty simple...constantly overloading an engine will shorten its life...period...no debate...for example if you used a regular 1500 series truck to tow a 12,000 lb. trailer every day how long do you think the drivetrain in the truck will last?....

if the new props do not allow your engines to reach the recommended WOT rpm range the engines are being overloaded....maybe not by a lot, but enough to probably make a difference over time....

cliff

 

[keokie]

GT,

I edited my previous post. I meant over propping "increases" combustion chamber temps.

To your other question about running at 4K all day.... That is often the struggle with boats. Getting it propped so that you are not overloading the engines, but still able to maintain a desirable speed at an acceptable rpm level.

You will typical burn up a motor if you run it in an overloaded condition. And it won't take long to do it. You will wear out a motor sooner if you are running at excessively higher rpm (Frank's 80% or less rule is what the motors are generally built for).

The tough part is some boats just cannot stay below 80% and still perform in a satisfactory manner. If that is the case, you are better off exceeding the 80% then you are to over prop. Before I resolved myself to that, I would be certain I have the best props for the situation, I am running the boat properly, everything is mechanically sound, and my expectation of speed is appropriate.

By us, Lake Powell is a very common destination. It is at 3700' elevation which takes a toll on performance. On top of that, there are few marinas so you need to have enough supplies to be self-sufficient. Further, it is a big lake with lots of ground to cover if you want to see it. Improperly propped boats often won't get on plane. And many correctly propped boats have motors that are too small and need 4K rpm or more to barely stay on plane.

 

[gtfireftr]

GT,I edited my previous post. I meant over propping "increases" combustion chamber temps.To your other question about running at 4K all day.... That is often the struggle with boats. Getting it propped so that you are not overloading the engines, but still able to maintain a desirable speed at an acceptable rpm level. You will typical burn up a motor if you run it in an overloaded condition. And it won't take long to do it. You will wear out a motor sooner if you are running at excessively higher rpm (Frank's 80% or less rule is what the motors are generally built for).The tough part is some boats just cannot stay below 80% and still perform in a satisfactory manner. If that is the case, you are better off exceeding the 80% then you are to over prop. Before I resolved myself to that, I would be certain I have the best props for the situation, I am running the boat properly, everything is mechanically sound, and my expectation of speed is appropriate.By us, Lake Powell is a very common destination. It is at 3700' elevation which takes a toll on performance. On top of that, there are few marinas so you need to have enough supplies to be self-sufficient. Further, it is a big lake with lots of ground to cover if you want to see it. Improperly propped boats often won't get on plane. And many correctly propped boats have motors that are too small and need 4K rpm or more to barely stay on plane.

Thanks for the reply. I have heeded the advice of the consensus. I went back to the 16p prop that gave me the 4800 WOT, at the sacrifice of speed. I think I may try a 17p and see if it can be the happy medium of all. I am sure 3-4 mph will not make an impact on my life. Lol. Hopefully it will increase the longevity of the engines as well. How many rpm can I expect to loose by changing 1p?

 

[blaster]

Why not send out the 4 blade props to be tuned? It could be as simple as reducing cupping on the blades. Even if you invest $150 each you will get the rpm's you need and more low end thrust.

 

[gtfireftr]

Why not send out the 4 blade props to be tuned? It could be as simple as reducing cupping on the blades. Even if you invest $150 each you will get the rpm's you need and more low end thrust.

When I bought the props I was told that I could swap for a $35. fee... So I had 18p... went to 16p, and I think 17p will be just right... if I cannot get to the appropriate WOT I will put the 16p back on and that's that.....

 

[ttmott]

You should prop to the torque curve of the engine not RPM; especially for gasoline fueled engines. The difference between diesel and gasoline is the gasoline (especially fuel injected) will adjust fueling based upon load and not necessarily throttle setting. Secondly, there are duty cycle requirements for engine RPM / Power settings; for example at full throttle you might have a 10% duty cycle where you should only run 1 minute full throttle out of every 10 and at 3800 RPM a continuous duty cycle. Combustion temperature is not necessarily determined by RPM but rather how much work is being demanded from the engine; work is heat and anything the engine produces other than rotational energy is shed as heat that the cooling system must deal with. The second principal factor in combustion temperature is the fuel / air mixture; the richer the cooler the leaner the hotter. The really good thing about fuel injection that fuel / air mixture is very well optimized regardless of the RPM / Power demand. So, in short your propping (over or under) of a gasoline engine won't necessarily shorten engine life simply due to propping but it may demand operating the engine off it's optimal torque curve which will carry inefficiencies that will change the life cycle of the engine and require operating outside of the manufacturer's duty rating.

 

[Lazy Daze]

Something is striking me as strange, here. It's not about the advice you've gotten - that's all dead on. Especially the points about overloading (not able to reach spec'd range) being more detrimental to the engine than simply running it at an elevated RPM (but still able to get to the spec'd range). But the numbers you are posting in relation to the props... something is off there. First, if you lost 600RPM's, that means that you went UP in pitch, not down - and that much of a difference, even changing to a 4-blade, shouldn't yield a BETTER hole shot.

And then there's this "When I bought the props I was told that I could swap for a $35. fee... So I had 18p... went to 16p, and I think 17p will be just right... if I cannot get to the appropriate WOT I will put the 16p back on and that's that..... " That's even more confusing since it sounds like you're saying that your 3-blades were 18P and you went DOWN to a 16" 4-blade (which gave the low RPM, which is impossible), but then you will likely stay with the (wrong) 16p, although I thought you had agreed that this was not a good idea?

Note that I'm not attacking you, or anything like that. But there appears to be discrepancies which make helping you hard. It could be that you just mixed up some numbers when you wrote them down here. But based on what has been written, something is not adding up. I also thought that having 16p (or even 18p) 3 blades on there, and getting into your spec'd MAX RPM range didn't sound right. I looked up what props would have been on there from the factory and, sure enough, they would have been 21" 3-blade. This continues to confuse things as there is no way you should be able to put a 16p or 18p on there and get LOWER RPM's (or even the same, for that matter). You would certainly get a better hole shot, but your RPM's would sky rocket and your speed would be lower.

 

[Creekwood]

I think he meant that AFTER ditching his 3 blades he started trying the 18" 4 blades and went from there.

 

[gtfireftr]

Something is striking me as strange, here. It's not about the advice you've gotten - that's all dead on. Especially the points about overloading (not able to reach spec'd range) being more detrimental to the engine than simply running it at an elevated RPM (but still able to get to the spec'd range). But the numbers you are posting in relation to the props... something is off there. First, if you lost 600RPM's, that means that you went UP in pitch, not down - and that much of a difference, even changing to a 4-blade, shouldn't yield a BETTER hole shot.

And then there's this "When I bought the props I was told that I could swap for a $35. fee... So I had 18p... went to 16p, and I think 17p will be just right... if I cannot get to the appropriate WOT I will put the 16p back on and that's that..... " That's even more confusing since it sounds like you're saying that your 3-blades were 18P and you went DOWN to a 16" 4-blade (which gave the low RPM, which is impossible), but then you will likely stay with the (wrong) 16p, although I thought you had agreed that this was not a good idea?

Note that I'm not attacking you, or anything like that. But there appears to be discrepancies which make helping you hard. It could be that you just mixed up some numbers when you wrote them down here. But based on what has been written, something is not adding up. I also thought that having 16p (or even 18p) 3 blades on there, and getting into your spec'd MAX RPM range didn't sound right. I looked up what props would have been on there from the factory and, sure enough, they would have been 21" 3-blade. This continues to confuse things as there is no way you should be able to put a 16p or 18p on there and get LOWER RPM's (or even the same, for that matter). You would certainly get a better hole shot, but your RPM's would sky rocket and your speed would be lower.

I will try to explain it better........
I initially started with a 19p 3 blade vengeance prop....the performance was lackluster at best....
Purchased a 4 blade 18p, which by all accounts overloads the engines..... hole shot was much better, cruise faster at a lower rpm... but could only get 4100 WOT.
I was sent a 16p, 4 blade.... this allowed me to reach 4700-4800 WOT, but cruise at a higher rpm.....
This is why I said I thought the 17p is in the middle and perhaps what I need.....
The reason I went down in rpm, AND had a better holeshot was I had a tremendous amount of prop slip with the original 3 blade props...
Sorry for the confusion.... Thanks for the info

 

[Creekwood]

Sounds like you are on track. The only other thought is to make sure that you consider what happens when you fully load the boat. If you are testing with a lighter than normal load (less than the normal number of people for example) then you probably want to get to the top end of your WOT range so when you load it up it comes down in the range and not below.