Speed vs RPM, my math isn't mathin.

[PierreR]

I agree 100%

The only real issue at hand (I think) is whether the fixed gear drive train is at OEM spec (right propped) or over propped or under propped. I think we can all agree that the OP is not over propped.

In practical terms the only impact of changes for the OP would be to cruise RPM. If bringing things to OEM spec lowered cruise RPM I'd take that as an unqualified benefit.

One of the nice things about discussing a Krogen 42 is that there is so much available information on the boat. With a PC of 0.59 the design is optimized for a 7.5 knot cruising speed and that is where I would probably operate it. The Ford engine is not a really slow turning engine and 1,900 to 2,000 rpm is not pushing it. The boat is optimized for it. He is damn near at book values for a new boat and his boat is not new and probably heavy.

The hull is so efficient at the speeds he is traveling at that over propping the boat is likely to result is less optimal conditions for the engine greatly added expense and virtually no fuel savings.

 

[Jeff F]

The hull is so efficient at the speeds he is traveling at that over propping the boat is likely to result is less optimal conditions for the engine greatly added expense and virtually no fuel savings.

I've never suggested over propping.

 

[PierreR]

I've never suggested over propping.

True, you actually did not.
Purely from my point of view I inferred that you did. I guess my reasoning for that was that slip is more a function of speed until cavitation comes into play. From everything I am calculating he has to have very close to the correct prop to get the numbers he is getting. I could be wrong but I don't think so.
The only way to reduce rpm is reduce diameter and increase pitch without cavitation. less efficient though. The second way is change the gear ratio driving it (over propping) and the third is to over prop without cavitating.

Selecting a prop is a compromise of what rpm you want. Generally about two thirds of the rated rpm at the speed you will travel at most of the time.

The problem as I see it says that its easy to do worse than his current set up but hard to do better.

 

[SteveK]

I've never suggested over propping.

Perhaps correct propping should be the goal? But even that is subjective.
I operate neither under or over loaded. With WOT @ 3000 my operating range, my zone happens to be about 1500 to 2100 which translates to 7-10 knots. I can do more or less of course. The engines are loaded but not overloaded.

 

[Jeff F]

The only way to reduce rpm is reduce diameter and increase pitch without cavitation. less efficient though.

Why would a 28x22 prop be any more or less efficient than a 28x20 prop? There is no need for diameter change.

This has nothing to do with prop efficiency, and everything to do with operating the engine in the appropriate rpm range. If the engine is currently running at higher rpms than OEM spec to support a load that is 25% of total rated capacity I think it's hard to argue that it's fully optimized.

 

[PierreR]

Based on what you just typed I blew up the graph to see what the top end is. I thought I read 3,000 but I think it's 2500 so 1,650 would be more appropriate but not at the expense of efficiency. Given a 20" pitch there is no way around 1,900 rpm. As speed increases slip decreases so I suspect the top end with a low prismatic coefficient is going to top out about 2,500. The only way to reduce engine rpm is to increase pitch to 24". To increase pitch by 4" you would need to decrease diameter to 26-27" You would have to do the math to check for cavitation. You are probably giving up 2-3% efficiency to do that. It could be worth it for the noise reduction at only about 0.1-0.2 gph increase in burn

 

[Jeff F]

Based on what you just typed I blew up the graph to see what the top end is. I thought I read 3,000 but I think it's 2500 so 1,650 would be more appropriate but not at the expense of efficiency. Given a 20" pitch there is no way around 1,900 rpm.

So far so good...

I'm having a hard time believing that there's something magic about the existing pitch, and that there's a material difference in prop efficiency between 20" and 24" pitch in a 28" diameter prop. Your point about the need to reduce diameter confuses me. And it's not clear right now what the appropriate pitch change should be. It might only be 2". It depends on how under propped the current installation is.

As I said earlier, I'm not a prop expert. But if you take the existing prop to a prop shop and ask to increase the load at cruise speed by 250 rpms I don't think that would pose any challenges or necessarily lead to a decrease in prop efficiency. And based on what we know so far it seems like an appropriate course of action for the OP to explore, after verifying through the WOT test the degree (if any!) that they're under propped.

 

[PierreR]

Increasing pitch without decreasing diameter is the definition of over propped. To drop to 1,650 rpm you need to increase pitch to 24" You will no longer be able to reach 2,500 rpm. Especially not with a PC of 0.594.
You likely cannot re-pitch 4". Decreasing diameter of the prop 1" is equivalent to about 3" in pitch in terms of prop power absorption.
You can run a more efficient prop by running higher rpm because the diameter is larger and pitch is smaller. 1,900 rpm is not unreasonable for a 2,500 top end on that hull design. The last 250 rpm will pile on 40 % of the power because the KK 42 hull design is very efficient at low S/L ratios.. It's nothing like a semi displacement hull. Not even close
The only practical reason to decrease rpm in that hull design is to reduce noise but you will need to compromise on a less efficient prop. In a semi displacement hull you are already starting with the compromised smaller diameter prop to prevent cavitation because of the need for much higher pitches to give a higher speed. Definitely so in planning hull. The result is you will have the same speed at lower rpm and higher power requirements.
Look at my boat against his. I have a full displacement hull but the prismatic coefficient is around 0.63. My boat is optimized for cruising at 8.6 knots. The KK 42 is optimized to cruise at 7.5 knots. My water line length is nearly the same, my weight is slightly higher and I have stabilizers. That translates to around 55 hp to his 31 hp at 7 knots. I am getting that from two engines of 112 hp each driving 26X24 propellers with the same 2.91 reduction gear. At 7 knots slip will be nearly the same for both boats but I can run the 7 knots at 1,650 rpm producing 27.5hp per engine. I have the space for 28" props but I am not optimized to run at 7.5knots, I am optimized to run at 8.6 knots at 1,950 rpm with a maximum at 2,500 rpm. Both of our designed optimum speeds set by the prismatic coefficient and set near 1,900 rpm. As you get into semi displacement hulls you will run lower rpm for the same 7 knots but its also not linear because as speed increases, the propeller becomes less efficient with decreasing diameter and increasing pitch.
Fluid dynamics is not linear. Navier-Stokes partial differential equations representing fluid dynamics are pretty brutal and I have probably aged out of them but I understand what they represent.

 

[Jeff F]

Increasing pitch without decreasing diameter is the definition of over propped.

Not according to everything we've discussed so far. If you are under propped the goal should be to increase prop load. Increasing pitch is the easiest way to do that.

If I was under propped by 4" pitch and added 2" of pitch with the same diameter I'd still be under propped, just less so.

 

[PierreR]

I will just about guarantee you he is not under propped. At 24" of pitch he would need about double the HP to reach 2,500 rpm

 

[Jeff F]

I will just about guarantee you he is not under propped. At 24" of pitch he would need about double the HP to reach 2,500 rpm

Ive never suggested that they need 24" pitch. That was your number.

The OP cites 2700 RPM reached quickly at full throttle with a verified tach. That combined with what is arguably a high cruise RPM strongly suggests to me that they're currently under propped by all common definitions.

 

[Jeff F]

Duplicate

 

[DDW]

Ive never suggested that they need 24" pitch. That was your number.

The OP cites 2700 RPM reached quickly at full throttle with a verified tach. That combined with what is arguably a high cruise RPM strongly suggests to me that they're currently under propped by all common definitions.

It doesn't just suggest it, that is the definition of underpropped if we accept your definition above.

Generally speaking for maximum efficiency at any rpm, you want more load on the engine at any rpm below max, than the "correctly propped" prop gives you. This is why variable pitch props exist.

 

[PierreR]

Selidster, I cannot verify the top end rpm on your engine. I have literature that says 2,500 and literature that says 2,800 rpm. At least the bottom graph that you have on range is not correct. Have you actually called with serial numbers to find out what you actually have. What year is your boat? What did your current WOT test reveal?

 

[Jeff F]

I operate neither under or over loaded. With WOT @ 3000 my operating range, my zone happens to be about 1500 to 2100 which translates to 7-10 knots. I can do more or less of course. The engines are loaded but not overloaded.

To DDW's earlier point, if you can get to 3000 RPM, by definition your engines are under loaded at 2100 in a trawler. If you had a way to reduce rpms further with the same load it would be a net good. But you don't.

 

[Ivan on TubTub]

I have a 40 foot marine troller with twin Ford Lehman 120s. I set my engine RPMs at 1900 and my normal cruising speed in calm Weather is 8.2 kn. If I’m adjusting for current and or wind, I can easily drop down into 6.9 kn range. I don’t see any problems with your math. The one thing you have not taken into consideration in your calculation is the gear reduction in your transmission. In my case, I am a 2 to 1 ratio. This might solve your under prop size problem because you’re not actually spinning at your calculated engine RPM. If you can find your ratio on your data plate on the transmission run that calculation and see if your math adds up. Good luck.

 

[Selidster]

Selidster, I cannot verify the top end rpm on your engine. I have literature that says 2,500 and literature that says 2,800 rpm. At least the bottom graph that you have on range is not correct. Have you actually called with serial numbers to find out what you actually have. What year is your boat? What did your current WOT test reveal?

There is a tag on the valve cover that states 2500 max working rpm. I haven’t confirmed the block serial number. My boat is 1980. I have yet to find wot. Last time I did it I cracked and ruined the manifold. They are difficult and expensive to find. But I will soon.