Doing some prop pitch numbers ...

[Sven]

After our recent non-sailing experiences I've become somewhat obsessed with figuring out not only how much fuel we have but how much we are using.

In a recent discussion with a very experienced cruiser he questioned our cruising RPM, he thought 1,900-2,200 RPM was way too high for a Perkins 4-108M. I went back and re-checked ...

47 SHP @ 4000 RPM for high speed pleasure craft
45 SHP @ 3600 RPM for pleasure craft
37 SHP @ 3000 RPM for commercial

so my recollection of 3000 was correct. 80% of that would be 2400 which should mean that we are running at low RPMs if anything.

So, assuming that we have a 2:1 reduction in the transmission and using our 8" pitch propeller ...

Inches per revolution 8
Prop RPM 1000
Inches per minute 8000
Inches per hour 480000
NM per hour (knots) 6.58

Of course, that's assuming 100% pitch to speed conversion and in flat water we're probably seeing 5 knots at 2000 RPM (1000 RPM at the prop) which would mean about 75% of the theoretical pitch to speed conversion.

So, are we under-pitched or right on target ?

If our tank is really only 55 Gallons instead of the 80 gallon spec we have another problem too, but it sure would be nice to get some of these numbers under control



-Sven

 

[rwthomas1]

Limited experience with this but this is what I've been told: With the throttle wide open in dead flat water the engine should not be able to reach its top governed RPM. The prop should effectively load the engine and act as a "governor". I was told to pitch the prop so that the engine almost reaches top governed RPM. I believe my boat is underpitched as the engine will easily reach the 2800rpm maximum in flat water. I'm thinking in my case 2700-2750rpm would be perfect in my case. Do you know if the engine will reach its maximum with the existing prop? Do you know where the governor is set? It would appear from your post that Perkins simply increases the max RPM for difference applications, thus extracting more HP. Even if the governor is set higher than you would like just pitch the prop for the max RPM you would like and then adjust the cruise RPM to 80% of that. Clear as mud? RT

 

[Sven]

Clear as mud? RT

Actually, it makes perfect sense.

The RPM to SHP numbers were definitely interesting. Makes you wonder about the 80% rule unless it is 80% of full load RPMs which is what you are describing.

Thanks,



-Sven

 

[rwthomas1]

I should also add that the "method" described above is supposed to work as it allows tuning of the prop to a specific hull. Not all hulls are identical, loaded the same, the engines may or may not be in the same condition, etc. Basically the differences can be factored in. That makes sense to me anyway. RT

 

[Alan Gomes]

Getting the right pitch

Limited experience with this but this is what I've been told: With the throttle wide open in dead flat water the engine should not be able to reach its top governed RPM. The prop should effectively load the engine and act as a "governor". I was told to pitch the prop so that the engine almost reaches top governed RPM. I believe my boat is underpitched as the engine will easily reach the 2800rpm maximum in flat water. I'm thinking in my case 2700-2750rpm would be perfect in my case. Do you know if the engine will reach its maximum with the existing prop? Do you know where the governor is set? It would appear from your post that Perkins simply increases the max RPM for difference applications, thus extracting more HP. Even if the governor is set higher than you would like just pitch the prop for the max RPM you would like and then adjust the cruise RPM to 80% of that. Clear as mud? RT

I believe you have stated the matter well. The only thing I'd add to this is that you'll also observe that the engine will have to work a bit to hit those last couple of hundred rpm. If the prop is under pitched, it will just effortlessly and quickly hit the top rpm set by the engine's governor. If it is over pitched, as the E26 I just purchased currently is, then it will be well shy of the engine's top rpm as set by the governor. (My Yanmar 1GM has a top rpm of 3750-3800 but with the present prop I can just barely get over 3000 rpm, at which point the engine complains by putting out black smoke.) I plan to remove some pitch from the prop so that it will be able to hit around 3650 max, with the engine having to work a bit for the last few hundred rpm.

--Alan Gomes

 

[rwthomas1]

I believe you have stated the matter well. The only thing I'd add to this is that you'll also observe that the engine will have to work a bit to hit those last couple of hundred rpm. If the prop is under pitched, it will just effortlessly and quickly hit the top rpm set by the engine's governor. If it is over pitched, as the E26 I just purchased currently is, then it will be well shy of the engine's top rpm as set by the governor. (My Yanmar 1GM has a top rpm of 3750-3800 but with the present prop I can just barely get over 3000 rpm, at which point the engine complains by putting out black smoke.) I plan to remove some pitch from the prop so that it will be able to hit around 3650 max, with the engine having to work a bit for the last few hundred rpm.

--Alan Gomes

Yes, you described it better than I did, but thats exactly what I meant. My 5432 will run to the governor on flat water without any problem so I believe the prop to be underpitched. I'll be turning the screws on it next time it comes out. RT