Propeller size vs engine torque vs horsepower

I have a propeller/propulsion dilemma on 49’ Elling E4, and would appreciate some guidance and input from the diesel & propeller engineer’s reading the forum.

Boat specifics:

LWL: 49’; D: 4’; B: 14’; Displacement: 41,000
Volvo D6-435 I Rated 435BHP@3500RPM; SHP: 422; Gear ratio: 2.5:1
Max torque: 772ftp @ 2500RPM, dropping to 644ftp @ 3500RPM.
Effective mean pressure: 349.9psi @ 2500RPM, dropping to 289.4psi @ 3500RPM
Max combustion pressure: 2843psi @ 2500RPM, dropping to 2756psi @ 3500RPM

Data from Sea Trial at Survey: Using 4-Blade Prop – 27” Pitch: unk

RPM's 680 1350 2200 2420 (WOT)
GPH (from gauge) 0.3 1.1 5.6 21
Spd (GPS 3.4 6.9 9.8 14.8
Miles per Gal 11.33 6.27 1.75 0.70
Range @ 95% of 400g 4306.7 2383.6 665.0 267.8​

Data after propeller replacement 5- Blade Prop - Pitch: 27 x 21
RPM's 1000 1500 2000 2500 3000 3500 (WOT)
GPH (from eng specs) 1.1 2.7 5.2 9 14 21.9
Spd (GPS) unk 6.3 8.3 9.5 11.5 15.5
Miles per Gal unk 2.33 1.60 1.06 0.82 0.71
Range @ 95% of 400g uno 886.7 606.5 401.1 312.1 268.9

My observations from reading the data:

1. With the 4 bladed prop, the WOT of 2420RPM’s is very near the max torque of the engine, however the fuel burn of 21gph is identical to running at 3500RPM’s of the 5 bladed prop.

2. It is understood that the GPH determined from the gauge might be inaccurate.

3. It is understood that the GPH from the engine manual for the 5 bladed prop might be inaccurate too.

4. Based on the data above, it appears the 4 blade propeller is more efficient and offers much better range up to the hull speed of the vessel of about 9.2 kts.


My question is:

Will the engine be damaged by using the 4 bladed propeller vice the 5 bladed one, knowing that the engine will never reach it’s rated RPM of 3500? Most of my cruising will be done at hull speed. Although I’m not planning on running WOT often with a fuel burn of 21gph, I don’t want to risk engine damage by having to run WOT should I choose to do so.

In advance, thanks for the information-and my apologies if I've placed this in the incorrect spot, as this is my first post to YF.

John

Cap'n J,

Thanks. I always enjoy your comments and suggestions. And in discussion with a couple of prop shops, they are coming in with sizes very close to what you indicated: 27" x 20.5", and 27" x 21".

What is driving my query is the change of RPM's discovered by replacing the electrolysis damaged 4 blade propeller (used during sea trial) with the original 5 blade propeller that was on the boat, and discovering that WOT is now 3500RPM. As I dig deeper into "propeller science", it sends me over into "diesel science", and to me that's just a complete "black hole" unto itself!

Although I'm not an engineering wiz, I'm trying to get my head wrapped around the declining torque curve above 2500 RPM's. That 2420 figure was near max torque for the engine, so I'm not understanding how it would cause premature failure by using the engine at lower RPM's (hence less piston travel) yet at the strongest torque.
The spec sheets follow.

Thanks again for taking the time to explain.
John

Many thanks to Capt J and Norseman for their helpful responses. Clearly they are speaking from their vast experience.

Onboard the merchant ship I’m currently on, the senior engineers – the guys that speak diesel as a primary language 24/7 – are saying (with the caveat they don’t know the Volvo D6 dialect) that if the engine is run within the torque power band, there should be no downside. They obviously concur with not running WOT all the time, and concur with running at 80 – 90% to properly load the engine. That way there is not turbo-barking, and all the sub-systems are happy.

The power band for the D6 taken from the manufacturer’s torque information [drawing a horizontal line through the graph’s bell curve] is from 2000 – 3000 rpm. At 2000rpm, it’s 100kpm(993Nm), rising to 105kpm(1047Nm) at 2500rpm, and decreasing back to 100kpm (980Nm)at 3000rpm. By WOT, 3500rpm, torque is down to 88kpm(873Nm).

So, from an engine operating status, how is the apparent over propping bad in this case? If running at 2200rpms, going 9.8 kts, consuming 1.75 gal per mile, with torque about 3kpm less than max isn’t that better use of the engine’s power?

I may be dense here, but I’m seeing the engine just using a comfortable amount of torque to thrust a lot of water astern, with minimum piston travel and fuel burn. The turbo’s kicked in – not working too hard & not barking – and life is “good” for the engine.

If I’m not tracking this engineering situation properly, hopefully someone with a “PHD” in diesel engines, and with a specialty in Volvo D6’s, can realign my thought process,.

- - Leaving deployment in Korea; next stop FL!!