Cruising propeller

[Smarten]

Hi,

I am evaluating the idea to equip *temporarily* my boat, a semidisplacement 55 LWL trawler, with a large cruising propeller to try to limit engine damages due to low RPM/load prolonged usage and secondary improve prop efficiency.

My plan is, in fact, to cruise around 8.5 KNT getting a mere 95-100 HP from one engine only at about 1000 RPM. In order to limit carbon growth I would install a propeller capable to load the engine at about 70-80% of the maxim power available at that RPM (I guess about 150 HP). The engines are 6V92 450 HP. The tranny ratio is 1:1.48. The max propeller diameter installable is around 37" with a 15% clearance.

I know the risk of overloading the engine I will install an EGT to monitor the exhaust temp for that purpose.

I don’t know if the envisioned approach it’s of real effectiveness about engine related problem and the gaining if propeller efficiency will

Furthermore, do you known of any countermeasure against carbon growth and other potential damages deriving from low RPM usage? Additive in the fuel? A modified thermostat to maintain the cooling temp at appropriate (higher) level?

I would appreciate your comments and suggestions

Lino

[marshmat]

I'm not sure if this is an issue with your diesels... with the small gas engines I'm used to, the major concern with low-RPM operation is that the combination of low speed and high load increases the force on the crankshaft and rod journals, thinning the oil film on the journal and wearing out the rods. Carbon will always be a pain in the you-know-what; the best solution to that is to keep the motors tuned up and running in their ideal rpm range, and to use only clean fuel.

[cyclops]

Sounds like your engines are to powerfull to operate at your desired speed. Why were they picked so large? Only solution to keep them, is a variable pitch prop. Or 2.

[Smarten]

Thank you for your support.

The boat is a semidispl trawler so is pretty common to have too much power at displ speed and never enough at higher.

Variable Pitch props are too much expensive but also being rich pretty difficult to retrofit in my case.

I would probably I would avoid the need of a variable pitch prop setting in advance the RPM to stay during cruise and knowing in advance what would happen opening the throttle ...

Regards

Lino

[PowerTech]

A 6v92 450 has 2 little dry turbos and a intercooler doesent it?I think so i know the 500 somethings do.if it does get rid of the tubos they suck any way dry tubos arent worth a damn.leave the intercooler in there in wont hurt any thing and get small injectors they are asey to instal and are cheap.then you won,t over feul.you can run it how you want to.you can bring that motors H.P. doun as low as you want to with the injector size.A detroit with no turbo and small injectors will run forever.It will be a huge heavy,noisy, stinky ,oil leaking uneficient pig but it will run a long time.try finding a 2 or 2.5 to one allison gear out of ships and harbors or someplace.I bet you can get a gear cheap we sell compleat running but wor out detroits for around 2500 bucks when we get them and thats with gear.what i am saying is detune the motor put a low gear it it and swing a huge wheel you will put along fine.Or do the white thing and put a deere in it.

[FAST FRED]

Your plan is fine , but you may get a hassle if the engine has a turbo. The turboed units dont like very low speed operation , carboning up and having other maladies.

Many engines are used to run Prime generators at 1200 RPM and you may be able to get the cont. loading power at 1200 from the eng mfg , and use that to size your cruising prop.

Beware of going too low rpm as only a few hundred more rpm may begin to overload the engine.

Little is saved at 1000 rpm compared to 1200 , but at 1200 you might be able to pull the huge cruising prop at 1800 , using the "pleasure boat" load limit restrictions.

Smaller injectors do help on our 6-71 we took out 90's and installed 60's and save 1/2 gal an hour at 8K. About $40 each for a matched set , easily owner installed but you will need a real DD guy to "run the rack".

I would attempt to LRC the boat at Sq rt of the lwl times 1.1 for lowest fuel burn.

Once the ratio gets to 1.2 the fuel used never pays for such a tiny speed increase.

If you have a GPS , doing a graph of speed vs tach readings may show the "sweet spot".

FAST FRED

[PowerTech]

your motor is not to big for your boat .but by modern standards it is phisicly to large for it's power output.but anyway.I hope your motor is non turbo if it is you are fine.But a 1 and a half to 1 rear box is crapy for that aplication.at least a 2 to 1 ,better yet a 3 to 1.what motor and gear came out of the boat? And are you just trying to load down your engine with the high ratio to keep the RPM low?

[Smarten]

Hi,

Thank you for your support.

Some answers and some further questions.

PowerTech:

My engines are turbo intercooled. They have only one turbo blower collecting both banks exhaust. Those where a J&T Ti version initially rated at 535 HP now 450 due to injector replacing (now are 9225) and other not documented modification by Detroit Diesel during last overhaul. I think now engines are hybrid between J&T Ti and DD Ta.

Regarding the trannies are TwinDisc MG 510-A. Those models where available in different ratio up to 1:2.44 in the same case. At moment I was unable to find the pignons to make the conversion and TwinDisc was not collaborative. I was used to know their business were more lucrative on the spare parts than on new equipment but is my case I think overall value is maybe so little to urge their customer care too.

For professional reasons do you have any contacts to get an answer about the feasibility of trannies modification?

It is you advise to skip the turbos for the passages? The aspirated version have bigger compression rate. It’s relevant? How complex is the temporary modification to skip the turbos? With no modification I will carbon the blower only or destroy the turbos unit at all?

Fastfred:

I have installed the Floscan and my plan is exactly to go 1.1 the hull speed. I will also have a sail to gain something from the monsoon coming from rear at true speed of 10-15 knts.

It seem you get better efficiency moving to little injectors. I mean littlest injectors seem more efficient (better spraying patterns or what?) than larger at half throttle. There is physic behind? Do you think I will get same results with 6-92 ?

Thank you again for your competent suggestions

Lino