Prop question

[BrianG]

The engine should be able to reach the max RPM for that engine. Usually it is recommended to run the boat for long periods at no more than 80% power. You should be able to run 100% for 5 to 10 minutes without overheating.

Okay thanks, I misunderstood what some of the comments then.

 

[FF]

"Boat diesels should run at full rpm? I'd always heard 90% of peak rpm."

Look at the mfg. specks power output / time varies .

90% for a modest time is OK for some engines for a limited min on a pleasure boat.

WAY !! Lower Power is for 24/7 operation.

M-1 to M-4 is the table to look for.

If the engine is a transplant , agriculture , truck, taxi it may not have power/ use ratings.

Then just divide the engine cubic inch displacement by 3 for a safe long term power output.

 

[rslifkin]

Max continuous output depends on the engine in question. Some manufacturers give an RPM that's safe to run constantly (assuming the engine is propped to their specs). Others don't.

As a safe rule, for any engine without a known max continuous rating and that's not a heavy duty design intended to run at very high output all the time, prop it to reach rated RPM or better and then treat max continuous as 80% of max achieved RPM. That's a bit on the conservative side for a lot of engines, but there are very few engines out there (gas or diesel) that can't handle that usage (80% RPM will typically be 45 - 60% power).

 

[SteveK]

Most engines are run at 60-80% of max HP at normal cruise. That also seems to be the efficient range for fuel consumption. Over propping adds some speed within that range, the reason it is done. Low end torque engines can handle the extra load.
Diesel engines are better than gas in (S)D boats. JMO

 

[Nomad Willy]

SteveK,
I tried to get in the 6-65% load range but failed. 70-75% isa lot of work being done and very few small pleasure boats and trawlers like ours would tolerate that. I’m just about right on 50% load at 2300rpm. Occasionally run 2500 but I doubt if it exceeds 1%.

But commercial applications are way different. If they can get the job done w an XBC engine at 80% load and it’s rated for continuous operation there .... the’ll do it. They won’t be inclined to say “it dosn’t sound awesome at that load” or “I love those screaming XBX engines””.

Steve, how does overpropping ad speed when it always reduces power? From an historical perspective on TF overpropping was done in the early years to increase millage or/and range. It does of course but only a small bit. It usta be seen as something that old salts or guru’s did to bend the rules and (to a degree) cheat. It was probably seen as “outsmarting” the manufacturers and NA’s. And that one would need to be extremely knowledgeable to do that.

But I could run a 25hp engine at 80% w the blessing of all marine power engineers. Assuming it was rated for that.

 

[SteveK]

Eric, I run my twin 120's at 70% or 1650-1750 range doing average 8 KN. I check wot each trip to learn if growth is taking hold etc. For clarity I have 24/18 CR port and 24/17 SB. different due to different trans ratio.

Over propping, what is it? We may all have a different idea.

Over propped means the engine cannot achieve its max rated RPM. Under propped means the engine reaches max RPM too easily

Why do we do it, also may mean something different to each of us.
For instance have you ever replaced a standard OEM prop, with the identical SS prop and not noticed an increase in speed? Same diameter, same pitch, but more speed because the SS does not flex as much and retains performance under load.

Over propping our (S)D boats is similar (I think) in that we increase diameter and/or pitch to overcome prop deflection and maintain the desired result.

cupping will increase speed, as the pitch "can" be increased by cupping, and since the pitch has is increased, engine RPMs should reduce too

The lower the prop pitch, the better your hole-shot. However, this comes at a price: top speed. The lower pitch makes the engine reach maximum rpm at slower speeds. Conversely, a higher pitch will deliver greater top speeds, but slower acceleration

Reduce power? Here is where I am not following your thought. IF you achieve WOT within 2-300 of the book wot then I am running to specs (according to others who spoke to Brian re Lehman). IF I am on spec with wot then I most likely would run a higher rpm to maintain speed as the prop is not biting water the same, less diameter and/or pitch means it has to turn faster.

It has been established I think that the more rpm the more fuel used, so we over prop (insert your interpretation)

 

[FF]

"Yes, but the noise vs load thing applies to both gas and diesel. Think about a generator. Constant RPM, but it gets louder as you increase load."

The competition in selling noise makers is not enough to force the assemblers to modernize.

A small Honda creates DC power which is run thru an inverter.

This allows the RPM to vary with the load , reducing fuel burn , noise and wear. .

The "off grid" folks have been doing the same with large bus alternators and larger inverters. The voltage control from a welding unit operates the throttle on the engine , so great AC sine wave power can be created , just above idle.

When boaters and RV folks demand better noisemakers , and vote with their check book , ihe assemblers will take note.

It only takes one to modernize , and the rest are forced to follow.

 

[rslifkin]

Inverter generators are a mixed bag. They're absolutely more efficient at light load, but as you get closer to full load, a standard generator is typically more efficient, as the inverter setup has some losses and it's only at light load where it gains you enough to make up for those losses.