Question on Engine/Prop load

[marks737]

Hello gentlemen. I’ve read your great articles regarding propeller load and engine life. I have a 2005 Sea Ray Sundancer with Cummins QSB 5.9 380’s. First diesel boat and love it. Just got back from a 6 day cruise and paid close attention to my fuel burn as it relates to the performance chart that you reference. At a comfortable cruise power (about 2450-2500 RPM, about 75%load) which yields about 21knots, it seems I’m burning 2-3 gal per engine more than the chart specs. Total fuel burn at this power setting is around 27 gph. Chart says it should be more like 23gph total. I just hauled the boat today to check the bottom and it was spotless, props were perfectly clean as well. I ran the boat up to full throttle and I’m getting just a hair under 3000rpm for max which seems right. Props are 21×23 four blade.

My question is referencing the chart which is not boat specific. How can the same numbers apply to any boat with these engines? Boats vary in weight.
A heavier boat should require more load from the props/engines, no? Not sure if I’m slightly overloading or if I’m ok. Any input would be appreciated.

Thanks!

Topic

[Rob Schepis]

marks737 wrote:

For example…if the same engines and props are driving two different boats. One weighs 20,000lbs and the other 35,000lbs. Wouldn’t the engines on the 35,000lb boat be under a much greater load/fuel burn to achieve the same RPM’s?

“Same engines and same props”, well then yes for sure but those two vessels would never use the same gearing/propping.

[Steve Lewis]

clarity…….

Rob was saying something different and nuanced. Weight is one of many factors, not the only one, that present load to an engine through the propeller. Here is a great article from Tony about this. Also, search my name and you can read some of the posts I wrote about my actual experience and boat performance as it pertains to propeller load and resulting fuel burn. I have the same boat that you do, 1 year younger though, and the gas engines. Same theory applies.

Propellers Move Boats, Engines Just Turn Them

Since transmission ratios are fixed, the way to dial in your setup is to tune the propeller so that you are under the published curve. The weight, windage, bottom condition and a host of other things all converge together and place a load on the drive train. That load is generated by the spinning of the propeller to generate thrust resulting in the movement of the boat. SO based on lots of conditions, your engine will be asked to develop power at any RPM based on those conditions. The goal is dial it in so that the amount of power demanded is below the published curve. The realistic way to do that is tune the prop.

[marks737]

Thanks again Steve. I’ll definitely do an RPM/Fuel burn chart. If you don’t mind, I’m still a little confused as to how weight has no bearing on load and fuel burn. For example…if the same engines and props are driving two different boats. One weighs 20,000lbs and the other 35,000lbs. Wouldn’t the engines on the 35,000lb boat be under a much greater load/fuel burn to achieve the same RPM’s? Just trying to understand.

[Steve Lewis]

Props

Tony recommends finding a reputable prop shop in your area and then have them service the props. I have not read any thoughts from Tony or the rest of the SMX crew about how taking an inch or two out of a prop weakens it. If you are worried about it then you can always get your props tuned and dialed in and then actually buy a new set of props that are the same spec as your newly tuned props and then keep the tuned ones as spares. Always good to have spares of the important stuff.

I am learning about the spare parts list that I should keep for my own boat. I learned the hard way on a couple of items recently on a cruise from Marblehead, OH to Erie Pa for the annual Blues and Jazz festival.

My guess, and I am sure others will chime in, is that tuning a prop by taking a couple of inches out will not materially degrade the strength of a prop. Ask around at your marina to see who uses whom for prop work and you will find a good shop. it would probably be a good idea to complete a full performance chart. Run your boat from 1000rpm to WOT in 200rpm increments and record your speed and fuel burn at each RPM increment. That way you have a good solid chart that you can show your prop shop and then focus on tuning for your cruise RPM range. That is where the bad stuff happens. Getting the fuel burn below the curve for your engines in the cruise rpm range will take care of WOT RPM and get you going on dialing in your boat the Seaboard Way!

Cheers!

[marks737]

Thanks for the great advice Steve. I think I’ll definitely take some pitch out of the props this winter. Yes I do have a 390 Sundancer 2005. We’re located in brackish water at the end of a river in NJ. I do a Salt Away flush once a month. Does taking modifying the props weaken then at all?

[Steve Lewis]

Next move

When you haul your boat out for winter storage it would probably be a good idea to have your props tuned a bit by having 2 inches of pitch taken out. This will allow your engines to spin up easily to the 3070 that Rob is referencing and unload your engine in the all important cruise RPM range. You may need to spin the engines a little more like 2550 – 2600 to get your cruise speed but the engine will not be working as hard by not burning as much fuel. This will get your fuel burn back into spec.

If you want longevity out of your engines then getting your props tuned would be a good first step. After that you should take a serious look at all of the attachments. The raw water pump. Change those out to the SMX version as the stock Sherwoods have known issues. If you are handy with tools then you could even do that on your own over the winter months. Order the parts from SeaBoard and have at it. You have QSB’s and the water pumps should be easy to reach on the top aft of your engines. Have your after coolers serviced the seaboard way. Remove, Clean, Lube, Pressure test, etc….. Lots of articles on the site about that. Also have your heat exchangers cleaned. Then take a serious look at the exhaust architecture.

Take some good high res pictures with a digital camera, not your phone, and post them so that we can see the overall condition of the engine compartment and especially the exhaust. You “might” have the wet elbow that is suspect at best and we can make some recommendations.

BTW….. do you have the 390 Sundancer by chance? Are you in Salt or Fresh water?

[marks737]

Thanks Rob. Not sure what my next move should be. I probably put about 50-75 hrs on the boat each season. Should I just not worry or look into reducing prop pitch an inch or two?

[Rob Schepis]

It’s not about the “weight” it’s pushing, it’s about the prop load put upon it, which is dialed in with proper gearing and prop dia/pitch.

So you’re about 15% over the fuel burn spec. Should be able to tighten that up. Max rpm —- No, not a hair under 3,000rpm, she should exceed 3,000, something in the range of 3,070. Your load% should settle in at the 95% range when she’s at WOT.

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