420 Sundancer Prop

[Rocket]

I am planning to replace both props on the boat and recondition the old ones for spare. My Marina is telling me that they seatrialed the boat and motors at WOT are turning 2750 RPM with a laser tach, which is pretty much what i get on the helm gauges. The motors are rated for 2600 RPM and he claims the new props should be set up to run the motors at 2650 max. What do you think, should we divert from the original specified prop? They claim that i will pick up an extra knot on my cruise.

 

[osd9]

Those motors are spec'd for 2600 WOT RPMs. Most recommend slightly underpropping to see 2650 WOT RPMs. HOWEVER, make sure you do your WOT runs with FULL fuel, water, stuff, POBs and a clean bottom. If, with the boat loaded as above, you still see 2750 WOT RPMs, repitching to 2650 WOT RPMs would not hurt the motors and you would indeed see a slightly faster cruise speed.

What is your current curise speed, in knots, at 2250 RPM?

 

[Rocket]

Depending on how the boat is loaded and current, anywhere between 22 and 23 knots.

 

[osd9]

I've ridden on only one 420/44DA but have read just about every thread here with their regard. Your cruising speed is inline, albiet, 'maybe' a knot or two shy of, what most others here are seeing at 2250 RPMs. Hopefully, the 'real' 420/44DA owners will chime in.

Back to your original question. You must FIRST confirm how the yard WOT tested your boat. Was it fully loaded? That's a VERY BIG unkown right now.

If the boat wasn't FULLY loaded for the test runs, then I personally would redo the WOT runs again, and ensure a full load before making any decisions.

If the boat WAS fully loaded, you have two choices.

1. leave the prop's pitch alone, keeping the motors nicely underproped and then, when you need a little more speed, bump the throttles.
2. repitch for 2650 WOT RPMs and gain a knot or so at the same cruise RPMs you cruise at now.

Either 1 or 2 will work.

Tony Athens from boatdiesel is a VERY big proponet on underpropping to achieve Rated WOT RPMs plus 100-150 RPM. He frequents CSR and hopefully will see this post and comment. I recently read a report that he posted from Cummins who now also agrees with the Rated ++ setup.

If it were me, I'd leave the pitch alone and add some cup.

Hopefully others who are more expert with props and Cummins will comment.

 

[Rocket]

When they seatrialed the boat it had 1/2 fuel full water and 2 people. I have ran this boat at full load and hit the 2750 on my helm gauge so i dont think the load is an issue. The Yard sent my information a prop guy in LI and i am awaiting their recommendation. I really cant go by 44da numbers as they have large motors and props altogether, even the 2005 420 da had the 480 motor and differant size prop than mine.

I am a member of BD and will do a search on their website.

 

[Jaws]

Rocket: Make sure that your RPM readings are accurate. I have the same motors and added Aetna digital tachs this year as accessory displays. They are accurate to one RPM and proved that my Sea Ray tachs were off by almost 100 RPM at cruise. My actual numbers are WOT 2620 and 2641 so I am a bit over-propped but not enough to spend money on. The people on BoatDiesel would prefer around 2700 RPM but there is a lot of safety built into the WOT figure. Generally I cruise at 85-88% of the 2600 RPM figure which works out to 21-22 knots in the 460DA. My Cummins mechanic said these mechanical motors can go 90% all day and 100% for one hour every eight.

If in fact you are pulling a true 2750 RPM on each side, you are under-propped but you may want to leave things as they are.

Hope that helps.

 

[Jack]

Rocket: Make sure that your RPM readings are accurate. I have the same motors and added Aetna digital tachs this year as accessory displays. They are accurate to one RPM and proved that my Sea Ray tachs were off by almost 100 RPM at cruise. My actual numbers are WOT 2620 and 2641 so I am a bit over-propped but not enough to spend money on. The people on BoatDiesel would prefer around 2700 RPM but there is a lot of safety built into the WOT figure. Generally I cruise at 85-88% of the 2600 RPM figure which works out to 21-22 knots in the 460DA. My Cummins mechanic said these mechanical motors can go 90% all day and 100% for one hour every eight.

If in fact you are pulling a true 2750 RPM on each side, you are under-propped but you may want to leave things as they are.

Hope that helps.

How hard were the tachs to install? Do you have any pictures? I would love to see them. Thanks.

 

[mistercomputerman]

Here is what they look like in a 410DA dash. Note that the chrome trim ring had to be machined as the tach is too small for the original Sea Ray hole. Also, note that the sync gauge is not working (had to replace the Teleflex sending unit).

http://www.aetnaengineering.com/8905-LCD-tachometer.asp

 

[Rocket]

Helm gauges are pretty Close, the marina took readings with independent laser tach off the belt flywheel. Ill report back what the prop shop recommends. I need to change the props anyway, just a matter of deciding whether to change spec.

 

[Jaws]

Jack,
Mistercomputerman has a nice in-dash setup but I decided to use the Aetna tachs as auxilliary for now, preferring instead the look of the Sea Ray instruments in the dash. I'll take a photo and post it next time at the boat. I have a door at my feet when at the helm for access to electronics. I made a box from "starboard" which screws to the inside of the door. It simply drops open at a good angle when I'm running. There are fuse links and and switches beside the tachs. Two advantages to the setup is that I can easily remove the box for use on another diesel boat (just have to reset the dip switches) and I did not have to change the senders. The (shielded) wire simply piggybacked off the existing tachs.
As said I'll post a photo soon.
James

 

[Jack]

Thanks James and Rick for your info. Look forward to the pics James.