RPM's

[TruckerTim]

I have been posting in several areas about my potential purchase of a 2000 450 Voyager and appreciate the response but here is another question that came out in the survey.

Has the 480 Volvo TAMD74P that are marked as 2600rpm max but during sea trial it would only achieve 2135 port and 2152 starboard.

I have read post about different props keeping boats from hitting top speed but I don't think over or under sized props would effect hitting to RPM's...

Looking for causes as to why it may not hit 2600, or is that not realistic in the 450 setup and should I be concerned over the lower RPM achievement?

Lots and lots of questions and I don't even own it yet!!!

Thanks

[km1125]

The first question would be "how clean is the bottom"?? Not sure if it's in salt or freshwater, but certainly if in salt it doesn't take much time at all to build up enough on the bottom to affect performance.

I would not expect or suspect prop size first, unless a previous owner changed them for some reason. I would first suspect the bottom condition, then the performance of the engines. Do you know if the engines are getting full boost on the turbos?

And with that big a difference, I would definitely be concerned.

[Sgodfrey]

In general terms the process for checking loading is something like:

(1) Find the engine load curves/specs for the engine in question. All my experience is with the Cummins 6CTA. The specs tell you the maximum RPM on the engine under no load. In the case of the 6CTA it is about 3,000 RPM
(2) Put the engines in warm mode and carefully advance to full throttle without engaging the engine in gear. If everything is normal you should achieve 3,000 RPM plus or minus. On a mechanical engine you need to check things like the throttle cable and the WOT RPM stop.
(3) After you confirm all is good on engine under no load in step 2 take the boat out loaded up with fuel, water and people. Carefully advance to WOT hoping you don't suffer a major issue - if the props and loading are good on the 6CTA you should achieve something like 2600 to 2700. The Carver specs on the 450 with the 6CTA is 2650. If you don't achieve the 2650 then the fun begins - confirm the bottom is clean, watch the engine smoke, boost pressure etc.

One thing of note is you do need to confirm you have an accurate RPM reading. I purchased an inexpensive strobe RPM tester and confirmed that the digital tachs on my Voyager 450 were spot on.

Of all the things you have asked about so far this mostly likely the most important thing to get right. You should confirm exactly what RPM should be achieved under load for the Volvos - if you don't achieve that RPM it is possible that you are overloading the engines and that is not a good thing.

Not sure if I mentioned I just completed a 700 mile trip on my 450 in a variety of conditions (ICW, NC bays and inlets, Deleware Bay, Atlantic off NY) - we had a blast and really enjoyed the 450 - biggest surprise was how much we ran the boat from the lower helm. That was helpful in the "rough" conditions and when we got chilly!

Good luck
Steve

[TruckerTim]

Thanks to both of you.

The bottom was done in Aug and was clean when we hauled it. There was some tube worm growth on the running gear and intakes but not severe.

We could hear the turbo's kick in, (one is new) and all that sounded normal from my experience with diesels in trucks but not sure if they are getting full boost.

[buster53]

Source of the post

I have read post about different props keeping boats from hitting top speed but I don't think over or under sized props would effect hitting to RPM's...


Thanks

Over or underpropping will absolutely affect max RPM’s. Yes, make sure bottom and running gear are clean. That is #1.
Assuming bottom is clean and engines are functioning properly, then check the props. Over propping 2-3” will easily take 4-500 RPM’s off the top end.
In my many years here and other boating forums, I have found there are some boaters who absolutely believe over propping is a good thing. Maybe the previous owner was one and made the change to higher pitched props.

[TruckerTim]

Source of the post

Source of the post

I have read post about different props keeping boats from hitting top speed but I don't think over or under sized props would effect hitting to RPM's...


Thanks

Over or underpropping will absolutely affect max RPM’s. Yes, make sure bottom and running gear are clean. That is #1.
Assuming bottom is clean and engines are functioning properly, then check the props. Over propping 2-3” will easily take 4-500 RPM’s off the top end.
In my many years here and other boating forums, I have found there are some boaters who absolutely believe over propping is a good thing. Maybe the previous owner was one and made the change to higher pitched props.

Thanks, Buster. So higher pitch takes away RPM's. Good to know. I will ask him if he changed them. He's the second owner and has taken great care of the boat mechanically.

Question- Does the higher pitch hurt anything? 40 years in the trucking biz yet I feel like a complete fish out of water with these motors!

[km1125]

Source of the post
...Question- Does the higher pitch hurt anything? 40 years in the trucking biz yet I feel like a complete fish out of water with these motors!

Depends on the load and how the boat is used. Normally the boat is propped so the engines can reach maximum RPM with the boat at a light load, and reducing from maximum RPM gives you a good cruising speed and a nice compromise on the engine life (as max engine speed is a detriment to long life). This also places the engine in an appropriate part of its power curve so that it can push the boat up on plane. Too large of a pitch and the engine won't be able to develop the HP at that RPM to get the boat on plane, so the boat also never reaches a good cruising speed.

On a heavily loaded boat (much more than when it was created), then even the stock prop might be considered "overpropped". Heavily loading can result from stuffing the boat with a lot of comfort things, plus adding things like a hardtop, extra furniture, or a dingy and motor (and/or hydraulic swim platform) that were not on the boat initially.

If the engine is struggling to get to cruising speed, then likely you're also bogging the engine, which is VERY BAD for most engines.

Now, if you never get up on plane and typically cruise below hull speed, then overpropping can add some speed and economy. You still won't go over hull speed but the engine doesn't have to develop much power to get you there. Turning excessive RPMs to get to that speed is just a waste.