Increasing rpm's without changing prop?

[Beefer]

Boat is a 1984 SportCraft 222 walkaround, weight approx 4500lbs (per nada). Engine is a Merc 3.7 488 (170) 188hp. Outdrive is Alpha I Gen I. Prop is a Quicksilver SS with 19 pitch, which I'm not sure if that's the pitch originally on the boat.

Owner's manual says WOT should be between 4400-4800 rpms. I seem to only be able to get it to 3900-4100 rpms. Speed (GPS) is 31-33mph. I feel I should be getting a little more out of it.

What can I do to increase rpm's (and speed) without major changes? Yes, a $300+ prop is a major change according to the Admiral.

Spark plugs that are in her now are basic Champions. I have AC RapidFire plugs, but not sure if that will make a difference. I don't want to change the plugs if there is no reason/benefit to doing so. Plugs look good (clean, good color, good gap).

Any suggestions of simple things I could try? I've only had her for 2 weeks, and the first week was consumed by troubleshooting a fuel problem, which turned out to be a need for new fuel system filters. Thanks!

 

[Don S]

Re: Increasing rpm's without changing prop?

What can I do to increase rpm's (and speed) without major changes? Yes, a $300+ prop is a major change according to the Admiral.

You can't, the prop is what controls WOT on boats.

 

[Beefer]

Re: Increasing rpm's without changing prop?

So a prop is the only thing that determine's the rpm's and speed? So there wouldn't be something that isn't performing up to par? What is the downside of the lower rpm's at WOT? Is it just a performance and fuel economy issue?

 

[Don S]

Re: Increasing rpm's without changing prop?

The prop controls the WOT of a properly running engine and a properly trimmed outdrive. Of course things that are bad can cause lower rpm, but it's going to get expensive throwing new parts at it in hopes of raising your rpm's up by 500-600 rpm.
You are lugging the engine. Overworking it. Can cause burned valves, overheats (NOT a good thing on a 470) and is just hard on the engine.
Here is a list of things you can check before you change props.

http://forums.iboats.com/showthread.php?t=295070

 

[Beefer]

Re: Increasing rpm's without changing prop?

Thanks Don!

 

[John_S]

Re: Increasing rpm's without changing prop?

So a prop is the only thing that determine's the rpm's and speed? So there wouldn't be something that isn't performing up to par? What is the downside of the lower rpm's at WOT? Is it just a performance and fuel economy issue?

Downside is that you are lugging the engine, and can significantly shorten its life.

Is a too high pitch prop the only thing that can cause low wot rpms? No. If everything else was in good order, than the prop is what you change to get the rpm correct.

There is a list in the "sticky" threads above on all the things that need to be checked for low boat performance. They include low engine performance, excessive weigh in hull, and bottom growth.

The first step is to figure out what the performance you should be getting with your boat and power package. It seems like a very heavy boat for the engine size. An approximation can be made with calculators (you will need your drive ratio), but the best input would be from other owners. I suspect you are correct that you prop is not the original. Finding out what was, may help solve most of the puzzle.

PS: OK, slow typer and Don already responded.