Prop question 28 x 28 vs 28 x 31

[Bob Quinn]

How does he know he's under propped now? He's never had it to WOT before or after changing props. You're assuming things not stated. Did they run it up during survey? What RPM did it make? I don't think we know that.

Correct I have not run it up to wot yet.

I started to wind it up the other day but it was smoking us out so bad I backed off at the admirals request.
Need to be going into the wind and get it burned out!

I'll be making about a 4 hour run to another port this Friday, I should get a chance to open it up a little.

[cww]

Correct I have not run it up to wot yet.

I started to wind it up the other day but it was smoking us out so bad I backed off at the admirals request.
Need to be going into the wind and get it burned out!

I'll be making about a 4 hour run to another port this Friday, I should get a chance to open it up a little.

That's normal for Detroits. It's sucking up all the oil residue that's built up in the intake from slow speed running. Leave it up on plane until the smoke clears, can take anywhere from a few seconds to much longer depending on how long it's been since you did it. Once they're running clean you can go back to running around at 8 or 9 knots.

These engines have a grate in the side of the liners for the air intake instead of intake valves. At slow speed the airflow isn't strong enough to stop a miniscule amount of oil getting squeezed off the side of the rings into the intake due to the 90 degree angle between the rings and the grate. If you never run the boat up it will start filling up the airbox to the point that oil gets recirculated back into the cylinders along with the intake charge air. Then the engines will start smoking. How bad this condition actually is for the engines is a point of debate, I've never arrived at a clear answer. There are guys who are adamant about keeping them clean, and then others always pop up to say they've put 10,000 hours on ti's at 9 knots and never had the first problem, so who knows. Probably one of those things where everybody sees what they want to see. I have heard it can contribute to cylinder glazing. I don't like smoke around the dock, so I clear mine out regularly.

If you've been running slow all day just put the boat up on plane for a few minutes before you put it back on the dock. Then the oil doesn't have a chance to build up in the intake. The wife will cope.

[Pascal]

I ll repeat what others have said: run them up and see what your WOT is. It should be 2300-2350 fully loaded and clean hull/running gear. Make sure you have full tabs down (and make sure they work)

If you re running more that 2300-2350 you ll probably loose a bit of top speed but it’s not going to make a difference in fuel economy at hull speed. It take x HP to push a boat at hull speed, a couple of hundred RPM won’t make a difference in fuel burn.

What you don’t want is to be over propped (say running only 2100 at WOT) as it is not good for the engines