Need to be at full throttle to stay on plane

[Mark Tindal]

Hey all,
I just took a long weekend away with my 1990 220DA Sundancer. 2 PAX, a tender and some basic food and supplies and a couple of duvets. She has the 5.0L Mercruiser, Alpha One leg and a 19P prop.

I reckon I burned about 18 gallon an hour on the two hour stretch across the Atlantic from Penzance to Scilly Isles. Why? Because for most of the journey she needed me to keep the throttle either full on or maybe a bit back from full. I mess with my engine trim after I get on plane and get about 21 or 22 mph at about 3750 rpm which is about full throttle. If I then ease back the throttle to 3500 rpm she'll stay out of the water at 16mph-18mph but often just dips back in and I need to push WOT and start again. Very annoying!

A bit of help in understanding the rpm to prop pitch to plane expectations and speed would really help me understand what I should be expecting and if it's not a prop thing, maybe I need a tune up or is it a servicing thing.

The underside and prop is clean and clear. I. do get a belt squealing for the first few minutes after startup. One of the trim tabs doesn't fully retract (about 2 inches) so I even. out with the other one until I get her out the water. Engine sounds good expect I hear a very high pitched squeal, almost inaudible. Can't think of anything else.

Any and all constructive comments really appreciated.

 

[Lazy Daze]

Is this a new issue, or has it always done this?

Your Max RPM is WAY low. It should be in the mid to upper 4,000 range. It could be a prop issue (over propped)... maybe a fuel quality issue... maybe time for a tuneup.

Your speed vs RPM sound roughly correct - and the speed at which you fall off plane sounds reasonable, too.

What pitch/blade prop are you running? How does that compare to the original prop (check the prop matrix on Sea Ray's website)?

What would really help - go back out with just you and a light load. Full throttle and keep trimming up till speed no longer increases or the prop ventilates. THAT's your true max RPM.

 

[Mark Tindal]

Is this a new issue, or has it always done this?

Your Max RPM is WAY low. It should be in the mid to upper 4,000 range. It could be a prop issue (over propped)... maybe a fuel quality issue... maybe time for a tuneup.

Your speed vs RPM sound roughly correct - and the speed at which you fall off plane sounds reasonable, too.

What pitch/blade prop are you running? How does that compare to the original prop (check the prop matrix on Sea Ray's website)?

What would really help - go back out with just you and a light load. Full throttle and keep trimming up till speed no longer increases or the prop ventilates. THAT's your true max RPM.

Thanks Lazy,

It was a river boat before I took her to sea and I'm not sure if there has been some tinkering. (See pic)

Re the prop: It's a 19 pitch, can't see diameter but the serial number says 48-16544 19P.

Re the RPM: With no load I still only get up to about 3,800 RPM but I do get up to about 26, sometimes 27mph on the GPS. The one on the dash is long gone.

My goal is to be able to stay on plane without having to keep the throttle wide open. Is it reasonable to expect this engine and boat combo to do around 20knots at 3000 revs at about half throttle then 30knots at WOT. That's my ideal, I don't need more top end than that on the ocean unless I plan to bring back Miami Vice.

 

[Lazy Daze]

What does "river boat" mean? Does that mean you never opened her up before?

19P is at least in the ballpark - and I see that it's a Merc prop so at least you know you have a reliably made prop. The other thing that could be going on is that the engine is just plain tired/worn out. A compression test is easy to do - and while not always completely indicative of the health - it's a good starting point and may be very informative.

Generally speaking, yes, you should be able to at least get 30knots out of it - probably a little more. Using the drive trim and tabs wisely... 3,000 RPM's might be doable - certainly 3,500. BUT that 3,500 should NOT BE ANYWHERE NEAR full throttle. 3,500 should be an efficient cruise RPM.

You WANT to be able to get full RPM/Speed out of it so that when you are running at mi-range, you are running efficiently.

Check fuel quality, check compression, report back. Check tune-up items - or just replace if there's any question and you don't know when they were done.

 

[Golfman25]

You definitely have issues. How long have you owned the boat? Did this issue just start? Good news is you're not making WOT at 3700. Bad news is you're not making WOT. You need to start inventorying everything you know. Actual Prop vs. spec. prop., last tune up, condition of spark plug wires, etc. Then start working from there. Good luck.

 

[Espos4]

Also check to see that the throttle blade is in fact wide open when the throttle lever is pushed all the way forward.

Of course, do this with the engine off.

 

[Maggieiscrazy]

My 240da runs 25-30 mph at 3000-3500rpm and I’m at less than half throttle. I get almost 3mpg. I never touch the trim tabs. I have a bravo 3 I don’t know how much difference that makes.

 

[Thornton69]

Your fuel consumption is reflecting the top end of throttle range. Sounds like you are down a cylinder.