Can't get on plane?!?!

kingfisher23

kingfisher23

New Member
Jul 7, 2014
38
NC
Boat Info
1981 310 Express
Engines
Twin 5.7L Mercruiser V8s
Straight drives
Hi all!

I'm running twin 260 (I think) Mercruisers, circa 1981. I have a power problem that I can't even begin to diagnose.

With the throttles wide open, I'm running right around 3500rpm and 25ish mph. At this rpm and speed, I'm WAAAY back on the stern. In order to flatten the bow out I have to run with my trim tabs all the way down, which drops me down to 22mph and is probably murdering my gas consumption. I don't have any adjustment in the throttle cables, but I definitely feel like I need to be running at a higher WOT than 3500. I can't find anything on Sea Ray's website about the prop/WOT specs for these engines.

This is with a quarter of a tank of water and a half tank of gas. No weight distribution issues. According to PO, the props are correct pitch and everything. Hull is not fouled or dirty.
 
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A few more details may help. Did this problem just start happening? Was it a flatter ride when you tested the boat assuming it was a recent purchase? What is your cruise speed, RPM and fuel burn?

Also, What boat are moving up from? The effect you describe may POSSIBLY be normal for a 34' boat as you have much more fiberglass in front of you than a small/shorter boat. Does sound like you have a potential issue with the engines/props IMO as the boat should do better than 25MPH at WOT.
 
The problem may not be new. I just purchased the boat. When I took it for the test drive, the throttles were not properly synced so I couldn't run it above 2800 or so. The port engine would max out WOT at 2800ish and the starboard would run 2800 at about half throttle. I've synced the throttles up (mostly), and they now max out at 3500.

I don't know about cruise speed or fuel burn yet. I'm COMPLETELY new to boats in this class. My only experience with powerboating is a 17' runabout we had 8 years ago. So maybe these handling characteristics are normal for this boat. I just feel like I should be able to plane out with minimal trim if my throttles are wide open. As it is now, with wide open engines and minimal load the bow is so high I can't see over it. The only way to get it down far enough to enjoy driving is to drop the tabs all the way.
 
I had the same boat
it was the first boat I had where I had to use full tabs otherwise I couldn't get the bow down.
 
Saverio what did your engines WOT at? Top speed? I just want to make sure I don't have some underlying engine issue.

I don't have a generator in mine. Just the waste tank aft of the engine.
 
I'm not well informed about the 5.7 engines, but I do believe that your WOT RPM should be closer to 4800-5000. Considering the fact that you have straight drives, you need more RPM to go faster than someone with outdrives, so that may be the source of your problem.
 
You have either a engine issue not allowing higher rpms or propped wrong. I assuming the props should be 17" x 19 pitch. If your shafts are bent or really poorly aligned this can also have effect. I would suggest you haul it and start with basics.
 
I don't have the proper book going back that far, or I would look it up for you. But you can contact Sea Ray and they might be able to tell you what your "OEM" prop for that boat/engine would be and also what the WOT RPM and Top Speed would be. However, I think somewhere in the mid-30's is probably about right for speed. RPM is a little lower than what SRC noted (that would be for newer engines). RPM is probably more like 4600 at the top end.

Can you at least get that RPM in neutral?

Unless I missed where you mentioned it, what's the condition of the plugs, wires, cap, etc? That would probably be the first thing I looked at.

Yes, running with full tabs is not too uncommon on older hulls. At least to the point where it gets up on plane and going a little faster.
 
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Silly no-power or rpm question but how old is the fuel?
Did it have ethanol in it that may have separated?
Ours acted that way when we unknowingly received ethanol fuel at our marina and it sat for three weeks - sounded and ran fine, would not plane with no power and low WOT rpm's.
Fuel had separated and had to be removed. Ran great afterwards.

Just another of the many possibilities. :huh:
 
When u full throttle what is sync gauge doing. My one engine was lagging and starving for fuel. Found carburetor screen clogged
 
Is the bottom clean?
If mine sites for a few weeks I have the same condition until I get the bottom cleaned off.
 
Thanks for all the suggestions! I'll try to provide a bit of clarification to maybe help narrow the issue. I'm out of town on business right now so I can't get out to test anything until this weekend.

My engines are running pretty much in sync. I over-adjusted the throttle so they're not completely synced up at the moment. I don't have a sync gauge, so I match the tachs and the sound of each engine to get them running together. The bottom is pretty clean, just a light coating of algae from the lake water. The gas was just filled in March, 91 no-ethanol. I don't know what my WOT revs are in neutral. I'll investigate and report back. The shafts are straight and properly aligned. The plugs, caps, and wires are about two years old according to the previous owner. They're slated for replacement during my big service in November, along with the belts and impellers.

I've heard that older hull designs sometimes require full trim to cruise on plane, but I can't reach a planed-out angle under any circumstance except idling or full trim.

Saverio, were you really only seeing sub-20mph top speeds? That sounds way underpowered.

I didn't have a survey done before I purchased the boat, for a myriad of reasons. Not the least of which was I have no way to pull the boat out of the water and I can't find any surveyors who would do one in the water. I don't regret it, even if it turns out I do have major engine issues. I got a killer deal on this boat, and there were no other options. I wasn't really shopping for a boat, but I found something way beyond what I could have afforded to purchase otherwise.
 
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Just FYI, WOT is tested under load, not in neutral.
 
Yes I was high teens low 20's. My 454 had close to 1000 hrs so I didn't push them. It's a heavy boat. I had an Owen 6.5 kW vac flush and an ac unit in rear. Plus bench seat. With that said my 390 is faster and steers so much better.

as your pushing the throttles forward start to trim the bow down. There a few second delay
 
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