Mercruiser 5L won't go over 11kn at full throttle

I have a Sunrunner 37ft with 2x 5L Mercruisers, my initial problem was that the port engine wouldn't rev past 3000rpm but yesterday to my surprise it went full throttle 5000rpm, BUT, it did not go past 11kn, . 2 weeks ago I had it going to 20kn with port engine @3000rpm starboard engine at 4200rpm.
Any ideas guys ?

what drives , motor history, service records

Bt Doctur said:

what drives , motor history, service records

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sterndrives,bravo2, I have no service records, motors have 500hrs, before I bought it I had them change one of the props I don't know what type they replaced it with ' starboard has prop speed' port hasn't... the port leg looks like it needs a really good clean, antifouled 6 mths ago, compression tested 3mths ago all was good.......yesterday when I had it at around 4000rpm it would go up to 5000rpm without me increasing power and then it would started shuddering . I would have to slow down to idle before shuddering stopped.

37' boat with 5.0LTR engines is way under powered.

In any case, Lots of RPM's and no speed can be as simple as a bad propeller or as painful as a bad engine coupler.

I'm surprised you think it's way underpowered. There are lots of Sunrunner 5L's on the the market as STD .

White Pointer said:

I'm surprised you think it's way underpowered. There are lots of Sunrunner 5L's on the the market as STD .

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That is why they are on the market...........

A boat that size should have twin big blocks or diesels........

It is a pricing decision not a power to weight decision....

had the hull, legs and props cleaned the other day. took the boat out today and had it going 26kn @ 4200rpm, could have gone faster but it was listing to starboard side at that speed....

With proper engines, you would have been going more like 26 kts @ 3200 RPMs.

A good guide to cruise speed on any planing hull, is that the boat climbs up on its bow wave while getting on a plane, and then the nose drops slightly, i.e., the boat levels. At that point, you should be turning no more than 65 to 70% of you maximum wide open throttle RPMS ( assuming boat engines rev to within the specified WOT range), preferably 50%. If you are running more RPMs @ that "leveling" point than the above referenced 70%, you are under powered.

Further KGHOST's comment.... I call those boats "boat show specials", i.e. boats equipped to sell at the lowest price possible to unsophisticated buyers. Usually with lots of glitz and the smallest and cheapest engines they have the nerve to equip the boat with... usually raw water cooled, just to save a bucks.

sand hits the nail on the head. Like how low of a price can we make it sell even if the performance is way down.
My 24 weekender had twin inline 6`s swinging 13 3/4 x 21 P props Throttles set to 3K and it would come on plane and do 32 mph. Maxed out at 61 mph but at 38 gallons per hour its hard to take.

Bt Doctur ...24" weekender ? I would not even fit my esky on that. No point in buying a boat that does 61mph @38g/h and you can't afford it. I'm quite happy cruising with family and friends at a speed of 24kn @ 80 lph .

..."That is why they are on the market..........."

Hah! Good one, Jack, and right on the money.

No way a boat that size should have such puny engines. For all intense and purpose, they are pounding their little hearts out moving so much weight--and that causes all kinds of hell. Anything not up to spec is gonna cause a serious lack of power from the engine(s) as they are working do damn hard to begin with.

Jeff

hey guys. when I said "there are lots of Sunrunner 5L's on the market as STD" it was suppose to mean there's a lot of that model out on the water.

FYI top speed of 33kn and cruise at 24kn i'm more than happy.

BTW thanks for your help with my initial enquiry.

That's spectacular performance for 5.0 engines.

Jeff

White Pointer said:

hey guys. when I said "there are lots of Sunrunner 5L's on the market as STD" it was suppose to mean there's a lot of that model out on the water.

FYI top speed of 33kn and cruise at 24kn i'm more than happy.

BTW thanks for your help with my initial enquiry.

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33kn and 24kn are fine, so long as the engines aren't running at more than 80% of MAX OBSERVED WOT RPMs @ 24 kn and @ 33 kn you are within the SPECIFIED MAX WOT rpm range.