volvo penta md1-20 low revs under load

[Irmascherkid]

Hello one and all, i trust this finds you all in good health..
Ok i have a question for the great mind hive regarding low rpm when engine is under load and throttle is to the stopper  ?
For the first time in what feels like an eternity ( well last sail was at new years eve ) i managed to untie the lines and nipp off for a little jolly lol, but to my dismay i found that the engine was realy pi#s poor with power, now my thinking was "ok you have what can only be described as a floating meadow ( and yes i hang my head in shame at the amount of grass that i have accumulated over the 6 months i havent gotten out to play ) but surely anything on the prop will vamoose pretty quickly"! but since there was little wind i found myself trying to get the engine to rev past 2200 rpm while in forward, it goes to the max 3200 rpm when i disengage the gear lever and only have throttle but when the engine is under load max is 2200 rpm and thats flat out! any thoughts are as always greatly received.         

[Symphony]

You have answered your own question. Dirty bottom and prop, probably barnacle encrusted will have that effect. Lift and clean then all will be back to normal! Investment in Coppercoat and particularly coating the prop with Velox antifoulingpaint.net/applyingvelox.html will reduce the chances of it happening again after long periods in the water.

[Salty]

There is also the possibility that you may have picked up a piece of old rope that someone may have discarded or, more likely, a piece of floating polypropylene rope from an unmarked fisherman’s crab/lobster pot. Unfortunately this kind of fishing often uses very poorly marked pots, and cheap polypropylene rope usually floats waiting to catch out anyone who hasn’t noticed it. Rope around your prop will significantly reduce Your prop and engine rpm when your engine and prop are both engaged. Try running your prop astern and if you have a rope and are lucky it just might unwind. I was lucky when it happened to me !!  If you can’t dislodge it you might have to dive, or employ a diver to go down and check the prop, or find a suitable place where you can dry out safely at low water. Done that too🤪

[Symphony]

Or fit a rope cutter which deals with most of that. Not infallible but works most of the time.

[Irmascherkid]

Thanks guys, i will have a proper look next time i get down to the boat and will see if i can get a camera in to see if there is anything wrapped around the prop or if it just needs a good scrubbing. Just wanted to ask for opinions incase something else was afoot. 

[Symphony]

The other thing to consider is a blocked exhaust elbow which displays similar symptoms. However, this does not usually happen suddenly nor as dramatically.

[Irmascherkid]

That could well be Symphony! but surely if the exhaust elbow is starting to block then i presume that the back pressure would be the same under load or not under load? 

[Symphony]

No. if not under load the engine is almost literally producing no power despite opening the control lever fully and reaching full rpm. The engine only start using fuel and producing power (and therefore pressure and exhaust gases) when a load is placed on it. With a boat engine this load is generated by the propeller trying to move the boat.

From your description the propeller can't move the boat fast enough because either the prop is fouled or the extra drag of the hull fouling - or a combination of the two.

The elbow may well also be partially blocked such that it is not noticeable in normal use with a clean bottom and prop because you only draw less than 50% of potential power to move the boat at cruising speeds of say 5 knots.

First thing to do is clean the bottom and prop, then if you still can't get revs under load the elbow is next in line. If it is severely blocked you should also see a dirty exhaust and possibly excess unburnt fuel in the exhaust gases.

[Mirror45184]

Tend to agree with Symphony, check the exhaust water flow if there is low water flow or there is more steam than usual, I would suspect the exhaust elbow. Replaced min when the engine was about 7 years old.
Cheers

[SorinCT]

No. if not under load the engine is almost literally producing no power despite opening the control lever fully and reaching full rpm. The engine only start using fuel and producing power (and therefore pressure and exhaust gases) when a load is placed on it.

Engines at 0 load always produce power enough to counter the friction and acceleration of moving parts. Depending on the engine size and construction this varies. For a propulsion engine if load is applied (engaged in gear) the load is permanent and fuel is used much as the throttle is pushed. There is a simple way to test if it is fuel quality issues or high resistance issues, just rev up the engine out of gear.