Fouling and engine speed

[Andy744]

Hi,
I'd like to run this past some of the more experienced motor boaters. (Which for me is almost anyone).

My boat was pressure washed and surveyed before I bought it 6 months ago and made 33kts top speed in the sea trials.

Six months later, my engines are short of my maximum RPM by 500 RPM (2500 is the max) and I can only make 2000 RPM and of course a much lower top speed.

So presumably there is fouling on the hull.
1. My question is, does fouling on the props & hull reduce the RPM that engines make?
The local mechanic looking after my boat tells me that It must be fouling that is causing the reduction in RPM. All other indications show there is no problem with the engines.

I just find it hard to believe that if you put 575 hp into a propeller that it won't turn at any speed you like!
2. Is my mechanic correct?

Thank you,
Andy
Sealine T50, with 2 X Volvo Penta D9 - 575

 

[rafiki_]

Andy, my experience is that the fouling on the props and sterngear is more likely to be the culprit. No matter how much horsepower you have, if the prop is fouled, the hydrodynamic will dictate the performance.

 

[Andy744]

Hi Rafiki, thank you, I'll start believing the experts!

 

[neale]

Yep fouling will do it. I once went from 30 knots to 12 knots in three months. Engine revs from 4800 to about 2000. One bottom scrape later and 30 knots was resumed.

 

[Andy744]

Hi Neale,
Thank you for that info.
That is quite a drop in performance that you experienced in just a few months.
Also for myself, I think that when my boat hull was pressure cleaned for the survey they must of washed off any remnants of anti fouling.
The mechanic suggested that this was the problem but, one month the performance was good and next month there was a huge drop and I found it hard to believe that the performance would go down hill so quickly.
I've been having unpleasant thoughts about engines problems but, your explanation has worked wonders.
Cheers,
Andy

 

[wipe_out]

You will also be burning MUCH more fuel at equivalent speeds with a dirty bum..

 

[Bandit]

Your mechanic is correct.

I have just had my boat out of the water after 7 months in the water for a service, pressure wash and antifoul etc.

So before and after figures exactly the same fuel, water and Stores etc.

18.5 tons 2 x 370 hp semidisplacement



RPM. ....................Dirty..................Clean
2500 rpm...............18.8knots ......... 20.4 knots

Max .......................20.8 knots .........23.7 knots

MBM did an article a few years go clean bottom or 12 months in the water 25% more diesel per nautical mile!

 

[jrudge]

Boat as fouled Easter.

Lifted under warranty 10 days ago. Got them to acid the props. I look at life as litres per nm which fell from 15 to 12. I had just done 350 miles odd so the cost was nearly 1000 lts of diesel. That was just props and rudders

 

[Bandit]

Boat as fouled Easter.

Lifted under warranty 10 days ago. Got them to acid the props. I look at life as litres per nm which fell from 15 to 12. I had just done 350 miles odd so the cost was nearly 1000 lts of diesel. That was just props and rudders

Did you pressure wash hull?

 

[jrudge]

No. That would have removed anti foul in places and the hull was clean.

 

[rafiki_]

Cost 10+ knots.

 

[Andy744]

Thank you for the replies!
I just could not see a reason for the reduction in engine performance after only a month.
Such a simple explanation and some interesting examples.
My boat is being lifted tomorrow for a clean and coat of anti fouling.

 

[omega2]

Had the hull been cleaned prior to this photo? if not then you need to find a shallow sandy/oozy bit of ground and whiz the props in it. that amount of fouling is crazy.

 

[Bandit]

You would be surprised how much even slime on a hull causes drag, clean stern gear ie props, shafts rudders and P brackets but spray hull with bleach and leave an hour before re-launch, this will kill the slime which will drop off without harming the antifouling.

 

[jrudge]

Good tip as ever from the forum ! Will bear in mind next time. Thank you.