Effects of hull/prop fouling.

Leighb

Leighb

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What are the likely effects of hull and/or prop fouling. Would it only affect the maximum rpm achievable at WOT or would speed through the water be affected over the whole rpm range?
We were launched in January and there is obviously some fouling, can’t see the props but the barnies may have taken up residence, I have noticed that at WOT max rpm is 2 or 3 hundred rpm down, also at a steady 3000rpm speed is only around 11 to 12 knots instead of 14 to 15 in previous seasons with a clean hull. Would it need a greater throttle opening to achieve intermediate speeds also?
 
The effect of fouling on props and sterngear is massive. It would knock at least 10kn off my top speed, and cost a lot of fuel. I launched in March, then lifted and cleaned the sterngear earlier this month. Well worth it. There was a lot of growth. I have never found fouling of the hull to be a big impact on drag.
 
Leighb said:
What are the likely effects of hull and/or prop fouling. Would it only affect the maximum rpm achievable at WOT or would speed through the water be affected over the whole rpm range?
We were launched in January and there is obviously some fouling, can’t see the props but the barnies may have taken up residence, I have noticed that at WOT max rpm is 2 or 3 hundred rpm down, also at a steady 3000rpm speed is only around 11 to 12 knots instead of 14 to 15 in previous seasons with a clean hull. Would it need a greater throttle opening to achieve intermediate speeds also?
Click to expand...
Seem like you have answered your own question.
More throttle for same speed equates to more fuel
 
I had the very same issue a few weeks ago.
Pushed to full throttle and couldn’t get anywhere the normal cruise speed and rpm was down by 300-500rpm.
So last weekend I scrubbed the props clean and so hopefully I’ll find out tomorrow if this is the cure.
Strange thing though was the rear of the blades seemed clean.
 
Having access to load and EGT Info enables you to visually see the effect of prop fouling and to some extent hull fouling .
Its vital you don’t exceed 100 % or cruise at near that .
Normally with reasonable clean stern gear EGTs are 560/580 ish @ 80 % Load which attains 27 knots @1760 rpm .
When it’s fouled like when arrived after lock down travel travel restrictions ( June ) eased ....
100% Load was @ 21/22 knots and EGT s soared north of 650 momentarily if I attempted to go any faster .
Talking 1400 rpm . = WOT .
WOT when clean is 2260 ....engines rated for 2200 .
I just stopped ceased pushing it ....could have but had both screens set on the relevant deltas.
fuel burn was in excess of 125 L / hr .....per side .

Did a manual scrub @ anchor of the stern gear with a dive bottle which improved the situation , now 85 % load made 25/26 knots .100 would have made nearly 30 knots .

I never normally exceed 80:% load .....lots of American marine engine sites in terms of longevity tend to throw that figure up .
Others may , but I am not inclined to disagree with that ethos .I have access to load so may as well use that info .

MAN are pretty good at engine info with there MMDS system .
Realise a lot of you out there are running blind .....prematurely risking shorting the life of the motors by overloading them running with fouled sterngear / hulls .

So after a professional 3 hrs + €300 diver scrub and water blast of the hull + stern gear she’s now as per just after an annual lift .
My 1780 rpm is now 77% load .....less drag .Burns 80 odd litres a side and pushes the boat at 27 knots cruse , EGTs down a100 to 550/560 ,
1700 rpm gives a nice 26 knot with a clean bum and 75 L / hr
1830 rpm 30 knots cruse load exceeds 80 to 85/86 % , fuel burn is near 95 L per side .

I rarely do north of 1800 rpm ( 2200 rated ) in the interests of machinery longevity.
WOT flat out , with a clean bum she will touch 37 knots , How ever 27/28 is fine by me and hopefully the machinery will stay uneventfully free from issues .Thus far in my 6 th season ....this strategy seems to be working .
 
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russ said:
Strange thing though was the rear of the blades seemed clean.
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Yes it’s the hubs that clean up first .Its also the hub area that tends to cavitate .
Here is a pic after my scrub and running round @ 24/25 knots ....see the golden coloured hub areas where they have self cleaned.
547F9C5F-3946-42F2-AC50-16806F344304.jpeg
 
russ said:
Strange thing though was the rear of the blades seemed clean.
Click to expand...
If by "rear of the blade" you mean the aft side, that's the one with the highest water pressure while under way.
So, more than likely it just cleaned itself after you ran the boat at WOT.
The forward face instead doesn't generate any thrust, and is "just" screwing into the water with much less pressure.
That's the reason why it doesn't self clean as much as the aft side, BUT it still can add a lot of drag, if fouled.
 
I find it’s not just forward that is slow. The bow and stern thrusting becomes sluggish as well and gets even harder to overcome beam winds.
 
Out this afternoon to test the engine temps after an impeller failure last week(non VP rubbish)and also what the consequences of cleaning the props were.
The cleaning made a huge difference although the port engine struggled to match the rpm of the stb engine within the same time period. Eventually matched the rpm at 3400-3500 @ 20-21kts.
(AD41P engines)

The forward facing part of the blades were the cleanest sides and aft facing sides were covered in small crustaceans. Which I assume is the working face of the props.
 
Just a thought, for outdrives has anyone tried placing a plastic sack (used for rubble or strong bin liner) over the drive, any pitfalls apart the obvious of forgetting it was fitted. Basically raise the drive to make it easier to fit over and leave a note on the dashboard as a reminder! I guess it would greatly reduce fouling particularly the dreaded barnacles.
 
Had one of theses 12 years now .
Paid itself back in buckets .
Its just something you should keep on top ....stern gear hygiene.
Talking 1-2 M only .
Beuchat Boat Kit Set | Hull Scurb Kit for Yachts | Travel Dive Kits for Boats

When I had a stern drive boat , could just reach by hand the props from the bathing platform, with them trimmed up.
When both trim pumps behaved :) .
But even so the rest of the leg , cavitation plate , water inlets grills etc etc would need a fettle too once past the 1/2 way point of the summer .
 
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