How mush speed loss due to weed??

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markspark7

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I recently took delivery of a bayliner 288 with a 5.7 mercruiser mpi magnum with bravo 3 outdrive.
The boat has only done 230 hrs and was imported from Florida around 3 years ago and sat on a dry stack for the best part of the last three years so it's never been antifouled.
on its first shake down run i was able to do approx 30 knots at around 4200 rpm.
Since then it's had a couple of issues such as a sticking thermostat and the gear lube pipe came off so Ive not really run it.

now that these issues have been resolved i took it for another run today and it would only do 3000 rpm wot and approx 15knots
fortunately I had a Bayliner guy handy who checked the engine management system but couldnt firnd any issues as I was convinced that the engine guardian system was active.
He put it down to a build up of green weed on the hull and that the props where over sized (22's)

How much speed would you expect to loose due to a mucky bottom??

Regards

Mark
 
you can loose a lot of speed owing to fouling on the bottom 10 knots or more. I assume the boat is now anti fouled. If not it could be like the hanging gardens of Babalon.
 
kashurst said:
you can loose a lot of speed owing to fouling on the bottom 10 knots or more. I assume the boat is now anti fouled. If not it could be like the hanging gardens of Babalon.
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no not yet as I only did the second test run today. I'm due to due it next week but we have no lift out facilities for big boats at Scarborough so i have to do it on the hard standing which dries out at low tide for around six hours . Really didnt think it would loose that much speed.
 
kennyh said:
The question is, how long has it been sitting in the water not antifouled?
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Around 3 months, although its brushed regular to keep it down.

What about the props, would they need to be changed as the dealer reckons that 22's are really for slow rivers etc and not high speed runs?
 
Weed would not cause that level of speed loss especially as you have been brushing but more importantly it would not reduce the rpm by what seems a huge amount to me. Something is holding that engine back in my view

Dennis
 
GSL said:
Can not think there is a reason to change props until you have got it running back at 4200 rpm on WOT.

Graham
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completely agree , it was really a general question regarding prop pitch to normal operating speeds.
If i'm going to be doing 20-30 knots would i be better off having a smaller pitch like 19'-20s or just stick with the 22's?
 
I once had a new leg fitted to my 285 which is of a similar size to your 288. Had no time to antifoul so went straight in. Six or seven weeks later, boat could not not go above 16 knots - if it could get on the plane at all. Had it lifted, large blobs of growth attached to the lower leg. Pressure washed, back to 25-30 knots...
 
EugeneR said:
I once had a new leg fitted to my 285 which is of a similar size to your 288. Had no time to antifoul so went straight in. Six or seven weeks later, boat could not not go above 16 knots - if it could get on the plane at all. Had it lifted, large blobs of growth attached to the lower leg. Pressure washed, back to 25-30 knots...
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did you have reduced engine revs as well?? I tend to think that whilst I agree that the weed would reduce speed , I cant see how it would reduce engine revs as well..
 
Hang on..

Your engine recommended rpm at full throttle is 4400-4800. That is with a clean hull and drive AND normal load.

You almost were there with the 22's so if you at that time were carrying extra load., you might be just right.

As the prop is the only thing converting engine power to forward movement it is pretty efficient at it. So, naturally, if the boat is held back by growth, the rpm will be restricted accordingly. Just like towing a huge trailer with a household eco car: Simply can't cope.

You ideally should reach the upper rpm (4800) so if you are below that with normal load you should consider a lower pitch prop. 1" lower pitch will produce 200 more rpm so a 19" would be right if your load is kept.
 
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Bigplumbs said:
Weed would not cause that level of speed loss especially as you have been brushing but more importantly it would not reduce the rpm by what seems a huge amount to me. Something is holding that engine back in my view

Dennis
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Oh it would. Very easily.

There is not enough power to overcome the resistance, so revs are down and the prop can't turn as easily. Clean the bottom and the speed/revs will come back.
 
Now you got me thinking about the whole revs thing and I am not sure you are correct. The car analogy is not correct because tyres won't spin on the road. Water from the prop will push backwards so the revs should still reach full.

I fly a lot or Rc planes and we check the prop revs at full throttle with the plane being held stationary and they will reach full or just below full revs

Also imagine tying your boat to the dock with a rope and opening her up full are you honestly telling me she will not reach close to full Rpm

Yes the boat will be slower with weed because if drag and weight but the revs should not be lower

Am I correct or barking up the wrong tree
 
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Bigplumbs said:
Now you got me thinking about the whole revs thing and I am not sure you are correct. The car analogy is not correct because tyres won't spin on the road. Water from the prop will push backwards so the revs should still reach full.

I fly a lot or Rc planes and we check the prop revs at full throttle with the plane bein held stationary and they will reach full or just below full revs

Also imagine tying you boat to the dock with a rope and opening her up full are you honestly telling me sh will not reach close to full Rpm

Am I correct or barking up the wrong tree
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Barking up the wrong tree.

We have all experienced this. You loose revs and speed with fouled hull/stern gear.

My boat is 500rpm and 6knts down at the moment due to fouling
 
You are barking totally up the wrong tree, weed will reduce speed AND RPM,
Bigplumbs said:
Now you got me thinking about the whole revs thing and I am not sure you are correct. The car analogy is not correct because tyres won't spin on the road. Water from the prop will push backwards so the revs should still reach full.

I fly a lot or Rc planes and we check the prop revs at full throttle with the plane bein held stationary and they will reach full or just below full revs

Also imagine tying you boat to the dock with a rope and opening her up full are you honestly telling me sh will not reach close to full Rpm

Am I correct or barking up the wrong tree
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Whitelighter said:
Barking up the wrong tree.

We have all experienced this. You loose revs and speed with fouled hull/stern gear.

My boat is 500rpm and 6knts down at the moment due to fouling
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If the actual prop is fouled I would agree that this will lower engine revs but if the prop is clean the effect with a fouled hull will only be slightly lower revs due to the prop not being able to unload because of the slower forward speed of the boat

Think about it. If I put a 5 hp outboard on a super tanker if would still rev it's heart out it would just not push the tanker very fast

Force equals mass x acceleration, equal and opposite reaction and all that stuff we learnt at school

Dennis
 
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markspark7 said:
did you have reduced engine revs as well?? I tend to think that whilst I agree that the weed would reduce speed , I cant see how it would reduce engine revs as well..
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Your engine has a hard connection to the prop, so if the prop is struggling to push the boat due to fouling, then the engine speed is dragged down as a consequence.
 
I can't comment on the technical aspects, but I do know that on my old Fletcher 17GTS with 3ltr Mercruiser, the engine wouldn't spin up and I struggled to 8kts once and it was all down to fouling. Lesson learnt.
 
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