Mushroom Vents - Effective Area

[rupertbear]

I have failed to find any statement of the effective ventilation area of mushroom vents, which has surprised me as many people must have needed to know this for their ventilation calculations.

I found this in an old post from Radiomariner:

'8000 mm² (80cm²)is more than a standard 4 inch mushroom with a 1 inch gap between shoulder and cowl, or two mushroom vents with half inch gap.'

Can anyone point me to anything further?

Thanks

[Flyboy]

30 minutes ago, rupertbear said:

I have failed to find any statement of the effective ventilation area of mushroom vents, which has surprised me as many people must have needed to know this for their ventilation calculations.

I found this in an old post from Radiomariner:

'8000 mm² (80cm²)is more than a standard 4 inch mushroom with a 1 inch gap between shoulder and cowl, or two mushroom vents with half inch gap.'

Can anyone point me to anything further?

Thanks

It's all in the BSS requirements. Chapter 8   https://www.boatsafetyscheme.org/media/180428/bss guide 2005 complete web.pdf

Edited March 31, 2018 by Flyboy

[WotEver]

3 hours ago, Flyboy said:

It's all in the BSS requirements. Chapter 8   https://www.boatsafetyscheme.org/media/180428/bss guide 2005 complete web.pdf

Except for two points. Firstly, that’s not the current BSS guide. This is:

https://www.boatsafetyscheme.org/media/268789/ecp-private-boats-ed3_rev2_apr2015_public_final.pdf

Section 8.9 and appendix K.  

Secondly, I don’t see any calculations which cover the OP’s question i.e. the effective ventilation area of a mushroom vent.  

Edited March 31, 2018 by WotEver

[Flyboy]

37 minutes ago, WotEver said:

Except for two points. Firstly, that’s not the current BSS guide. This is:

https://www.boatsafetyscheme.org/media/268789/ecp-private-boats-ed3_rev2_apr2015_public_final.pdf

Section 8.9 and appendix K.  

Secondly, I don’t see any calculations which cover the OP’s question i.e. the effective ventilation area of a mushroom vent.  

Then I suggest the OP refers to the mushroom vent manufacturers or suppliers data.

Here's a common one that specifies 5800 sq.mm  :-   http://aquafax.co.uk/product/n-73000-mushroom-vent-fixed-brass

Edited March 31, 2018 by Flyboy

[WotEver]

Also worthy of note that if your vents are adjustable then the area at its most closed position will be used. If they can be closed completely then they will be disregarded. 

A strategically placed pair of nuts on the spindle can prevent them being closed, or limit the amount by which they may be closed. 

[ditchcrawler]

40 minutes ago, WotEver said:

Also worthy of note that if your vents are adjustable then the area at its most closed position will be used. If they can be closed completely then they will be disregarded. 

A strategically placed pair of nuts on the spindle can prevent them being closed, or limit the amount by which they may be closed. 

Our share boat had short lengths of copper pipe on the threaded section.

[WotEver]

13 minutes ago, ditchcrawler said:

Our share boat had short lengths of copper pipe on the threaded section.

Perfect

[cuthound]

On our last shareboat, I had an over zealous BSS inspector disregard the mushrooms that had fans in them, despite my protests that he should look at the amount the blades restricted the opening and apply a percentage reduction to a normal mushroom. I also pointed out ventilation is advisory for private boats (the boat was legally licensed as a private boat as the managing companyour isn't own any of the boat).  

He then left and returned when the next co-owner was on board. The next co-owner was not as well informed as me, so agreed to have a large vent cut into the back door.

[ditchcrawler]

3 hours ago, cuthound said:

On our last shareboat, I had an over zealous BSS inspector disregard the mushrooms that had fans in them, despite my protests that he should look at the amount the blades restricted the opening and apply a percentage reduction to a normal mushroom. I also pointed out ventilation is advisory for private boats (the boat was legally licensed as a private boat as the managing companyour isn't own any of the boat).  

He then left and returned when the next co-owner was on board. The next co-owner was not as well informed as me, so agreed to have a large vent cut into the back door.

But a vent in the back door wouldn't mitigate insufficient high level ventilation, which he was complaining about in the first place. I did hear of one case where fans were fitted the examiner wanted moor low level ventilation calming that the running fans would cause a negative pressure inside the boat and draw CO from the fire

[Keeping Up]

4 hours ago, cuthound said:

On our last shareboat, I had an over zealous BSS inspector disregard the mushrooms that had fans in them, despite my protests that he should look at the amount the blades restricted the opening and apply a percentage reduction to a normal mushroom. I also pointed out ventilation is advisory for private boats (the boat was legally licensed as a private boat as the managing companyour isn't own any of the boat).  

He then left and returned when the next co-owner was on board. The next co-owner was not as well informed as me, so agreed to have a large vent cut into the back door.

The best solution for that is to replace the BSS examiner.

[haggis]

44 minutes ago, Keeping Up said:

The best solution for that is to replace the BSS examiner.

I think that is the second time recently that I have read on here a suggestion that if you are not happy with what a BSS examiner says, you should get another one. I presume from this that the first examiner doesn't inform the BSS office of a fail. If he does, when a second examiner sends in a pass will the BSS office not question things? However, if the first examiner doesn't inform the BSS office that "problem" , if it is one won't arise. 

Just asking out of curiosity as I hadn't heard the suggestion of employing a second examiner raised before.

haggis

[WotEver]

44 minutes ago, haggis said:

I think that is the second time recently that I have read on here a suggestion that if you are not happy with what a BSS examiner says, you should get another one. I presume from this that the first examiner doesn't inform the BSS office of a fail. If he does, when a second examiner sends in a pass will the BSS office not question things? However, if the first examiner doesn't inform the BSS office that "problem" , if it is one won't arise. 

Just asking out of curiosity as I hadn't heard the suggestion of employing a second examiner raised before.

haggis

How it should work in my opinion is an appeal to head office, pointing out that they have an examiner who is ignoring the guidance. Unfortunately it doesn’t appear to work like that. 

[Eeyore]

12 hours ago, WotEver said:

Except for two points. Firstly, that’s not the current BSS guide. This is:

https://www.boatsafetyscheme.org/media/268789/ecp-private-boats-ed3_rev2_apr2015_public_final.pdf

Section 8.9 and appendix K.  

Secondly, I don’t see any calculations which cover the OP’s question i.e. the effective ventilation area of a mushroom vent.  

Appendix K says something along the lines of carefully measuring vents to determine the actual area. Take the dome off and measure the area of the support for the dome ( basically four rectangles and a circle ); subtract this figure from the area calculated from the inside diameter of the vent to give the actual area available. You can then use this figure to calculate the optimum position/height for the dome.

Examiners apply “derating” percentages for grills and fans; but these percentages do not appear in the guidance (well not in anything i’ve found so far), perhaps i’m not looking hard enough.

Edited April 1, 2018 by Eeyore

[Detling]

Our examiner said that the grill was an automatic 50% de-rating, even though as a thin wire grill is only obstructs about 10%, he said this applied whether it was a grill or a louvre, but he said nothing about the fact they could be screwed down, although they never are. Each to their own.

[Nightwatch]

This is all very well, but I can't get my mushrooms off to allow derusting of the roof. Screws well embedded. Impact driver? 

[rusty69]

1 minute ago, Nightwatch said:

This is all very well, but I can't get my mushrooms off to allow derusting of the roof. Screws well embedded. Impact driver? 

Or Drill.

[rupertbear]

On 01/04/2018 at 01:45, Eeyore said:

Appendix K says something along the lines of carefully measuring vents to determine the actual area. Take the dome off and measure the area of the support for the dome ( basically four rectangles and a circle ); subtract this figure from the area calculated from the inside diameter of the vent to give the actual area available. You can then use this figure to calculate the optimum position/height for the dome.

I had considered working it out for myself but I need to know what the 'accepted' method might be.

For example, your suggestion seems fine at first (assuming that I have understood it correctly) but would remain the same no matter how far up or down it was screwed.

In the absence of anything better, the suggestion that I trawl the web for examples of such vents with quoted areas seems the best way forward.

Thanks for the responses.

[ditchcrawler]

3 hours ago, rupertbear said:

I had considered working it out for myself but I need to know what the 'accepted' method might be.

For example, your suggestion seems fine at first (assuming that I have understood it correctly) but would remain the same no matter how far up or down it was screwed.

In the absence of anything better, the suggestion that I trawl the web for examples of such vents with quoted areas seems the best way forward.

Thanks for the responses.

That would depend on how close the cover came down towards the roof and upstand, it is possible it could be completely closed.

[WotEver]

Just now, ditchcrawler said:

That would depend on how close the cover came down towards the roof and upstand, it is possible it could be completely closed.

Hence you need to prevent that if you want the examiner to count them.