Volvo Penta sterndrives: vertical water spray to the sides...

[mnts]

Hi,

I'm having an issue which unfortunately I have not had chance to trace down yet.
on a new to me boat I added a custom extended platform and I noticed that with engines trimmed down I get a vertical water spray on the platform (in the area about where the drives are). As there is lower air pressure behind the boat, this spray is pushed into the deck as light drizzle. However if you drive long enough things start to get wet inside. If I tilt engines higher spray is being reduced. When I have them very high (too high), then it is no longer there. I should have filmed it with a gopro bellow the platform before the boat was taken out of water, however did not find a good moment for that...

Now, while watching some videos on youtube I noticed one drone footage and it shows one boat with short platform having some significant vertical spray up and to the sides (from around the transom shield).
It started me thinking that it could be same with my case that this spray is hitting my platform extension and climbing up over the rear edge...

I attach a photo with some still images I made form the video I found.
here is the video (not my boat) (that spray is best seen starting from 2:33min):

anybody knows where what is this spray, is it some problem with the drives or that's how it is designed? in this case I would need to install something to divert the water flow of the extended swim platform.
appreciate your comments on this.

 

[QBhoy]

Wonder if the trim tabs are having an influence, should they be deployed at all.

 

[DavidJ]

Interesting that it’s on your boat and the one in the video so clearly a design characteristic. I wonder if it’s worth contacting the factory because it might be an issue that they have solved.

 

[mnts]

interesting... on same broker's youtube channel I found another Bavaria boat with single engine (newer model). and it also has similar spray happening.
here with one engine it does not seem to be interfering with the trimtabs/interceptors - but more looks coming from the transom shield.

link to video (from 0:38 min):

 

[oldgit]

Some flybridges have a notorious problem with low pressure around the stern of the boat.
Friend had Fairline Brava, 37 under no circumstances would you leave the saloon patio door open underway at any sort of speed, unless you wanted the cabin filled with light coating of oily muck and interior reeking of exhaust fumes..

 

[mnts]

here is Sealine S28, so it's not manufacturer specific I guess.

 

[mnts]

ok, I think my expectations are too high - it's just how it is. when I started looking at this, so a lot of boats have this issue...
I think most dramatic found so far is on this Sunseeker Manhattan 52:

I suppose it is different type of spray from the sides, but I'm sure nobody would want to open the canvas covers with such spray from the sides on the swimplatform

 

[mnts]

and also the famous Project Sunseeker "RED" is having similar spray. so I guess if there was a canvas closure on it (why would anybody do that?!!) - it would also get a good amount of spray on the sunbed...
(starting 2:55min)

with this I believe I stop my exploration, seems to be an issue with sterndrive boats, so need to think of some solution to divert this spray under my swim platform.

 

[ss2016]

Could it be that the trim tabs are a little down and that because the water under them is pressurised it is 'escaping' upwards at the sides? ( I hope I have explained what I mean!)

 

[mnts]

Could it be that the trim tabs are a little down and that because the water under them is pressurised it is 'escaping' upwards at the sides? ( I hope I have explained what I mean!)

yes, I think it's very logical that it might come from the trimtabs, however if you see the photos I attached for Bavaria 28 you can see that it has this spray from the drive's transom shield and that boat has interceptors instead of trimtabs, and they seem to be not engaged, but still there is the spray coming. so it looks like something from the drive. maybe some water climbs up the drive and then gets blown away apposed to an actual pressure spray.. but I'm just guessing..
also thought maybe there could be some exhaust release, but normally that's pointed down...

 

[ss2016]

Looking at my stern drive it is clear that once the water comes off the back of the hull it is going to hit the stern drive. VP have shaped this to minimise the issue, but it still has to either accelerate (like the air over the top of a wing) or find a lower pressure route. The only direction for that is up where there is no water. If not the trim tabs then the drive itself is surely the cause.
( With aircraft you see little vertical fins to stop the air going around the wing tip, high pressure below low pressure above.)
There seems to be a slight correlation in the photo between where there is least growth and where the water would be accelerating to 'escape'.

 

[MikeB.]

I know the Bavaria in the op there certainly isn't any problem with spray coming into the boat to worry about unduly. Certainly no more so than any other boat of that size/speed I have been on.