Aerotec transom splash issue.

[Fenlander]

Weight... hmm not sure as it's 8-10kg more which is about the difference between a Tohatsu 9.8 and my Mercury 15 and they felt the same... light on the tiller.

Perhaps it's the blooming great anti-ventilation plate holding all that splash down!

I'll observe (or feel) more carefully next time out.

[Max...]

Just back after a week on hols (no internet) and this forum is more anally-Aerotec than ever! My anti splash plate for the BAT-Mariner/Mercury splashing ended up falling off so I'll be making a simpler one this year. No amount of height adjustment cured it for me, some sort of plate or deflector is essential.

[69cmw]

I had a yam 15hp on my aerotec and the splashing was really bad, put a 25hp yam on it and the splashing was pretty much gone.
Plus it was a lot more fun with the 25hp ...

[Firstrib]

I am still trying to work out why I had zero splash on my Aerotec 380 at full or any speed and then on the next trip, had lots of splash at full speed only. I did change the prop but it was identical. I wonder, if the floor was done up a little differently on the straps on the non splash trip. I am going to monitor this closely. 25HP

[Fenlander]

I think any small change can alter the splash... as you say perhaps floor pressure and also how your kit & tank weight is spread about.

[Firstrib]

Hi Fenlander, there cannot be a more forensic discussion on splash than this! Having thought about your comment about tanks, YES this was a change. When I had two 22L tanks end to end and two up, I had no splash. When I had the 22L tanks side by side under the seat and two up, I had splash at full speed (only). I think this might have changed the shape and weight of the floor, so will put end to end again this time and report back.






I can't wait to get my outboard back from Chas Newens!

[Max...]

Shame Locozodiac has gone off the radar - he was the master of SIB/engine height set-up and splashy OB techy do-dah...

[chipko]

Started this response on another thread but thought more relevant here.
Always suffered the dreaded 'splash' to a degree although never into the boat so more of an annoyance than the real problem many suffer. To try and bottom this out I knocked up a couple of temporary transom raisers at 50mm and 65mm.



Only tested solo this weekend so findings not fully conclusive yet, though I fear the underlying cause is not just engine height. Splash is worse on a lightly loaded boat and on mine only at near max speed.
Have been running 25mm riser as stock, now tried 50 and 65mm. No top speed gain (21knots) with any, splash is getting less with more height but not yet fully eliminated. Not suffering any significant ventilation even at 65mm so may be room to go higher though reluctant to do this. Also trimming either side of optimum makes little difference to splash, but significant to performance.

In my opinion it's the disturbed water exiting off the concave transom/v-floor edge curling up and inward hitting the lower leg. The AV plate sits nicely at the surface of the main water flow with 50mm raiser, but the intermittent secondary flow hits the leg randomly and above the splash plate. This is what causes the splashing. Photo below indicates this soft corner between the transom and floor. Note: 25mm riser in pic.


Below are a couple of crude sketches to try and indicate water flow off the Aerotec and a typical SIB.


Have tried a splash plate in the past, which helps, but would rather eliminate splash at source rather than mask the problem. However, now I know the splashing is not significantly affecting performance, then a combination of raising the engine to optimum and then a splash plate to deflect the pesky secondary intermittent water flow, may be the way to go.
The quest continues.

[Max...]

That is a very well argued diagram - I'd always thought it was somehow the deep V specifically at the transom (unique to Aerotec) that caused it but maybe the edge curve as you say?

My anti splash guard stops it coming in 100% - just directs it out back. Sadly no added water jet type velocity is gained....

[EDIT[ Just been out to look at mine sitting on it's side and you have to be right - the shape is almost perfectly designed to direct water off that curve as it meets the V and straight up in the air.

[chipko]

Thanks Max. Would love to know the thinking Bombard had behind the concave transom. Surely if it had a straight V to match the floor it would give a nice 90 deg. hard edge to the joint. Hope it's not just to save material when cutting transoms from a standard sheet!

[Max...]

Originally Posted by chipko

Thanks Max. Would love to know the thinking Bombard had behind the concave transom. Surely if it had a straight V to match the floor it would give a nice 90 deg. hard edge to the joint. Hope it's not just to save material when cutting transoms from a standard sheet!

Possibly - they don't ever seem to want to spend any money on tooling, procedures or refinements. We all agree the Aerotec is pretty impressive and hugely capable but they really need to add a few inches to the beam, re-design the transom shape and height and change the blooming floor to anything but white! Two handles on the bows to match the rear would be handy too.




Pic here (my floor is slightly deflated for storage so not as tight as normal):

[chipko]

Same with mine Max. Nice taught hull with a saggy bottom just where it counts.... Nobody likes a saggy bottom!
I'm tempted to duck tape (lots of layers) a plastic angle to each side to temporarily form a hard angle between hull and transom. Worried it may damage hull/engine if it gives up at high speed though. Also don't relish cleaning glue residue off.
Edit; A fairly firm foam filler packed in the void internally might just pack it out enough to help a little and not damage anything.

[Max...]

I made this:

http://www.rib.net/forum/f50/rigging...tml#post715124

[chipko]

Yes, been impressed with your deflector in previous threads. I also bodged a temp one some time ago. It did work to some degree.

[Fenlander]

I hope monkey doesn't mind me linking his image but just noticed that the Futura has quite a similar transom shape... but has a much neater squared off end to the floor where it attaches to transom...

https://c1.staticflickr.com/8/7428/2...28ba0262_o.jpg

[Fenlander]

Just looking at the diagram and comments of Chipko in post #48...

I wonder if the way the water leaves the floor is very sensitive to the "angle of attack" of the floor. Not sure if I'll end up explaining well what I'm thinking but...

If the boat is very evenly loaded... perhaps even a little bow heavy (which is close to our situation) with enough weight that the bow will not lift much at all for small waves... then the water exiting from under the floor in the transom area will come out quite level.

If you are one up or laden such that it is bow high and/or bow light so it will lift at the slightest wave then the transom will be "digging in" which will mean that exiting water will tend to be going round a slight corner and be squeezed out with more upwards force??

[simonafloat]

Originally Posted by chipko

Same with mine Max. Nice taught hull with a saggy bottom just where it counts.... Nobody likes a saggy bottom!
I'm tempted to duck tape (lots of layers) a plastic angle to each side to temporarily form a hard angle between hull and transom. Worried it may damage hull/engine if it gives up at high speed though. Also don't relish cleaning glue residue off.
Edit; A fairly firm foam filler packed in the void internally might just pack it out enough to help a little and not damage anything.

I thought about trying to stuff some dense foam into the back corner. Thing is my 420 doesnt suffer too badly, only slight splashing when going full tilt with one up. I always have it pumped right up to the max, even if its slightly below pressure the splashing and flex is a bit offputting

[Big Al C]

I did these mods to the transom two years ago not had a problem with the water coming over the top since,

Transom mods aerotec 380, Pictures
Some pictures of the transom height mods I've done to the areotec. Two heights for two different engines, 25mm for the mercury 15hp and 50mm for the Johnson 30hp. With the engine sat in-between the plate it can only go in one place so centralizes the motor every time. Have used 8 of the 9 same screw holes in the transom so its not laced with holes.
Just need to got 2 bolts to finish it off.







I've since dropped the height down to 40mm as now using Tohatsu 25.


This is from a post 2 years ago but don't know how to link it in.

[Max...]

http://www.rib.net/forum/f50/transom...res-61289.html

[Max...]

Bumpsville for this topic, so much useful stuff was discussed here.

Has anyone done any further work or mods on this in the past year?