Aerotec transom splash issue.

[Fenlander]

Over the years before and after owning an Aerotec I've read lots of posts on splash over the transom. I think its agreed the root cause is the way the water flow from each side of the V-floor meets and bursts to the surface bouncing off the outboard leg and over the transom into the boat.

The degree of splash seems to vary from mildly annoying to a real flood.

Here's one past thread... http://www.rib.net/forum/f50/bombard...ght-35926.html

And on post #7 of this thread Chipko shared an image of a splash shield he'd seen... http://www.rib.net/forum/f50/old-use...ons-68875.html

And Max tried a couple of mods in this thread... http://www.rib.net/forum/f50/bombard...s-62326-3.html

Last year by adding transom raiser blocks I eventually managed to get the splash to an acceptable degree while minimising ventilation on turns.... a best compromise. It did seem to completely stop this issue though I'd need to block the transom up to a degree that would need inner/outer outboard pads raising... and to accept some ventilation in tighter turns.

Well this year mine has stopped splashing for a very unexpected reason... an outboard change. When we first used my new Suzuki DF20 EFI over the weekend transom splash was the last thing on my mind but as soon as we'd done enough running in hours to get on the plane I suddenly realised... no splash.

(To be totally factual I did have on one of the transom raisers from last year which I thought was a 20mm but just measured to find it's a 10mm... not enough height to help with the old Mercury)

In the combined image below you can see the much larger anti-ventilation plate on the Suzuki (left) than my old Mariner 15 (right)... and that it starts at the leading edge of the leg compared to the Mercury's starting close to the trailing edge. Perhaps more important is that the upper water deflector plate is also much larger (more so than is clear from the image). In fact the upper plate is getting well on the way to the size of the wooden one Max made that fell off.

I've added a second image taken from the net that shows this better.

[Neil Harvey]

Thats interesting David. Whats the shaft length? I think the merc is 17" as a ss and not 15"?

I am wondering if a pair of those fins might help with the splash issue for those of us not considering an ob upgrade...

[Fenlander]

I'll measure the exact length tomorrow.

I know Chipko has the same Suzuki/Aerotec combo and I think he reports a little remaining splash but of course part of it can be down to load and trim setting so perhaps by luck all my factors coincide to give a good result.

[jeffstevens763@g]

so thinking the cav plate bigger sorts out the splash why not fit a dole fine to engines that suffer splash back which aids lift too?

[Fenlander]

Jeff I'd wondered that... well at least some sort of mini one.

However I can't say how much of the result is the increased anti ventilation plate size and how much that larger than normal water deflector plate above the anti ventilation plate.

[jeffstevens763@g]

Originally Posted by Fenlander

Jeff I'd wondered that... well at least some sort of mini one.

However I can't say how much of the result is the increased anti ventilation plate size and how much that larger than normal water deflector plate above the anti ventilation plate.

i used one years ago to get a sib on the plane with three divers on board with success but how much top end we lost i dont know but that wasn't the issue so a result for us,
the one we had was a wrap around not two separate fins if they are still available with a slight mod you dont have to drill the cav plate with just a strap across the back if that makes sense.
i dont like drilling the cav plate as it shows you have been needing more out of the engine and its possibly had an hard life plus corrosion issuess.

just a thought if anyone is into making one a piece of HDPE with a slot in it for the cav plate slid on will work, start fairly large and keep cutting it down till you get the right size the slot can be made with a soldering iron to quite a good finish and the sides that are cut smoothed with a wood smoothing plane.
cheers

[Fenlander]

Just measured.

The Aerotec transom is 15.5" and the Suzuki DF20AS is 16.75" or 17" from top of transom to anti-ventilation plate depending how you sight along it.

Wonder what the Mercury (and any other outboard Aerotec owners have) measures?

[jeffstevens763@g]

Originally Posted by Fenlander

Just measured.

The Aerotec transom is 15.5" and the Suzuki DF20AS is 16.75" or 17" from top of transom to anti-ventilation plate depending how you sight along it.

Wonder what the Mercury (and any other outboard Aerotec owners have) measures?

interestingly i have always set my engines strait on the transom top with the cav plate between one & two inches below the keel this new boat has two 1 inch spacers allowing to raise the cav plate to level with the keel if desired i suppose thats all relative to displacement though.funny in the boat hand book no mention of setting up an engine at all, the transom of the mercury 365 is 16 inches to the keel bottom.

[Neil Harvey]

Originally Posted by jeffstevens763@g

interestingly i have always set my engines strait on the transom top with the cav plate between one & two inches below the keel this new boat has two 1 inch spacers allowing to raise the cav plate to level with the keel if desired i suppose thats all relative to displacement though.funny in the boat hand book no mention of setting up an engine at all, the transom of the mercury 365 is 16 inches to the keel bottom.

The BAT manual I now have gives some mention to it, and says you may need to add a shim to get the correct height, as short shafts come in different lengths, then mentions cavitation if too high and splash if too low... so they know about it!

[Neil Harvey]

Originally Posted by Neil Harvey

I am wondering if a pair of those fins might help with the splash issue for those of us not considering an ob upgrade...

Originally Posted by jeffstevens763@g

so thinking the cav plate bigger sorts out the splash why not fit a dole fine to engines that suffer splash back which aids lift too?

Dole fins, that's the word I was looking for... great minds Jeff

[jeffstevens763@g]

it would be interesting to see what difference it actually makes on the rise and fall on a sib engine height?
not that keen on the doel fin Neil if it worked significantly i think the manufactures would have changed the cav plate by now, when we fitted one we were right on the line for weight perhaps too much but that was then,but lift equals friction so top end must suffer.
if i had the splash problem i would fit a necklace of bristles like they use in fish filters or in gutters to stop birds getting in, above the waterline the bristles are close enough to stop the water passing light weight & cheap held with bungee cord

[chipko]

Originally Posted by jeffstevens763@g

it would be interesting to see what difference it actually makes on the rise and fall on a sib engine height?

Well in my humble opinion....lots. Plus or minus as little as 10mm either side of optimum can have a dramatic effect on performance and/or splashing.
Most boat/engine manuals usually state AV plate within 0 to 50mm below keel. With on water testing we usually end up with AV plate about 10mm lower than keel. This gives us good performance, no splashing, but occasional ventilation out of tight turns. It's always a compromise but this set up suits us for slight swell and chop....our usual conditions.
Obviously trim is also critical....we set this up so AV plate is as close to horizontal when on the plane.
Well worth a bit of effort/faffing to get this right as the performance gains can be surprising.
As Fenlander quite rightly mentions the Aerotec does seem to have its own unique set of splashing problems, although never had this come back into boat! Luckily, we seem to have virtually eliminated any excessive splashing with both our Suzuki and Tohatsu motors. I think the Suzuki lower leg profile with large AV plate, upper deflector plate and pointed leading edge really help negate the problems caused by the Aerotec deep V transom.

[jeffstevens763@g]

Hi chipko really interested in your findings re height have you got any figures.
I always set up strait on the transom top and second hole up on trim I don't suffer splash or cavitation but I might not be getting the best out of the rig any input helpful.

Cheers

[simonafloat]

On our aerotec the height is quite critical. Being a 420 version it takes a long shaft motor but it still needs exactly 30 mm of packing to stop the splashing, and if it's just me aboard then nothing stops the splashing completely. However, the transom on ours is considerably taller than a 380 so the splashing never finds it's way in.

[jeffstevens763@g]

So displacement makes quite a difference to the normal set up does trim compensate at all when single handed.

[simonafloat]

Originally Posted by jeffstevens763@g

So displacement makes quite a difference to the normal set up does trim compensate at all when single handed.

No, nothing I can do with height or trim stops the splashing completely when 1 up.

[Fenlander]

Agreed. Even with a tiller extension I've not been able to get a level trim one-up and there is no doubt the splash was quite fierce with our Mercury last year in this situation.

Of course there are two elements to the splash... the amount that comes over the transom and the confused short rooster tail that fall harmlessly behind. With the Mercury last year we reduced 90% of the water over the transom with riser blocks but never lost the wake splash.

In the image below you can see with the Mercury on the left despite not having a fountain into the boat there is still an extra central spray that fills in the normal tapering V wake. This is rising to about the height of the outboard hood. On the right with the Suzuki this is pretty well non-existent.

[Neil Harvey]

David,

That's interesting. What height shim gave you the best result?
With the Tohatsu, I was using about 40mm last time out, and it was quite good in terms of splash, certainly no waterfall as with only a 20mm ish shim, not sure about the rooster tail. But, Certainly it had cavitation.

I just measured the Tohatsu shaft length and it's more like 18" compared to the Merc 17", so it kinda makes sense the Merc does not need such a large shim.

I also just measured two yammies, the 20hp SS is also 17", so hopefully the same size shim as the Merc will suffice. The 25hp ls, which will never now be used on the BAT 380, is about 22", which makes sense.

(I am sat here wondering if I will ever find a reasonably priced larger BAT, to justify keeping the 25 for!)

[Fenlander]

Do you know I've genuinely forgotten the final preferred shim size with the 380/Mercury combo last year... over 30mm... possibly 35mm?? I remember we got to the best compromise and I was due to raise bother inner/outer pads to suit... then the weather turned... I took the shims off forgetting which ones I'd used and put the outfit away for the winter. I had several of differing thickness all drilled in the same place so I could use them in combination to try a range of heights.

It will be interesting to take the 10mm shim off next time we are out with the Suzuki and see if that changes anything.

[SeanKent]

It's interesting how the aerotec always needs a tramsom shim.

Neil how comes you wouldn't use your 25 on the aerotec ?