I did the test.
But then if you want to trust people who did not bother test or interpret their effect, or just by feeling it's up to you.

Their effect is crap. People have been confused so much, because seaman is a high complex class of sailors.
Just because it adds directly the SD, doesn't mean it has an huge effect. The proof are the other boosting ability.

Go on the test server even if you boost the seaman, stack 7 seaman on one CV. You will realize they are 7 seaman lvl 120 with 30% vet and max experts that only adds tiny effect on crews. It's a joke seriously.

Now for more serious, we did test back there with 3rd party programs that could check the boost ability on gunners.
And the boosting was a joke...
Tested with debuggview....

Also, back there, I purposely posted a video about seaman and BO effect, the extra guideline adding was also a joke.
If you can dig my post in general topic in the archive, that might gives you more information.

The mystery sailor.

Mythical algorithims and complex formulas divided by obtuse calculations and multiplied by irrelevent statistics all based upon insignificant data and discredited assumptions.

but if you boost him and add vets....

Let me know when you figure it out.

Since we are at it, I want to know if high potential seaman partially/fully does the job of medic (saving lives)?

I have 120lvl IJN E/B/V/E seaman with 170 vets and I noticed that it miraculously saves some sailors from crew losses.
For example, 1 vet loss in 3-4 months and expert losses are really tiny (3-5 max per battle if crew losses occure)...

So I need your thoughts/research about it, to proove/dissaproove my theory.

I remember reading somewhere that a seaman increases the other sailors ability by 3%.
Where the writer got this information was not mentioned. 3% boost of ability is a small number
and might be hard to detect. No change in stacking seaman might imply that there is no cumulative
effect if you stack. One would think that higher level seaman might have higher "boost" then
3% unless the 3% boost is for the Chief seaman. This would imply that the lowest "boost" for a second
seaman would be 1% which would be hard to detect.

It appears to me from using seaman that they impact Soft Defense. Perhaps the "boost" of other sailors
by the seaman is only to their Soft Defense ability or perhaps only to certain tasks.

For example, does aircraft refueling go quicker with a seaman in the support slot? Does reloading go faster.
(Seems like I've read an old post where someone said it occurs.)

I've been wondering about the seaman ability also.

There is some Synergy to the SDE by using a seaman. Synergy means the sum is greater then the parts.

In this case, I took all crew off of one of my ships then put my !st seaman on. The SDE was a total of 30.
I then took him off and put a repair man on. The repairman by himself was 26.

Putting the repairman and seaman on the total was 58, not 56. The sum total was greater by 2. I did this
with various sailors and always the total SDE was greater by 2 then the total of each one individually.

This was a quick test. The ship was a CV2.

When I put the entire crew on the CV and summed the SDE, the addition of the 1st seaman increased
SDE by +6 over the sum of the individuals. It appears to be a cumulative affect. In this case, a 5% boost
in Soft Defense for this ship. Doing the same test On a FFY with lower lvl sailors, the Boost to SDE was about 8%.

However, it is not ship lvl that effects the SDE boost but the sum total of the SDE of the onboard sailors.

From the comments in this thread, it does appear that
if seaman save lives they are at least a "defensive" sailor which would fit the boost in SDE.

Some questions to consider.....
Do they increase gunner ability as to reload speed and spread?
Do they decrease aircraft refuel time?
Do they impact Pilot Ability?
Does a seaman play a more important role on different ships such as an SS? (Enhance sonar or torpedo reload)

Don't know the answers to these....but, this simple test shows that seaman do have some sort of effect.

This test is repeatable and you are free to try it on your own ships and sailors.

I'm looking into the Seaman issue too. primarily to boost my Fighter Pilots on my UK CV (low T3 Levels). is it actually worth it to drop a Seaman for boosting FPs up? and if so, what's the best method for doing so (I'm a little vague on the implementation of it unfortunately).

my thought is to use my olives to get an Elite FP (13 stat one), boost with one of my leftover boosts, and turn into Seaman @ level 20.

is this correct? or am I better off just dropping the Elite FP as an Elite FP and leveling up to add to my ship's crew at a later date (currently have 3 TBs and 3FPs on Illustrious, coming up to Arc Royal)

Noone has figured out yet what stat a seaman uses to improve the other sailor, if it's acc that improves acc and reload to improve reload or whether it is one stats ( likely pot if it's 1 stat) that improves all other stats. And then we don't know the mechanism of the improving, so we don't know if having a higher base stat has any influence in raising the boost.
All we know is a seaman works and that only for reload you can calculate pretty close how much of an ability boost a seaman gives.