Did the naval architect screw up?

Same boat - 52 ft / 15.6 metre power catamaran operating in Thailand - it looks like the bows have had major surgery.

Went from a wave piercing bow -

to whale bows


Any thoughts on why etc and what changes it would make to performance and handling?

 

What is the NA error that you have detected?

 

I haven't detected an error - I asked did the NA screw up? - because I can't imagine that they would replace the bows with ones that are pretty different like that unless they had to - both due to fabrication expense and time out of the water / off the run for the boat.

 

OK, thanks, but then, I still don't understand your question.
I suppose the designer was trying to achieve, with the same deck length, more waterline length, which may (I'm not claiming) give more speed for the same power.
It could also be that, although such "different" bows didn't improve anything, fashion or the owner's whim prompted them to be installed.
In general, if displacement doesn't change, length improves speed.

 

I will word my question a different way - can anyone see or think of a likely reason that would justify pulling a vessel off the run to have such a large change made to its bows given costs for fabrication and lost ticket sales?

 

It looks like the new bows would add considerable volume/buoyancy at the front of the vessel - but it didn't look like it needed it. If it was a private boat then owner's whim might come into it, but if everything was working fine then I couldn't see the owner of a commercial boat paying to make change for fashion or a whim - and risking the boat potentially not performing as well or worse fuel economy or other problems.

 

Ok.

 

Are you sure that isn't just a trick of perspective between the two photos? Another pair of before/after shots taken from, say, a dockside would be definitive.

 

The bows have definitely been modified - these pictures might help -

Before -



After -




Its definitely had some Botox

 

It’s clearly a matter of reserve buoyancy, but I would have added some rake as well.
Some photos of the boat underway, loaded, and in rough seas would show why.
The tunnel is very bluff in the bow, it would not be a pleasant experience to stuff that into a wave!

 

Well, if nothing else, they've made it uglier. I hope they're happy with it now.

 

It isn’t likely reserve buoyancy as all the changes appear to be well above the waterline, although perhaps a bit if the boat was grossly overloaded.

What I see as the major difference is positive rake of the bow and the knuckle would reduce splash. So, more likely the design change is to make it a drier boat.

I happen to have some experience in this as last year, I modified bow extensions to positive rake. The reason for this is to avoid catching crab pots or lines, weeds, etc.

 

It does rather look like they changed the bow from a sportscar version to the box truck version doesn't it!

 

As much as I liked the fish look of the negative rake, I decided to match the upper bow. If you look at the total linear dimension of forward thin edge; the positive rake is also less edge, at least the way I constructed it here.

But if they are operating where lines are a concern; that could be a part of it.

And I mispoke, I said expertise, but I meant experience. Edit coming.

If you look closer at mine, the extensions are smaller as you get to the waterline, and in my case, I would get more displacement as kapn suggests.

 

If you look at this picture, and extend the line upward; you can see how there would be more displacement for’d. I believe the reason they use the ?dihedron? is to reduce drag. I think some splash is coming off my front above 5000rpms. Not sure where, need to videotape it.

look close at the front where the scum line is, and you can see the waterline is well above where the hull is returning ?inboard for lack of proper terminology

It could be the photos of the boat in question are lightship running, and when the boat is loaded it was wetter. I’m guessing. @Ad Hoc would know.