Battling the Bulge, with a Trim?

[jeffstevens763@g]

i would probably put a child stern and adult forward but are you saying that the floor shape also is the keel shape do you think, and that in the rough its better since the floor can straiten in the troughs between waves [somewhere to go] whilst on the flat its not got any room to move [totally supported]?

[kaman]

I am aware of the bulge being described forming within my Honwave T3.8.
Can't say it bothers me.
As I tend to be in and out of the boat quite a lot when foraging around the West Coast of Scotland I decided to put a rubberised foam floor in the Honwave to protect the floor.
Can't say I much notice the bulge with it in.
The floor also help conceal the fuel line which does annoy me as it runs accross the middle of the floor.
Might help as a psychological barrier if nowt else.

[Stigomery]

Originally Posted by jeffstevens763@g

do you think, and that in the rough its better since the floor can straiten in the troughs between waves [somewhere to go] whilst on the flat its not got any room to move [totally supported]?

OK - so my theory is that on flat calm you have a seal around the hull. When the revs get up there is a suction under the boat, essentially the prop is pulling the water out from under the keel and causing the bulge at the rear, this in turn causes the front (under the petrol tank) to be forced into the void/pressurised area under the aft/before the prop and you get the characteristic concave/convex wave shape in the floor.

In rough water the suction cannot build up as the seal around the hull is incomplete and thus the effect is significantly reduced...

That's my take on it anyway...

[Fenlander]

In section 5 of that link it describes the initial formation of a concave area under the boat floor due to the pressure of the water on a flexible material. That this then affects the hydrodynamic behaviour of the boat and then this pressure "wave" will pass to the transom before it is expelled rearwards and the cycle can start again.

It refers to rough water breaking this cycle which makes sense as the effect of the waves is massively more than the deflection seen in the floor.

http://eprints.soton.ac.uk/183099/2/...r_Halswell.pdf

[jeffstevens763@g]

now you see its a plaining hull soft ish granted when i look down mine its a gull wing shape allowing air in creating lift [similar to tunnel hulled boats] yes it must get trapped to some extent re the soft ish material but as you move forward the pressure increases to a point then travels to the stern and out this must be an ongoing process, with my ali floor i dont see it but can feel it with slight surges bit like prop slip.so in the rough that cycle gets interrupted and so released more often preventing the pressure build and the bulge forming to such a degree it is noticeable

[Fenlander]

I don't think it is so much that the front is being sucked down that's just the comparison with the bulged up area near the transom makes it seem like that. Usually as the bulge reaches the transom that's when the boat can feel bogged down on power... like a brake has been put on.

At this point it's developed what in a hard boat is called a hooked hull. Unless a specific design feature on the odd boat this can be a mould error when built or a failing of the structure on an old boat... it reduces performance.

[Stigomery]

I forgot to add that I had the floor pressure bang on 0.8 bar or even slightly over when tested after 30 minutes on the water so floor pressure was correct during all test runs.

I'm going to play around with the trim next time and see if that aids in any way.

I just thought of another variable... I was running without a seat in the boat, I wonder if that allowed the tubes to splay slightly and therefore exacerbate the problem?

[Fenlander]

Seat a long shot possibility... it tightens the Aerotec.

Was it quite flat water as a matter of interest?

[Stigomery]

Not particularly, this was the roughest day but none were anywhere near mill pond.

[Stigomery]

Originally Posted by kaman

I am aware of the bulge being described forming within my Honwave T3.8.
Can't say it bothers me.
As I tend to be in and out of the boat quite a lot when foraging around the West Coast of Scotland I decided to put a rubberised foam floor in the Honwave to protect the floor.
Can't say I much notice the bulge with it in.
The floor also help conceal the fuel line which does annoy me as it runs accross the middle of the floor.
Might help as a psychological barrier if nowt else.

What size engine are you running and what's your ice box full of - a nice bit of ballast to keep the floor down perhaps ;-)

[kaman]

Hi Stig I'm running a Tohatsu 20hp 4 stroke.
My seat is always attached as it carries my fishfinder.
The box carries tools, fishing gear, spares, filleting knifes, first aid kit, my man sized packed lunch etc. Even got room for beer😊
I must say that the bulge is very miniscule - barely noticeable in fact. I always repressurise the tubes and floor after 10 mins in the water.

[Stigomery]

Originally Posted by kaman

My seat is always attached as it carries my fishfinder.


I must say that the bulge is very miniscule - barely noticeable in fact.

Ditto the fish finder/plotter.

Hmmm, I wish mine was... so I'm going to try running with 2 seats in the next time I go out and see if bracing between the 2 tubes adds rigidity and lessens the hump...

Watch this space...

[jeffstevens763@g]

I get the seat acting as a brace and would say it does a lot of good as I said before the tube line does seem to have moved in the picture I might be wrong but for the bulge to appear something has to give does the boat narrow or widen hence the seat helps either way?

[Poly]

Originally Posted by Stigomery

When I was on the Scottish SIB trip I hardly suffered the bulge, but the seas were far from glass like.

I arranged those waves especially so that you folks with blowy-up-boats wouldn't get frustrated...

[Yabadabadoo]

Just my tuppence, the 3.2 honwave I've got with me and the missus, who isn't much heavier than a tank of fuel - shame in this case - I get that bulge you mention no matter what, pressures all ok. I've chosen to accept it, knowing full well a solid hull wouldn't drag so much, it's just the way it is for now.. Maybe the Scottish scenery was a distraction?

[Yabadabadoo]

It'll take some searching but there was a thread I read a year or so back, said the poster he knew a thing or two about the dynamics of a boat and suggested plying out the inside of the hull/deck, to push down the bulge. I cannae change the laws of physics https://youtu.be/nfZ12UGiisM look forward to hearing the results