Thoughts and Bumblings to make my sib 'better' ?? Comments Please

ruddles · 25 September 2008, 10:33

Ullo all again.

I have had many inflatable boats and ribs in our years of cruising on our ketch. My few posts indicate that I have a new honda 3.2 Air V Floor with tohatsu 18 two stroke.
After reading many threads here, gaining valuable tips (Like using a 12v tyre pump to get to final pressures with a suitable manometer etc.. also, the problems that occur with tohatsu 2 stroke 18 design in the area above the anti vent plate (Causing my splashing issues !.. I will try both raising the transom, and or fashioning a small ali plate to fit over the offending area to provide a smoother flow...

Anyways, I bin thinking.. and that is always dangerous lol

....

How to make the otherwise super appearing design of the Honwave 320 IE 'better' ?

One of the things readilly apparanty is the flex in the floor around 18 inches to 2 feet forward fo the transom, even if correctly inflated.. this is apparant when using the max recommended power - or even a tad more oops...

So, how to improve things..

Here are some thoughts for your valued consideration, there may even be someone who can provide the parts I want (If practical - and I think it is !) - bonding pvc materials is not one of my better skills !.. doh..

Anyways...

How to stop the apparantly inevitable flexing of the air v floor ?.
I got this idea from my yacht with fully battened mainsail...
Why not add a pocket of pvc material, suitable constructed and bonded to the floor from the transom to around 4 or 5 feet forward of such, one each side of the v floor, around 4 to 6 inches wide. Inside of this slips a grp or say, 10 mm marine ply strip to form a stiffener to the floor section in the area of most underwater action when planing ?. - hope that makes sense ?.. little weight added, flexing controled by thickness, no need to even insert them if you dont want to, no chaff due to the batton being inside a pocket and suitably rounded on the edges with anti chaff provided.

Please offer your thoughts, it seems eminently sensible and productive to my tiny mind .

If there is anyone skilled in the repair of pvc materials who can produce such items to a brief design, I would love to hear from them, it may even be a saleable item.

Having such inserts would also offer better location poiunts for fixing the fuel tank or adding structures to the air floor.

Hope all this makes sense lol

..
So, if someone can produce a compatible,strong pocket and advice of fixing it permenently to the air floor in a neat way, please let me know.

Pondering mode subdued for a while.. need beer...

Ruds.

MustRib · 25 September 2008, 16:29

Hello Ruds
I have the bigger brother to your Honda SIB the 3.8m air floor.
The floor on that flexes as well, particularly when the boat gets on the plane, but its not much, sure you notice it when your feet are on it, but it doesn't seem to affect the boats handling and doesn't feel unsafe.
I would say it doesn't deflect more than a couple of inches maximum in the centre of the floor in the "planing area" in front of the transom.

I dont think adding any battens to the floor will help matters, and to be honest if its freaking you out so much I would trade it in and get a solid floor model.

How much is the floor deflecting by then?

Andy

Locozodiac · 25 September 2008, 16:48

If you are not using a manometer to inflate your air floor, get yourself one and inflate to max pressure recommended by manufacturer, or use electric inflator that can be pressure adjusted, Bravo type. check owners manual for refference. Once done, try sitting inside of floor if this is a confortabole driving position for you, will flex slight less.

Happy Sibbing

ruddles · 25 September 2008, 21:10

Hello guys
Thanks for the input.
First, yes, I use a quality manometer and modded tyre inflator to get the correct pressures.
As for the amount of flexing, yes, around a 2 inch upward deflection. it sis certainly not 'freaking me out' lol

.. just that it is not helping performance. You can feel the boat 'drag' when the deflection starts and the speed does not increase, The last twist of gas on the towy 18 2 stroke is a fair kick, and is not being utilised.
It would seem perfectly logical to me that the deflection is causing upset to the water profile under the boat, hindering performance.
Ruds

Locozodiac · 26 September 2008, 00:33

Quote:

Originally Posted by ruddles

Hello guys
Thanks for the input.
First, yes, I use a quality manometer and modded tyre inflator to get the correct pressures.
As for the amount of flexing, yes, around a 2 inch upward deflection. it sis certainly not 'freaking me out' lol

.. just that it is not helping performance. You can feel the boat 'drag' when the deflection starts and the speed does not increase, The last twist of gas on the towy 18 2 stroke is a fair kick, and is not being utilised.
It would seem perfectly logical to me that the deflection is causing upset to the water profile under the boat, hindering performance.
Ruds

What is your sib max engine rated to: 15 HP ? If so, 18 will exceed, in that case don't full throttle to lessen floor bending up-down as if you were using a 15. That's the issue about air V floors & air deck sibs too.

Happy Sibbing

ian parkes · 26 September 2008, 00:39

I had a 3.8 too and noticed the same thing . My floor seemed to sucked downwards . I think its just something any SIb does but you just feel it more on the air floor of the honda because you are sitting / standing directly on the bottom of the boat rather than above it on a ply or air deck.

the futura i had also seemed to hit a wall at about 18kts whick took lots more power to overcome the hull flexing. using all of the power you have is probably a bit of a waste as half of it probaly gets you 90% as much speed .

still a great little boat and lots of fun though

ruddles · 26 September 2008, 09:24

Hi again guys,
As for the sib being rated for 15 and using 18, that should not be an issue, its only a guide

.. like using a 40 on my Southern Pacific 3.1 ali rib lightweight.. what a toy hee hee..

Re the actual flexing, any one into hydrodynamics here ?.. what do you think is actually happening, it is easy to say 'thats the issue with air floors sib etc' but it is often something that can be improved, sometimes not.

As Ian said, it 'feels' more like a suction it is as if the transom area and froward are being sucked down lower, not the aft middle being pushed up in its own sense, when it starts to occur you feel the boat lower into the water and, you lose performance. It is very easy to dimiss sibs as simply not ribs and they will do this, however, the same as lowering transoms, raising transoms, a bit of work may yield profitable results. and, I like a challenge

It may even be a matter of adding a couple of flexible flow assisting strips under the boat, but one would need to know what exactly is happening. I dont buy into it being simply an air floor issue, just yet

. and I dont think or believe for one minute that the manufacturer looks at the design from a hydrodynamic point of view in great detail. Vortexs on aircraft are a major issue and often a simple deflector solves the issue. I remember a microlight aircraft I used to fly that suffered from a deflection of the aft boom over a certain speed. The manufacturer solved the issue by adding a simple 1 inch deflector in the cockpit area to solve a problem 10 feet backwards.

What sibs actually do NOT suffer from this ? I would be interested to know, is it possible an issue with the aft projecting floor pieces (supposedly for spray protection) on the Honwaves ? are they acting as deflecting trim tabs at speed. I mean, they are a similar size to trim tabs, but apparntly not desinged to be such, if these flex upwards at speed, does it shove the transom down ? that would be an easy one to fix with a small wood plate above each one and a stiffner to the transom. Is it that, for example, the base of the transom is being deflected inwards slightly causing a kink in the keel area ?, if so, a transom to bow stiffner (similar to my novorania sib with wood keel) but made from grp could be added internally, only needs to be around 2 inches high and chaffe protected, simple to add, simple to remove and weighing only a few pounds if foam cored. would not hinder inflation or deflation at all really.
Again, I like a challenge, there has to be a really simple solution to this he said ...

PeterM · 26 September 2008, 18:32

LInking this to the 'flow' under the 'hull'. Are you getting a nice smooth laminar flow under the first portion & then you will get the flow ' break' away from the surface of the hull & rotate giving you an area of distrubed flow which is trying to get out to lower pressure the quickest way it can - up via the floor ? ..................

Or it the floor at the bow ( with a slight concave) 'tunelling' water under the stern , being held in ( albeit slighly) by the tubes & creating a higher pressure of water and again trying to push the floor upwards as a result. Rather than lift the stern as you would in a rib due to the ability of the hull to take the load, its just flexing the floor - in essence you are trying to ask the rear of the floor to take the full weight of you/ engine/ boat / petrol etc. - It just wont handle it ................

I would expect both these possibles would only show at hight speeds when the real effects of lift/ drag from the hull at planing speed would come in to play.

Just a couples of ideas to to put out there to play with . Logic would say the only way to fix it is to get a more solid floor ! Back to the pros/ cons of an air floor of which you can choose a side depending on what is of prime importance.

ruddles · 27 September 2008, 09:11

Hi Blackroady
Interesting thoughts and very much appreciated, and I agree 100% that the only way to fix this is to get a more solid floor, but am convinced it does not mean a total solid floor.
Did some more tests / observations yesterday with a borrowed budda to add weight at various points

. End result is yes, its a laminar flow issue due to flexing of the hull which in old english is cocking up the planing surface lol. During movement of the friendly Budda the problem was exactly the same.
My deductions so far, it is not a problem with horizontal stiffness in the hull floor, it is one of longitudenal stiffness. Whar 'APPEARS" to be happening is that as the power increases the leverage from the thrust of the prop below the transom to the fulcrum ON the transom is causing the transom bottom to move inwards very slightly, this in turn allows the floor to flex in the weakest area, vis a vis the flatter portions aft of the seat area, the boww having a deeper v section is unneffected. This upsets all flow under the boat and Captain Cooke's thing a wee bit.

Vewwy interwesting.. so, is it possible to remedy or vastly improve ?
Over the next few months I intend to try a few things, first is just to add longitudenal stiffness in the form of a totally temporary internal keel as per my old novorania from the 80's, suitable padded of course, and probably made from a scrap of 18 mm ply, from transom to bow tube and following the floor contour. If this does the trick, then I will have to consider a two part grp internal keel stiffner in an inverted t shape, around 50mm wide and 50mm high.
Will let you know how things progress.

PeterM · 27 September 2008, 14:07

I have used a kids float thingy ( 5ft long , 4inch diameter foam) in the past to 'fully stretch' the floor in my sib - seemed to work well & give the fabric under the air floor more tension. This was when I used to keep it infated , now it gets let down I cant be bothere to put it in - my bombard has completely flat floor so so V to consider - unless i out the foam in .

ruddles · 03 October 2008, 12:25

Hello all
Well, I made a test....

AND, it works well..
Heres what I did, no piccies at present cos I stripped it out to make a marine ply version.
10mm Plywood floor section extending forward 1.3 metres from the transom with a stiffner down to the keel underneath to stop the flexing, weighs next to nothing and the floor area is fairly flat section in this area.
Results....
No flexing to speak of, 5 knots increase in top end speed !.. now get 24 knots on the gps and the boat doesnt SUCK down into the water as the floor flexes. previous top speed was around 19 knots and the last third of gas made it worse. (Using the tohatsu 18)
I now think it could take a 25 no worries lol...
Now need to remake the test ´floor´insert in marine ply with protected anti chafe edges and padded keel spacer.
The whole sib felt completely different, far more responsive and superb even at WOT with the 18. No digging into the water as the floor flexed upwards.
Will post piccies in the next month or two as I remake the insert and am aiming for a total weight of the mod at less than 3 Kg´s. A couple of Stainless steel bracing rods from the transom should be the icing on the cake. It will also provide a solid mount for the steering system, the whole thing easy to add, easy to remove, light and simple and still retaining a reasonable degree of flex for the whole SIB.

Happy camper now. still a light weight unit.. now, If I can just find a good 30 to 40 hp light outboard Mohahahaha!!!!

Ruds

Andy JC · 03 October 2008, 14:20

I'd be interested in seeing some pics of this.

I have a Honda 3.8m with a 2 stroke 25hp and also find the 'wobbly floor' a bit disconcerting. I think I can picture what your describing and it sound feasable to me, I've had similar thoughts myself.

If it's an easy mod then I reckon I'd be up for giving it a go.

Post some pics when you can

ian parkes · 03 October 2008, 21:21

or you could sell the air floor and buy a hard floor version , the 4m alloy floor is the same price as the 3.8 .

longjohn · 04 October 2008, 11:05

Quote:

Originally Posted by ian parkes

or you could sell the air floor and buy a hard floor version , the 4m alloy floor is the same price as the 3.8 .

But surely the air floor actually has a more rigid floor than the ali or wood floor models? With the ali/wood floor models you have an inflatable or solid keel stretching and keeping taughtish, a large expanse of 'flappy' fabric. No mattter how taught there will still be a fair degree of give/"flappyness"

With the air floors (as in Hondas anyway) - the floor is a lot more rigid over the entire surface (if at the right inflation pressure) - sure there is a little flexing - but that will only ever be overcome with a solid hull.

I have to say I am very pleased with my Honwave 3.8 air-floor - handles very well - was at sea last weekend with four large adults aboard I'm sure a combined weight of well over 50 stone and it had no problem planing.

Did a quick gps test with only me aboard and had no problem getting high 20's mph ( actually forgot to check if i set it to read mph or kts....

Andy JC · 04 October 2008, 13:28

Quote:

Originally Posted by ian parkes

or you could sell the air floor and buy a hard floor version , the 4m alloy floor is the same price as the 3.8 .

I considered the hard floor at the time or purchase but it was surprisingly heavier. The 3.5m ally floor weighs 78kg compared to 53kg for the air floor. As I have to carry the boat up a steep pebble beach the weight factor is crucial.

If the above mod only adds another 3kg then it might be worth considering.

ian parkes · 04 October 2008, 18:56

Quote:

Originally Posted by longjohn

But surely the air floor actually has a more rigid floor than the ali or wood floor models? With the ali/wood floor models you have an inflatable or solid keel stretching and keeping taughtish, a large expanse of 'flappy' fabric. No mattter how taught there will still be a fair degree of give/"flappyness"

With the air floors (as in Hondas anyway) - the floor is a lot more rigid over the entire surface (if at the right inflation pressure) - sure there is a little flexing - but that will only ever be overcome with a solid hull.

I have to say I am very pleased with my Honwave 3.8 air-floor - handles very well - was at sea last weekend with four large adults aboard I'm sure a combined weight of well over 50 stone and it had no problem planing.

Did a quick gps test with only me aboard and had no problem getting high 20's mph ( actually forgot to check if i set it to read mph or kts....

Yes I think you are correct the air floor does give a more rigid hull than the air tube type .
Iam amazed at the speed you guys are getting , my honda 3.8 with a honda 20 would do about 18 flat out with a bit of a load , and my futura mk3 with 25hp did just over 20 with 3 people in .
My 4.1 rib with a 50 only does 30

samt · 04 October 2008, 19:30

Glad to see your idea has worked.

My old air floor sib was always slower than the wooden floored sib I had before. Same engine, despite being lighter the floor wasn't as well stretched, and caused drag when planing.

24knots is JUST possible with the tohatsu 18, assuming you are running a 10inch pitched prop, and reaching top RPM. The prop calculator gives 0.07% slip which is unlikley, so I expect this speed was recorded running with the tide.

Specs from here: http://www.watertoyz.com.au/outboard...roke-18hp.html

Prop Calculator: http://www.rbbi.com/folders/prop/propcalc.htm

Ian, I got 32knots with the mariner 40 on my searider. So I would think if you tried a few different props you'd find a few more knots with your 50.

ian parkes · 04 October 2008, 20:49

Ian, I got 32knots with the mariner 40 on my searider. So I would think if you tried a few different props you'd find a few more knots with your 50. [/QUOTE]

Iam not too bothered about top speed on such a small boat with tiller steer , i wouldn't be able to use more speed very often unless it was flat calm
its on the standard alloy prop but revs to about 5500 so its about right .

Best thing is still gets that speed with 5 people in the boat or towing a donut with 3 adults on it and still comes out the water instantly like a rocket