flex in SIB longside, front triangle

[snakeeater]

Hi,

I am new to this forum and to inflatable boats. I read loads of threats during the last weeks and got a lot of information.

Finally I decided to order a 3.3m SIB and a 30hp engine.

Now what I am considering is to bolt bars onto the alu SIB floor to make the boat stronger in length direction, considering the engine power.

Good idea or not a good idea? Does a SIB need flex in the long direction? I suppose at least for the alu boards in the middle it should be good. How about the front triangle? Does this have to flex (how much max)? If the front triangle does not flex there might be more load on the tubes, right?

Basically I wanted a RIB, but a trailer transport and storage are not an option at this point

[Peter_C]

Do you have a pressure gauge? If not you need to go and buy one. Most problems related to boat handling are pressure related.

Also if you are inflating every time a nice electric Bravo pump is going to be in order.

What horsepower is the boats max recommendation?

[snakeeater]

Well, the 30hp should be just on the limit. I am not really afraid it might fall off.

No handling issues so far, did not even assemble the boat, since i am waiting for the engine.

I am just a tech and like to pimp my stuff

So second part of the story is i want to fit some seats and a console. Question is do i have to fix the seat on one panel and the console on another panel, to allow the panels to bend slightly? Or can i put the items always between two panels with the bars to fix the panels solidly together, so the whole floor is like the stiff floor of a RIB?

So, the mechanical stuff aside, does a SIB have to flex (as a whole or just the triangle)? Or is it fine to reinforce everything? I want to modify if there is no argument against it.

[Max...]

3.3m and 30hp sounds like a threat to me....

[snakeeater]

I am aware that 30 hp are a lot for the boat. I take the safeties serious and will not crank it up right away of cause. Will drive it on a quiet lake

So basically that's what I am asking: if you are sitting in that boat, what is the threat? Which failure points have to be considered.

I will load the boat symmetrically and spread the weight over the length (forward controls).

Obviously the transom is a concern. I will bolt this with angles to the last aluminum plate.

Other failure points I see are the connections between the other floor plates. So i want to install the bars. Here I am not sure if there might be negative effects I am not aware of, so I started this thread.

Third: the tubes could be ripped off when breaking a wave. In my case on a lake it will not happen. But I would still like to have this covered. Maybe glue a second layer of PVC to where the tubes attach to the floor.
How could I effectively add straps?

Other things to cover?

[Max...]

We all love tinkering and others might disagee but if you want a RIB buy a RIB, trying to turn a SIB into a 'franken-boat' RIB will not a RIB make and seems fraught with hassle and potential problems. Once you have added all the weight and parts you describe and that will make the SIB 'non-deflatable' how will you transport it? Regardless how will you handle a 30hp engine - presuming this is even a two stroke it's a hell of a lump to lift, transport and mount.

A 4m Searider sounds likes a much more suitable buy?

[snakeeater]

I understand your concern.

I got the engine mainly because I want to upgrade later. It is not that much of a problem to swap the boat, but if the engine is too small for the next bigger boat it seems costly. That being said i could imagine a 4m as an option, just thought I don't need that much boat at the moment. But as you say it it makes sense. Maybe I will later look at a 4m zapcat to have sth. different that makes use of the engine. And some day hopefully a RIB of cause.

Regarding the engine (2stroke) I can easily limit the throttle to less hp, and reduce torque with a higher pitch prop. then I have a 15hp with 20Kg ballast. I will start slowly, promised I am not hunting for speed records. I know this boat is not made for it.

My question is also not how to make this boat ready for high speed.

As I said I want to make myself a seat and a console. I would also do this with a 15hp engine. Now my question is just: is it better to put each on a separate floor panel without any influence on the rigidity of the boat. Or place them in a way that the panels are stiff together? Would this make the boat better (more stability, the tubes do not have to hold bending force) or make it worse (increased stress on the collar of the tubes).
Especially: does the front triangle have to flex?
I do not want to make a crazy boat beyond its limits

No worry about the mechanical stuff, i will have everything reasonably strong and modular and with my means transportable. Otherwise no point in doing this.

[Landlockedpirate]

Over on YBW.com they are always overpowering Sibs and Ribs, but its done in a careful and considered way, well most of the time Having said that, 30hp on a 3.3 sounds a lot. Most 3.3's have max power of about 15hp, doubling it is going to make it interesting and I think your transom and the join between the transom and the tubes will be the weakest point.

In reply to your question, with a Sib, the stiffer the better.

I got hold of a 3.1m Zodiac with floorboards when I was a kid and fitted a 8hp Yamaha. At 1st the front triangle floor board used to bend upwards when running at full power and into waves, this had the effect of halving the speed. My Grandad, a retired aircraft engineer, fitted titanium rods along the length of the boat and it was transformed, faster in all conditions and it felt much more rigid.

I know this is a rib, and a bit smaller, and its 'only' got 30hp !

http://youtu.be/TA9khGGbQlc

[Festinghouse]

thats just plain stupid

[kerny]

Originally Posted by Festinghouse

thats just plain stupid

I agree. Is this a wind up

[kerny]

Originally Posted by snakeeater

So second part of the story is i want to fit some seats and a console. Question is do i have to fix the seat on one panel and the console on another panel, to allow the panels to bend slightly?

It would like being on the catapiller at the fun fair at Belle Vue back in the 70's great fun though

[Landlockedpirate]

Originally Posted by kerny

I agree. Is this a wind up

Not by me, as well as the video that I have posted, I have also seen a Suzumar 2.1 with a 25hp 2 stroke on it.

I was trying to illustrate to the OP that overpowering is not always a good thing.

[Jeff Hutcheon]

Originally Posted by kerny

I agree. Is this a wind up

He's got some bottle, its a couple of bobs worth of motor to end up at the bottom of the dock!!

[snakeeater]

Well, I am not looking for danger. So better get a 4.2m sib? At this point still possible.

But threat question would be the same. How to fix stuff. Thanks landlocked pirate.

Edit: Is a 3.8m 1.7wide with 50kg rated transom still too crazy?

[snakeeater]

There seem to be lots of people using 25-30hp on 3.8m boats. Are 3.8m that much of a difference to 3.3m I guess the length does not matter too much on the plane, can also be done with trimming. Weight ist also similar. So is it safer because of its width (more support when cornering)? I could imagine a wider boat could also be more unstable regarding chime walking. Please help me understand the dynamics behind.

So is it only the transom to worry about? That's obviously just a mechanical problem that can be calculated.

[Festinghouse]

Originally Posted by snakeeater

Well, I am not looking for danger. So better get a 4.2m sib? At this point still possible.

But threat question would be the same. How to fix stuff. Thanks landlocked pirate.

Edit: Is a 3.8m 1.7wide with 50kg rated transom still too crazy?

i would say that you would get away with that (3.8 + 30hp). i would also say that if you were to try both boats out on the water, you would notice a big difference in their handling and stability with any engine on the back.
as for fixing stuff down then why not use bolts glued in place from underneath the floor so as to keep them captive, then slot your seating etc down over the bolts and fix using wing nuts(?) and washers?

[MKII_CGT]

Originally Posted by snakeeater

Hi,
Now what I am considering is to bolt bars onto the alu SIB floor to make the boat stronger in length direction, considering the engine power.
Good idea or not a good idea? Does a SIB need flex in the long direction? I suppose at least for the alu boards in the middle it should be good. How about the front triangle? Does this have to flex (how much max)? If the front triangle does not flex there might be more load on the tubes, right?

I would think that any floor reinforcements to stop the floor flexing could only be a GOOD thing.

The lest the floor flexes the better right? Or am I wrong?

[Fenlander]

A wood or alloy floor as long as it's in decent order and properly fitted is rigid enough for the purposes of a SIB. It doesn't matter how much additional stiffness you add to a sectional floor because underneath it there is still a flappy PVC/rubber floor with a minimal V section formed by a squashy inflatable keel.

[snakeeater]

I have to decide on the boat now, so please one more time your honest and constructive advice.

I will get the 30hp 2s.

Will be driving on lakes and rivers only, no big waves.

Payload will be 250kgs on average, 200kgs minimum, evenly distributed.

Driving for fun, not to haul stuff or get from A to B.

I definitely do not worry about the transom. I will make a massive aluminum mounting bracket to to fix engine bracket and transom to the alu floor boards. Will secure the engine to the tube. No matter which boat.

I will get smart tabs for handling and balance.


Choices of boats:

3.3m long, 1.5m wide
considerations: Several people here and on youtube seem to drive 3.3m boars with 25hp 2s. With two people that seems to be just right, not scary at all.
Size seems right for my storage and handling possibilities.
With smart tabs it will handle like a slightly longer boat?

or 3.8m, 1.7m wide
considerations: Everybody in this threat says 3.3m is crazy (I take your experience very serious)
But will it still be fun to drive with 250kg load? (get on the plane, cornering)


It all comes down to the handling. I do not want to break speed records while jumping around like a tennis ball with the keel up high in the air.

I want a boat that handles precise and swift and goes as fast as it is stable, taking into account flat water, well distributed weight and smart tabs.
I am not expecting wonders out of these. If you still say 3.3 is too crazy I will listen.

Sorry, just hard to imagine for a noob that there is that a big difference between 3.3 and 3.8 in handling. Please convince me

[MKII_CGT]

Get the 3.8m boat and put the 39hp on it. Done.