SIB bouncing

[jimbojambo]

Hello all

I've only had my SIB for a few weeks, but notice a bouncing at about 30kph+. It is not wave based and even happens on flat lakes. It seems to be when I have one passenger up front, but I have not tested this in detail.

Any ideas if this is a form of resonance? If so, can I do anything to supress it? It is no biggie, but makes the passenger uncomfortable.

It's a OMC Express 305 with a Mariner 15M (short shaft) on the back.

Loving the new hobby though

James

[jyasaki]

Does it seem similar to this?

http://rib.net/forum/showthread.php?t=33402


jky

[jimbojambo]

Thanks for the speedy reply. I'm not sure it's cavitating, so to speak, although at other times there was definitely over-revving going on (which I put down to a form of cavitating after seeing a lot of air by the prop). There were no signs of the wooden floor arching either, that I could tell...

I must admit I only inflated the 2 tubes and keel by "touch", so they pressures may have been a bit off (although were firm to the touch, and seemed pretty good).

I'm just wondering if there's anything I should experiment with (trim, weight distribution etc) that might stop this bouncing. I'd estimate there was around a foot of bounce every 1 second - regular as clockwork when at speed.

[jyasaki]

Well, don't know exactly what's happening with yours, but my experience with a SIB showed that they like weight forward at takeoff (to help keep the nose down while it climbs out of the hole), then prefer it further back once the boat is on plane.

In my case, having weight forward caused the boat to fall off plane much earlier (at a higher speed) than if the same weight was further back; don't remember it causing bouncing or anything like that.

jky

[jimbojambo]

Has anyone else experienced this sort of behaviour? I'm wondering whether it's a design fault (hopefully not!)

[Locozodiac]

Originally Posted by jimbojambo

Has anyone else experienced this sort of behaviour? I'm wondering whether it's a design fault (hopefully not!)

It's normal for sibs to bounce when having all weight towards transom, if sibbing with 2, secure tank at bow, move sibbers passing middle lenght. Try trim angle at 2-3 hole out from transom to have perpendicular engine/boat when on plane. Have had the same combo and performed exactly as described. Inflate tubes to recommended factory settings, buy a pressure gauge, push/pull yellow model.

Happy Sibbing

[jimbojambo]

Thanks for the replies. I have recently heard about "porpoising" and am wondering if this is the same thing... I've bought a pressure gauge and will be taking on the bank holiday weekend to see if correct pressure helps (it was quite a bit under when I tested it).

I've also noticed that the transom looks to be not 90 degrees to the boat, more like 95 or maybe 100. This might mean the engine is too far trimmed out, maybe? I've built an 18mm plywood wedge to put behind the transom plate to counter this angle, I'm hoping this makes a difference if the pressure does not. But, I can't see how being too far trimmed in could make it bounce (or porpoise) when it's usually caused by being too far trimmed out.

Hmmph.

[theo53]

Originally Posted by Locozodiac

It's normal for sibs to bounce when having all weight towards transom, if sibbing with 2, secure tank at bow, move sibbers passing middle lenght. Try trim angle at 2-3 hole out from transom to have perpendicular engine/boat when on plane. Have had the same combo and performed exactly as described. Inflate tubes to recommended factory settings, buy a pressure gauge, push/pull yellow model.

Happy Sibbing

I had the same oscillating problem with my sib (suzumar 3.90 alu 15 hp suzuki 2t) at full speed in calm water. It even ripped off one rivet from the aluminum floor. I ve noticed that distributing the weight in two too far apart (bow stern) worsened the problem. Also less air (less rigidity) worsened the problem (keep in mind that you cannot make a SIB totally rigid). Also wrong trim worsened the problem. The solution that worked for me is to not use the speed where this oscillation occurs and ... put my friend (passenger) at the bow for planing and then ... moving him a little closer to the center. I did not tamper with trim (its manual and inconvenient in any case)