Sib tube bending

[Ronaldzodiac]

Hi,

My sib is bending (see the red line in the photo) when getting in plane. When I go in a straight line in forward and increase the gas, around 20 km/h (I think when it’s getting in plane), the tubes are bending roughly 80 cm from the transom to the front. I don’t know why that is.

Situation:
I have a 1972 sib of 4,70 meter (rated max 60hp). It have an old mercury 45HP 2 stoke on the back with hydrofoils. The outboard doesn’t have an electric trim, but the trim angle can be set before taking off. (This is done with a pin that goes though the engine mount). The tubes (2,4) and the keel (2,2) got the right pressure. I’m sitting on the front bench and got a battery below me (total 100kg). On the back seat I got the fuel bag of around 20 kg.

What could be the reason? Am I sitting to much in the front for a sib? Are the hydrofoils doing this because the outboard isn’t in the right angle?

All advice is greatly appreciated. Hope to hear front you!



upload photos and share link

[Limecc]

You could remove the hydrofoil which is providing additional lift outside the manufacturer's design parameters however I don't believe you have a problem that suddenly started, it's just a SIB limitation. Ie live with it or get a RIB. Overpressurizing the tubes might help but I wouldn't risk that except you can set the pressure whilst at temperature on the water, often there's a pressure drop.

[Fenlander]

A sib of this length with such a large outboard is outside my experience so about all I can think of is...

The pressures you mention... is that 2.4/2.2psi or 240/220mbar?

Are you happy your gauge reads correctly... some are way out.

I assume you have a wooden sectional floor with stringers... this should resist bending... what happens to it when the tubes bend?

When you say it bends 0.8m from the transom to the front do you mean it bends up by 0.8m or that it bends 0.8m along the tubes from the transom?

Is this bend a distinct kink or a gradual banana shape?

[brveagle]

I'm going to throw something out there and see if it applies. You may already know this, but when a boat is about to go on plane, the nose will raise substantially in the air. Pretty much what you are describing. Once on plane, then the nose drops down. It sounds like you are approaching that point where the bow lifts up and then throttling back. It also could be the placement of your motor trim. I.e. may need to lower it to get the nose down.

[jeffstevens763@g]

i 've ran 5m sibs with 40hp on with no problems if your pressures are psi you're low, i surmise that the floor is all ok with stringers to stop the floor flexing & keel blown up too

[Ronaldzodiac]

Thanks for all your replays. It helps and it forced me to rethink assumptions.

Originally Posted by Limecc

You could remove the hydrofoil which is providing additional lift outside the manufacturer's design parameters

That might be a good idea. I bought the motor with the hydrofoils, but haven’t tested the boat/motor without it. Today I watched almost every Milpro fc-470 video, but in none of the videos I found a hydrofoil. It would be better to the take that thing of.

Originally Posted by Fenlander

The pressures you mention... is that 2.4/2.2psi or 240/220mbar? Are you happy your gauge reads correctly... some are way out.

Yeah it’s the 240/220 mbar. I made a gauge by myself with industrial components. It’s accurate :-)

Originally Posted by Fenlander

I assume you have a wooden sectional floor with stringers... this should resist bending... what happens to it when the tubes bend? When you say it bends 0.8m from the transom to the front do you mean it bends up by 0.8m or that it bends 0.8m along the tubes from the transom? Is this bend a distinct kink or a gradual banana shape?

I got the original wooden floor with stingers. Everything is setup nice and tidy. It’s not bending up by 0,8m but the visible bend on top of both tubes are 0.8m along the tubes to the front from the transom. No banana, but more like a kink. Sorry I don’t have a video, my phone battery was dead.

Originally Posted by brveagle

It also could be the placement of your motor trim. I.e. may need to lower it to get the nose down.

I think you’re right, it’s all about the right trim. And the use of hydrofoils will probably magnify the problem with a wrong motor trim. But I have no idea what the right trim should be. What is a right motor trim for such a boat for inland waters (see option below)?

(The horizontal red lines in the 3 setups are in accordance referred to as the horizontal fins on the tail. The motor trim in the pictures is a bit exaggerated for clarity. As compared to the position A, position b is only 1 hole futher than position A, and the position c is only two holes further than A.)
gas near my current location

I tried all options below, but all with hydrofoils mounted. I guess it should be position B. In position C I got lots of cavitation. On advice of some others I raised the outboard up with 2.5 cm. If position c is advised, I can bring the outboard down and try again.

I will get the prop wet later this week. Than I can try the boat without the hydrofoils. I’m looking forward for advice for the right motor trim.

[Locozodiac]

With Sib inflated to its recommended factory working pressure on all air chambers and deck load evenly distributed, trim the motor to sit at 90 deg (fully vertical) on transom. Go for a wot run on flat calm water cond if possible, pull head out side of transom and check at which lower leg height is hardened middle water flow passing by. This is of extreme importance if wanting to get the best water performance out of that oldie combo.

The ideal transom/motor height match is when water flow skims right under the small upper plate once Sib is on plane at speed. We don't know if in need to raise or lower said motor to match that spot on motor/transom height. If you vary the trim position to other trim seting from ideal 90 angle check what will happen as seen on pic 5.

If motor sits at 90 deg and doel fin rides parallel to water level once sib planes fast enough will be OK, BTW how is that large Sib being driven, tiller or with a front drive wheel with remote throttle and gear shift ?

Can you upload a nice pic of the rear floor area adjacent to the transom to have a look.

Happy Boating

[Ronaldzodiac]

Originally Posted by Locozodiac

check at which lower leg height is hardened middle water flow passing by. This is of extreme importance if wanting to get the best water performance out of that oldie combo. The ideal transom/motor height match is when water flow skims right under the small upper plate once Sib is on plane at speed.

Very helpful information. The photo’s with instruction are very clear. The next time I will check this. I am very curious.

Originally Posted by Locozodiac

We don't know if in need to raise or lower said motor to match that spot on motor/transom height.

Based on advice of an other member of this forum I raised the engine, he advised to raise the engine with 5 cm. I could raise only 2.4 centimeters. This was the maximum height to get bridge clamps still on the original transom protector.

Btw, I found the reason that the tubes were bending. Both floor stingers were torn. I got one new stinger and I will try to glue the other one.

Originally Posted by Locozodiac

BTW how is that large Sib being driven, tiller or with a front drive wheel with remote throttle and gear shift?

Hard to say right know haha. There were still a few problems that I am now resolving.
1) The keel was still leaking (after 15 patches already).

2) The original zodiac fuel bag leaked on a seam.

3) The floor stingers were torn.

4) I tried to use the original wire steering system in a hydroplane way, but wasn’t great. I was still waiting for my Motolita Teardrop steering wheel and was using a simple self made pierce of wood as a steering wheel.

5) Concerning the outboard I blown out the crankshaft seal. The fuel pomp seals were leaking and get an overheating alarm after 30 minutes. Despite the cleaning of the carbs thoroughly, there was a blockage in the nozzle needle. I only had a prop of 13” but was still able to get 48 km/h.

A few days ago, I bought a Teleflex NFB 4.2 Rotary (for up to 235PK, so should be fine). I received the Motolita steering and found an adapter for between Teleflex and Motolita. I fixed all the engine problems and bought a fuel filter with water separator. Glued the keel and the flexible fuel bag. I found a prop with a 14" pitch, but still looking for a 15" pitch. All I have to do is paint the stingers and the wooden wheelhouse and then everything can be mounted again.

Relatively unimportant, the new flagpole from zodiac has a different thread. I cut thread into a round piece of wood as an adapter between the boat and the new flagpole.
The condition before:

[Locozodiac]

Ronaldzodiac, need to fix all those mentioned issues before going for a final water test. Raising the motor on transom as per recommendation is a wild shot as won't know at which lower leg height is the current water flow passing by. If passing too low near lower AV plate combo will work OK on flat calm water cond at straight water courses, but will experience awfull prop aeration on choppy, windy, white caps water conditions and at tigh close turns at speed.

Sit motor back at 90 deg fully down on transom, have other mate kneeling, sitting next to transom to have a look side of motor, check at which lower leg height is water flow passing by at full throttle. Will probably need to remove the back set for a comfy looking positiom out transom...

If flow is skimming right under the small upper plate or near it will be a perfect motor/transom height match as seen on my previous 4th photo. This test must be done on flat calm, no wind water cond if that's possible.

Ideal is to count with a combo that's working spot on on all type of water conditions and not only on plain, flat ones. Report your findings...

Happy Boating