Sib Owners

timboli · 06 May 2004, 20:35

Last year,I bought a Bombard 335 aerodeck,partly as a tender to use with a tohatsu 3.5,but also to take in the boot and explore rivers.This combination has worked fine and will even plane one up,however curiousity got the better of me yesterday and I tried it with an Evinrude 15 shortshaft I bought some time ago with hydrofoil type fins fitted,it planed with 2 adults but seemed to want to vent the exhaust at planing speed and attempts to use with more then half throttle caused the prop to cavitate and lose grip,and the boat consequently to slow until I throttled right back.The prop isnt perfect but isnt wrecked and had no problem on a flatacraft force 3 rib.
The dealer suggested it was a problem with this type of boat and I try a coarser pitch prop.
I was wondering if any other SIB owners had suffered similar problems as I have been in a bombard 3.8 with a 9.9 which planed easily with no problems.

Ribald · 06 May 2004, 21:20

Where's Mr Hart when he's needed?

Colin F · 07 May 2004, 09:56

two things i'd look at:

1) take the foils off - you never know, the lift they give and the fact the boat is vey light couls cause ventilation
2) maybe look at the trim angle of the engine

I have a Yam 280sti (airdeck) that i can get to plane (1 person) with a yamaha malta (3.5hp), but like you get the ventilation. I think it is because i have to sit right at the front of the boat to get it to plane, maybe lifting the stern.

Not sure but worth a go as these cost bugger all, just a bit of time fiddling around.

hope it helps

Colin

Richard B · 07 May 2004, 10:02

I used to have a 3.6m SIB with a Tohatsu 15 which also had hydrofoil thingies fitted. Never had any problems like you describe, although the previous owner had fitted a coarser prop which was in good condition. I think that the dealer's advice is probably quite sound.

Keith Hart · 07 May 2004, 15:45

Hmmmmmmmmmm...

Well my 3.4m sib has a 15hp short shaft engine and it is okay on the plane.

If I don't have my balast in the front (Noel) then I use an 80l kit bag full of stuff and a full fuel tank to keep the front down. I have thought about fitting the foil thingys but do not like the idea of drilling the engine bits to fit them.

If it has the standard prop then that should be okay.

My best guess is as Colin says - the engine trim. I have played about with mine and found that incorrect trim will cause the prop to cavitate. Take the foil things off, then you should try is adjusting the engine trim.

One thing you mention that puzzles me a bit: I think it is because i have to sit right at the front of the boat to get it to plane, maybe lifting the stern.

This makes me think that you may have the engine trimmed out too much as usually the boat would plane easier with your weight at the back, thereby helping to lift the bow over the wake.

Keith (SIBs are great) Hart

timboli · 07 May 2004, 18:42

Thanks all for your comments,I,m a bit worried about using it with the 15 without another person as its so light(around 30 kg),I did try shifting my weight but my wife was right at the front,I fitted the doel fins as the other boat,the flatacraft tended to porpoise,they completely cured this on the flatacraft,the outboard was raised slightly to align the outboard with the bottom of the hull,and achieved around 18 knots.
Next time I have time to play with the bombard,I will try removing the fins and trimming in.
The boat does sound like the exhaust is starting to break surface with the 3.5 planing and me sitting slightly(armstretch) forward.

JCW · 07 May 2004, 20:00

Timboli,
I use a mariner 15HP two stroke on my Zodiac 3.4m fastroller. No problems with prop slip in calm conditions and tops out at 20 knots (one up) with the standard 9" pitch aluminium prop. I have used my 6HP 4 stroke aux outboard on the same sib and I experienced lots of prop slip. The cavitation plate on the 15HP is twice he size of the 6HP however.

If you have some manual trim adjustment, try trimming in in as much as you can and try that. I presume your sib is designed for a short shaft outboard and you are using a short shaft?

timboli · 07 May 2004, 20:36

Yes,short shaft engine and sib designed for short shaft engine according to the dealer who sold it to me

Clipper · 11 March 2008, 10:20

howdy,
i also have a similar problem, i think? my SIB does the porpoise thing, going up and down, with the majority of the time the nose is pointing at the sky, i thought about the hydrofoil, but would prefer another alternative. the trim was all the way to the bottom, with most weight at the back and some in the middle. i still dont think i have experienced planing due to this problem. would a different pitch in prop help? why would it make a difference? and what is pitch?

prairie tuber · 11 March 2008, 13:18

a) Double check the hub prop to make sure it is not spun. This is far and away the most likely problem.

b) Check the fabric bottom (paticularly near the transom) to make sure there are no loose flaps of fabric

c) Make sure you have the right air pressure in the floor.

d) Don't load up weight on the front, distribute it more evenly. If you have weight cocentrated at the very front and very back (from the motor & operator) the center is going to flex like crazy and cause alot of porpoising - especially with an airdeck. Also, having a person sit at the very front is dangerous.

Nick Hearne · 11 March 2008, 16:39

This thread is 4 years old!

Clipper · 11 March 2008, 23:39

thanks prairie tuber,

"Double check the hub prop to make sure it is not spun. This is far and away the most likely problem." - not sure what to look for, is the hub what holds the prop on? how would i distinguish whether it was 'spun'?

i should have mentioned that i have a ply floor, not air deck.

nick,

as a newbie i am trying to get as much information out of the older posts as humanely possible, without nagging you experienced guys with things that have been covered a number of times. though many of the threads are just a little short on the info i need personally. i am determined to get my bearings around SIB's, how they work, what impedes their performances, what enhances it, maintenance, outboard basics etc.....

prairie tuber · 12 March 2008, 04:19

Most outboard props have a hub mechanism inside of them to protect the drivetrain of the ouboard. This is in place of the sacrificial shear pins that were used on older outboards. The prop hub mechanism involves a splined metal bushing that slides over the prop shaft that is surrounded by a rubber bushing that is tightly wedged between the splined bushing and the propeller housing The concept is that if the prop strikes something solid such as a rock, the hub inside will give way and spin rather than snap the driveshaft or damage the engine.

Once that hub has been spun, it will still have enough 'grip' to cause the prop to turn under low power (so you can limp back to shore), but as you apply more power the hub will start to spin again. The benefit of this system is that you don't have to try to change out shear pins in the middle of choppy water. The disadvantage is replacing the hub mechanism (or whole prop) is a whole lot more expensive than replacing a shear pin. You should be able to find some schematics of this by googling 'outboard propeller hub' or something along those lines.

As for checking if the prop hub is spun, have a look at the prop. If it looks like that rubber bushing has been twisted up, it has been spun. If you're still not sure, take an indelible marker and mark a straight line along the metal bushing outwards to the propeller housing. Take the boat for a test drive, then look at the prop to see if the splined bushing is still aligned with the propeller housing.