Sib with 20hp

[CHRIBS]

Hi all I have bought a second hand sib with a Honda 20hp outboard, it starts without problem every time but when I’m on full throttle it sounds like it’s been knocked into neutral and revs high, as soon as I ease of the gas it recovers and is fine but I can’t run at full speed as it keeps doing this .....any ideas ?? Thanks

[Fenlander]

Which Honwave did you get?

BTW no need to ask the same question in two places as it confuses which thread to answer.

[CHRIBS]

Oh I’m sorry I’m new so forgive me...it’s a 3.5m with 20hp Honda

[Fenlander]

Hey it's fine... no way of saying without it sounding like a grumble but it does help if all replies are together. In the other thread Poly has wondered about a slipping prop hub. Another possibility is it is an effect you can get with SIBs if they are underinflated particularly the keel (I assume you have the alloy floor model with sausage keel under?).

Even when everything is spot on certain OB and SIB combos with certain loading balances in certain sea states can do this due to the prop losing its grip.

This happens with SIBs due to the flexible nature of the hull.

[CHRIBS]

That’s really kind I will check all the pressures and see if this helps I will also get the prop slip checked ...they both sound sound advice and thanks so much

[Lee1]

Your cavatating, try moving your position in the boat to balance the weight, as above if it's a V floor this is one draw back of the v floors

[Mikefule]

Based on the limited information, my first thought is that once the engine reaches a critical speed, the prop is sucking in air, meeting less resistance, and therefore allowing the engine to spin faster. I had this on my 9.9 when I tried a different tilt setting as an experiment.

This sounds more likely than a mechanical problem with the engine.

Try the tilt one notch lower and if that doesn't work, try 1 notch higher.

It may also be the trim of the boat. Where you sit, and where you put any heavy kit can affect the trim, raising or lowering the transom by an inch or two.

[Fenlander]

>>>Your cavatating, try moving your position in the boat to balance the weight...


Cavitation is often spoken of in these situations but it's not the correct term... no big deal though as we all know what someone is trying to describe. Cavitation is a sort of boiling of the water adj the blade surfaces usually due to prop damage or the wrong prop for the boat. It can be felt as a vibration and can eat the paint and prop surface away with pitting.

If it's not hub slip but the prop taking air as Mike describes it's called Ventilation. This can be due to the wrong OB height or trim and in hard boats set up correctly is only usually found on tight turns. But as I explained above the flexible nature of SIBs can (can, not will) trigger this far more easily if some set up factor is wrong... or just down to a particular type that is very prone.



>>>if it's a V floor this is one draw back of the v floors


No that's totally wrong... it has nothing to do with V floors.

The most prone SIB design to triggering ventilation is the flat air floor with a low pressure sausage keel. In certain makes/setups it's really hard to stop this happening.

Better but the next most prone are the shallow V air floors like Honwave (and your Waveline Lee). Usually but not always the longer the model the more likely the problem can be.

Less prone to ventilation is a well set up alloy floor SIB with low pressure sausage keel.... like the OPs... and that's why I think we can quite easily talk him through sorting it.

The SIB very least prone to ventilation is the Aerotec as it is a true V hull for its whole length which by its design doesn't trap dollops of air under the hull. In all Nm we have covered at sea in our Aerotecs ventilation and the associated prop slip has never been an issue.

[CHRIBS]

Hi thanks for This I have looked at the prop today and the castle nut was loose enough for me to turn with my hand ! I wonder how much to tighten and see if this helps as well as the other suggestions . I will also look at the outboard angle as it’s currently set on the pin nearest the boat

[Fenlander]

Obviously good to tighten the prop nut correctly... to lose the prop would be most unfortunate! But as it's on a spline it won't help the "spinning" effect.

Yep try all the things we've advised and I hope you can report a cure.

[jeffstevens763@g]

Never whitnessed it before but I noticed compressed air escaping from the beam of gurnard boat air being trapped in the gull wing shape of the keel and tube has to go somewhere I guess some boats suffer it traveling past the prop in large amounts causing slip/cavitation bit like dipping the clutch on your car whilst underway.

[Lee1]

The 3.5mtr is a big boat for a SIB I feel a larger more weighted engine might be better, are you using a 2 or 4 stroke engine, as there is a big difference in weight, I doubt it's a faulty prop as it would slip the bushing under pull away. What your experiencing could also be if you do not have a trim pin in place, many people buy outboards with the trim pin missing and because the engine sits fine without it (some models do) they don't know it's even missing. Does the engine become lose on the transom despite retightening it? You can test to see if your engine leg lock is working right by going into reverse, if the engines rises up on reverse the trim bar is missing or the locking hooks to that bar are broken or even missing, if you have shallow drive option on your outboard this could be stuck in the active position or even broken thus the leg is not locking to the transom bracket, if your engine leg doesn't lock to the transom mount your engine is rising out the water on full throttle, you won't be able to see this happening, I suspect this is the problem, if your leg doesn't lock to the bracket you have two forces fighting each other, the force of the forward motion of the thrust and the drag and rising of the leg that's not locked to the transom bracket rising slightly at an angle out of the water, i.e. You are running on a very slight shallow drive once the speed and force of the water makes your leg move back.

Please post various pics of the engine mounting bracket and upper leg.

[Fenlander]

Lee you are misleading someone who is new to the forum and looking for helpful advice and it's out of order.

3.5m is not large for a SIB... it's midrange. A 20hp is the perfect engine for it. It doesn't heed a heavier engine... that's plain daft.

You say you are an outboard engineer yet the OP says he's a Honda 20hp and you have to ask if 2 or 4 stroke!

In the 20hp range there isn't a big weight difference... that Honda 20hp weighs about the same (little less if anything) than the average 20hp 2-stroke from the 2000 era.

[Lee1]

With no pictures to go on I asked relevant questions, I have people bring me engines that they said were 20hp on the phone and they turn out to are repainted 10hp 2 strokes with new stickers or even new hoods, the poster is new to boating and I do not know what he actually has brought!
3.5mtr is big for a SIB most common sizes are 2-3.2mtr size therefor mid range is 2.7 I see a lot of 2.4mtr SIBS out on the water!

After 3.2mtr most inflatables boats users go for RIBS not SIBS.

It is the general agreement that after 3.8mtr a SIB flexes to much. So 3.5mtr SIB is big in the regard of SIBS!

[Fenlander]

You are on a wind up for sure.... if you knew SIBS you'd know there are dozens of makes/models above 3.2m that are true SIBs.... probably over 50% of the members on here have SIBs over 3.2m.

You seen this thread... all three guys with true SIBs... 3.65m, 3.8m and 4.3m... on the sea doing proper sibbing.

http://www.rib.net/forum/f16/the-bes...tml#post772647

Re the OB size there are indeed 3.5m SIBs rated to over 20hp but the OPs is rated max 20hp. The sweet spot for overall sea performance for a SIB is usually between advised (usually one size down from max) and max rated hp.

[Lee1]

So anything under 3.2 is not a true SIB?

No I disagree, a SIB is a boat that is brought and chosen because it folds up for vehicle transportation, the most common sizes used are 2.7-3.2 of course many use 3.4-4.2 meter as well as many use 2.4mtrs but the bigger 3.2 plus require bigger engines and the set up will then only fit in larger vehicles.

This thread is about helping the guy sort out his suspected cavitation, my points and suggestions are valid and correct, no 100% right suggestions can be provided without pictures of the actual engine!

And on the flexing issue here's a 3.2 metre flexing nicely:

https://m.youtube.com/watch?v=wQTk1FWw-Ek

[Poly]

Originally Posted by CHRIBS

That’s really kind I will check all the pressures and see if this helps I will also get the prop slip checked ...they both sound sound advice and thanks so much

You can test the prop slip yourself. Search forum for slip and tipex and you’ll get instructions.

Originally Posted by Lee1

So anything under 3.2 is not a true SIB?

now surely you are trolling because nobody could read that in what was written?

No I disagree, a SIB is a boat that is brought and chosen because it folds up for vehicle transportation,

someone better tell The Gurnard, and everyone else who keeps them on a trailer...

the bigger 3.2 plus require bigger engines and the set up will then only fit in larger vehicles.

or on a trailer... but the OP seems to have the right size engine so you are chasing geese.

This thread is about helping the guy sort out his suspected cavitation, my points and suggestions are valid and correct,

except that it is definitely NOT cavitation although it might be ventilation. Still your point about pictures or better still a video are quite right.

[Fenlander]

>>>Quote: the bigger 3.2 plus require bigger engines and the set up will then only fit in larger vehicles.

>>>or on a trailer...

Or in the case of my 3.8m with 20hp (which is as bulky as a 30hp 2-stroke) in the back of my daughter's small Honda Jazz still leaving room for three folks in the car.

[Lee1]

A SIB outboard and passengers in a Honda! I would like to see that! I just about fit a 2.7 sib with a 7.5 and 20hp outboard, fuel tank, life jackets, gear for a beach day etc in a freelender with the rear seats down!

Why trailer a SIB when you could be trailering a RIB? When it comes to trailering is either trailer your 2.7 or 3.2 Honwave for instance which is nice to do which is often part time trailering for a summer holiday etc and not fine full time, (it's getting ready and testing yourself for the RIB plunge lol) for instance then such folk often going back to packing it away in the vehicle for shorter outings, or trailering full time in which case (other than the zodiac fans) a RIB would be the trailer choice, I know people trailer a honwave 3.2 but many many more have it in their boot! I see more RIBs in the 3-4meter size trailered than SIBs!

[CHRIBS]

Hey guys thanks for all the help. I will try and take a video when I’m out of it doing the fault. I am also going to tighten the castle nut and adjust the trim as well so fingers crossed but will definitely put video and pics on