Strange problem with SIB hull/ Keel ....?

[TheMadMariner]

I've been chasing this ghost for a year now, and it's driving me nuts.

It's about an ~ 17 feet military grade SIB, made by Germany-based DSB GmbH.
It's a SIB with the usual inflatable keel and floorboards. All boards are original and in good shape. Now for the special part; the floorsystem consists of two more rubber-coated boards which you attach to the transom and pressed to the hypalon hull.
Together, these two boards form a upside-down V, pointing forward. Object of this; is to rigid the hull at this point. The inflatable keel is then placed on top of that, and the SIB is further constructed as any other.

Now; when inflated correctly and mounted with a 40 hp Merc this is a video of the performance;

zephyr+ vlinderplaten steiger - YouTube

So it planes pretty well, but once up, the thing starts an irreversible cadence in which it keeps bouncing up and down. Remember, this problem occured a year ago and I have since then tried and tested all the obvious; weight distribution, motor trim, motor mounting height, varrying the pressure of the inflatable keel, countless checks of seams, reattached the keel to the hull which had let go at some places. All to absolutely no avail. The problem seems really basic and seems to be lurking in the very structure of the boat....

Now, after spending some time vertically under the trailer, it dawned to me that the hull doesn't seem right. It doesn't look like it's been stretched very well, what normally the inflateble keel will take care for I think ....?
Furthermore, at the point where both rubber-coated boards meet and form the V, it seems it is pushing the hull too much, creating a concave.
See pictures attached:

I got few tip's from the only official dealer here in Holland, but he couldn't suggest any instructions specific for this type of boat.

So this quest is still unsolved... and becoming an increasingly frustrating pain in the arse for me.
Any, (and I mean any) hints and tip's are welcome!

Thx,
Chris

[captnjack]

The bouncing up and down is called "porpoising". Its a fore-aft dynamic instability similar to the port-starboard instability called "chine walking".

The concave bit of the hull is making it worse. What is essentially happening is the center-of-lift when on plane is shifting fore and aft across the concavity. The concave bit of the hull makes this worse since the center of lift won't smoothly transition across this part. Every boat has a shift (aft) in the point of lift as speed increases, its normal. The concavity is interupting this though, making it abrupt and leading to the propoising. Your task is to keep the center of lift from moving across this section of the hull (solving the problem although it may create new ones) or slowing down how fast the center of lift moves aft so it transitions across the concavity slower and/or keeping the center of lift on the aft side of the concave bit.

3 possible solutions:
1) switch to a prop with more stern lift (light weight but expensive, hard to experiment, may not be available for a 40hp)
2) add weight to the bow (sand bag, cheap and easy to try, but annoying and heavy)
3) add a hydrofoil to the engine cantivation plate

Basically you are looking for slight downward force on the bow to keep the boat from riding up out of the water as the center of lift moves aft in spurts then crashes back down again.

[bigscott]

Hi chris.

Im no expert but I would say theres too much weight close to the transom and not enough at the bow. When I dive off my Bombard I often have all the weight at the back when im making a short hop to the next reef, and it does the same as yours in the video

[TheMadMariner]

Originally Posted by captnjack

The bouncing up and down is called "porpoising". Its a fore-aft dynamic instability similar to the port-starboard instability called "chine walking".

The concave bit of the hull is making it worse. What is essentially happening is the center-of-lift when on plane is shifting fore and aft across the concavity. The concave bit of the hull makes this worse since the center of lift won't smoothly transition across this part. Every boat has a shift (aft) in the point of lift as speed increases, its normal. The concavity is interupting this though, making it abrupt and leading to the propoising. Your task is to keep the center of lift from moving across this section of the hull (solving the problem although it may create new ones) or slowing down how fast the center of lift moves aft so it transitions across the concavity slower and/or keeping the center of lift on the aft side of the concave bit.

3 possible solutions:
1) switch to a prop with more stern lift (light weight but expensive, hard to experiment, may not be available for a 40hp)
2) add weight to the bow (sand bag, cheap and easy to try, but annoying and heavy)
3) add a hydrofoil to the engine cantivation plate

Basically you are looking for slight downward force on the bow to keep the boat from riding up out of the water as the center of lift moves aft in spurts then crashes back down again.

Thx for your fast response! What you say certainly makes sense;

- I have yet to try a hydrofoil.
- Are these stern-lift prop's a special kind or just the right alloy and pitch? I am currently sporting a 14" pitch alu.
- I have tried to add weight to the bow, by directing my fat friend to sit in front. The porpoising stops as expected, but adding throttle won't add speed then.
To the contrary; when trying to hit max RPM then, it does result in the prop starting to ventilate violently, losing all grip. It does that at aprox 5000 RPM at a speed of 20/ 25 miles, which is pretty crap performance. i.m.o.

My theory here is that the obstruction to the hull (possibly the concave) is such extremely it can't be overruled and is just getting worse when adding more force (=throttle)
When porpoising, I can spot the water spraying from under the hull going directly sideways. Most horsies the Merc produces is wasted on this obstruction, I reckon.

But even so, I can't possibly imagine how I should rectify the concave....?

[jyasaki]

Are you inflating the keel to whatever it is spec'd for? Tubes as well? Insufficient pressure dow weird things to SIBs, and pressure may alleviate seemingly big problems to less noticeable ones.

Peter Chandler had a similar problem last year, and I think it came down to a soft keel tube. Might PM him and see exactly what cured his problem.

jky

Sorry; Should have said that Peter's username is Peter_C

[Slaphead]

I have probably what is the equivalent of your sib called an Avon CRRC 520 which is a heavy duty MOD/Navy type with inflatable keel in the form of a long bladder under a slatted aluminium/hypalon roll-up floor which gives the outer hypalon floor a V shape. The service speed of these boats is around 20 knots with a 40hp outboard. Bear in mind that at speeds above this the sib hull will form a gull wing shape rather like a Shetland cabin cruiser and try to take off Donald Campbell style. The boat only weighs 180kg and so if you try to go faster than what the boat is designed for you are going into untested waters and interferring with the aero/hydrodynamics. When it starts trying to flip, the boat is telling you to ease off the throttle.

[C-NUMB]

Hi,
How did the engine height impacted on the porpoising? And how high was it at is highest?
Sometimes playing with various propellers, pitches, can reduce or solve the issue. A hydrofoil is a pretty cheep so i would start there together with borrowing various props.
By the way, what happens with more cargo, lets say 4 persons on board?

[captnjack]

Originally Posted by TheMadMariner

Thx for your fast response! What you say certainly makes sense;

- I have yet to try a hydrofoil.
- Are these stern-lift prop's a special kind or just the right alloy and pitch? I am currently sporting a 14" pitch alu.
- I have tried to add weight to the bow, by directing my fat friend to sit in front. The porpoising stops as expected, but adding throttle won't add speed then.
To the contrary; when trying to hit max RPM then, it does result in the prop starting to ventilate violently, losing all grip. It does that at aprox 5000 RPM at a speed of 20/ 25 miles, which is pretty crap performance. i.m.o.

My theory here is that the obstruction to the hull (possibly the concave) is such extremely it can't be overruled and is just getting worse when adding more force (=throttle)
When porpoising, I can spot the water spraying from under the hull going directly sideways. Most horsies the Merc produces is wasted on this obstruction, I reckon.

But even so, I can't possibly imagine how I should rectify the concave....?

You'd want a prop with the blades raked aft less. You'd have to ask a propellor specialist/supplier if there's even one available for a 40hp with less rake than you have. Most people are trying to raise the bow, you are trying to lower it.

Your heavy friend did the job it seems, and yes with more wetted hull in the water top speed will be less. Every solution to propoising will involve getting the bow down and slowing you down. Some slightly more than others.

Originally Posted by Slaphead

I have probably what is the equivalent of your sib called an Avon CRRC 520 which is a heavy duty MOD/Navy type with inflatable keel in the form of a long bladder under a slatted aluminium/hypalon roll-up floor which gives the outer hypalon floor a V shape. The service speed of these boats is around 20 knots with a 40hp outboard. Bear in mind that at speeds above this the sib hull will form a gull wing shape rather like a Shetland cabin cruiser and try to take off Donald Campbell style. The boat only weighs 180kg and so if you try to go faster than what the boat is designed for you are going into untested waters and interferring with the aero/hydrodynamics. When it starts trying to flip, the boat is telling you to ease off the throttle.

Agree, my Bombard C3 was the same way. They are rated for quite a bit of horsepower relative to their weight to be able to push a heavy load of people or gear, not to go really fast.

[m chappelow]

Looking at the video it just looks like you need more weight at the bow and as was also said earlier you will only get about 20 knots out of it top speed .

some large sibs heavy duty workborts/military specs ect are designed for long shaft outboards especially if its a deep keel right to the stern/transom,
though the engine leg /cav plate may look the right on the wood transom the under water profile of the keel may be much deeper.

another cause is that it may have gone out of shape for some reason ,(hogged)or sat on trailer and been full of rainwaterfor months on end causing the bottom fabric to stretch and to go concave when in the water .
one uk rescue organisation used to put their sibs on a jig each year to see if the hull was out of shape.

[TheMadMariner]

Thank you all for your thoughts on this, I really appreciate your effort.

1) Pressure
Alas, the problem can't be solved by correct pressure, the outer tubes are pressured right, and the keel has been pressurised both above- and below manufracturer recommendations. The porpoising stop's when slightly deflating the keel, but getting on plane does take considerably longer and feels like pushing a barge. It just doens't get better.
Once on plane with the soft keel, the prop will start ventilating when pushed above ~ 15 knots. Inflating the keel above recomm. pressure will only worsen the porpoising effect.

2) Prop's
Changing prop's certainly will change performance.... slightly.
I have tested a 11" pitch prop with a 4/5 man crew. Motor does get to it's rev's, ansd boat behaves pretty normal, speed roughly estimated at 12 knots.

3) Speed,
The SIB is pretty rigidly build and weighs a bit more then average. Despite this, I was once easily overtaken by a 7-feet something SIB with a 6 hp. I could hardly hold back my tears at this point...

From military specs I found this boat would be capable of 30 knots, and around 20 knots with 6 man/ 600 kg on board.. might even be measured in the rough seas they use them in all the time.
I am aware these aren't build for speed, but seeing these specs, mine should perform better then it does now...

But here's a twist;
- What if I construct the boat without the hull-boards so it would be like any other SIB, which I perfectly can, according to the dealer.

So I did, and here's a small vid of the test run:

DSB504-zonder-vlinderplaten.wmv - YouTube

Especially the last bit, you can see the awkward effect how the bow is pushed down in the water more then the stern is. In the vid he is revving to ~3500 RPM with a 14" pitch prop. More throttle will result in prop ventilation, and a effect that I can only describe as the SIB desperately wants to dive in the water bow-first. Reducing throttle and lowering speed and the boat will recover from this. Speed are simular to other construction.
Again; changing trim&mounting height won't make difference here.

My personal opinion about this; it must be the same problem, only with two appearences, the hull-boards making the difference here.

Any thoughts...?

[TheMadMariner]

Originally Posted by m chappelow

...another cause is that it may have gone out of shape for some reason ,(hogged)or sat on trailer and been full of rainwaterfor months on end causing the bottom fabric to stretch and to go concave when in the water .
one uk rescue organisation used to put their sibs on a jig each year to see if the hull was out of shape.

BINGO; when I first bought it, I took out all the floorboards for maintenance, leaving the hypalon fabric half inflated on the trailer. It could well be hogged by during that period.
Didn't know the fabric could even be stretched like that. But now you're saying...

Is there a way to test it, and how can it possibly be restored?

[TheMadMariner]

Has anyone experience with the hogging & bogging phenomenon/ overtstretched hull on SIB's, and can it be fixed?

I am pretty sure that is my problem, considering the slack in the hull when fully inflated..

[C-NUMB]

Originally Posted by TheMadMariner

Has anyone experience with the hogging & bogging phenomenon/ overtstretched hull on SIB's, and can it be fixed?

I am pretty sure that is my problem, considering the slack in the hull when fully inflated..

Not 100% sure i understand the construction of Your boat. But if the issue is that there is too much "slack" in the bottom fabric, you might try to tighten it? This by adding some material between the fabric and the wooden V pices. If You have a few of those closed cell sleeping pads try to put that stuff between. No idea would it work but at least the fabric would be tighter and a the test is cheap?

[frankc]

Years ago, the fabric stretched in our Avon with a wood deck and performance became eratic. While considering a replacement boat, a lady at Achielles, who was familar with that type of problem suggested I replace the front floorboard section with a longer piece to make up for the stretch.

Put in a new floorboard section and the problem was solved. Have no idea how your floorboards fit, but a loose bottom will ruin the ride of a boat. It takes very little stretching of the fabric to cause the floorboards to require attention.

[TheMadMariner]

Thank you lads for all input!

In the last 24 hours, I've received more and better tips from you guys then I did in all phone call's to both prof workshop's and dealer's. That's why I love this forum.

@ frankcc, that's definatly worth trying, because when I take out the front thrustboard and decide to go for a ride, the boat is porposing just as bad as he would with the front board installed.
Save to say the current thrustboard isn't helping in stretching the boat.
But how did you figure out how much length you needed to make up for the stretched hull?

Thanks,
Chris

[sinner]

So, the motor height (cav plate even with transom bottom) and changes in trim had no change? In your last video I looks like the trim needs to be a little more out, away from the transom to lift the bow a little. Unfortunately for us sib owners, it is necessary to change our trim very often as out load changes. Also, as stated earlier, if you don't have a gauge, get one as tube and keel pressure can change performance dramatically.

(I also have a bombard C3 like Captnjack, but have no performance issues. It has a tohatsu 40hp (maximum rating) and the thing screams! I haven't personally clocked it, but the guy I got it from swore he and his friend clocked it at 40 mph with a gps!)

[TheMadMariner]

@sinner,
I wish it did made a difference, but I have mounted the motor small bits higher and tested everytime, up to 1 inch above center-line keel. Changing between the trim setting with every correction. It all didn't made the difference I hoped for, and did not stop the porpoising. Also the weight distribution; been there done that, to no avail. You can repress the purpoising, but that simply worsen the performance.
Hope to reach similar performance of your set-up, I believe the topspeed of mine could well be ~32 mph with the right prop.

Next stop is the forward thrustboard, will keep you guys up to date on that....

[frankc]

MM, built a mock up from hardboard too long on purpose. Used the front shape of the old floor section to capture the correct curves. Then by trial and error cut the length of the hardboard until it appeared to fit too tight. Then used the hardboard as a pattern to cut the marine plywood which was then trial fitted and shortened as necessary to insure the boat bottom was stretched tight.

The handling improved to what it was when the boat was new. We still have the boat and many years from making the floorboard, the performance has not changed. I may be wrong, but after the stretching of the bottom initially, it does not have the capacity to stretch much now.

I would recommend something like this for your craft, as a loose bottom is the pits.

[sinner]

Originally Posted by frankc

....

I would recommend something like this for your craft, as a loose bottom is the pits.

No one likes a loose bottom!

[TheMadMariner]

Results incoming!

Today I achieved to extend the floor the lazy man's way; by jamming an 1 inch thick piece of wood between the two forward floorboards. (see 1st picture below)

After inflating the thing, the hull sure did seems more rigid then before.
I decided to go for a testride.

And, to my genuine surprise; the boat planed and didn't porpoise! Throttle up to an incredible estimated ~ 25 mph, @ 3500 RPM, before it start to bounce once again, but less violent then it would otherwise.

As you can imagine, I am one happy bloke now
It might be some work to achieve the exact lenght to make up for the stretch, but we're finally on to something.
I might mount the motor higher up a bit as well, to add performance. (see 2nd picture)
Any thoughts on that?