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Old 14 October 2011, 18:03   #1
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Strange problem with SIB hull/ Keel ....?

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;



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
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Old 14 October 2011, 18:36   #2
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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.
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Old 14 October 2011, 18:46   #3
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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
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Old 14 October 2011, 19:42   #4
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Quote:
Originally Posted by captnjack View Post
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....?
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Old 14 October 2011, 20:42   #5
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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
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Old 14 October 2011, 21:04   #6
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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.
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Old 15 October 2011, 02:36   #7
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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?
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Old 15 October 2011, 02:49   #8
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Quote:
Originally Posted by TheMadMariner View Post
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.

Quote:
Originally Posted by Slaphead View Post
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.
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Old 15 October 2011, 06:02   #9
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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.
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Old 15 October 2011, 07:09   #10
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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:

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...?
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