hello，we have a problem in stepped planning craft

It would be useful to have someone make a video of your boat while it happens. It should be made by a person standing on a dock, and the boat should pass sufficiently close to make well-visible the boat's trim, draft and conditions in which chines and steps work. Can you make and post such a video?

While doing it, you should take a precise note of engine rpm and boat speed. Without that, we can only do a random guessing of little use. And what engine does it mount?

 

Obviously my interpretation of "course instability" was, um, off track in this instance !

 

sorry, we have no video! I hope some one meet the problem(like your first video). and can give some advice .

 

Then I am sorry, it is very difficult to help you in these conditions.

Try to check if steps are correctly ventilated when the problem happens, an asymmetrical or alternating obstruction of ventilation intakes (vents) might be one of self-sustaining causes. That's all I can say at this point.

 

it's my fault。can you give me some reason leads to this problem.

 

I have sent you a private message.
Cheers

 

you will need to give a much better description than what you have written !!
is the boat rolling from side to side or wagging its rear end from side to side like a fish swimming !!.
I live in china so You tube is pointless give me youku instead !! or post me a reply and send video on QQ153771661 !!!

 

It is doing all of the above imo, fishtailing, chinewalking, which means there is no need for a cocktail shaker on board.

 

Typical of ventilated steps !! they really are a pain in the behind ! if its fish tailing id be looking seriously at the steering for slack and parts that are worn !! the rudders isn't doing its job and there should be trim tabs as well !! big ones !! Id be all for making an in fill and doing away with the steps of certainly reducing there size because they obviously not working properly !! at 38 knots who needs a ventilated bottom any way ??

 

Can chine-walking occur in a boat this size at just 20 knots ? I'd have thought it would have barely lifted out of the water at that speed.

 

I would take the 20 kts claim with a grain of salt. Jimhongfei has given us a range of speeds, 20-30 kts, which are imo a more attainable info.

A 4.6t boat, lwl 10.5 m (approximately) at 20 kts speed gives an L/D ratio of 6.3 and a volumetric Fn of 2.5 , which means a low planing speed. Perhaps even a semi-displacement speed, at that L/D ratio. A video or a photo of the boat running at that speed would be really necessary, in order to understand what happens to steps in that condition.

For example, I can imagine a following scenario (among several others):
- for some reason at some point a port or a starboard step gets ventilated, the other not
- the non-ventilated step creates drag and downwards suction
- boat starts to roll and yaw, further increasing the initial asymmetric step ventilation.
- the counteracting hydrodynamic "spring" (righting moment from the buoyancy and hydrodynamic lift) push the hull back to the other side.
- and so it goes on.
But it is just a conjecture, there is absolutely no reliable data in this case to support it (or to reject it). Several other factors can create a similar effect, a wrong weight distribution being one of them.

Cheers

 

fill them in and paint over it !!

why would any one with half an ounce of common sense what to ventilate a hull that big for ?? tell me ??? is its to reduce skin friction ?? is it to make the boat go faster ?? simply what ever the reason its not working is it !! like I pointed out fill the steps in and do away with them and I will almost bet the boats will perform much better .
need some close up pictures of the actual steps and also the rear of the boat with rudders and props etc etc

Picture taken of the steps that are in the mould could be a better option to look at because the forward sections could be lower than the after sections so filling to get to a fair shape could mean a lot of fill and fare work to be done even to the point of cutting the section out of the mould forward of the step to get a fare line to mate up the foreword and aft sections

 

jimhongfei, what is the deadrise angle of the hull bottom at the transom, and what is it at midship, or forward of the step?

The photos are tough to study, but it looks like the deadrise angle decreases from midship towards the transom. That is called a "warped plane" design and usually isn't very fast. 20 knots would be a good speed for such a boat. Does the boat not want to flatten out above hump speed like it should? The bow won't drop as speed gets higher? More power needed than you calculated? So you trim the drives down and then it gets unstable?

 

what ??

the guy that designed it should have his name up in big red lights warning people to keep away from him !!!! along with the guy that got sucked in to buying the thing !! its a case of cutting the hull at the chine and doing a complete rebuild !!

 

If it has only "moderate" deadrise at the transom, the 0.8m draft mark would indicate somewhat more than the 4.6 tonnes quoted. A 38' Cigarette weighs about the same and has less draft and a steep (24 degree ?) deadrise. All with only 8 feet of beam. Something doesn't add up.