New 6M HT boat porpoising

Hi there Guys
Can someone please give me some insight on below...

I have just bought a brand New Atomix 600 HardTop Fibre Glass boat which
I have had fitted with 2006 Suzuki DF140 4 Strokes. The Hull is about 950kg's with deadrise at transom 23degrees and transom angle 16degrees, has a deepV Hull.
Here is the issue I am having....Dead calm water conditions she goes awesome , gets up to 60 odd klm/h but the minute I hit abit of chop she starts to bounce and if I do not reduce speed the bounce gets bigger... I reduce the throttle and she is ok. I have had this boat in water a few times now and have tried trim in and out with tiny adjustments and a lot of patience to see if any improvement... no joy
The boat manufacturers has totally ruled out weight distribution and design issue as they many of these boats out there since 2009 and also pair Mercury 150 4S with Enertia prop to these hulls which performs perfect. The Mercury 150 4S is 15kg heavier then my DF140 so this why Atomix has ruled out weight distribution issue. They have had this boat up with them for a week or so and they have cleared the Hull from having any issues and they are happy with how they have installed the outboard (Set up). They tell me I need a prop with bigger CUP similar to Mercury's Inertia 3 blade prop which is 14.5 x 17 SS they say this prop has a big cup. Unfortunately the Mercury's Inertia does not fit Suzuki DF140 due to Suzuki having smaller Hub diameter.... been tried with the fitting kit.... no joy..... Hub and the blade in the prop are too big.
They are telling me that my factory prop is not managing to hold the bow up or give the transom a stable lift.
I spoke to a couple of propeller guys here in NZ and their initial thought, I have weight distribution issue..... Which I believe is true as if no one sits at the transom seat but stand inside the cabin by the helm.... she is ok (re photo of the boat).
I am very confused....Could propeller make this difference and what do you suggest.
My factory Suzuki prop is 3x14x17 and on this blade she sits @6000 which is within range. I have also tried a 3x14x21 Suzuki prop which did not help.... the revs dropped to 5000 (Suzuki DF140 rev range 5600 x 6200) and she porpoised as usual.

I am not a newbie at boating and have owned a couple of fibreglass boats.... Have never encountered this issue. Love fishing so have being out in all sorts of condition over years.

The sweet spot is a touch up (Maybe a inch or so) from being all the way down but the minute I hit a tiny bit of chop or if someone is sitting by the transom she starts to bounce .
Can anyone help.... please advice with your thoughts.
Regards

 

Propellors are low down on the list of causes of boats porpoising. Judging by the photo, the boat does not appear to be sitting low at the stern. So seemingly weight distribution is within bounds. Is the motor height "right" ? I doubt you'd be getting the 6000 rpm if it was too low, which might contribute to porpoising. Maybe the hull is just a little more bouncy than what you are used to. One thing that surprises me is you say it porpoises worse in chop, this has not been my experience with boats porpoising, it is usually worse when there is just a little gentle undulation on the surface. You are not confusing porpoising with vertical accelerations due to waves ? I can't imagine what the cause might be, if it is classic porpoising, but if trim tabs can't get rid of it, nothing will, that I can think of. A different prop could give some extra lift, but tabs will give a lot more.

 

Welcome to the forum.

You need to truly identify what's happening, before you start throwing parts at it. That is about the max for the boat and you should be hitting around 62 - 65 KPH with it.

In spite of what the boat dealer is telling you, I would consider moving a some weight around to see if you can get her CG in a more optimized location, because it sounds like you are "driving over center" with this boat at WOT and in a chop.

Yes, a prop change with more cup can help, though it's more than cup that will help, it's likely both rake and cup that will satisfy the boat's needs. You should pull the full prop spec's on the Murcury prop, including cup and rake and see if you can get similar with the Suzuki.

I don't know what's available in your area, but I have a prop guy that I see when I run into these issues. He'll drive the boat, then have me drive the boat past him as he stands on a dock taking pictures of the boat underway, blasting past him. Afterward, he can tell what the boat is doing, (trim angle, squat, etc.) and a reasonable estimate of a prop can be made. This often means a number of props are tried, until he "dials it in", each having a blast past the dock with him aboard and ashore, checking the same things he previously did. Most of the time he guesses right and matches the prop with RPM after his first "guess". I say guess, because this is accurate, as fine tuning, especially if there's a problem, is about experience more than math. The math will get you close and most boats can live with close, but if there's a need for fine tuning (like your boat), you need a real prop guy, to spend half an afternoon playing with it.

 

An "interesting" critique of the hull here:
http://www.boatadvice.com.au/atomix-600cc-review/
It is a relatively lightweight and deeper than usual vee at transom hull, with a reverse chine. I am wondering if there is some convexity to the bottom sections forward. A bit hard to tell from the pics, but I'd rather not have that.

 

Light boats can have their CG altered pretty easily, so again, move stuff around and try the boat in various trim condisions. Also, if you drive a light boat too hard in rough condisions, comfort and ride will go down the dumper, unless you back off the throttle to suit condisions.

 

Thank you Sir,
The boat went back to the dealer and they had her for a week or so..... They have confirmed that the settings are all as per what it should be and the change of prop is what is required.....unfortunately for me they not interested in help get the right prop and doing the trials.
Inhave spent a good amount of time trying on water.... Looked very carefully the conditions, speed , wind and can for most definite confirm that she is porpoising... Infact a couple of times she porpoised in calm-ish condition while going against the wind... Not dead calm but a little ripples.
I have a 4 Blade pro pulse prop coming to try so will see how she goes.
Regards

 

Thanking you sir, got 4 blade prop coming so will see how it goes

 

I wonder what the width is from chine-to-chine at the transom ? I take it the outboard is transom hung, and not set back on a pod ? If you have a relatively narrow boat with a podded engine, it won't help porpoising. One possible fix if your prop doesn't do the job, would be something like a Permatrim bolted on to the outboard.

 

Again, porpoising is not typically a prop rake issue, but a trim issue. Boat dealers don't have any idea about boats, other than selling you one. They can install stuff, if they're familiar with it, but for the most part, they really don't have much clue about diagnosing things, especially if it's out of their general wheelhouse.

Your boat is relatively light for is size and shape, so the prop might help a little, but I'll bet all you need to do is move the batteries (or something similar) to get the CG in a more conducive location. You've already had confirmation from your prop guys that they suspect the same, so why the need for the new prop, when moving around some weight is a lot cheaper? Bring aboard two concrete blocks, which weigh about 15 - 16 kg each. Place each up by the forward seating area and take her out for a good WOT blast and see how she acts.

A ventilation plate foil might help a little too, but again I'm guessing this is more a trim issue than propulsion. A "Permatrim" will help hold the bow down and make the trim a little more sensitive, but the changes will be modest on that type of hull. The same would be true of a different raked prop, which might hold the bow down a touch and help the boat "get up" quicker.

 

Thanking you again sir.... I will get her out again in a day or two and post with result. Thank you again for your time.
Regards

 

I think that ideally the dealer should go out in the water with you and show that the boat runs well.

 

The boat dealer has already made plenty of excuses, obviously is typical of most and doesn't have any staff, that can honestly diagnose the boats issues. I find this pretty common down here, where there's lots of dealers, but the real technicians are in the repair and speciality shops.

 

In my relatively limited experience of boats going in that speed range (and slower), PAR's advice is good. Try either the concrete blocks or a person up forward. In one case on a Boston Whaler/Dell Quay dory type, it required a person leaning forward over the bow half a meter to stop the boat porpoising!!!...
In that case it was a cheapo copy hull and twisted to boot.....

The genuine Whaler/Dell Quay hulls work properly throughout their selected power range in my experience, no porpoising and good stability through chop. Not had any problems with a number of V hulled RIBs' either. One thing worth checking is the exact angle of the fence plate of the engines, as this can do some odd things if not set right. It may prefer an 'in between' setting rather than one of the (5?) hole settings you usually get.

 

Fence plate ? What are you referring to, a splash plate on the motor leg ? I know sometimes having the motor too low can start porpoising, what the mechanism is I am not so sure about.

 

Yeah, it's a ventilation plate foil, which has been called a lot of things over the years. Without pictures and possibly some video of this puppy underway, it'll be difficult to make much more than basic suggestions. I would assume the boat dealer hung the engine with the plate in the appropriate location for this hull, but without pictures.