36 foot, $300,000 boat porpoises to no end

At my wit's end here! Owned this RHI (that's rigid hulled inflatable) for almost a year now. Supposedly a great reputation for handling rough seas..... NOT. Tell me, how did the manufacturer screw up their BOAT DESIGN to impart this problem unto me.

This boat is a porpoising backbreaker and so far our only method of enjoying an acceptable ride is FULL application of 18x12 trim tabs... which puts a damper on economy and makes for quite a bit of spray on a windy day.

A few particulars:
Length: 36'
Weight: 13,000lbs (no fuel), 18,000lbs (17 people, full fuel)
Power: Twin Yamaha 350, 30" shaft
Deadrise at transom: 23 degrees
Bracket: Armstrong, offset from bottom of the hull by 3 inches.
Fuel tank: 400 gallons, 12 feet long.

Here's a video of the boat's performance, no trim tabs engaged:


Boat was originally furnished with the 30" shafts mounted on the lowest mounting hole. We've since raised them 2 mounting holes for zero to nominal improvement. We originally had 15.5" 3 blade props. Thought the 16" 4 blade was gonna be a sure improvement. Maybe a little. We moved a life raft from the far aft to the top of canopy forward. Thought that was going to be the ticket. Just a little bit of improvement.

The boat does not seem to be stern heavy (see first photo). It rides poorly whether there is 2500lbs of passenger payload in the front or not. It struggles mightily to hold onto a plane, only assuming a proper running posture in excess of 30 knots. There is no mid-range fuel economy on this boat. It's a flat curve. Take a look at these depressing numbers from my own personal test:



My suspicion revolves around this Armstrong bracket... this is the first boat of its class ever to be fitted with one. All Ribcraft 11 meters that exist in the UK have either transom mounted outboards or inboard diesels, and allegedly, do not have the problem I have.

Ribcraft USA, the manufacturer of my particular boat, has been content to suggest to me that trim tabs are the solution. Meanwhile, we are getting .7-.8 MPG on a class of vessel which performance bulletins suggest should be achieving at least 25% better than that.



Here is a photo of where the motors sat when they were mounted on their lowest hole position:



My ideas for improvement at this point involve either redesign of the bracket to be a hull extension or a dramatic change in motor height with the current bracket. Armstrong insists though, the bracket was designed for 30" shafts, so I can only go up 1 or 2 more holes.

Again, we are dealing with two problems here:
1. Porpoising
2. Fuel economy/inability to plane until 30 knots (unless full tab is applied)

Lastly, I own a 24' and 30' Zodiac both of which ride and perform flawlessly, so I have a benchmark.

Would appreciate any input as to why these performance issues are occurring and what can be attempted to fix them.

Thank you

 

Photos can be deceptive, but the engines appear to be too low. I'd suggest get a straight edge about 2 metres long, align it with the fore and aft, and place it against the bottom of the hull proper, and measure where it meets the outboard leg. It should be well under the cavitation plate, (not the splash plate above it) by at least a couple of inches. It does appear you have plenty of holes left to raise the motors.

 

Is it the Pods Fault ?
Should it have have such a sudden rise after the actual Transom, instead of continuing the keel line of the main hull ?
I didn't read all of your notes, but should the motor angle should be trimmed with the Props further forwards if the cavitation plates are full on ?

 

Does the boat make her designed lines at rest?

If the boat is below designed lines at rest, the transom angle is wrong and you would need to wedge. Not inexpensive mind you, so be sure, but Ribb ought to be able to tell you design lines and you just mark them and check on the dock, so it costs nothing. If you trim the motors up and porpoising worsens; this is a good bet. If it stays the same with a slight trim up; less likely.

But with the 1500 pounds back there; it is wise to check. And if you are below design lines; one must ask why..

And let's be honest here, needing 30 to get out of the hole is horrid, so I would ask for a lot of prop recommendations as well. You probably already went to the four blade for lift, but the other typical remedy is to then also pitch down.

That boat ought to plane on one engine and you need to plane earlier. You may still experience wot instability, but designing for better hole shot ought to give you more stability before porpoising begins.

Finally, I had a boat that would always porpoise and never had great hole shot and I added hydrofoils. The hole shot was greatly improved and the top speed suffered about 3 mph and the boat stopped hopping. They are an inexpensive, albeit bizarre, solution. The difference can be remarkable. If you go this route, then pitching up might be needed. I realize I have presented two opposing solutions, but hydrofoils are drastic eh! I do not like the ability of the foils to grab weeds. My helm response also improved with the foils, really changed the boat.

 

by the way, when you check design lines, check the bracket as well as the transom to make sure they agree as a matter of semantics, and at rest would be no load

Do you have different results with passengers?

I am a bit curious as to trying to design for the passengers, but then reporting the data from a single man.

 

OK, I see there are before and after photos of the engine height, and they have been raised. If the tabs have stopped the worst of the porpoising, I'm not sure there us a lot you can do, except maybe go up to the last hole, if there is one left. Engines too low drag badly. You have a couple of heavy engines perched out there, and nothing in the way of support under them, and the beam of that boat at the chine does not look generous to me, to be tolerant of weight aft. Additionally, your long fuel tank may be causing a shift aft in COG as it drains. You might have noticed that.

 

Also, find a prop shop that will work with you and test pitching down.

 

That pod is creating problems for the props. In your last pic. it becomes clear that the bottom panels of the pod have a negative trim angle compared to the hull bottom panels. As long as the flow attaches to the pod bottom, there will be a zone of low pressure which tends to increase the angle of attack ("aoa"); and increasing propeller submergence. Now the ventilating air flow into the propeller disc is cut off, leading to increased propeller thrust, which leads to acceleration and the hull (including propellers) rising closer to the surface.

At some higher speed, the sides of the pod become unwetted, allowing ventilating air into the propellers, changing their mode of operation into a ventilated semisubmerged state. In this mode, there is an increased vertical lift from the props and a reduced horizontal thrust, adding lift at the rear, thus producing a nose-down torque, and causing a deceleration. Since the propeller thrust curve is non-linear in ventilating mode, the instability leads to periodic nose-up/nose-down movements.

In your video, filming the wake, you can see the antiventilating plate emerging when the nose goes down. This is where there is a sudden increase in hull drag, at the same time as the propeller ventilating is increasing, leading to a thrust reduction and your head is going forward.....

The pod bottom should be parallell to the hull bottom, possibly with a 50 mm step at the transom. With this in place, you will have far better options to find a suitable propeller shaft position. I'd guess that you may find a suitable engine height with the ac plate roughly 25 mm above the pod bottom level. Use a long ruler, place it along the hull bottom and see where it meets the outboard leg in the present configuration, and send us a picture, please.

 

You are pretty angry about this.
Must be frustrating.

Did you have the opportunity to test drive this $300K boat before you bought it?
A year you've been struggling with this.
It does it with, or without, passengers.
Were trim tabs fitted to help or were they already there?

Engine trim alone should eliminate all porpoising.
Who put that pod on and why doesn't it simply extend the hull lines?

 

Thanks all, for the responses, questions, and answers thus far. I appreciate it.

This company which makes the brackets, Armstrong, touts their offset and sloped design as an improvement on a simple hull extension bracket. Apparently it works well for other boats. I did not really have a choice/trusted the builder when they sent me this boat with this type of bracket furnished with it.

Are the "design lines" from the drawings? Here is the drawing of the bracket dimensions and angles. I not know off hand what the angle of the boat's transom is:

True.... the beam of the rigid hull is narrow to support the payload. And also the fuel tank I believe does cause a COG shift aft when we are having to plane up at a steep angle. I believe those factors exacerbate the additional problem as it relates to the mounting of the propulsion.

Thank you! This is a logical description of the possible problem occurring from a physics perspective. So the bracket is causing some sort of suction or increase in aoa at the point of attempting "rotation" from low speeds to a planing speed. Hence why a full 2,000 of RPM is wasted trying to free the bracket from the bad flow dynamics. The two Zodiac boats I own have outboard brackets which are totally parallel to the bottom and we don't have this problem at all.

I would like to be able to provide you with the picture requested but I don't own a trailer for this boat; the service yard charges me to use their quad axel trailer when I pull the boat for impeller changes. So it may well be a month before I'm able to do the ruler and hull bottom demonstration.

No test drive.. the boat was built specifically in accordance with the USCG regulations for passenger use, hence, deposits were required at various stages of construction since it wasn't just a factory production boat, it was a special order. Also this was the first 11 meter produced in the United States by this company. With passengers and payload the porpoising does not go away. Trim tabs did not come standard with the boat. I first applied 14x18 tabs, we later went to 18x12 for an improved aspect ratio. The Armstrong pod allegedly has a great reputation among other applications and so the builder commissioned them to build the pod for this boat.

I'm not sure what it means to observe a boats "designed lines at rest". I also forgot to mention we added -2.5 degree transom wedges for seemingly small improvement but nothing major. Trimming motors certainly makes porpoising worse. The props I feel are good, we make the manufacturer recommended top RPM of 6,000 and propping down would put us over redline. I like the large diameter and four blade in this twin engine application. Agree on the hydrofoils helping but most manufacturers do not recommend them for an Yamaha 350 because of the large loads placed on the engines because they power very heavy boats.

 

I feel for you.

I had a custom pod made by Armstrong for my 25HP houseboat years ago,
and they completely screwed it up to the point I didn't pay for it and sent it back.

I have thousands of hours driving Zodiacs and Zodiac style boats and
I've never seen one where the pod didn't follow the hull lines flush.
Any chine hopping or porpoising could always be eliminated by
adjusting engine trim angle.

I would think the manufacturer is on the hook here to help you fix the problem.
If they won't, I'd go to the media and embarrass the hell out of them.
That should get some action out of them.

Cruising in economy mode should give double the speed you're seeing in the
3000 - 3500 RPM range.

 

Call up both companies separately from "one party consent" States (Hawaii sounds good for this) Recording Phone Calls and Conversations - 50 State Survey :: Justia https://www.justia.com/50-state-surveys/recording-phone-calls-and-conversations/

Pretend to be Chinese or something and absolutely loaded with money but pretty clueless about boats, and obviously "bitten by the boat bug", and after making billions in construction in China and Dubai, you want to buy a FLEET of large RIBs for your island estates. Get them to transfer you to "the boss". Ask them about their "commitment to customer service and satisfaction" etc. Guaranteed they will start saying things like "we have always had a 110% commitment, if for ANY reason a customer isn't 110% happy with a custom build, we'll buy it back for 110% of cost", etc. Then let them know who they are really talking to, and that they are live-feed on Youtube.

Ex-employer pulled similar stunt over his warranty on his F-350 A/T. He goes into the dealership shop and asks about getting a cheap rebuild A/T (knowing that is what they will try to pawn-off on him if THEY are paying). Salesman goes into full up-sell mode and tells him "we don't waste our time or yours installing that junk, etc, brand new factory Ford is the ONLY way to go, etc". Then he breaks the news that he has an extended warranty that covers the A/T. He said the salesman suddenly didn't look to happy. This a guy that buys a brand new top of the line truck every 3 yrs like clockwork from this same Ford dealer, and they still try and cheat him every which way they can.

Tell these boat companies you will be "back charging" them for R/D costs, since they seem to be using you to fund their experimentation program. Did they have you sign a bunch of extra papers saying they don't recommend this setup, and you are taking full responsibility for going against their recommendations etc. Such is pretty common and proper in construction. Recently saw one about creating a tripping hazard because a client wanted rather rough (but really cool looking) flagstone on a walkway.

PS- did "specifically in accordance with the USCG regulations for passenger use" have anything to do with the non-flush pod design?

Good news and bad news: Bad news is with two companies involved both will blame each other before helping you. Good news is if you sue both companies, they will have to spend big money suing each other if they don't help you. I'm pretty sure you could sue each of these companies for $10K individually for total of $20K between them "on the table" in Small Claims, but probably better to sue for whole $300K+ in real court. Only after they get the court papers will they help you. What would cost be for whole new pod, and sea trials?

 

I'd disagree about the pod being the problem, there are thousands of boats with the negative angle of attack pods, the idea being to reduce wetted area and have the props working in "clean" water, less affected by the hull, for an increment of extra speed. It is possible that the pod is set a little too low, but I think the real problem here is weight, there is a lot of it, being carried on a narrow, deep-vee bottom, which explains the high speed to hold plane, and the need for tabs. The COG being too far rearward is probably also a factor. I'd be interested in the distance from chine to chine aft, 18,000 lbs on a boat that length can't be supported happily by a skinny deep vee bottom. You are being forced to run too high a cruise speed to keep it in top of the water, and your fuel figures are bad as a result of that. You could switch to a pod that is just an extension of the bottom, but I'd say make it the full width of the hull, if you do. Whether that will be enough, I would not be sure about.

 

What he said!

That pod/extension, is an abomination on that hull.

 

The pod is not "creating problems for the props", the problem is a heavy boat on a skinny deep-vee bottom, and the LCG probably too far back, with that pod that provides no lift underway. The majority of bolt-on pods are of the reverse angle type.