"This is what happens to an IPS propelled boat when it hits a rock!"

You can see tests of Zeus in a sandbar (soft) grounding and hitting a concrete pier at both low and high speed. I saw it at Miami and the video is now posted at http://www.cmdmarine.com/ , look for Zeus Video at the bottom.

Volvo made a serious error in the design of the IPS breakaway system. It does not work properly at low speed or in soft groundings. It can work in a high speed impact, but only if the boat hull is strong enough. If you ask an IPS rep, they will tell you that you need to be going over 20 knots. Problem is that you can sink the boat going slower than that.

Zeus was designed and tested to work in any situation. Volvo would never agree to a side by side comparison. It would make their mistake too obvious.

Hi,

Welcome to YF

I notice that in the video you provided the link to that they state the concrete obstacle height was "adjusted carefully" to ensure impact with the drive.

Such carefully controlled rocks are not that common in the wild so it still remains to be seen if these do really work as well as it says on the tin.

So how do you know all of this..? I am in the process of selecting a drive system and this is new to me.

Like the lawyers say, "Don't ask a question unless you already know the answer."

The videos that probably best reflect real life are the ones we will never, ever, see.

So call me skeptical ...

I asked CMD and Volvo a lot of questions at the boat show. They don't say anything about each other, but they are surprisingly forthcoming about their own drive if you are asking specific questions.

No drive can completely protect the boat - you could punch a hole through the hull without ever touching the drive. Zeus is tunnel mounted, so they don't project down nearly as much as IPS. That makes it harder to hit just the drive. Therefore the test was "adjusted carefully" because it isn't easy to hit them without hitting the hull first.

The most important thing that I learned was the 20 knot limit for IPS. If the drive can't break below this speed, then it either has to stop the boat or the hull will fail instead. Would you want to stop your boat in a couple of inches going 20 knots?

Actually, a dead stop from 20 knots is more common than many would care to remember. Just saw a 50 SR do it the other day right after he blew by me almost swamping me.

The hull is of course reinforced where the IPS is fitted, but I will check this with Volvo Penta on monday.

It even happens with jets. See post #39 here: http://www.yachtforums.com/forums/g...ag-harbor-3.html?highlight=Sea+Tow+Sag+Harbor

What do you mean by a dead stop? Stopping in a few feet is very different from stopping in a few inches - the g force is 10 times less. If you hit a granite shelf with the drive, it either has to break or stop the boat in the distance that the drive can bend.

Even the early magazine articles about IPS tests said something about the dummy in the boat getting thrown out. I think I would rather have the drive break.

That boat with the jets hit the hull first. Doesn't matter what drive you have in that case!

A 30,000 lb boat doing 20 knots has considerably less energy than a 60,000 lb boat doing 20 knots and whole lot less than a 15,000 pound boat so that 20 knots figure is meaningless.

A large heavy boat moving slowly might wipe a drive off the bottom without doing much other than shuddering while a lightweight speedster might come to an unpleasant halt at the same speed. I am curious how they derived that magic number and why it is being used.

It should also be noted that drives rarely hit flat walls. When the forward motion stops upward movement of the transom will likely begin.

I think the absence of flat walls is why the video showed the drive hitting an angled plate.

I do not know what boat weight Volvo assumed when they came up with that number, it is a good question. I do know that since IPS breaks at a higher speed than Zeus (CMD said something like 5 knots), you have to have a much stronger and stiffer hull to use an IPS drive than a Zeus. The boat pictured in the beginning of this thread obviously wasn't stiff enough and let the drive pull out.

From the layman's perspective, I'd expect that a 60000# boat and a 15000# boat would not only have different power, but differently spec'ed drives in terms of size, mass, and power handling capabilities. If a smaller drive didn't shear with less force against it than a larger drive did, I'd be left scratching my head over whether the smaller drive was overbuilt and thus caused damage to my lighter hull, or if the larger drive was underbuilt and failed at too low an impact.

The break away feature has nothing to do with the size of the unit. The unit is supposed to stay intact. It's the shear bolts that are supposed to do the break away. Theoretically Zeus' props being 3" higher in the pocket and facing rear should somewhat minimize damage over IPS from a hit that doesn't remove the drive, but an object is just as likely to come around the side of the cone to find the prop anyway as it is to gently roll under the skeg and safely out of the way. In fact encountering the spinning cone & blade tips may even be more likely to push an object off. (Just speculation).
My big concern is that partial disconnect and failure to seal. Possibly they need different strength shear bolts mounting the front and rear of the unit with the rears weaker.

I agree with Seafarer, it makes sense to have a drive on a bigger, stronger boat break off at a higher force.

IPS doesn't use bolts, it breaks through a large casting. I don't think they could easily lower the breaking force as with Zeus.