Rodman 41 IPS drives

[mjf]

Over the holiday had a look at this craft.

Not sure if others have looked at the IPS fitted to a boat yet.

However I think Rodman have missed some of the point...or maybe manufactures have to build boats around the IPS from scratch and not simply fit this system to existing hulls.

On the Rodman the engines sit right aft with the drives where the steering gear would be on a conventional shaft drive boat.

Either side of the engines is void - completely wasted space.

The fuel and water tanks are under the saloon floor - where the engines are found normally.

So, you lose the big laz that is boat normally has.....

The IPS had better be v.good indeed to make up for the loss of important storage.

Anyone else seen this?

 

[DepSol]

Re: Rodman 41 IPS drives

Yes I saw the R41 at Ancasta and this boat was designed to take IPS rather then IPS just being slotted in. True you do lose the Laz but from what they were saying the handling more than makes up for it.

Makes docking a boat a piece of cake.

Should be trying one soon and let you know what I think. But for 300k inc VAT for a 3 cabin boat I think its well worth it.

Dom

 

[Deleted User YDKXO]

I agree with you entirely and said so at LIBS time. One of the big advantages of IPS is that it should allow boat builders to redesign the accomodation to take advantage of the fact that the engines are positioned further aft, something Rodman have failed to do. An example of what could be done is the new Sunseeker Manhattan 50 which, even though it doesnt have IPS, has its engines much further aft then normal which allows a full width master cabin to be positioned amidships - much bigger than the poky forward cabin normally found in this size of boat
I guess Rodman would say that the other advantages of IPS in terms of increased speed/economy and manouvreability are still present so the boat is still worth having. Personally I would'nt agree and, if I was in the market for a new IPS boat, I would wait 2-3 yrs for new designs to appear and, of course, for the technology to prove itself

 

[oldgit]

Maybe the drives must go right aft to get weight and balance correct.
This will then give all the usual downsides associated with outdrives.The centre of gravity will be well aft,at least if the drives and power units are towards the middle of the boat surely it would be better balanced ?

 

[Deleted User YDKXO]

You're right oldgit. IPS drives have to be placed further aft as the drive goes forward from the transmission so the centre of gravity of the boat will be changed compared to the shaftdrive version. As with V-drive boats the layout of other components usually changes in compensation eg fuel tank, batteries, gennie and aircon plants are often mounted forward of the engines rather than to the sides or aft

 

[oldgit]

Just an observation,but if the engines are mounted right aft ie.Z drives and out drives etc,then all the stuff needed to counterbalance it for instance gennys and other heavy stuff will have to go much futher forward.So ending up with two mass concentrations at both extremes of boat like a sort of seesaw.Add in a variable such as fresh/foulwater/fuel fuel tank and ? Did I not recall Kim mentioning a boat(B...w...d ?) which became an unstable beast if bow mounted fresh tank was full or something.That C of G needs to be in the middle.
So moral of story is.....Nice heavy midships engine with shafts ...........perfick /forums/images/graemlins/grin.gif

 

[Deleted User YDKXO]

The engines wont be as far aft as with sterndrive boats. In the Rodman, they were under the patio doors, so about 4 - 5' further aft. I was talking to the Rodman salesman at LIBS and he said Rodman had experimented a lot as to where to place the engines because even minor differences affected the trim and handling of the boat
IMHO, designers and builders will take some time to understand how best to fit IPS because I think the handling of an IPS boat could be very different to a shaftdrive one

 

[mjf]

Re: Rodman 41 IPS drives

On the boat I saw the drives were astern of the engines.

What I orginally thought was that the engines and drives would be where the laz is located (drives either in front or behind) and the E/R space would become a full width cabin with the tanks moved across the whole boat length. the saloon would have to rise a little but lots of additional space in a 40ft boat.

The Rodman has lost space............period!

 

[tcm]

not that much space saved, tho ?

um, ips saves the space of the gearbox - not the entire engineroom. Surely on the uk rodmans they'll seem a bit empty cos no aircon and gennies - which is wot the space at the sides of the engines is for? The idea is faster/more mpg, not lots of wine storage space for london types.

Altho the handling is of course affected by the position the engines, it surely can't be that super-critical in the scheme of things with a 1000kg+ of fuel and another 400+kg of water sometimes being there, and sometimes not. I mean, the ideal positon must be a compromise

 

[Deleted User YDKXO]

Re: Rodman 41 IPS drives

The drives are astern of the engines but the props go forward from the transmission rather than backwards like a sterndrive. The essential difference is that IPS pulls the boat forward rather pushing it. I agree with you on the current design of the Rodman. It has certainly lost space

 

[Deleted User YDKXO]

Re: not that much space saved, tho ?

I'm going to get flamed by some mechanical engineery type here but, IMHO, the forces acting on the hull are quite different with an IPS boat compared to a conventional shaftdrive one so, potentially, the handling could be quite different. The reason for this is that IPS pulls and shaftdrive pushes. If you consider the moment around the point at which the IPS drive is connected to the hull, it is in a clockwise direction (looking from the portside of the boat) effectively pulling the stern of the boat down and pushing the bow up. With shaftdrives, the opposite is the case because the shafts are always at a down angle, the effect being to push the stern up and the bow down. The second point is that, because of the duo prop fitted to IPS, I think the grip on the water of an IPS drive is much greater than shaftdrive so any forces acting on the hull due to the IPS will be larger than shaftdrive (if you think about it this is obvious because IPS boats are faster so the force imparted by the IPS drive must be greater)
The guy on the Rodman stand at LIBS told me that Rodman played around with the positioning of the engines a lot because not only was the fore/aft trim affected but the sideways trim was also affected when turning - basically he intimated that with the engines in a certain position the boat heeled alarmingly when turning
I guess these different forces can be counteracted by re-positioning of other heavy components within the boat but I would'nt be surprised if we get postings in the future on this forum from owners of IPS boats complaining about the scary handling characteristics of their new boats.

 

[mjf]

Re: not that much space saved, tho ?

I would not want my wine near the engines thank you!


I guess from memory that there is space enough for a/c and genny on the sides of the engines.

 

[tcm]

light flaming for Deleted User

ah, i can pass on jfm's flaming that he gave to me for having similar ideas

forget for a moment the fact that a prop is attached to the front of a shaft or the back - just think of the prop spinning. As it spins, the water in front is moved behind. It simply can't be "pulled" cos the water molecules aren't connected together very well, ie pulling a teaspoon of water out of a cup doesn't bring the rest of the wqater in the cup along with it. The water is pushed, whether IPS or conventional - the extra efficiency is cos of minimal underwater appendages and presumably cleaner flow on to the prop - no p-bracket.

Thus there's no partic different forces acting - just greater power delivered. Or (more likely) they can have smaller engines with IPS - and that actually is gonna be the "saving" - cheaper boats that still do a bit more or a bit less than 30knots, not 30 knot boats doing 40 knots. For this and above reason, no scary handling should result, other than er it might be a scary boat anyway.

 

[tcm]

Re: mjf\'s wine

very wise not to have that wine of yours near the engine, cos with it being in plastic bottles they would probly all melt, dya think?

 

[Deleted User YDKXO]

IPS Theory Revision No1

Yup, I remember this flaming from last time but actually it does'nt make any difference to my theory. Whether the IPS pulls or pushes, the moment acting around the point of connection to the hull is still in the opposite direction to shaftdrives because the IPS force acts through a lever arm but I now concede that this going to depend on where the CoG is. At least I think so. I'm digging myself a hole here so I'm ready and waiting on the BBQ to be flamed by more learned forumites

 

[Its_Only_Money]

Re: IPS Theory Revision No1

Well I think your comment has some merit - after all some of the efficiency gain of IPS/Sterndrive vs shafts is that the prop axis is parallel to the plane of travel so effort/energy isn't wasted trying to lift the boat through the vertical vector of the force up the shaft.

The vertical vector presumably will act upwards at the shaftdrive gearbox so if this is behind the vessel's CG then the effect is to lift the stern. Whether the vertical force acts by an amount that means your avg shaftdrive has a hull designed with this lift in mind I don't know.....if they are then would a hull have to be subtly redesigned to provide the missing stern lift to correct the trim change when IPS is fitted /forums/images/graemlins/confused.gif or maybe it gets lost in the grand scheme of moving all the weight around??

Agree push/pull makes no difference as you say.

 

[jfm]

Re: IPS Theory chapter 2

Deleted User I'm afraid you need to hop up onto the barbie . Leaving aside the fact the IPS propellor is vertical and the shaftdrive is angled a bit, which does have an effect, there is no difference in the drive forces or moment exerted by the drive onto the hull with IPS. There are of course local differences, as loads are applied to the hull where the drive gear is bolted to the hull, or on the gearbox of a shaft drive boat, but that doesn't change the combination of forward forces and turning moments exerted on the hull as a whole, and hence can have to effect on the handling of the boat.

Hard to explain, so here's an example. Imagine an outboard attahced to a boat via an outboard bracket. As the engine runs there are various thrust forces and moments etc on the hull, and these affect trim and handling. The forces are transmitted to the hull through the attachment point/bolts on the transom/bracket. Now imagine you made a really wierd outboard bracket that bolted to the hull at the bows, and had a long steel girder arm (a very stiff one) going back from the bow, arching up over the superstructure and swooping down at the transom, and the outboard was bolted on to this arm by the transom. The outboard is in exactly the same physical position and orientation relative to the hull as it was before, only it is not bolted to the transom, the only attachment bolts are by the bow.

The forces on the hull are now applied via the bow attachment bolts. So, would the handling change? No, it would be the same. The local stresses where the bolts are attached to the hull are different, they have moved from transom to bow. But the thrust forces and moments taken together are exactly the same whether the engine is attached at the bow or the transom, so long as the position and orientation of the propeller relative to the hull remains the same.

For same reason IoM's comment is wrong where he says "vertical vector presumably will act upwards at the shaftdrive gearbox so if this is behind the vessel's CG then the effect is to lift the stern". The vertical vector does not act at the gearbox, it acts at the propellor. It is transmitted to the hull as a combo of forces exerted at the gearbox and P bracket, but all those exerted forces and moments equate to a vertical vector at the propellor. Hence there is upward lift if the propellor is aft of the CoG, and it doesn't matter where the gearbox is relative to the CoG.

Right, Planty needs to flame me now if I recall /forums/images/graemlins/grin.gif

 

[Deleted User YDKXO]

Re: IPS Theory chapter 2

I'm still digging and still waiting to be fried. I accept your o/b engine analogy but its not applicable here.
Do we accept that if an object is pushed by a force that does'nt act through the CoG of that object, the object will rotate?
Firstly, I think the down angle of the shaftdrive system is significant at least as far as the Deleted User IPS theory goes. There is clearly an upward component to the driving force exerted by the props which, assuming the CoG is forward of the gearbox mountings (or whatever component is designed to take the thrust) results in the stern of the boat being pushed up. Secondly, the direction of thrust will probably not pass directly thru the CoG so a moment will be created. In which direction this moment acts depends on the position of the CoG
With the IPS system on the other hand, because of the vertical part of the transmission, there is a short lever arm over which the prop thrust acts which will certainly create a moment about the CoG and I would submit that this moment will be substantially different to that set up by the shaftdrive system because of the different lever arm length and the greater force that can be imparted by the IPS system. Probably larger and in a different direction
Different moments, different handling characteristics?

 

[tcm]

Re: frying tonite

ah, even i didn't get this far off track. The effect of the force is dependent upion the positon of the force relative to the cg, not the route by which it is transmitted.

For example, the abilty of an ornery spanner to turn a nut on a bolt is the same as a flat plate a mile wide provided that the force and nut are in same place realrtive to one another as berfore and (if zero bendiness) the spanner wd be excatly the same whatever shape you cut out of that big flat plate-like spanner.

With ips, the way in which it is kneccted to the boat makes no difference, as long as it connceted strongly enuf not to break. So, the prop could be cneccted to the hull by a massive clamp around the sides of the hull, or a shaft that went from the engine six feet fortward and then six feet backwards before reaching the prop - and indeed this is exaccerly what my v-drive does - no difference in handling (apart from me being crap, but anyway) provided the prop is in the same position, relatively, to COG as shafts.

 

[jfm]

Re: IPS Theory chapter 2

Mike, just nipping to pub, will revert. The beer should help