Distance between props and handling/performance

[Pseudomind]

There is a well known yacht maker who besides adding distance between the props when he designed his boats also insisted on a very shallow draft.

He put the engines as far to port and starboard as he possible could as well as having the shallow draft.

Does anyone here have any idea of his logic behind this. I can understand a shallow draft allows one into waters where others cannot go, but at the same time, this would also seem to work the other way, against one for offshore use to a degree. I have seen boats from 45 foot to 59 foot with this design.

I am only asking out of curiosity (yes, I know about the damn cat) and not for any better/worst arguments with this design. I am asking for knowledge sake.

Thanks

:thumbsup:

 

[Four Suns]

I'm not sure what performance effects (in a cruising dynamic sense) having the engines far apart from each other but there are two things that having engines as far away from the centerline that come to mind. The first is the draft of the boat will be shallower as the props and shafts (or I/O units) are further up the "V" on the hull and still in the water freestream. The second is there is a longer moment arm from the centerline when doing maneuvering so the boat will be more responsive around the dock (it's like having a long handle on a wrench vs. a short handle). I used to have a 2001 380 DA and putting one engine in reverse and one in forward was "ho hum" and required me to goose the engine throttles to make in turn. On my 480 DB, the props are further apart and one in reverse and one in forward spins the boat around without touching the throttles.

 

[CSR_Admin]

A third benefit is a roomier engine room between the engines.

 

[Sea Gull]

This is one of the big advantages of the IPS/Zeus systems. The props can be placed where they perform best and independently of the engine placement. This gives designers a lot more to work with.

 

[dr.j]

Much greater control in tight spaces at low speed ie: docking in reverse with shifters.

 

[Southpaw II]

Depending on the weight of the engines, and the size (length and beam), having engines further apart would give the boat greater stability on the water. There would less tendency to roll because of wider weight distribution. The only downside I see is that having props, rudders, or outdrives space further apart would afford them less protection from underwater damage because they wouldn't have the keel of the boat to guard them.

 

[Sea Gull]

Not so fast, Southpaw. If I remember my high school physics correctly a boat would be more stable with the weight concentrated in the center not spread apart as you suggest.

Think of kids on a see saw. With one at each end once can push off and if they hold their feet up they can bounce up and down for a long time. If they both sit in the center, side by side, the motion would subside much more quickly.

 

[Dirtguy]

That's right but it's called "Polar Moment." You would want the benefits of either at different times. But the largest polar moment would be best for stability. It takes more upsetting input to rock the boat if they're further apart.

Hence the upscale Cats becoming more popular for fast big ocean working platforms.

Your analogy about the see saw is correct, except that it takes less energy to get either side in the air when their closer together. You'd like it to be tougher to upset in the fist place.

And as far as the draft, larget higher speed boats are pulling the drive further out of the water for better efficiency. Take the Arneson surface drive for example. Ot the Weismann drives. Shallower, faster, more efficient.

dg

Kind of all makes good sense.

 

[Sea Gull]

Yikes, there is a lot going on here. I think my see saw analogy works for the discussion of weight distribution of a given see saw. By changing the fulcrum (two hulls instead of one) you confuse the issue. Yes an upright catamaran is more stable than a monohull, but that is like laying the see saw across two saw horses instead of one - not fair!. Also, surface piercing props are a bit outside the discussion at hand.

 

[tmhudson2]

In any sea condition that would cause the boat to pitch, having the props further outboard and higher on the deadrise of the deep-vee hull may cause one or both props to be out of the water, or at least partially so, at times. Not good for control. But if your primary concern is shallow water cruising, and ease of turning in tight situations, the trade-off could be worth it. All boat design is a series of compromises.

 

[Southpaw II]

My point was that under identical hull configurations, engines spaced further apart would roll less than those spaced closer together. Obviously, there are limits as to what a designer can do with a given hull, given beam, deadrise, and general shape, etc. I wasn't considering catamarans, or the fact that props could come out of the water, or any other variation of the question. Consider a trawler, or any other boat that an be configured with one or two engines....which one will roll the most?

 

[Four Suns]

This is an interesting discussion...

I thought "polar moment" had to do with structural analysis and how cross sectional shapes resisted deflection? I thought moment of inertia had to do with dynamic motion... but what do I know.

In the world I came from, "stability" refers to the ability of an object to return to a desired equilibrium state when it is displaced. I don't see how the moment of inertia is going to make something "roll less". It'll make something "roll with less frequency" if the weight gets pushed out but the angular displacement is going to be the same if the same forces are acting on the object through the cycle (from what I recall of dynamics... but I may have missed a square root of 2 somewhere...).

The "roll stability" on a planing hull is going to have to do with the deadrise angle. It's like dihedral on an airplane wing. The more a boat rolls to one side, the lift on the side it is rolling towards increases and the opposite side lift decrease causing a counter force to always right the boat and bring it back to equilibrium. Placing the engines outboard is not going to change the "stability"...

The stability of displacement hulls (or a planing hull not on plane) are dictated by center of buoyancy and center of gravity relationships which is much different than a hull on plane.

From a practical sense (and not academic like the above comments), I don't think moving the engines a few feet outboard is going to have an effect on roll frequency a normal person would notice.

But I also think an electric boat is a stupid idea....

My 2 cents.

 

[Woody]

So would the perfect boat have a self adjusting center of mass?

 

[Jim240]

You're on the right track Gary... but I'm pulling up old brain cells too.

Polar moment of Inertia, think of J... as in Tc/J. It's a structures thing related to torsional stress. Moment of Inertia is used by both the stress heads and the dynamics guys, but the meaning is a bit different. For stress guys, it's an area thing for calculating stresses. For the dynamics guys, they use a little mass in the equation for their I's.

For two objects of equal mass, the one with a greater percentage of the mass farther from the center is going to respond slower to an input force, and have a lower natural frequency about the axis it's spinning, rolling. The example that comes to mind is skater spinning... she lets her arms out, she slows down; pulls them in, and she speeds up.... puts them back out and slows... all without adding or removing energy.

In a 20,000 pound boat with a pair of engines, trannies and shafts weighing 2,000 pounds each, that's 20% of the weight being moved about, if it were an extra foot apart... I could calculate that but it would require me to go get my HP and I don't do that very often anymore!

 

[tobnpr]

My brain's fried just from trying to follow some of this, but IMO engine location is a trade off...
Further outboard, more space between engines, but less space against the fuel tanks (in a typical saddle-tank setup).
Better dockside handling, but greater risk of cavitation as both points already mentioned.
In a deep-v, bringing the engines further outboard also raises them, reducing vertical clearances and raising the COG (probably not enough to make a difference?)

I believe most builders address the draft issue with prop pockets more so than engine configuration/shaft angles. My 390 draws only 28"- less than my 22' walkaround did with the engine down. But, cavitation is sometimes an issue in certain sea conditions...

 

[RollerCoastr]

Not to fry your brain further Tob, but what you experience when your props suck air is "ventilation". Cavitation is really more about an absense of water near the blades, so it can occur when your props aren't exposed to ambient air.

You might ask why the add-on "wing" sometimes mounted to the shaft of an outboard is called an "anti-cavitation plate" instead of an "anti-ventilation plate"?

I have no flippin idea, but I guess this is sort of a hijack anyway...

 

[skibum]

Did any of you guys see the episode of Ship Shape TV where they installed 2 of these Mitsibishi gyrosopes on a 57' Bertram?

Has nothing to do with distance between props or hull design, but it uses a "control moment gyro" to stabilize the boat.

Michael

 

[Pseudomind]

The reason I originally asked was due to reading this statement from the online ad. I am not implying anything is incorrect, I was more curious as to what difference this placement made, as it would seem to me if safety is an issue, more builders would do the same with their hull designs

Thanks for all of the replies

:thumbsup:

Low Center of Gravity
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[chollyjohn]

Greater distance between props has its advantages with the steering and overall handling of a boat at slow and fast speeds. Another way of saying it is that a wide beam has its advantages cause it allows for greater distance between props. Anyone familiar with twin screw inboard will see a noticable difference with a wider spread. Take for example when you back up. if you reverse the starboard engine the boat nearly goes back with little deviation. however, the port engine reverse with have a tendancy to bring the stern to starboard in addition to backing up. The difference being the rotation of the props are opposite. Twofold, keep in mind that the srtarboard screws turn clockwise same as a single screw boat and those who have single screw notice the boat will slightly move to starboard when backing up.
With the engine being farther apart its like having BETTER leverage than a twin screw with engines closer. An experience captain can use this to his advantage when trying to moor sideways along a dock, between two boats.

Shallow draft was always preferred by boat owners that needed access to shallower water. i.e. fisherman, crabbign boats...with the usual dead rise flat bottom boats and sharp bow cut....and later the pleasure boat industry wanted to adjust the deep V hulls to allow boaters who had opportunities to beach their boats often.

Sea Ray developed the tunnel drive NOT for performance. It adds nothing to performance (there are articles by Sea Ray's engineers who developed this tunnel drive) The sole purpose was to develp a shallower draft and also lowering the degree of deadrise and still have good deep-V perfromance. There have been modifications to Sea Rays tunnel like bottoms since it was engineered and it is also noted that the design fell short of slow speed turning manuverability. Again, there's my two cents. Small note: Penn Yan had its own tunnel drive design; not sure which boat manufacturer influence who?