Midships Thrust - Why not?

[Scott Carter]

A theoretical boat has it's propulsion mounted at about or just aft of amidships. Assuming all structural, ergonomic and other issues that are readily made right, what are the relative merits and drawbacks to the following CONCEPT? I capitalize that because I don't want some of you to get hung up on the fact that the beer tank is smaller than the fuel tank. This is a concept...it being that there are, among the drawbacks, some attractive features of having a steerable midship drive as well as a rudder.
In my sketch I've used an outboard engine in a well which is steering wheel controlled and a rudder which is tiller controlled. Again, don't remind me that the guy's hat is silly.
I have some ideas of the drawbacks in terms of steering under way as well as steering in slow situations and tight quarters. For convention's sake, lets assume this is a boat between 30 and 40 feet with the proportions as illustrated (Rhino sure was worth the investment, don't you think?).
I'll be building this boat or one similar, so even suggestions as to the merits of slight modifications would be helpful. Has anyone here done this before? I sure would like to hear about your mistakes first.
Cheers,
Scott

[clanning]

Hi Scott:

NOT an exhaustive list...

Pros:

gets weight out of the stern;
provides access to the stern for boarding, whatever.

Cons:
in a strong swell, the lower unit could be lifted out of the water (guess it would be worse in the stern);
cavitation;
takes up room in the cockpit (part of the strength of an outboard is to get the space/noise out of the way);
structural - makes the keel structure discontinuous (but composites make it easier to accommodate for this).


Mercruiser offered a long driveshaft for their Alpha One at one point -- Limestone 24 used it. The engine was mounted in the centre of the boat like an inboard, and a driveshaft extended to the outdrive which was mounted to the transom (in the conventional manner).

Why not a straight inboard?

(Are you planning on hanging the head over the stern? :eek: )

Chuck

[rwatson]

From observations of hanging an outboard in a well on another boat, at any kind of speed, you get a lot of water forced up the stern wall of the well, that slops into an open boat.
So, I bet the drag associated with an open column in the hull would be considerable.
The Norwalk Island sharpies hang their outboards in a well (at the stern), and they have a couple of 'flaps' to close the opening when the outboard is raised and not in use.
I think the biggest Con in the equasion is the loss of performance, manouvarability and functionality on what appears to be the sole method of propulsion

[kengrome]

Scott, you didn't mention speed so I'm going to assume you're NOT talking about a planing speed boat. If this is true any objections regarding the boat's planing performance are moot.

Having an outboard in a well in the middle of the boat, especially one that turns, will give you better control over the boat's steering. I might suggest moving it even further forward so the steering can be done from both ends of the boat rather than from the middle and the aft end, but not so far forward that the engine has to run in air-entrained water or ventilates.

Some boats (the small Bartenders for one) have a "plug" that fits around the engine's lower end to help seal the boat at the waterline and keep the water in the well from sloshing around and being splashed into the boat. You might want to fit such a plug around your engine's lower end as well.

[tom28571]

Scott,

You mentioned some drawbacks but what are the advantages of this rig. I can't find any.

[marshmat]

Scott,

Firstly, provided you're willing to take the risk that things won't be quite as expected, experimental craft can definitely be a really cool proposition. Witness Bolger's "Query" and its bow steering system.

I can see an interesting close-quarters handling property here, in that while under engine you would have somewhat greater separation of the yaw and lateral translation motions than you have with a stern-mounted drive. (Steer with the engine and she'll likely slide sideways without turning too much; steer with the rudder and she'll turn without sliding much.)

There may be disadvantages in efficiency. The well for the engine is the obvious one. It would be helpful to use a cover like Kenneth suggests, or something like the "bomb bay" doors that a VO 70 has around its canting keel, to streamline the flow here.

It is my understanding that one of the reasons a propeller is generally placed near the stern is that the prop wash carries tremendous vorticity, many orders of magnitude greater than is present in even a turbulent boundary layer on the hull. By flowing this rapidly spinning, highly turbulent prop wash over a large stretch of underbelly at a higher speed than the free stream, not only is the viscous drag on the hull increased near the prop wash, but some of the prop's thrust is lost as the water flowing off the prop is dragged along with the hull. Placing the prop near the stern where its wash will not interfere with the hull helps to reduce these losses. Don't quote me on this yet, though, as I have to check on how this situation actually plays out in the literature.

Your beer tank appears, upon closer inspection, to actually be some 20-25% larger than your gas tank. I'm reminded of an early ad for Chrysler's "Stow n' Go" seats in the Caravan, in which they pointed out that with the middle seats in the up position, the bin into which they fold can hold 220 (IIRC) cans of beer.

[Scott Carter]

All good answers guys.
This is a concept of a drive system, please disregard the absence of a head or any other creature comforts (except the noise thing is a good point, though the well would be covered and sound-shielded).
The structural concern of having a discontinuous keel member is easily remedied with a little thought.
It is correct that I was not considering a planing boat. This is a "sunset cruise for two + driver" boat.
The plug at the base of the well is a good thought. I'd intended to angle the lower edge of the aft wall of the well to ease the inherent turbulence/water surge upwards.

Tom, re. advantages which, understandably, might have slipped under most people's radar, are definitely application specific. I'll divulge them here so that everyone can stop wondering "Hey, wait a minute, have I been overlooking this great idea all along?"
The boat style we'll be using is based on the Thai longtail drive boats prevalent in this part of Thailand. This dictates the stern shape (fine ended on both ends). An outboard on this shape would look very out-of-place (though it has been done).
So to an inboard and tiller steer. I've just built one of these and I'm not satisfied with some of the aspects of its tight docking performance. The tiller is needed for aesthetics in a classic fusion west meets east look. The principals for this boat would almost insist on it.
Advantages? It seems to me that this is a close cousin to 4 wheel steering in a car. Sometimes all four wheels turn the same way (at speed) sometimes opposite for tight cornering while going slow. That could be listed as a universally applicable benefit, except that at speed you would want the two steering systems in harmony, while there would be situations (docking springs to mind) where, if you engaged the two systems opposite to one another it would effectively just move the boat laterally (like a thruster). A benefit more specific to this application is that the technical level of most mechanics here is quite low. An inboard is trapped in the boat and subject to anyone with a monkey wrench tinkering with it. An outboard, while typically more complex, is a discreet unit, readily lifted out and taken to a qualified outboard shop or manufacturer's dealer. May sound strange to you not here, but it's true and real.
Thanks for the input everyone. I value any other ideas.
Scott

[Ari]

Hello Scott.
The idea to put out board engine in 'engine well' like what your drawing shows, only good if the boat doesn't need to be beach.Tried it before.Slow speed is o.k cannot plane the boat. Those with 'engine well' on the stern is actually for the safety purpose for doing works on the engine that get foul up with debris, while a few others are running- in the case of two,three, four or more engine mounted on the transom;With that kind of tiller you doens't need to have the stearing wheel.A small four stroke outboard is quiet enough, might not need noise insulation box.

[MikeJohns]

Apart from upsetting your hydrodynamic flow and weakening your structure I would be concerned about noise and fumes and losing the best part of the boat to an outboard well.

[kengrome]

An inboard is trapped in the boat and subject to anyone with a monkey wrench tinkering with it. An outboard, while typically more complex, is a discreet unit, readily lifted out and taken to a qualified outboard shop or manufacturer's dealer.When the outboard is removed it becomes less of a target for thieves too of course ... :)

What outboard are you thinking of using, a high thust 9.9 perhaps?

[Pericles]

Scott Carter,

Central power and steering exists already in circular dodgem boats.

See picture top right. http://www.wicksteedpark.co.uk/

It would be like sitting on top of a pod drive. :D

http://www.schottel.de/eng/r_produkte/SPD/uebersicht.htm

Pericles

[rwatson]

It would be *nothing* like a pod drive - no hydrodyanmic footing, and limited to a 45 degree turning circle.
For the xtra building work and unknown performance issues , still doesnt make a lot of sense to me. The design appears to be a candidate for some kind of retractible sail drive ideally.

[Pericles]

The units at Wicksteed use outboards that swivel 360 degrees without limitations, so the kids can steer in all directions without rewinding, so to speak. They're great fun!

Sitting on top of pod drive is a joke. I am aware of the necessary connections that limit movement. Perhaps http://www.schottel.de/eng/r_produkte/SPJ/uebersicht.htm is more suitable?

Nought new unda sun lad. (Northern dialect)

Pericles

[Scott Carter]

More excellent observations and links. Thanks again to all.
Kengrome, I don't know what a high thrust 9.9 is, but if it refers to a 9.9 hp motor then that's about 1/8 of what I was thinking. A 75 hp outboard would push this boat to about 8 to 10 knots and that's our principal's goal (top end, not 'sunset cruising'). This (someone correct me if this is incorrect) precludes a standard saildrive as a candidate (don't they go just up to 45 or 50 hp normally?). Plus, as I said, having a 4 stroke in a well ventilated well(thank you for your 'fumes' observation MikeJohns) does approximate a plug-and-play power source. 360 degree steering isn't the goal, but those are interesting links you guys sent.
Has anybody ever seen a boat like this (near-midships thrust with tiller steer)?
One other thing, MikeJohns...it's been suggested that the outboard well cover would make an excellent cocktail table. Now what's the best part of the boat, eh?
cheers again to all,
scott

[kach22i]

Have you seen the Cutfinger thread?

http://www.boatdesign.net/forums/showthread.php?t=20437

[SamSam]

With a well you should have it long enough so you can tilt the engine out of the water when not in use to lesson corrosion and barnicle problems, and when in use to untangle ropes and stuff from the prop. With an enclosed well you need ventilation for fumes but also enough fresh air supply so the engine runs right. It won't run good with not enough air or air full of exhaust fumes. Maybe even fan driven ventilation.

As to steering I would think steering by turning the engine alone, it might not steer at all depending on if the fore and aft underwater resistance was balanced. When an outboard is on the stern you turn it right to go left. When it is mounted in the front, you turn it left to go left. There is a pivot point in there somewhere. If you have the engine at the pivot point, it seems you might have a problem.

The rudder will have to be big since you would have no direct prop wash over it to help it turn the hull.

If you do end up with poor performance, making the beer cooler really large might compensate. ;)

[nero]

When the boat passes over a wave, the level of the water in the well will rise and fall with it.

Maybe design the area of the hull around the motor as a flat pannel. Then you could make a cylinder shaped well and mount the outboard into a second smaller cylinder that seals around the shaft of the outboard. This would give you the advantage of 360 degree trust.

Poorly described, but perhaps you get the idea.

[rwatson]

If a sail drive won't provide the power, why not do what Norwalk island Sharpies, Bolger and a few others - stick a well at the stern so that the outboard is secure, hidden and can actually work like it was designed to - including wash against the rudder,security, positive steering effect etc etc.
This is a proven, viable elegant way to preserve a traditional vessel appearance, and avoids many of the problems associated with centre wells.