Twin engine, twin rudder and doors on a sailing yacht

Hi all,

I have 2 questions, one concerning the use of 2 engines powerering 2 props (one turning to the right and 1 turning left), and the second question about the "doors" to the cabin from the cockpit.

The engine first:
I am designing a 48 foot sailing yacht, and am thinking about installing a 80HP diesel engine with a single prop, but last evening i was thinking about why not use 2 engines with 2 props, the engine power for each engine could be much lower then the 80 HP, say 2 times 40 HP and one can (if wanted) completely eliminate the wheel effect, or use one engine in reverse and have 2 times the wheel effect. The question is wether i can divide the needed power by 2 or am i thinking wrong there. Furthermore, when using twin rudders, i will always have pressure on both rudders. I think having a single prop with twin rudders will give too little pressure on the rudders at low speeds. I am also wondering what 2 engine not on the centerline will do with stability (they are a bit higher due to the roundbilge shape, not much tho).

Are my thoughts correct?

About the doors (i don't know the correct english terms for these parts), all boats i have sailed on have sliding plates (mostly 2 separate parts) used to close the doorway to the cabin from the cockpit. These are slided into slots and then a topplate is slided over it. This never closes very well, and on a nice day, the parts are always laying around. They have to be stored somewhere to prevent them from going overboard or go through the cabin. In case bad weather comes up, you will have to slide the pieces in quickly, especially with high seas in the night when you are on watch and are moving in and out of the cabin. The risk of having a wave come in from behind with the sliding parts out of the opening and not being able to replace the parts quickly is not a nice thought. Why are designers not using regular doors when designing? My design has a doghouse, to keep me warm and dry in heavier weather, so height for the door really isn't an issue, but i have never seen this. Anyone any ideas or thoughts about this?

Thanks for your reply, i will post my design shortly, so you all can take a look at it.

Grtz,
Arvy

 

Re: the double engines -

First of all wjat's the use? Specifically in a sailingvessel?

You are making the propulsionsystem more complex, more than twice as expensive as it should be and necessary; you'll spend more fuel than required and on top of that you double the costs of maintainance. For what?

A lot of waisted effort, I would say. The wheel effect? I am sailing, not motoring.

About the (set)boards in the companionway.

Here I am with you.

However, some yachts don't have them and just have regular doors. You are free to design whatever you want. Look at the Standfast design of Frans Maas, than you will understand what I mean.

 

I think, it also depends where you want to sail your boat.
Almost everywhere in the med. you drop your anchor and tie the bow or stern to the dock. Walking, single props usually alert all other boats already tied at the dock, particular from $ 500,000 upwards.
Another alternative, sometimes used on catamarans is a single engine driving a hydraulic pump and two hydraulic motors. Because it is hydraulic, the energy losses aren't that bad, compared to a mechanical gear box. Or you use electric motors propulsion and use the engine to charge the batteries.
I assume, when you design your own boat, you have some technical skills and can keep a more complicated system running. Nevertheless, the additional costs are quite significant and I rather would consider a bow thruster (Vetus), if you have to motor in tied spaces.

 

If you are looking for the superior docking abilities of twin engines. Efficiency is gone. Dragging all that weight and fuel. Install bow AND stern thrusters if you really want the extra power in winds and current. A controlled electrical power take off, ( PWM, pulse width modulated ) from the main engine does the job @ 50 amps. @120 volts = about 8 hp. But the low speed torque would equal a 80 to 140 hp engine for docking duty cycles. Control would be by a control that looks and feels light a light dimmer switch. zero speed thrust is limited by the capacity of the alternator and Amperage capacity. 2/0 wire would do for a start.

 

The bigest disadvantage to twin engines is all that crap hanging down unprotected like a motorboat.

If you cruise you will run aground , or have the pleasure of pot/trap lines that can be caught so easily.

In addition if the boat is EVER to sail you will need 2 sets of folding or feathering props , at about $2000+ each!

For that type of cash I would contemplate a single engine with an adjustable pitch prop.
This will save fuel when you chose to motorsail , or can be cranked for full power motoring easily in the cockpit.

Rather than 2 engines I prefer a BETTER single engine, first choice in small power is the John Deere and its derivatives,Twin Disc tranny and dry exhaust with keel cooling . The NO MAINT setup.

A single rudder of a good size will be less drag and more efficent at going to windward , and is fine for docking ,as it has been for almost a century.

KISS!

FAST FRED

 

Thanks for the replies so far.

I guess you guys convinced me of not using twin engines and twin rudders. The best reason is giving by Fastfred, and that was the comment about the drag caused by the twin rudders AND I forgot that i had to have 2 feathering props for the drag caused by fixed props would be terrible. But then again, i was only playing with the idea and trying to come up with reasons to either do it or don't do it. One of the main reasons was the docking, but those problems can easily be fixed with a bow thruster and perhaps even a stern thruster.

@stewi: yes, i have an engineering background, tho i studied mechanical engineering (university level) and not naval engineering. I have worked some 4 years with a yachtbuilding consultancy agency. so i am convinced i can design a good ship (my old boss is also going to take a look over the design ). Considering electrical propulsion i don't think this will be a good idea when in some unfortunate case you will have to motor a long way and still be able to generate some 80 to 100 hp, i don't think that would go very well.

@D'artois: I tried to search on the internet for a standfast design by frans maas, but i never found anything (pictures, drawings) that show me what you mean, perhaps you have some pointers/links?

Grtz,
Arvy

 

One advantage of twin engines is that, it you get good with them, they may in some cases eliminate the need for a bow thruster.

Whether you use one engine or two, an Autoprop or controllable pitch prop is a real plus for sailboats. Because the Autoprop automatically optimises for the speed it's turning relative to the water flow around it, more energy is converted to thrust, and less to "prop-walking", in both forward and reverse.

 

I have seen the term "prop walking". Give a example of it in real life. Please.

 

Companionway Closure

One of my favourites has a single-piece washboard that slides up and down to close the companionway opening. It slides down into a recess, and there is a latch mechanism (accessible from both deck and cabin) permitting it to be locked at several levels of closure.

This way, the stowage of the board is not an issue, and it is always immediately at hand for use.

Instead of my poor description, have a look at a Swan 45 -they use this system.

 

on crusing yachts I've been on there is usually a large peice of vinyl or other water proof material to cover the companionway entrance

works really well and you don't have to bother about putting boards in while underway