one prop instead of 2

[YuriB]

Hi,
i have twin prop boat, and i'm going to remove right prop and tunnel, leaving only one on the left side. There 2 rudders on the boat.
Some friends of mine don't like the idea and arguments mostly like "this never has been ever done before".
As for me i don't see any proplems. Does anybody else have an experiense riding a big boat on one prop?

[apex1]

Use common sense Yuri and you will see why that is not a good idea.

[YuriB]

Common sense tells me there is no crime in the whole idea.

[gonzo]

The boat will run at an angle and will turn one way fast and little or none the other. If you want one engine, install it in the middle like it should

[mydauphin]

Good ideas if you cruise your boat in a round lake and you can remove prop on side that always needs turning into.

[Stumble]

Yuri,

There are a number of very good reasons not to remove the second prop, unless you want to move the remaining one to the center of the boat. This while possible would demand a redesign of the central framing to carry the loads of the now central engine.

Some of the problems with removing only one engine:

1) Matched engines counter-rotate with props to match. Removing one of them will destroy this balance and cause the boat to want to turn all the time

2) Because the boat wants to travel in a spiral the rudder will have to be turned to counter act this causing increased drag. The increased forces could also lead to premature rudder failure

3) Boats with 2 engines are designed to have that much horsepower. Assuming this is a planing hull, you may eliminate the ability to get up on a plane at all

4) Low speed manuverability will be drastically reduced since you can no longer use opposing engines to spin the boat.


And these are just off the top of my head. Give me a few minutes to think about it and I am sure there are a hundred more.

[YuriB]

Thanks for your notes, Stumble.
No it is not planning boat, it is 90ft GRP minesweeper, that will have new engine.
Distance in between centerline and the prop shaftline is 4ft, so there is not that big turning momentum to the hull. Also, usually on the single prop vessels, the prop drags a hull aside, towards its rotation direction, so that if i arrange rotation of the prop CCW, it will at least compensate the turning momentum.

Yes the first what comes in a mind is that the boat should curcle when goes on one prop, but what i heard from the people which had one of two engines breakdown, the ship goes rather straight. Even though some constant deviation from the curse is normal because it always happen due to the wind, waves, etc. so i would not much worry about rudders.

[masalai]

Just run your twin system at HALF revs and you will get the dreamed of economy and not stuff up everything... and the outcome will be the same speed.... or somewhere in that vicinity...

Otherwise, sell the boat and get one that more appropriately meets your needs....

[YuriB]

masalai
we, humans, got used to change things to meet our needs.
If you have 2 engines running at half revs, you're not only use your boat space in unefficient way, but also abuse your engines, because diesels are designed to run with nominal or higer load.

[masalai]

Likewise the boat was designed and built for 2 engines - would you remove the front right and the rear left wheels on your car?

[FAST FRED]

The largest hassle you will have is the vessel will be harder to control as you accelerate to a speed the rudders will operate at.

Same hassle docking , it may get really strange with one prop attempting to stern walk the boat , and little rudder control,

Underway should be no hassle , although with only one prop wash the steering at lower speeds will take more rudder angle.

FF

[gonzo]

I like mydauphin's idea. There must be some round lakes around there. Otherwise, you could cruise the Mediterranean in whatever direction you boat wants to go.

[apex1]

The black sea is´nt bad either and probably closer to Yuri´s jetty.

[masalai]

Become a politician, then going around in circles will seem normal/appropriate?

[mydauphin]

Remove both propellers and install a center jet drive of whatever size you want, then if you ever change your mind put propellers back.

[FAST FRED]

"Remove both propellers and install a center jet drive of whatever size you want, then if you ever change your mind put propellers back."


"it is 90ft GRP minesweeper, that will have new engine."

I think if he has a spair $100,000 he would be installing a second engine.

At displacement speeds water jets is only selected by military folks ,
who value the shallow draft , more than they value fuel consumption, maint, and installed service life.

FF

[YuriB]

Idea was not to change something in the desigh and simplify the whole concept. Yes i don't have spare 100K, but will find some to put in bow thruster that will improve control over the vessel while docking and at slow speed.

[apex1]

Yuri, install two smaller engines instead of one and you get happy. Leave the bowthruster, no twin engine boat needs that, as long as one has the skills to handle a boat.
you found that nobody here was a friend of your idea, so you should think twice about it.

Regards
Richard

[mydauphin]

"Remove both propellers and install a center jet drive of whatever size you want, then if you ever change your mind put propellers back."


"it is 90ft GRP minesweeper, that will have new engine."

I think if he has a spair $100,000 he would be installing a second engine.

At displacement speeds water jets is only selected by military folks ,
who value the shallow draft , more than they value fuel consumption, maint, and installed service life.

FF

I know, I was joking ....

[Submarine Tom]

Yuri,

All the power to you my friend.

For the record, I think it a terrible idea.

However, you think it a good option.

So, try it and see what happens.

All you have to do is remove the starboard (right) propeller and give it a go.

You may want to put a barrel of water on the port side to simulate the missing starboard engine.

Please be sure and let us know what you think after you've logged a few hours .

Tom

[Willallison]

I'm with our submariner here... whip off a prop and see what happens. Ok - most of us are well aware of the consequences, but Yuri obviously needs convincing.
Whilst it's not the ideal set up, I agree with FF - the only real difficulties will arise at slow speed. There are plenty of displacement boats that go around dragging one prop.
Installing 2 smaller ones as apex suggests may indeed be the best work around - though you'll almost certainly have to change to different props at the least.

[mydauphin]

How can anybody have a 90ft minesweeper and want to go with one prop, sideways. It is like buying a Ferrari and putting smaller tires on one side.

[Willallison]

We seem to have missed the most important question here.... why?

[mark775]

Familiar with glacier bay cats? IMO, about the only small cats that work, as they are displacement. The guy who built those is now building cats with one engine to good effect.
The minesweeper will never be a great boat on one off-centerline engine but obviously money is a consideration and props are removed all the time on boats transiting to Alaska to no ill effect. I don't think it'll be all that bad, certainly not dangerous or anything, and the fellow can decide later if it's what he truly wants. I like the two smaller engine option but apparently it isn't one. Re-distribute the load if an engine is pulled.

[apex1]

I like the two smaller engine option but apparently it isn't one. Re-distribute the load if an engine is pulled.

Why should´nt that be a option Mark?
It is common practice on almost all conversions of ex Mil. vessels due to their too mighty engines. A prop and gear setup of course has to be part of the conversion.
And the statement in post #7 was pretty clear "new engine", so, I claim my advice to be at least a very sensible and cost effective one!

Regards
Richard

[mark775]

You're right. I was assuming a money problem that fades away with post 7 - and two little ones would be cheaper than converting to one new big one...no question.

[YuriB]

Apex1, original engines were rather huge, low revs and taking almost half of the hull space. Now i will move the smaller Cat 3208 engine further to bow into small compartment but will have to add one more shaft and bearing). I will use the boat as lake cruiser for holiday once a year (for 2 weeks) and all the rest of time the lake is frozen. So i don't see any reason to equip it again as military ship with twin engines and propellers (exept if i get into money troubles and decide to sell it to Somalian pirates for their business))

You may be surprised, but still it is possible to buy in USA Cat3208 (bobtail) with 100SMU for $2800. This is FirePump engine.

[Willallison]

2 weeks per year!!
In that case, do away with both engines and stay in the marina....
Even better, sell the boat and charter for 2 weeks - it would be 1000% cheaper than mooring and maintaining a 90ft boat.
Seriously Yuri... are you having us on...?

[YuriB]

No, i want to have my own boat, not charter. I hate the idea, really.
Mooring the 90ft boat cost 100 euro per month (at least while they are not going to rise the prices). We all here in the North live like this (seriously, all water frozen or weather is too cold for pleasure sailing). In many cases people spend weekends on their boats cooking BBQ and drinking vodka during winter season.
Well,
this is a little bit different from what you call marina, but still..:)
http://yuke.users.photofile.ru/photo/yuke/95158084/xlarge/99807119.jpg

[apex1]

Yuri
you can have such engine for free at some places.
And your idea of moving the engine compartement is a true nonsense at least!

I like to follow Will to ask: what is your purpose? having wasted our time?

[YuriB]

At the time the post was created, there was a question from my side. Thanks for all who shared their opinions.
The others waste their own time as long as they ask THEIR questions that outside of the scope of the subject)).

[Willallison]

Yuri - a few gentle jibes aside, we are all here to help as best we can - don't take offense at anyone's comments, they're (usually) made in good faith.
Apex is almost certainly spot on in regards to moving the engine room. The cost of doing so would exceed the cost of installing 2 smaller engines.
And I was only 1/2 joking when I said rip the pair of them out. For the amount of travelling you're talking about, you could get towed. It's simply not worth the expense of making major changes to the vessel unless you're going to get some kind of return from doing so - be that financial, pleasure, or whatever...

Now, back again to my original question.... why?
I think you've answered that to some extent - you want to free up the space currently occupied by two oversized engines. But what else are you trying to achieve? Do you intend to do a complete makeover of the vessel, for instance. And what do you want it to be capable of when you are frinished?
These are fundamental questions - there's always more than one way to skin a cat - give us your desired outcome and maybe we can help to provide the best means of achieving it

[YuriB]

Hi Will,
sorry if it sounded offeensive, was not supposed to be.
On this ship, it is easier to use another compartment for engine room, because existing one is full of pipes, cables,pumps and whatewer else stainless steel and aluminum staff that takes long time to remove. And it is really huge. Yes i do complete makeover of the vessel.
When it comes to financial side, let me do the calculations myself, as i'm better informed on the local costs.
Towing the vessel trough the river to the lake with all its briges and dams by the way -that is expensive.

[FAST FRED]

You might consider a thrust bearing at the end of the first from the propeller shaft. That will allow you to use any engine and connect it with a truck style drive shaft,locating the replacement engine mostly in line with the old.

IF the existing transmission ratio is correct , you might mount it where it is and drive the trans with a simple truck shafting .

Should parts and cost be a concern.

FF

[mydauphin]

I did the same Yuri, I removed 1000hp engines, put in two 250hp, and now I have big bathroom and extra engine room. I am still looking for thrust bearings. Fast Fred any ideas on thrust block for 2 -1/2" shafts used or something. Aquadrive and Python, too expensive.

[FAST FRED]

Fast Fred any ideas on thrust block for 2 -1/2" shafts used or something. Aquadrive and Python, too expensive.

That's because there MARINE.

Go to an industrial bearing supplier and see what he has for your 2 1/2 shaft.

Should be 1/10 or 1/20th the price of "marine".

FF