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Old 19-09-2018, 02:06   #1
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Replace inboard engine with two outboards?

Hi

Sounds crazy but hear me out: I am thinking about buying a yacht but almost all of them have really old inboards. The price of getting a new inboard diesel is almost as much as the yacht itself. This is for a 28 footer. So I thought: what if I have two strong brackets made, buy two 6 HP long-shaft outboards, and get rid of the inboard altogether? Two outboards are each lighter than a bigger single one, and this was the initial thought of having two rather than a single outboard.

Crazy idea? Maybe not, I think there are definitely some advantages:

1. Even if there are two outboards, they will be cheaper to buy than a new inboard, especially because there are no installation cost, except a couple of brackets.

2. You have two engines, if one breaks down you still have another one left.

3. Because each of the two outboards is light, one can be used for the dinghy.

4. Easy to replace off-the-shelf in most countries.

5. The weight is likely to be somewhat less (together less than 100 KG) than an inboard.

6. You do away with an inboard prop, which is a drag, can get caught on things like seaweed, and could leak.

7. Cooling is all internal and does not make use of raw seawater which can corrode or block things when there are algae in the water for example.

8. A lot of space saved where the inboard used to be (this can be used for storing jerrycans with fuel).

9. You can use them for steering if there is something wrong with the rudder.


Negatives:

1. Higher fuel consumption (outboards do not run on diesel - unless there are some, please let me know, could not find any on the web)

2. Fuel is more flammable.

3. You may have to mount / unmount the outboards from the stern bracket, especially in a storm.

4. Even if they have a long prop shaft, they may not reach the water properly especially in big waves and when sailing offshore.

What do you think, any comments? Anyone know of people who have done offshore sailing using outboards (on small sailboats of course).

Thanks

Hugo
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Old 19-09-2018, 02:42   #2
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Re: Replace inboard engine with two outboards?

I owned a 27-foot sailboat that I made a bracket to hold two outboards and a trolling motor. It worked great. I left the 15 horsepower original outboard on the bracket at all times. And the second outboard was a 6 horsepower short shaft that I normally had mounted to my dinghy so I could only use it in calm conditions on the sailboat. But even if I didn't use it it was nice to have a bracket to secure it to when I was sailing. If I had to do it again I would definitely go with longshaft altering the dinghy transom. One added benefit if it was flat calm and I wanted to save fuel I could motor with the 6 instead of the 15. both were two strokes which are more efficient at higher RPMs. If you make up your own bracket make sure you put the mount far enough back from the transom so that you can tilt the outboard out of the water and the cover doesn't hit. Besides the cost savings, zero drag when sailing, never having to clean the prop, easy replacement, total accessibility, I doubled my storage space having the whole engine room filled with long drawers.
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Old 19-09-2018, 02:47   #3
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Re: Replace inboard engine with two outboards?

There are many boats in Europe that use outboards as their mechanical propulsion. I've never seen a boat with two outboards. You don't mention the weight of the boat you're interested in, but 6hp ought to be adequate for a 28-footer. If you are considering a sailing boat (you say 'yacht') then you will never need two motors. The sails are your primary means of propulsion. Most people use the engine just for getting in and out of harbour or in a flat calm. If 6hp gets you to your hull speed (somewhere around 6knots, I'd guess) in flat water there's no point in having two. It might require one 8hp motor, but then the weight becomes a factor. Incidentally, 100kg hung off the transom is much worse than 100kg inside the boat. Offshore, I would remove the motor from the transom.
The prop being lifted from the water by waves is a common difficulty, even with long-shaft motors. There are boats which have a well in the cockpit or after locker which is a better solution if it's possible.
I've owned an Achilles 24 with a 4hp outboard mounted in a cockpit, which was adequate in such a light and handy boat. It would move us at about three knots in flat water, but wasn't much use in a head wind or sea. The biggest drawback to it is carrying a large amount of explosive fuel. I now have a 33-footer with an inboard and much prefer it. If conditions are not conducive to sailing a Diesel engine will do a very good job of getting you from a to b. (Purists will be recoiling in horror, here!).
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Old 19-09-2018, 03:34   #4
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Re: Replace inboard engine with two outboards?

Actually number 7 in advantages is wrong. Outboards only cool with raw seawater, there is no closed cooling system at all. If not regularly flushed with fresh water they are subject to cooling system clogging from salt deposits.
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Old 19-09-2018, 04:29   #5
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Re: Replace inboard engine with two outboards?

Number 7 is incorrect.....outboards cool exclusively with raw seawater.

Negatives number 1 is incorrect. There are diesel outboards. But they’re heavy and expensive.

http://www.klaxondieseloutboardmotors.com/

And possibly of suspect quality in the small sizes. There are larger ones produced with big name backing though
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Old 19-09-2018, 04:47   #6
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Re: Replace inboard engine with two outboards?

Plus storing large quantities of petrol inboard isn't a great idea. Petrol vapour is heavier than air and will sink down into the bilges and remain there presenting quite a hazard.

Rather than two 6 hp engines, why not use a single 9hp sail drive outboard. The have slightly different gearing and propellers to suit a heavier but slower yacht rather than a light inflatable dinghy.

However, don't make any decisions until you have bought the yacht. Yes you may find an old diesel engine on board, but it might continue to run for another decade completely eliminating the problem. Our Volvo 2003 is 30 years old.

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Old 19-09-2018, 04:52   #7
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Re: Replace inboard engine with two outboards?

Here all outboard powered motor boats have two.


No reason why you can't do the same on a sailboat .... BUT .... the sailboat has some sails? Uh?



Hence one auxiliary may be all you need. 99% of the time.


Eh???



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Old 19-09-2018, 09:32   #8
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Re: Replace inboard engine with two outboards?

I would go with propane fuel. The biggest downsides would be big swells and the ability to have a quiet conversation in the cockpit under power. Let us know how it goes.
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Old 19-09-2018, 09:49   #9
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Re: Replace inboard engine with two outboards?

-Two outboards just un balance the boat hanging off the rear. Causing it to hobby horse etc. One is bad enough.
-Outboards dip in and out of the water in a chop badly. Just be aware of that.
-Just "any" outboard can be a real PITA on a sailboat, hard to reach to start, or to shift through the pulpit, etc, maybe constantly turning itself. If doing this go with an expensive high thrust OB with cable shifting and cable speed control and battery start.
-It's a sailboat, you dont need two OB's. Get one good one.
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Old 19-09-2018, 09:52   #10
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Re: Replace inboard engine with two outboards?

Number 8

Never store Gas (Petrol) below in jerry cans. Firstly it is extreemly dangerous due to explosion and fire issues. Gas vapour is lighter than air and is subject to combustion from electric switches (unless they are marine grade), open flames, etc.
Unless the fuel is kept in a properly built in and vented tank it will probably void your insurance.
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Old 19-09-2018, 09:54   #11
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Re: Replace inboard engine with two outboards?

Quote:
Originally Posted by Sailmonkey View Post
Number 7 is incorrect.....outboards cool exclusively with raw seawater.

Negatives number 1 is incorrect. There are diesel outboards. But they’re heavy and expensive.

Diesel Outboard Motors 5 to 80hp 4stroke 2,3,4 Cylinders water cooled

And possibly of suspect quality in the small sizes. There are larger ones produced with big name backing though
Re: Diesel outboards. I have only ever seen one. It was the noisiest bastard you ever heard.
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Old 19-09-2018, 10:33   #12
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Re: Replace inboard engine with two outboards?

Quote:
Originally Posted by Cadence View Post
Re: Diesel outboards. I have only ever seen one. It was the noisiest bastard you ever heard.
The OP didn't say noise was a concern
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Old 19-09-2018, 10:53   #13
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Re: Replace inboard engine with two outboards?

you have an old engine, Just start an account to budget for a new one. Of course have the current engine checked out before you buy the boat. if you start putting a little money aside every month, in a couple years ,it won't be a hardship if the engine goes out. You might not want to cross oceans with it until your confidence in your boat and engine is up..
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Old 19-09-2018, 10:55   #14
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Re: Replace inboard engine with two outboards?

Quote:
Originally Posted by wolfesmy View Post
Number 8

Never store Gas (Petrol) below in jerry cans. Firstly it is extreemly dangerous due to explosion and fire issues. Gas vapour is lighter than air and is subject to combustion from electric switches (unless they are marine grade), open flames, etc.
Unless the fuel is kept in a properly built in and vented tank it will probably void your insurance.
I'm sure you meant to say that gas vapors are heavier than air.
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Old 19-09-2018, 11:17   #15
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Re: Replace inboard engine with two outboards?

There is a huge and for most people decisive difference in the performance of diesel engines over outboard engines and that is that the torque of diesel engines increases with lower rpm while that of outboards decreases. That means that in heavy weather on a sailboat an outboard is of limited use whereas a diesel gives you that extra punch to climb the wave that could otherwise knock you over.
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