Two engines driving one propeller ...

I am interested in the possibility of coupling two inboard engines to a single propeller shaft in a boat. I don't want to complicate matters by trying to synchronize the engines, that just makes things difficult. I prefer instead to use a small differential like those on rear wheel drive land vehicles if possible ...

By connecting an engine to each side of the differential -- where the drive wheels would normally be attached -- it seems like I could run either engine (or both engines at the same time) to turn the center drive shaft, and the differential would handle the RPM differences between the two engines. If this works, it would make the use of small cheap industrial engines super-easy for powering boats that would normally need a single larger engine.

Some people will say it is more efficient to use a single large engine rather than two smaller engines. They are correct, and if fuel economy were the only issue here then that's what I would do. However, two small industrial diesel engines cost a lot less than a single engine twice the size, and two engines provide redundancy should one of them fail. Both of these issues are 'real world' concerns for many people, so it seems there are actually good reasons to think about this type of drive system.

I am considering such a system for powering a boat that needs an inboard engine of roughly 20 HP. I can get two 10 HP diesels for 1/4 the price of the larger 20 HP engine, and small differentials are very cheap as well -- lots of them are imported here as surplus parts from tiny little Japanese pickup trucks.

If you have any thoughts on this type of drive system -- good, bad or otherwise -- please feel free to post them. I hope to get enough feedback on this concept to decide whether or not to try it ...

 

Two engines driving one propellor

Kamusta Ka Kengrome. I question wether running two 'half-sized' engines will prove as cheap as running one 'full-sized' engine of the same capacity.
You will also need to consider double exhausts and double cooling systems plus an awkard configuration which will reduce your 'working' space.
But overall your idea seems feasible - apart from the apparent economics.
Hunt around for a second/third hand Landrover gearbox from one of the 'constant 4X4' drive models because they have differential facilities for duel drive when in low/high range 4X4 drive.
You might also consider fitting a prop with variable pitch to lesson the strain when switching from twin engine 'power' to a single.
But personally I'd go straight for the larger engine from the start.

 

twin diesel one prop

Reminds me of an idea i once had:

http://boatdesign.net/forums/showthread.php?t=11378

 

Hi Bergalia,

Thanks for your views and comments, I appreciate everything you said. I feel like I should address the issues you raised so you have a better idea of my concept and goals, so here goes:

Running two small engines will not be as fuel efficient as running one larger engine, that's true -- but I suspect that it's going to be very close so I'm not worried about a minor drop in fuel efficiency with two small engines. Besides there is both fuel economy and "purchase price economy" to consider here. If I can build and sell the boat for a couple thousand dollars less simply by using two small air-cooled industrial engines, this means more buyers will be able to afford it, and perhaps the boat will sell in greater numbers because of the lower price.

Also there is the issue of having a backup engine. With one engine you have no backup if it fails, but with two engines it is unlikely that both will die at the same time, so effectively you have a backup engine to get you home in an emergency. This appears to be an attractive safety feature especially for people who go offshore fishing and in other situations where a main engine failure could create an inconvenient or even a life threatening situation.

Double exhausts and double cooling systems are no problem in this particular situation. The engines I'm talking about are 10 HP air-cooled diesels and each engine has its own cooling and exhaust system. If I do this, I will probably build a box to enclose the engines, then run a separate fan to ventilate the engine box -- probably via a v-belt and pulley off the propeller shaft. I can draw fresh air in from beneath the seat so the air will have minimal moisture in it, then the exhaust air will be blown out the holes in the top of the transom which will be baffled by a dorade vent type system to prevent water from splashing into the engine compartment.

I intend to install the engines in the rearmost portion of the boat anyways, so whether I use one large or two small engines the same space will be occupied by the engine compartment. The small air-cooled engines would likely be easier to install than a larger water-cooled engine too, since water cooling generally means a keel cooler and/or wet exhaust and all their related complications, expense and potential problems.

Maintenance on the larger water-cooled engine would likely cost a lot more too. The small engines I'm talking about can be repaired simply by unbolting them, taking them out of the boat and dropping them off at the nearest lawnmower / small engine mechanic's shop. A single large engine that weighs twice as much and is less common will certainly be more difficult and more costly to repair.

Here's a picture as an example of the type of engine I'm talking about, as you can see it is nothing more than a plain small industrial diesel engine:



This one comes from China and weighs about 106 pounds so two of them add a couple hundred pounds to the boat -- about the same as a single 20 HP diesel engine depending upon which manufacturer's engine you use.

 

Hi SeaSpark,

I saw your thread yesterday, thanks for drawing my attention to it again. I noticed that you drew a variable pitch prop in your sketch (did you actually mean a controllable pitch prop?).

I've been trying to contact the suppliers of controllable pitch props via email but so far I haven't received any response at all. It seems like a CPP would eliminate the need for a transmission, so even though they are expensive the entire system might not cost that much more -- and the CPP would probably be lighter in weight than a transmission, too.

 

kitchen rudder

Think i meant CPP i'm not a native English speaker.

A cheap (easy to fabricate yourself) alternative for a CPP in your case could be the "kitchen rudder" http://www.boatdesign.net/forums/showthread.php?t=418 an underestimated device.

I'm worried about the noise of two air cooled diesels and the amount of cooling air they need. Placing them in an insulated box may help but marine (water and diesel resistant) insulating materials are very expensive and it needs an expert to make them give good results (especially the air intake/exhaust). Placing them on rubbers and perhaps a belt drive can also help to keep the noise down a bit.

 

kengrome,

Inovation is good, but I think if you are looking to sell boats, tried and true is the way to go when starting out.

Why do almost all commercial fishing vessels use single conventional diesels? Because it is the most practical way to go. Diesels do not break down that often.

Buyers will be put off, in many cases, by something they may percieve as "experimental". If you've got the cash, build a prototype & put it through some serious testing. Establish a reputation & track record. Until then I'd build any vessel for sale with more conventional propulsion features.

Just my two cents worth.

TGoz

 

Regarding the use of an automotive differential, the propeller shaft will spin at 3-4 times the rate of the inputs, and when one engine isn't running, it will change ratio to 1 1/2 to 2 times the speed of the single engine.
Using one engine will consequently give you one half and not one quarter of the power.
A better plan would be to use two centrifugal clutches, one for each engine, and sit the engines side by side with one centered. The connection of the second engine on the side could then be by way of chain to a sprocket on the prop shaft.
Synchronizing is then ensured, but if you want to run two engines at different speeds it will get far too complicated.
Another way would put the engines equidistant to the centerline and run chain drives from each, which will allow any ratios you like, dispensing with a gearbox, unless you already have matched engine RPM with the prop.
What about reverse? If you want reverse, the gearbox will have to be behind the two synchronized engines.
In the end, you might consider a single engine with a gearbox to get a reverse gear and clutch----- back to square one.

Alan

 

Hi SeaSpark,

Thanks for the info on the Kitchen Rudder. I have heard and read about them, and someone actually built and tested one in this or another boating forum recently. He discovered some vibration or oscillation issues and decided to put a regular rudder on his boat until later when he has more time to work on the Kitchen Rudder.

I like the idea of a Kitchen Rudder as well as a Controllable Pitch Propeller, they both eliminate gearboxes and provide F-N-R without varying the engine speed by redirecting the propeller thrust in a different direction. The Kitchen Rudder looks like a not-so-easy item to build however, and the CPP is (from what I have heard) very expensive and probably not made for boats as low powered as this one.

The noise of two air cooled diesels concerns me too, but I'm pretty sure I can find a way to make them reasonably quiet inside an engine box. I won't know until I try of course ...

 

Construction

Just had a look at your website do you have a reliable source for marine ply and epoxy overthere?

The kitchen rudder does not look overly complicated compared to the construction of a complete boat, can imagine the vibration issues.

Sound insulation is a complicated matter, have had to do with it in industrial installations. Do like the simplicity of air cooled engines.

 

Hi TGoz,

Your suggestions are very practical, thanks.

This is supposed to be an affordable boat to build (or buy) and own. The problem is that diesel engines in the 20 HP size range cost 5 times as much as TWO diesel engines half that size. This price differential has motivated me to consider two engines instead of one -- and a way to couple them to the single propeller so I do not have to modify the rest of the boat.

I can always use two engines each with its own drive shaft and propeller. This would allow for two smaller diameter props which would further enhance the boat's shallow water capabilities, so it's not a bad idea from that perspective. But two small engines in a small affordable boat probably does seem like a gimmick, especially if I use diesels.

 

Hi Alan,

Thanks for the info on automotive differentials, their gear ratios seem to make them unacceptable for my proposed idea. My reason for suggesting them was because they are an affordable planetary gearbox that can handle two different speed outputs (or inputs) to/from a single drive shaft. That's how I thought I might eliminate the problem of engine synchronization ...

 

Yes I do, and both are very inexpensive and very high quality ...

 

I meant one quarter and not one half the power.

 

A similar thread here;
http://www.boatdesign.net/forums/showthread.php?t=16688