Best Old Diesel For 500 -600 hp?

I must confess that this is only the 2nd time I've heard of someone cunverting a twin screw recreational vessel of similar type to a single. The first was many years ago when I guy created a clandestine dredge for poaching clams from commercial beds. He crept along the bay for a short time before getting busted. What are your expectations for performance and handling or your reasons for doing this?

Of your choices, the CAT 3406 is a very very durable engine and was used on a lot of generator sets that ran non-stop. A 12v71 is a good choice also. I wouldn't go near a 6v92, and a 6v71 at the hp you're looking at is really pushing the engines design envelope, but a great engine at lower hp's.

Hi,

If these are truck or land based engines you can get I would also suggest you look at what it's going to run you to marinise one of them.

Using the truck radiator in the bilge of a boat is not going to work for you. You are also going to need marine exhaust manifolds (possibly) and water cooled risers. You are going to need to run a heat exchanger. Unless you are in freshwater or do not care about long term longevity then you may just use raw water to cool it also. Typically a single 600hp is going to be faster and more efficient than 2- 300 hp engines. You might be best buying a totally blown up marine engine and using the parts and pieces off of it. However, this is not a simple engine swap out like you are thinking. Not to mention a new shaft, propellor, rudder etc.......

12-71 in a 36' Roamer? You want to do this on the cheap and run 28mph? ...twin to single, you're looking at a lot of $$$ to rearrange the running gear. Seems like an expensive way to save money. Why not give up some speed and drop in a couple of older 3208 RTO's?

Hi,

You might find some problems in getting enough air for the engine to run and cool itself with that setup in such a small engine space.

There are systems where the engines are mounted above the deck and self contained in what looks like a stumpy 20 ft container, maybe you could use the super structure of your vessel to house it instead and move the cabin to the engine space.

These units are radiator cooled and the only connection to the boat is where the unit is bolted to the deck.

http://www.veth-motoren.com/vervolg_full.php?pagina_id=101

There are a lot of these getting around in Europe so some have no doubt found their way to the Oil Patch in the GOM maybe you can find a used one there and add your choice of driving iron to the front.

No need to worry about any of those expensive shaft and strut mods with one of these puppies.

Alternatively can't you just beef up the transom and drop on a couple of big outboards?

Hi,

The engine you choose will no doubt have a lot of rejected heat that can usually be dispersed to the atmosphere in a land based vehicle.

Once you have the engine in a closed space like an engine room you will need to have an air flow sufficient to get the heat out of the space.

It reads like you are going to get into a pretty interesting off the beaten track project there.

When you get going please take the time to drop us some updates and photos.

I for one would be interested in seeing how it works especially the remote mounting of the radiator, I have seen this on a few Emergency Gensets with Electric Fans and none worked how it would have it the radiator was bolted on to the front of the engine.

What are you going to use for a gearbox and shaft arrangement with this single engine application?

If you can shoehorn in a 16V71 those things have a sound of their own through short straight pipes.

Good Luck, I look forward to reading follow-ups.

A dry exhaust will produce a ton of heat, also in the engine room and especially where it goes through the transom would be an area of fire concern.

As for remote mounting the radiator, unless you mount it in the open on the hardtop with electric fans, which has plumbing issues of it's own and cost issues. You're not going to get the airflow that you need. When it is mounted in a truck, think of all of the CFM of airflow the radiator is currently getting.

Your best bet, if you do decide to take on this project would be to marinize the motor. There are tons of used DD or even CAT 3406 marine parts all over the place.

Any of the engines that you are considering are entirely too large and heavy for a 36' boat. By the time you get the engine out of the bilge high enough to clear the oil pan and flywheel housing it will be so far forward that you will have the bow buried. There is really no reason to add the weight and complexity of the radiator and fan setup when you can use a heat exchanger or keel cooling. You won't need anywhere near 600 hp to match the power of the old engines. The torque output is much greater with the diesels. A single 300-370 hp 903 cummins would be plenty of power. I know a charter boat owner that took out a pair of Yanmars in a 36' pacemaker hull and put a 903 in it thinking he would save on fuel . He ran it one year and went back to twin Cummins and loves them. One thing you may want to keep in mind is the noise and vibration. I don't know if you've spent much time on a boat with a loud engine but it can get rather tiring quickly. You don't have enough planing area to carry a bunch of weight. If you cant't get the boat on top it's not going anywhere no matter how much power you put in it.

Not sure about that. Do you mean you are not getting the answers you wanted? There has been some good advice on this topic so far.

If we assume you have solved the problem of how to convert to a single screw and fit a large enough propeller under the boat to push it at 28 knots then we are left with the engine issue.

The best place to start is your current power requirements. When you convert to diesel you generally need a good deal less rated horsepower. Those gas engines may be hp rated at 4500 rpm but often cruised at 3000 rpm. What are your current cruise numbers with the gas engines? Fuel burn at cruise?

The second big item you need to look at is the weight of the engines and transmissions. Total up what your current setup weighs. I can't see you going bigger than a 3208 or a 671 without a major weight penalty. A couple of 225 hp cummins B series engines would probably be more in line with what you need.

As for cooling since this is a steel hulled vessel you might want to look at keel cooling. Especially if you are able to fabricate it yourself.

Good luck

I was afraid that what I wrote would come off sounding rude, so I hope you accept my apology. I thought about prop size after I made the first response but, that has just been addressed. If you are thinking about doing this you need to draw out your bottom profile, with shaft angle, stuffing box location, etc. on a couple of sheets of plywood laid end to end to see if you can get the wheel under the boat. Most engine manufacturers have installation blueprints available. The problem with a large diesel is the size of all of the other systems involved. The starting system is larger. The water intake and exhaust have to be enlarged. The shaft size will likely need to be increased. It's all doable but, alot of times it's not less expensive. It is also difficult to convince a future buyer that a single screw boat is reliable, not to mention the handling which can be a pain if the wind is up. I didn't mean to rain on your parade, anything can be done with enough time and money. We put a 370 hp Cummins in a lafitte skiff style bay shrimper. The kid would run it up and down the river at about 30 knots.

1st, maneuvering a 36' steel Roamer with a single I/O would have your neighbors clearing the marina. 2nd, the connection would be fairly insane if even possible. 3rd, I have doubts if the small prop on a single I/O would even push that boat faster than rowing it. 4th, how good are your welding skills since with the original idea you're talking about closing 6 holes in your bottom, and opening 3 more, plus building an engine box and probably cutting out part of your keel and possibly moving some deck supports. With an I/O you're talking about cutting a hole in your transom, and pushing your motor back till who knows what will happen to your stability. Dumb question, why are you not just rebuilding the existing motors or replacing them with twins? This really sounds like the kind of project that ends up sitting in someone's yard for 20 years unfinished.

How about putting a 20hp outboard on the back, then mounting a mast and sails?

Hi,

Or something like this.

http://en.wikipedia.org/wiki/Long-tail_boat

Do it the easy way ...

http://www.harbormastermarine.com/Outboard_Drive_Photos.htm