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Old 06-08-2013, 04:03 PM   #1
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Minimum Outboard Size

I was reading one of the forum threads and someone mentioned that the fact a commercial vessel originally had two big high horsepower diesel engines didn't mean you needed that much horsepower to drive it for recreational use. I am considering a budget repower on a 56' v-hull trawler that weighs 38 tons. What would be the minimum size outboard motor(s) I could use to safely power a boat of that size? Speed is not a priority, just safety (it will not be taken out into the open ocean). It originally had two 180 horsepower diesels, now removed. Yes, I am a newbie to both boating and this forum so please excuse me if this is a dumb question or is covered elsewhere in the forums.
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Old 06-08-2013, 04:20 PM   #2
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Your issue is likely going to be propeller diameter not horsepower. Trawlers in that size will likely be turning a 24" or greater prop(s). While you might make it work, it won't be very efficient as there will be a lot of prop slippage as the propeller(s) won't be big enough to take a proper bite in the water.

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Old 06-08-2013, 05:22 PM   #3
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I see workboats with 2 115hp Honda 4 strokes pushing 60 foot steel barges with a crane and a load of pilings around the intracoastal.

There's doable and then there's what's safe for you in your situation (what you will expect out of it.)
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Old 06-08-2013, 07:18 PM   #4
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Originally Posted by ckpetrus View Post
I was reading one of the forum threads and someone mentioned that the fact a commercial vessel originally had two big high horsepower diesel engines didn't mean you needed that much horsepower to drive it for recreational use. I am considering a budget repower on a 56' v-hull trawler that weighs 38 tons. What would be the minimum size outboard motor(s) I could use to safely power a boat of that size? Speed is not a priority, just safety (it will not be taken out into the open ocean). It originally had two 180 horsepower diesels, now removed. Yes, I am a newbie to both boating and this forum so please excuse me if this is a dumb question or is covered elsewhere in the forums.
First I'll say don't do it, it's a really bad idea (in your case). It's one thing to haywire stuff together if you have a lifetime of experience with such things. But, as you are new to boats, best to start with a typical and complete vessel (that's not going drown you). There are so many boats available at very low (giveaway) prices, the uncertainty and unknown expenses of some wacky conversion are not sensible.

Haunt craigslist and ebay, something will turn up......
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Old 06-08-2013, 07:41 PM   #5
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First I'll say don't do it, it's a really bad idea (in your case). It's one thing to haywire stuff together if you have a lifetime of experience with such things. But, as you are new to boats, best to start with a typical and complete vessel (that's not going drown you). There are so many boats available at very low (giveaway) prices, the uncertainty and unknown expenses of some wacky conversion are not sensible.

Haunt craigslist and ebay, something will turn up......
Thanks for the words of warning. The boat itself is actually a done deal already. My wife and I are planning to use it strictly as a liveaboard so the idea of "mobilizing" it is just optional/theoretical at this point.
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Old 06-08-2013, 07:53 PM   #6
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I looked at Craigslist myself for the first time the other day and the sheer amount of large boats for low prices amazed me...If I had the $ I would buy a few and ship them back to Oz...mucho $ to be made !
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Old 06-08-2013, 08:28 PM   #7
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Do you have any idea of what’s involved in living aboard a boat?

Do you understand marine sanitation requirements and laws?

Do you know and understand what kind of shore power you have? 30 Amp 120 volt? 50amp 120 volt? 50 amp 240 volt?

Do you have a grasp on how dangerous it is to you and your neighbors in owning a boat that can not be safely moved? Think about fires, storms, and broken docklines. You are a danger to anyone near you.

My guess is you don’t know what you don’t know.

Then you are fortunate that a well known and respected Naval Architect - Tad Roberts - gives you advice that would cost hundreds of dollars in a face to face meeting, and you dismiss it out of hand.

Sorry to seem so negative, but I do not want to read about your drowning.

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Old 06-08-2013, 09:36 PM   #8
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Do you have any idea of what’s involved in living aboard a boat? ,,,,Sorry to seem so negative, but I do not want to read about your drowning. -Mike
Sceptic. You sound skeptical, but I agree. Still, if a lot of us didn't "wing it" in our initial boat experience, there would probably be a lot fewer members to caution others against making many of our same mistakes. Listening to advice like yours saved me a lot of unnecessary strife.

Petrus, I can appreciate where you are coming from. You just want to have the mobile option if needed, right? My guess is that if you are going to live-aboard her anyway, I'd spend my money having it properly towed in the case of simply wanting to move the boat a short distance. You'd probably need twins to maneuver it to any degree, and that would be mounting, hooking up steering, throttles (unless remote electronic which would be $$$). If I absolutely had to move it and the engines were gone, I'd tow it. Like you said, the priority is not speed, it's safety. With a 38 ton vessel, there's no way to be reasonably safe without a properly placed, properly rated powertrain in operating condition. Live aboard the boat and check out all the other systems that still exist. If it's worth the investment to make it move on its own, then do it. Otherwise, save the money and the risk.....there are just too many other options existing out there.
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Old 06-09-2013, 12:23 AM   #9
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Then you are fortunate that a well known and respected Naval Architect - Tad Roberts - gives you advice that would cost hundreds of dollars in a face to face meeting, and you dismiss it out of hand.
I can't argue with you on the point of my relative lack of knowledge but I do take issue with the statement that I dismissed Mr. Roberts feedback. I received his message loud and clear, but the fact is that I've already committed to the purchase of the boat so his suggestion to shop around, while clearly good advice, is not an option for me. Just to give a little more history on the boat, the previous owner had lived aboard it for 8 years and only moved it from its slip (professionally towed) to have the bottom repainted. My wife and I will be more than happy to do the same if that is our only option, but the sale of our home will give us a small influx of cash with which we could potentially repower the boat so we are exploring our options. Again, I am a complete novice so all advice, for and against, is greatly appreciated.
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Old 06-09-2013, 12:46 AM   #10
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Petrus, I can appreciate where you are coming from. You just want to have the mobile option if needed, right? My guess is that if you are going to live-aboard her anyway, I'd spend my money having it properly towed in the case of simply wanting to move the boat a short distance. You'd probably need twins to maneuver it to any degree, and that would be mounting, hooking up steering, throttles (unless remote electronic which would be $$$). If I absolutely had to move it and the engines were gone, I'd tow it. Like you said, the priority is not speed, it's safety. With a 38 ton vessel, there's no way to be reasonably safe without a properly placed, properly rated powertrain in operating condition. Live aboard the boat and check out all the other systems that still exist. If it's worth the investment to make it move on its own, then do it. Otherwise, save the money and the risk.....there are just too many other options existing out there.
You got it, and if towing is the best bet, I can accept that. I'm just thinking out loud here as, like most boat owners, I would prefer some level of mobility. The boat still has the rudders in place, so I was thinking it might be possible to simply lock the outboards in place and use the original rudders to steer. Bad idea? I can accept that too!
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Old 06-09-2013, 06:13 AM   #11
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IF "weighs 38 tons." is actual weight , not the tons found on the USCG doccumment which is a measure of volume , about 2 hp per ton to 3 hp per ton will move you at 6.5 to 7 K.

So about 80- 120 HP total rated OB should do. Two 40's or two 60's .

That assumes you can actually get the hp into the water.

Small outboard props wont be efficient so fuel consumption might be high for the speed you get,1 mpg perhaps

I would steer the outboards by connecting them somehow to the existing rudder system..

Always nice to be able to leave for a marina fire , or just to relocate as the seasons change.
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Old 06-09-2013, 06:49 AM   #12
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Nowhere do you mention the state of repair of the existing diesels nor their existence?

If the fuel tanks and running gear are OK (shafts, bearings, props, rudders and steering) then you have several affordable options.

1. Repair/rebuild existing diesels.
2. Replace with much smaller, used/rebuilt diesels with deep reduction gear to turn those big props.
3. Replace with gas motors - not really recommended but less money than outboards and will work much better.

To move that boat at 5 knots should take very little HP if efficiently delivered to the water - that means through big, slow turning props.

Since you've committed to the purchase of this boat to be essentially a "floating home," it is essentially purposed the same as those barge types one finds in Saulsalito and elsewhere. My guess is none have any means of propulsion. I recall a recent court battle, I think in Florida, between the municipality and owner on the issue of boat vs. floating home. The city impounded the "boat" and had it destroyed. The owner won, too late, and the city had to pay him for damages.

I bring that up since the city then passed an ordinance that "boats" in the marinas had to be self propelled.

Other than to sneak past such an ordinance, I cannot recommend outboards. Work a deal with local Sea Tow or Tow Boat US to tow you as necessary.
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Old 06-09-2013, 06:51 AM   #13
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There are millions of craft on the water around the world that fit no recreational boater's or "yachties" concept of what they know or understand.

I work in a business where I move many "engineless craft" of all sizes and shapes.

They are not difficult to move and are no more a danger to others than the yachts next to me with their engines torn apart for repairs.

People live in houses all over the place with no concept or knowledge of plumbing or electricity....many homes don't meet all the building codes either ......without mass casualties every day.

As FF pointed out...around 100 hp will move you around...not great but just for minimal mobility. I would spread that hp out to the corners as twins to help in control.

...or...just be like the tens of thousands of other dock condos without engines and pay for a towing company to get you where you need to go. If it's a regular thing...more than 2x per year or longer distances than a lift to a slip....then engines mght pay for themselves in the longer run.
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Old 06-09-2013, 07:28 AM   #14
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Rjtrane, the O.P. did say in post #1, no engines.
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Old 06-09-2013, 08:17 AM   #15
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The boat in my sig is a 65'x22' wooden behemoth. I purchased the boat after a fire had destroyed everything but the hull and cabin exterior. One stipulation of the purchase was to get it gone (July is hurricane season around here). We moved it with a 32' silverton with twin 454's. That was the scariest ride i've had in a long time. The rudders don't work until you reach a certain speed, so don't count on your rudders being of any use. Sea tow wanted almost $6,000.00 to move it about 10 miles up a river to my land. If you must move it my best luck was tying my 28' hydrasport with twin 250hp yamaha's to the side with the motors even with the stern of the big boat and use the motors and the turning capability to maneuver the big boat. I learned that the heard way. Be sure to have a lot of big fenders and do not stick your hand out. Chances are if you move it your going to have some inexperienced help and legs and arms will snap like dry twigs. I don't know if you have insurance but a liability policy would be good to have in place. I had none and it made the trip all that scarier. Good luck.
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Old 06-09-2013, 08:42 AM   #16
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The boat in my sig is a 65'x22' wooden behemoth. I purchased the boat after a fire had destroyed everything but the hull and cabin exterior. One stipulation of the purchase was to get it gone (July is hurricane season around here). We moved it with a 32' silverton with twin 454's. That was the scariest ride i've had in a long time. The rudders don't work until you reach a certain speed, so don't count on your rudders being of any use. Sea tow wanted almost $6,000.00 to move it about 10 miles up a river to my land. If you must move it my best luck was tying my 28' hydrasport with twin 250hp yamaha's to the side with the motors even with the stern of the big boat and use the motors and the turning capability to maneuver the big boat. I learned that the heard way. Be sure to have a lot of big fenders and do not stick your hand out. Chances are if you move it your going to have some inexperienced help and legs and arms will snap like dry twigs. I don't know if you have insurance but a liability policy would be good to have in place. I had none and it made the trip all that scarier. Good luck.
LMAO...Swampu, you just became my hero...
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Old 06-10-2013, 06:13 AM   #17
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With no wind a 13ft B Whaler with a 25 will tow it slowly and it can be put allongside a dock with the whaler moved to the stern , used "on the hip".

Only VERY modest power is required for a no wind tow.
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