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Old 09-08-2021, 05:51 PM   #1
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Reowering a twin engined boat with smalller engines?

As I look through the "For sale" listings I see a number of very nice looking boats that are around 40 years old powered by almost 40 year old engines.

Now those engines have to be reaching the end of their service life with parts being hard to come by and marine mechanics charging B.O.A.T. prices. Two new or even rebuilt big engines look like they could easily run into 6 figures by the time the dust settles.

Now I have no desire to head off into the sunset at 15 to 30 knots so I wonder "Could one repower with much smaller engines?".

All the charts I look at suggest that those boats might move quite happily with roughly 1/6 of their original power. So if they had 2 x 240hp diesels originally then 2 x 40 should be quite OK?

Never going to plane but should get close to hull speed.

And a boat that was built to manage 500 hp should be good enough to manage 80 hp, even after 40 years.

Now I've already repowered one yacht quite successfully so I know it's not that big a job. New engine bearers can be fabricated. Propellers can be cut down and repitched and even new smaller ones are not going to be that costly. Exhaust systems were never blindingly expensive.

Could even get a few dollars selling the old engines.

Might need to add another half ton or so of ballast though.

So has anyone ever done this or seen a boat on which it has been done?
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Old 09-08-2021, 06:34 PM   #2
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Its been done. First, if you have the talent then you can do it. If you pay a yard to do it then the numbers will never add up. You still are going to loose in the resale picture but if you keep it for 10 years then the lack of resale won't mean much. The real key is to start with the right boat. You want a boat that is well equipped and in good shape but suffered a catastrophic failure. This is not easy to find, such boats are often repaired. Usually what you find is a poorly maintained boat that nothing works on and the engines are only the beginning of the financial mistakes.
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Old 09-08-2021, 07:23 PM   #3
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If the goal is using less fuel, go slower with the current engines. They'll last longer and it's much cheaper than changing engines. Old engines run at lower rpm can last decades. A grossly under powered boat can be hard to sell.
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Old 09-08-2021, 08:49 PM   #4
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Most, if not all, of the bigger engines are turbos, and I was under the impression that turbos all have to be run at a high % of full power to keep on working properly.


Running a large turbocharged diesel at less than 10% of total power may not be a good idea?
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Old 09-08-2021, 08:58 PM   #5
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Originally Posted by Boracay View Post
Most, if not all, of the bigger engines are turbos, and I was under the impression that turbos all have to be run at a high % of full power to keep on working properly.


Running a large turbocharged diesel at less than 10% of total power may not be a good idea?
It depends on the engine. Some do poorly under sustained light load. Others are fine as long as you run them harder occasionally. And some just don't seem to care. Newer engines usually tolerate light load better.

Keep in mind, some hull forms need more speed to perform well in certain sea conditions. So if you swap to small engines you may hurt handling in some cases. Heavy following seas come to mind. It's one of the only times where I wish for a little more power at max continuous in my boat.
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Old 09-08-2021, 11:04 PM   #6
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I always thought re-powering an old Hatteras with the big 8cyl Detroit Diesels might be viable if you squint a bit and do the work yourself. Those things drink so much fuel above hull speed many owners don't spool them up anyway. If you found one in need of a rebuild & repowered with, say, 80hp JD's you might be able to get some credit for the swap when you sold. I don't know that you'd ever get stupendous fuel economy out of that big heavy semi-displacement hull, but you could end up with a very comfortable and reliable boat.
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Old 09-09-2021, 12:33 AM   #7
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The reliability, peripheral parts availability, internal parts availability/rebuildability, and availability of rebuilt engines totally depends upon the type of engine. Many Cats, Detroits, Lehmans, and Perkins, you won't have any trouble with. Of course, many engines may be hard to service or like-kind replace (including some from those very same manufacturers).

And, many engines, though old, are low hours and have a tone of time left in them. Of course, some engines, especially on neglected boats may be neglected. And, some engines, especially on higher performance boats may have seen heavy use.

It all depends.

There might be good reasons to repower any boat, newer or older. Sometimes just wanting to do it is one of them. But, I wouldn't assume it is needed just based upon age. Condition matters a lot.
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Old 09-09-2021, 09:10 AM   #8
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I did what you are describing with my single engine boat. Went from a 450 HP Cummins to a 135 HP John Deere. My boat runs very nicely between 6 and 8 knots. At 7 knots I'm using about 40 HP. In my situation the fuel consumption was cut in half at 7 knots.

You can read about it in this thread:

https://www.trawlerforum.com/forums/...fit-19105.html

The areas of concern for you are transmission ratio and supporting a heavy drive shaft. In my situation, I used the same transmission and shaft, while flattening the propeller pitch. Mating the existing shaft to the transmission for a 40 HP engine could be a big hurdle. While you can reduce the diameter and pitch of the propeller, there comes a point where there is too much slippage and maneuvering is compromised, especially when docking. A 4:1 transmission ratio would be perfect for my boat, but it doesn't exist until the transmission gets very large.

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Old 09-09-2021, 09:19 AM   #9
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You might also consider resale value. Seems like there are a lot of people out there (I'm not one of them) who think that they need all that horsepower, or maybe even more. I often see ads for boats where the seller is so very proud of the monstrous engines that he put on his semi-displacement trawler. As if he plans to go water-skiing behind it.

In any case, I suspect that drastically reducing the available horsepower, like you are talking about, would also drastically reduce the resale value of the boat. Personally, I would be more inclined to go for a somewhat less drastic reduction in horsepower. Perhaps from 500hp down to 200-300hp.

Good luck, whatever you decide to do.
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Old 09-09-2021, 11:14 AM   #10
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You may wish to try to find a single engine boat to repower for displacement speeds. Added bonus is that by installing a high gear ratio tranny then you'll be able to spin a relatively larger diameter prop which will be more efficient as well.
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Old 09-09-2021, 11:29 AM   #11
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Eight - maybe ten years ago I saw boat for sale on the east coast that had such a mod.

The boat was a GB 36 and the engines were Yanmar JM series (55hp).

Very close power to the single engined standard boat.
I would love to have seen a series of tests re speed and fuel consumption and other related input like weight reduction. Nobody (to my knowledge) has ever done a single/twin comparison that was anymore than dock talk. Or comparing a boat w two FL’s to a boat w one. Rediclous .. as then one is comparing a boat w 240hp to a boar w 120hp.

I think the difference in performance is only relevant to the small differences in the installations. Not worth talking about yet there’s prolly been hundreds of posts about the single being more efficient. If there’s a difference worth talking about I’d like to know what it is. But if there is I think it would be something simple like propeller blade area per square inch of blade or diameter or … you fill in …

I read the above comments and will add that the 36GB I talked about above sold rather quickly. Had it been on the west coast I would have been interested in buying it.
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Old 09-09-2021, 04:26 PM   #12
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And when things go pear shaped ...

The other concern that I have is when the weather turns really bad it's good to have lots of power.



Something to punch through 40 knots and confused rough seas.


It would be nice to have a big reduction gear and to swing a large propeller.


Unfortunately there don't seem to be that many suitable gearboxes with a large reduction.
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Old 09-10-2021, 06:34 AM   #13
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It would be nice to have a big reduction gear and to swing a large propeller."

"Most folks happily give up a slight bit of efficiency to save 1.5 or 2 ft of draft a huge prop would require.

Also once over 30 35ish in diameter ,props get REALLY! expensive.
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Old 09-10-2021, 06:47 AM   #14
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Quote:
Originally Posted by denverd0n View Post
You might also consider resale value. Seems like there are a lot of people out there (I'm not one of them) who think that they need all that horsepower, or maybe even more. I often see ads for boats where the seller is so very proud of the monstrous engines that he put on his semi-displacement trawler. As if he plans to go water-skiing behind it.

Good luck, whatever you decide to do.
I repowered an old Mainship I from the oem Perkins 160 hp to a Cummins 6BTA at 270 hp.
It made a whole new boat.
Not only did the speed increase, but it also became MUCH MORE EFFICIENT.
Cruise speed went from 8 knots to 12. Which kept it from "wallowing" in the trough with quartering seas.
At 11.5 knots with the new engine the mpg was the same as it was at 8 knots with the original engine. I was getting about 3.2 mpg and with the old engine 2.3 mpg
It wasn't just a home run it was a grand slam.

And when I sold the boat (2005) it went in 3 or 4 days at my asking price.
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Old 09-10-2021, 07:30 AM   #15
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It would be nice to have a big reduction gear and to swing a large propeller."

"Most folks happily give up a slight bit of efficiency to save 1.5 or 2 ft of draft a huge prop would require.

Also once over 30 35ish in diameter ,props get REALLY! expensive.
Plus there's a limit to what will fit under the hull without a big compromise in shaft angle, etc. on most boats. Particularly if it's not a single with the engine really low in the bilge.
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Old 09-10-2021, 08:58 AM   #16
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That's why we bought a boat with Ford Lehman SP135's. Easy to service and inexpensive parts easy to find. No turbos, runs at trawler speeds - will last for many thousand more hours.
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Old 09-10-2021, 09:29 AM   #17
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I repowered an old Mainship I from the oem Perkins 160 hp to a Cummins 6BTA at 270 hp.
Seems like a reasonable upgrade. Especially since the Mainships tend more towards the "semi" side of semi-displacement.

I was thinking more of the guy who replaced the 130hp engines in his Gulfstar 43 (which leans very much more towards the "displacement" side of semi-displacement) with engines three times that size. That's just idiotic. And I've seen plenty of ads with similarly excessive re-powers. Something I just don't understand.
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Old 09-10-2021, 10:11 AM   #18
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I repowered an old Mainship I from the oem Perkins 160 hp to a Cummins 6BTA at 270 hp.
It made a whole new boat.
Not only did the speed increase, but it also became MUCH MORE EFFICIENT.
Cruise speed went from 8 knots to 12. Which kept it from "wallowing" in the trough with quartering seas.
At 11.5 knots with the new engine the mpg was the same as it was at 8 knots with the original engine. I was getting about 3.2 mpg and with the old engine 2.3 mpg
It wasn't just a home run it was a grand slam.

And when I sold the boat (2005) it went in 3 or 4 days at my asking price.
What was the hull speed of the Mainship? you are describing an originally underpowered boat. That have been a pet peeve over the years.
Those smaller boats that could not get on plane thus burned more fuel trying.
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Old 09-10-2021, 10:25 AM   #19
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That's why we bought a boat with Ford Lehman SP135's. Easy to service and inexpensive parts easy to find. No turbos, runs at trawler speeds - will last for many thousand more hours.
Love our Lehman 120's! Just bought four new heat exchangers for $480. Try that on a Cat 3208TA!

I actually wish our CHB41 had the FL80's though. 240hp is overpowered for us. WOT gets me to ~10.5kts versus our standard 8kt cruise. I have never been in a situation where that extra 0.5-1.0 kts the extra 80hp buys us is useful.

Straight sixes are nice though for their natural balance. Still, if I repowered (unlikely as you point out) I'd probably go with a pair of smaller Yanmars or JD's. I'd rather have a single honestly - I bet a single 120 would do fine for our boat, and it would certainly make changing the impeller & injector pump oil on the stbd engine a lot easier!
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Old 09-10-2021, 10:34 AM   #20
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I actually wish our CHB41 had the FL80's though.

What is the model# of the 120. Mine are 2714E which means the injector pump is economy mode, or a different pump and they are rated at 108HP each. Works well since I rarely use all of it anyway.
The 2715E is 120 HP
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