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Old 07-11-2016, 12:56 PM   #1
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twins below vs one

Engines. power and re-power will be the largest issue of having a larger boat.

even under 30ft I'm finding twins below; I like twins!

But! This is a big expensive issue for me, as I'm sure it is for anyone coming from sail or smaller engine boats.

$10,000 +- a pop is not small change and probably why many older boats are on the market.. engines getting old. although some have been re-powered

Is it very unusual to take a single engine boat and install twins? Seems easy enough. given that fact that trawler types have way way more below deck space then sail boats.

this is the one big issue that may keep me from going for the trawler type boats.

What you and others have done or will do is of great interest to me!

I'm thinking and know how.. get 2 little yanmars or kubotas.

Oh. what transmission ratios are used on mid range speeds?
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Old 07-11-2016, 01:19 PM   #2
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Not practical to convert a single engine boat to twins, for many reasons. Just buy the boat that has the number of engines you desire.

And don't discount singles. Many boats run singles and are very reliable.

Without doing the analysis, most gears are around 2:1 ratio unless you are doing something unusual.
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Old 07-11-2016, 01:24 PM   #3
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Originally Posted by DeniseO30 View Post
Engines. power and re-power will be the largest issue of having a larger boat.

Is it very unusual to take a single engine boat and install twins? Seems easy enough. given that fact that trawler types have way way more below deck space then sail boats.
I think you're getting way ahead of yourself here... A full re-power is relatively rare instance in a boat that's been maintained well. Mine's been re-powered after 14 years (14 years ago). I don't know why but expect it was some sort of catastrophic failure as this is not normal. I expect it cost $20K Plus (Volvo TAMd-P). Many folks here have original engines going on 40 years with good maintenance.

On putting twins in a single, I'd forget that. While it could be done, it would be outrageously expensive and exceed your specified boat budget by a factor of 3, 4 or more...
I have a local friend who replaced a single pod drive with 2 diesels. It's a large cat and he is at the $100K mark and rising on his restoration..
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Old 07-11-2016, 01:25 PM   #4
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There are well established protocols for sizing engines in boats. The specifics depend on the hull form, displacement and desired operating speed. Once you have decided on power, you can start worrying about things like prop size, blade count, prop type, prop pitch and finally transmission gear ratio. Everything is interrelated , so changing one thing requires changes in all the other variables. For the smaller engines I think you are referring to, there are limited choices of transmission and hence gear ratio. Consequently you may find yourslf dealing with quite different propeller choices with different engines.

One argument against going to two smaller engines is that those engines tend to be higher revving than larger engines typically used in trawler type boats. Hence, you will be running at higher rpms with resulting greater engine noise.

Finally, small diesels in the 50 hp range tend to cost around $10K each. Altering a boat from a single (say Lehman 120) to twins in the 50-60 hp range will cost a lot more than the cost of the engines. First you will have to remove the existing engine and drive train. Then you have to glass up the shaft tube. The next step will be to remove the existing engine beds. Depending on the boat you may also have to rebuild the hull grid to accommodate the new engine beds for the twin engines. When that is done you will have to install the new engine beds and then beef up the hull at the attachment points for the struts you will need to install. When that is done you will need to install new shaft tubes and stuffing boxes. After that you really should remove the old single rudder and replace it with two new rudders located behind the new propellers. You will also have to remove the old exhaust and install two new exhausts. You will also have to run new engine control cables and install new shift/throttle levers. Finally the two small diesels together will certainly weigh less than the old single, so you will need to reballast the boat to get the trim right. Overall, I doubt you could have this sort of change done for under $50K and more likely $60K.

The last straw will be that changing to two small engines will likely make it harder to resell your boat since many buyers like more horsepower. You may even decrease the value of the boat.
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Old 07-11-2016, 01:26 PM   #5
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For most displacement yachts Twins are like putting 8 wheels on your car...


PS this photo is from the aft section of Laura Dekkers Jeanneau Gin Fizz, they installed an second engine complete with drive line.
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Old 07-11-2016, 01:29 PM   #6
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I agree with impractical. One thing I'm finding is BIG differences in Horsepower Same boats. I've seen for example. Albin 27's with 60 up to over 100 HP.

TD thanks for the reminder and yes, I do know this;
"There are well established protocols for sizing engines in boats. The specifics depend on the hull form, displacement and desired operating speed."




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Old 07-11-2016, 01:42 PM   #7
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Don't fret over the engines being "too big". Nothing wrong with running a Ford Lehman 120hp at 30-40hp. In fact, they seem to like running there.
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Old 07-11-2016, 01:51 PM   #8
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This Lehman seems to be a very very common and reliable engine, Tell me more
and from reading the forum there is an outfit that many of the members use for parts and more.
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Old 07-11-2016, 02:09 PM   #9
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I agree with impractical. One thing I'm finding is BIG differences in Horsepower Same boats. I've seen for example. Albin 27's with 60 up to over 100 HP.


[/I]
My Cape Dory was offered with the 100 hp Westerbeke, the 200 hp Volvo and a 225 HP Gas Chrysler. From My experience the Westerbeke would have been the ideal engine, though most of the CD's were spec'd with the Volvo. Go figure. In our society, bigger is better, though not necessarily in Engine selection..
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Old 07-11-2016, 02:39 PM   #10
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For most displacement yachts Twins are like putting 8 wheels on your car...


PS this photo is from the aft section of Laura Dekkers Jeanneau Gin Fizz, they installed an second engine complete with drive line.
that is what is know as wing engine. It is a small auxiliary engine installed as an emergency power. It is more often seen in ocean cruising trawlers nnd not often seen in a sail boat.
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Old 07-11-2016, 02:40 PM   #11
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Bigger CAN be better. I had an old 34 Mainship I with the OEM Perkins 160 hp diesel.
I ran that engine for 8 years.
Then I repowered with a 270 HP Cummins. Made a whole new boat, much faster when I wanted it, and much better economy when I wanted that.
The best of both worlds.

And I won't enter the single vs twins debate, but consider the room you will have to maintain a single engine vs twins.
Also many boaters (not all) are more willing to do PM on a single simply because the cost (and time) is half.
Those are just my observations after 25 plus years of boating.
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Old 07-11-2016, 02:47 PM   #12
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Repowering worked very well in my refit. But I'm probably the exception and not the rule. Was able to sell my old engine for almost what I paid for a new smaller one. Then I did most of the install myself. If you have to pay someone to do it, the additional parts (exhaust, raw water system, fuel system, etc) and labor will be more than the engine for the size you're considering.

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Old 07-12-2016, 08:51 PM   #13
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I owned a marine repair business and did some new builds. We converted some tugs to twins, very expensive. Engineering work, new engine beds, lots of new plumbing, wiring and so on.
In an existing boat it is always cheaper to rebuild than replace unless you're doing the work, and maybe not then.
Most commercial fishing boats, freighters, tankers are single screw.
I have a twin screw because I like the handling.
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Old 07-12-2016, 10:13 PM   #14
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I'm in the market soon for a trawler an and it will be a single screw.
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Old 07-12-2016, 10:15 PM   #15
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DeniseO30,
Twins are best. I think more trawlers are twins than singles and they do'nt spend all that money expecting nothing for it. Why would so many people pay all that extra money if they did'nt think they were better.

On Yacht World a few years ago I saw a GB36 w twin 55hp Yanmars. Far far less weight and about the power of a GB36 single. But that may not be enough power. If it's only 4hp per ton the hull of a GB36 could be inappropriate because that amount of power may be approprate for a FD boat. And if it's enough power for a FD boat the hull of a GB36 should be FD .. not SD. The power and the hull may not match. In this case I think it's close but 80hp would probably be about right for a 36' FD boat. And since the GB36 is a SD boat whereas 10or 12 knots would be very approprate for the hull both amounts of power fit the hull .. but just barely. They fit the marketing demands of the 70's better.

Most boats are overpowered and a repower presents the opportunity to get it right. But powering a boat w too little power may make it hard to sell. Every boat is designed for a specific range of power. But when you see a boat offered w engines where the higher power is double what the lower powered boat has one may not powered correctly. The power should always match the hull. The power range is so narrow to be almost nonexistant for FD boats and wider for SD. The notion that you ca'nt have too much power is nonsense. The right amount of power is the only responsible route to follow.
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Old 07-12-2016, 10:18 PM   #16
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It's all good conversation for me and something I enjoy. what I actually do will I'm sure be far far more practical then anything I write. I am quite sure I will stay under 30 feet too!

My idea for the bucket list trip is from Cape May to maine. If I can do it one time with a friend I'm sure I'll be more willing to be a snowbird. (per another discussion)
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Old 07-12-2016, 11:08 PM   #17
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I was hell bent on a single but am now leaning toward twins. As the navy seal motto goes "2 is 1 and 1 is none" meaning always have a backup.
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Old 07-13-2016, 01:09 AM   #18
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When I think of twins on a less-than-30-foot boat, I picture twin outboard engines.

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Old 07-13-2016, 07:03 AM   #19
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Quote:
Originally Posted by DeniseO30 View Post
Engines. power and re-power will be the largest issue of having a larger boat. Even under 30ft I'm finding twins below; I like twins! This is the one big issue that may keep me from going for the trawler type boats.
You're over-thinking the engine issue, when in actual fact it will solve itself. As others have said, most reasonably (not even perfectly) looked after and serviced diesels will run almost forever. The debate of single versus twins has waxed and waned forever on this forum, and after all that, (you can look them up with the search function), you will find there is no winner, either is fine, subject to personal preference on the part of some, but more usually decided by what the boat that decides it wants you as their next owner has.

So, re the engine thing...just relax and go with the flow. You boat will choose you, and then you will be happy with whatever engine it has. My boat has a single Lehman 120hp, and I love it. We've had folk with two of those who love them. We have had some on here with two of those who always wanted something faster, but were happy to live with what they had, because they are that damn reliable. As I said...not really an issue. The boat will decide how many engines you get/have...
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Old 07-13-2016, 10:23 AM   #20
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Very well said Peter,
For better understanding you should be aware of some of the details and know enough to not buy an unsuitable boat but for the practical side of the issue Peter is right and expressed it well. Sometimes marketing forces promote manufacturers to make odd combinations but most all boats are designed by a NA and are fine. But some boats and engine combinations are more suited to some buyers than others.

A twin has advantages and so does a single. I have a single. But if I had an unlimited pile of money I'd make it into a twin. But then I would have a different boat too. But it's up to all of us no mater how much we know or think we know about boats to use knowledge to guide us in choosing boats but perhaps practical considerations will often be the stronger element of choosing.

Knowledge or assumed to be knowledge may dictate what to buy but you may likely buy what comes up on YW or what appears in your local harbor. It may have the wrong number of engines and you may be wise to throw out what you know or think about that and buy that wonderful boat. Some are not open minded enough to make that leap.

One last thought. Twin engined boats 30' or less is so unusual the likelyhood of having to choose on this # of engines issue is very unlikely
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