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Old 09-28-2012, 06:45 PM   #1
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Let's play a game... Repower

Let play pretend for a minute.

Bess wants Skinny Dippin' to go faster. The reasons are not important, but everyone here knows why and I don't completely disagree with them.

We currently have a single 135hp Perkins with a semi-planing hull and cruise at 8mph and burn 2gph. Sure, we understand that we will spend many thousands and only gain a few mph, but let's pretend I am ok with most of that. The way we see it, it's cheaper than a new boat. We want to be able to go about 15-ish mph after we are done.

What are we talking about here? Is it even possible? I want to consider a "modern" motor. How much hp? Will it fit in the space? New prop and shaft? Do motors come with transmissions? Anyone do an engine swap themselves? How long does it take?

Pics of the engine bay and hull shape below and thanks in advance for playing pretend with us. Although, we ARE considering it.

Tom-
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Old 09-28-2012, 07:05 PM   #2
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Tom,
I'm in Ketchikan going south.
Looks like there's more than hull shape in the pics. Hull in pic is too dark to tell much anyway.
The "game" will be easy to play. Just consider your disp and compare to other boats w the same basic hull form (ther'e are many) and when you find one that has sufficient power to attain the speed you desire go engine shopping. But going 15 is going to take a LOT of power.
Personally I think the weight of your hull will dictate that you should get another boat light enough to do the speed you want w reasonable amounts of power.
If you do repower consider new fuel tanks while the eng is out. Twin engines may also be better.
Repowering can be fun if you're at least kind of a gear head.
GB boat may provide a good base for comparison as they have quite a number of high power hulls similar to yours. If you can find out their disp much could probably be learned.
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Old 09-28-2012, 07:28 PM   #3
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If I had to do it, I wouldn't repower with anything not made today. It looks like you may have the space for a 5.9 Cummins 370. With a Deere you'd have to have at least a 6135 series to get that much HP, and it may be too tall, heavy too. A 6LYA 370 Yanmar might do it....a bit lighter in weight. All those would be major pricetags. You can find 375 Cat 3208 takeouts, but the life might not be that great and isn't worth rebuilding. You'd save money with another boat. Of course, we're just playing a game here. Twins, of course, would be a whole other story. How about a couple of big outboards from "7"? These are 557 HP.
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Old 09-28-2012, 08:03 PM   #4
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Here's something to think about. Carey of this forum has a 36' lobster boat. I suspect it is considerably lighter than your boat. His cruise speed is about 15 knots. To do that, he has a 420 hp Cat in-line six diesel. As I recall his fuel burn at that power setting is something like 10 or 12 gph. Which is why today he cruises the boat at 9 knots.

You currently have 135 hp in a heavy boat. Logic would indicate that you are not going to see anything near the speed you'd like to go unless you get up into the 400-500 hp range.

Single-engine GB36s were powered wtih 210 hp Cummins (for the most part) in their final years. And they were 8 knot boats at a sensible cruising rpm. At the end of it's run the GB42 used a pair of 450 hp (I think) Cats and they could cruise at 14 or 15 knots with a total fuel burn of about 24 gph.

In my opinion, while you could repower with a somewhat more powerful engine I don't think you're going to see a substantial increase in speed unless you go to a different boat altogether. So even going up to 250 or even 300 hp, I suspect you will spend a lot of money to achieve not so much.

We've been kicking the idea for awhile now of having our boat completely worked over including a repower. But while we would get another couple of knots with the new engines, that's would not be the main reason for the repower. It would be to get rid of very old, inefficient, heavy, noisy, polluting, and short service interval engines.
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Old 09-28-2012, 08:06 PM   #5
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A prop chart and a speed versus rpm plot for another semi-planing boat in that weight/length/hull grouping would give you a ball park idea. The prop chart for our 44', 30,000 pound semi- planing hull says it takes about 300 horsepower to maintain 15 knots. The 500 installed horsepower gets 18 knots. Think I saw a thread with some prop charts a while back.
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Old 09-28-2012, 09:18 PM   #6
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Throw enough horsepower at it and see if it flies. How about 930?
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Old 09-28-2012, 09:22 PM   #7
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19,000 lbs from the closing paperwork. I'll get a better pic of the hull up if you think it will help.
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Old 09-28-2012, 09:28 PM   #8
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Tom,

We have pretty much the same boat. I've got twin Cummins 6bt with 210hp each. I once did 15.5 with half full tanks, a following sea and a good stiff wind behind me. It's possible but you can hear the diesel pouring out of the tanks.

I don't know about modern engines, mine are 1987, but something to consider is maintenance. If I were to lose my starboard starter, I don't know that it would be possible to replace it. I have maybe three inches between the engines and the tanks. Maybe midgets with super long arms.

Sell your beautiful boat and buy a bayliner. I feel icky even saying that!

E
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Old 09-28-2012, 09:30 PM   #9
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Post your realistic weight (19,000 lbs doesn't sound right), and your water line length and I can run it through boatdiesel's calculator. But my guess is that it will take as much as 300 hp to go 15 kts. 300 hp is about 16 gph. That will take a Cummins C or similar to put out that much hp for any length of time.

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Old 09-28-2012, 09:32 PM   #10
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I'm 20,000 pounds with twins, sounds about right
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Old 09-28-2012, 10:31 PM   #11
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Sell your beautiful boat and buy a bayliner. I feel icky even saying that!

E

AHHHH!!! Ouch. I just want to be able to go 10-15...not all the time, just when we're in a hurry, or crossing the gulf steam. I like slow and efficient...but damn wouldn't I like to be able to get where we're going quicker.

The problem is loving your boat so much, that repowering is actually a consideration. Or....do we start the list of "want - don't want"?
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Old 09-28-2012, 10:56 PM   #12
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19,000 is the only number I have. You think I am lying about it? Nope. Sorry. Next time I pull her out at Wayfarrer's Cove (in two weeks) I will try to get a weight for you. She's only 35' with a single. She's light in the loafers.

Anyway, if 300hp would do it, is that realistic? Baggi has 400hp, but two 2000 pound motors to push and does almost 15 downhill in a stiff wind. "Close" to 15 would be in the range. Where would I buy engines such as this in/near North Carolina? Or do I just Google Shop? Not like I am actually shopping for such a thing.
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Old 09-28-2012, 11:06 PM   #13
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If it was me I'd talk to diesel shops in the area. When Carey had to repower his lobsterboat he worked with one of the better diesel shops in the PNW. They represent a number of different makes--- Deere, Cat, Lugger, Cummins, and more. They made recomendations and helped Carey with his decision. And they did the installation.

There are a lot of considerations to be taken into account. Usually the engine stringers have to be modified or new mount brackets for the engine fabricated to align the engine with the shaft. The existing transmission may or may not be usable with the new engine. New exhaust system components will be needed. The engine's wiring for sensors, instruments, etc will be different than the old engine's so that's something that will have to be dealt with.

Other components--- plumbing, heating, AC, batteries, etc.-- may have to be re-routed or reconfigured to accommodate the new engine.

So there's a lot that has to be done. Some people can and want to do this sort of thing themselves. But I think most boaters have it done if they are going to do it at all. There are a lot of learning curves involved and unless you've got prior experience you'll be starting out at the bottom of all of them. A good diesel shop, however, will be well up them if not at the tops.
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Old 09-28-2012, 11:39 PM   #14
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I don't see 15 kn as desireable. Either cruise efficiently at 8-10 or plane at 20. The latter is not sensibly achievable for most trawlers.

I'm playing the repower game right now. My fuel tanks leak and have to be replaced, and that means removing the engines. Stuff will get removed next week, so choices need to be made soon.

Given the cost of removal and reinstall of engines it makes sense to at least consider repower. The easy option is to replace hoses, recondition starters, pumps etc and put the old Cummins 555 back in. Trouble is, the list of things to prudently replace gets long, and cost mounts rather quickly. And I would still have old motors at the end, and the wrong size to boot. Way too much power for normal cruise, and only 15 knots (and 34gph) at WOT. And parts are expensive and perhaps will become more difficult to source when the inevitable R&M issues arise over the next few years.

So I've convinced myself. I think I'll go with new 6BT's, 210HP. JD would be 50% more for the engines and have higher install cost given the change instruments and wiring. Lugger is more pricey again, indeed downright expensive. I cant see that the ECU engines will be that much better given I'll likely run at about half rated/installed power. ECU really comes into its own when running at low power/rpm eg displacement speeds in a planing boat.

I figure the repower game is pretty rough. Make sure you are sitting down when you read the yard's price quotes. I'll do it only because the boat is a 10-15 year keeper.
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Old 09-29-2012, 12:27 AM   #15
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Gonzo, if you're looking at about 300 HP and want a modern design take a look at the Yanmar 6LPA-STP.
http://www.ellisboat.com/docs/Engines/6LPA-STP.pdf

My boat's builder is using them now and claims it will push my boat up to 25 MPH (about 22 knots). My boat is about half the weight of yours.

I too want more power and was playing with one of those prop calculators. I put in my boats current horse power, length and beam. It was pretty close predicting the speed I actually get. When I plugged in the 315 HP of the Yanmar darned if it didn't come up with 22 knots. You may want to play with one of those calculators.
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Old 09-29-2012, 12:35 AM   #16
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The problem is that you don't have any real world comparisons to go by, with your exact boat.

For example the Grand Banks had smaller engines, then moved to larger engines as the market shifted. So you can reasonably predict what a GB will do with larger engines.

My boat (totally different I know, yes a Bayliner...Ick ) will make 15 knots at 2400 RPM fast cruise. I seriously thought about dropping to twin NA cummins engines when I repowered because I didn't and don't cruise at 15 knots very often.

You might re-think the larger engine unless you really want your boat to become a science project. Nothing wrong with hull speed.
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Old 09-29-2012, 12:41 AM   #17
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It would be to get rid of very old, inefficient, heavy, noisy, polluting, and short service interval engines.

Good god I love that engine!
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Old 09-29-2012, 03:18 AM   #18
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Gonzo, you're looking for a different kind of boat. I'd look to sell your present boat and purchase one that fits your needs. Perhaps you should get bids for repowering your boat. I'd expect the price of a boat fitting your needs and the sale value of your present boat would be relatively financially appealing compared to regutting your current boat.

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Old 09-29-2012, 06:25 AM   #19
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The ENTIRE drive train will need to be replaced.

A shaft for a "135" hp diesel that operated normally at 35HP will be far smaller than a 500 hp defuler would require.

So the first repower question is how much room do you have for a larger diameter prop?

Than add the prop , shaft , strut and stuffing box to the cost of the engine.

Don't forget 4x the power will require different engine beds , another big cost.

Far cheaper to sell the boat and buy a boat built for the speed (probably more V than a "traweler") and the heavier duty hull to take the smashing at speed.

Old sport fish are cheap today , as no one but you wants the fuel bill.

1/2 mile per gallon ? Enjoy !!!!
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Old 09-29-2012, 07:04 AM   #20
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Originally Posted by Besslb View Post
AHHHH!!! Ouch. I just want to be able to go 10-15...not all the time, just when we're in a hurry, or crossing the gulf steam. I like slow and efficient...but damn wouldn't I like to be able to get where we're going quicker.

The problem is loving your boat so much, that repowering is actually a consideration. Or....do we start the list of "want - don't want"?
I'm in the doubtful camp....many boats ride that semi-displacement mode at a little over 10 knots and seem happy all the way up to their max (which depends on hp).

Long for their beam boats like the Shamrocks I'm used to and other types like many lobster yachts seem to do well in the modes you want...real slow and up on that "slow plane"...but not many others have I ever seen happily cruising along in that "mid-range".

Just hard to make a boat do it all...I think your quest really involves another boat...I think to do what you are looking at is at least a $70,000-$100,000 mod for all the new stuff..not sure what old stuff you could sell off or how much you want to do yourself or could do on the cheap (used engines and parts)...
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