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Old 05-16-2013, 10:46 AM   #1
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Trawler outboard conversion

Hi there. I own a 1981 hiptimco ed monk 40 +4 aft cabin soft chine trawler with twin ford lehmans. I'm considering removing the tired old engines, running gear, fuel tanks etc.... And replacing it all with a pair of 60- 90 hp outboards. I usually cruise every other month and travell a maximum of 100-130 miles at a time between 5-9 knots. I understand the weight distribution problem. Just wondering whether this would work. And whether the fuel consumption would be reasonable.

Thanks

Ian
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Old 05-16-2013, 01:00 PM   #2
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I contemplated that idea after the sudden demise of my single Lehman that croaked...for use on our local lakes/rivers it makes sense...especially one on each corner of the transom, deep enough to not be affected by any fore-aft rocking. I was looking for twin 60-70 hp 25" shaft four-strokes...we have 40-50-60 foot houseboats around here with single outboards and with some acquired wind-compensation skills, their skippers do just fine. I have since bought a reman Bomac replacement Lehman but haven't installed it ...the outboard route may be workable for me yet!
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Old 05-16-2013, 02:42 PM   #3
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It's been done before:

Mercury Outboard Powered 65' Motoryacht (no joke!) - YachtForums.Com
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Old 05-16-2013, 03:21 PM   #4
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Ha ha. Mercury race motors @ 280 horsepower each on a trawler. I think not. I see photo shop
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Old 05-16-2013, 03:21 PM   #5
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Ian:

Your fuel consumption will nearly double, maybe from 3-4 gph at 8 kts to 5-7 gph with the twin outboards. But since you are only going 15 hours every other month your annual fuel bill will be relatively low, about $2,000 or so.

Two new outboards will cost $15-20,000 plus maybe $5-10,000 for fuel tanks, outboard brackets, hoses, controls, etc. So the total bill would be $20-30,000.

Still I would seriously consider repowering to new diesels and count on getting some of the difference back at resale time. A big trawler with outboards isn't going to sit well with most buyers.

Have you looked at replacement diesels? I think a pair of Beta Marine 75 hp marinized Kubota diesels will cost about $30,000. Installation, mounts, shaft mods, prop mods, new fuel tanks will add maybe $10-15,000.

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Old 05-16-2013, 04:42 PM   #6
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I'm not a propulsion engineer, but while the outboards may have similar horsepower to your diesel, I would bet even a pair of them would have far less torque, and would be spinning smaller props at higher speed. Performance lightly loaded in calm water would probably be fine, but heavily loaded and/or trying to make headway against a head wind might be compromised. You might want to talk with someone who really knows propulsion systems.
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Old 05-16-2013, 04:46 PM   #7
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Ha ha. Mercury race motors @ 280 horsepower each on a trawler. I think not. I see photo shop
Follow the link and read on ...
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Old 05-16-2013, 04:53 PM   #8
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I have often wondered about the viability of big outboards on heavy displacement hulls. Take an outboard designed to produce (say) 70hp when bolted to the back of a boat doing maybe 30 knots. Can it produce anywhere near the same horsepower on the back of a 40ft trawler with a 9 knot hull-speed? What happens when you try to reverse or maneuver such a beast?

Endurance: Just saw your post - similar thoughts eh?
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Old 05-16-2013, 05:00 PM   #9
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Yeah, similar thoughts Mike. The hp of the engine will be the same regardless of what boat it's on, but while hp gives you top speed, it's the torque that actually does work (such as moving a heavy load into the wind). I don't know the torque output of a typical 60-90 hp engine, but it is certainly far less than a diesel of the same hp. Smaller props, even spinning at higher speed, probably won't have the same bite in the water.

I recall some diesel outboard out there years ago, I wonder if any are still produced...
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Old 05-16-2013, 05:19 PM   #10
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Quote:
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Follow the link and read on ...
Click the link - it's well worth the read.

Spoiler alert: the picture was taken in Miami before the start of a salvage trip to Texas!
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Old 05-16-2013, 05:23 PM   #11
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I think the smaller props on the outboards are going to suffer from much lower propeller efficiency, so even though the power is the same performance is going to suffer. Also, where do you store all the fuel? Diesel boats are not equipped with engine room blowers that are required on gasoline boats.
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Old 05-16-2013, 05:29 PM   #12
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"... where do you store all the fuel?"

Follow the link, read the posts.

Hint - the big blue thing on the aft deck is a fuel tank.
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Old 05-16-2013, 05:31 PM   #13
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I was talking about the original poster.

Gad that blue fuel tank is ugly!
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Old 05-16-2013, 05:39 PM   #14
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OK, I followed the link and read the first page. It was a kludge to get a damaged boat home. But I wouldn't recommend doing that as a long term solution.

I think the OP should replace the worn out Lemans with new or rebuilt diesels. Switching a diesel boat to gas is a big deal, unless you ratchet strap a bid blue fuel tank to the aft deck.
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Old 05-16-2013, 07:37 PM   #15
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You could actually calculate the thrust and volume of water moved per unit of time for the diesel vs outboard. For a heavy trawler, I think thrust is more important than simply "horsepower". I would imagine that a high torque diesel spinning a big prop (even at lower prop speed) would have vastly more thrust than a low torque outboard spinning a small wheel at high speed.
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Old 05-16-2013, 09:27 PM   #16
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There are a couple of reasons why I don't want the inboards anymore. In Malaysia there are not many good marine diesel mechanics. I'm not so good with my hands. My impellers keep getting eaten up for some reason (5 sets last year). I get rubbish stuck around the prop every time and have to employ a diver Togo down. There are a lot of illegals drift fishing nets in this area and I've had them wrapped around both my props at one time.

There are so many out board specialist over here. It would be easy to service without much issue. As long as the boat can cruise at 9-10 knots at around 6-7 g/p/h I would be happy. But what concerns me is whether the boat would steer well with the smaller props. Or would smaller props going faster equate to the same thing.

I'm ok to live with the conversion cost if it works.
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Old 05-16-2013, 09:31 PM   #17
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I saw this powerboat heading up the river near Ft Myers a couple of years ago. From the actions of the guys aboard I got the impression it was sort of a seatrial.

The sailboat picture was take at The Wharf Marina in Alabama. I'm posting just because it seemed strange to me, a sailor in a hurry or afraid to lose an engine while becalmed. Obviously a doctor maybe he has to get in quick for emergencies.
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Old 05-16-2013, 09:53 PM   #18
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Remember my "Lehman replacement" thread?

Two new 173hp Iveco's for $32K.

Shoalwaters wrote;
I have often wondered about the viability of big outboards on heavy displacement hulls. Take an outboard designed to produce (say) 70hp when bolted to the back of a boat doing maybe 30 knots. Can it produce anywhere near the same horsepower on the back of a 40ft trawler with a 9 knot hull-speed? What happens when you try to reverse or maneuver such a beast?

Sailboaters do it all the time BUT w a full disp hull. HP is hp and will determine how fast you go BUT ........ gear ratios, prop size and many related things will help or hinder thrust and speed. An OB will have a small prop but if the prop is ideal (pitch & dia) the engine will turn it's rated rpm and develop it's rated power. The OB prop shaft speed and prop clearance is for 30 knot boats. Not ideal at all for slow and heavy boats. Slow full disp. boats will, however move along at amazing speeds w very little power. For example a Willard 40 will make 7 knots on about 23hp.

I don't think torque has anything to do w it. Torque can bring you up to speed or stop your boat while making a landing but to propel da boat one needs hp. takes hp. So I agree w Shoalwaters ... maneuvering the beast will be much less responsive.
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Old 05-17-2013, 05:34 AM   #19
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This is a 64' Blue water with twin 4 stroke 250 HP's- owner says it does 25 plus MPH across the Flats down in Miami...?

here is his Ad on YW first page
1992 BLUEWATER YACHTS 64 Mega Yacht

VIDEO--YACHT RUNNING 25 mph ACROSS 2 FOOT FLATS ON YOUTUBE AT

You Tube Video Link- "ISLA BOAT 120621"

NEW 250 HP 4 STROKE MOTORS WITH 10 HRS WITH WARRANTY

NEW 600 GAL. TANK

This Yacht IS THE ONLY ONE EVER MADE--Floats In 1 1/2 Foot go where no YACHT can go--RUNS 25 MPH WITH LESS THAN HALF THE FACTORY HORSEPOWER

THERE ARE NO THRU HULL HOLES TOTALLY DRY BILGE 55'OF STORAGE FROM THE BOW TO STERN AND NO WATER IN BILGE MEANS NO MOLD OR MILDEW SMELL
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Old 05-17-2013, 06:12 AM   #20
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Can it produce anywhere near the same horsepower on the back of a 40ft trawler with a 9 knot hull-speed? What happens when you try to reverse or maneuver such a beast?

The problem is prop efficiency.

There is demand down in the 15 hp area for high thrust outboards for sail boats , but no demand when 80 -100 hp is required.

Finding the outboard engines with the highest gearing (3-1 not 2-1) and the largest diameter propeller will be all thatt can be done.

Sure it will burn more than a properly sized diesel inboard but so what?

Manuvering will require more RPM , to get the same effect , again so wghat the diesel at 1000 or the outboards at say 2000 will have the same effect , far better tho when docking as the outboard engines swivel.

The local advantages would seen to out weigh any disadvantages.

After everything is cleaned out the boat may need trimming ballast to restote a level attitude.

No need to ballast back to the old water line , just level.
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