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Old 02-04-2020, 09:23   #31
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Re: Diesel to 4 Stroke outboard conversion in displacement hull.

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Originally Posted by rbk View Post
If you’re going that in depth with those requirements I would seriously consider a Yanmar 50 diesel outboard $$$ but might fit your needs perfectly while maintaining decent fuel economy. You can expect 1.5-2 gph Or about 1L/km at cruising speed with gas and severely reduced range.
I've only seen one Yanmar diesel OB. It was the noisiest thing you could imagine.
Sounds like more horses than he need anyway.
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Old 02-04-2020, 09:48   #32
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Re: Diesel to 4 Stroke outboard conversion in displacement hull.

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Stu, I think you have the decimal point in the wrong place?

Thx. Wouldn't be the first time. I moved to Canada 3 years ago. You should see me with km and miles!!!! And ml and cups...
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Old 02-04-2020, 14:42   #33
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Re: Diesel to 4 Stroke outboard conversion in displacement hull.

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Originally Posted by Stu Jackson View Post
That sounds incredibly high for fuel consumption.


1.5 litres per hour = 5.68 gallons per hour
.....


You did the math wrong. 1.5l =0.4gal. Which strikes me as kind of low, that’s 15nm/gal for a power vessel.
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Old 02-04-2020, 14:56   #34
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Re: Diesel to 4 Stroke outboard conversion in displacement hull.

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Originally Posted by Cadence View Post
I've only seen one Yanmar diesel OB. It was the noisiest thing you could imagine.
Sounds like more horses than he need anyway.
I meant the new ones. I haven’t seen one in person but I can’t imagine they’d be all that bad. 50hp at max rpm (2400 I believe) but you’d be looking at 1/2 that and still be in proper rom range with plenty to spare. I haven’t looked at their mpg either but imagine it’s better than gas.

https://www.yanmarmarine.com/dtorque/
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Old 05-04-2020, 12:00   #35
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Re: Diesel to 4 Stroke outboard conversion in displacement hull.

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Ahoy there,

I have a Roberts Longboat 21 displacement motor cruiser that currently has a midship mounted 27hp diesel with shaft to the prop achieving 6 knots at 1.5ltrs per hour.

As I am planning a refit of the boat, I am considering removing the diesel and fitting a high thrust 4 stroke outboard in a though hull well with the prop in the same position of the current prop. It would be fixed with the current rudder remaining in use. I also plan to fit a ballast tank and batteries in the keel where the diesel would be. (Hoping for adjustable ballast as the boat is on a trailer).

I am hoping for some input into required hp and any handling concerns for this radical change.

Thanks in advance for any help.


I’m struggling to understand the benefit of the outboard well. Looking at the boat’s plans I can see no real difficulty in fitting an outboard on the transom and saving yourself a LOT of very difficult fabrication work.

I acknowledge that maintaining fore and aft trim will be difficult, but I’d say less difficult than the modifications required to get an outboard in a well near the stern, which, incidentally, will have at least 80% of the same trim issue.
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Old 10-04-2020, 08:25   #36
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Re: Diesel to 4 Stroke outboard conversion in displacement hull.

Your Roberts 21 is a fine looking boat, and similar to the 22’ cruiser I’m building. My plan is to launch this summer, with an outboard hanging on the stern.

I’ll can offer some feedback, and my rationale in opting for an extra long shaft, high thrust outboard.

Firstly, I am a diesel fan. I’ve owned an Albin 25 with a 27hp Yanmar. Brilliant, efficient boat. I’ve also had a 120hp Lehman in a larger trawler, and a Volvo saildrive in a Bavaria.

But...for a small displacement cruiser, an efficient modern 4 stroke is my choice for these reasons: 1 Space saving in the cabin, 2 cost, 3 quietness, 4 I’ve reserved the inboard space for a future electric drive

For my 19’ 6” waterline, hull speed is calculated as 5.84 knots. It takes about 6 diesel hp to achieve that speed.

My hull had engine beds for a 2GM Yanmar, under 15 hp, but it would have consumed a lot of cabin space. I’ve been aboard a few similar boats. The diesel takes up too much space, and it is noisy!

There are plenty of 10T sailboats driven by an outboard on a bracket. It works. Fuel consumption is not a significant issue. You’ll burn 4 litres/hr vs 2 litres with your 27 hp diesel. Your (proposed) outboard in a well will be quiet and efficient.

The downside? Gasoline onboard. An important safety issue, but manageable.

I presume this is a dyi project. Here are a few photos of my project.
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Old 10-04-2020, 08:37   #37
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Re: Diesel to 4 Stroke outboard conversion in displacement hull.

Think I saw that boat in Ft Myers Beach around 2011. You own it then? Interesting boat. (The yellow junk rig)

I highly recommend the Yamaha T8. The ones built around 2003-2005 were assembled in France with likely Italian castings. Mine has about 2500 hours on it and runs great. Carb is simpler and more reliable than the new 9.9 which is same displacement. Now made in Japan supposedly, alloy does not seem to be as good a quality. Went thru a series of these motors on a day charter cat in St Augustine.

If you need a bigger motor the Yamaha 25 is available as high thrust and fuel injected. Don't know much about them yet. The Honda 15 is also very nice, but nearly the cost of the Yamaha 25.
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Old 10-04-2020, 11:08   #38
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Re: Diesel to 4 Stroke outboard conversion in displacement hull.

More wood for the fire; I have a 1980 Catalina 30. Two years ago I removed a 38 year old 11 hp diesel and all its high dollar components built around it. Mounted a aft lift bracket and installed a Suzuki 9.9 four stroke with electric start and tilt.

Fuel burn at hull speed is 3/4 of a gallon fuel (2.8 liters) per hour.

All in cost running was just under $3100. Now if you can rebuild your current engine and all necessary components needed from the prop to the fuel tank and needed labor for $3100 dollars I'd say go for it.
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Old 10-04-2020, 12:31   #39
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Re: Diesel to 4 Stroke outboard conversion in displacement hull.

Hi there Boatguy30,

Yes, we were in Fort Myers Beach for a couple months on a mooring, but it was 2008. I have owned the boat since building her. Click image for larger version

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Old 10-04-2020, 12:59   #40
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Re: Diesel to 4 Stroke outboard conversion in displacement hull.

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Yamaha has a new high thrust, extra long shaft fuel injected 25hp that may do the trick. I believe it swings a large 12” prop and weighs less than 140 lbs. Would work great on a displacement hull.
I like this suggestion.
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Old 10-04-2020, 13:53   #41
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Re: Diesel to 4 Stroke outboard conversion in displacement hull.

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Originally Posted by GILow View Post
I’m struggling to understand the benefit of the outboard well. Looking at the boat’s plans I can see no real difficulty in fitting an outboard on the transom and saving yourself a LOT of very difficult fabrication work.
An outboard in a well will handle seas way better than one on a transom bracket. With seas kicking up much past a couple of feet immersing a bracket engine becomes a reality. Good thing is you probably wouldn't need the engine as there would be enough wind to sail. An engine in a well won't be subjected to the large gyrations of a pitching stern and be safely used in most conditions. Yes, if seas get really lumpy even an outboard in a well could be flooded. Had an outboard in a well on my first boat and never had a problem using it even in some pretty steep ocean waves.
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Old 10-04-2020, 14:47   #42
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Re: Diesel to 4 Stroke outboard conversion in displacement hull.

I have seen St. Pierre Dories with a long well for an outboard that allowed for the engine to be tilted up, which protects the engine from fouling, corrosion and allows the boat to be beached. This may help as an FYI:
https://www.boatdesigns.com/26-Ultra.../products/892/
I would think your boat with that high thrust 25hp would enable you to get a little extra cash farming yourself out as a tug.
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Old 10-04-2020, 14:53   #43
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Re: Diesel to 4 Stroke outboard conversion in displacement hull.

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Originally Posted by roverhi View Post
An outboard in a well will handle seas way better than one on a transom bracket. With seas kicking up much past a couple of feet immersing a bracket engine becomes a reality. Good thing is you probably wouldn't need the engine as there would be enough wind to sail. An engine in a well won't be subjected to the large gyrations of a pitching stern and be safely used in most conditions. Yes, if seas get really lumpy even an outboard in a well could be flooded. Had an outboard in a well on my first boat and never had a problem using it even in some pretty steep ocean waves.
The OP's boat is a powerboat, not a sailboat.
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Old 10-04-2020, 15:00   #44
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Re: Diesel to 4 Stroke outboard conversion in displacement hull.

Just as an FYI here is an example of a St. Pierre Dory with outboard in a well and no rudder. Looks like the ultimate gunkholer and beach camper and it's an ancient seaworthy design.
27' St. Pierre Dory, a seaworthy wooden boat
( I have no stake in this company)
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Old 11-04-2020, 09:41   #45
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Re: Diesel to 4 Stroke outboard conversion in displacement hull.

Shortly: do not do it.
1. The theoretical max speed of your hull is ~5 kn. If you push it with let say 50-80 hp, you convert your nice boat unto a monster, which will create wakes like a tsunami. So you might increase your speed to 7-9 kn, but it would burn a lot of fuel and esthetically be ugly.
2. Doing so, you are taking engineers, who designed your boat as idiots.
3. What do you want to achieve? Speed? - buy a planing hull or semi one. Economy? - yours is very good with 1.5/h.
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