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Old 10-21-2021, 12:47 PM   #1
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The most efficient outboard I can put on my hobo houseboat?

I'm having a 1979 hobo houseboat completely rebuilt from scratch. I was planning on maybe a 25 to 50HP outboard.


How do I figure out the most fuel efficient option? Speed is not much of a concern. 10 knots is more than plenty.


I'd love a tiny diesel but I don't see how that is possible from my research unless I fork out like 50K for something like a dtorque 111.


You can see the hull and design of the boat here -


I am planning to attach an outboard motor bracket to the rear so I have more room off the back porch.


Suggestions?
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Old 10-21-2021, 01:40 PM   #2
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As long as you are not trying to plane the boat out, you don't need much power but I would go with a 40hp four stroke for practical reasons like having additional cylinders for a smoother idle and the largest alternator possible. Make sure that the mounting bracket used is a floating type that will support the weight of the outboard and prevent the stern from squatting. Neat project, good luck.
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Old 10-21-2021, 05:40 PM   #3
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I think the cost of the outboard is going to be more significant than the efficiency. You may want to do some rough calculations ( or set up a spreadsheet ) to look at combined cost of motor and fuel based on the number of hours per year you estimate you'll be operating. At a few pennies per hour it takes a lot of hours to offset an engine that costs a few thousand more.
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Old 10-21-2021, 06:38 PM   #4
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I had the 70 hp Yamaha which is the highest hp in that model range on a 23' tiny trawler and it worked great and was quite fuel efficient. You don't need that much power if you are just cruisng your houseboat at 6 kts or so. The 50 hp version would work fine and would last, well not forever- no outboard lasts forever, but it would last a long time in that service.

As noted above, do attach it with an Armstrong or similar "floating" bracket that will provide flotation and support the extra weight of an outboard. Even better would be a full hull width support like my Pompano 23 seen to the right in my avatar.

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Old 10-21-2021, 06:52 PM   #5
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You can start with a 9.9 extra long shaft meant for sailboats. Electric start and remote control. If it isn't enough, sell it and trade up. But you are not going to make 10 knots with it.
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Old 10-21-2021, 06:52 PM   #6
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IMO, it's not going to plane, so it's a question of how fast you want to go. Assuming 25' length, 5 knots will be easy, and efficient with very little fuel. A 15 HP Suzuki would be my recommendation. You will get some battery charging with the electric start version and remote steering, shifting, and throttle are available. My rough guess is 5 to 10 HP to go 5 knots.

To go six knots will require twice the HP and fuel consumption of going 5 knots. At 7 knots it will probably be 4 times the HP and Fuel consumption of 5 Knots.

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Old 10-21-2021, 09:51 PM   #7
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And welcome aboard. I hope we will hear from you as you move forward in this project.
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Old 10-21-2021, 11:47 PM   #8
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Yanmar made a few diesel model outboards, I believe from 15-40hp. Stopped making them in the mid 2000s if I remember correctly. Not light, and about triple the cost of a new equivalent size outboard but super cool and very long life expectancy and very efficient. Used a lot for near coastal fishing fleets. Last quote I got for a new rebuilt one with a one year warranty one or two years ago was 12k+ shipping for the 40hp model.
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Old 10-22-2021, 12:03 AM   #9
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36hp ya mar outboard diesel for sale on eBay right now. Type in yanmar d36
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Old 10-22-2021, 12:30 AM   #10
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If I were to buy an outboard motor now it would be a Suzuki. As to efficiently between brands of motors the difference will be almost insignificant. Get the best deal on the motor that has the best support in your area.
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Old 10-22-2021, 12:35 AM   #11
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I use a Suzuki 9.9 on a 22 foot fishing boat to troll with. In fishing trim, with people, weight is around #6000. A little over half throttle gives 5 knots. Wide open maybe 6. Most of the time I am salmon fishing at 2 knots. Very low fuel use for me.
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Old 10-22-2021, 02:12 AM   #12
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I'd target a high thrust 9.9 or 15hp outboard. A high thrust outboard has a larger prop and lower gearing vs. a standard model. You won't make 10knots with that size but they would be economical. Here is one for example Suzuki Marine - Product Lines - Outboard Motors - Products - DF9_9BT - 2012 - DF9_9BT EFI

You may be able to find one used and even more so if you are around a sailing community. These are popular on sailboats that use outboards.

To go faster or if you have strong currents to deal with then you'll probably need to get into the 25-40 range. You have the benefit of a fairly flat bottom so that will help lower the amount of horsepower you'll need. See if you have a buddy with a portable outboard that you could clamp on the back for a test. Whatever outboard you find you can always buy a smaller pitched prop to get more "power" vs. top speed. What gets tricky is the amount of weight you have on the boat will change the power requirements. If it's just you and a friend that's much different than you and 5 friends in a small boat.
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Old 10-23-2021, 07:02 PM   #13
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Thanks everyone for the replies. I'm leaning towards a suziki 25 because it's the smallest I can get with electric trim and tilt.
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Old 10-23-2021, 07:15 PM   #14
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Honda anything.
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Old 10-25-2021, 05:32 AM   #15
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" I'm leaning towards a suziki 25 because it's the smallest I can get with electric trim and tilt."


Most displacement boats need the outboard adjusted only once , as the speed and trim underway do not change.
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Old 10-25-2021, 05:39 AM   #16
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Quote:
Originally Posted by FF View Post
" I'm leaning towards a suziki 25 because it's the smallest I can get with electric trim and tilt."


Most displacement boats need the outboard adjusted only once , as the speed and trim underway do not change.

Except when you run aground! Power tilt becomes a very nice feature at that point.
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Old 10-29-2021, 04:28 PM   #17
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You're on a trawler forum so you're getting trawler answers. This is NOT a displacement hull, it is a planing hull. I'd go with a Yamaha 60 or Honda 60 big foot model. They have the lower unit of a 90 so they can swing a bigger prop which is more efficient. They'll also have a bigger alternator.
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Old 10-29-2021, 04:40 PM   #18
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Yeah... I ended up with a choice between a Mercury 20HP or the Mercury 50HP Command Thrust which is designed for houseboats and pontoon boats.


Got the 50HP. It's slim picking for motors right now and prices are rapidly shooting up so figured I better grab it now.
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