Outboard Vs. Inboard Diesel Semidisplacement

Hello Guys,

We are in process deciding what kind of propulsion will ran in our new project.

And I dont get a lot of stuff on web, so i hope somebody can help me.

Hull is/will be semidisplacement 39 foot boat with7500 kg(15000lbs), we have idea to put two 115hp outboard engines or less.

I know what are basic advantages of outboard engines.

So do we stay with conventional DIESEL SHAFT ENGINE or put two OUTBOARD engines,speed is not so relavant.

Thanks


Best regards

Miha

 

"I know what are basic advantages of outboard engines."

There are also disadvantages like fuel consumption, life expectancy and above all NOISE!

 

I doubt noise would be an issue in deciding against outboards, but for a boat of this size and type, diesel seems the obvious selection.

 

I ran into the same thought process several years before building my boat. I wanted diesel outboards in 150 hp range. Ha ha... the biggest I could findelete was 27 hp yanmar, and those were very expensive and getting was problematic and they could get discontinued at any time.
So decide for inboard diesels with surface drives. Gas was never an option for me, safety, and fuel economy far outweigh any benefits of regular gas outboards. Another issue would have been prop depth, size. Outboard prop size and depth are very limited, not good for powerboat of this size.

 

Noisy outboards are a thing of the past. An inboard diesel will more likely generate more noise inside the boat. The cost of inboard diesels will be much higher than that of outboards too.

 

I have a modern Yamaha F100 with EFI. What keeps me from opening the throttle for more than 1/3 is not just the digital fuel flow indicator but also the noise!

 

You should try an Evinrude then.

 

Lightly on topic, JetPac makes diesel outboards in the 150hp range and has for awhile http://sealionmarine.com/products_en.php

 

Make sure you have enough prop diameter (blade area) to push (and also stop) the boat. A rough guide I have seen for twin screw is 1/2" dia for each foot of boat so you would use 2 x 20". But, your boat is light (15,000 lb) so a pair of outboards might work -especially with 4 blade props and you don't want much speed... say up to 9 knots.
Many of the new outboards are very quiet so noise should not be a problem. You could always fit a sound hood over them

 

Presumably the boat in question would have a cruise speed in the mid-teens (knots), which is about the minimum outboards work well, but I doubt in the size needed, the gear ratios/prop diameters available would do the job, especially if a lot of use is anticipated.

 

Jet propulsion ? Probably going to be less suitable at these speeds.

 

I did look at them, but not appropriate or efficient for a 60 foot boat. I would have loved to get rid of engine room, and put propulsion in pods outside boat. General idea is good, but in the end big props, need big shafts, deep enough underwater to work.

 

Seems like a pretty difficult conversation engage in without a little more information from the MihaS, what is the boat going to do for a living and how many hours a year ect.?

 

The difference in my opinion is certainly cost. The benefits are known regarding the efficiency and longevity of diesels; but the re-engineering and rigging of diesels will pay for at least 2 sets of new outboards- even 4 strokes.

I would consider the addition of significant weight; which may decrease performance at the same horsepower.

Personally, I went diesel from outboards because I have come to the conclusion that I have no respect for my hard earned money and time. (Also fuel where i am was $.0.50 a gallon when I started) So I went with twin 440 Yanmars tweaked on the dyno to 456...EPA not an issue. The boat weighs in at about 16500 with 60% fuel and 6 people and I still get well over 50 knts and cruise at 48.

But everything diesel is expensive....just an (1) injector can set you back $400 USD.

Good luck

 

I test and fix my own injectors on diesels... but outboard mechanics are not any cheaper.