Modification of boat plans

[johansen]

I am looking at and considering purchasing the plans to build this boat. The current plans call for a 50hp outboard motor. I want to know if it is possible to convert this boat to use an inboard 50hp compact diesel motor such as an Yanmar or even a Nanny diesel, I believe by Kubota? Please take is easy on me, as I don't know that much about boats, and this question may be way out of line. Any info would be greatly appreciated

[Gilbert]

Certainly it could be done.
But if you don't know how to do it you would need a tutor or someone to draw up the installation for you.
I would suggest you try to find a boat that is very similar but is designed for the inboard engine and buy the plans for that design. There is a good chance someone on this forum will suggest such a design, so hang around and see.

[johansen]

Gilbert, thanks for your reply. I look forward to hearing from someone to see how to go about doing this on the boat I describe, or on a similar boat.

thanks again....

[PAR]

This isn't that uncommon a conversion, though it will require a designer spec up a set of engine beds and do a basic weight distribution analyses.

Give Joel a call (the place where you're looking at plans) and see if they have a monohedren hull, about that size arranged for an inboard.

Of course, the inboard with a straight shaft will eat up most of the cockpit space. Possibly a V drive arrangement.

[Willallison]

The obvious question - to me at least - is why? The outboard will likely be cheaper, a 4-stroke will be damn near as cheap to run, and as PAR has pointed out, will probably result in a more practical boat. Plus you can tilt and trim an outboard, making beaching and trim easier...
You may, of course, have a good reason....

[johansen]

Well, I decided to steer my attention towards a different boat of roughly the same dimension as the previous boat that I listed. The Bravado from the Glen L Website allows for the option of an outboard, or, inboard with outdrive, weighing up to 650 lbs. This boat, it seems to me, accommodates an inboard engine quite well, leaving all purpose room on either side of the engine. What are your thoughts on this boat in general? I am especially interested in your thoughts about the construction method of this boat. Efficiency, is the main motivator for an inboard (diesel) with outdrive. Biodiesel sells for 30% cheaper than petro diesel down here and it is nearly as efficient. You can make your own for much cheaper as well, if you chose to do so. Your input and comments are greatly appreciated.

[Gilbert]

There are more things to go wrong with outdrives than either outboards or conventional inboards due to their complexity.
Just thought you should know.

[bear]

Most small boats are designed by the marketing department -- an outboard is therefore the obvious choice, as the customer can easily chose the power he wants, without alteration to the boat -- the boat therefore appeals to a larger customer base. There are times however when an inboard diesel is a superior choice.

I don't know the size of boat you are looking at, but judging from the recommended 50 HP outboard, it is probably 16-20'. In my case, I had downsized from a 38' SF with twin V-8 Detroits to a 28' express with twin gas, and now wanted a boat in the 20' range, with a diesel. I picked the boat I wanted based on underwater shape and boat design, and ordered the plans knowing that I would build it only if I could change the plans from an outboard to an inboard diesel. Because of the style of the boat, it looked silly with a modern outboard hanging off the back. After receiving the plans (there were no line drawings, just templates), I used a simple graphics program (not CAD) to draw the profile of the boat to scale (using 16 pixels to an inch as the scale). After proving I had room for everything, I emailed the designer describing in detail the changes I wanted to make, and asked for his blessing -- he said "Go for it!"

The alterations were not extensive: raising the cockpit sole 1/2", sistering the plywood frames with mahogany floors, notching and bolting stringers to the floors, and adding knees from the stringers to the transom.

Interestingly the inboard design provides more usable space than the outboard version.

You certainly wouldn't need a 50HP diesel to provide the same oomph as the 50HP outboard, and it would likely change the balance of the boat too much. I opted for a 30HP Kubota which I expect will provide performance close to a 50 HP outboard.

There are disadvantages to a diesel. You'll have to go up a few sizes to get a gear with the correct ratio. Diesels in the 30HP range generally go into much heavier boats so the ratios are wrong in the smaller gears. Another disadvantage of a diesel in the limited RPM range; you'll probably need a trolling valve to slow it down for docking! :-) Then you'll need an interlock for the trolling valve so you don't fry the gear! :0-) Then you'll need...