Twin I/O to Twin Outboard Conversion Project

[fallguy]

The stern will rise to eq of buoyancy and mass..maybe that is what you mean, but technically, not displacement.

Your CoB moves back.
Your CoG moves back.

 

[tpenfield]

The stern will rise to eq of buoyancy and mass..maybe that is what you mean, but technically, not displacement.

Your CoB moves back.
Your CoG moves back.

Yes, correct, that is what I mean. In this instance, the hull will rise at the stern so that buoyancy = displacement.

 

[tpenfield]

FWIW - I just did a little bit of modeling on my 3D software to check my assumptions on the change in buoyancy as the hull rises/lowers (out of)/(in to) the water.

Here is a 1" thick 'slice' of the hull (without the extension) at the water line . . . The model calculates the volume (cu. ft.)
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. . . consistent with my assumptions/estimations.

 

[tpenfield]

OK - Labor Day weekend is upon us in the USA. I probably will be using the boat a few times over the weekend. Then, the boat will come out for the season and I can begin the actual work. Probably will start by taking the engines out. In order to pull the engines, I need to take the rear seat assembly off first. It is pretty heavy . . . I'd guess 400 lbs.

So, my first step will be building a gantry to do the heavy lifting . . . rear seat, engines, etc. One of the design considerations is that the gantry needs to span the beam of the boat (10' 10"). I plan on using a 12' span of lumber, netting out 11' 1" of clearance. I plan on using 2 2x10 joists with bracing and doubled-up 2x6 lumber as shown below . . .
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Thinking/hoping it will be enough . . .

 

[fallguy]

Plenty. I made one 16’ wide with 3 2x12. It could pick up a ton, but I was scared.

Not sure what the top intersect is, but it cannot pinpoint.

Mine was actually a double gantry.

 

[tpenfield]

I think the engine weight is around 900 lbs. each. I'll also need to re-position the chain hoist along the span for each engine . . . IIRC, the engines are 35" apart.

 

[DogCavalry]

A number of design details for your lifting frame are worth pointing out:
Make sure of your load paths. All loads should be carried through timber, not fasteners.
Any flaws in your cross beams must be on the top edges, not on the bottom edges. Place accordingly.
If there is too much bending in your cross beam assembly your can add upper and lower flanges to make it an I-beam.
The structure you drew has great strength in the vertical plane, but is less able to handle side loads. Visualize the whole process so you don't drop anything.

 

[BlueBell]

Thinking/hoping it will be enough . . .

What, no praying?

 

[DogCavalry]

On second thought, hire a truck with a HIAB. Faster, cheaper, safer.

 

[comfisherman]

Boom truck here is 175 hr for the cheapest one, clock starts when he leaves his lot. If you have lots of little pulls it adds up, for single big lifts its not bad. Would think some overhead lifting would be nice to have on site.

 

[tpenfield]

I have a 19' sailboat rebuild coming up after the outboard conversion. So a gantry on site will come in handy.

 

[montero]

Gantry is always a cool thing.

Let's get back to the conversion. Are you confident the boat will work well with that kind of conversion ?

What I try to said earlier , you need some kind of hull elongation . Using Coosa and some glass : who do you believe more ? When you consider strenght .

If I will make this conversion , I will make GFRP hull elongation with Coosa or plywood transom only . 2x350hp.

 

[tpenfield]

Here are some quick comments below ( in bold) and I plan on posting some illustrations later. . . .

Gantry is always a cool thing.

Let's get back to the conversion. Are you confident the boat will work well with that kind of conversion ? I am confident that the boat will work well, better than it does now. I am also understanding that it may need some fine tuning in terms or weight balance and trim settings. There is no way to tell for sure, but I am confident in what I am doing.

What I try to said earlier , you need some kind of hull elongation . The planing surface of the hull will be about 3' 4" longer than before. The additional planing surface will not be the full width of the transom, but I am going to make it as wide as I can without too much transom alteration ( 6 feet wide vs. 9.5 feet transom width). I have planned a 2" 'step' in the planing surface to make it more efficient. Not sure if folks here have any different approaches on having a step vs. 'no step'.

Using Coosa and some glass : who do you believe more ? When you consider strength . I believe in fiberglass

If I will make this conversion , I will make GFRP hull elongation with Coosa or plywood transom only . 2x350hp. I'm going to be glassing over the Coosa board and going with 2 x 300 HP engines. I think the elongated planing surface will make the boat plane more easily and not require as much HP as it has now.

Illustrations to follow soon. . . .

 

[tpenfield]

One of the challenges of this project has been the unusual shape of the boat's transom vs. the desire to get more planing surface. . .

Here is a image of a 3D model of the boat's transom area.

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The dimensions show that it is inset quite a bit from the stern of the boat and the center 'flat' area is only 58" wide vs. the overall width of 114".

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A traditional bracket that was secured to the flat part of the transom could only be 58" wide. Certainly enough width to mount the engines, but not enough to provide a decent planing area, IMO.
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Here is the transom area with the trim tabs . .

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I decided to work with the 72" width, which would mean the the mounting surface would no longer be flat (Orange shaded area). Potentially, I could mount/adhere the platform to a greater area of the transom, including the underside of the swim deck.

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Here is an illustration of the mating area of the bracket (shown in orange)

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A traditional bracket, made out of aluminum would need an air gap (space) on the top surface to avoid trapped moisture and corrosion of the aluminum. If I make the bracket out of fiberglass, it could be adhered and fastened to the underside of the swim deck as well as the transom.

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Here is an illustration of the bracket as it would be installed to the transom.
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The bracket would be secured with adhesive and fasteners on all mating surfaces, including the top. this will provide a greater area of support than a traditional bracket installation the is relying on an area of the transom for support. Shown are the 2" step in the running surface and the 40" (+/-) of additional running surface.
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Here is an illustration of the engines mounted on the bracket.

 

[tpenfield]

A couple more illustrations for your viewing pleasure . . .

The will be about 80 fastener locations (probably 3/8" SS bolts) . . . not sure I'll need this many, but we'll see.
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Then there is the transom bracing. The boat manufacturer send me some pictures of the bracing that they do for their outboard versions. My bracing design is similar, but should be lighter & stronger due to the 'cross support' piece at the top of the transom. The added bracing is shown in white (below). . .
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I should be taking the boat out of the water soon, then the 'fun' will begin.