I/O (Sterndrive) Conversion to Outboards

[redneck joe]

You could probably preheat the resin in a warm bath to help too... Starting with uncatalyzed resin at 75-80 degrees may help?

Totally ignorant question but would the more rapid cooling be detrimental?

 

[Pmt133]

Totally ignorant question but would the more rapid cooling be detrimental?

I don't know. I know guys chill poly when doing layups in the heat... I'd imagine it would work fine the other way. Once it kicks... it kicks in my experience with it.

 

[tpenfield]

My preference is to start cold and add heat once the glass is laid up. I’m thinking if the resin were to start warm, it would tend to kick too fast and cooling it down would be too late.

I’m using VE resin, so it’s open time is a bit longer than poly resin.

 

[Pmt133]

Yeah. Even at 1% when I was glassing at 100f that stuff was cured by the second layup (half hour or less). Poly was hell to work with when warm.

You could always bump up the catalyst too. 3% is still acceptable for most resins at the right conditions. Though I understand why you'd want to be closer to 2.

 

[Scott Danforth]

ted, buy some infrared heat lamps

 

[tpenfield]

I got out there early this AM to check the glass/resin. It cured OK.

Here is a pic of the 4 pieces of the contact area set in place and glassed at the edges/corners.

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I used some cardboard on the transom as a 'spacer' to mimic the fiberglass thickness, then stuck the Coosa pieces to the cardboard. It seemed to work OK. I figured the cardboard was about 3/32", which would give me about 0.09" thickness offset to account for the glassing on the transom-mating side.

With a couple of swift tugs, the thing came off. Looks OK . . . probably as good as I was going to get.

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I put the center support in place to see how well it matched the transom angle . . . Looks good!

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No gaps . . .

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I need to check some dimensions and then start putting the rest of it together. The transom 'plate' and the top plate are going to have some decent-sized cut-outs for fabrication and assembly access. I'm thinking that I won't make the cut-outs until much of the box structure is fastened together.

 

[tpenfield]

I moved things to a more sunny location in the yard . . . I think the sun will be enough to cure the resin.

I installed the main (center) support. This piece I am glassing in - just so it stays put during assembly of all the other pieces.

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[tpenfield]

It's a good thing that I was paying attention in Geometry class (way back when) . Figuring out the shape & dimensions of the outer supports has been a bit challenging, but I think I got it now.


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I'll have to make an adjusting cut in the center support to match the outer supports. (I knew it didn't look right . . .)

I'm trying to capture this shape (below) for the transition from the planing surface to the outboard transom.
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Hopefully this will be right . . .

 

[redneck joe]

You gonna extend chines?

 

[flashback]

All this gets bolted on to an existing transom that to my knowledge was not designed to carry much weight.

 

[tpenfield]

You gonna extend chines?

Not extending the strakes (if that what you are asking) The extension will have a somewhat mild chine (as shown). I might give a little bit of 'lift', but not sure.

 

[tpenfield]

All this gets bolted on to an existing transom that to my knowledge was not designed to carry much weight.

This has been covered along the way . . . the transom gets additional support bracketing, much like the factory does on their outboard versions.

 

[tpenfield]

Morning talk . . . While I wait for it to warm up outside.

Some folks (here and elsewhere) have expressed concern about sterndrive transoms not being as strong as outboard transoms. Probably true, but I like to work with facts & figures as to how much stronger. So, I did some web searching and did not come up with much.

So, I threw some numbers at it and came up with some approximations . . . This would be for 350 HP in a boat going 41 mph (i.e. 60 ft./second (YMMV) ) and having a sterndrive vs. a 30" shaft outboard. It looks like the torque on the transom is about 2/3 more with the outboard (a lot having to do with the shaft length).


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From this, my rule of thumb would be the need for the transom to be about 2X stronger.

 

[Pmt133]

Is the core thicker on the outboard version or is there just more internal bracing?

 

[flashback]

Thanks for clearing that up for me, I suppose a couple of knees fastened to the stringers in the stern should do it.

 

[Scott Danforth]

to expand on Ted's #433 post. an I/O is pushing the boat from the bottom of the transom and the hull, where the stringers and motor mounts come together. this is trying to pry the bottom of the transom off a boat with a 15" lever arm the majority of the weight of the motor is supported on the stringers and motor mounts

an outboard is trying to pry the transom off the back of the boat with a 30" lever arm and push from the top of the transom, and all the outboard weight is on the top of the transom

this is the primary reason that hulls with outboards have a fixed HP rating and the same hull with an I/O is not rated. simply because outboards put 2x to 5x the forces into the transom than an I/O does.

@tpenfield , make sure you have good knee braces. I would do a bulkhead dead center on your outboards splitting the mounting pattern

 

[tpenfield]

Is the core thicker on the outboard version or is there just more internal bracing?

I don't know for sure, but it may be worth a look at the guts of some outboard boats.

My understanding is that transom thickness is 2.00-2.50" . . . so similar to sterndrive transoms.

 

[Scott Danforth]

core material is just the filler between lamination schedules. the strength is in the lamination schedule. the transom is the same between I/O and OB. just more bracing in the OB versions

 

[tpenfield]

The big differences (that I can see) comparing center consoles (outboard boats) to sterndrive boats . . .

Center consoles tend to have the main stringers higher up (taller) and meet the transom closer to the top of it.

Sterndrive stringers meet the transom much lower in order to accommodate the engines.

Outboard boats have 100% of the engine weight on the transom, sterndrives have 80% of the engine weight inside the boat. Outboard weights tend to run about 50-60% of sterndrive weights.

The dynamic load of the engine on the transom can get to 5X of what a sterndrive transom may see in extreme conditions.

I can probably calculate what a 5G load would look like vs. the static (1G) loads . . .

 

[tpenfield]

In other news . . .

I'm going to be on a 'fixed income' starting this month and the Admiral retires in 4 weeks.