Twin I/O to Twin Outboard Conversion Project

Thanks for the info on your experience. What model Double Eagle was it? I'd like to review the specs of the boat.

 

I don't know. It was ~20 years ago. The boat is long gone.
Big enough to be certified for 14 people. Sorry, that's all I got.

 

Probably a Double Eagle 34 Flybridge Sportfish (?)

This is a 1989 with twin Volvo sterndrives.

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Another possibility is the 22 Command Bridge, but that would be overloaded with 14 people.

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Anyway, if it was either of these boats, having structure up top, I could see why going to twin outboards would be a significant change to the CG.

 

The knee braces will really help with the stress you were going to be imparting on that joint. Make sure you still tab the horizontal portion inside and out as there will still be stress there on the outside edges during turns. Or, if you opt away from the knees, make sure to tab the area well as there is a concentration there.

5.2” up with the vcg is a bit, but I think the flybridge of those other boats was the big driver of their tippiness.

Not to make you nervous, but I’d be more worried about pitching or props coming out (in ocean swells) than roll.

 

I'm thinking that there will be a period of sea trialing, adjusting, and getting used to the boat's new handling characteristics. Many of the multi-engine conversions that I have reviewed needed such. . . more so on the triple and quad conversions than the twin engine conversions.

 

Can you calculate/estimate the center of buoyancy change vs LCG? Just curious. If you already did and I forgot; sorry in advance.

 

EDIT: my bad; I misread your post. I did some buoyancy calculations earlier on in the thread.

My CG calcs are below. . .

Yes, I did. I put a picture of the original and projected CG in post #194.

No worries . . . I didn't post the calculations so as not to bore people, but here they are. . .




The Calculations . . .

(12” x 1400lbs) + (19” x 2250lbs) - (12” x 400lbs) = 54750 in-lbs

54750 in-lbs = ? in x (11000-2250+1400+400) lbs

54750 in-lbs/10550 lbs = ?

? = 5.2 inches​

 

Sorry for the confusion on my last post . . . watching football while reading posts

Anyway. I did some calcs a while ago and determined that the static water line will be up about 2.5" . The CG impact on stability really comes into play while the boat is on plane. No good way (for me) to calculate that. I think it will be a wait-n-see as to how well the boat runs in its new configuration.

 

Here is a diagram that combines CG and CoB (Center of Buoyancy), in both a static and 'tipped' positions. The nice thing about CG and CoB is that they are aligned vertically while the boat is at rest, and tend to seek alignment to the extent that they can as the boat moves in any direction.

My general thought has been that the beam of my boat is wide enough to compensate for a slightly higher CG. Basically, as the CG is higher relative to the submerged portion of the hull, it gets closer to the CoB while heeling over, as shown in the 'tipped' diagram. I've done similar analysis for my sailboats over the years in terms of their ability to right themselves from a knock-down.

One factor that is not shown is the effect of propeller thrust from a lateral perspective while turning, a factor that is common to all power boats. I often use this lateral thrust to quickly level my boat while underway in choppy seas.

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Anyway . . . taking a look at the CG has been a good exercise to raise awareness and any concerns. My thoughts are that boats with a fair amount of structure above the main deck, such as flying bridge designs, should have a careful review of changes to the CG in considering a re-power conversion.

 

Quick Update . . .

I am on to ribs and gussets in the internal structure of the extension pod. My plan is to do as much of the internal structure before I put the planing surface and engine mount pieces in place (orange areas shown below), which will limit internal access.

 

Progress Update . . .

I have got most of the internal ribs and gusset cores installed. The fillet party will begin soon. . . following by glassing of the internal details (gussets and ribs).
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The next major step will be to install the planing surfaces and get those glassed in. The are a few more gussets to be installed on the internal side of the planing surfaces.

I may see about weighing the extension pod at this point to determine if I'll be on target for the estimated weight of 400 lbs.

 

I weighed the extension pod as it is now . . .

168 lbs.

I estimated that I have about 186 lbs more materials to add, bringing the total weight of the Ext. Pod to ~ 354 lbs. Probably another 50-75 lbs of transom reinforcement in the boat, making the total 400-425 lbs.

 

Weep holes in the bottom of the stringers are installed . . . Fillet party was real 'fun' . . .
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Ted,

A thought, this may have already been addressed in your thread, have you had a surveyor or insurance agent perused your progress?
In the interest of maintaining a good relationship, it may be prudent.

Isn't filleting fun!

 

I have discussed the changes with my Chief Insurance Officer (Aka Admiral). We will update the engine power on the policy and take it from there.