Twin I/O to Twin Outboard Conversion Project

I am modifying the bracket design to provide an air gap between the underside of the swim deck and the top surface of the bracket. I'll have a few 'tabs' to fasten to the lower edge of the swim deck in several places along its radius. This approach should be easier to design/fabricate, because the transom angle does not have to be as precise. I measured the transom angle to be 12.5˚ +/-. The manufacturer of the boat has their bracket with a transom angle of 16˚, which I think is a bit excessive. (Probably to give the outboards extra 'in' trim). I probably will go with a 14˚ transom angle at the bracket as a compromise between the 12˚ and 16˚.
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Also, the maker of the bracket for the Chappy 327 states in their product information that they use a combination of 6061 and 5052 series aluminum. My nautical engineering sources tell me that 5083 series would be better ( accordingly more expensive, but not a deal breaker ).

As a next step, I may try to make a fairly accurate 3D model of the transom of my boat. It has a lot of unusual shapes and angle and is deeply inset under the swim deck.

 

I arrive very late but before spending 100 k at least plus hundreds of hours of work for installing outboards, I would consider simpler solutions.
The problem you have exposed is one very common on the kind of boats you have. Rather heavy, deep V, and rather short in length, boats like this I've seen dozens in Mexico with the same problems.
There are several lines to explore:
1) Lose some weight if possible taking out all the no needed items.
2)The Mercruiser engines are pretty good with heat exchangers so the engines are cooled with a true coolant and are running hot at the good optimum temp: 92 degrees Celsius. If the engines are cooled at only 50°C with sea water they do not work as well plus having the problems of salt deposits and internal corrosion. The ECU must be tuned for the 92C. The air entries, air filters and exhaust circuits have to be checked, the engines must breath at their full potential, specially at acceleration. Many engines do not run at their full potential.
3) I do not know what transmissions are used, but I can say that on rather heavy motorboats the Bravo 2 with 17 to 18 inches 4 wide blades propellers solve many planing problems. Too many boats have too small diameter propellers with too much pitch, so they can't accelerate as the propeller is cavitating while the engine is overloaded. The Bravo 1 uses like the 300-350 hp outboards 15.25 inches propellers, and that's to small in diameter for a good efficiency with 300 more hp.
4) Last but very important, wide, strong big trim plates with hydraulic actuators are often the solution as they modify the dynamic planning surface to get the boat out of the hole. Combined with the transmission trims, that works 90% of the time
All that is far less expensive than 2 new outboards plus the pain and time of installation, plus solving the new problems that will appear.
Personally I prefer largely the Mercruisers over the outboards. The mercs are simple good old V8 cheap and easy to maintain. Spares are rather cheap. Easy to rebuild. The 6 liters 300 hp well tuned had a very good specific consumption under the 200gr/hp/hour. Well maintained they last far more, on the patrol boats of the Mexican Navy the 6 liters could go to 2000 hours.
Installing two outboards is a hard and expensive task, please think over that twice. I won't enter in the problems of such a mass hanging behind (longitudinal polar inertia) with the center of gravity getting higher (lateral dynamic stability) plus the torsional problems added to a boat designed and calculated for inboard engines.

 

In Australia many people convert their Bertram 25s to dual outboards. Problem is lifting the C of G higher and making an already tender boat worse. Many fit Seakeeper Gyro stabilisers to reduce the effects of the repower. Something to think about.

 

Thanks for the continued comments . . .

@JonW A friend of mine has the same boat and installed the SeaKeeper gyro . . . He didn't have anything good to say about it. I'd do a wait-n-see on a gyro.

@Ilan Voyager . . . a few bits of information in reference to the points made . . .

The engines run at 74˚C (165˚F). I run a De-scaler solution every 1-2 years to keep the internals of the cooling system clean.

The outdrives are the Bravo 3 with 24P prop sets. The prop sets have the 4-3 blade combination.

The engines are the new(er) in-house built 6.2 litre engines, CAT exhaust, drive by wire, etc. So, parts, etc. are not as economical as the prior GM-based engines. (My previous boat had the twin GM 454 (7.4L ) and I could get fairly economical parts for it.)

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I do have trim tabs . . . They are 12" x 18". I may consider extending them.
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I have taken about 400 lbs out of the boat through various projects . . . Battery Upgrade, removing a redundant refrigerator, minimizing extra stuff in the boat.
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The Chaparral 327 that went through a conversion is the best example I've found so far. WeldingWorld, Inc. Makes the bracket. My boat would need something very similar, and I've taken a few pointers from what was done, as mentioned in earlier posts
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Overall, I may wait another year, as the boating market is getting soft and I believe better deals on engines, etc. may be had in 2027 and beyond.

 

With all due respect, the boat in the last photo looks little strange. What special solutions are there in your boat that make it impossible to sell?

A well-considered purchase of a new outboard hull is probably the best idea.

If you insist on the modification, extending the hull with composites is a good idea.If you want only aluminium bracket it should be full beam and full float hull extension.My question is than strenght of connection between hull and bracket .Your initial problems with boat handling are mostly due insuficient lenght of the hull.

 

Besides, I think that selling a complete original boat with attractive extras can be successful.Selling just the drives is a different story.

 

Yes, I tend to compare boats by their weight vs. hull length. Not a totally scientific approach, but it gives some insight. I consider it analogous to wing loading on an airplane.

My boat is 10-20% 'heavier per foot' than many of the similar boats. The best of the boats that I have compared my boat to is the Cobia 330 DC, which is about 20% lighter per foot. There are some boats heavier per foot than mine, but my boat tends to be on the high side.

Anyway, I've got time to consider the options.
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