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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I just came across this conversion . . .



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Interesting design with the extensions on either side of the motor mount area. Those are needed for stern boarding access. I will probably want something similar.

 

Yes, it's a Chaparral, so I wouldn't touch it

 

Quick Update . . .

The boat will be coming out of the water for the season soon. So, I'll be 'diving' into this conversion project. Over the summer, I have refined the design a bit. Attached are a few drawings for your viewing pleasure.

My plan is to make the extension/bracket out of Coosa cored fiberglass. Based on my parts layout, it looks like I'll be using 6 sheets of Coosa. The estimated weight is around 475 lbs., which initially seemed like a lot (to me). By comparison I estimated the weight in aluminum (5083) and came up with 630 lbs. Cost of the materials is estimated at $5,000.

I'm going to be getting quotes to narrow down the pricing on the re-powering, but I'm anticipating that the final pricing may not be until the shop sees the boat ready to go.

First steps will be taking out the engines & drives and getting the materials to fabricate the extension. I have a special technique in mind for making an exact fit to the odd-shaped transom of my boat. We will have to see how that works out.

Here are my latest drawings . . .
Even though Mercury is shown, I plan on pricing out Mercury, Yamaha, and Suzuki. Probably stay in the 300 HP single prop versions of those motors.


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You will be running in turbulence?

Not sure the offset is wise that close to the vee.

 

One consideration I have is with the flotation of the extension/bracket and change to the static waterline, given its fairly large size and underwater profile. The software that I am using for the 3-D models has the ability to calculate volume of an object. In order to get the volume function to work, I had to make a separate file with just the below waterline shape. Here is a picture of the 'underwater' model, which is 4" underwater at the transom ( as shown).
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I had 'guesstimated' the underwater volume to be about 24 cubic feet, but the actual model calculated it to be 32.8 cu. ft. So, I expect some rise of the static waterline from the extension. The extension itself will lose 2.55 cu. ft of displacement for every inch of rise . . . more significantly will be the loss of displacement of hull as it rises at the stern.

I know some folks test launch their boat after the hull work is done to get an idea of the static waterline. Since there will be about 1,300+ lbs. of engines and rigging added to the transom, I'm not sure if a pre-launch would prove much . . . other than a water tight test.

 

I did some rough estimations on the amount of rise in the static waterline from the added displacement. . .

The boat will need to displace about 172 cubic feet of water to float at its projected weight. The hull extension will add about 32 cubic feet more displacement if at the original depth of submersion.

The hull draws about 24" , so on average each 1" of submersion will yield 8.5 cu. ft. of displacement. The hull profile being triangular in shape . . . yields about a 17 cu ft/inch change at the water line and a zero cu. ft./inch change at the keel. So, loosing the 32 cu ft of extra displacement from the extension will need about a 2" rise of the hull. Given that the extra displacement from the extension is at the stern . . . the rise might be more in the 2.5 - 3" range
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I could probably do a 3D model of the hull below the water line to get a more precise calculation, but not sure it is worth doing given that 2-3" seems pretty typical. I'd anticipate storing more gear in the newly formed storage area of the engine bay to balance things out.

 

the extension will lose 2.55 cuft for every inch of rise

hmmm

Are you saying it will lose displacement on the conditional if it rises above static waterline? Or if your calcs are wrong?

Probably you are adding some aft buoyancy which would tend to lift the stern a bit. Is this the worry then? My hunch is that it’ll net out pretty well with engines moving. Worst case, some ballast or leave the battery location a tbd.

 

In post #1 I did some calculations on the changes to the overall weight and movement of the balance point (CoG). These calc's were not considering the displacement volume of the hull or the extension, just merely the weight. Having now calculated the volume of the extension at a 'probable' water line and seeing that it will be fairly large (32 cu. ft and 2,000 lbs. of displacement), I wanted to make an estimate of how much it will change the static water line (i.e. raising the boat by the stern). Obviously, the extra buoyancy of the extension will cause the stern to rise some. Essentially, the stern will rise to the point of equilibrium in weight vs. displacement. My numbers point in the direction of 2-3" of rise as compared to how the boat sits in the water now. I think this is a manageable amount, and as you say - stuff can be moved aft to trim how the boat sits as needed.