Twin I/O to Twin Outboard Conversion Project

Greetings . . .

After much consideration I have decided to do a conversion of my Twin Inboard/Outboard (Mercruiser sterndrive) powered boat to Twin Outboard power. It is a Cruisers Yacht (CY) 338 Mid-cabin Bowrider (33' LOA, 10' 10" beam, 11,000lbs. Dry weight, twin 350 HP sterndrives)

In a prior thread, I considered adding Transom Pods to address some planing issues, and more lately I have also become tired of the maintenance of the sterndrives. Thus, I am now on the track of an outboard conversion to address both planing and I/O concerns.

As an alternative approach, I have thought about selling my boat and buying a similar boat with outboards as a way to make the transition. The considerations being that I really like the layout and amenities of my boat and in getting a different boat I would potentially be doing one of the following:

1) Get a boat that falls short of my current boat's amenities/layout. (I have looked extensively)

2) Spend approximately $150,ooo more to buy the outboard version of my boat. (which was made for the last few years of the boat's 9-year production run). I would then probably spend the next couple of years making the boat my own with all the things I have done to my existing boat.​

So, all these things considered, I have decided to convert my boat to outboards. I have been in touch with the boat manufacturer (who have been helpful in providing information) and I also have been talking with my nautical engineering friends about doing such an outboard conversion. The outboard versions of the CY338 had the swim deck area modified (reduced) to accommodate an outboard bracket that placed the transom at about the end of the existing LOA. (I believe CY did this so as to minimize the changes to the existing balance (CoG) of the boat as compared to the sterndrive models.) This design required the swim deck to be notched/reduced to accommodate the tilt-up position of the outboards. Both the top and bottom molds of the boat were changed accordingly.

As a matter of preference, I plan to have the bracket extend further out beyond the swim deck, to retain the swim deck space, avoid alteration of the hull & cap sections of the boat, and provide some additional planing surface.

In pictures . . . my plan is to go from this: (current boat)

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to something like this:

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The bracket would be such that it has some planing surface (with a 2" step up from the existing hull) and then be notched for the last 18-20" leading to the transom.

Since this would most likely move the CoG towards the stern, I wanted to approximate how much of a movement there would be, considering the removal of the sterndrive engines, adding of the bracket and the outboards.
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Looks like about 9" towards the stern . . . I can deal with that.
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The bracket itself would need to be of a custom design, since the existing transom is quite recessed under the swim deck, far different from many of the typical OB conversions. The bracket will probably look something like this. . .
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A nice thing about this design is that the bracket, although being quite large, can be fastened to both the existing transom, and the underside of the swim deck. The bracket will have buoyancy (probably more than what's really needed) and about 3.5 ft. of planing surface.

I have design drawings and factory pictures from the boat manufacturer as to how they modify the sterndrive version to accommodate the bracket and outboards. . . It includes the typical knee braces tied from the stringers to the transom, etc.

My plan is to run the boat for 1 more year as a sterndrive, complete my research and design work, then start the actual conversion next September (2025). The sequence of events/phases will probably be:

1. Remove rear seat/engine hatch of the boat
2. Remove the outdrives and engines
3. Remove the engine controls (throttles, joystick, etc.)
4. Clean up the engine bay and helm as required
5. Final design of the bracket
6. Have the bracket fabricated at a local Marine welding shop
7. Perform transom modification (DIY - since this is in my wheelhouse)
8. Get price quotes for twin engine (300-350 HP) and controls purchase/installation
9. Have bracket installed
10. Have the engines and controls fitted
11. Re-install the rear seat/hatch
12. Shake-down cruise

I am figuring on a budget of about $80,000, which would include the credit for trading-in (or selling outright) the sterndrive packages (Mercruiser 6.2L DTS/Axius, (CAT), SeaCore (Closed Cooling), Bravo 3 drives.

Continued research/analysis between now and next year will include:

• Bracket design (planing surface vs. notch)
• Re-powering options . . . (Mercury may not provide the JPO option (joystick) in the after market - OEM only). Suzuki/Yamaha with Optimus controls for a fallback. (any re-powering Guru's feel free to chime in). I really want to maintain the joystick feature that my boat currently has, and I am learning that the joystick/digital controls adds quite a bit of cost ($15K ish) vs. the traditional installations.
• Other issues/considerations as they arise.

TIA for any experience to share.

 

15 for joysticks another 55-60 for budget 300hp outboards doesn't leave a lot for glasswork unless mercruiser inboards have a high resale value in your location. Skew towards the 350s and joysticks and it eats up 80k fast.

 

Yes, I figure $100K gross and -$20K for the 2 engines and outdrives, to net out to $80K. I do the glass work, and I work for beer.

 

How many hours on the inboards?

Can you state the planing issues you refer to (often)
and how the conversion will fix it.

And what if it doesn't fix it, then what?
Either way, you're going to miss the performance of those duo-props.

Thanks

 

Do these questions help with providing advice on the project or the design?

 

The 300/350 hp section is kinda a tough one. The 350 duo suzuki had almost completely decimated all the banked goodwill that suzuki built with the 300ap. Saw a ton of power head and lower unit issues over the first few years. Am told by the sales guy that suzuki has resolved it, but new boat builds locally are clearly avoiding them. The 300 duo uses lower octane fuel, has a lower compression ratio fuel system and supposedly the reworked duo lower unit. It's only a couple thousand more than the 300 single that's been the gold standard here for the last decade. Have one friend who charters and does tours who puts 8 to 10 hrs a day on an outboard. He's gone to the 300 duo from its release and loves them, it's a good enough endorsement it's probably where I'm leaning for my own boat. He had 300 ap setups for years the 350 for about 8 months (would have been less if others were available) and now the 300 duos for a couple years.

We don't really use mercury outboards up here, they don't seem to live long enough to gain traction up here. See a few big bore 600s but not many.

The new Honda 350 looks good, lord knows the early 225s were heavy but last forever. They aren't a lot heavier than the suzuki, maybe 40 pounds each... but they are physically huge motors. In my local they haven't been adopted yet as they were introduced about the time we started our economic downturn. Only 5 grand more than the suzuki duo, not bad for 50 extra hp.

Yamaha has a new 350 that's a v6 rather than the 8. Probably a good thing as it's based on the 300 architecture. That 350 was a pain with the flywheel issue drove a lot of guys nuts. Another friend has a higher hour set of v8 350s, who's most efficient cruise range lands in the range of rpm that causes the flywheel issue. He's looking to repower and it's down to the honda and new yam 350. Was all set on the honda but the 350 is 626 pounds and would shave off more than 300 pounds in the stern. Granted they are 4500 more each than the honda so it's not making his decision easy. Think if the honda was a little more competitive in the price range it would dominate as nothing else is close in weight.

 

Thanks @comfisherman . I appreciate the insight on the Suzuki duo's. I think I will get price quotes for Mercury, Suzuki, and Yamaha. I can certainly take a look at Honda as well. I have noticed the new Yamaha 350 V6. My concern would be where in the RPM range they are getting the extra 50 HP. One factor in deciding on an engine brand will be which brand is the best choice for digital controls and joystick docking in the re-power market.

My plan is to reconfigure the boat/structure, have the bracket fabricated & mounted, then have the engines/controls done as a re-power.

 

On the joystick front all have some version or way to interface with the mainstream systems. Have been a bit surprised at how much the integration can cost. That's something I'd certainly make sure to add in the quote. I've only done joystick install on a jet boat, it was absurdly expensive (more than the more mainstream outboard systems). Don't have a lot of experience with them, commercial side usually shoots for a more streamline approach.

My suggestion of the yamaha mainly comes from the weight, it's always easier to find a way to hang less weight off the stern and get balance. Although your doing significant enough surgery that anything can be accounted for. I'd say yamaha isn't likely to release a lemon and it should be a good one... but the 350 that preceded it had the never resolved flywheel issue...


From what I'm told by guys who have used various versions of all the above, the duo links up at cruising speeds and lower a lot better. For me I'm fingers crossed it's enough to offset the 50 hp loss.... we shall see.

Our use case is usually slower than down south given our higher prevailing winds, certainly changed what's needed in an outboard. Friend who is replacing 350s has his most efficient cruise between 34 and 35.5 knots.... a speed brutal when traversing our chop and normal swell. Lots of time were searching for more efficient slower cruises, that certainly skews my perspective.

 

No, but the answers do.

Thanks

 

Great!. I'm looking forward to any/all input on the project.

 

As far as engine/boat performance goes. The waters that I boat in (Cape Cod, Buzzards Bay, USA) are usually choppy having a fair amount of fetch from the Long Island sound. The typical south-west wind whips up 3-4 foot seas in the afternoons. As such, I don't get much opportunity to go over 27-30 mph (25 knots). So, not overly concerned about top-end speed (which is currently 45-47 mph with the I/O engines . . . 40 knots). There are occasions where we have lots of people on board, so the dual-props are attractive in that sense.

I have not found any hp/torque curve information on the outboards. There used to be some information available on the I/O packages, but I know the manufacturers have gotten away from publishing such information.

I see that the Mercury engines (V8, V10) have a couple of extra cylinders in the 300-350 HP range as compared to the Yammy and Suzuki engines (V6, V8). It might be good to compare the torque curves of those engines to see if there is a significant difference.

In the end, I think engine choice will come down to cost & features. I am expecting Mercury to be the highest cost and would think Suzuki would be the lowest. It seems that Suzuki engines are often the choice on re-power and conversions because of the price-point.

I am also thinking that I may need to favor new power packages - rather than used - since used engines could result in having to 'Frankenstein' various components together.

 

Question for those following along and may have some insight to the bracket design aspect of the project. . .

I have seen various approaches to bracket design in terms of flotation, planing surface, hull steps, etc. Each aspect probably having advantages and disadvantages.

Some brackets are designed merely for some extra flotation (at rest).

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Some have what may be considered a planing surface, although most are stepped up from the main hull surface, and provide such at the center portion of the hull.

Bertram 25 . . .

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Formula 280

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I even found one that was full-width of the hull and had no step, acting more like a hull extension. ( this boat seemed to have too much buoyancy in the stern once it was in the water . . . not sure how it performed. )

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The approach that I am planning with my boat (as shown in the earlier posts) is to have a bracket with a width of approximately 1/2 of the overall beam of the boat and about 44" of planing surface that is stepped up about 2" from the original transom. (similar to the Bertram 25 and the Formula 280 conversions shown above).

Any thoughts or experience on these various approaches?

 

I think before you start you ought to do a trim analysis. My bias is this way because of all the problems I’ve had with my prototype boat and trim problems the other way.

Your boat looks like it is not level inside at static rest in the pictures.

When you add the extension/pod, the buoyancy from the pod kicks in late at 2” up or worse, so I am super worried about your trim.

I read your original post and you responded with a 9” lcg move aft, but you made no calculation for how far below dwl you’ll end up. But really, you want to know what happens to 1st (static) trim hanging all the weight back there…

I’d do the hydrostatics more thoroughly before anything.

My hunch on your pictures, is the hull extension version was the way they had to go to keep static trim..

 

Thanks for your input.

I have and will continue to consider at least a couple of things, perhaps more.

1) Static water line
My rough calculations are that the bracket will provide more buoyancy than what is needed at rest, so the boat will rise somewhat in the water. I can perhaps put together some tricky math to calculate such, keeping in mind that as the transom/stern area raises out of the water due to bracket buoyancy, the hull itself then looses buoyancy. If I had to make an educated guess, I'd expect the stern to rise about 1".

As a comparison, here is an 'in-the-water' picture of the boat that received a full-width bracket (lots of buoyancy). It is a bit difficult to see, but it looks like the stern was raised about 2" and the bow is down about the same amount.

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Prior to the conversion the boat sat 'evenly' in the water as referenced by the bottom paint line. Of course, where goes the center of mass, so must the center of buoyancy.

2) Dynamic waterline/planing
Various information that I have encountered indicates that when you use a bracket to extend the outboard mounting further beyond the hull/transom, you can raise the outboard about 1" for every foot of extension (so 1:12 ratio). I'm not sure how much science vs. trial and error has been involved in arriving at this 'rule-of-thumb', but so be it.

My assumption is that the water rising up behind the transom while on plane is what allows the outboard to be raised, but of course this would all be speed dependent. Planing at 40 mph would be 58 ft/second, so with a 2 foot extension, the outboard lower unit would be 0.034 seconds (34 milliseconds) later than a traditional transom mounted motor. My quick math indicates that the water would need to rise at 4.9 ft/second to make that happen (i.e . . . a 2" rise, 2 ft back, going 40 mph) . . . That rate does not seem unrealistic.

I also am considering the amount of step-up the bracket will have from the main transom. A 2" step and a 44" planing surface would net out about a 2˚ pitch (up) of the bow, without factoring the water rise after the step. In all likelihood, factoring in the water rise after the transom/step it would tend to put pressure on the bracket, reducing the bow-up attitude.
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Anyway, a bunch of things to consider, but one thing that sticks in my mind is that you don't see the folks doing these conversions as the types with scientific calculators and pocket protectors . It seems like most conversions are done in a more intuitive manner.

 

Quick Update . . .

I came across an outboard conversion and bracket for a boat very similar to mine. It is a Chaparral 327 SSX, which I considered when shopping for my CY338.

Here is the boat after the conversion . . .

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and here is a look at the bracket . . . notice the space (air gap) between the bracket and swim deck.

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One take-away from this example is that the bracket design/installation is such to have a space (air gap) between the swim deck and the top of the bracket. I am thinking that this is done (vs. a bracket that mates to the underside of the swim deck) to avoid the possibilities of trapped water and the related corrosion issues.

I may apply this idea to my bracket design, yet still have an attachment point at the end of the swim deck via a flange of some sort. This would make the fabrication and installation a whole lot easier, because the angle of the transom and the underside of the swim deck would not matter.

While this particular bracket is similar to what I am planning, it is too small for my boat, but the idea of an air gap is useful in working towards a final design of the bracket for my boat. The bracket in these pictures (weldingworld.com) is priced at $8,200. So, it is also a good budgetary reference point.