Twin I/O to Twin Outboard Conversion Project

[tpenfield]

For anyone crazy enough to take on a project like this, here are some final cost numbers. . .
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If I had to do it again, I could probably shave $10K off the bracket build-out, but the re-powering would probably be more.

 

[tpenfield]

Here are my initial performance numbers. I'll be doing a second run of tests with a bit more trim up, since I was not trimmed very much.

Time to plane and planing speed are greatly improved.

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Prop slip is really low . . . more so than I have seen with the sterndrive engines.
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[tpenfield]

The Savitsky analysis that the Naval Architect did showed that the boat would come up on plane at about 18 mph, and the test results are consistent with the analysis. . . pretty much 18 mph and 2700 RPM. These numbers were with light load, about 50 gallons of fuel, and typical gear (300 lbs.)

 

[BMcF]

Awesome. Your results are generally similar to those I had when I repowered my Rampone 25 deep-V hull, from a pair of 140HP Volvo inboard straight drives to a pair of 150HP Yamaha outboards on a transom bracket. Huge performance improvements across the entire operating range and less than 3" additional aft depth/trim. Your attention to detail and fabrication quality has been impressive.

 

[tpenfield]

I am hoping to get more in the 5,500-5,700 RPM range and 50 mph for speed. We shall see. The re-power company will help decide on any prop changes, etc.

 

[fallguy]

How much fuel? How far can you go offshore?

 

[tpenfield]

How much fuel? How far can you go offshore?

I was doing the test runs with 50 gallons of fuel. I can go off shore and keep going until I run out of fuel.

 

[fallguy]

I was doing the test runs with 50 gallons of fuel. I can go off shore and keep going until I run out of fuel.

I mean what kind of range do you think you can get? I’m curious because my range is a little less than I had hoped is all. And I’m trying to decide an action.

 

[tpenfield]

I mean what kind of range do you think you can get? I’m curious because my range is a little less than I had hoped is all. And I’m trying to decide an action.

Not sure of range because I have not gotten mpg figures yet. The prior configuration got 1.2 mpg, so that would mean about 60 miles out on the ‘1/3, 1/3, 1/3’ rule.

 

[comfisherman]

Looks good, got completed, appears to work good... heck of a project and thread. Thanks for keeping us in the loop, it has been both entertaining and informative.

Would love to see a Pic of it ripping across the water.

What was the a team quote... I love it when a plan comes together.

 

[tpenfield]

Thanks for your kind words, I have enjoyed sharing this project and I hope it will benefit others doing similar projects.

Here are a couple of pictures at the mooring. The outdrives clear the water at full tilt, which is a challenge with some of the outboard installations.

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[tpenfield]

A couple more pics of the wiring/rigging in the (former) engine bay . . . I still need to install some level flooring and arrange some storage . . . maybe a couple of totes. For now I just have one of the tables in there.
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The white unit next to the battery box is the dry bilge system that I just installed. It works pretty well in getting the last inch or so of water that the main bilge pumps cannot get.
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The steering system is the fully electric (Optimus EPS), so it does not need hydraulic pumps and the associated hoses, which would be in the engine bay.

 

[tpenfield]

Here is a graph that might be interesting . . .

I looked up the BoatTest data on the 'standard' CY338 Outboard version that came out in 2019 (Verado 350 w/ 16x18 propellers). I often feel that BoatTest speed data is slightly overstated, or perhaps done in overly ideal conditions. Anyway, I compared the data that BoatTest published on the boat vs. what I have recorded so far on my boat (Honda 350 w/ 16x19 propellers).
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In terms of the boat coming up to speed, the extension bracket is making a huge difference, and probably 5-7 mph difference in the planing speed. My boat is nicely planing at 3,000 RPM, while the stock 338 OB is still trying to get-out -of-the-hole. I'll have to see what this translates into fuel consumption, etc. But . . . I think this chart demonstrates the biggest benefit to doing the conversion with an extension (i.e. additional planing surface) vs. just a bracket.

 

[tpenfield]

Quick Update . . .

With a good amount of up trim, I got 50 mph @5300 RPM. So, that hints at the engine height being too low. I hooked up a remote camera, so I could look at the water stream while on plane . . .
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Not the best picture, but the height seems to be low, because the ventilation plate is really not visible.

Anyway. . . I'll be doing another run at it this week as the sea conditions are supposed to be calm for a couple days. I'm just wondering about any good/better ways to get a visual or tell how low the ventilation plate is vs. the water stream.

FWIW - There is about a 30" setback of the engine's lower unit to the end of the running surface. The engine was set 1" higher than the running surface, but I'm thinking it may need to go up an additional 1.5" to get to the [1"/12"] 'rule'.

 

[fallguy]

I turned the pic for you is all. For your situation, I’d have used a manual jackplate with the amount of uncertainty you faced. You still can if rigging allows it and no new holes?