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I’m looking at a B26 i/o and thinking about pulling out two Merc six’s and Alpha legs and redoing the transom to go to a single Merc 8.1 with Bravo leg. Any thoughts about performance or whether this is a good idea? I wonder how many RPM I’d be running to get what cruise?

You would see a big improvement in performance and fuel economy. Way less drag, the only thing you lose is the redundancy of a slow putt home if an engine dies or you wreck a prop and maneuverability at low speeds, oh and maintaining two engines and drives instead of one, you'll lose that too!

In terms of RPM and props and gear ratios for the leg etc.. if you give the weight and length to the merc dealer they should be able to work out what gear ratio and what prop.

Thanks Rocket. I know that even two fresh 5.7s in the twin inboard model have to both be working quite hard to get 22 or 23 knots (3200 rpm). This speaks partly to a bit of inefficiency with inboards but also to how much hull these boats have in the water (why we like them) with the full deep v. Is the efficiency of an i/o really that much better that a single big block v8 would see that kind of cruise without needing to be wound up to a 3,800 rpm to get a cruise of that kind of speed? That is faster than I’d want to spin a motor long term.

I have two B25's that were converted to single v8 duoprop i/o's.
One has a big block rated @ 375hp, the other a small block rated @ 350hp.
They both easily cruise @ 25mph to 35mph and top out in the low to mid 50's.
The duoprop o/d tracks straight and true in forward and reverse around the docks.
I would love to do one with the 6.2L ho rated @ 430hp.

Speed to RPM is very linear aswell.

Thanks John. In the end I bought a twin inboard B26 rather than the I/o because it was in much better condition. I am considering what to do for repower. Putting fresh small blocks under those engine boxes would be the easiest. But I will investigate going I/o for that boat. Surgery and a bigger project for sure but better fuel economy. Could even go with a jack shaft and put the new motor closer to mid ship for better weight distribution. The shaft would go through the current tank location so I would need to put tanks where the motors originally were. I’m open to advice on this including suggestions to stick with the original twin inboard configuration.

Chris,

Run the number on one of the prop calculators.
Use what you have from these motors to confirm the approximations with other power configurations.

If it were me...I'd stick as close to stock configuration as possible to avoid opening the can of worms even further.
I believe it's better to use a boat then work on one...

Hi Chris,

Conversions are expensive and a lengthy process.
The transom on your i/b boat most likely would need to be redone/beefed up.
Jack shafts would reduce the amount of fiberglass work required, but they're not cheap either.

Quickest and most economical solution is obvious.
But were talkin' about a boat, and sensibility isn't always the number one priority.
I'm surprised the ...gasp....o/b guys haven't chimed in yet.
If you spend a lot of time in skinny water, that's definitely an option to consider.

Best of luck,
John

What does your boat have in it currently?

Flexman 5000 the boat has the original Crusader 165 V6s. So about 3800 rpm a side to get to about 19 or 20 knots. I’ve seen lots of B26s with V8 power. Not sure if these are all repowers. Did Bertram ever spec 8’s in the 1983-84 model run I wonder.

It’s no small job (upsizing of shafts (from current 1”) and exhausts from current 3” is needed too).