replacing inboard to outboard engine

[semaan]

hello,
please i replaced an inboard engine (mercury 250Hp) with outboard DF140
I built an aluminum bracket and screwed the engine to a suitable height (one inch higher for each 10cm off original transom)
engine works well, water level reach to cavitation plate when running
but i noticed water goes up for engine sides (please watch the attached video to clarify my point. P.S video is before lifting engine to normal height but approximately situation still the same)
also take note that i installed two fixed tabs (also shown in pics)
thank you for help

[Moonlighter]

From the video, it looks like the engine may be set too deep.

Does the boat get onto the plane OK?

What revs can you achieve at full, wide open throttle (WOT)? Trimmed out?

If it does plane well, find a smooth river and run it at a good medium/fast cruise speed, usually around 4500-4800 rpm. Trim out the engine to optimal position. Go down the back and look over at the engine leg, while someone trustworthy holds the wheel!

If the engine height is OK you should be able to see the anti-vent plate (the large plate directly above the prop sometimes wrongly referred to as the cavitation plate) should skimming the surface - it should not be submerged in the flow at that speed. If it is submerged and not visible, the engine needs to be lifted until it is.

See the sticky thread at the top of this forum re props and engine height for more info.

[semaan]

rpm at WOT reached 5000
yes the anti-vent plate is skimming the surface now after i raised engine up.
but water still pumping on both sides
i did not try to trim engine out ( i will try that again) but i need to be sure if that would be because of tabs? because engine now is high enough i suppose

[Harper]

It is difficult to tell for sure without seeing the boat from the side view, but it appears that the motor is sitting well behind the back edge of the trim tabs that you installed. It could be that the water diverted by the tabs is creating a ridge of water about where the prop is turning, effectively making the motor sit much deeper in the water column than it would without the tabs. This could also cause the "pumping" of water up the sides of your motor. Those trim tabs might be a little bit too wide for your setup (or perhaps not wide enough. It's very hard to know the best way to approach this without a lot of trial and error). It also appears that you might have too high a pitch on your prop. You should be able to attain 6000-6100 RPM at WOT.

[Moonlighter]

Agree with Harper - especially that you need to reprop to get the engine to at least 6000rpm at WOT.

Why did you decide to repower a boat that previously had a 250hp inboard with a 140hp outboard?? Seems a strange thing to do, and unlikely to produce a satisfactory outcome dropping so much horsepower??

[jHydroman]

I believe you will find that the surface of your trim tabs is way too large and the fixed angle is too great and you are creating to much drag. I would expect that trim tabs are pushing you bow lower than desired, which increases the wetted surface between the bottom of the boat and the water that also causes poor performance. You might be surprised on how much the trim can effect the WOT and you engine RPM. It will be difficult to determine the optimum trim tab angle without being able to adjust then when under way.

JIm