Do I need an anti-ventilation plate on my 26M's Suzuki DF60A

[45Plus+]

Well, I had the opportunity to get my 6 foot 5 inch 185 lb son to fold himself up into the v berth and it did not have much impact on boat level or speed vs rpm performance. I was still doing about 12 - 14 knots at WOT at 5500 rpm with ballast in, three adults in the boat, two 6 gallon tanks of fuel and not much else in terms of weight in the boat.

There's a photo below of the motor hanging off the stern in what my boat trim instrument shows as "level". I tried to upload a MOV file of the video of the action off the stern but it was not permitted. At 4-5 knots, the water flow from under the boat and past the engine is pretty smooth. At 10 knots +, there is lots of reversal of water back towards the transom and lots of seemingly aerated water washing around behind the transom and around the prop. The anti-cavitation plate on the lower unit of the engine is below the surface (likely in aerated water though). With these conditions I think its apparent that the engine is having trouble getting a bite into the water - and hence my original question about adding a bolt-on hydro foil type of thing to the lower unit to try to limit the aeration going on back there.

There's a 26X at my boat club that has a fairly large DIY aluminum plate bolted to its lower unit anti-cav plate. I will have to chase down the owner and find out why he (or the PO) put it on and impact on performance. He may have had similar challenges he was trying to address.

Perhaps on a related note, when I was launching the boat, I noticed the trim tab on the lower unit of my engine was positioned at a 45 degree angle off the center line of the engine. I thought this a bit curious but left it as is to see if there was a reason for this seemingly extreme positioning. My steering is pretty tight and straight-on at all speeds (well, straight on for a Mac that is - there is continuous input required to hold course but I am getting used to it - and its much better given my replacement of the steering cable and installation of plexiglass washers on the rudder bolts to eliminate the loose fit with their brackets). Given that the tab is to counter the action of uneven force of the prop on the direction of the boat, I don't think I will mess with it - and its not likely a cause for the performance problems I seem to be experiencing - correct me if I am wrong on that.

[Russ]

I'm not sure I get more than 14mph at WOT which for me is 4800RPM. I'm probably overpropped but this is good enough as I rarely run WOT.

Correct about the trim tab thingy. It's to counter the sideways blade pressure the prop creates. The folks at BWY told me they set it to where they found works best. It's about 45degrees.

I fixed the Attachment function (the tab below). I just posted a test video. Try posting your video.

[Be Free]

Russ,
WOT at 4800 RPM sounds overpropped but you probably have great low speed control and great mileage around hull speed. That's not a bad setup if you are not running at high speed very often.

You have me wondering about my trim tab. I could not say what it's set to but I don't believe it's anywhere near 45 degrees. As long as you are not having to fight it you are probably set right for your engine.

Bill

[Russ]

Russ,
WOT at 4800 RPM sounds overpropped but you probably have great low speed control and great mileage around hull speed. That's not a bad setup if you are not running at high speed very often.

You have me wondering about my trim tab. I could not say what it's set to but I don't believe it's anywhere near 45 degrees. As long as you are not having to fight it you are probably set right for your engine.

Bill

I'm sure I'm overpropped. My original prop was worse. I believe BWY props their boats for sea level and salt. I'm at 4500 feet and fresh so I downsized a bit.

I don't know if that trim tab is at 45. But memory wants to say it's close to it. I do still get some sideways blade pressure if I let go of the wheel at higher RPMs. Maybe I should try changing it.

[45Plus+]

Thanks for fixing the site to enable posting of videos Russ.

I'm posting the videos of the relatively smooth flow out from under boat at around 4-5 knots and the second shows what happens at near WOT - quite a lot of thrashing around going on back there....

[Be Free]

Russ,
WOT at 4800 RPM sounds overpropped but you probably have great low speed control and great mileage around hull speed. That's not a bad setup if you are not running at high speed very often.

You have me wondering about my trim tab. I could not say what it's set to but I don't believe it's anywhere near 45 degrees. As long as you are not having to fight it you are probably set right for your engine.

Bill

I'm sure I'm overpropped. My original prop was worse. I believe BWY props their boats for sea level and salt. I'm at 4500 feet and fresh so I downsized a bit.

I don't know if that trim tab is at 45. But memory wants to say it's close to it. I do still get some sideways blade pressure if I let go of the wheel at higher RPMs. Maybe I should try changing it.

I forgot to check my trim tab today so I just went out with a flashlight to check. I'd give it 5 degrees max.

At your altitude you would definitely have to tweak a carb but I'd expect fuel injection to adjust on it's own. If you've already gone to a lower pitch on the prop this might be as good as it gets.

[Russ]

I'm posting the videos of the relatively smooth flow out from under boat at around 4-5 knots and the second shows what happens at near WOT - quite a lot of thrashing around going on back there....

Still hard to tell from the video. It goes slow motion and I can't really see much.

Question: Are you sure it's tilted fully down? The first video, hard to tell, but looks tilted up a tad.

Mine is always as far down as it will go.

[Russ]

At your altitude you would definitely have to tweak a carb but I'd expect fuel injection to adjust on it's own. If you've already gone to a lower pitch on the prop this might be as good as it gets.

Yep. It's fuel injected. I think this is as good as I'm going to get. I also wonder if the tach could be off. Next year I'm going to try installing the NMEA connector to my chart plotter and see if I can get some digital tach readings.

[Be Free]

I'd definitely trust the NMEA data over the tach if the tach was originally from an earlier engine. When I replaced my older BF40A (carbs) with a BF40D (fuel injected) my old tach always read significantly lower than the NMEA data was reporting.

[Russ]

I'd definitely trust the NMEA data over the tach if the tach was originally from an earlier engine. When I replaced my older BF40A (carbs) with a BF40D (fuel injected) my old tach always read significantly lower than the NMEA data was reporting.

Yea, my tach is original analog and I don't have confidence it. If I tap on the face, it moves the RPMs up a bit.

I'm hoping the NMEA will also provide other useful data.

[Be Free]

It will. The only hard part will be deciding what to actually show on your console. Fuel consumption rate and range are a couple of my favorites.

[45Plus+]

Hi Russ (et al)

I did have longer videos but they showed much the same thing - relatively laminar (?) flow coming out from under transom and around the engine at 4-5 knots and the rather chaotic backwashing that happens at higher speeds.

With respect to the angle of the engine to the transom, I think that is where it is positioned to show "level" on the trim gauge. I will try to get it to hug into the transom more and see how that might help the boat respond - at least the prop will be deeper in the water even though the level gauge might imply I am digging the bow in. We will see. Perhaps its a feathering of the trim that is needed (or at least tried) before I bolt on an anti-ventilation plate. I hopefully will be back on the water Wednesday.

Cheers

45Plus aka Duncan