To "Foil" or not?

[LGF]

Kay so the dragon fly mkiii apparently is a little on the heavy side relative to its physical size.  Now as the boat came with a 115 v4 2stroke Yamaha in peak condition, it certainly is quite capable. The Yamaha had an "Atwood" hydrofoil, split Delta wing type installed on it.


The issue I have is this:


When she's on the plane, one is supposed to tap back on the juice to about running @ 3/4 I was told. Then one is supposed to start trimming the motor out the water to 1. Lessen the drag the motor has, 2. To get the boat to run that little faster as to ensure that no excessive drag (on the hulls' side of things) are present. This is all relative to the boat not porpoising at the instances where the above are present.


What I am getting, is that at the moment that the motor gets into that sweet spot, where it sounds comfortable and not overstrained (optimum drag ratio on hull and motor) it then starts porpoising. I tried remedying this by moving three batteries to the anchor hatch right u front. I also moved some other weighted items to the front and it didn't help. What i was told,I need to do to stop the porpoising, is to go heavier on the throttle, which I do but that does not do much for the porpoising.  So currently all I can do is trim the motor back in, but immediately it sounds like the motor is doing heavier labor, it doesn't sound like the motor is as happy.


The question now, would a slightly larger hydrofoil like the SE 300 Sport, which is slightly bigger than the two "Atwood" fins and also angled downwards as opposed to the "Attwood's" upward angles, make a significant difference?


PS, the Yamaha is fitted to a jackplate with a 6inch offset and the anti cavitation plate of the Yamaha is already running slightly higher than bottom of the hull, not sure but if I remember correctly it's around 1 inch. These were specs I found on various sites dealing with jackplates etc etc and that's how the setup was when I got it, so I haven't changed anything yet. Could an endeavor to alter of the motor height also perhaps resolve the issue?


The goal is not to increase speed but rather to have the motor running in that space where I can here it's happy without running @ WOT.

[Cam Mundy]

Try taking the foil off completely and see what happens. Generally boats porpoise when they are overpowered or very badly trimmed as the boat is literally trying to climb out the water. On very high powered boats you use trim tabs to counteract this.
Sea trial with and without the foil ignore WOT and all that bollocks just start out with the motor fully trimmed down, increase speed till the boat pops onto the plane once on the plane trim the motor up you should see a gain in the RPM, different hull like different amounts of trim et voila you now have your boats comfortable cruising speed.

[LGF]

Thanks Cam, never thought of that however, the hull is quite heavy.That gain in rpm is exactly the "sweet spot" I was referring to. Will give it a bash over the weekend. I have installed the SE Sport hydrofoil and will see what that does, if the same, I'll remove and retry, if nothing changes with the "sweet spot", I'll start messing around with motor height on the jackplate.


What I also need to do is to see exactly how high or low the cavitation plate is sitting in the water when on plane. I suspect that it's just a tad too low. Will see if I can't record with Senoir "GoPro on the stick."

[John F]

Are you running a 3 blade or 4 blade prop?

[LGF]

4 blade stainless

[LGF]

Not sure of the pitch can take a pic quickly, still @ home!

[LGF]

Herewith some pics, pitch, hydrofoil curvature and size and also position above the prop.

[LGF]

The Verdict on the new hydrofoil is sort of complicated. Here's why:


1. The boat is on plane about 60% faster.
2. The "sweet spot" is now more of a band than a spot.
3. Since there's no fuel consumption gauge, one have to go on the engine sound and it's sounds happier for longer.
4. There has been an increase in top-ends of around 13km/h on Botha the boat speedo and the GPS.
5. The porpoising is considerably less, but here's where it's gets complicated.


It was a mirror smooth surface when I tested it.
There is some porpoising at a specific point but once trimmed past that it smooths out and doesn't bounce anymore.
As I said this was on smooth water. So I expect different results on more choppy water.


I could not gather the courage to trailer and relaunch the boat after removing the hydrofoil and/or adjusting the height of the motor with the hydrofoil on and/or off. I would have had to do this alone and as the boat ramp is quite narrow I would be hindering other boaters launching. So that'll have to wait for a next time when I can have somebody in the bakkie to assist with all the trailering and relaunching to make these adjustments.

[MIKE PIKE]

Way i see it Foils are designed to assist with an "ass" heavy boat or if its slightly underpowered to get up on the plane quicker , and then come back on the power once up and running ie will keep yuo on the plane at a lower power setting , they do kill the top end slightly  cause of slight increase in drag , not sure on the porpoising though , but fine tweaking the trim should eliminate it at some point.

[LGF]

I for one have questions about the jackplate. Being at an offset, I am very sure it has something to do with the porpoising, then there is the suspicious drag the foil might have as it is most probably running deeper (on plane) than most of the little I could see pictures of hydrofoils position whilst boats are on plane.


I need to take a vid in order to see where this thing is running.
Removing the foil and then see what the results are when on plane. Similarly I need to take a vid to see where the cavitation plate runs on plane, this would be the only way to really determine where the motor ito height should be.


Pretty sure if that, the motor height, is sorted, the rest like running with or without the foil will be easier to fine tune.

[LGF]

haven't had time (actually a crew) to do the various planned tests, however, the boat have been on some competition water since the installation of the new foil, AND:

1 thing I can add is that it is generally much much more stable in flat to moderately rough water.
Another is the foil pushing the boat (hull) level when on plane and keeping it there. This was not the case with the previous foil and what was worse was that each time a different person got on as crew as opposed to being alone, the boat would change the way it sat in the water especially when on plane. Now it does not happen anymore, regardless of the shift in weight. AND Yes the boat is as balanced as can be wrt weight distribution, if I sit between the two seats the boat is perfectly level in the water, even just drifting under zero power.

Still need to do the tests though, need a proper conclusion.