Limited to 9.9hp

[dazk14]

Re: Limited to 9.9hp

We all know this combo isn't ideal. Let's get that out of the way.

I'm going to shoot from the hip here and say/recall the merc/transom angle -even in the lowest tilt pin setting - doesn't give very much negative trim...as compared to some other brands.

I have ALL the outboards that have been talked about in the shop(9.8,9.9,15,20,Tohatsu,Suzuki), so I could check tomorrow if I have a moment...

Regardless, you can run without the tilt pin and pickup much needed extra angle to lift that stern and get on plane. In fact it'll look like there's an extra tilt pin(fixed) below the moveable pin. Run it in the tilt mode, so the pawls aren't trying to grab the pin.

That, and likely your favorite planing aid should get it done. Easy to test. Pull a pin, report back.

You also have a prop ventilation issue at planing speeds(losing bite,rising rpm) and running that cav plate 3/4 above is not ideal. I'm too old to call it an AV plate. Maybe a slight hook to the hull.

If it is still an issue you(any handy man) can easily add some cup to the 7" merc 3 blade to eliminate it. It'll then run more like an 8" pitch.

 

[slowleak]

Re: Limited to 9.9hp

I certainly agree that this is not the ideal solution, but being the lake here is restricted to under 10hp, and going with the larger hull makes it worse, (short of buying a brand new flat bottom bass boat), I don't have much of a choice but to optimize my current set up.

The best running position so far is the second hole from the transom, going to the last hole stuffs the bow in and it plows water bad. Removing the pin would no doubt make that worse.
Any further testing will have to wait till the temps come back up here, its about 12 degrees outside right now and the water is hard again.

I never really attain 'planing speeds' with this, the difference between 3/4 throttle and wide open throttle is only how badly the boat wallows while it attempt to get on plane. Speed doesn't seem to increase or decrease, the boat just gets lower.

Again, the hull is not hooked, the bottom of the boat is perfectly flat, no dents, no damage, no lip. If the hull was hooked, I'd probably not have this issue as a hook at the rear of the hull would in effect push the stern upward much like those fixed tabs do.

I can't see how the prop is ventilating with the stern sinking so low under throttle, its nearly two foot below the surrounding surface water. I could be led to believe that there's some sort of turbulence forming due to the huge wallow it forms.

Something that I did notice after running another boat last week, the Starcraft with the 9.8hp makes a massive wake, no matter what I do, even at super low speed with that motor, if I hang a big electric trolling motor on the stern, there is almost no wake.
When I ran my buddies boat, it makes almost no wake, until I hang my 9.8hp.
Something else I forgot to mention, and I'm not sure if its of any importance, if I turn hard with my boat, with the 9.8hp on it, it gains speed, the motor doesn't change RPM, the boat just goes faster as it banks into a turn. Its as if its getting closer to climbing up on plane when making a hard turn under full power.
Also, after thinking about a few test on the lake, there's a shallow section where if I go real slow and don't pull up the motor, the skeg will touch bottom. Yet under power, with the boat wallowing in the stern, I can pass over that area without stirring the muddy bottom. So that tells me the boat is rising up to some degree, yet it doesn't seem like it since there's a wall of water on both sides of the boat at the rear. If the boat was indeed 'dropping' into the water as water is pumped from below by the prop, the motor should in theory be lower. What we always do in that area is either make real certain your on plane when crossing that area, or come in hot and yank the motor up and drift over it. Its only about an 8' wide muddy topped bar across the rear of the lake.
Also, if I look back and down over the motor, I get splashed in the face by the churning water off the lower unit, there's not a defined 'trail' of water behind the boat, just a wide, deep wake forming about 3' off the back of the boat. This happens on any hull I've tried this motor on with a similar rounded bottom shape. The dead flat bottom of the Polar mod V hull I tried left nearly no wake, just a slight ripple, even under full power. I'm 100% serious when I say the wake is HUGE, if I passed a canoe or sailboat within even 300 yards, chances are they're going under. I've owned larger boats, over 30' and not a one of them made a wake like this little 14' boat does.
With the hydrofoil, the wake is flatter, and more controlled, but just as tall. The hydrofoil seems to tame the third and fourth swell the boat creates without it.

 

[slowleak]

Re: Limited to 9.9hp

Finally, some warmer weather today!
I did some experimenting today, I had been thinking about the idea of fixed tabs but have concerns that in effect don't they do the same thing as a hooked hull???
Either way, I took a length of 2x2" thin aluminum angle, about 20" long, I made two cuts about 8" from each end, then drilled a hole where the drain plug sits, then added two bolts to catch the edge of the hull so it can't rotate. I plugged the drain from the inside and use a threaded plug on the outside to hold the angle in place. I bent the two end tabs on the angle downward about 20 degrees or so.
(My thought was that if the angle iron pulled free, the inner plug would keep the boat from taking on water.)
The angle was secured solely by the drain plug for now.

I put the original 9x9p original prop on, and since I didn't have a passenger today, I put five 5 gallon buckets of sand just ahead of the middle bench to simulate a passenger. To my surprise, the boat jumped right up out of the water almost instantly, and stayed there until I came to almost a complete stop. Top speed seemed faster as well, I didn't mount the GPS or fish finder since I was just going to test the theory of using tabs of some sort. What I did notice was that the prop now put out two small rooster tails off each tab, which sort of flattened the wake. The boat still made a huge wake but it was much flatter on the outer edges.
I then went back to the dock and unloaded the sand buckets, with just me in the boat, and I got almost the same result but at full speed the prop would lose grip or cavitate. If I locked the throttle and moved forward it got worse with just me.

At speed I had to stop a few times and tighten the makeshift tabs but this was only to see how the boat would react.
This pretty much tells me that tabs of some sort are probably the answer but they don't need to be very big, a few inches or so long and maybe 6 to 8" wide would be fine. Maybe wider vs longer would also work better.
I actually expected a big loss in top speed but I think that because so much more of the hull is out of the water, the lessened drag of the hull far exceeds the drag caused by the two small tabs. My thought on making the aluminum tab was to match the angle and width of the Hydrofoil's rear edge but without all the lift on the outboard mounting bolts and all the surface area drag of the thick plastic fin. I also taped over the seam where the aluminum tab met the hull.

Now to find something permanent. I like the idea of the aluminum tabs so I can bend them to get the right adjustment.
I played around with adding a few degrees and taking a few away but the original bend worked the best.
Ideally I'm sure getting them farther from the motor will work better and even maybe make the boat more stable side to side on plane. I did notice however that turning was affected, the boat took much more input from the motor to change direction but went dead straight with little correction otherwise. The overall drop or bend of the tab I made is very slight, maybe 1/2" down at the rear edge of the tabs. I think a wider tab with less drop would work the same but with less drag but I'd have to test a few ideas that way.
For today, I was just glad the thing stayed in place and worked. I had concerns that the water resistance would rip the plug out and send the whole thing into the prop or to the bottom of the lake. This is why I put a plug in from the inside as well.
With the sand buckets back in the boat, on plane, I noticed that if I moved forward too far it would begin to stuff the boat real hard and fall off plane, it just couldn't keep all that weight up in the air. With the tabs, the motor didn't become hard to steer like with the hydrofoil.

There's definitely a big difference between the tab idea and a hydrofoil, the tabs seem to be the way to go.
I didn't bother trying it with the three blade prop, I figured it was just too slow to ever get the boat up to enough speed to gain any lift, and if it did, it would be too easy to over speed the motor.
I could tell by where the water spray started that most of the boat was up out of the water, without the homemade tabs the water spray starts out from under the first bench seat area, with it, its just slightly ahead of the last bench seat.
I attached a quick drawing of what I made, now to see if someone makes something similar or do I have to just make my own. Something I was thinking is that it would be nice to have something I could manually adjust to match the load each time out. Maybe a simple jack screw with a small crank or key to turn it?

Another thought is that if this minor change makes this much difference with the weakly little 9.8hp Mercury, what would it do with a bigger motor?
I was also talking to an old boat racer and he said that simply extending the hull's bottom surface on both sides would do the same thing without the drag of downward facing tabs.

 

[oldboat1]

Re: Limited to 9.9hp

hey slowleak -- saw your trim tab design, and it looks pretty good to me. I also did a search for the old spring loaded tabs (think they were sometimes called "self adjusting"), with some limited success. You might do a google search if you haven't already. It looks like the modern versions have gas actuators rather than springs (which could be OK), but maybe you can locate a salvaged set of the old spring actuated ones. A search here on iboats should turn up a few options for the gas loaded types (Nauticus, and maybe others). The newer types seem to run $120 to $130 and more.

anyway, think you are on track with the tabs, whether your design or the store bought types above. I would try for simple -- can see some advantage, though, to having tabs that adjust for the hull speed. Seems to me when you slow, the boat would settle evenly, and likewise would remain level speeding up (might be a tad better than your stationary ones in that regard, but not sure).