Prop is spining free during acceleration and turning

[sundaypaper]

Hi Boaters,

This has been going on since I bought my boat in 2012. Switch to a 4 blade prop and performance got better but it still happens.With the trim fully down, I can hit full throttle from a stop and it hangs in there as long as I don't turn coming out of the hole. At higher speeds, 4000+ rpm, it seems reasonable. Think about taking the kids tubing, this is the situation where it can show up, the rpm go up, boat is slowing down, I hear the engine revving up but it normally won't reach redline, part of that is me pulling back, but it doesn't "shoot up" fast. I'm not hearing popping sounds and I have confirmed the prop is in the water. It acts like my motor is spinning its tires. Typical loading is 2-4 people, 30 gals of fuel. Just an opinion, this Mercury outboard has big torque. I think the prop is too small in diameter.

Props:
Solas Amita Rubics 4 blade, 19 pitch x 14.25
stock black diamond 3 blade, 19 pitch x 14.75

top speed for both props (fresh water) is about the same, 5900-6000 rpms, 50-51 mph, fully trimmed out, clean bottom, full tank, 1 person, think first spring run on a calm lake

2012 Mercury 150hp FourStroke, inline 4 cylinder, 3 liter, fuel injected, EFI
2012 Starcraft 19ft deck boat (tri-hull), 2750 lbs with outboard + people and gear

Thanks for your help,
Neil

 

[alldodge]

Have a pic of the motor full down showing how it meets with the bottom of the hull?

 

[Maclin]

What is the trim at when turning?

 

[ajgraz]

If I am understanding this correctly, sounds like the prop is ventilating in turns. I'm with prior two posts...possibly the motor is mounted too high on the transom, or you just need to trim in a bit more on turns.

 

[sundaypaper]

It seems trimming up makes it worst. The outboard is in the bottom hole so it can't go any lower. I don't have a picture but I have looked at before when out of the water. It looked to me that the plate was even with the boat bottom (flat pad) but I don't see the plate when on plane at about 3500 rpm, around 20 mph.

 

[Maclin]

I have to trim in to almost 1/8 when going into a turn with this new to me boat or it can ventilate depending on how sharp I am trying to turn. If I trim in too far it leans uncomfortably into the turn. First boat I have had to worry about that, still getting used to it. Maybe yours is tuneable, but maybe just needs that kind of pilot input too.

 

[alldodge]

Without a pic I'm guessing but think the motor might need to be tucked in a bit more. If not adjustable may need to get some V spacers

 

[sundaypaper]

I was boating this weekend and paid attention to what I'm doing with the trim, you are correct that I need to trim in more. This is for another topic but the reason why I trim out is the hydraulic steering, its really not very good. My steering is hard and I need to trim up to get the steering easy and even, trimmed in is hard and uncomfortable. The boat handles well with the trim out as long as I don't make too sharp of a turn. Had cables on my old boat and was disappointed in the hydraulics when I first got the new boat but I got use to it and didn't think about it much anymore. Trimming out works good when taking the kids tubing, steering back and forth. The problem is when I feel the need to do a death spiral. Clearly I need to trim in, a lot, to make it work well. Perhaps I'll look into my hydraulic steering, right or wrong, I rather have it easy and even at all trim positions, then I would go tubing with the trim in most of the time. I'll look into the motor mounting in the fall when on land. Thanks for your help!!

 

[Texasmark]

If I am understanding this correctly, sounds like the prop is ventilating in turns. I'm with prior two posts...possibly the motor is mounted too high on the transom, or you just need to trim in a bit more on turns.

+1......the prop is sucking on incoming water to provide thrust from it's spinning. Too close to the surface for the load it has to pull reduces the incoming water in the normal fashion and it looks for an alternative source. Surface water is just inches away and it goes that route. Putting air with the solid water that is supposed to be flowing across your blades reduces the load on the engine and allows the rpms to increase with a decrease in mph. The harder you punch the throttle, the more pronounced this problem is. Tucking in the trim helps to control that problem to some degree.

If tucking all the way in doesn't solve your problem and you don't have a hook in your hull, or barnacles, or a keel or depth finder transducer in the wrong place or high dead rise at the transom, then I agree with "sunday paper", your engine is mounted too high.

If lowering it is not an option, then I would go for a different prop with 2 things: Cupped blades (tail end of blades is bent up slightly) providing better grip, and lower pitch which would lower the load per rpm and reduce the possibility of ventilation. Going to a 4 blade or 5 blade prop just puts more turbulence in a smaller amount of water per blade and aggravates the situation.

Finally something like a Whale's Tail would increase the path length from the surface to the prop and reduce ventilation and would also help your hole shot and slow speed cruising in assisting in keeping the transom up and the boat on plane at a lower speed....like trim tabs do (that don't solve the prop problem). They can get in the way for other boating activities and personally would use as a last resort.......50+ years of boating and never had one!

 

[elkhunter338]

The merc. 150 shortened the mid section of the motor so your motor might be mounted too high, can you lower it? I just went thru something similar with my merc. 150. I built the transom 1.5" higher (26.5") because most guys were installing jackplates to raise the motor, well it turned out mine is now slightly too high and doing something similar. I went to a 4 blade piranha prop with hydro bits and this help my problem out significantly. Great company. Make sure if you order a prop you specify you want blades with hydrobites.
I still need to lower my motor but that will require a transom modification. If I shift 2 people to the back of the boat things are better.
It also seems these merc OB's don't trim in as much as you would think. I am considering building a bracket to get the motor farther behind the boat 2", but also angle the bracket so I can trim the motor in farther.

 

[Sea Rider]

It's an OB/transom mismatch height issue, if OB would run at the Tip Top transom spot trimmed to 90? while combo is on plane gliding on flat water level at speed, prop should not aerate at close turns. Choppong transom down to lower OB for prop to deliver full forward thrust it's a tedious manual time demanding work of art many boaters won't go that route...

Happy Boating