Boat will hop right on plane if you slam it down turns about 5800 rpms at full throttle can trim motor up and get to about 6000 rpms before prop blows out. Motor is hung on the last hole, as low as it will go on transom. All indications from dealer and my experience is that it is propped right.
Problem…motor wants to cavitate between the 3500 to 4200 range particularly if you baby it while accelerating or if you try to run in the range and if you are moderately loaded. Extremely noticeable if in a moderate chop. Originally thought prop was slipping on shaft but have tried my spare aluminum prop that is the same pitch and have same results.
Does not matter if boat trim tabs are pushed all the way down or all the way up. Also changing load distribution in boat does not seem to change performance either.
Indications of the problem include excessive vibration of the motor, deeper pitch sound from motor and sound from prop. The RPM range with the problem is a good cruise speed on this boat particularly if it’s copy. Wondering if perhaps a hydrofoil like the non drill pro sports may help?
Original motor was a two stroke and it would cavitate real bad in a turn, when we repowered we with the big foot so we could sling a larger prop and it no longer blows out in the turns unless you turn extremely sharp which of course would be expected.
'Seems like if the prop was ventilating, you’d see your rpms increase when the prop broke lose. And, you’d lose thrust = hull would want to slow and drop off of a plane. Does she do that?
Is your prop a vented prop? (Has little hole in the side near the front that allows exhaust out.)
If so, might the vent insert have gone missing?
If not, it does not sound, to me, like the prop is ventilating. (Aside - outboard motor props do ventilate. I haven’t done the math, but, I don’t think it is possible to create the pressures on an outboard prop blade surface to cause cavitation.)
Do you have any hull penetrations forward of the prop that might be disturbing the water going into the prop? Bait well intake or other?
17’ Henry O Hornet w/ Johnson 88 spl
26’ Palmer Scott project hull
14’ Bentz-Craft w/ Yamaha 25
Chris I am going to try and run it tonight and recheck RPMs it’s been a while since I have run full throttle. Also real curious how it runs with just me in the boat.
Palmer it is not a ventilated prop. Rpms do not spin up it just vibrates like crazy. No obstructions ahead of motor but at that particular rpm range you would think I have a cluster of oysters growing off the bottom.
Boatpoor that’s exactly how it acts but it certainly seems to be trimming all the way down. The transom bracket does not have a stainless stop pin in it that runs from side . It has two stops/rubber covered parts that are molded into each side of the backet and the back of the motor comes to rest on those from it appears when it’s all the way down.
I ran it tonight. Turns 5800 RPMs at full throttle trimmed all the way down. Can trim up just a touch and can get to about 6200 before it starts to blow out. Runs right at 34-knots. Chris does this sound right or do I need more bite? I contacted Hewes when we installed motor a few years ago and asked them if they had any experience with this motor boat combo. They referred me over to power Tec and they suggested one of their 4-blade props (can’t remember pitch) to the tune of close to $500! I know that four blades give more transom lift and I am not certain that is exactly what I needed their second recommendation was a 19-pitch and that’s what we went with from the dealer. (Not power Tec but Mercury)
After tonight I figured out it does not cavitate as bad with just me in the boat. Still a fair amount of vibration at 3500 rpm which is right at the transition of it going onto plane. When it’s “slipping/vibrating/cavitatating (whatever) the RPMs do not shoot up like it does when it blows out when trimming up too high at full throttle but does have a similar sound.If I tab down/bow down it doesn’t help actually makes it vibrate worse much like there is a heavy load in the boat. If I ease up to 4000 to 4200 bow totally flattens out and runs good. When coming off plane no vibration unless you slow to 3200 to 3500 then it vibrates until you vary the RPMs up or down.
As long as you jump up on a plane quick none of the vibration is apparent or it’s so short lived you don’t notice it.
Sure sounds that it is sitting to high on the transom! Is it a 20" or 25" shaft? What motor was on it when it was new? Might want to see if you have the right thrust washers? I once changed props and unknownly added another washer that made the prop stick out further and was sucking water in and loosing torque, after removing prop i noticed I had 2 washer on the shaft before the prop, after using just one it ran like it should.
Sure sounds that it is sitting to high on the transom! Is it a 20" or 25" shaft? What motor was on it when it was new? Might want to see if you have the right thrust washers? I once changed props and unknownly added another washer that made the prop stick out further and was sucking water in and loosing torque, after removing prop i noticed I had 2 washer on the shaft before the prop, after using just one it ran like it should.
This is what I would encounter when moving an engine up too high…
Keep us updated, I’d like to hear what you find wrong.
Motor is mounted in lowest hole possible. Top of bracket where it 90s over is pretty much resting on the top cap of the transom. It’s a 20-inch shaft same as original. The hub assembly for the aluminum prop is different from the hub assembly for the stainless steel prop and therefore each one has a specific thrust washer that matches up and is installed. Prop seems to be sitting all the way down on the shaft and the only play I notice when pulling on the prop is the small amount of play in the actual prop shaft.
I agree it does act like it’s not hanging low enough or is not trimmed all the way down, but short of installing a jack plate I cannot figure a way to get it any lower. Was not sure if the increase in size of the cavitation plate by using a hydrofoil may hold more water around the prop and give more traction or do I need a prop with more bite? I don’t care about top end speed just want it to perform well at all RPM ranges. That 3500 to 4000 range is ideal when I pull the kids on a tube or if I am running in a moderate chop.
I’m not convinced this is a prop slippage issue. What else could make the motor run rough that would be rpm range related?
Dropping a cylinder for some reason?
Some kind of internal resonance due to lose or failing parts?
Weird fuel flow dynamic?
I have no idea. But, this does not necessarily sound like a prop misbehaving or ventilating.
17’ Henry O Hornet w/ Johnson 88 spl
26’ Palmer Scott project hull
14’ Bentz-Craft w/ Yamaha 25
Palmer I sure hope you are wrong. I am thinking this may have been occurring since we installed a few years ago and I am starting to notice it now because I am more aware of it and because my kids are growing so the boat is more loaded and I am running in this range more often.
Chris the foot is stamped 13x32 not sure what that correlates to? Ratio looks to be 2.38:1
Prop is a Mercury 832830A45 P19 it is their aluminum prop. The stainless prop is a Quicksilver 48 16316 A5 19P
Just went to utube and pulled up bigfoot and command thrust! Seems your motor should run a prop made for a V6 which is larger than the standard 115 prop, check what you are running and adjust up in dia. see the reports on Google:mercury marine 4 stroke command thrust!
There used to be a plastic ring that sat between prop and foot, it rode on the base lip of the prop essentially, was thin maybe a 1/8-inch white poly ethylene. I noticed when a swapped props a while back that it was no longer there. My Yamaha 250 does not have that nor does my mercury 30hp tiller so I didn’t think anything of it. Does that have something to do with my problem perhaps?
contender1 you stated the original motor blew out on turns i have never run the big foot but sounds like engine is still to high the cavitation plate on your motor should be the same as the bottom of your hull maybe an inch or so higher i had the same problem with my offshore boat with a 29" bracket according to what i have read for every 6" motor is moved back you can raise engine 1" well that being said i bought a 25" shaft and it did not work had to get a 30" problem solved dont know if this will help but just my experience with a simular issue
that is the correct hub size for that motor.
how much space is between the cavitation plate and the blades of the prop as they swing around?
you may need to increase diameter, but just know that you definitely have an issue, and it needs to be solved, so you are doing the right thing.
