Hard to get to plane when towing a tube

[QBhoy]

Hi

I'm thinking something isn't quite right here.
Having had a 150 V6 and a 200 V6 on and 18ft boat...these ran with a 23" and 25" prop. I am aware that gear ratios might vary, but perhaps not in this case ?
Having a 25" on the boat, I could understand perhaps being a little slow to plane when towing (I didnt find this to be honest. Managed ok), but having a 19", the thing should be like a rocket ship taking off out the hole.
Also, if I've read this right...you say you are getting 50mph at 3500 rpm. I'm not sure this is likely at all, especially with a 19" prop.
Just a thought. As suggested, be good to get proper rpm and gps data...unless you have and I've missed some of the posts. I'm on the phone...not ideal.

 

[QBhoy]

Just FYI, the boat I had managed 60+ with the 150 and knocking on 70 with the 200hp and the 25, from memory. Slightly different hull though.

 

[jimmbo]

Well now, don't I feel stupid! Worst part is that I thought of asking that question before, but I figured "nah, there's no way to take the measurement off something that isn't flat". Guess I was wrong. Lol. That'll teach me... ok so I could do the test again, but judging by the fact that the area you've indicated is a lot lower (at least thick enough to fit a drain plug, so what, maybe 3+ inches), the motor must definitely be high enough, maybe even too high.
!

You measured what you thought was the transom height and it was, 18". so add the 3+ or so and that gives you 21+". If you are mounted solid on the transom you should not have any major issue with it venting(the boat manufacturer would ensure the design would function) with your current prop. If you want to go higher well your current prop is not going to cut it.

Last week there was another post with a similar setback on it. I don't know if that was yours or someone elses

 

[sammith]

QBhoy: Thanks for the comparison. Quite a few posts ago, someone made the comment that the gauge would read wrong because the motor that was on the boat from the factory is a Force motor that has a 20 pole vs my current merc which is 12 pole would definitely be the cause of the flawed RPM reading. I still have to confirm once I get my hands on a tool, but it's estimated that my RPMs are more around 5800.

Ok, so I've got a lot of homework to do: Confirm tach reading, try and change my current tach setting to see if it'll work (found out the old Force motor was 4 cylinder, so I've got somewhere to start), and I guess from there I might just try a slightly different prop (depending if my RPMs will allow it) and go from there.

 

[QBhoy]

Fair enough and no problem sam.
I d say in normal scenarios given the boat and engine set up, a 19" is too wee for it, pitch wise.
This still begs the question of why it's struggling to plane. As mentioned it should fire out the hole in seconds....

 

[sammith]

No Title

Fair enough and no problem sam.
I d say in normal scenarios given the boat and engine set up, a 19" is too wee for it, pitch wise.
This still begs the question of why it's struggling to plane. As mentioned it should fire out the hole in seconds....

Ya I'm still puzzled myself, but this is my first boat so I don't really have anything to compare it with. I've considered the possibility that I'm just overloading it, but I contacted Bayliner (since there isn't an information plate on the boat itself), and they told me maximum person weight is 900lbs, and with gear and everything 1500lbs. I've only gone tubing once so far, but I'd say that between 3 adults and 2 toddlers, we were probably at 550lbs of people and I won't venture to guess how much weight in cargo we had, but we pretty much brought the strict minimum we needed (other than a small cooler with snacks for the kids that my wife managed to get on the boat, so it can't be very heavy at all).

Just as a quick update, I went and pulled the tach off the boat. Here's a picture of the backside and I've included a picture of a picture I found on the internet that seems to be the exact same (or pretty darn close). As you can see, my numbers are non-existent these days, but compared to the other picture, I would say my tach is in the 2 maybe 3 position. Unfortunately this doesn't tell me much since most of the documentation I find on Faria's website seems to be geared towards newer tachs and even if I find something that's similar, it sometimes refers to the different positions as being cylinders, while other times it mentions poles. So I contacted Faria, gave them the boat and (old) engine info, gave them the "6B21" number stamped on the back, and hopefully they'll be able to help me learn more about the tach. If not, then it'll be the good ol' trial and error method!

 

[NYBo]

Once again, the number of cylinders is irrelevant! It's the number of poles in the alternator. Here's a link to a Faria document: https://www.manualslib.com/manual/8...otor-TachometerAnd-Tach-Hourmeter.html?page=3

 

[sammith]

Once again, the number of cylinders is irrelevant! It's the number of poles in the alternator. Here's a link to a Faria document: https://www.manualslib.com/manual/8...otor-TachometerAnd-Tach-Hourmeter.html?page=3

Right, I forgot that you had mentioned that (been reading all cross-eyed tonight). Assuming I can figure out the positions on my tach, when you said to set it to 6, do you mean to set it to 6 pole (setting number 2 on the bottom right of the page you linked)?

 

[NYBo]

Right, I forgot that you had mentioned that (been reading all cross-eyed tonight). Assuming I can figure out the positions on my tach, when you said to set it to 6, do you mean to set it to 6 pole (setting number 2 on the bottom right of the page you linked)?

For a Faria, you set it to 5 if it's a 7000 RPM outboard tach, 4 if it's a 6000 RPM I/O tach with 12-pole O/B option. Either way, it's the setting farthest clockwise. Don't force the knob. Push it in first if there is a lot of resistance.

 

[sammith]

For a Faria, you set it to 5 if it's a 7000 RPM outboard tach, 4 if it's a 6000 RPM I/O tach with 12-pole O/B option. Either way, it's the setting farthest clockwise. Don't force the knob. Push it in first if there is a lot of resistance.

I guess I'll have to tinker around with it then. Yesterday I tried turning it (slightly depressing every time) to find the stops in order to figure out where it was set, and well I got a weird surprise: it doesn't seem to have stops. I was able to (gently, without forcing it) get it to go to any position around the circle.

 

[sammith]

No Title

Curious as I am, I took the tach apart. I posted a few pictures if anyone else if curious. Here's what I learned:

There are 4 settings, it's normal that it can do a 360 (it just keeps going 1234 1234 1234....), having the arrow pointing directly to the right is setting #3 (so I think it matches up with the other picture I had found online). I think that's about it... that was fun!

 

[sammith]

Getting back to something more prop related, I've been doing some reading on props and I think I'll try a 4 blade prop as it seems that these are better for low-speed planning, hole shot, acceleration, etc. All things I'm looking for. Of course this sacrifices some top speed, but I was ready to sacrifice this anyways. From what I've read, going from a 3-blade to a 4-blade, I should be going down 2 pitch and I can either stay at my current diameter or even go up a bit. Taking these into consideration, I've narrowed down my search to 2 different props:

- Piranha 14.516A-4 (14.5 diameter, 16 pitch). I'm pretty happy with the composite modular propeller I have now, so I'm sure this one would be nice. The diameter of this one is the same as my current prop and the pitch is down 3 from my current 19 which sounds ideal to me.

- Quicksilver QA2182x (Mercury 48-834850A45). OEM Prop, pretty much can't go wrong with that. Diameter 15, pitch 16. Aluminium. Seems like a pretty basic "gets the job done" prop.

Anyone have any comments on these choices? I've done a good amount of reading and comparing, but I'm still new when it comes to this.

P.S. I haven't gotten my hands on a tool to check RPMs yet, but I'm taking the boat out again today, so I'll be able to check if changing the tach setting had an effect.

 

[ondarvr]

Don't get any preconceived ideas on what the pitch on the new prop should be, wait until you get actual results from a working tach.

 

[QBhoy]

You defo need to figure out what the deal is with your current situation first before thinking of getting a 4 blade...especially if you are thinking of going down in pitch.
Every instinct and bit of knowledge I have in me is screaming out to me that a 19" is underpropped and I wouldn't even be giving going lower a thought.
I'm not sure, but there is a small chance that the drive ratio is much lower than the norm...but it would be unusual.
Is it 100% a 19" you have ?

 

[sammith]

No Title

Is it 100% a 19" you have ?

I definitely agree that I need to figure things out before buying another prop, I'm just being proactive with my questions since I want to be ready if/when I get the RPM puzzle figured out (coming off vacation Monday, so I'll be able to check for an RPM tool then).

Here's a picture of one of my blades. I've compared the markings with Piranha' s website and confirmed it's 19".

 

[sammith]

Don't get any preconceived ideas on what the pitch on the new prop should be, wait until you get actual results from a working tach.

My understanding of pitch is that you want to keep an eye on RPMs at WOT. So for example, if my RPMs are maxed at WOT with a 19" (hypothetical scenario), I could still choose to go down pitch to more or less whatever I want (depending on my needs) so long as I keep my eye on RPMs since I can't safely go to WOT anymore.

Is this right? That's how I've understood it, but I might have completely missed the mark.

 

[QBhoy]

If your rpms are maxed.....then you can't go down in pitch. That would increase the rpm even further. Probably by 3-400 rpm.

 

[QBhoy]

Well...physically you could...but it would be crazy to do this. Haha. Don't understand why you would want to it need to. All being well.
Just doesn't all add up somewhere along the line. Something is wrong.

 

[ondarvr]

My understanding of pitch is that you want to keep an eye on RPMs at WOT. So for example, if my RPMs are maxed at WOT with a 19" (hypothetical scenario), I could still choose to go down pitch to more or less whatever I want (depending on my needs) so long as I keep my eye on RPMs since I can't safely go to WOT anymore.

Is this right? That's how I've understood it, but I might have completely missed the mark.

​It would be true for you, if, and only if, you knew exactly what the current RPM's, speed and prop are, you need all of this info to even make a guess at changing the pitch. You don't have that info now. Wait and gather all the facts first, then look at choosing a prop.