Top speed seems slow....

[yooper]

I thought that this would be covered somewhere, but I searched and found nothing... I pulled a slalom skier yesterday (which I rarely do) with my '13 Mojo. I set the speed at 32, and I couldn't reach it. I was topping out at 30.8 or so, at about 5200 RPM.

I guess I really don't remember what my top speed was last summer... I rarely top it out, but 30 seems awfully slow. I tried another run later, without a skier and the speed control off. Same results. Only a couple passengers, ballast and bilge empty, prop (acme 1235) in good shape.

Boat seems to be running fine. Any ideas? Anyone have a '13 Mojo with the same prop? What is your top end?

[smorris7]

I have a 12 mojo with the 1235 prop and top speed is around 38.

[yooper]

I have a 12 mojo with the 1235 prop and top speed is around 38.

Yeah, that's what I thought.... Very strange.

[mmandley]

Speed it relative to, is your speedo calibrated?
How much weight
How good a fuel
What Prop
What weight in the boat.

I have a very large prop on mine for hole shot on surfing and weighted wake boarding, plus we tend to have larger crews.

I think my boat tops around 34

I would start with what prop are you running and how much weight was in the boat including people.

V drive boats in generally don't go much over low 30s though

[yooper]

Speed it relative to, is your speedo calibrated?
How much weight
How good a fuel
What Prop
What weight in the boat.

I have a very large prop on mine for hole shot on surfing and weighted wake boarding, plus we tend to have larger crews.

I think my boat tops around 34

I would start with what prop are you running and how much weight was in the boat including people.

V drive boats in generally don't go much over low 30s though

Yes, I know that V-Drive boats aren't known for top end. The problem is, it is now much slower than it used to be, for no apparent reason. The Speedo is correct, and everything else you mentioned I covered in my original post.

The gas is the same as I have always used.... ethanol free premium. Although I'm still on my first tank of the summer.... Do you think gas can make that much difference?

[viking]

Seems slow to me! Most re-proping will give better whole shot if slammed with ballast and reduce top end speed. But most OEM equiped props will be in high 30's to 40mph at WOT for all around performance. That has just been my experience.

[mmandley]

Yes, I know that V-Drive boats aren't known for top end. The problem is, it is now much slower than it used to be, for no apparent reason. The Speedo is correct, and everything else you mentioned I covered in my original post.

The gas is the same as I have always used.... ethanol free premium. Although I'm still on my first tank of the summer.... Do you think gas can make that much difference?

Personally I wouldn't use Non ethanol unless you get it for the same price as 89-91. Non Ethanol is great fuel don't get me wrong, but it it doesn't provide any real benefit over normal fuel unless your not using it for long periods of time.

I have non ethanol in my town but its $1 a gallon more then normal fuel. I only use it at the end of the season. Just something to consider. As long as you are burning fuel through the boat and its not sitting more then a couple weeks at a time your fuel system won't gum up.

On another note, yes Old fuel can make a huge difference, As fuel ages it loses Octane and octane is the ability to reduce Knocking in the engine. So older fuel with less octane will knock sooner and in our engines we have knock sensors, and these will reduce power in the engine to keep it from knocking.

I personally run 89 in my boat as that's what its called in it. I don't use Premium 91 because I get no benefit from it, the boat still sucks just as much fuel and run exactly the same.

Now in another instance my Vette, if I run anything less the Premium it has less power, less MPG but my tires seems to last longer. IDK LOL

[uniwarking]

I'm fairly sure my Mojo with the 1235 runs top speed around 40, stock OJ was closer to 45. I'm assuming you have the 345 surf engine, if so your WOT RPM is right where it should be.

In simple terms, since these boats have very simple gear transmissions, if your engine is running at the high end of the spec WOT (4600 - 5200) and your prop is the same as mine (ACME 1235) you should be traveling at the same speed as me. There are other factors at play here that could alter speed... But if you had a ton of weight in the boat or drag from another source you wouldn't think you'd be achieving 5200 RPM.

Did you look at your speedo as well as the digital cruise speed? Did you pull up a phone GPS speed app to be sure? I'd hope it was a speed calibration issue.

One other less likely thing to check... Is everything alright with your prop/propshaft joint (key in keyway, nut tight to prop, etc.)?


Sent from my iPhone using Tapatalk

[yooper]

I'm fairly sure my Mojo with the 1235 runs top speed around 40, stock OJ was closer to 45. I'm assuming you have the 345 surf engine, if so your WOT RPM is right where it should be.

In simple terms, since these boats have very simple gear transmissions, if your engine is running at the high end of the spec WOT (4600 - 5200) and your prop is the same as mine (ACME 1235) you should be traveling at the same speed as me. There are other factors at play here that could alter speed... But if you had a ton of weight in the boat or drag from another source you wouldn't think you'd be achieving 5200 RPM.

Did you look at your speedo as well as the digital cruise speed? Did you pull up a phone GPS speed app to be sure? I'd hope it was a speed calibration issue.

One other less likely thing to check... Is everything alright with your prop/propshaft joint (key in keyway, nut tight to prop, etc.)?


Sent from my iPhone using Tapatalk

Yup... You're thinking the same as me. I do have the 345 and I do have the 1235 prop. No extra weight or drag, but like you said.... that would likely affect RPM.

I didn't check the speed with GPS, but the digital and analog read the same, and as far as "feel" it was going 30, not 40. My speedo isn't off by 10mph.

I'm kind of perplexed.

[yooper]

Fixed. Here was the resolution. After really wracking my brain, and talking things through with several people, I isolated the problem to my prop. It had to be.... The problem was, why? Last fall, one of my last days out, I was on a really crappy boat launch, and was taking my boat out. The trailer was sitting crooked and the boat did not self-center itself properly on the bunks. When I powered up, the rear end went to the side and the prop hit the prop guard.

I was thinking that I got the prop repaired after that, but after going through this problem over and over in my mind, I remembered that I had the prop repaired the summer before, from a different ding!! I remember thinking I dodged a bullet at the boat launch because hitting the frame of the trailer caused no visible damage to the prop. I didn't think anything of it this spring because there was no vibration, and everything felt right. Everything seemed fine until I tried pulling a skier and needed to go over 30mph (which I don't do all that often). What happened was that the prop hit bent each blade equally... keeping the prop in balance, but effectively reducing the pitch, thus the lower speed.

I took the prop to the repair shop, he got it back in spec, and I'm back in business...

Case closed.

[smorris7]

Thanks for the update. Makes perfect sense.

[uniwarking]

Glad you got it figured out! I hit the prop guard once too... not a fun ordeal!

[kaneboats]

Yup, glad you got it sorted and it's not really a power issue.