Prop for more speed?

[tdadvertising]

Hi guys, getting better at barefoot, and my Moomba Outback will only do around 36 with the stock prop (assuming 13x13). Can I get a couple more mph top end with a different pitch? What pitch should I try, and will it wreck my slalom experience? I'm also a course skier, just need to get that boat going 38-39

[Andre]

How many revs are you turning?

I remember Outback having like a base 310hp engine wich should get you way (3-4 mph at least) more speed...

Double check what prop you have also.

[tdadvertising]

It's the 310 but it's a reman engine. Maxes out I think around 4k and you can hear it starving for more air. It's a stock prop so I would imagine 13x13 but I will look. Even the original motor it wouldn't do over 38 without a tailwind.

 

[Andre]

Find it kind of low,both 38 mph and 4 k rpm but never been in a Moomba.Maybe someone with one will chime in.

Can you edit title to add ''Moomba''? Recent tune up?

 

[UWSkier]

Something sounds wrong with that boat. Assuming it's an Indmar, WOT RPM should be closer to 5000.

[BraceMaker]

Sounds like you have too much prop boats don’t starve for air they usually have unlimited air, prove it by running hatch up spark arrestor off.

 

You need to have less prop till your boat over revs before a steeper pitch because right now you are load/drag limited not rpm limited.

 

2 engines doing this suspect rpm gauge off speedo off or maybe a fuel system problem with boat.

[Ski_Dad]

i don't think the issue is your prop unless you have something odd on there. You should easily hit 40 on that boat. A picture or numbers off the prop would help and if you have perfect pass double check that RPM.

 

has it had a tune up in a while - cap, roter, plugs/wires, etc

 

oh and are you judging the mph by the speedo or a GPS - that could account for some as well.

[teammalibu]

Put a fat sack in the back seat and try it!

[6balls]

Tune up (and look over by a good mechanic--compression test etc), if that doesn't do it or find a problem, pitch down for higher rpm and hopefully more speed. 36 wide open is all kinds of wrong for that boat.

[tdadvertising]

Thanks guys, I'll get a pic of the prop and put a tune on it and start from there, then chime back in. I agree, I've owned a Nautique & 2 ProStars and never had a sub-43mph inboard

[BraceMaker]

Pics aren't very helpful of props hard to tell much. I use the bike analogy you are peddling uphill as hard as you can but you are maxed out. Can you shift up to a harder gear and go faster or do you need to shift down and get your cadence higher to speed up. Once you find where you can't peddle any faster you need to get a steeper gear to slow your cadence.

 

Can be prop and can be poor engine performance

[GregHind]

If all you can get is 36mph in that boat then your engine is almost certainly not even making 200hp. Don’t assume what the prop is. Check. A 13x13 should be ok but ultimately a 13x12.625 or 12.5 might be better.

When you are that far down on power it could be that 2 spark plug leads are swapped onto the wrong plugs, spark is not advancing or some other simple but major setup problem is killing power. That last 5mph takes a LOT of power and you are probably missing the last 8mph meaning you are probably close to half power.

[S1Pitts]

Once you get it all tuned up don't ignore the bottom of the boat. During our spring covid isolation I cut, polished and waxed the blazes out of my boat including the bottom side. I picked up another 3 mph and now I don't need WOT to yank my butt out of the lake. I was shocked at the increase of overall performance.

[vtmecheng]

@S1Pitts is right, it doesn't take much to disrupt the flow of water. That said, I agree with the tuneup as a first step for @tdadvertising. No reason he shouldn't be over 40 mph even if the bottom was really dirty.

[tdadvertising]

Thanks guys. I'm going to start with a full tune. Meantime here is a photo of the writing on the prop. My analog tach is right on with my perfect pass tach until we get up to 30mph. Wide open Perfect pass says I'm running well over 5,000, and being a course skier I think it's the accurate tach. 34 feels like 34 in the course compared to other boats I ski behind.

 

[Andre]

Well over 5000 rpms? How much the analog and PP shows at 34 mph?

Rule of thumb, a 13x13 prop should spin about 3300-3350 @34 with 2 persons crew and a 180 lbs skier.

If you're turning that at 34 and top speed is 36,you're not running 5000 plus rpms for sure.

 

Edit:Rereading myself,with different cupping of the prop,the rpm range could go up a bit more then 3350 i think.Like up to 3450 i guess...

Not a prop expert.

[BraceMaker]

@tdadvertising you just have to sort through a few things linearly.

 

Actual RPM is very important to know - it is difficult to make prop choices if there is any ambiguity in the number.

 

The prop you're showing is a 13x13 but it is also clearly a CNC prop and based on number its a michigan wheel ambush 4 blade - I don't have any sort of chart for MI Wheel - but my instinct tells me that is a lot of prop for the boat. The ACME 543 1" prop is recommended for your boat and that's a 13x11.5 3 blade - so I wouldn't be surprised at all that it would drop your RPMS a bunch - so this is where getting back to the actual RPM reading really matters.

 

Do you have an automotive multimeter or timing light with an RPM trigger? If not for 15 bucks you can get an optical tach to test it.

 

I assume the boat has a 1:1 transmission but it could have a reduction drive transmission - I'm not familiar with the options moomba sold.

 

 

 

 

[DW]

A 4 blade prop would not to be the best option for your application. Typically, 4 blade (v 3 blade) props can offer a better holeshot but also are ~1-2 mph slower on the top end. With a CNC 4 blade prop, which has a lot of bite, good chance a 3 blade will increase top speed a couple of mph. Barefoot boats tend to run 3 blade props, 4 blades are more suited for slow speed activities to hold better speed at the lower speeds.

 

Having stated the above, a 4 blade can easily hit low to mid 40's for most typical ski boats so I would look a bit deeper as others have said to unlock all the power you should have. You noted a reman engine, you should verify that it has the appropriate marine camshaft as on a SBC it is specific and not an automotive one. When doing your tune, verify ignition timing, plug wire orientation and I would suggest a compression test and if possible a leak down test. Is it fuel injected or carbureted, if the latter ensuring correct jetting may be an issue and if carbed, make sure the secondaries are fully opening.

[GregHind]

If that prop was an acme 13x13 and you were running 5000rpm you would be running 50mph

So it sounds like you are propped wrong or under developing horsepower or both and your tach seems miles off. That’s way too many variables - basically so far it could be anything. Check of PP is set to v8 4 stroke. I doubt PP is reading wrong. You could try the inverted tach setting on it to be sure it stays the same.

 

If you really are running 5000rpm at less than 40mph then you need a new prop. Your engine won’t breathe above 5000rpm and will run out of puff. An acme 13x12.5 should be close but ask the factory or acme or oj props what they recommend before buying. Or just borrow a known good prop from a similarish boat.

[Orlando76]

What transmission?

Carb or EFI?

Elevation?

 

13x13 CNC prop on a 1:1 is typically a lot of prop to pull, motor needs to have major cojones to pull it. If its a reduction gear, you’re all sorts of wrong.

[tdadvertising]

Hey guys, thanks to everyone who chimed in on this problem. We took some of the earlier advice and started with tweaking the timing, and that did the trick. She'll do 40 now and that's more than enough for me to long line barefoot, which was the goal. I greatly appreciate this forum and the expertise people are willing to share