Couple of questions about propellers...

[IDBoating]

(posted these in the middle of another thread but got no responses, so giving them their own)

New 2009 24Ve, EX343 CAT engine, stock 537(?) Acme prop:

1) I tried a few full throttle starts today. When I go WOT from a dead stop, I get a pretty strong shudder/vibration for a while until the boat is underway. The whole boat is shaking; you can feel it through the entire hull/deck. Is this what cavitation feels like on an open prop boat? I don't feel it except at or near WOT from a standing start.

2) The engine specs say WOT RPM's should be 5000. My WOT RPM's are 4700 (which yields 40 MPH on the dot). Generally this means a prop change is in order, but we're planning to install a ballast system for wakesurfing. However, wakesurfing is done at much slower speeds. Should I be concerned today about WOT RPM's?

Thanks!

[turbonine]

1) That is not what cavitation feels like. It feels more like a slip along with a clunk sensation. It will probably only be experienced in turns when heavily weighted with the 537 prop.

How many hours? If new, most manufacturers recommend an enginge break-in period.

[IDBoating]

How many hours? If new, most manufacturers recommend an enginge break-in period.

~19 hours now. Had about 16 when I received it. The recommended break-in procedure includes ramping up to WOT which is when I feel this "shudder" or vibration.

However, to be clear, I do NOT feel it when at full speed and I do not feel it when I ramp up slowly to full speed. It only occurs under WOT from zero speed, and lasts until the boat reaches (estimate) 10+ MPH after which everything is smooth as silk. That's why I wondered if this is what cavitation feels like on an open prop boat - because it seems to occur only when the hull is under maximum acceleration, when the prop RPM's are highest but the actual water speed is lowest. As that differential diminishes, so does the shudder/vibration.

BTW, I visually inspected the prop and other running gear when we pulled the boat last night. Everything still looks perfect, so no imbalance of the prop is expected. Besides, I'd feel that at WOT regardless of hull speed.

Ideas?!?

[turbonine]

Maybe some slight cavitation is happening.

What is the water depth?

Does it happen in all TAPs positions?

What happens if you roll into the throttle? Giving it about 2/3 until the boat is almost on plane, then going to WOT.

[IDBoating]

What is the water depth?

Average 40-50 feet. Never less than 10-15 feet.

Does it happen in all TAPs positions?

I didn't notice. Good question, though. I likely won't have the boat back in the water until next Spring but will experiment then.

What happens if you roll into the throttle? Giving it about 2/3 until the boat is almost on plane, then going to WOT.

The shudder/vibration does not occur unless you go WOT immediately, or almost immediately. If you "roll into" it, it ramps up smoothly with no vibration at all.

Thoughts?

[talltigeguy]

IMHO, not reaching top RPM's is a good thing and means that your prop is about right for high speed cruising. But who cares about high speed cruising? I care about low end grunt with a fully sacked out boat.

So on my boat, I do reach the rev limiter at 42 MPH or so, but I don't care. My hole shot is terrific.

[rensink]

If your thinking of loading your boat down with a lot of weight reproping will help a ton check into the ACME 1235 one of the most common and best upgrades I think.

[IDBoating]

IMHO, not reaching top RPM's is a good thing and means that your prop is about right for high speed cruising. But who cares about high speed cruising? I care about low end grunt with a fully sacked out boat.

Agreed, we don't spend that much time at WOT but I'm concerned it's an indication of a problem. If so, I'd rather fix it now than let it progress to something worse.

So on my boat, I do reach the rev limiter at 42 MPH or so, but I don't care. My hole shot is terrific.

The PCM engine docs say redline is 5200 but 5000 is preferred. 4000 is the max recommended for "extended cruising". I'm hitting 4700 so I'm well within the max, and my lake isn't big enough to do any "extended cruising" {grin}. But I do like to run my engines at WOT occasionally just keep them clean.

[IDBoating]

If your thinking of loading your boat down with a lot of weight reproping will help a ton check into the ACME 1235 one of the most common and best upgrades I think.

Yes, that's on my list. Lots of positive reviews here for that prop on this boat. That's why I asked about repropping above... the 1235 is supposed to improve things for heavily ballasted boats, but this problem is occurring at WOT and slow speeds. Those aren't the conditions for wakesurfing, so I'm not sure the advantages of the 1235 will address this shudder thing.

BTW, isn't there an Acme dealer here on this site? I'd like to keep the money in the family when I buy the 1235.

[talltigeguy]

The PCM engine docs say redline is 5200 but 5000 is preferred. 4000 is the max recommended for "extended cruising". I'm hitting 4700 so I'm well within the max, and my lake isn't big enough to do any "extended cruising" {grin}. But I do like to run my engines at WOT occasionally just keep them clean.

Never heard of a 'preferred WOT'. I would assume that to get that extra 300 RPM you would have to go down in pitch. Sounds like a 1235 is in your future.

I hope someone can commment on what PCM means by 'preferred'.

[turbonine]

My hole shot is terrific.

Understatement by far. That thing jumped out of the water with barely any throttle at all. It was awesome.

[Ewok]

[IDBoating]

Exactly. Based on that chart, I'm below the "minimum full load" RPM's. Odd that Tige would ship the boat with a suboptimal propeller, eh?

I suspect a 1235 is in my near future. Is there a market for used Acme props in perfect condition, to offset the expense of the new one?

[turbonine]

Exactly. Based on that chart, I'm below the "minimum full load" RPM's. Odd that Tige would ship the boat with a suboptimal propeller, eh?

I suspect a 1235 is in my near future. Is there a market for used Acme props in perfect condition, to offset the expense of the new one?

Get the 1235. Keep the 537 as a spare in a weekend saver kit.

[Ewok]

It probably has to do with the altitude you are using your boat in. FL is basically sea level and Spokane area is about 2,300' above seal level. The new prop will probably help. I might have to look into getting a new one too when I move up there. I might order the weekend kit and keep my current prop as a spare.