Prop question

[Sroberts882]

Bought me a 2000 larson with the 3.0. boat currently has a aluminum 3 blade 14.25x21 prop. The boat gets out of the hole pretty good with just me and my wife in the boat but it's never just us in the boat. The boat is mainly uses to pull the kids around the lake on a tube or knee board. I damaged my prop the other day so I'm currently in the market for a new one. Trying to decide between 3 or 4 blade prop. What 4 blade prop is equivalent to my current prop to keep the rpm the same. Also when you want to test your prop rpm at wot do u need everyone in the boat like it's usually being driven or just the boat operator. Thanks

 

[alldodge]

What is your WOT rpm with you and the family aboard?

 

[steelespike]

W0T rpm and gps speed?
Drop 2" and get a 4 blade.

 

[jimmbo]

Also when you want to test your prop rpm at wot do u need everyone in the boat like it's usually being driven or just the boat operator. Thanks

Well there are several schools of thought on that. One is to make the boat as light as can be and then prop it so the engine is running right at the top of the rpm range. Doing so should guarantee the engine will not be over revving if you let someone else operate it as the boat will usually be heavier. Another way is to prop it for the same rpm under your normal usage. This usually results in a better holeshot but may allow over revving under light load conditions. Still another way is to deliberately under prop the engine to gat even better holeshot, but an eye needs to be kept on the tach to ensure the engine is not over revving. I am a little leery of that method, on non computer controlled engines as rpm/load/ignition_timing/fuel_mixture could be incorrect due to a lower than expected load on the engine. Given a choice, a slight over rev is preferable to over propping, resulting in an over loaded engine

 

[Sroberts882]

Don't know wot rpm and gps speed. I'll take the boat out when I get a chance to test it. Do u think my prop with the damage it has will get me accurate results. Well was going to lost a pic of prop but it says file is too big.

 

[Sroberts882]

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[Sroberts882]

No Title

 

[jimmbo]

Bent blades will affect rpm more than a few nicks. FWIW, the prop pictured looks like a Michigan

 

[Sroberts882]

The water wasn't as smooth as I wanted it to be, it was windy but it was the only chance I would have to go in the next few days. Not sure about my results. It was me and my wife and a full tank of gas. The first few times I got
4400rpm@40 mph
4700rpm@40mph
4800rpm@39mph
All the above the rpm would shoot to 3000rpm then climb.

Plained out around 12mph

Then started getting readings.
5600rpm@37
5000rpm@39

Idols around 10,000rpm

 

[jimmbo]

10,000? well it won't idle for long at that RPM. Your other readings indicate a high slip rate and/or excessive ventilation. Does the prop have vent holes?
Planes @ 12mph? Exactly how rough was the water?

 

[Sroberts882]

What rpm should this motor idle at. I going to go check on a rpm gauge when I get off work to make sure I'm getting the right readings. What is involved in hooking up a temporary rpm tach. Water was choppy. Could the damage to the prop in the pic above cause any of it.

 

[Sroberts882]

What would cause the slippage.

 

[alldodge]

What rpm should this motor idle at. I going to go check on a rpm gauge when I get off work to make sure I'm getting the right readings. What is involved in hooking up a temporary rpm tach. Water was choppy. Could the damage to the prop in the pic above cause any of it.

Idle should be 700 rpm in gear with boat in the water. When out of gear the rpm will be a bit higher

Damage to the prop will increase slip

What would cause the slippage.

Many things but most common are:
Incorrect trim
damaged prop (bent, dinged, chipped)
weight of boat, people and gear (The heavier the boat gets the more slip will happen)

 

[Sroberts882]

My bad I added 1 to many 0's. It idles around 1000rpm maybe a tad under when not n gear

 

[Sroberts882]

So instead of doing another test with the damaged prop do u think a 4 blade 19p will be ok with this boat and motor. It has a 21p prop now.

 

[alldodge]

It might if you could use a bit more lift, but might be better with a repaired/replaced 21 3-blade if you don't need more lift. A prop can work great on one boat with a given type of boating, and the same prop on another boat might make things worst. If you can find a shop to loan you one for trying, that would be the way to go

 

[Sea Rider]

With 19 pitch combo will run, but doesn't mean will rev around full wot revs with current load. Plain theory doesn't apply while selcting prop pitches. All props needs to be water test. It's near rocket science dialing best prop performer.

Happy Boating

 

[Sroberts882]

It lifts good with me and the wife but with the people in the boat besides us it takes a lot longer

 

[alldodge]

The lift I mentioned is how high the stern is out of the water. If lift gets to high the prop will ventilate, maybe not going straight but maybe turns. A prop is pushing the boat forward along with trying to climb out of the water.

Many bass boaters use a 5 blade prop, but also have a jack plate to lower the drive, just as an example

 

[Sroberts882]

No Title

Is this everything that should be on the shaft