Prop pitch vs. Dia. Question

[helimech2003]

Hello all.
Need some help deciding which prop to get. First. The boat is a 94 Chaparral 1830 SS. 4.3l LX Alpha one gen 2. 1.81 ratio. Used for skiing, tubing, wakeboarding etc. Not to concerned about top speed.

The boat came with a quicksilver stainless 3 blade in a 14.25 X 17. This prop has a TON of holeshot but I can only run about 3/4 throttle turning 4600 rpm. I used turning points prop calculator and it suggested a 14.25 x 19 4 blade husler. Bought it and installed. It was pretty sluggish outta the hole and I could only get 4100rpm out of it.

So here's the question.......
I was going to go to a 14.25 x 17 4 blade husler but, I also see where they suggested a 15x15. What would be the benefit of going to a larger diameter/ shallower pitch over the 14.25x17???

Like I stated above. It's used mainly for watersports so, I'm looking more for holeshot. I would go back to the stainless 3 blade that came with it as it does what I want it to but, with that prop trimmed up even slightly it starts to porpoise horribly. I didn't realise how bad it was till i put on the turning point 4 blade and could trim it way out before it started.

 

[Scott Danforth]

leave the prop you have for watersports

 

[ahicks]

Like a 4 blade prop, increased diameter will give you more blade surface area for a better "grip" on the water. As well, that extra "grip" generally has a downside - it increases drag.

In your case, nobody cares much about that extra drag as the emphasis is on speeds below where that drag really become a big factor.

I'm confused over the comments about trimming the boat out and porpoising, with the 3 blade blowing out easier and porpoising more, than the 4 blade. What is it you were trying to accomplish here? You can increase the ability to trim the lower unit out with increased cupping if you're feeling the need. The other thought is the amount of trim required to max speed is going to kick your pants when you turn or slow down. Max trim amounts are going to change with speed.

If you want to go 4 blade, use the data you got with the one you tried to get another that will allow the boat to run the 4600 you're looking for (the 14.25x17). How the 14.25x17 would compare to the 15x17 is something you'd have to try. Not sure anyone is going to be able to predict that - or - the effect it would have pulling a skier. Part of the difference between these 2 props may lie in the shape of the blades. For instance, the 15x17 could be a mickey mouse eared pontoon boat prop?

 

[Sea Rider]

What's the min-max wot rom range factory stated for that engine to start with ? You like boating while lightly, medium light or heavily loaded ?

Happy Boating

 

[achris]

Hello all.
Need some help deciding which prop to get. First. The boat is a 94 Chaparral 1830 SS. 4.3l LX Alpha one gen 2. 1.81 ratio. Used for skiing, tubing, wakeboarding etc. Not to concerned about top speed.

The boat came with a quicksilver stainless 3 blade in a 14.25 X 17. ...

4.3LX in an 18 something foot boat should be turning a 21" prop with revs to spare! I was running a 21" with my 4.3LX, and that was in a HEAVY 20' cabin cruiser, and I was still hitting 5000rpm. And holeshot was more than acceptable. I once tried a 3 blade Laser II prop with the vent plugs removed. If you're looking for 'Polaris missile' type acceleration, that's your prop!

Stay away from Turning Point and Solaris props. Known for inconsistent results and poor blade design. Stay with the props made by the same people how made the drive. Laser II and High-5 both have vent holes and are both 'prop of choice' for the ski fraternity.

Larger diameter is the OPPOSITE way to in which you wish to go. Large diameter is great for soaking up huge amounts of torque (usually made by BBC engines), and SLOWING the revving of the engine. You want to get that engine up into the 3000rpm + range (the power band) as quickly as possible, which is why vent holes work so well.

Chris......

 

[jimmbo]

I have used OMC/BRP RAKER props and Merc Enertia props on my VP drive with great results. Aftermarket props were less consistent.

 

[helimech2003]

4.3LX in an 18 something foot boat should be turning a 21" prop with revs to spare

That's what I was thinking too.
I traded for the boat over the winter and was told that it had a rebuilt engine in it. Looking at the engine ,it does look to have been pulled (new freeze plugs and clean metal in areas along with fresh paint) but that doesn't mean much. Prob going to do a compression check on it and see where its at. If its weak I'll just run that 17" pitch quicksilver for the season and rebuild over the winter.

 

[helimech2003]

I'm confused over the comments about trimming the boat out and porpoising, with the 3 blade blowing out easier and porpoising more, than the 4 blade.

Yes. I start out with the outdrive trimmed all the way down. As soon as it get on plane and starts picking up speed I'll start to trim it up. With the 3 blade at 3900rpm, it will take very little trim before it starts to porpoise. I noticed with the 4 blade that it could take a lot more trim at the same rpm's and lift the bow a lot more before it started.
I was always taught that once on plane and at a steady RPM to trim it up till it starts to porpoise then trim it back down a little and, that's the "sweet" spot.

 

[jimmbo]

Trimming can change depending on the prop, less trim is needed/tolerated with a bow lifter, while a stern lifter will need more to get the bow up to reduce wetted area. Other props are able to lift the entire boat vertically out of the water and still raise the bow.
The trim out till porpoising, then trim in till it quits rule generally applies, but a caveat ‘or until it blows out’ should be added