Largest prop

[maxumiain]

Hi folks
I recently bought a Regal 233 with a 5.7 v 8 on alpha 1 Gen 2. We travel with fellow boaters and only cruise at low speed. 8 to 10 mph. 15 at most. All posts are about top speed etc. Excuse my ignorance but is it possible to reduce the rpm at this lower speed by altering the prop. If so what is the best prop size, 3 or 4 blade to cruise at that low speed and reduce rpm's. I'm sitting at 2000 to 2200 rpm for this speed. It's a little noisy and thirsty for this speed. I currently have a 14 21p I think.
Thanks in advance
Iain

 

[steelespike]

Your motor needs to be able to rev freely within its wot rpm rating. If you try to force it to a lower rpm with a prop its not good for it.
What is the wot rpm and gps speed right now.
8,10 or 15 are all probably the worse for practical cruise speed.6 mph would probably produce the best gph.(gallons per hour)
Best practical cruising speed is some where above planing speed.
A 4 blade might at least make it easier to maintain speed and control. Usually suggested to drop 1" in pitch going to a 4 blade.

 

[jimmbo]

Is your prop vented? If so you might want to plug the holes.

 

[maxumiain]

Thanks for the replies. I have only had the boat out a few times so far. Don't know what wot speed is as never tested it. Was just wondering as my knowledge of props and their characteristics is non existent. I thought that if I increased prop size I could reduce rpm for low speed. Probably will never be on the plane so may not have to worry about wot. Again have no idea whether my prop is vented or what the difference is.
Thanks,
Iain

 

[H20Rat]

Don't know what wot speed is as never tested it. Was just wondering as my knowledge of props and their characteristics is non existent. I thought that if I increased prop size I could reduce rpm for low speed. Probably will never be on the plane so may not have to worry about wot.

The WOT test isn't to tell you what is happening at WOT, it is just as much about what is happening at the rest of the rpm range. Think of it this way, you could drive a manual transmission car around in 3rd gear at 1000 rpm all day long, but the engine would not be happy and you would be doing damage. The WOT test tells you if you are lugging the engine or not by running too much of a load.

You mentioned 10-15 mph... That is a very tough speed for many hulls to run it. Right on the edge of planing, where you are plowing a mountain of water. Smart tabs will help, as well as a stern lifting prop, but don't expect miracles. It is better to either slow down under 10, or speed up until cleanly on plane.

Looking at the mpg stats for your boat, the WORST possible speed for MPG is 9.9 mph. (2.0 mpg) Speeding up to 17.2 mph which must barely be on plane, puts it at 3.4, and 23.3 mph goes to 3.6. Best on-plane mpg is at 38 mph, at 3.8 mpg. At least as far as MPG's, you are burning more fuel at 9.9 than holding it wide open! (2.5 mpg)

Your boating buddies might have different hulls that are happier at that speed. I know bayliner used some semi-displacement hulls through the years that are very happy at cruising at non-planing speed, and don't have the dramatic break in between plane and non-plane.