Do I have the wrong prop?

[SFT2]

I've been looking around at props out of boredom and have found that most of the manufacturers are recommending a different diameter and pitch than I currently have. The boat is an 89 Maxum 1700SR with a Mercruiser 3 liter and Alpha I. The prop is a 15.25x15, but Michigan Wheel, Turning Point, and so on are recommending less diameter and more pitch, up to a 14.25x23. I've accepted the fact that I need two props, one for skiing/tubing/etc, and one for cruising/going fast/etc, but isn't 8 inches of pitch a bit much? I'd rather not spend a bunch of money on props that won't work, naturally, but I would like to get a bit more speed. Is there a general rule of thumb like take an inch of diameter, give an inch of pitch (works that way with airplane props)? Does anyone have a similar size, weight, and design boat with the same power and a good prop choice?

 

[Silvertip]

Re: Do I have the wrong prop?

Your boat can run anywhere from 14 to about a 15-1/2 inch diameter prop. That said, props of the same pitch from various manufacturers can be significantly different in diameter. It's all about prop design. Next, prop diameter changes generally affect performance less than pitch. So the only way to determine which prop is best for your boat, load, and application (towing water toys, skiing, etc) is to try what you have and note the wide open throttle rpm and speed. The prop is the transmission (one speed) for your boat. It has to get you out of the hole and provide decent top end and at the same time, allow the engine to run in its recommended wide open throttle rpm band (4400 - 4600 rpm in your case). If it revs faster than that you need to add pitch (1 inch = about 200 rpm). If revs slower than that you would reduce pitch. Although diameter would alter RPM somewhat, it should not affect it as much as pitch. Now the problem. If you switch prop manufacturers and style, as well as changing pitch and diameter you have introduced about four variables. The best bet is to work with a prop shop or marina that lets you try before you buy. But be a sport and buy from them when you find what you like. I think you are pretty close with the prop you have now. But post some numbers as without them we have nothing to work with.

 

[BAYLINER185]

Re: Do I have the wrong prop?

Just for some #'s for you

My BAYLINER 185 18 foot open bow w/ 4.3 merc and alpha 1 uses a 14 X 23 pitch and my WOT is 4200 RPM top speed of about 53~55 MPH. With a SS prop.

Same sice prop in Aluminum will run WOT at 4600~4800 RPM and yeald a 50 MPH top speed.

You boat is close to the same design as my BAYLINER and I have looked at a lot of boats this summer that are close to the same design and they all had 23" pitch props.

Sounds like you do have room to work. Also I was talking to someone this summer and he said I problably wouldnt be able to ski with this prop and would need to lower down in pitch in order to pull a skier properly.
I really dotn know why but it doesnt matter as I dotn ski any way so to me it makes no differance. But your train of thought for having two props is common place if you do a lot of differant things with your boat.

 

[SFT2]

Re: Do I have the wrong prop?

Thanks for the help guys. Went out and got some numbers today. WOT, for the very short time I could use it without getting hammered, was about 4500 rpm and 39.something MPH by GPS. She was stumbling a bit at WOT, so I'm not going to treat that as fact just yet. I didn't just nail the throttle out of the hole, but she had no problem getting on plane. Could be hull design, prop choice, or the hydrofoil (the Attwood version with the dihedral. Looks cool, might help, and mostly gives at least a little protection if the kids get too close to the prop), but right at 20 MPH had her on plane every time. I do need to adjust the trim tab, though. Had to hold the wheel close to 90 degrees to starboard going north and about 45 degrees going south. She turns a whole lot better to port, too.

I did learn one very important piece of info. Wide open throttle is just about useless on my lake. Way too choppy. Cruising at about 4000 got me 30-32 MPH and a smooth ride, and I was quite happy at that speed. I know I have a lot to learn about trimming her, but I think once I figure that out and get my current props rebuilt I'll be pretty happy.