Correct prop?

[KRH1326]

I have a 1976 24' Winner Flybridge. Estimate weight around 5000 lbs. loaded.
OMC 235 HP Electric Shift Stringer. Ford 351W.

Upper 21.16 Lower 15.23 Total .86 Overall ? 1.16

WOT supposed to be 4200 - 4600 RPM.

I am currently running an OMC 382017 which is a 14d x 12p aluminum prop.
I am limited to pin drive type props.

I achieve 22 mph @ 4200 rpm WOT.

I cruise to and from favorite fishing grounds 3000 - 3500 rpms, which gives me about 18 mph.

Are these numbers good for my size, shape of boat?
If I want to try to gain some mph, should I be considering a change in pitch?

 

[KRH1326]

Found weight info, and inserted into original post.

 

[KRH1326]

Re: Correct prop?

Found Gear Ratio info, and inserted into original post.

 

[steelespike]

Re: Correct prop?

I'm not acquainted with the OMC drive but I feel you should have more speed and be able to turn a larger prop maybe not a whole bunch more but maybe a 15.
A 12 seems really low for a stock prop with little room for lower pitch for heavy loads.
I could be way off base. Is the motor in excellent tune with a compression test?
How is the hole shot?

 

[KRH1326]

Re: Correct prop?

I am very critical of keeping her tuned. I constantly am tweaking timing, checking points and condensor, etc. I will throw on the timing light out of boredom.

I rebuilt this motor from the block up just a couple of years ago. I did a wet and a dry comp test last season, I just can't lay my hands on the numbers right now, still looking. The numbers were high and tight, like a fresh build should be.

As far as hole shot, she is no ski boat, lol ! No seriously, she does step right out and is responsive.

I have tried to follow the hows and whys of prop calculation, but I seem to struggle with it.

If I have a WOT of 4200 rpms right now, with the 12 pitch prop, even putting on a 14 pitch, theoretically, should drop my wot as low as 3800, if I have the jist of it right.

Book says I should be turning 4200-4600, so maybe I don't quite get it yet, but I'm trying.

 

[steelespike]

Re: Correct prop?

Maybe I'm under estimating the bulk of your boat.
Its probably a deep V and perhaps loaded with lots of gear.
You might verify your tach.If you wanted to get to 4,600 you would have to go to a
10" prop seems rediculess to me.

 

[KRH1326]

Re: Correct prop?

Yes, I would call it a deep vee. Right now, as it is my ongoing project boat, it is fairly stripped out, however, I do have a screaming load of tools aboard. On long fishing trips, once I get baits over the side, I start tackling my to do list. So actually I do have tools and materials aboard.

So do the numbers that I posted seem reasonable? I would like to squeeze out 5 or so mph more, but maybe it's not possible. My wish would be if I could get 20-22 mph @ 3000-3500 rpm. That would make my day.

So now I ask, is this possible? If so, how do I go in pitch? If I do go with a 14 p and my wot drops to 3800 or so, what would the mechanical ramifications be?

I will first set up another tach test again. I did this last year, too. My engine analyzer matched my tach at that time OK, but may have changed.