Propeller Sizing/Boat Speed etc.

[Bob Perkins]

Hi Everyone,

I've been out playing with the boat and doing some testing.

We ran the boat through the various RPM ranges and recorded the MPH. (Tach is digital w/motor, Used GPS for MPH - so we are confident in the recorded numbers)
I then went to a prop sizing site and used their calculators to calculate prop slip because I thought it was an interesting number.

Here is what I came up with:
RPM MPH Prop Slip
1000 8.1 34%
1250 10.0 35%
1500 13.9 25%
1750 16.7 22%
2000 20.3 18%
2250 24.3 12%
2500 27.5 11%
2750 29.7 12%
3000 31.8 14%
3250 33.6 16%
3500 35.9 17%
3750 37.1 20%
4000 38.4 22%
4250 39.6 24%
4500 40.9 26%
4750 42.5 27%
5000 44.6 28%

All of the sizing information says that over pitched props will never reach redline and under pitched props will over rev. (un)fortunately?, My motor has built in redline protection and will not rev past 5K.

So... my question is, how to I determine if I'm using the correct prop? Certainly this one seems to work fine - I feel as though if I had more pitch, I could go faster at the top end - and it feels like I should be too.

Thanks for any help,
Bob

[pcford]

So... my question is, how to I determine if I'm using the correct prop? Certainly this one seems to work fine - I feel as though if I had more pitch, I could go faster at the top end - and it feels like I should be too.

Thanks for any help,
Bob

Prop sizing/pitch is a bit of a black art. What I do is find a similar boat and use that prop.

Your choices in wheel diameter are typically restricted because of space available. Take a guess and go to the middle of the range....pitch can be changed up to 2 inches if I am not mistaken.

[dmg]

Bob

I'm sure others will respond, but, probably would help some to know your engine, H.P, any reduction gear, displacement, and whatever is maybe shown on your print.

As a comparison of sorts, mine is ( or will be ) 28', 340 H.P. Chrysler 440, 1:5 to 1 reduction, 4250 lbs. or so disp., 16 x 24 3 blade prop, advertised top speed of 50 mph., just guessing on rpm limit of 4800 to 5000.

Dave

[kc8pql]

My motor has built in redline protection and will not rev past 5K.

So... my question is, how to I determine if I'm using the correct prop?

If the governor limits the RPM before the throttle reaches the full throttle stop, you're under propped. As you noted, if the engine won't reach redline, you're overpropped. I like a prop that maxes out just below redline, maybe 100 RPM's below..

[Bob Perkins]

Hi All,

I should have added details:

300HP, 23' Boat, 1:1 transmission, ~2400 pounds. Right now it is a 13x13 full Nibral Cup prop. Bottom is flat at transom (no vee).

I'm hunting on ebay for a 13x15 just to see if I can get another prop and give it a shot. Buying a new one just to test would be too much $$$

Thanks all

[pcford]

Hi All,

I should have added details:

300HP, 23' Boat, 1:1 transmission, ~2400 pounds. Right now it is a 13x13 full Nibral Cup prop. Bottom is flat at transom (no vee).

I'm hunting on ebay for a 13x15 just to see if I can get another prop and give it a shot. Buying a new one just to test would be too much $$$

Thanks all

13x13 sounds close. You could repitch to 13x14....and perhaps add a cup?

[mmd]

According to my Q&D (Quick & Dirty) prop calculator, you are missing some top end and spending some valuable horsepower turning innocent seawater into angry froth. I think that you should be able to gain about 7-10 mph if you increase pitch a bit, maybe as much as two inches. I'd find a used prop in a pitch range between 13 and 15 inches and a DAR of around 0.77-0.80, and use it as the "trial horse", re-pitching it until you find the sweet spot. Then have your original prop pitched to the proper degree and save the used prop as a spare. Be aware, though, that increasing pitch for top end will be at the cost of power out of the hole, so it may affect your get-up-on-plane time.

[Bob Perkins]

Thanks Guys,

MMD - Great idea - I'm going to hunt down an old 13x15 and see how it goes. Then I'll get the good one repitched if it makes sense.

Now I need craigslist or eBay to come through.

[Lew Barrett]

Check Second Wave in Seattle to see if they have one in your desired shaft diameter. They have piles of old props there.

[pcford]

I really don't understand the reasoning behind getting a used prop and putting it on as a trial horse. What is the advantage?? Unless the prop is in perfect shape it is not going to give a good result....in other words it will have to be reconditioned. Why not just repitch your prop. I am almost positive you can repitch two inches. You certainly can repitch an inch. With the addition of a cup in the blades this might be adequate. Perhaps better, as the cup gains efficiency at higher rpms. Kinda like an automatic transmission.

Buying another prop for a test seems a waste...

[Bob Perkins]

PC,

I don't have an issue with getting a second prop as a tester - as long as it doesn't cost TOO much. My 13x13 has cup now.

If I re-pitch the existing prop and find I've made matters worse - Can I un-pitch it? Or have I ruined a good prop?

FWIW: I know I'm splitting hairs at this point - I could leave the prop alone and be fine.. I just wanted to hit 50MPH. The boat is so light and has plenty of horses.

Thanks again - you guys are a great help.

[mmd]

PCFord, the "trial horse" prop serves two purposes: Firstly, repitching a prop requires that the blades are cold-bent, and this stresses the blade metal, case-hardening it. This is not good and will encourage cracking, especially if finding the right pitch entails several re-pitchings. By using a spare prop for this exercise, the good prop only needs to be re-pitched once, minimizing any stress imparted on the prop. Secondly, at the end of the exercise, Bob ends up with a spare prop, the benefits of which should seem pretty evident.

[pcford]

First, in my experience, if you have done good research, finding the right wheel/pitch for a boat should not take several repitching sessions.

I don't doubt the problems may occur as you mention. However, with the size of wheel which is referenced, I have never seen it.

[Lew Barrett]

I was given a free pitch job (if required) by Northlake when they sold me my new wheels. He expressed that he could pitch up 2 inches at the time, but I think I'm pretty close right now so haven't taken him up on it, but I've retained a niggling suspicion I could pull a bit more as I have so much power down low and hit redline so easily. In the case of my boat it wouldn't be to gain extra speed, just to know we wee properly geared for most efficient and easiest running. Without doing it, it will remain a bit of a question though. In any case, with a bronze wheel, re-pitching up to two inches is not an issue.

However, Nybril is a different beast and is harder to work. That might bear on this issue to some extent.