Performance Differences Between MaxProp Sizes and Pitches

[zboss]

Hi,

I am looking to replace my existing fixed prop with a Maxprop.

The recommended prop by Westerbeke is a 18" x 8P and this is what was originally fitted on my vessel but it was old and needed to be replaced. I replaced it 2 years ago with a 17" x 11P under the recommendation of a prop shop.

However, I want to move to a Max Prop to improve sailing efficiency. Maxprop originally recommended a 3 Blade 17x10 but I have found that my existing 17x11 doesn't seem as efficient as the old 18x8 seemed to be.

Also, when fully laden, our waterline length is higher than the designed waterline length and the boat is much heavier, hence why I think that extra inch helped a lot.

I asked about moving up to an 18" maxprop and he said that best I could get was 18x9.

However, I am still a little confused as to what kind of performance difference I would notice between the two maxprop sizes - 18x9 and the 17x10.

What do you guys think?

[Pauls]

Two things.

First, Maxprops are adjustable pitch. You just buy based on diameter, the pitch is adjustable to anything you want. One of the nice features of the Maxprop.

Second, getting prop recommendations on a forum is a really poor way of optimizing your prop. Prop recommendations take into account a lot of detail, including the clearance around your prop shaft, your actual sailing displacement, engine hp, reduction ratio, and maybe personal preference for how you want to run your engine. If you want to be sure you are making the best choice I recommend you buy a copy of The Propeller Handbook by Dave Gerr. It will enable you to make the best choice on your new prop and you will end up understanding why that choice was made.

[zboss]

Originally Posted by Pauls

Two things.

First, Maxprops are adjustable pitch. You just buy based on diameter, the pitch is adjustable to anything you want. One of the nice features of the Maxprop.

Second, getting prop recommendations on a forum is a really poor way of optimizing your prop. Prop recommendations take into account a lot of detail, including the clearance around your prop shaft, your actual sailing displacement, engine hp, reduction ratio, and maybe personal preference for how you want to run your engine. If you want to be sure you are making the best choice I recommend you buy a copy of The Propeller Handbook by Dave Gerr. It will enable you to make the best choice on your new prop and you will end up understanding why that choice was made.

Thanks for the info.

MaxProp has already been given that information and they came back with those recommendations but I don't feel I fully grasp the theoretical difference between the two recommended props.

[Pauls]

OK. I'll explain some of the trade-offs that MaxProp looked at when making their recommendation:

1. The larger the prop the greater the efficiency, so long as the prop is working in a range that is efficient. What that means is that as the prop gets larger it needs to turn at lower RPMs. This efficiency keeps increasing up to a prop diameter of about 1/3 the beam of the boat. But you can't get a reduction ratio that high, and the sailing drag would be ridiculous.

2. A prop needs enough clearance between the prop tips and the hull. Typically this is spec'd at 15% of diameter as a minimum. This is because when the prop tips run too close to the hull the pressure wave surrounding the prop slams into the hull and creates noise and vibration. On most boat this is what limits the prop diameter.

3. There is range of diameter/pitch ratio at which a prop is most efficient. The farther you move out of that range the more the efficiency degrades.

These are things that someone looks at when picking a prop for a boat. To summarize, bigger is better, but the factors listed above place limits on how big the diameter can go. I'd assume that Maxprop looked at all these and came up with 17" as a good compromise. Perhaps 18" is better. Again, Gerr's book will give you what you need to make an informed judgement on that. If you don't want to go thru reading the book, then I'd suggest you trust your experience. If you found the previous 18" prop was better on your boat, I'd buy the 18"

[exMaggieDrum]

FWIW - I think my fixed prop is too big because I can't get my engine to full RPM. You want to be able to get to full rpm on your motor. A bigger prop than works on your boat basically limits the motor, i.e. it is too big a load on the motor. I get black smoke from this problem. I have a 17x14 prop. I contacted Flexofold with my data (not yet asked MaxProp but I will) and they recommended a 16x10 prop.

But this depends on a lot of factors as Paul noted, including transmission ration, the engine and its specs, etc. BoatDiesel.com has a calculator that I used but you have to pay for a membership. There may be others.

On my previous boat I had a MaxProp and I changed both the transmission ratio and then the pitch. It made a dramatic difference in fuel efficiency and speed.

Bigger is not necessarily better. It may be worse or it might be better.

[alansmith]

Is Maxprop any different than Autoprop?

[skipgundlach]

Originally Posted by alansmith

Is Maxprop any different than Autoprop?

Very much. There's a good article I just read (maybe linked in CF?) about tests of many different folding and feathering props.

As to diameter and pitch, you can adjust your pitch to whatever works, previous comments about clearance factored. Tugboats have massive thrust, but at very low pitched props. Racing boats have massive thrust, but at very high pitched props. If you can't move your boat through the water at a motor speed commensurate with performance for that motor, you're either over sized or over pitched or both.

In that article, a 3-blade MaxProp (which we have) did the best in astern propulsion, but also had nearly the worst stern walk. All the folders (or nearly, I forget) handily beat the fixed prop in forward effectiveness (speed at a given RPM or top speed at WOT), and drag was very significantly better.

An interesting data point was that, in controlled circumstances, the freely rotating fixed blade (assuming your tranny could do that safely) was markedly less drag than one which was locked.

Here's the link:

Folding and feathering propeller test

[zboss]

Originally Posted by skipgundlach

Very much. There's a good article I just read (maybe linked in CF?) about tests of many different folding and feathering props.

As to diameter and pitch, you can adjust your pitch to whatever works, previous comments about clearance factored. Tugboats have massive thrust, but at very low pitched props. Racing boats have massive thrust, but at very high pitched props. If you can't move your boat through the water at a motor speed commensurate with performance for that motor, you're either over sized or over pitched or both.

In that article, a 3-blade MaxProp (which we have) did the best in astern propulsion, but also had nearly the worst stern walk. All the folders (or nearly, I forget) handily beat the fixed prop in forward effectiveness (speed at a given RPM or top speed at WOT), and drag was very significantly better.

An interesting data point was that, in controlled circumstances, the freely rotating fixed blade (assuming your tranny could do that safely) was markedly less drag than one which was locked.

Here's the link:

Folding and feathering propeller test

I like having prop walk - its the only way for me to turn my cut-away full-keel properly in tight marina's - without it I would be screwed so I have learned to use it to my advantage.

[Chuck Hawley]

I was a boat captain (fairly novice at the time) of a 67' "sled" designed by Ron Holland. To get rating credit, we used the largest two-blade Max Prop that we could find, which was 900mm or about 35" as I recall. To allow that to work, we had a V-drive with a very large reduction ratio (something like 2.8:1) and a steep shaft angle. The engine was a Perkins 4-236.

Prior to launching, I came up with the idea that 20" (or possibly degrees, I can't remember) of pitch would be correct (as I recall, it was a long time ago), and so I assembled the prop with the correct letter designations inside the hub. It turned out to be a good choice, but the prop was set up to go forward in reverse and reverse in forward due to the V-Drive, so I had to run the transmission in the wrong direction to get the boat to move. Lesson #1. However the engine seemed like it was able to achieve max RPMs (2500) with this setting.

The next time I hauled the boat (frequent occurrence with a large race boat), I reversed the "handedness" of the prop, and tried 22" (or degrees) of pitch to see if that would allow more speed. The boat was a completely different animal. The engine bogged down, accelerated slowly, and couldn't get over about 2,000 RPM. All this was due to a 2" change in pitch (smallest increment on a prop this large). On the next haulout, I went too far the other direction and reduced the pitch to 18" (or degrees). The boat could not get out of its own way. Top speed was reduced by at least a knot (from 9 knots to 8 knots) and the engine appeared to be racing at all RPMs.

Finally, I returned the pitch to 20" and left it there.

My conclusions:

1. Having an adjustable pitch prop is really wonderful if you don't have lots of sisterships from which to draw conclusions. This would have been an expensive mistake.

2. Assembling a 35" propeller with tight tolerances while holding it above your head in a boat yard is challenging.

3. Naval architects will do anything to get a rating advantage.

4. The steep shaft angle must have destroyed any efficiency arising from the large prop diameter. Presuming that it was a 20 degree optimum setting, the pitch difference on the top blade vs. the bottom blade was significant.

Cheers,

Chuck

[Cheechako]

I've had a bunch of Max Props. Probably wouldn't buy anything else ....at similar prices anyway. I think either diameter will work for you, I would buy the largest diameter that still leaves 15% clearance to the hull. They have been very fuel efficient props for me and superb stopping power. I'm with Z boss as far as prop walk goes on a cruiser. I need that prop walk!

[alansmith]

Skip, thanks for the instant gratification on this practical article comparing the different manufacturer's props. It was very informative. Thanks for the link.

[redsky49]

Here is a prop calculator:

Vicprop - Propeller Calculator

I have used it just to see if it validates my seat-of-the-pants conclusions. It did in my case.

I have had one feathering prop in my boating experience. The biggest difference was reverse thrust, which was noticeably improved.

Also once had a 2-blade folding prop when absolute speed was paramount. Hated it!

[Krilleonnova]

I got my propcomments in under the diveboard thread can some computer
guru pls move it

Krilleonnova

[cacalack]

Don't know a thing about these folding props, or adjustable either, but have run many, many small (<25') power boats in rental fleets and can tell you the main consideration in that context: get the largest diameter possible given clearance requirement, then match pitch to engine rpm. You want the pitch that, when the boat is loaded "normally," will allow the engine to operate in the max range given by the manufacturer. So, on an 150 hp outboard, for example, rated 150hp @ 5,000 rpm, you want to be able to reach 5,000 rpm with the throttle in the corner. (Of course you'd almost never run it there.) I have to suppose this principle holds for low-rpm inboard diesels, too. I can tell you from vast experience, it makes a world of difference when you're running I/Os or outboards.

Good luck.

[zboss]

Originally Posted by Pauls

2. A prop needs enough clearance between the prop tips and the hull. Typically this is spec'd at 15% of diameter as a minimum. This is because when the prop tips run too close to the hull the pressure wave surrounding the prop slams into the hull and creates noise and vibration. On most boat this is what limits the prop diameter.

When you say "15% of diameter minimum"...

If I select an 18" prop... that would mean 15% was 2.7" - would that mean 2.7" on each side or 1.35" on each side?