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Old 05-26-2012, 11:27 PM   #1
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New Prop - Clean Bottom

This is the propeller I've been wanting for quite awhile now. It's a Michigan MP that's made in this country and is listed as a commercial workboat prop. I remember it was almost universal in the 50s and 60s on fishing boats and other commercial vessels. Most fish boats look more like trawlers that the fish boats of the 40s and 50s and so do their props. Most of the fish boats in Thorne Bay have big wide flat transoms and flat bottoms just like contemporary trawlers and if you've got any ideas about why it may make interesting conversation.

I did some running about a couple hours ago and the new MP prop is smoother and has more reverse thrust but only slightly. It seems to perform at about 2250rpm exactly the way the other prop performed at 2300rpm. That fits very well with the fact that WOT rpm is down 100rpm from 3000. So I'm now 100rpm over propped. It's very noticeable in the harbor as I need to spend more time out of gear. This symmetrical (blade) propeller with (apparently) no rake is known to be a bit less efficient than the more usual skewed props found on most boats. Doing speed/rpm comparison runs in both directions it seems to perform at 2250rpm about exactly as the other prop did at 2300. But that should be expected by loosing 100rpm at WOT. So my conclusion at this first act point is that it's not much different that the other prop other than everything happens 50 to 100rpm slower and it is noticeably smoother. I think if I reduced the pitch 1" I'd be at 3100 at WOT so I'm inclined to have (at a good prop shop) cut away a tad bit of blade area to pick up 100 to 125rpm. I consider being under propped 25 to 50rpm to be ideal.

I put the boat on the grid at the last moment so did'nt have time to go to town and get new zincs but the boat should be hauled in 4 to 5 months. I'm satisfied that I did get the prop changed and I borrowed a pressure washer from a friend so now we have a clean bottom and a bit smoother prop for the trip south.
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Old 05-27-2012, 01:13 AM   #2
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Looking good, Eric. It appears that the new rop has "Prop Speed" applied (?).
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Old 05-27-2012, 02:25 AM   #3
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Giggi,
No it has nothing on it. Didn't even put my usual ArmorAll on. The whole surface has a rather rough "commercial" look and feel about it. No ball of wax but seems to be a good prop. The very rounded and soft curves of the prop seem to match the whale-like lines of Willy's aft end. Should be (as they say) "good to go".
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Old 05-28-2012, 06:09 AM   #4
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"I consider being under propped 25 to 50rpm to be ideal."

Why , do you operate at max rpm during the usual cruise?

Many "commercial" work boat props suffer from the casual finishing yours shows , but the added blade area over most yachty props is what is usually desired.

The commercials are running 4-1,5-1 or 6-1 reductions and will frequently have 500 RPM on the shaft , if that, so surface finish is less important.

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Old 05-28-2012, 12:15 PM   #5
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FF, wrote:
"Why , do you operate at max rpm during the usual cruise?"

You know better than to say something like that Fred. Of course not, but if I've got the right amount of power on my boat I may want (or even need) to operate for short periods of time (say 10 min) at 100% load and if I'm not over propped I can do that without any worry about damaging my engine. More importantly I can run at 85% or 90% load for 12 hours and not worry about that either. When I bought my engine I "bent over backwards" to make sure I had an engine small enough that I could load it over 50% during normal cruise to insure that I would be operating an engine fully warmed up.

Re the prop finish most yachty props are probably finished in a more polished state than others because we like them shinny and pretty. The edges of my MP aren't rounded or made to be like a knife either. The edges are ground off flat. The flat edges are about 1/64th of an inch wide and make 2 sharp edges themselves. This may be "casual finishing" as you call it but on the other hand it may produce the lowest drag and have other benefits as well. Polished props and chrome plated cleats may serve no real purpose other than to please the yachtsman. I have several chrome plated cleats, I'm a yachtsman and I like them but hardly anybody is going to see my prop. And the prop I'm sure is going to perform perfectly well wether polished or finished w/o polish and there's a slight chance it will be perform even better. To this yachtsman the blade shape and the commercial heritage is more important. I could polish it myself but I want to see if the coarser finish is more prone to fouling. If it is I'll polish it the next time it's out. Which means I'll probably Polish it the next time it's out.
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Old 05-29-2012, 06:25 AM   #6
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"And the prop I'm sure is going to perform perfectly well wether polished or finished w/o polish and there's a slight chance it will be perform even better."

Perhaps , the Herrishoff rule was to imagine the prop was blown up to the size of the boat , barnicles , scratches and all.

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Old 05-29-2012, 07:24 AM   #7
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Don't know if this applies to props, but when I worked in plastics, we discovered that mirror finish cooling rollers made the molten plastic stick to them. Our solution was to dull that finish slightly which acted like a release coating. Maybe they work better with a dull finish?
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Old 05-29-2012, 10:33 AM   #8
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And a float plane accelerates better with a light chop on the water. The pilots say glassy smooth water is "sticky". Something Marin can relate to.

I frankly think it just dosn't make any difference and those buying a commercial propeller don't need a shiny thing to get attracted to.

Sailor of Fortune are you accustomed to seeing polished props on tugs?
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Old 05-29-2012, 11:31 AM   #9
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Quote:
Originally Posted by manyboats View Post
And a float plane accelerates better with a light chop on the water. The pilots say glassy smooth water is "sticky". Something Marin can relate to.

I frankly think it just dosn't make any difference and those buying a commercial propeller don't need a shiny thing to get attracted to.

Sailor of Fortune are you accustomed to seeing polished props on tugs?

Here's a link that shows a couple of divers "polishing" propellers on a cruise ship in Mexico. It took them 4 hours to do both props.

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Old 05-29-2012, 12:00 PM   #10
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Well to go to all that trouble there must be some benefit. Gives some credit to the notion that polished propellers work better.

When I get south I'll ask a prop man.

Barnacles DO impede performance and it's possible those divers were just whack'in off barnacles.
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