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Old 09-23-2020, 08:05 AM   #21
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RS your Chris Craft must be similar to my Tolly for weight and Haul design? What do you have for engines? Props?
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Old 09-23-2020, 08:07 AM   #22
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In general, more blade area (from more diameter and/or more blades) will improve hole shot as well as low RPM thrust. More blade area will also reduce WOT RPM with no other changes. Pitch will determine RPM and also speed. But sometimes adding blade area and reducing pitch reduces prop slip enough that you don't lose any speed at a given RPM.

Also, if efficiency is the goal, your most efficient slow cruise will likely be in the 6.5 - 7 kt range, as you'll be pushing much less water than at 8+ kts.

For my boat (38 feet, 33.5 foot waterline, twin 454s) I typically consider slow cruise as up to 1300 RPM (around 6.5 - 6.7 kts in flat water) unless I'm pushing a good headwind where I may bump it up to 1400 - 1500. Fast cruise for me is typically 3300 (17 kts) unless I have a reason to run a little slower. 3400 is generally considered max continuous for 454s. WOT speed this season was about 25 kts at 4200 RPM (a bit slow for this boat), so something is causing me a little extra drag (but nothing major). I remember years ago seeing this boat hit 27 kts at WOT (which should give about 4300 RPM with these props).

I've got 2.57:1 reduction transmisssions behind my Mercruiser 454s and 22x25 3 blades (turns 4200 - 4300 at WOT). Original props were 22x26, but it was a little overpropped with those (just barely hit 4000 RPM), so the 25s were swapped on. I'm currently hunting for a set of 22x25 4 blades, as it could use a little more blade area. If I don't come up with any, I'll probably take the other 3 blades (the original 22x26) and have the de-pitched to 25, as they have a bit more blade area than the currently installed props.

EDIT: Weight and hull design wise, the boats are probably similar. Mine should be about 26k lbs with full tanks and a normal load of stuff. It's a low deadrise (10*) modified V hull, pretty much looks like an old Hatteras with a shorter keel. From a picture I found, your hull looks pretty similar.
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Old 09-23-2020, 08:25 AM   #23
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RS Great info!! I'm also new to the 454 world, we should have comparable burn rates? Do you keep track of average burn say from 7-17? Wide open is not even a question on burn?
The prop info is good, being that I am going new I think the 4 blade is a good fit to check the sound and vibration issues off. Diameter seems to be the money question to nail it first time, as you can re pitch for minimum $
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Old 09-23-2020, 08:30 AM   #24
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Yeah, diameter is the bigger question. I'd expect you'll be either 19" or 20" in a 4 blade (definitely 20" in a 3 blade). But knowing gear ratio should help confirm that.

I don't have flow meters, but from my own estimates and some numbers I've found posted elsewhere, I figure my burn at 3300 RPM to be around 30 gal/hr (give or take a little). At 1300 RPM, it should be around 5 - 6 gal/hr. Dropping to 1200 should get it definitely down to 5 gal/hr or less and 1200 is still over 6 kts on my boat if I'm remembering correctly. I usually see about 5.4 kts at 1000 RPM, but efficiency drops off with load on a gas engine, so I don't think fuel burn per mile would really improve much by slowing down that much more.
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Old 09-23-2020, 09:51 AM   #25
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Okydowky wrote;
“ My goals:
Less Vibration
Less Prop Noise
Fuel Economy”

Oh I see economy there. Didn’t know that was an important factor.
Back to the three blade prop.

Pitch/dia ratio is important. Too much pitch and you get too much water at the blade tips spilling over the tip to the backside of the blade = lost efficiency.
Too much dia and/or # of blades = too much blade area and skin friction will be too high This is commonly found on trawlers. Too many skippers think dia. is everything and wind up w too much blade area and friction as a result.

Talk to a good prop man (or woman). Ask around. You can call Michigan Wheel as a good option. If you had a 14” dia prop about 12-13” of pitch would be good. Nine inches of pitch would be too little pitch. 11” pitch would result in high tip losses.
And the ideal ratio varies w power loading and speed. High speed and light boats require more pitch than dia.
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Old 09-23-2020, 10:05 AM   #26
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FWIW, assuming 2:1 gear ratio and 25 kts WOT speed, the Michigan Wheel calculator suggests a 21x18 3 blade or 19x19 4 blade for a 4300 RPM target RPM at WOT. Once we know gear ratio and what RPM and speed it hits at WOT currently, it'll be easier to get closer.

As far as blade area, it's a balance. Too much and you lose efficiency due to more drag (this hurts top speed too). Too little and prop slip gets high, the boat is hard to get on plane, doesn't run as well as it could, and loses efficiency. And maneuvering at low speeds can be crappy with insufficient blade area as well.
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Old 09-23-2020, 06:14 PM   #27
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Thank you again for this! Both your knowledge and logic is making sense. The 19x19 seems like I’m on the right track. This would give me more tip to hull clearance for hull noise and 4 blade should be smoother for vibration.
It seems we need to know WOT, to make the final decision on diameter at least.

For measuring shaft taper, Are tapered shafts all standard? Shafts are 1 3/4”, the prop shop only wrote down 1 3/4” bore on the new prop estimate he gave us.

Good and better brand of props? Michigan come up a lot in conversation...?

And yes Willy efficiency is in my list, but I’m a realist if I can save a half gallon an hour that would be great. I know there isn’t a prop to be made to save me enough to notice. Lol
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Old 09-23-2020, 06:57 PM   #28
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1/2 gph reduction is easy .. in a way.
Just back off on the throttle.
But the three blade will deliver more efficiency.
Re the noise above the prop through the hull ....
Put some lead shot back there .. above the prop blades. In the bilge. Make the hull twice as thick there w matt and resin or pour in some concrete. And you could put lead pieces in the concrete. You may want to address the possible issue of removal of whatever you put there though.
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Old 09-23-2020, 07:54 PM   #29
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I had to use a four blade prop because I was limited in diameter and a three blade prop of big enough to absorb the horsepower wouldn’t fit.
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Old 09-23-2020, 08:01 PM   #30
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Extra blade area is unlikely to hurt fuel burn at low speed on a planing boat, as the shaft speeds are pretty low (500 rpm for 6.5 kts on my boat, for example). It's at high shaft speeds up on plane where it matters more.
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Old 12-15-2020, 02:26 PM   #31
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Took a while to configure the prop diameter and pitch but I think I found one we will be able to work with.
Question now is Overseas or American made? Bang for your buck?

Any thoughts or experience on over seas metals and workman ship? Other than costs is the most noticable difference right now.

I have had issues in the past with nuts, bolts and other overseas deck hardware not lasting. My gut is telling me no, but the prop shop obviously is saying they have had good results.
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Old 12-15-2020, 03:11 PM   #32
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You may want to look at this calculator.
https://vicprop.com/displacement_size_new.php
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Old 12-15-2020, 03:32 PM   #33
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A prop screws through the water.

For a single revolution, the boat screws a distance.
The size of the blade is your traction. A big sportfisherman may go 40" per revolution, and take 1000 hp to do it. And he turns it fast, maybe 1:1 or less, 2500 rpm.

My Lehman turns I think 23" and takes probably 40 hp. I turn it 400rpm, similar to yours.

I have a prop with 1" more screw. At the same rpm I get a 10% speed increase. 7 mph to 8. Takes slightly more fuel, which makes sense.
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Old 12-15-2020, 05:41 PM   #34
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Taiwanese or North American

Has anyone had experience with Overseas props? Right now I got a price for the props I think we need and it was quite reasonable, but found out they are coming from Taiwan.

I get from fellow boaters! "hell no" "cheap is good". I am not sure what there quality would be like, I have had some experience with poor over seas quality products before.
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Old 12-15-2020, 09:08 PM   #35
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Quote:
Originally Posted by Okydowky View Post
Has anyone had experience with Overseas props? Right now I got a price for the props I think we need and it was quite reasonable, but found out they are coming from Taiwan.

I get from fellow boaters! "hell no" "cheap is good". I am not sure what there quality would be like, I have had some experience with poor over seas quality products before.

I ALWAYS stick with US made props. They seem to be better balanced, and stronger.
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Old 12-15-2020, 10:28 PM   #36
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I read someplace that every time a prop blade comes close to a part of your boat there is an "incident". A very minor occurrence between the water, bubbles and the hull of the boat. With a three bladed prop, you get three of these "incidents" per revolution. If you have a full rudder like on a single screw trawler you would get six of these "incidents" per revolution.( Three against the hull, three against the rudder support of the keel.)

With a four bladed prop, you would get four of these "incidents" per revolution, eight with a full keel.

With a four blade prop the "incidents" would happen top and bottom at the same time as one blade passes near the hull and its opposite blade passes near the keel.

These so called "incidents" are minor, tiny events. (If they occur at all) But taken cumulatively it equals nearly a million "incidents" an hour at 1800 rpm, Several million an hour in a high reving go fast boat.

If an "incident" has no more effect than a tiny air bubble hitting a prop blade or a tiny drop of water hitting the hull or whatever tiny comparison you can imagine, multiplied by several hundred million on a cruise it really could contribute to prop noise, vibration or some other disturbance.

Of course, I have no idea if there is anything like this self described "incident" but it is food for thought.

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Old 12-16-2020, 01:02 AM   #37
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Most Taiwanese boats likely had Taiwan props, that`s a lot of props. I found Taiwan made boat stuff better than their immediate neighbour`s,eg. anchor windlass switches. Unless politico/economic reasons matter,if price is much better and availability good,choice seems clear.
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Old 12-16-2020, 01:04 AM   #38
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Pete,
I believe such “incidents” do in fact occur.

But I also think these occurrences happen so as to make a 4 blade about as smooth as a 3 blade and visa versa. But the blade tip design, mostly the width of the blade tips will make a 3 blade less efficient than a 4 blade. There’s lots of little variables that cancel each other largely out.

So when choosing a 3 blade or 4 blade most or all of these “incidents” should be overlooked. Just choose the number of blades to achieve optimum blade loading and a good balance of pitch/dia. ratio. To not have too much pitch or diameter is far more important that anything else.
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Old 12-16-2020, 07:01 AM   #39
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"Question now is Overseas or American made? Bang for your buck?

Very pleased with Ahoy props, price and quality.

Taiwan workmanship seems far better than mainland communist products.

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Old 12-16-2020, 08:01 AM   #40
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Probably "Overseas or American made" could be another thread which would get plenty of play. But..since you asked:

The cost to ship a boat from Taiwan, or China or Italy would probably be about a quarter of the new, retail purchase price. So before we can talk quality of parts, workmanship and labor, building techniques, etc. we need to consider final price.

A boat which is overpriced just will not sell. By overpriced I mean comparing apples to apples, not comparing a Mainship to a K.K.

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