Increasing prop diameter

BillD

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25 Terry Jason with Cummins 370 power
I NOW some of you guys love to "do the math", "use your experience" or simply "make a best guess" !! :D

Assume hulls 1.,2. and 3. are the same hull. Weights of all 3 hulls are the same.

One of my "virtual DEs" can swing up to a 28" prop.

Here is the known data (1. and 2. ) of same hulls

1. 440 hp 3300 rated 2:1 gear 24X26 with #4 cup

2. 420 hp 3000 rated 2:1 gear 24X35 no cup.

3. Bill D's "virtual hull"

480 hp 3400 rated 2.5 : 1 gear..... what prop ??????????

Hulls 1. and 2. go plenty fast enough @ cruise.

My virtual DE hull does not require "more cruise speed" but I'd prefer to put a "more efficient prop" on this build"

Bill D
 

Powderpro

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For Bill's virtual DE build, assuming a real world weight of 17,000 lbs, I would start with 26x28. If going with a 28" prop, go with a 28x26. That's what boatdiesel calculator says using an "average planning" hull and 17,000 lbs. Actually boatdiesel says about 1.5" more pitch than what I suggested, but my experience has been that the calculator is assuming you want max 100% load, and for most of us, we would rather have 94-96% load.

I've said this before, but my personal experience has shown that the boatdiesel calculator is really close if you are accurate on the weight and using the "average planning" hull. On my Wesmac for example, I used 17,500 lbs as the weight (I know the real weight of the Wesmac) and the calculator said I needed 33" of pitch. Currently I'm at 30" of pitch and only 88% load WOT. 33" of pitch would probably have my engine at exactly 100% load. My personal experience with a shaft speed of 1,220 RPMs also tells me that 1" of prop pitch is worth 3-4% of load. That figure has worked on my last 4 boats, so I'm gaining a lot of confidence in it.

Now you shaft speed if you go with a 3,400 RPM engine with 2.5 reduction will be 1,360 RPMs, so the 3-4% load per 1" pitch may not work out exactly, but it should still be fairly accurate. With a faster shaft speed, it may end up being 4-5% load per 1" of pitch.

If it were me, I would steer clear of a 3,400 RPM diesel. It just doesn't make sense to go with such a high revving engine when they make ones that redline at 2,600 (QSC) or even 3,000 (Iveco). The more RPMs you run, the more vibrations you get. And with a 3,400 RPM motor, you will have to cruise at 3,000 RPMs to get the 21 or 22 knot cruise that you want. At 2,600 RPMs you will be doing about 18 knots, and I know you won't be happy with that.

No one takes my advice (except my wife), but if you decided to take it, I would go QSC 500hp if you truly want a 22 knot cruise where the engine is below 70% load. Another engine I would be really interested in is the Iveco NEF570 6.7 500hp version. That would also give you a true cruise of 22 knots or better and your load % would still be at 70 or below. Does Cummins make a 6.7 that redlines at 3,000 ? I think they make a 425hp version don't they? That would also be a good engine to go with. Also, I would use at least a 26" diameter prop, 24" in my experience is a waste of time, and if you want really good efficiency and cruise, I would go 28". You can still use a 2" AQ22 shaft with a 28" prop.
 

Powderpro

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Bill- I forgot to mention, but on my 34 CB, that was a QSC 500 with 1.75:1 reduction, 26x28 prop. Had the boat been more completely finished with goodies and had more fuel in it, I think a 26x27 would have been a perfect prop, or a 26x26 if you wanted to be real safe. At sea trial I was very light and I think we were about 95-97% load at WOT with the 26x28. That shaft speed was pretty fast at 1,514 RPMs at WOT (2,650 rpms/1.75 reduction).
 

lobstercatcher

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I NOW some of you guys love to "do the math", "use your experience" or simply "make a best guess" !! :D

Assume hulls 1.,2. and 3. are the same hull. Weights of all 3 hulls are the same.

One of my "virtual DEs" can swing up to a 28" prop.

Here is the known data (1. and 2. ) of same hulls

1. 440 hp 3300 rated 2:1 gear 24X26 with #4 cup

2. 420 hp 3000 rated 2:1 gear 24X35 no cup.

3. Bill D's "virtual hull"

480 hp 3400 rated 2.5 : 1 gear..... what prop ??????????

Hulls 1. and 2. go plenty fast enough @ cruise.

My virtual DE hull does not require "more cruise speed" but I'd prefer to put a "more efficient prop" on this build"

Bill D

If you are really thinking out efficiency ,,You should think how fast you want to cruise at and work backwards to require what will achieve what you are trying to accomplish. For instance.. if you want a easy cruise speed of 20 kts and a pushing cruse speed of 28 kts why put a engine and equipment in the boat that will do 30 kts and 36 kts?

Max Diameter is suppose to produce the easiest efficiency. Some hulls produce hull knock with larger wheels so you might want to stay smaller to reduce that chance.

How fast do you want to easy cruise at, your high cruise speed and wot?

Also, is there a hull speed on the boat you want to build?
 

lobstercatcher

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What is hull knock?

It is when the blade of the wheel gets too close to the hull and creates noise or knock as it passes the hull with each passing of the blade.

There probably is a formula somewhere that gives the distance in % of the blade diameter that a clearance should be maintained between the blade tip and hull.
 

Powderpro

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15% of the prop diameter is the magical number I always hear, but I had 11% on my 34 CB and it was smooth. We could have had more tip clearance on the Calvin, but there was a small miscalculation and we ended up with 2.875" clearance on a 26" prop. On my boats I try to maintain at least 12% and they have all been smooth.
 

captainlarry84

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KristenFormer Charter Captain
I NOW some of you guys love to "do the math", "use your experience" or simply "make a best guess" !! :D

Assume hulls 1.,2. and 3. are the same hull. Weights of all 3 hulls are the same.

One of my "virtual DEs" can swing up to a 28" prop.

Here is the known data (1. and 2. ) of same hulls

1. 440 hp 3300 rated 2:1 gear 24X26 with #4 cup

2. 420 hp 3000 rated 2:1 gear 24X35 no cup.

3. Bill D's "virtual hull"

Found this one a little late. my #s

480 hp 3400 rated 2.5 : 1 gear..... what prop ??????????

Hulls 1. and 2. go plenty fast enough @ cruise.

My virtual DE hull does not require "more cruise speed" but I'd prefer to put a "more efficient prop" on this build"

Bill D

Hull #1 should really have a 2.5 gear with a 28 X 27
Virtual hull should have a 28 X 30

Hull #2 Jury is out on your number @ a 24 X 35 no way on that more like a 24 X 28 would fit or better a 26 X 26
 
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