whipper
Well-Known Member
If you have a 15 inch hub vs a 14 inch hub will the 15 inch hub have 1 inch shorter blades in the same pitch as the 14 inch hub?
James
James
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Q about hub and pitch?
- Thread starter whipper
- Start date
GotMyAlly
Well-Known Member
Never seen a 15" hub?????
Normally the hub (or tube) is somewhere in the 3" - 4" range. The 14" or 15" number is typically the overall diameter of the prop (or blade length X 2 + the diameter of hte hub).
If you had a 15" diameter prop and a 14" diameter prop, each blade would be 1/2" shorter on the 14" prop, all else being equal.
Normally the hub (or tube) is somewhere in the 3" - 4" range. The 14" or 15" number is typically the overall diameter of the prop (or blade length X 2 + the diameter of hte hub).
If you had a 15" diameter prop and a 14" diameter prop, each blade would be 1/2" shorter on the 14" prop, all else being equal.
whipper
Well-Known Member
hmmmm? I see lets try that again.So if you have a large hubed prop would the blades be the same length as a smaller hubbed prop in the same pitch? Is that what you are saying that with a larger hub the blades would be shorter? Besides the diamiter the pitch is the distance the prop travels in one rotation threw the water so i thought that with a larger hub the blades would be shorter yet still have the same pitch if the overall diamiter was the same as a smaller hubbed prop with longer blades, diamiter would be equal from the center x2 as you said. My therory is that with a larger hub and shorter blades I would get a longer life out of a prop and not loose to much top end with a 4 blade in this aplication because I could gain rpm not turning as much blade area while still retaining the same pitch. James
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GotMyAlly
Well-Known Member
Pitch & blade length are unrelated measures. Pitch is the angle of the blades. You can have 24" with 13" diameter or 15" diameter and the pitch would be the same while the blade length changed. Blade length doesn't have anything to do with pitch.
In my experience, you can turn more pitch with a small hub (over hub) because the exhaust gasses cause the prop to slip at slower speeds, whereas with a big tube all your exhaust gasses go through the hub and the motor bogs.
I'm not sure about blade length vs. prop life....but trophies are a thru-hub prop and most guys that surface them can't keep ears on 'em.
In my experience, you can turn more pitch with a small hub (over hub) because the exhaust gasses cause the prop to slip at slower speeds, whereas with a big tube all your exhaust gasses go through the hub and the motor bogs.
I'm not sure about blade length vs. prop life....but trophies are a thru-hub prop and most guys that surface them can't keep ears on 'em.
whipper
Well-Known Member
Thanks GotMyAlly thats what I always thought to. I found this artical off aromarine research and was trying to figure it out.In this artical it talks about the rake and cup and other variables in regards to pitch of course but it says under pitch that "pitch is the theartical distance a propeller has to travel threw water in one revalution"? that threw me off a bit. Hear is a link to that artical.It also talks about the vent holed props a bit. James
http://www.boatbuilding.com/article.php/designofpropellers2
http://www.boatbuilding.com/article.php/designofpropellers2
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whipper
Well-Known Member
I just figured it out, the rotation they are talking about in pitch is the forward motion as per rotation.A 30 pitch prop will move the craft more forward in a single rotation than a 28 for example. I was getting screwed up thinking rotation in terms of diameter,not motion sorry about that! I never broke it down in terms of single movements just taking advantage of the the knowledge that a 30 will be faster than a 28 relatively speaking in most cases. The rake and cup factors in relation to pitch will again come into the factor for figuring out the design elements of a prop. I'm studying this because there is so much more to a prop than just trying them out till you find one that works OK or that some one else says it works good on there hull. There is a lot of weight to that but I cant help but think with more knowledge of why they work and the hydromatics of different styles will make me feel what is happening under and behind me so I can use the designs more efficiently. GotMyAlly thanks very much it took me a while but I understand what you are saying now! I always thought that the angle of the blade was the pitch but I got all screwed up reading this artical.The only limiting facter in blade length would be hitting your skeg! I see that with longer or shorter blades the you can have the same pitch on either. It would figure that surfacing a chopper style hub with longer blades would have a better area for that aplication and I would guess that the distance the tips are apart would play an important roll when surfacing aswell? I will look into that part of it. The closer the tips are and the with of the blades,rake and cup come into play as far as airation 3 blade or 4?Hmmm. James
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