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Old 28 April 2006, 10:11   #21
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Quote:
Originally Posted by Yuley
The reason they go so fast is 'cos they've got a 250HP lump on the back and a larger diameter prop. (Dont confuse pitch with size)

By the way, your prop pitch and size should be stamped on the hub of the prop (Thats the bit through which the exhaust gasses pass)

The pitch will normally be proceeded or followed by the letter K



Cheers
Blimey! So the QE2 must be quick then!! Big old props are for big boats.
Most of us guys with 75hp+ will be running 14" diameter props most race props are no more than 15". My big old slow Searay ran a 22".
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Old 28 April 2006, 10:50   #22
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Quote:
Originally Posted by BogMonster
OK a slightly related question here

If my leaky orange bathtub does 35 knots on a 19" prop then surely to get 60-70 odd knots that some people say they get, must need either a very coarse prop (like about 35") or an engine that does about 12000rpm?

or am I missing something??

I don't need to know, just curious!

...gear ratios too.
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Old 28 April 2006, 12:36   #23
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I was talking to yamaha about props (need a change ourselves) They sam its roughly 250-300rpm per inch of pitch thats the yamaha ones though. This is incredably useful stuff for me anyway. Hope it helps you.
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Old 28 April 2006, 15:10   #24
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It is, but it still doesn't work, because it then asks you to prove who you are by entering valid credit card information which then doesn't work because there was no option available to tell it that my Mastercard address is actually in the Falklands and not in the UK....
As I understand it there is no way for the vendor (ebay) to verify the address to which a UK c/card is registerred - so you may be able to enter any old crap and still authenticate.

I don't remember needing a C/card to register the account anyway.
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Old 28 April 2006, 15:50   #25
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Originally Posted by Polwart
As I understand it there is no way for the vendor (ebay) to verify the address to which a UK c/card is registerred - so you may be able to enter any old crap and still authenticate.

I don't remember needing a C/card to register the account anyway.
I think it is a new idea to prove you are who you say you are and cut down on Ebay scams. Don't know what the problem was but it would not authenticate me anyway. Might try again one day - but I got the item I was looking for (one of those shake up and down ball bearing in a tube syphon pumps) from Mailspeed Marine so no immediate need.
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Old 29 April 2006, 14:32   #26
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I have found the pitch, was looking in the wrong place, doh!!

14 x 17 is stamped onto the prop by where the retaining nut is, so I guess that means it is 14" dia and 17" pitch, finer than expected.

What do you folks reckon, try a 19" and see what happens? 17 to 21 seems like a big jump to me?

I just did an Excel sheet to play with numbers and I get 35.75 knots on a 17in prop at 5500rpm to be 93% efficiency (i.e. 7% slip) so based on that, a 19in pitch would give 40kt and a 21in would give 44kt, assuming the engine still reaches 5500rpm which of course it may not.

What is the max I am likely to get from a 115 in practice, total weight of the boat is about 700kg (350 hull 150 engine, fuel and not so little ol' me) if that makes a difference.
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Old 29 April 2006, 19:11   #27
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definately try a 19 before you jump to 21"
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Old 01 May 2006, 14:37   #28
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Have had this on my harddrive for a while. Even get to post it on occasion. Note that this is a general guide, and if you go more than a few inches at a time, things may get really wacky:


Ok here is what some general rules are.

General rules for props:

switch from aluminum to SS (all dimensions are the same) should drop WOT RPM by ~100
an increase in diameter of 1/4 inch should drop WOT RPM by ~100
an increase in pitch of 1 inch should drop WOT RPM by ~150 (range is 100 to 200)

3-blade vs 4-blade
4 blade props have some advanteges over 3 blade. If you have like material, then 4 blades promote stern lift, give better mid range fuel econ, slower top speed, better holeshot, are more balanced and dont vibrate as much and allow you to mount your engine higher due to more blade surface area in the water.

Generally, the heavier the boat the more blade surface area you want, not more pitch. So, rather than going up in pitch, go up in diameter or number of blades or a combination of the 2. This promotes less slippage, but costs you top end. Make the boat more efficient in cruise rpm range.

These are REAL general. You must consider that manufacturer A's prop might have identical dimensions as manufacturer B's prop, but there will be a large difference in WOT RPM's. This is usually due to the variation of blade width, cup, hub flare... There are many things that can cause a difference.

The other thing that could cause such a difference is the current at the time of your test. Maybe if you were able to run the other direction with the props in question, you would see a speed decrease of 5 mph. This would make your average lower and affect your calculations. With out the second pass, you shouldn't count the data. unless you can compare the one way pass with these props to the same direction one way passes with all the other props. Then the comparisons would be level.

Edited by physicistkev 4/26/2005 9:33 AM
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Old 01 May 2006, 14:46   #29
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Quote:
Originally Posted by jyasaki
switch from aluminum to SS (all dimensions are the same) should drop WOT RPM by ~100 NO - STAINLESS PROPS ARE USUALLY LOWER DRAG AND WILL INCREASE THE RPM

4 blade props have some advanteges over 3 blade. If you have like material, then 4 blades promote stern lift I THINK YOU MEAN BOW LIFT
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Old 02 May 2006, 13:15   #30
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Roy;

First off, I didn't write the little treatise; as I stated, I found it on some other boating forum and kept it for reference.

On the SS vs. Alum, SS has less drag due to thinner blades (which will cause higher rpm), but also much less flex, which will reduce rpm. The writer is stating that the overall effect going from Alum to SS, blade shape (except thickness) being equal, is going to be a slight reduction in rpm achieved. Add in that most SS props have greater cupping and you'll reduce the rpm more.

On the 3 vs 4 blade, I believe he meant stern lift. Also written as planing at lower speed, planing faster. Not sure how a prop will lift the stern, but I assume it means the boat gets moving faster, so it gets on plane quicker. I also suspect that stern lift and bow lift with respect to this discussion may be the same, but taken at different points during the accelleration curve.

http://www.marine-outboards.com/choosing-propeller.htm

jky
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