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Old 01 February 2003, 04:22   #11
Country: UK - England
Town: Portchester, Hants.
Length: no boat
Join Date: Nov 2002
Posts: 584
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Prop Info

Hi Guys,

From the caluations supplied My max speed was calculated at 48knots allowing for 10% slip = 38 knots. On the water I recorded 37.6knots on gps @ 6000 rpm. This was with 2 adults on board, full tank 50lts, plus equipment.

I will try and work out Demitri's formula to include boat weight ect. and let you know how it compares.

p.s. Anyone going to the Gosport Boat Jumble on Sunday? I know Allen P. will be there flogging his wares.

Roll on summer

Aging Youth
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Old 01 February 2003, 04:27   #12
Country: UK - England
Town: Hamble
Length: 9m +
Join Date: Dec 2002
Posts: 2,256
Originally posted by crazyhorse
Maybe Im asking a silly question but does the weight of the boat make any difference and the water line length and the waterline beam are needed to work out the theoretical speed. of the boat?

Cant see any of that on the formula thingy,its brill though,just not sure whether its accurate?
Crazyhorse, I see from your quote you are having a bit of a job grasping this. It really is just simple maths and common sense, but the answer is FACT.

People used to use the term single or double screw when talking about props, thats because they are a screw. The "pitch" is the theoretical distance a prop moves in one revolution. This is how it works.

Max revs (per minute) is 6,000 divided by ratio 2.33 =2575. (here is where the magic 1056 bit is used , 36 x 1760 / 60 = 1056) 2575 is the amount of inches it would travel in one minute, divide by 36 is the amount of yards, divide by 1760 is the miles, then you multiply by 60, this is to make the calculation into an hour, end result is max speed in mph = 48.77. Now this is theoretical, you have to subtract slip. Lets call this the S factor. Now I know what the S factor is for my boats, because I know all the variables, revs, pitch, ratio, speed. If you don't have the boat, you can't work this out. The S factor is obviously determined by hull design, weight, drag waterline length and many other thing, most of which a Naval Architect will claim to know, but you will have to wait till you have your boat in the water to find out if he was correct, or if he has overcharged you!!

It is better to remain silent and be thought a fool, than to speak and remove all doubt!
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Old 01 February 2003, 05:44   #13
Country: UK - England
Town: Salisbury
Length: 5m +
Join Date: Dec 2001
Posts: 459

I was going to reply along the same lines, the variables that Crazyhorse was mentioning are simply built into the Slippage factor

We all know that if you strapped the same engine to a 40 barge and then a rib, it would give different speeds

Your calculation can only be regarded as a theoritical guide, based on the assumption that a propellor screws its was through the water, very much like a wood drill screws its way into a piece of wood.

It is however a simple one and an excellent guide.

I believe however that a propellor in fact does not screw through the water it behaves more like a wing and flies through the water, other than at slow speeds. This may also effect your equation.

Why are propellors always painted black ? Because its the magic they have in them
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Old 01 February 2003, 06:23   #14
Country: UK - England
Town: Blackpool
Boat name: To Exi
Make: new sib 4 man
Length: 8+ft
Engine: Mariner 4hp long shaft
Join Date: Nov 2002
Posts: 1,012
I do love a bit of Fact when I see it.

Im a whole lot the wiser Now.

Thanks Diggler and others.

I have to go as I feel a bit of slippige comeing on!.


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