Propeller Info

Aging Youth · 30 January 2003, 19:19

Hi everyone,

I currently have a Humber Attaque 5.3m with a Mariner 90Hp 2 Stroke with a stainless Mercury Marine Laser II fited.

The serial No. on the reverse is 48-16990-20 My questions are:-

1. Can enyone identify the size & pitch of this prop or tell me a way of measuring the pitch/diameter?

2. Who manufactures/supply these props.

I have reviewed the various comments made on propellers especially the formula for calculating prop selection, (is dimitri a mathmatician?) I do not know what what my starting point is.

Out on the water on sunday and with 50lts fuel and two adults recorded 38knots on gps @ 6000 rpm (Rev counter). My manual recomends to not exceed 5700rpm.

So am I over reving but engine? engine wanted to rev freely, could have gone a bit quicker sea conditions prevalent at the time.

However not worried to much about flat out top speed much rather have crusing speed at 3000 rpm 25-30 knots to maximise fuel consuption on cruises.

Any thoughts or comments are always welcomed.

cheers

Dirk Diggler · 30 January 2003, 20:26

Your part no 48-16990 is for an early 13 1/4" dia X 20"pitch Laser.

The details can usually be found on the hub. The prop is made by Mercury, and can be purchased from any Mercury/Mercruiser/Mariner dealer. If your motor is over revving, you ought to increase pitch by a couple of inches.

To calculate max speed you need to take max revs and divide by ratio, multiply this by Pitch, divide by 1056 and this will give theoretical max speed, deduct percentage of slip, around 10% is common, and you have top speed.

Aging Youth · 31 January 2003, 08:02

Dirk,

Thanks for the reply, now I know what size my prop is I will give dimitri's formula a try & of course yours.

Can you tell me what you mean by ratio is it the gearbox ratio?

Cheers

Dirk Diggler · 31 January 2003, 08:39

Correct, from memory I believe the ratio of your gearbox is 2.33:1.

Your "theoretical" top speed at 6000rpm would be as follows;

6000/2.33= 2575 x 20 (pitch) / 1056 = 48.77 mph.

You then have to allow for prop slip. I don't know what this is on your boat, but 10% is quite common, so your max speed would be 44 mph

Aging Youth · 31 January 2003, 13:30

Dirk,

Many thanks for the tech info, this will be a great help.

So for now I will consider this thread closed.

Happy Ribbing

matiboy · 31 January 2003, 21:20

excellent little formula

anybody wants it in an Excell spreadsheet format which includes a different range of pitches , mail me and I'll forward it (as only 17k attachment.) Thats bytes not price.

Tells me if I want my rib to do 80 mph, somehow I'll need to get my engine to rev to 12,000 rpm and change to a 21 inch prop !!! LOL

DJL · 31 January 2003, 22:26

The formula seems spot on for my boat

I have made a webpage that does the calculations for you

http://ribs.loughlin.tv/propeller/default.asp

Daniel

crazyhorse · 31 January 2003, 22:32

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?

DJL · 31 January 2003, 22:41

I'm sure weight and a hundred other things affect the top speed of the boat. But like most formulee you have to leave out some variables.

I guess if you know WOT RPM, max speed, gear ratio and pitch for your current setup you could work out the % slip and then use that to work out max speed using different pitch props

crazyhorse · 31 January 2003, 22:58

Im interested to see what the Kad works out at,I will get the imfo and see what she says using your formula.

I know what the Speed worked out is by a Naval architect ,so it will be of interest to see what your formula says,and then nock off 10 % as Dirk says for slippige and see what is left.

They use a complicated formula that costs money to work out,so a comparison will be of interest?.

Aging Youth · 01 February 2003, 10:22

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

Dirk Diggler · 01 February 2003, 10:27

Quote:

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!!

matiboy · 01 February 2003, 11:44

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

crazyhorse · 01 February 2003, 12:23

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!.