Basic Prop question

[ROBENDOG]

Hey guys have been reading a few threads on prop size......I am chasing a layman's explanation of advantages/disadvantages in relation to different prop sizes.

Recently purchased my 1st boat, a 6m plate cuddy with a 200hp Yam on the back.

It came with two props. I could not tell you the sizes, brands or anything. One is big, one is small.

Big one - WOT - 5200 - 42knts.

Little one - WOT 6100 - 52knts.

Have not been out enough to really compare fuel usage.

Which is more fuel economic?

What are the different uses for the different props??

Thanks
Robendog

[TimiBoy]

http://www.ausfish.com.au/vforum/sho...d.php?t=129003

Have a look at this link to learn a little. some data there on my prop change. I don't know much about it beyond that, except to say that now I have made the change, I see no use for my old prop except to carry as a spare.

I suspect the same will be due for you - the one which allows WOT of 5200 is just too damn big, and it's only practical use will be as a spare

[FNQCairns]

The most economic would be the little one because it has doubled the life of the engine, for fuel cannot say the spread is so wide in rpm so could go either way, will depend on how fast and at what rpm your favored cruise speed is.

cheers fnq

[cormorant]

Hi

The first thing is to find out what revs you motor was designed to do at WOT wide open throttle.

Outboards don't have different gear ratios like a car gearbox so they require a prop that allows the motor to rev out to it's maximum revs. If the motor isn't set up so it can achaive it's rated WOT it is labouring - sort of like going up a steep hill in 5th gear at 40km'h. Your motor won't last.

Search on the site there is heaps of info as you aren't the first to ask.

Different props are like your car gearbox ' s gears each are designed for a different use be it fast accelleration or ecconomy or pullinbg power. The required characteristics determin the prop's shape and size.

A props size and design or part number is usually written on the hub wher the exhaust comes out. It will real 15 1/2 X 23 for example which means the diameter of the prop is 15.5 inches and every time it turns it in theory would move the boat 23 inches forward ( pitch).

Tell us what you prop dimensions are from the numbers and if they are branded . solas, viper , tornado etc etc.

The other big issue with props is that they require the motor to be at the correct height when mounted on the transom to get the correct performance and handleing characteistics of your boat.

Tell us more and do a bit of reading on the threads.

[ROBENDOG]

I will find out some more info and get back to you.

FNQ what do you mean about smaller one doubling the life of the engine??

Thanks
Robendog

[ROBENDOG]

How do i figure out the model???

It is a 1998 200hp V6 saltwater series II.

Have downloaded a manual but.........

It has WOT for 2 x 200hp engines

S200TRX - 4500-5500
P200TRX - 5000-6000

[bar raider]

Hey guys have been reading a few threads on prop size......I am chasing a layman's explanation of advantages/disadvantages in relation to different prop sizes.


What are the different uses for the different props??

Thanks
Robendog

I'm interested too!

what does just changing the pitch do ? or just changing the size do ? and why do both?

thanks

[FNQCairns]

I will find out some more info and get back to you.

FNQ what do you mean about smaller one doubling the life of the engine??

Thanks
Robendog

Decreased angular/frictional forces within the reciprocating portions of the engine + lower piston crown temperatures at all rpms = a longer lived engine by a long shot over one doing it hard in relative terms, esp applies to 2 st engines and esp to constant duty engines like ours in effect are/usually are.

Sorry dunno howto tell those two apart.

cheers fnq

[ROBENDOG]

Hey guys thanks for the explanations so far......

I had a look at the two props.

One is a SOLAS 14 x 19.

The only marking I can find on the second is 17-M.

I have included pics of the props and also where the prop sits as I have seen a lot of discussion about leg length.

Still unsure of my WOT as I am unsure what model engine I have (see my last post).

Thanks heaps for any help. Cannot thank you guys enough for all the advice you give. It is great to have people to bounce ideas off without being charged an hourly fee. Your advice is appreciated.

Robendog

[FNQCairns]

Rob by the look of your pics put the big prop back on raise the engine 4-6inches!! and hold on

Could only need 3 inches as i cannot tell by the pic where your vent plate is in relation to bottom of the hull with any certainty, your setback pod will make all the difference.

cheers fnq

[ROBENDOG]

Ok. So i want the prop sitting just below the pod? with the SOLAS prop on. Can you explain what this will do?

Ben

[Hydrotherapy]

I'm interested too!

what does just changing the pitch do ? or just changing the size do ? and why do both?

thanks

An increase in pitch will always mean a decrease in diameter (and visa versa), prop manufacturers use a formula (which includes cupping and rake) to determine the correlation between the two. The only way to change this formula is to have a prop custom made.

The golden rule with props is to prop for WOT, too big a pitch will cause lugging (like driving a car up a steep hill in overdrive, and cause the problems that FNQ outlined), too small a pitch will cause slippage and overrevving.

There are some advantages in using a smaller than recommended pitch, you will slow down your trolling speed and you will have much better hole shot (slippage can be a good thing!) for skiing but you need to be very mindful of overrevving at WOT.

Roughly, 2" increase in pitch will result in a drop of 400RPM at WOT (and visa versa).

[Hydrotherapy]

The solas has a diameter of 14" and a pitch of 19".

The alloy prop is a genuine yammy with a diameter of 14 1/4" and a pitch of 17".

[FNQCairns]

Ok. So i want the prop sitting just below the pod? with the SOLAS prop on. Can you explain what this will do?

Ben

Possibly more than now, run a straight edge along the bottom of the hull then extend this edge with a string line past the cav/vent plate - is it above or below the string line? How much? Rule of thumb says 1 inch higher than level for every foot back to the outboard leg from the end of the planing surface.

cheers fnq

[cormorant]

Model number should be on a plate attached to your motor mount where it bolts to the hull. Should have production date , model number etc on it.