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.:confused:

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/showthread.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.

[ROBENDOG]

A photo of the plate with numbers on it is attached.

The manual I have downloaded discusses a S200TRX and a P220TRX.

There is a bit of difference in WOT between the two (4500-5500 or 5000-6000) and I cannot tell from the plate which it is.


Sorry for the gumby Q's guys, I have s*#tloads to learn.


Hydro just clarifying the SOLAS is a 14 x 19 - diameter of 14? or 15?

FNQ - when you said raise the donk and put the bigger prop on did you mean bigger diameter or bigger pitch. Going by Hydro the Yamaha Prop (17-M) is the bigger diameter but the SOLAS is the bigger pitch. Also you said hold on! What should I expect to happen with these changes.

Thanks
Ben

[Bilopete]

I'm only new at this game as well so please forgive me if this question sounds stupid.

My 2nd hand outboard came with 2 props (one's a spare) but how do I know if either of these two props are ideal? Can I assume the outboard manual gives you preferred prop dimensions? If so, i sthere an easier way of finding out instead of contacting my Evi dealer?

(By the way its a 35hp elan evinrude 1997 model with 3 cyc).

Cheers

[Hydrotherapy]

Sorry, typo.

The solas is 14" diameter.

[Outsider1]

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

If your motor is the Saltwater series then it is a S200TRX model.

P200TRX is the Pro Series Model;

http://www.marineengine.com/mfr/yamaha_models.html

Cheers

Dave

[ROBENDOG]

Thanks Dave:)

[ROBENDOG]

So with the genuine Yam prop it is revving to 6100. The WOT is 4500-5500 from what I can figure out. Over revving cannot be good? I was told this was the prop it came with new.

[FNQCairns]

Ben I was referring to the prop that gave you the least RPM ATM because if you now raise the engine you should pick up some more and the prop that is pulling 6000 odd may be too small/high reving.

Long way to go yet but of the 2 the bigger prop is the one to use after the raise.

BTW over reving is not real bad for tests etc just not the place to be on a trip, you have control over the throttle BUT in theory your engine has a tev limiter so unless you diconnect this to go further you are not strictly over reving but you are outside of the marketed manufacturrers range they use to obtain the hp sticker on the cowling

cheers fnq

[ROBENDOG]

Thanks FNQ.

For anybody following this thread I will post result as I progress through my investigations.

Will get to measuring as outlined by FNQ on Monday. Post results and pics Monday night.

If I am lucky might get a couple of hours fishin in Monday also:D .

Ta
Ben

[ROBENDOG]

Ok I measured the setback and calculated the required rise of the engine….I think.

I will include an explanation of how I achieved my results, for clarification that I did it right, and also for those who are new like me.:)

If I have mucked something up, I apologise for any bum steers, and look forward to being corrected. I have no knowledge other than what I have obtained surfing the web.

I did a bit of research on the www and found a bit of discussion on this topic. A few differing opinions, but the advice I have received on this thread stands up to majority of opinion.

Why use a pod/spacer/bracket to move the donk back from the transom. Seems this gives more room in your boat and also allows the prop to spin in calmer waters, hence performing better. How

Pic 1 – donk mounted on transom. Lots of turbulence coming off the rear of the planning edge.

Pic 2 – moving donk back places it in smoother water.

Why does the donk have to rise when you attach a pod?

Basically plane speed is the speed required for the boat to overtake it bow wave. Now the bow wave (does it change its name now) is behind the boat it increases the depth of the water behind the boat in a bell shape. Hence the donk can be raised a little. Why is this important? Because the more leg in the water the more drag. So raising it reduces drag.

Pic 3 – Bow wave in front

Pic 4 – Bow wave behind.

There were a few different opinions on how to calculate how much to raise the donk. Some said 1:1, some said 1 foot back = ¼ inch rise………the bulk had the same ratio mentioned by FNQ ……for every 1 foot setback = 1 inch raise. From what I can gather there is no set rule as every boat is different but this 1 foot setback = 1 inch rise is a good place to start. Basically you want the cavitation plate to be running parallel with the water surface when you are on the plane. Correct???? So start with a 1foot = 1 inch ratio, then get out and have a look how she runs.

So this is what I did……

Pic 5 and Pic 6 – mounted string on bottom of rear planning edge and ran it back parallel to the bottom of the hull.

Pic 7- Measured from the back of the planning edge to the front of the leg – 92cm (3 foot, 1 inch….ish)

Calculated that the cavitation plate should be raised three inches (3 foot setback = 3 inch raise).

Pic 8 - Cavitation plate was already sitting at 40mm above string so therefore a further 35mm (1.4inch…ish) raise is required.

Questions

Will this 35mm raise really make that much of a difference??

As you can see in pic 9 I can move up a further 2 bolt holes. This about 1,1/2 inch?

Any other pointers????

I hope this post is correct and is of some help to somebody else wondering about this issue.

If this is all correct I will post results when I have made the adjustments and probably have a few Q’s about actual prop changes………:)

Thanks
Ben

[FNQCairns]

Looks all good to me, the 1 inch/1foot is a rule of thumb, when the water exits the rear of the boat it immediately tries to equalise in heigh with the water around it, the distance back relates to this because of the other factors of influence, the amount of hull below the natural water level and the speed of travel.

Lighter boat/flatter hull = less height needed at end result.

There was, just a day or so ago a pic of a v19 setback podded boat that illustrated height a deepish V with some weight.

Point is yours is an individual, anyone that tells you where will be best is guessing, raise it yourself and test to see, 1 inch can make a big difference, then another 1/4 inch and it can suck -literally.

cheers fnq

[FNQCairns]

I would go up the 2 holes in your instance for a test, if to high it will, ventilate a on turns, catch air and loose grip in lumpy water with some speed behind it - it will do this more than a transom mount or hull extension pod will so you will need to temper your expectations to suit.

It may also when at WOT while triming out give lots of bow lift and a nice increase in extra top end, trouble is the work the engine does at cruise will not reflect the extra rpm gained up top so a person runs the risk of overproping for cruise but correctly proping for wot.

When you test be sure to get accurate rpm and gps readings at your cruise speed as well as wot, you can include a 5k also.

cheers fnq

[ROBENDOG]

FNQ - ta.

I will make the two hole change and post an accurate account of RPM and speed, and also WOT.

Can include a 5K???????

Ben