merc,mariner,tohatsu 20 4 stroke prop size

[sparky666]

Hi
I have been considering a prop size change, my mariner 20 4 stroke came with a 9.25 x 10 prop which delivers 20 mph on my 4.0m SIB exactly what mercury quotes for this prop,has any tried an 8.25 x 11 or 11.5 with this set up? mercury state a possible 26mph with an 11.5?
GOOD IDEA OR A WASTE OF MONEY???

[frankc]

It will only go as fast as the horsepower will allow. The power must allow it to operate in the RPM range specified for the motor.

Just for giggles, try the following formula for theoretical speed. 10-13% slip covers most hulls:

RPM x lower unit ratio x prop pitch divided by 1056 = theoretical speed in MPH less 10-13% slip. My rig has a 11% loss of speed due to slip for instance.

If you assume ample power, you will find the formula pretty dependable. However, the power of the motor will dictate just how fast the boat will go. If you select too much pitch, the motor will not reach proper RPM and will be developing less than rated hp. You need at least one test to see what the motor is capable of with your present prop.

I would not buy another prop without seeing what the present one is doing.

[Locozodiac]

You must install an induction tach and check max wot rpm readings with you alone wigth current prop before playing with different prop pitch to start with. Wild guessing is costly and can ruin your engine in a very short term.

Happy Boating

[frankc]

I would add that your present prop seems to be pretty close to ideal, but only a tach would prove that. As Loco points out, don't waste money until you know the facts.

[sparky666]

Hi went out today with an induction tacho and in choppy water reached 6020rpm and 26 mph peak on gps and running against the tide 23mph , quite pleased with that for a honwave 4 ,
I think the rev limiter is set at 6000 as seemed to sit solid at 6020rpm

[sparky666]

Ps tacho from eBay is brilliant at £12 delivered , does run hours and service reminder too,
Steel developments do a 9.25x11 bit less pitch than the mercury 11.5
WHAT DO YOU GUYS THINK??? :-)

[frankc]

If you are turning 6000+ RPM, you need a prop with more pitch, not less.

[sparky666]

hi
i agree i currently have a 9.5 x 10
mercury go from 10p to 11.5p

[frankc]

The rule of thumb is a change of 150 to 200 RPM per inch change when changing pitch. A 1 1/2 inch change from 10 inch to 11.5 inch would drop your RPM from 225 to 300 assuming the same efficiency in both props. What is the operating range published for your motor?

The closer you come to the middle of the range, the more HP the motor will develop. The 11 1/2 inch looks doable, as many new motor specs are for a 1,000 RPM range.

[sparky666]

I will look up the rpm range
I wonder if the rpm is hitting the limiter therefore there may be some more slack to be had

[yam man]

Originally Posted by sparky666

I will look up the rpm range I wonder if the rpm is hitting the limiter therefore there may be some more slack to be had

5250-6250 rpm range

[sparky666]

The rpm range for the mariner is 5400-6100
So a 1 inch rise possibly more??

[sparky666]

This was taken from handbook so I guess this is correct

[Locozodiac]

If max wot is 6.1 K stay there, bear in mind that as soon you add more weight or passengers the rpm will drop accordingly. Besides nobody in it's riight mind runs wot all the time, just for a quicker plane if moving more weight or passengers.

Have been reading some tech discussions stating that going lets say 1 pitch down will not rise rpm 150-200 more, seems way more, don't plan investing on a new prop to find out this...

Happy Boating

[frankc]

How about an actual test result to muddy up the water.

15 HP Yamaha 2 stroke, 14 ft fiberglass boat yielded 5500 RPM with 9.75 pitch prop and 6,000 with 9 in pitch prop. That 3/4 inch difference in pitch yielded 500 RPM difference which agrees with your observation.

Oh, and I am one of those not in his right mind. All my 2 stroke motors below 18 HP have been normally run at WOT. They are tools to me which are kept in tip top condition, as I am a fisherman not a pleasure boater The larger motors are run at cruising speed.

I am a prop nut who changes props for different purposes. My rigs have one prop for heavy loads and another for general purpose. I like to get all the goody out of my motors without straining them When we sold the 15 HP Evinrude, it had three props for different services, all of which were useful from a light inflatable to the 14 footer with grandchildren aboard. Someone stole the fourth prop which was a barge prop used when the motor was on a large boat as an auxiliary.

Come to think of it, we have three props for the 30 HP Merc. One is fine for a heavily loaded boat, the other is for my wife and I fishing, and the third ended up as a disappointment good for high mileage but not much else. Too much diameter and cavitates easily.

[sparky666]

I agree normally carrying passengers etc will reduce the boats performance radically especially with only 20 hp to play with but i often go twenty or thirty miles on each outing lightly loaded and dare i say at full throttle .
my engine has burned on average 2.3 litres of fuel per hour since new, I like to cruise at 18 mph and my plan was to use the lower part of max rpm (5400) to achieve this speed to save on wear and tear and give a higher top speed (everybody wants that)
My boat weighs around 160 kg including engine and gear ,the manufacturer select the prop for an average boat and load and i am guessing that the average boat and load used is heavier than this.
On family days the standard prop could be refitted

[sparky666]

The volume lever is there to be used as long as the lube is good why not

[frankc]

The only way you will know for sure is to try the prop. I do agree with Loco that you are not in a bad way with the prop you already have and that it is a good choice capable of good service with a heavier load in the boat.

By my actual experience with props on my 15 HP, I saw a 500 RPM drop with 3/4" change in pitch, so the common rule of thumb can lead you astray.

[sparky666]

What difference did your change make to your boat performance

[Locozodiac]

Frank,

Good to read that you have lots of spare props to change accordingly for overall weight you will be carying, that's the way to go and quite an investment on it's own. So you agree that 150-200 +,- per pitch increase or decrease is not a rule of thumb, right.

Factory delivered props on portable motors usually delivers full factory stated max wot rpm and sometimes slightly passes when used on light boats, that's ok.

My Tohatsu 18 used on a 420 light rib produces max stated 5800 rpm with me alone, with 3 boaters the rpm drops to 5500, will nearly lug the engine with 4. As this is not the club rib, just my personal toy, usually go out myself and extra boater.

Running between 3/4 to full throtttle, the speed difference is not that great, max 1-2 knots, but fuel consumption increases quite a bit running over 3/4 for extended time periods. Have my + vote if you need to reach fishing spot ASAP and fuel cost is not an issue for your economy.-

Have seen 150-200 rpm per pitch difference advertised on prop shops, seems it's kind of way wrong rule of thumb...

Happy Boating