Prop change for more top speed.....?

[Sixxgrand]

Hello members, it has been quite some time since my last post. Largely because since I installed the carb gasket for a 30HP 2 stroke Merc / Tohatsu into my 25HP it has run like a top, with power to spare and a crazy strong holeshot on my Naden 16FT aluminum boat. My GPS indicates 54KPH at 5830RPM.

I would like to increase the top speed with a prop change.

My manual states that a stock prop in my long shaft is 9.8 X11.3 and that I can install a 9.9 X 14.2.

Please all chime in here. I run a light aluminum boat with myself and sometimes my spaniel.

What increase in top speed can I expect in moving to a diff prop.
What about these four blade ProPulse propellers that I am reading about.

All information, comments and details appreaciated.

Again I am running a new Merc Tohatsu 30HP two stroke on a 16ft Naden aluminum boat which weighs about 120 pounds.?

Kind thanks all!

--Jeff

 

[steelespike]

Need the year of the motor. 2015? You should confirm the prop size before changing.Numbers usually appear at the base of the blades or under the prop nut.
Your present prop is darn close to perfect. 33.55 mph is very good. You definitely wouldn't go to a 14" pitch. It would lower the wot rpm too much and your hole shot would disappear.
A 12" or 12 1/2" pitch. A 4 blade isn't the best choice for top speed.
You may be able to improve speed a little by adjusting trim and raising the motor.The anti vent plate(just above the prop)should be even with
the bottom to about 1" above.Trimming up a notch may improve speed.If one notch is too much you could buy or make wedges to make 1/2 a notch trim.
While a aluminum boat is light all those rivets and convolutions can slow things down.

 

[Sixxgrand]

Need the year of the motor. 2015? You should confirm the prop size before changing.Numbers usually appear at the base of the blades or under the prop nut.
Your present prop is darn close to perfect. 33.55 mph is very good. You definitely wouldn't go to a 14" pitch. It would lower the wot rpm too much and your hole shot would disappear.
A 12" or 12 1/2" pitch. A 4 blade isn't the best choice for top speed.
You may be able to improve speed a little by adjusting trim and raising the motor.The anti vent plate(just above the prop)should be even with
the bottom to about 1" above.Trimming up a notch may improve speed.If one notch is too much you could buy or make wedges to make 1/2 a notch trim.
While a aluminum boat is light all those rivets and convolutions can slow things down.

So I believe what you are saying is that I have a really nice set up and I should not mess with it. I am more than fine with that. Just checking in.
The year of the motor is a 2010 I understand. I can send pix if you give me your email address. It is the 25HP Merc Salt Water version, long shaft.
The position of the motor is perfect, I checked and double checked it. I ran a tach on the engine after I swapped out the restricted carb gasket and it is well within factory full out guidelines I must say that this combo really JUMPS out of the HOLESHOT.....often removing persons from their seats...really a very tourqye engine for a 30hp....well it says 25 hp....:0
Thank you for your prompt and specific comment to my inquiry. Sounds as though I am best to enjoy the set up I have - YES?

 

[steelespike]

You really need to confirm the prop size. Assuming 1.92 gear ratio 54 KPH (33.5MPH) isn't possible at 5830 rpm.
Assuming the GPS is accurate, guessing 12% prop slip your rpm could really be 6800 to make those speeds
Your amazing hole shot could be an indication the prop is too small.And there may be a need for a prop change.
It is also possible the prop is a higher pitch already.Slip is usually in the low double digits.
As we stand right now the only known accurate number is the GPS speed. If we can confirm the Prop size and the gear ratio
We can tell better where you are at.

 

[Sea Rider]

Jeff

You can install any prop as long that motor doesn't overly pass 5850 rpm at WOT. If so will be over revving the engine. That speed is awesome, need more speed, a 40 HP will do the trick. If engine sits at the sweet transom height probably will gain + 1 Km more ? You must be running the factory delivered prop, it's a medium size pitch prop which works fantastic on light aplications such as yours.

That speed could be possible in flat calm no wind water cond, my actual 2 stroke Tohatsu 18 HP does 40 Km/hour on a 430 mt Rib, just running 10 rpm less than its factory stated max 5,800 wot rpm. Personally would stay with your current prop as the engine already is hitting its top wot factory stated rpm with you alone and lightly loaded boat.

Happy Boating

 

[Sixxgrand]

Hello and thanks for the wonderful feedback thus far.
Following is an actual picture of my GPS showing the 53.4kph top speed of my 25HP Long Shaft Merc Tohatsu with the
30HP carb gasket installed running on a Naden 16' aluminum. I checked my notes from the information provided by my Tiny Tach and
indicated RPM was 5870 at wide open at the indicated top speed. Prop is stock.
Manual states the following:
- Propeller must be selected so that the engine RPM when cruising with a wide open throttle is
within the following recommended range: 25HP 5000-6000RPM / 30HP 5150-5850RPM

Since the ONLY difference between the 25 &30 hp is the carb gasket I always found that odd.

Anyway my goal in this communication is to request from experienced persons if a change of prop
will increase my top speed. I expect that a full throttle RPM of 5600 would still be acceptable?
What prop size can I move up to?
The manual indicates that my stock long shaft prop is 9.8X11.3.
It indicates that a light boat prop for this engine (the 25 and or the 30HP) is 9.9X14.2.

So to those knowledgeable - what top speed gain can I expect should I change my stock prop to a
9.9X14.2?

Where is the best place to obtain one of these props and I will need it shipped to Alberta.

Kind thank all!!

 

[steelespike]

I have no doubt your speed is correct with the GPS unless there is a strong current adding or subtracting its speed.
If we assume a 11.3" prop and 10% slip your rpm is 6679. Not 5830.Based on your amazing hole shot it could be a 11.3 prop.
Rule of thumb for slightly bigger motors 40hp or more is "ABOUT" 200 rpm change for each 1" pitch change.Your slightly
smaller 30 may be more sensitive to pitch change. It appears ,assuming a "11.3 "prop, we need to make up about 700 rpm.
A 14" prop Should drop rpm at least 600 perhaps a little more.Your hole shot will be slower but probably more "normal".
You may pick up some speed. Keep in mind when dealing with props the numbers are not carved in stone.
As far as where to get a prop check here at iboats if they ship to Alberta.? Otherwise a Merc or Tohatsu dealer in your area.
Perhaps a prop dealer on the internet.I just noticed You rpm at 25 hp and 30 hp.Typically when hp is increased within the same
displacement the rpm is higher for the higher hp.Did you get the numbers reversed?

 

[Sea Rider]

Tohatsu wot numbers for the 25 & 30 HP engines are correct. On the 30 is 150 rev less than the 25 model, don?t ask me why. Anyway whichever prop pitch you go after the overall speed increase will be just slightly more, assume not that wow speed compared to your actual top speed. That engine is revving top, don't know if next + pitch that should make your engine rev less will produce more top speed ? Will need to buy a new prop and test for yourself, probably will thrrow $150 into the blue...

BTW, have seen some 30 HP engines delieverd with just 20? advance timed and restricted carb gasket, others with 25? and carb gasket restricted. Still awesome to count with 5 extra ponies with a peanuts cost carb gasket swap..

Just curious, is the waterr flow at wot skimming sligtly under the upper small water deflecttor plate as in this pic ? If so, you're at the sweeet transom height for any Tohatsu engine. Once flow at speed engages there, engine will produce its top punch.



Happy Boating

 

[Sixxgrand]

I have no doubt your speed is correct with the GPS unless there is a strong current adding or subtracting its speed.
If we assume a 11.3" prop and 10% slip your rpm is 6679. Not 5830..Typically when hp is increased within the same
displacement the rpm is higher for the higher hp.Did you get the numbers reversed?

Point 1 RPM: I assure you my engine is not running 6679RPM. Fuel consumption is almost exactly as per spec at 2.74GPH at WOT, a little less actually due to my light boat. Also since the carb gasket swap the sound of the engine is identical. The holeshot improved dramatically and I picked up top end.

Point 2 WOT RPM Numbers: Those are per the manual which can with my engine!

Thanks for the great comments - please keep them coming.

 

[Sixxgrand]

Just curious, is the waterr flow at wot skimming sligtly under the upper small water deflecttor plate as in this pic ? If so, you're at the sweeet transom height for any Tohatsu engine. Once flow at speed engages there, engine will produce its top punch.



Happy Boating

As I do not have a picture but do recall peeking over the transom at WOT I believe that the water is skimming below that upper small water deflector as per your picture. Have a look at the two pictures below - that is the trim I run. When the boat is well up on plane it is obvious that it would be hitting below the small water deflector plate - I would guess about an inch below that plate.

 

[steelespike]

Tohatsu wot numbers for the 25 & 30 HP engines are correct. On the 30 is 150 rev less than the 25 model, don?t ask me why. Anyway whichever prop pitch you go after the overall speed increase will be just slightly more, assume not that wow speed compared to your actual top speed. That engine is revving top, don't know if next + pitch that should make your engine rev less will produce more top speed ? Will need to buy a new prop and test for yourself, probably will thrrow $150 into the blue...

BTW, have seen some 30 HP engines delieverd with just 20? advance timed and restricted carb gasket, others with 25? and carb gasket restricted. Still awesome to count with 5 extra ponies with a peanuts cost carb gasket swap..

Just curious, is the waterr flow at wot skimming sligtly under the upper small water deflecttor plate as in this pic ? If so, you're at the sweeet transom height for any Tohatsu engine. Once flow at speed engages there, engine will produce its top punch.



Happy Boating

According to the prop calculator the boat can't go 53.4 Kph (33.5) gps at 5830 rpm. assuming 10% slip.and 11.3 prop.
Does have amazing hole shot so the 11.3 seems reasonable.

 

[Sixxgrand]

According to the prop calculator the boat can't go 53.4 Kph (33.5) gps at 5830 rpm. assuming 10% slip.and 11.3 prop.
Does have amazing hole shot so the 11.3 seems reasonable.

Couple of questions:
- where can I obtain a copy of your 'prop calculator'?
- what is the top speed your prop calculator concludes with a 9.8 X 11.3 factory prop at 5870rpm?
- what is the top speed your prop calculator concludes with a 9.9 X 14.2 factory prop at 5870 rpm? (this I really wish to know...!)

Remember my Naden 16' boat weighs only 116 pounds, + fuel and me..and I polish and wax the back 30% of my hull every spring.

Anyhow the facts as I have them are:
- 9.9X11 factory prop (just read the prop itself)
- stock 25HP 2 stroke Merc with restrictor carb gasket removed / Naden 16' aluminum at 116lbs
- my NUVI GPS indicates 53.4KPH
- my TinyTach indicates 5870RPM at WOT
- amazing holeshot and run to top speed takes a handful of seconds...
The prop calculator needs to come for a ride in my boat. Evidently it requires calibration!

 

[steelespike]

I'll be back.
Couple of questions:
- where can I obtain a copy of your 'prop calculator'?There are a number of prop calculators on the net mine is a green page and sponsored by Polson Enterprises.
Put in any 4 of the 5 numbers and it figures the 5th. Only as accurate as he numbers inputed. The only numbers we can be absolutely sure of now is the gps speed and prop size. gear ratio should be 1.92. I'll be back.
- what is the top speed your prop calculator concludes with a 9.8 X 11.3 factory prop at 5870rpm?
- what is the top speed your prop calculator concludes with a 9.9 X 14.2 factory prop at 5870 rpm? (this I really wish to know...!)

Remember my Naden 16' boat weighs only 116 pounds, + fuel and me..and I polish and wax the back 30% of my hull every spring.

Anyhow the facts as I have them are:
- 9.9X11 factory prop (just read the prop itself)
- stock 25HP 2 stroke Merc with restrictor carb gasket removed / Naden 16' aluminum at 116lbs
- my NUVI GPS indicates 53.4KPH
- my TinyTach indicates 5870RPM at WOT
- amazing holeshot and run to top speed takes a handful of seconds...
The prop calculator needs to come for a ride in my boat. Evidently it requires calibration!

I'll be back

 

[steelespike]

Lost a whole post try again. Prop calculations 33.5 mph(54KPH) 1.92,11",5%=6500 5870,1.92,11",5%=30.3, 5270,1.92,14"5%=34.6 mph.
Each inch increase in pitch lowers rpm"ABOUT" 200 rpm. thats where the 5270 came from. I used 5% slip to help make the numbers work.
5% is possible but highly unlikely.Slip number is the actual distance the prop moves through the water(liquid).
A 14" prop theoretically moves 14" forward in one revolution less the percentage of slip.Very basically as prop rpm goes up slip
goes down and vice versa.
The numbers in your initial post don't work We know now that the prop,gear ratio,and speed should be accurate.
The usual culprit is the tach. My guess at your slip is very liberal on the optimistic side for speed.
While the tiny tach has a good reputation it does occasionally read wrong from wrong settings or interference from plug wires.
If the tach is accurate then one or more of the other numbers has to be wrong.

 

[Sixxgrand]

Thanks for your ongoing input.

Here is what I have done and I like what I see.

Prop slip is considerably less on very light boats. My boat is considered very light. Therefore I am inputting a prop slip of 5%. I can not input a prop slip of 0%.

The prop calculator indicates that my theoretical top speed at 5870 rpm with an 11" prop to be 30.3MPH or 48.76KPH. I am seeing an indicated 53.4KPH a difference of +4.64KPH / +9.53% above what the prop calculator is suggesting should be my top speed.

The prop calculator indicates that my theoretical top speed at 5870 rpm with an 14" prop to be 38.5MPH or 61.96KPH. When I add back in the 9.53% my theoretical top speed - predicated on my engine being able to still turn the same WOT RPM of 5870 should be 42.17MPH or 67.87KPH - a very nice increase indeed.

So I expect that my boat will run 60KPH+ with a 10X14 prop - well worth the $100. Since I never run heavy loads and never pull anything or anyone, I will certainly enjoy the higher top end as well as a higher cruise speed with improved mileage.

Thanks everyone. I will report back in the spring when I make this swap.

 

[jerryjerry05]

IF?? Your motor's mounted right, cavitation plate even with the bottom of the boat, the tilt is set right, the motors revving at the recommended rpm,s then your doing it right.
The props right and the speed is about what your gonna get.
I had 17p on a 85hp. Wanted more!!!! Changed to a 19p and got 3 mph more. The economy went more too. I used about 15-20% more fuel.

You can try experimenting with a 4 blade and even find an old 2 blade but the factory guys seem to know what's best.

 

[steelespike]

When increasing pitch you will lose rpm At "ABOUT" the rate I indicated.
You understand you can't put in 0% because it isn't possible to have no slip.
In fact it is highly unlikely to have 5% slip.
You tach isn't accurate Thats why it calculates to a lower speed.
Put in your numbers and calculate for rpm.
There is an error with your tach or the other numbers. You can't fudge with the numbers
and expect the results to work. This calculator works with outboards,inboard outboards and straight inboards.
very heavy and very light boats.
Your boat may be very light but I expect it has at least 5 keels, not conducive to great speed.
What is the motor rating on the boat.
I've been doing this for 15 years. When the calculations don't add up you need to find the error not fudge with the numbers.
I've found that when things don't add up very often it is the tach.
I believe the Tiny tach has a setting for wasted spark ignition if set for wasted spark and actually normal spark motor the results will be low.
It may help you to read "read this first" at the top of the prop forum.
Did you note my numbers based on your actual rpm,6500 the calculators speed results for 6500 and 5900.

 

[Sea Rider]

Tiny Tach must be set to 2 sparks per revolution that's what 2 strokes Tohatsu works with, if used with factory delivered settting will read half or near double of what have been reading so far...Assume Jeff has taken care about programming the tach correctly when installing it.

Happy Boating

 

[Sixxgrand]

Tiny Tach must be set to 2 sparks per revolution that's what 2 strokes Tohatsu works with, if used with factory delivered settting will read half or near double of what have been reading so far...Assume Jeff has taken care about programming the tach correctly when installing it.

Happy Boating

When I attached the Tiny Tach (purchased brand new just for this) I read the install information in detail. The tach showed correct rpm at idle. And it showed correct RPM at WOT for this model within 130RPM thus I doubt it is reading inaccurately. There is a fellow with an older 65HP on a 14FT fiberglass who says he does 60KPH and he just barely got by me in top end, couldn't touch me out of the hole....

I am concerned about the comment stating that although more top end was achieved - gas mileage plummetted. I expect that fuel consumption would improve - much like adding an overdrive to a transmission...

 

[Sixxgrand]

Changed to a 19p and got 3 mph more. The economy went more too. I used about 15-20% more fuel.

If you increased the pitch by two inches your engine rpm should have dropped by 400 rpm.
How did your fuel consumption increase by 20% by reducing rpm - on the same motor, pushing the same boat/weight?

OR - Did your rpm increase that much to achieve that extra 3mph?