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

[steelespike]

Speaking very basically a carbed motor wot gph will be about 12 to 14% of the rated hp. A 100 hp will use about 12 to 14 gallons per hour at wide open throttle.
Changing props shouldn't change wot fuel use. If boat speed increases a slight fuel savings may happen as you will get from here to there quicker.
MPG is changed by the resulting speed increase or decrease. Increasing pitch lowers rpm and may slow hole shot may be more sensitive to added weight.
Basically very little change and very likely will even out over the various activities.
As far as your initial numbers there is an error somewhere and its not the calculator. Assuming the gps is accurate at 54 kvh I feel your 14" prop will net about 38.7 mph This will depend heavily on how much rpm you lose with the 14" prop.

 

[Sixxgrand]

I will be very happy with 39 mph from a 25HP Merc !
Thanks for all the wonderful feedback. I will order up a prop and advise my findings in May!

 

[gbrowne9]

from the pictures of the motor, the motor is to low. the plate above the prop is 2-3 inches below the bottom of the boat. raise the motor and test again.

 

[Sea Rider]

If Jeff states that water flow at speed is passing an inch or so below small upper water deflector plate he should be all right, The ideal is to have water flow skimming slightly under deflector plate, skip what's stated on owners manual about cav plate even with hull, this is just a refference from where to start. In his case mission impossible as will need to chop his transom about an inch so to have water flow skimming right under small plate, which is the ideal transom height for any Tohatsu engines. (lower tail shape issues.)

The example used achieving 3 + KPH/MPH ? corresponds to a larger 85 HP engine which is a 3 cylinder, has nothing to do with a 2 cylinder 30 HP engine. Jeff, you plan to go for +1, or --1 in pitch size ?

A picture is no good indication of a perfect engine/transom height match as the angle/height of the camera in which the pic was taken can lead to show anything, that's why water testing is mandatory on initial set ups.

Happy Boating

 

[Sixxgrand]

The example used achieving 3 + KPH/MPH ? corresponds to a larger 85 HP engine which is a 3 cylinder, has nothing to do with a 2 cylinder 30 HP engine. Jeff, you plan to go for +1, or --1 in pitch size ?
Happy Boating

My intentions are to move from a 9.8 X 11 to a 9.9 X 14.2

 

[Sea Rider]

How about a 9.9 x 13.1 which is next to the stock prop ? You're already hittting top wot rpm, with the one you intend to go after will probably lower top wot rpm more compared to the next 9.9 x 13.1 ?

Happy Boating

 

[steelespike]

He feels his rpm is correct. I feel that one or more of the numbers is wrong You can't go 54 kml(33.5mph) at 5870 rpm even if you use 5% slip.
If we use 11.3 prop,1.92 ratio 33.5 mph,10% slip we get about 6500 rpm. Speed is by gps. 10% slip is my guess but still won't work with only 5% slip.
Assuming a 600 rpm drop with a 14" prop puts it close to either the top rpm using 6500 or the bottom rpm using 5870.
He says hole shot is amazing throwing people off the seat leads me to believe he is closer to the 6500.
He says his 16 footer is 116 lbs,lightest 16' Naden I could find was 280 lbs. ??

 

[Sea Rider]

By january end will probably test a 120 kilo, 4.20 mt rib with a 2 stroke Tohatsu 30 HP engine if buyer wants a Tohatsu engine installed, although have tested 30's on 420 lighter sibs, never have done it with a tach and gps. Now have both handy, should do at least 50 Km/h with stock prop on flat calm, no wind waters.

I'm a firm believer that Sixxgrand is achieving those mentioned speed and wot rpm. Stock props on light applications delivers near and sometimes exceeds a bit max wot numbers factory stated for that engine. Personally would stay with stock prop, that's the max you can take out of that horse without compromising : Long Live the Engine...

BTW, does the prop calculator takes in consideration such issues as water, wind cond, hull shape ? How science exact is the calculator between theoretical performance and real word wot rpm/speed numbers.

Happy Boating

 

[Sixxgrand]

...... Personally would stay with stock prop, that's the max you can take out of that horse without compromising the : Long Live the Engine...

Thank you for the kind and detailed comments....

In as far as engine longevity - from what I have read from prop manufacturers increasing my pitch by 2 inches reduced rpm by approx 400 rpm. If my 30hp engine can not pull its 5870 then having it run at 5470 - with the same or perhaps higher top speed - would that not both reduce fuel consumption and increase engine life through cruising at lower RPM?

Since I have running darn near 5900RPM at WOT and I have a very light 16' aluminum I am increasing pitch by 3 inches. I calculate that I will still be able to pull 5500+ rpms with a nice increase in top speed! while achieving reduced fuel consumption and lower rpm cruising.

 

[steelespike]

By january end will probably test a 120 kilo, 4.20 mt rib with a 2 stroke Tohatsu 30 HP engine if buyer wants a Tohatsu engine installed, although have tested 30's on 420 lighter sibs, never have done it with a tach and gps. Now have both handy, should do at least 50 Km/h with stock prop on flat calm, no wind waters.

I'm a firm believer that Sixxgrand is achieving those mentioned speed and wot rpm. Stock props on light applications delivers near and sometimes exceeds a bit max wot numbers factory stated for that engine. Personally would stay with stock prop, that's the max you can take out of that horse without compromising the : Long Live the Engine...

BTW, does the prop calculator takes in consideration such issues as water, wind cond, hull shape ? How science exact is the calculator between theoretical performance and real word wot rpm/speed numbers.

Happy Boating

To this point the calculator has been accurate It does suggest adding an inch for cupped props.I usually suggest taking into consideration winds and currents
if numbers refuse to work.I also ask about hull conformation when things seem really sluggish or too fast.It is a simple calculator that has served well for about 5 or 10 years.There are others that are very sophisticated calculators and do work well for more exotic applications.
The calculator calculates using 4 numbers to find the 5th. With accurate gps,gear ratio,tach and prop size you can tell if the speed is possible at the given rpm.Very often the tach is the culprit if the numbers don't add up. If you don't know one of the numbers you can find it with the other 4.
You can use it to calculate any one of the 5 numbers. Slip on the typical boat application is usually in the low double digits.
Heavy or pontoon boats would tend to be higher, light fast boats somewhat lower.
Exotic combinations are better figured with more sophisticated system.This system works for Joe Blows average guy boat and activities.
Any calculator is only as accurate as the numbers inputed.

 

[steelespike]

Thank you for the kind and detailed comments....

In as far as engine longevity - from what I have read from prop manufacturers increasing my pitch by 2 inches reduced rpm by approx 400 rpm. If my 30hp engine can not pull its 5870 then having it run at 5470 - with the same or perhaps higher top speed - would that not both reduce fuel consumption and increase engine life through cruising at lower RPM?

Since I have running darn near 5900RPM at WOT and I have a very light 16' aluminum I am increasing pitch by 3 inches. I calculate that I will still be able to pull 5500+ rpms with a nice increase in top speed! while achieving reduced fuel consumption and lower rpm cruising.

Lowering rpm can be good.Lowering it too much can be harmful Based on your numbers I feel a 3" drop should work putting you at the lower end of wot rpm.
You run light most of the time so should be ok at about 5200 rpm.
Fuel use, GPH(gallons per hour)will remain about the same regardless if a house boat or a runabout MPG result will depend on the speed.
Grossly over propped motors will suffer from stress and poor performance outside the typical ratings.

 

[Sixxgrand]

Lowering rpm can be good.Lowering it too much can be harmful Based on your numbers I feel a 3" drop should work putting you at the lower end of wot rpm.
You run light most of the time so should be ok at about 5200 rpm.
Fuel use, GPH(gallons per hour)will remain about the same regardless if a house boat or a runabout MPG result will depend on the speed.
Grossly over propped motors will suffer from stress and poor performance outside the typical ratings.

I would be willing to place a small wager that I will still pull close to - if not - the same 5870rpm with the 14" prop.
The hole shot is crazy on this combo. It runs to top speed from idle in perhaps 3-4 seconds. When running at half throttle and you crank it wide open it literally jumps to top speed at WOT in a couple of seconds. It is far under-propped in my opinion. Remember my boat is only 116 pounds, + fuel and myself, fishing rod and bait!

Have a prop enroute for less than $100 shipped...

 

[steelespike]

No argument from me,you are under propped.One reason I think you can gain some speed is because you appear to be so far under propped.
If your boat is 116 lbs.you must be able to hang it on your shoulder and carry it.

 

[Sea Rider]

Don't know about fuel comsumption running at wot rpm extremes, that's running min 5150 max 5850 rpm. At +5850 rpm that engine is fully delivering its 30 HP. How long will that engine last at that wot rpm running as if you were stealling the combo everytime it's used no one knows for sure, will depend entirely on the 2 stroke oil quality used and its correct fuel/oil 50:1 mix.

Very few run wot throtle on all outings, anyway if running + 5850 wot rpm, as soon you begin to add more weight into combo, rpm will drop accordingly. If running 5900 and increasing pitch by 3" should theoretically put you at 5300 rpm which is very close to lugging the engine, worst if adding more weight into combo. You'll be good running solo at 5500-5850 wot rpm. Check both rpm top speed and stay with the prop that achieves this, as this is what you're after.

Let's see what numbers you obtain from the prop size and pitch you have ordered. Usually everybody stays with the factory stock prop which is a medium pitched prop, works very well on light to medium light applications. unless propping right for special applications.

Happy Boating

 

[steelespike]

My info states 30 hp 2 stroke rpm range is 5150-5850 ,He stated 5830 originally, 5830 obviously if we remove 600 rpm we are at 5230. He stated early on he runs by himself and sometimes with his dog.