Possible gearbox trouble - oh woe

sarabande said:
Robert

pending a resolution of the prop issue by physical checks, can you plug in the figures to one of the online prop calculators and see if they come up with something wildly different from the one you have been given (FAL will surely be able to give you the figs for the prop) Is it on the invoice from them ?

e.g.

https://www.vicprop.com/displacement_size.php

and

https://veempropellers.com/propeller-calculator/
Click to expand...
I'll try using the data in FAL's invoice, but they may have used additional information which is not to hand.
Thanks
 
Pye_End said:
Prop was certainly my first thought as well, but I just wonder if you can be assumed that the engine itself is pumping out full power - eg. would fuel or air starvation give similar symptoms?
Click to expand...

The engine seems to be pumping-out full power. Makes the same racket as 3 years ago when the throttle is pushed fully down!!
I didn't have a tachometer then, but the Tiny Tach (just fitted) shows c.3500rpm which I'm hoping is what FAL have worked on when calculating the pitch etc.
 
The key thing is the shaft speed. Your engine is right at 3500 rpm but the gearbox reduces this, typically to the range of 1300-1800 rpm depending on the reduction. The two most common reduction ratios are 2:1 which will put shaft speed at the higher end and 2.5:1 which will be toward the lower end. A prop sized for one will be no good with the other shaft speed.

So, check what reduction ratio you actually have.
 
Robert Wilson said:
The engine seems to be pumping-out full power. Makes the same racket as 3 years ago when the throttle is pushed fully down!!
I didn't have a tachometer then, but the Tiny Tach (just fitted) shows c.3500rpm which I'm hoping is what FAL have worked on when calculating the pitch etc.
Click to expand...

But you said you were only getting half revs.

Revving it with no load (ie out of gear) will tell you very little in terms of overall power output.
 
Tranona said:
The key thing is the shaft speed. Your engine is right at 3500 rpm but the gearbox reduces this, typically to the range of 1300-1800 rpm depending on the reduction. The two most common reduction ratios are 2:1 which will put shaft speed at the higher end and 2.5:1 which will be toward the lower end. A prop sized for one will be no good with the other shaft speed.

So, check what reduction ratio you actually have.
Click to expand...

According to the man who sold me Khamsin in 2010 (when I asked him last year before ordering the new prop) it is 2:1 ratio. He put in a replacement engine in 2002, using the same new gearbox he bought/installed in circa 1990 (to a previous Kubota 13hp)
Sadly he died earlier this year, so I can't find out any more information from him.
 
Pye_End said:
But you said you were only getting half revs.

Revving it with no load (ie out of gear) will tell you very little in terms of overall power output.
Click to expand...

Only getting half revs under load (forward or astern).
I thought, perhaps ignorantly, that full revs in neutral was what the prop makers would require.
Certainly with the old prop on (up and inc to 2015) the engine noise at full chat ahead/astern was similar to the noise currently at full throttle out of gear.
Not scientific, I appreciate, but seems a good starting point?
 
Any chance the fuel supply is restricted? You could still get full speed in neutral, but possibly not enough fuel to maintain engine speed under load... Easy to change fuel filters to check that they aren't causing the problem... Or you could see if operating the fuel lever at the engine when in gear improves matters which would indicate a fault with the control cables...
 
Last edited:
OK, time to teach me what goes on in the gearbox!


I presume I am correct in accepting that the transfer of power/rotation from crankshaft to prop shaft is not via cogs but by some form of plate-faces.
If I'm correct, then:-
1. What does the gear lever do to change from neutral to ahead/astern?
2. If the plates are slipping (seems to have been discounted by posts above) what causes them to slip.
3. If the Teleflex cable is damaged/sticking, would that mean that something is not engaging fully, thus allowing the plates to slip?



I do understand that if the pitch of the prop is wrong the engine can't produce enough power (torque?) to spin the prop adequately to the deliver appropriate "thrust". ​
 
sarabande said:
Robert

pending a resolution of the prop issue by physical checks, can you plug in the figures to one of the online prop calculators and see if they come up with something wildly different from the one you have been given (FAL will surely be able to give you the figs for the prop) Is it on the invoice from them ?

e.g.

https://www.vicprop.com/displacement_size.php

and

https://veempropellers.com/propeller-calculator/
Click to expand...

O can find most of the data, but I'm guessing at #2
Does the following help/seem accurate? Is it helpful in my case?
[TABLE="width: 760, align: center"]
[TR]
[TD="bgcolor: #ffffff, colspan: 14, align: left"]Data Input

[TABLE="width: 500, align: center"]
[TR]
[TD]Waterline length in feet:[/TD]
[TD]22 feet[/TD]
[/TR]
[TR]
[/TR]
[TR]
[TD]Beam at the waterline in feet:[/TD]
[TD]9 feet[/TD]
[/TR]
[TR]
[/TR]
[TR]
[TD]Hull draft in feet (excluding keel):[/TD]
[TD]2.6 feet[/TD]
[/TR]
[TR]
[/TR]
[TR]
[TD]Vessel weight in pounds:[/TD]
[TD]3060 lbs[/TD]
[/TR]
[TR]
[/TR]
[TR]
[TD]Engine Horsepower:[/TD]
[TD]13 HP[/TD]
[/TR]
[TR]
[TD]Number of engines:[/TD]
[TD]1[/TD]
[/TR]
[TR]
[TD]Total Engine Horsepower:[/TD]
[TD]13 HP[/TD]
[/TR]
[TR]
[TD="colspan: 2"][/TD]
[/TR]
[TR]
[TD]Engine R.P.M. (max):[/TD]
[TD]3500 RPM[/TD]
[/TR]
[TR]
[TD]Gear Ratio:[/TD]
[TD]2:1[/TD]
[/TR]
[TR]
[TD]Shaft R.P.M. (max):[/TD]
[TD]1750 RPM[/TD]
[/TR]
[TR]
[TD="colspan: 2"][/TD]
[/TR]
[TR]
[TD]Number of shaft bearings (per shaft):[/TD]
[TD]1[/TD]
[/TR]
[TR]
[TD]Desired speed in Knots:[/TD]
[TD]6.5 knots[/TD]
[/TR]
[TR]
[/TR]
[/TABLE]
Horsepower Calculations

[TABLE="width: 500, align: center"]
[TR]
[TD="colspan: 2"] This will calculate the maximum horsepower and torque available at the prop(s).[/TD]
[/TR]
[TR]
[TD="colspan: 2"][/TD]
[/TR]
[TR]
[TD]Total available horsepower at the engine(s):[/TD]
[TD]13 HP[/TD]
[/TR]
[TR]
[TD]Total available torque ft/lbs at the engine(s):[/TD]
[TD]20 ft/lbs[/TD]
[/TR]
[TR]
[TD]Horsepower loss of 3% per gearbox:[/TD]
[TD]- 0.4 HP[/TD]
[/TR]
[TR]
[TD]Horsepower loss of 1.5% per shaft bearing:[/TD]
[TD]- 0.2 HP[/TD]
[/TR]
[TR]
[TD="colspan: 2"][/TD]
[/TR]
[TR]
[TD]Total horsepower available at the propeller(s):[/TD]
[TD]12.4 HP[/TD]
[/TR]
[TR]
[TD]Total torque ft/lbs available at the propeller(s):[/TD]
[TD]37 ft/lbs[/TD]
[/TR]
[/TABLE]
Speed & Power Calculations

[TABLE="width: 500, align: center"]
[TR]
[TD="colspan: 2"]Basic displacement speed and horsepower required[/TD]
[/TR]
[TR]
[TD]Displacement hull speed (1.34 X sqrt of waterline length):[/TD]
[TD]6.29 Knots[/TD]
[/TR]
[TR]
[TD]Minimum horsepower required at propeller(s) for Hull speed:[/TD]
[TD]6.7 HP[/TD]
[/TR]
[TR]
[TD="colspan: 2"][/TD]
[/TR]
[TR]
[TD="colspan: 2"]Calculations based on desired speed and available HP[/TD]
[/TR]
[TR]
[TD]HP required at propeller(s) for desired 6.5 knots speed:[/TD]
[TD]7 HP[/TD]
[/TR]
[TR]
[TD]Estimated maximum speed with existing 13 horsepower:
This is the speed we will use for the propeller size.[/TD]
[TD]7.95 Knots[/TD]
[/TR]
[TR]
[TD="colspan: 2"][/TD]
[/TR]
[TR]
[TD="colspan: 2"]At this point it is important to note that all of the calculations above are based on full RPM and HP. Most engines are rated to run at a percentage of thier full RPM. This is what will determine your maximum cruising speed. The propeller sizing calculations below are based on 90% of full RPM. This gives the engine some reserve power to allow for variable loading in the vessel.[/TD]
[/TR]
[/TABLE]
Propeller Size

[TABLE="width: 500, align: center"]
[TR]
[TD]Number of blades[/TD]
[TD]Diameter (inches)[/TD]
[TD][/TD]
[TD]Pitch (inches)[/TD]
[/TR]
[TR]
[TD] 2 Blade[/TD]
[TD]12.5[/TD]
[TD]X[/TD]
[TD]8.6[/TD]
[/TR]
[TR]
[TD] 3 Blade[/TD]
[TD]11.9[/TD]
[TD]X[/TD]
[TD]8.5[/TD]
[/TR]
[TR]
[TD] 4 Blade[/TD]
[TD]11.2[/TD]
[TD]X[/TD]
[TD]8.3[/TD]
[/TR]
[TR]
[TD="colspan: 4"][/TD]
[/TR]
[TR]
[TD="colspan: 4"]The propeller sizes shown above do not contain calculations for cavitation or blade loading.
If you find that the recommended propeller is too large to fit your vessel, you can try increasing the shaft speed. Failing this, you can reduce the diameter and increase the pitch at the expense of your propeller efficiency. The rule of thumb is 1 inch of diameter is equal to 1 1/2 to 2 inches of pitch.[/TD]
[/TR]
[/TABLE]
[/TD]
[TD]
spacer.gif
[/TD]
[/TR]
[/TABLE]
[TABLE="width: 760, align: center"]
[TR]
[TD="width: 760"] [TABLE="width: 760"]
[TR]
[TD="width: 760"][/TD]
[/TR]
[/TABLE]
[/TD]
[/TR]
[/TABLE]
 
Robert Wilson said:
O can find most of the data, but I'm guessing at #2
Does the following help/seem accurate? Is it helpful in my case?
[TABLE="width: 760, align: center"]
[TR]
[TD="bgcolor: #ffffff, colspan: 14, align: left"]Data Input

[TABLE="width: 500, align: center"]
[TR]
[TD]Waterline length in feet:[/TD]
[TD]22 feet[/TD]
[/TR]
[TR]
[/TR]
[TR]
[TD]Beam at the waterline in feet:[/TD]
[TD]9 feet[/TD]
[/TR]
[TR]
[/TR]
[TR]
[TD]Hull draft in feet (excluding keel):[/TD]
[TD]2.6 feet[/TD]
[/TR]
[TR]
[/TR]
[TR]
[TD]Vessel weight in pounds:[/TD]
[TD]3060 lbs[/TD]
[/TR]
[TR]
[/TR]
[TR]
[TD]Engine Horsepower:[/TD]
[TD]13 HP[/TD]
[/TR]
[TR]
[TD]Number of engines:[/TD]
[TD]1[/TD]
[/TR]
[TR]
[TD]Total Engine Horsepower:[/TD]
[TD]13 HP[/TD]
[/TR]
[TR]
[TD="colspan: 2"][/TD]
[/TR]
[TR]
[TD]Engine R.P.M. (max):[/TD]
[TD]3500 RPM[/TD]
[/TR]
[TR]
[TD]Gear Ratio:[/TD]
[TD]2:1[/TD]
[/TR]
[TR]
[TD]Shaft R.P.M. (max):[/TD]
[TD]1750 RPM[/TD]
[/TR]
[TR]
[TD="colspan: 2"][/TD]
[/TR]
[TR]
[TD]Number of shaft bearings (per shaft):[/TD]
[TD]1[/TD]
[/TR]
[TR]
[TD]Desired speed in Knots:[/TD]
[TD]6.5 knots[/TD]
[/TR]
[TR]
[/TR]
[/TABLE]
Horsepower Calculations

[TABLE="width: 500, align: center"]
[TR]
[TD="colspan: 2"] This will calculate the maximum horsepower and torque available at the prop(s).[/TD]
[/TR]
[TR]
[TD="colspan: 2"][/TD]
[/TR]
[TR]
[TD]Total available horsepower at the engine(s):[/TD]
[TD]13 HP[/TD]
[/TR]
[TR]
[TD]Total available torque ft/lbs at the engine(s):[/TD]
[TD]20 ft/lbs[/TD]
[/TR]
[TR]
[TD]Horsepower loss of 3% per gearbox:[/TD]
[TD]- 0.4 HP[/TD]
[/TR]
[TR]
[TD]Horsepower loss of 1.5% per shaft bearing:[/TD]
[TD]- 0.2 HP[/TD]
[/TR]
[TR]
[TD="colspan: 2"][/TD]
[/TR]
[TR]
[TD]Total horsepower available at the propeller(s):[/TD]
[TD]12.4 HP[/TD]
[/TR]
[TR]
[TD]Total torque ft/lbs available at the propeller(s):[/TD]
[TD]37 ft/lbs[/TD]
[/TR]
[/TABLE]
Speed & Power Calculations

[TABLE="width: 500, align: center"]
[TR]
[TD="colspan: 2"]Basic displacement speed and horsepower required[/TD]
[/TR]
[TR]
[TD]Displacement hull speed (1.34 X sqrt of waterline length):[/TD]
[TD]6.29 Knots[/TD]
[/TR]
[TR]
[TD]Minimum horsepower required at propeller(s) for Hull speed:[/TD]
[TD]6.7 HP[/TD]
[/TR]
[TR]
[TD="colspan: 2"][/TD]
[/TR]
[TR]
[TD="colspan: 2"]Calculations based on desired speed and available HP[/TD]
[/TR]
[TR]
[TD]HP required at propeller(s) for desired 6.5 knots speed:[/TD]
[TD]7 HP[/TD]
[/TR]
[TR]
[TD]Estimated maximum speed with existing 13 horsepower:
This is the speed we will use for the propeller size.[/TD]
[TD]7.95 Knots[/TD]
[/TR]
[TR]
[TD="colspan: 2"][/TD]
[/TR]
[TR]
[TD="colspan: 2"]At this point it is important to note that all of the calculations above are based on full RPM and HP. Most engines are rated to run at a percentage of thier full RPM. This is what will determine your maximum cruising speed. The propeller sizing calculations below are based on 90% of full RPM. This gives the engine some reserve power to allow for variable loading in the vessel.[/TD]
[/TR]
[/TABLE]
Propeller Size

[TABLE="width: 500, align: center"]
[TR]
[TD]Number of blades[/TD]
[TD]Diameter (inches)[/TD]
[TD][/TD]
[TD]Pitch (inches)[/TD]
[/TR]
[TR]
[TD] 2 Blade[/TD]
[TD]12.5[/TD]
[TD]X[/TD]
[TD]8.6[/TD]
[/TR]
[TR]
[TD] 3 Blade[/TD]
[TD]11.9[/TD]
[TD]X[/TD]
[TD]8.5[/TD]
[/TR]
[TR]
[TD] 4 Blade[/TD]
[TD]11.2[/TD]
[TD]X[/TD]
[TD]8.3[/TD]
[/TR]
[TR]
[TD="colspan: 4"][/TD]
[/TR]
[TR]
[TD="colspan: 4"]The propeller sizes shown above do not contain calculations for cavitation or blade loading.
If you find that the recommended propeller is too large to fit your vessel, you can try increasing the shaft speed. Failing this, you can reduce the diameter and increase the pitch at the expense of your propeller efficiency. The rule of thumb is 1 inch of diameter is equal to 1 1/2 to 2 inches of pitch.[/TD]
[/TR]
[/TABLE]
[/TD]
[TD]
spacer.gif
[/TD]
[/TR]
[/TABLE]
[TABLE="width: 760, align: center"]
[TR]
[TD="width: 760"] [TABLE="width: 760"]
[TR]
[TD="width: 760"][/TD]
[/TR]
[/TABLE]
[/TD]
[/TR]
[/TABLE]
Click to expand...

How can 13hp possibly attain 7.95 kt with 22 lwl
I cant get that spd with 37.5 hp & 31ft lwl with a clean bottom & prop
 
Robert Wilson said:
OK, time to teach me what goes on in the gearbox!


I presume I am correct in accepting that the transfer of power/rotation from crankshaft to prop shaft is not via cogs but by some form of plate-faces.
If I'm correct, then:-
1. What does the gear lever do to change from neutral to ahead/astern?
2. If the plates are slipping (seems to have been discounted by posts above) what causes them to slip.
3. If the Teleflex cable is damaged/sticking, would that mean that something is not engaging fully, thus allowing the plates to slip?



I do understand that if the pitch of the prop is wrong the engine can't produce enough power (torque?) to spin the prop adequately to the deliver appropriate "thrust". ​
Click to expand...

There isn't what you would think of as true clutch inside a marine gear box, it's akin to the synchromesh cones in a manual car gear box, allowing a limited slippage to stop the gears "crunching" as you move from neutral to ahead or astern. As has been said already, from the symptoms described (ie lack of revs in gear) that isn't your problem. If things were slipping inside the gear box you'd be getting bags of revs for little movement: think of a car, if you put it in gear and dip the clutch, the car will move slowly, the engine will rev fast and there will be a smell of burning clutch linings.... You're not getting anything like that.
What happens when you move the teleflex leaver is that a fork inside the gear box moves a gear wheel into a new position so that it transfers the drive to either the forwards or astern gears. To avoid the gears crunching together, there's a mechanism as I outlined above.
These gear boxes are pretty much either in gear or out of gear, there no halfway house with slipping gears loosing half the engines output between the input and output shafts.
It sounds to me, as it does to others, that there's a mismatch between the new prop, the boat and the engine. I'd be looking to switch back to the old prop and see of that lets you get back to you expected performance.

Edit. Just seen your posts re prop calculations. All are talking about 11 inch or so props, whilst your is 13 inches and 8 inches or so pitch. That's where your problem lies.
 
Last edited:
duncan99210 said:
There isn't what you would think of as true clutch inside a marine gear box, it's akin to the synchromesh cones in a manual car gear box, allowing a limited slippage to stop the gears "crunching" as you move from neutral to ahead or astern. As has been said already, from the symptoms described (ie lack of revs in gear) that isn't your problem. If things were slipping inside the gear box you'd be getting bags of revs for little movement: think of a car, if you put it in gear and dip the clutch, the car will move slowly, the engine will rev fast and there will be a smell of burning clutch linings.... You're not getting anything like that.
What happens when you move the teleflex leaver is that a fork inside the gear box moves a gear wheel into a new position so that it transfers the drive to either the forwards or astern gears. To avoid the gears crunching together, there's a mechanism as I outlined above.
These gear boxes are pretty much either in gear or out of gear, there no halfway house with slipping gears loosing half the engines output between the input and output shafts.
It sounds to me, as it does to others, that there's a mismatch between the new prop, the boat and the engine. I'd be looking to switch back to the old prop and see of that lets you get back to you expected performance.

Edit. Just seen your posts re prop calculations. All are talking about 11 inch or so props, whilst your is 13 inches and 8 inches or so pitch. That's where your problem lies.
Click to expand...

Thank you,
That is sort of what I expected to be the case, before my "fishermen engineers" started frightening me about "slipping plates"(no disrespect to them, they are always most helpful).

I shall be switching back to the old prop tomorrow or Tuesday, not Wednesday as it's a day of birthday celebrations (:eek: staring my eight decade....:encouragement:)

Roughly measuring my old prop:-
2 blade, 15" diam x 2" pitch

As mentioned above the new prop is stated as 9" pitch - surely one of us is wrong?
I understood the pitch is the distance from the lowest part of the blade angled up to the highest (not from boss to tip).
9" sounds incredibly severe compared to 2" ??
Perhaps they've made a typo on the invoice?
 
Robert Wilson said:
.
Roughly measuring my old prop:-
2 blade, 15" diam x 2" pitch

As mentioned above the new prop is stated as 9" pitch - surely one of us is wrong?
Click to expand...
Pitch is the theoretical distance moved by the prop for one turn, with no slip, rather like a screw-thread. Your old propellor won't be 2" pitch, it may be stamped on the hub if you look hard, eg. 15 x ?. Difficult for DIY measurement.
I wouldn't have thought that a 13" x 9" prop was too far out for your 13 hp engine if it's performing correctly. Beta suggest 12" x 9" 3-bladed for their B14 with a 2:1 gearbox. Certainly shouldn't limit it to <2000 rpm IMO.
 
Rather than rely on what you remember the previous owner remembering about the gearbox reduction, check for certain by looking on the plate on the gearbox (with manufacturers name, serial no. etc.). That will usually have the reduction stamped on it.

You may need a mirror, or camera or whatever, to read the plate in situ.
 
LittleSister said:
Rather than rely on what you remember the previous owner remembering about the gearbox reduction, check for certain by looking on the plate on the gearbox (with manufacturers name, serial no. etc.). That will usually have the reduction stamped on it.

You may need a mirror, or camera or whatever, to read the plate in situ.
Click to expand...

There is a plate atop the gearbox, but it's been painted over and neglected for a long time.
I'll attack it with suitable tools and solvents, with fingers crossed.

Looks like a long day squirmed into the engine compartment. Oh JOY :(

Thanks for your advice
 
Propcalc give a 12*9 so your 13*8 is about right (1" greater diameter but 1" more diameter is "good")

Your 2 blade is too big diameter as there is not enough clearance as you found. Pitch will not be 2", probably more like 7 or 8.

Your displacement is I think wrong as the figure should be kgs rather than lbs.

The prop size is calculated to achieve close to maximum revs and hull speed at the same time, so you should get around 6 knots at revs above about 3300 (you are a bit short on power for the displacement). This will allow you to cruise at 50% power (or 70% revs - about 2500) at 5 knots.
 
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