Propeller pitch problem with Jprop

westernman

westernman

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I have a Jprop and the boat yard have messed around with it the last time the boat was out of the water.

Now, I am getting a speed of just 5.4 knots forwards at WOT and hitting 3800 RPM - the max RPM for my engine. According to my calculations (assuming about 35 tons all up weight, and 45ft waterline length), I am using just 12 HP from the engine to attain this speed. The engine is 85HP.

OK so far so good, so the pitch is way off, and needs to be substantially increased. According to my calculations I should be getting 8.4 knots at WOT. So presumably the pitch should be increased in the ratio 8.4/5.4 right?

However, the bit I don't understand is that in reverse I get a maximum of 1800 rpm on the engine instead of 3800 rpm. If I increase the throttle beyond the position I initially get 1800 rpm, I start to get large amounts of black smoke for very little change in rpm - i.e. it looks like the engine is seriously overloaded.

Acceleration in reverse seems to be fairly good (for 35 tons). Acceleration forwards is very poor.
No idea what top speed in reverse is.

Is there anything I have got wrong?
 
Think I would start again from basic principles and recalculate the pitch based on you boat/engine data. Looking at the manual it seems that reverse pitch is the opposite of forwards, so if you have too shallow pitch in forwards it will be too steep in reverse, which will explain why it is overloaded in reverse and won't rev. The manual has a table to convert the notch position on the adjuster to pitch in inches. So you should be able to see what pitch you are set at and the number of notches to move to get your desired pitch.
 
Tranona said:
Think I would start again from basic principles and recalculate the pitch based on you boat/engine data. Looking at the manual it seems that reverse pitch is the opposite of forwards, so if you have too shallow pitch in forwards it will be too steep in reverse, which will explain why it is overloaded in reverse and won't rev. The manual has a table to convert the notch position on the adjuster to pitch in inches. So you should be able to see what pitch you are set at and the number of notches to move to get your desired pitch.
Click to expand...

Which bit of the manual makes you think that the reverse pitch is different from the forward one?
 
The right pitch would be:-

8.4 x 6076 x 12 x 3.3 / 3600 x 60 x 0.55 right?
I.e. about 17 in.

(8.4 knots, 3600 rpm, 3.3 gearbox ratio, 6076 feet in a nm, 12 in a foot, 0.55 real pitch vs theorectical)
 
Bit of a guess, because the manual is silent on setting the reverse pitch and you have such a big difference between forward and reverse which suggests that they are not equal pitch. If they were the same you would not see such a dramatic difference in achieved revs. Even if the pitches are the same you might get some variation in revs because reverse reduction ratios are sometimes different from forward, but not 3800 against 1800. I am also assuming that reverse performance was satisfactory beforehand.

I see the manual has a table at the end where you can enter the settings for the individual boat. Presumably yours has been set correctly in the past, so there might be a record of that setting somewhere.
 
I think the gearbox ratio is the same in reverse as forward - at least if I have the gearbox which corresponds to the documentation I have!
 
Just done a quick calculation using Propcalc and the data given. Suggests a 22*16 3 blade and a speed of 7.11 knots at 3600 engine rpm. Are you sure of displacement and LWL? 85hp is not enough power to give hull speed of near 9 knots with that displacement.
 
I wonder if the JProp pitch has been set for LH or RH (which ever is opposite to what the OP needs.)
The good news is that at least you won't have to take the prop off to change it.
(We have a JProp fitted.)
 
That thought had occurred to me, but would that not mean it would go backwards with the gear in forwards?
 
Tranona said:
That thought had occurred to me, but would that not mean it would go backwards with the gear in forwards?
Click to expand...

I don't think so. I think it would mean the wrong edge of the blade would be cutting through the water but I could be wrong.
As the OP is in the South of France it must be time to put the snorkle on.
 
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Tranona said:
Just done a quick calculation using Propcalc and the data given. Suggests a 22*16 3 blade and a speed of 7.11 knots at 3600 engine rpm. Are you sure of displacement and LWL? 85hp is not enough power to give hull speed of near 9 knots with that displacement.
Click to expand...

LWL is correct. Displacement may be a bit pessimistic - The crane registered 30 tonnes with half full fuel tanks, empty water and a fair bit of kit on board. Water would add another 1.2 tonnes when full.

I think the prop is 25 in (at least that is what the particulars say).

What is maximum speed with 85hp according to propcalc? 8.4 knots with my water line length is were the curve starts to get very steep in terms of hp required vs speed. I.e. 8 knots would be a lot less HP.
 
7.1 knots with 35 tons, 7.4 with 30 tons. 45' LWL and factor of 1.35 would give 9 knots. Suggests you need 150 hp the get 8.5 knots with a 25*17
 
Talulah said:
I wonder if the JProp pitch has been set for LH or RH (which ever is opposite to what the OP needs.)
The good news is that at least you won't have to take the prop off to change it.
(We have a JProp fitted.)
Click to expand...

That is a very good question.

I previously have been able to get about 7.3 knots at WOT. This seems to fit well with the figures from Tranona. However, even then I have still had a big difference in reverse.

In any case something is obviously wrong as I now only get 5.4 knots at WOT.
 
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