Transmission?

[John Gardner]

Today I did some investigating and thought you would be interested in the results.  A friend had the brilliant idea of using an optical tach on both the engine and the gearbox output, so I borrowed one from work.  The Hurth transmission manual on this website indicated that there is a label on the trannie to show what the ratios are forward and backward.  Great stuff.  I should have mentioned before, that I have a 25XP engine.

After a number of interesting contortions with mirror in one hand and flashlight in the other, I concluded there was no part number and no ratio label on my trannie.

I went ahead anyway with runs in neutral, and in gear, loading the engine by trying to pull my helical mooring out of the ground, first in reverse, and then forwards.  I didn't have the courage to pull against the mooring at max rpm.  I concluded that the boat tach was reading a bit low, and assuming no slip, calculated the gear ratios.  It just so happens that the calculations closely match the example shown in the manual (1.86 reverse, 2.05 forward), so I'm concluding no slip.  Max true engine rpm in neutral was 3217.

Then I went out and did some motoring.  There was just me, and I wasn't prepared to go below for long enough to make measurements, so I had to content myself with using the measurements on the mooring to estimate true rpm from the ship's tacho.  Two main conclusions were my knotmeter is under-reading by about a knot, and the max rpm under load is 3125 indicated, 2952 estimated actual (i.e. a loss of 265 rpm due to load).  Running at 80% of this value gives a true speed of about 5.1 kt.  I'll try to attach an excel spread sheet with all the details - I may be successful or not, but thank you for your interest and help.

Moored neutral   Tach   Eng rpm   Tran rpm    GPS   knotmeter              tach o'read      
                   1130   1076                                    idle           5%      
                   1500   1445                                               4%      
                   2000   1881                                               6%   Tach over-reading about 5%   
                   2500   2380                                               5%      
                   3350   3217                                    max rpm   4%
      
Moored reverse   1100   1016            546                         idle           8%   Gear ratio    1.86
                   2000   1896            1013                                    5%   Gear ratio    1.87
Moored forward   1100   1034            508                                    6%   Gear ratio    2.04
                   2000   1900            932                                    5%   Gear ratio    2.04

Motoring forward      estimated                                     % max rpm      
                   2400   2286                   5.0   4.0                      77%      
                   2700   2571                   5.5   4.5                      87%   Ktmeter under-reading by ~1 kt   
                   3100   2952                   6.3   5.0           max rpm   100%      
                           

[Ron Hill]

John : Thanks for the data.

You should have a HWB 50 Hurth transmission which has a ratio of a 2:1

I don't recall where the data plate is on the housing case, but it's probably painted over!  A thought

[Indian Falls]

I thought you'd like this...
Now you can see the potential for damage from running low on oil and overheating causing all those clutch discs and springs to call it quits.  Looks like there is a clutch pack for forward and one for reverse. 

[Ralph Masters]

Moored neutral WOT 2800. WOT forward underway 2300.

Ralph
Ciao Bella

[John Gardner]

Wow, all that stuff in such a little casing.  So much opportunity for "Whoops, seem to have a bit left over"

[David Sanner]

John my first thought is that your transmission is fine.
(Your RPM at WOT while moored seems to suggest it's not slipping)

2nd (assuming your test was on flat water, little wind or current)
you've either got a really dirty bottom or it might be time to look
into another prop if you want travel 20%+ faster.

I have a 25XP with about 2k hours.  With a freshly
cleaned bottom I do about 6.3kts at cruise rpm which
is just over 80% of my max rpm underway at WOT.
(I believe I also loose one to two hundred rpm when
in gear vs. neutral at WOT.)

If you're bottoms is a little bit dirty that could explain
a 1/2 kt loss but you're still coming up short.

Good news is that if you don't mind going a bit slow
you hopefully have to spend a dime.  If you want
to get an extra knot a nice folding prop is cheaper
than replacing your transmission and you surely
notice a nice bump while sailing as well.

btw, my old three blade 15x9 came in a few tenths of
a knot slower than my current geared folder and didn't
really do any better in a seaway.

[Ralph Masters]

John,
You weren't going up hill were you??