Hurth HBW 100

The ratio's are governed by the gear set, the reason for the different ratio in reverse is due to the idler gear that changes the rotation (this is also the reason these transmissions cannot be run in full load in the reverse ratio).

I don't think there is a quick & easy way to change the ratio's, and a new gear set won't come cheap. Sorry.
 
Graham_Wright said:
Thanks for the link.

My memory must be defective.

I will check the ratios this weekend. 1:1 seems not to exist.
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Very unusual to have a 1:1 on a box designed for displacement boats with modern high speed engines. The 2:1 and 2.6:1 options are the most common. The aim is to get a shaft speed in the 1500-1700 rpm range. Most modern diesels peak in the 3000-3600 rpm range and 2:1 is far and away the most common, although Volvo use 2.2:1. The 2.6:1 is used for the odd engine that peaks at around 4000rpm or more commonly where the boat requires a large diameter flatter pitch prop - usually heavy displacement long keel boats. For example my old Eventide had first a Yanmar 1GM then a Nanni 14. With a 2:1 reduction the prop would be 12", but with a 3.1:1 for the Yanmar and 2.6 for the Nanni I was able to increase this to first 14" fixed and then 15" feathering.

So, I expect your box to be 2:1, but if it is 2.6:1 then you will need to check that there is enough clearance to use the larger diameter prop that will be needed. The reverse ratio is the same for both, so if you do have a 2.6:1 it will probably be quite fierce in reverse at low revs and revs will be limited, unless you go for a feathering prop that can allow different pitch in forward and reverse.
 
Graham_Wright said:
Prop, engine and gearbox are as the designer specified. However, boat should achieve 8 knots, only manages 3.4!
Gut feeling is that prop is too big/coarse pitched.
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You have a considerable problem. Are prop. and bottom clean? What's the engine and what are the revs. at full power under-way? Prop. calc. programmes are available free on-line. to help with matching the parameters.
 
Graham_Wright said:
Prop, engine and gearbox are as the designer specified. However, boat should achieve 8 knots, only manages 3.4!
Gut feeling is that prop is too big/coarse pitched.
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Likely whoever specified the prop did not get it right. That amount of speed differential would not be just due to a difference in reduction ratio between 2:1 and 2.6:1. If you are only getting 3.4 knots then the engine is probably only achieving under 2000rpm - and sounding awful! 8 knots is a bitt Over the top - more like 7.6. The aim of prop sizing is to get a size that allows the engine to achieve close to its maximum rated RPM and a speed in flat water just over "theoretical" hull speed.

Not a lot of point in guessing, though as the only way to find out is to determine the exact ratio and then do the sums to get the prop size. Am I right in thinking you have a Countess 33 with a 35hp engine. If so then you might find this useful Vicprop - Prop calculator for Displacement and semi-displacement hulls

You can edit it if I have not got the details right by going back one to the data entry page. with a 2.65 :1 the size would be 17*11.

Hope this helps
 
The engine only achieves about 1200 rpm and I'm sure that is the problem. This weekend, I'll check the gearbox ratio and the prop pitch. I think the pitch is actually stamped on the boss (unless someone has repitched it!).
 
The problem is almost certainly the prop. You need both the diameter and pitch which as you say should be stamped on the boss. Easy to measure the diameter, not so easy the pitch! 1200rpm does not surprise me for that speed. Once you have the correct details to feed into vicprop you will get a good idea of the ideal size. suspect you will need a new prop as the current one seems too far out to get repitched. 1" of pitch is roughly 250-300 rpm
 
To limit any sort of healthy modern engine to 1200rpm max. would seem to require a gargantuan prop. mismatch. If the prop's clean I'd suspect a problem with the engine
 
While I agree that 1200rpm max is low, you might be surprised at how big a difference even what seems a relatively small change makes. The recommended prop for this boat (from the information I have) with a 2:1 reduction is 15*9, and with a 2.6:1 17*11 - that is 4"" in diameter/pitch or up 1200rpm difference.

No doubt Graham will come back when he has more information.
 
Tranona said:
While I agree that 1200rpm max is low, you might be surprised at how big a difference even what seems a relatively small change makes. The recommended prop for this boat (from the information I have) with a 2:1 reduction is 15*9, and with a 2.6:1 17*11 - that is 4"" in diameter/pitch or up 1200rpm difference.

No doubt Graham will come back when he has more information.
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I am!
Gearbox ratios are A 1:1.9, B 1:1.85.
Prop is 17 X 15.5. Spare prop is 17 X 10. (Recommended 17 X 11.)
Swop the prop and all will be well?
 
Not surprised it only reaches 1200rpm. No way you can swing a prop that size. As I suggested earlier a 15" would be more appropriate. However you need to do a more accurate calculation. I do not know what your engine is governed to. The information I have found varies with anything up t 4400rpm. I used 3200rpm, but the engine may not produce its notional 35hp at that speed. The faster the engine runs and the lower the reduction the faster the shaft turns and the smaller the prop you need. So at 2.61 and 3200 the shaft speed is 1226 rpm, but at your 1.9 it is 1684.

Next step is to check the actual rated hp and rpm of your engine and enter that plus your reduction ratio into the Vicprop programme.
 
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