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Splines and Tapers

Many of the problems associated with the fitting of a propeller to your yacht, relate to the shaft diameter, taper ratio, length of taper, key way, nut.

In the case of the Saildrive units - the the spline is parallel so has no taper or key. There is only one spline standrd in use which always makes fitting a spline on a Saildrive simpler than on a shaft with various tapers / keyways / threads.


Every unit that leaves the factory has been fitted over the correct spline prior to despatch. We have factory purchased splines permanently mounted at the factory, and each propeller is offered up to these to make sure that they fit correctly prior to dispatch.

If your new propeller does not fit the spline you should first check the following:

1. Is the spline clean of all dirt, burrs, and ribbons of steel from the previous fitting.

2. Underwater installations are difficult to clean as microscopic amounts of corrosion on the sharp edges of the splines may be enough. Calcification can build up quickly after cleaning and cause binding as spline tolerances are quite low.

3. Has the spline been altered flared, bent, scratched, distorted - perhaps by movement on a previous installation.

Saildrives only have two different sized nuts and are used as follows:
  • All Yanmar SD20, SD25, SD30's use M16 x 2 nuts
  • All Yanmar SD40, SD50 and SD60's use M20 x 2 nuts
  • Yanmar SD31's came with M16 x 2 initially - later models used M20 x 2 nuts
  • Yanmar SD60's first batch came with M20 x 2.5 threads produced in error

Virtually all Saildrives come with a standard SAE 16/32 Spline with 17 teeth of 28 mm diameter.

However manufacturing tolerances can and will vary so there will be variances between the individual splines from the various manufacturers.

Very early Nanni's and the smallest Bukh 10 hp units used a different smaller spline compatible with some outboards. Kiwiprops do not offer units for these splines specifications.


If your new propeller does not fit the taper:

1. Have you ordered the correct boss for the shaft sizing - The difference between a 25 mm shaft and a 1.000" shaft is just 0.040" and can only be measured with calipers or a micrometer, yet this will determine of the taper attachment standard is imperial or metric.

2. Have you ordered the correct boss for the shaft taper as we see installations where say a metric taper has been machined onto an imperial shaft and conversly. This simply will not occur on production vessels from reputable builders - but of course over time with shaft replacements differences can creep in.


Imperial shafts would normally be fitted with an SAE taper of 1:16 taper ratio that has been in use for over 100 years.

Metric shafts since the early 1990's with the introduction of the ISO standards have used the ISO 1:10 ratio.

Only on vessels built in the UK do we see what are termed Admiralty tapers of 1:12 ratio, but this psuedo standard does not specify taper length, keyway or thread Ø, type or length.

Consequently fitting these requires machining a blank bored boss and nut  to a bespoke fitting. 

The Kiwiprops approach to non standard shafts is always the same. Pull the shaft and machine to a standard taper. Using a steeper ISO 1:10 taper allows for an existing shaft to be retained and turned down - but does not address the keyway which will need to be remachined parallel to the newer taper.

The existing thread can often be retained - but it is important to contact Kiwiprops before embarking down this route as taper lengths and thread lengths need to be considered.

3. Have you ordered the correct boss for the keyway sizing

4. Has the taper, shaft and keyway been wire brushed clean and all burrs, swarf, paint residue been removed.

5. Is the key holding up the installation of the propeller. The propeller should not bottom on the key when installed. The propeller should fit into exactly the same position up the shaft, whether the key is in place or not. This ensures the boss of the propeller is not binding on the top of the key so preventing the unit sliding fully up the taper.

6. Is the key properly home, it should be realtively easy to install and remove from the keyway - insufficient depth of shaft keyway will cause the key to stand proud of the shaft and then interfere with the mounting of the propeller

If your new nut does  not fit the retaining thread on the shaft:

1. Have you ordered the correct nut. It is very hard sometimes to guess the TPI  and diameter of a thread. Expert help maybe required with a thread gauge that fits to the thread and displays the thread - but this does not determione diameter.

METRIC threads as used on all metric shaft diameters are measured both in Diameter of the thread and in Pitch per thread or the distance in mm between two adjacent " hills " of thread. The profile of all metric threads is the same. Typically specified as M16 x 1.5 for example on a 25 mm ISO taper. The M = Metric, the 16 is the diameter of the thread and the 1.5 the pitch in mm between indovidual threads. All metric thread profiles are standard so this is not specified.

IMPERIAL threads are measured in tpi or threads per inch along the thread. A 10 tpi thread as used on  UNC 3/4"  would expect to have 10 " valleys " every 1". In addition Imperial threads have numerous different thread profiles.

The most common are UNC ( Unified Coarse ), Whitworth and UNF ( Unified Fine )

While not unknown it is rare to see imperial threads on metric shafts and vica versa. It is not uncommon to see this in Scandanavia on very old vessels as the current ISO metric shaft standrads were  not adopted until the early 1990's.

Early Beneteau's pre ~ 1991 used M20 x 2 threads prior to the adoption of the now universal M20 x 1.5 of the ISO standard.

2. Is the thread worn, flared or has been cross threaded by the previous installation. An easy way to check this is to run a correctly sized die nut onto the thread prior to fitting the nut. Perhaps do this prior to ordering your Kiwiprop.

While bronze nuts can be tightened onto a thread that is slightly oversize - All Kiwiprops come with SS 316 nuts and stainless nuts require that the thread on the shaft be within tolerance or it will bind due to what is termed " galling " where the metal " picks up " and prevents the nut running smoothy down the thread.

Very often when turning a shaft taper the thread is subsequently machined in the lathe. Correct procedure requires what is termed a hardened die nut be run down the thread to ensure it is within specification and any high spots are removed. Failure to do this can cause a SS 316 nut to seize on the thread. The solution is to run a die nut down the thread.


We run into many queries with regard to these issues:

1. Don't accept what is written on the existing propeller or shaft - this is can now be wrong if the unit has been re-machined at some point.

2. The SAE/ISO specifications are quite clear and precise, copies are on our website. There are many installations we have seen where engineers maybe some years ago, have done a DIY specification to "make it work" which will not allow for subsequent standard mountings.

Turnng the shaft taper in a lathe then using the same taper set up to turn the propeller makes for a perfect fit - BUT of course does not mean the taper is to standard.

3. Make all measurements with a good quality micrometer. It is very hard to tell the difference between 1/4 inch = 6.35 mm and 6.00 mm ! Similarly with 8.00 mm and 5/16" = 7.94 mm used on imperial shafts. Some of the keys are not square but may be 8 x 7 Deep for example as used on ISO 30 mm shafts.

All thread diameters equate to the rod from which they were machined. Threads cut deeper than standard may reduce the outside diameter of the threads so allowance has to be made for this possibility.

4. A shaft drive installation, must have correct taper ratio, taper length, correctly sized and machined keys, keyways and set screws holding the key in place in all areas where torque is transmitted through the installation. Keys on shafts are typically made from Brass - not Bronze which is too hard to be extruded. They are thus subject to corrosion over time and should be regularly replaced or swithched to 316 stainless steel or Titanium which will need to be machined out of solid rod form.

5. In a perfect world - though not in general use today a shaft shold have a matching taper at the coupling end attached to the gearbox. The nut will prevent the shaft sliding aft under reverse thrust but also allows the shaft to be turned end for end so presenting new shaft areas to both the stern / strut / shaft log bearing surfaces as wear develops over time.

6. When all of the above has been checked - please call us, we will generally be able to help