Propeller size

[Hopscotch]

V 16 1/2 x 12 is stamped on my Albin 25
Propeller. Powered by a AD-21 engine
Is it possible to improve speed by increasing
Prop size.

[denchen]

I am not an expert, but I would think if you increased your pitch, it would cause the engine to labour and you would be slower plus do more damage to the engine. On my 25 I have a 1800 diesel with a 16 by 13 prop and the motor runs well.

[DesertAlbin736]

Am no expert, but seems that like any full displacement hull, the older non-semidisplacement models especially, hull length and beam is what limits vessel speed. 6.5 kts seems to be the sweet spot for full displacement early model A25s. Much beyond that and you're just wasting fuel.

As far as prop pitch, it's more about what it does for WOT (wide open throttle) RPM.

In that regard, I'd consult with this expert:

http://www.soundmarinediesel.com/Albin25Repower.shtml

[dkirsop]

I have the same propeller and engine combination on my boat. I also have the original dealer documentation that came with the boat when it was new. The propeller is the size that was originally specified for the boat. Personally, I find the propeller is well matched to the boat and 6.5knots is the limit imposed by the waterline length of the hull. The boat cruises very efficiently at 1800rpm and this corresponds to the 6.5knots when there is no opposing current. In the dealer literature there is a note that the propeller will improve fuel economy and performance if the diameter is trimmed to 16". This was based on a series of test runs which are documented in the supplementary manual. Personally, I am not convinced this is worth the cost as the performance improvements were marginal.

[don123]

In the dealer literature there is a note that the propeller will improve fuel economy and performance if the diameter is trimmed to 16". This was based on a series of test runs which are documented in the supplementary manual. Personally, I am not convinced this is worth the cost as the performance improvements were marginal.

That doesn't make too much sense, but I suspect it means that the boat is slightly over propped with the factory supplied 16 1/2 with 12 inch pitch . . . . or, maybe the tip to hull clearance is a little too tight so a slightly smaller prop would be advisable. To gain maximum efficiency, it usually makes more sense to reduce the pitch by 1/2 inch rather than reducing the prop diameter

As a general rule, so long as there is adequate room for the prop, a larger prop with less pitch will be more efficient than a smaller prop with more pitch - The larger diameter prop gets a bite on more water

But, if your engine is rated for 3600 RPM and you can run it WOT in flat water at 3500 to 3650 or so, your prop is perfectly matched to your hull/engine and there's little to be gained by changing anything. In theory, if there is adequate room for a larger prop, going up an inch in diameter (from 16 1/2 to 17 1/2) while going down an inch in pitch (from 12 to 11) *should* give a slight increase in efficiency. An inch of diameter is roughly equal to an inch of pitch. But . . . . you should maintain a prop tip to hull clearance of 12 to 15%, so you want to keep at least 2 to 2 1/2 inches between the blade tip and the hull - If that's all you have now (with the 16 1/2) then a larger prop wouldn't be recommended and if you have less than 2 inches with the 16 1/2, then trimming it half an inch as the factory suggested just might be the correct move

As dkirsop mentioned though, if the prop is already 'correct' any changes will only result in marginal performance increases and they may not be worth the cost. If you did want to reduce the diameter, you should have it done by a qualified prop restoration shop and that might cost you $100 to $200

Don

[Panhdjoe]

There is an interesting article in the most recent Passagemaker magizine about the advantages of being "over proped"