Re-pitching prop

[Woody]

I'm considering having our prop pitch increased. I think we should be able to get a bit faster cruising speed without using more fuel.
The engine is a 27 hp Westerbeke with 1.8 gear ratio. The prop is 15" D with 8" Pitch. With engine at max rpm of 3000, we get about 6.8 knots top. We normally cruise at 2700 rpm which gives us about 6.4 knots.
Since the hull speed is rated at 8 knots, and the original specs show economical cruising at 7 knots with a smaller engine, it seems to me we're just wasting fuel and going too slow.
Our marina suggested re-pitching to 10" instead of 8. Any opinions?

[don123]

Is 3000 the maximum rated speed for the Westerbeke? . . . . or is it the fastest attainable in your boat?

Where did you come up with an 8 knot hull speed for an Albin 25? That seems quite high for something with a waterline length in the 22 foot range - I would expect your calculated hull speed to be something under 6.5 knots

I would think your current set-up is pretty close to correct - If your engine cannot reach it's recommended top RPM with your current prop, you may actually be a bit over-pitched now, but if you can do an honest 6.4 at 2700, I would leave it alone if it was mine - That sounds really good to me

At any rate, going from 8 to 10 on the pitch is a big change - If 3,000 is your max now, you might not be able to run 2700 any longer with the 10 pitch

Don

[Woody]

Hi Don,
Thanks for the input.
Yes, 3000 is the max rpm for the Westerbeke.

The original Albin25 owners manual shows a graph suggesting a top speed of 8 knots (and with a smaller engine), and cruising efficiently near 7 knots at 600 rpm less than the engine's max.

Our boat won't reach 7 knots and the engine maxes out without any apparent strain. So in consultation with our mechanic (who seems very knowledgeable about diesels), we decided it was worth trying a larger pitch as it's clear that the engine should have the ability to push her a bit more. My hope is that we'll run lower revs at our usual cruising speed (and use less fuel), and have the option to go a bit faster if needed some times.

Anyway, the prop has been re-pitched and is installed. We hope to get her in the water next week, so stay tuned for the results.

[don123]

Since you were able to easily reach the engine's max RPM, you'll probably be happy with a bit more 'bite' in the prop and it may even give you a bit more speed

I just priced having the prop on my A27 re-pitched today - Since I'm changing from a 60 HP diesel with a 1.85 to 1 gear to a 100 HP with a 2 to 1 gear, more prop is called for. My current 17 inch is about the largest that's practical for a 27, so I'll be adding 2 inches of pitch as well. What did it cost you to have yours done?

Don

[Woody]

Don,
My Marina quoted me $200 for re-pitching. I haven't actually got the bill yet. They don't do it themselves - they send it out to a specialist.
He did a beautiful job on the prop - as well as changing the pitch, he removed all the nicks and polished it up so it looks new. Can't wait to see what difference it makes.

[Woody]

Our prop was re-pitched from 8" to 10". They also cleaned up all the nicks and it looks brand new!
But most important, we now get an extra half knot at a given engine speed. So we can just reach the theoretical hull speed of 8 knots at full rpm, and can cruise economically at 7 knots instead of 6.5.
I'm happy. Cost was $200.

[don123]

I just got our prop back yesterday. Re-pitched from 9" to 12" and it too looks brand new, but then there wasn't a nick or a blemish on it when I sent it in. The cost was $180, but I had to wait 4 weeks for it

They told me that it had one blade at 9, one at a little more than 8 and the third one was in the middle, so it should perform much better now that all three blades are the same

Don

[Woody]

Don,
Should be faster and smoother running! Let us know how it goes.