Westerly 33 and the Mercedes OM636

[gorb]

Can anyone here advise? We have just re-engined our Westerly 33 ketch with a reconditioned Mercedes OM636 engine. The hull is pretty weedy so performance is not sparking on the initial engine test runs.

We are about to lift out and will be having a look at the propellor. Traditionally, Westerly 33s were regarded as 'over-propped' and the old engine never managed more than about 2400rpm. A quick web search suggests that we should achieve a cruising speed under engine of about 6.5 knts, with a possible max of 7 knts. calculation suggests that hull speed is somewhere just over 7 knts.Has anyone here knowledge of adjusting the prop pitch, or even of fitting a variable pitch prop to a Westerly 33?

All comments and advice very welcome.

 

[Searush]

My Pentland 31 would do 6.5+ with the old 24hp MD2B and marginally higher with the new 30hp Yanmar. Over 2,600 rpm on the Yanmar she starts to squat and produces a pretty big bow wave. I would expect you to get 7-7.25 under engine with the 33 but never actually been on one.

 

[Tranona]

Just put propeller calculator into Google and you will get loads of sites where you can enter your own boat data to get a theoretical estimate of ideal diameter and pitch. Alternatively go to any Prop manufacturer and give them the data and they will do it for you. Specifically for your boat you might then try the Owners website to get some empirical confirmation of the estimates.

 

[Robin]

We had a Westerly 33 for 14 years and early on had the prop resized because the OM636 was as you say only reaching about 2,400 in gear under load, typical Westerly overpropping at the time. Interestingly the prop company found all 3 blades were different pitches and asked if we had any vibration, which we didn't. Their solution was to remove 1" of diameter (I think 17" to 16" from memory) and reset all 3 blades correctly to the same original pitch as stamped on the prop, I don't remember what it was now. The engine felt a whole lot 'livelier' afterwards and the prop kick astern was reduced although remained significant, plus now we achieved quoted max revs and therefore max hp under load. I don't remember the revs we used but we cruised at around 6kts for comfort and economy with a max of over 7.5kts. IIRC the fuel consumption was around 0.58gph.

We later had to re-engine and fitted a Volvo MD22L 50hp (Perkins Prima, painted green by Volvo) and had to fit a different prop to match that, same diameter but bigger pitch. We then cruised effortlessly at 1800/2000rpm to give just over 6kts at 0.5gph (much improved) and flat out nearly 8kts. The bigger pitch on the prop effectively feathered the blades more and reduced the drag under sail.

Our current boat with a 44hp Yanmar has a Brunton Autoprop which self pitches and I rate this highly as performance under power is excellent as is fuel economy and drag under sail is minimal by comparison with a fixed 3 blader. Ours came with the boat which was a bonus as they are rather expensive!

 

[gorb]

Robin - thanks, that's great. It's just the sort of experience I was hoping might be out there. I think the clear plan is to take the prop off when we lift out and consult a prop expert. I believe there is one near us in Glasgow.

Interesting point about fuel consumption. We reckon on about 1 gal per hour - you remember about 0.58 gph. The difference would be consistant with our over-propping and exhausting unburnt fuel. Cost of wasted fuel is about to go up greatly too!

Thanks for your help.

 

[Robin]

I suspect the fuel comsumption relates more to the chosen cruising speed because as you approach the theoretical hull speed the hp needed is on the hockey stick part of the graph. We chose to cruise at around 6kts, hull speed was 7.12kts (1.34 x sqrt LWL) so 6kts is 84% of that whereas 6.5kts would be 91.3% and well onto the steepest bit of the hp required curve. Factor in a weedy bottom and needing more revs to compensate and there goes the fuel burn!

On our current boat hull speed is about 8kts and we cruise at 6.5kts which is 81% of that and still only use around 0.53gph despite being 41ft and heavier, however if we up the revs to get 7kts we burn around 0.7gph, a third more fuel for just an extra half a knot. The beauty of the Brunton prop is that even a tiny bit of helping wind in the mainsail will push the boatspeed to 7kts at the same revs and fuel consumption, or we can reduce the revs and drop the fuel consumption whilst still maintaining 6.5kts.

It all comes down to the amount of hp actually used by the prop and there is a direct correlation between this and the amount of fuel burnt, I used to know the figures but can't remember them offhand.

 

[gorb]

As a result of tis conversation, we have just had 1.3 inches removed from the prop diameter after doing appropriate calculations. Results completely up to expectations! Cruising speed at 2200rpm up from around 6knts to 6.6knts. Max revs at full throttle now 2800rpm instad of 2200rpm, giving boat speed of 7.1knts.

Excellent company in Buckie did the prop mod while we waited, and charged £30!

 

[Robin]

Glad it worked on your W33 too! So if you stick to the old cruise speed of 6kts, now achievable with reduced revs, you should also see better fuel consumption.

Did you find the prop kick also less than before?