Sea Legs for SOHVI
Despite having been afloat for most of the summer, last weekend was the first opportunity I have had to dry SOHVI out and make a few changes.
First and most obviously, I set up a pair of beaching legs acquired courtesy of eBay.
These legs (type PL30) are made by the Yacht Leg Company and have the great advantage of being telescopic, which makes them easy to stow in the starboard cockpit locker when not in use.
Drying legs may be of limited use elsewhere, but here with 10m + tides, the ability to take ground safely from time to time is a real bonus. They are also very useful when laying up the boat ashore.
Some Summer shots:
Performance under power
A summer of cruising means that our new Vetus diesel is now fully run-in. During this time I have been unable to reach more than 2,650 rpm, compared to the designed maximum of 3,000 rpm. Consequently I decreased the pitch on the Variprop by approximately 1″, to 13″ (diameter is 22″)
Today we went out to conduct some speed trials and found that maximum rpm is now exactly 3,000 rpm. Whoopee! So what does all this tweaking mean in real life? Let’s look at some data:
Doing the math:
I commented previously that the performance curves for the Perkins 4.108 and the Vetus M4.45 were very similar. A quick conversion of units allows one to make the following comparison:
The Vetus is a little better than the Perkins below 2,000 rpm and much the same up to 3,000 – which is the maximum permitted, generating 42 hp.
The following speed v rpm chart represents the average of a two way run on reciprocal course in reasonably calm conditions:
Objectively that is slightly slower than I expected, but subjectively it seems fast enough, with the quarter wave right at the stern of the boat. Any faster and we’d be going uphill! The Vetus engine is also noticeably quieter than the Perkins (although I have not been able to quantify that with a db meter).
Interestingly the reduction of slope at approximately 2,000 rpm, or 6 knots, coincides closely with the theoretical maximum hull speed of 6.6 knots calculated using the empirical formula of 1.3 x √LWL (26′). It may also reflect the fact that a flat-blade feathering propeller is not as efficient at higher output as a true helical screw. This same propeller used on our old boat would typically incur a reduction of approximately 8% in top end speed compared to the fixed blade propeller.
Initially I performed a straightforward conversion from these figures into litres per hour, but the end result suggested a consumption considerably higher than other MS33 owners experience with similar sized engines. (I am indebted to Svein Lamark who first highlighted this difference – see comments below).
An exchange of emails and telephone conversation with Vetus UK clarified the situation. The consumption curves are theoretical in so far as they assume the engine is fully loaded at any given rpm. The reality of course is that for a given pitch, the engine is only fully loaded at one particular rpm, which in our case is 3,000 rpm. At lower rpm, the engine is nothing like fully loaded, or to put it another way, the propeller is absorbing considerably less horsepower than the engine is capable of delivering at these lower rpm. The actual amount of power absorbed will vary considerably depending on the hull design, displacement, sea state and other variables. Consequently real-world consumption at lower rpm is likely to be considerably less than I originally calculated. Vetus provided the following data as to what a ‘typical’ installation might return and which clearly illustrates the point above.
Fully loaded at 3,000 rpm, the M4.45 will consume in excess of 10 litres per hour. However, partially loaded at a typical cruising rpm of 1,800 rpm, the consumption is likely to be in the region of 2 litres per hour. That very significant difference demonstrates the real cost of pushing a hull past theoretical maximum hull speed: it is much more efficient to motor at a relaxed cruising speed. Interestingly the 2 litre per hour figure Vetus suggest above is very close to the 2.2 litre figure Svein achieves with his 49hp Yanmar installation (see comments below).
There is only one way to accurately report fuel consumption and that is to log fuel used against hours motored over a variety of conditions. In other words, to keep a decent log of fuel consumed during a season of cruising. Ask me in a year’s time and we’ll see whether I have been diligent enough to do this 🙂
It is highly likely that all of this simply serves to confirm what most MS33 owners will know already; that a cruising speed of approximately 6 knots is undoubtedly the most economical way to make progress.
Should I have left the pitch unchanged? We were obtaining close on 7 knots at 2,650 rpm, which was possibly more efficient. However, it created some black smoke suggesting the engine was overloaded and in the longer term I am comfortable that the engine is now set up as it should be.
Had we not experienced a gearbox failure, I would probably have kept the Perkins for as long as possible and been entirely happy with it. Having made the change, and despite the fact that the performance is really not much better than the Perkins, subjectively the experience is much nicer: the Vetus is undoubtedly quieter, smoother and cleaner. More importantly we are achieving the same performance as the old Perkins, but with the benefit of a feathering sailing propeller. Losing 75kg of weight has probably helped too, both in terms of sailing performance but also in our ability to carry all the usual cruising gear without depressing the waterline too much!