Nausikaa came originally with a Volvo MD7A 12 HP. As that engine kept giving me no end of trouble, I replaced it with a Beta 16 HP. Following the 4 HP/tonne rule, and with a nominal displacement of just over 3 tonnes (maybe 4 t when fully loaded), I figured it would be just the right amount of power. It was also the largest Beta engine that I could comfortably fit into the bay.
However, on a recent extended cruise down from Lowestoft, Enland to Brest, France (that is down the North Sea and English Channel), the boat felt significantly underpowered. Some of this is certainly caused by the wrong choice of prop, but that is a known factor which I am soon to remedy. Still, in perfect conditions; no wind and flat seas, I struggled to maintain 4.5 knots with the engine at chugging at 2.4 k revs. With any headwind or in choppy seas, I’m lucky if I make 3 knots. In this part of the world where tidal currents across channels and harbour entrances regularly clock 6 knots and more, that feels inadequate.
I was wondering what was the choice of engine size for other owners of the same model, and the results they got.
First, where did you get 3 tonnes from? Feronia weighs just over 5 tonnes on the crane with a full fuel tank, empty water tank, catering equipment, rubber dinghy, warps, fenders, a couple of bottles of wine, part bottles of gin and whisky, a few beers, bedding and some clothing. The engine is a Beta 20 fitted with a Featherstream prop. The prop is an inch smaller than Darglow advised, but it came off my last boat and was repitched to suit. I can’t quite wind it up to the full 3,600 rpm, but top speed is 6.5kts in flat conditions. At 2,200 I get a little over 4kts. If you were running at 2,400 you were doing quite well to make 4.5kts - that will be the benefit of a clean bottom. If you can’t go past 2,400 your prop is way out.
Thanks for your comments Andy. Confused though. 3 tonnes is the displacement of an F30 according to the literature; 7000 lb. Just over. At least the ones built by Tillotson-Pearson in the US. As far as I know (and I may be very mistaken!) engines and props are sized, in among other factors, according to displacement tonnage, not mass of the vessel. When I selected the engine, I added 30% displacement load factor to the nominal 3.1 tonnes, expecting it would put me more or less in the right ball park figure re engine size, following the 4 HP/tonne rule.
But you are right, the prop seems to be right out. This is very annoying as the prop specs were recommended by the manufacturer for my engine/gearbox combination. I have a two-blade 14 X 9 RH. On speaking with another supplier about this problem, I was informed that prop was inadequate, and the cause of not being able to rev the engine through its full RPM range; it starts cavitating beyond 2.5K RPM. They recommended instead a 3-blade 12 X 8.5! This supplier assured me with that 3-blade prop i could get 5.3 knots @ 3.500 RPM.
Evidently, switching from a 2 to a 3-blade prop will have implications whilst under sail, but if I have to compromise, I rather lose half a knot under sail and gain a knot on the engine.
You might consider a KiwiProp. It has three blades, with adjustable pitch, and which feather. Not quite so efficient as a fixed three blade one, but as cheap and also repairable. The feathering means no discernible loss of speed when sailing. We are very pleased with ours, which we have used for four seasons now. It needs greasing once a year, but otherwise low maintenance.
Regards,
Gerald
PS, Castaway has a nominal displacement of 5.5 tons, but is about 7.5 tons in cruising trim; this is the displacement that has to be moved through the water. Under power, we can get 6.5 knots max, and more revs just sinks the stern further. Sailing, we can get 8 knots in good conditions!
Thanks Gerald. I did at the beginning. Although Beta published guidelines recommended (at the time) a Kiwiprop as an option, I got in touch with Kiwi in the UK and they said it was not appropriate for my engine/gearbox combo; i.e. a Beta 16 HP with a TMC gearbox @ 2.1:1 ratio.
It’s a bit of a no-win situation with any variable pitch prop. I don’t have enough clearance between the end of the prop shaft and the rudder. And any folding prop is out of the question. Even if a Kiwi was appropriate, the length of pop hub would force me to cut a notch out of the leading edge of the rudder to fit. Same with a Darglow Featherstream. Maybe that’s what I’ll end up doing!
I was puzzled by the weight in the literature. Hard to sea how a boat with 1.5 tons of lead ballast and the high volume hull that the 28 CK has can weigh in at 3 tons.
The FeatherStream should fit without needing any modification. There’s two versions of the anode, pointed and flat. The flat one fits, but not the pointed one. Darglow would be able to recommend a prop for the engine you have, although they might observe that it’s a bit underpowered.
I recall including the Darglow as an option when I was researching prop/engine combinations, and the issue with the length of the prop hub, taking into account not only the distance between the hub and the rudder, but also the minimum clearance required between the prop an the keel. Will look at it again.
And yes, a Beta 16 is too small. Tough choice. Upgrading to a 20 HP (GBP 4.5 k new) plus a variable-pitch prop (another coupe of grand), plus fitting etc, will set me back 8-9 grand. At the moment, too much for my pocket, to spend in that little boat in exchange for a couple of knots.
Will start changing the prop, and measure relative improvement.
Many thanks Andy. This is very useful! Actually, I was looking at the prop this very afternoon (under the rain!) And taking measurements. Will get in touch with Darglow again and review options.
The boat is on the hard in Brest at the moment. I may have to take the engine out. The mounting bolts have worked themselves loose form the grp cradle. Probably need to be cemented in again. Then would be a good time to change the prop and (perhaps) upgrade the engine too. I don’t know how easy/hard would be to do that here in France, but needs to be done!
Thanks again
Rafa
Rafa
I have original yanmar 2qm15 13 hp motor.
I have no problem to cruise at 5.5 knots with max just over 6 knots in ideal conditions.
Not sure about prop size, it is two blade, I believe original one.
Clave’ s original volvo transmission gave up, an inability to repair led to repower. Beta more expensive here,and although I liked it, we went with Yanmar 2ym15. Higher rpm just takes getting used to. Spinning 16 inch two blade. Flat water 5.5 knots. Fat heavy boat with windage could use more power but I use the engine to get out of canals and no wind. We have hit 9 knots sailing but often moving at 4-6 knots. Plan to sail soon.
First-generation Freedoms are correctly motorized for navigation in the areas where they were designed.
But for areas with high tides and therefore strong currents, the engines in question are a little weak.
This is the case with the Yanmar 3GM D (22.7hp) on my Freedom 33, which weighs 13,200 lb (6 tons)
Now that it’s sailing in France, near Lorient, I can see that it lacks power as soon as the current increases.
And it would be worse in the Channel.
For my Freedom, I think I’d need a 35hp to be comfortable.
