Traditional get home systems on single screw powerboats are typically not very functional. They tend to deliver 50 to 60% of the normal cruising speed in calm conditions, and be essentially useless in a stiff breeze fighting a head sea. This has made no sense to us. Why pay the weight, cost, and drag penalties, and make life more difficult maintenance wise in the engine room, if you can only accomplish what can be done with a dinghy acting as a tug? Especially since we have a get home sail.
If we were going to have a diesel powered system we wanted to have a respectable passaging speed, and the ability to make progress to windward in less than ideal conditions.
We now have that system.
A few preliminary comments. To begin with, once the engine and drive line have 250 to 300 hours on them, the odds of a mechanical failure are low. Given the massive fuel tanks with which these yachts are fitted, FPB 64 owners have the option of waiting to buy fuel from reliable sources, making contaminated diesel unlikely. The risk of debris damaging the drive line is also low. So we are left with two scenarios: fishing nets, and a lightning strike which takes out the electronic controls and main engine CPU.
If a disabling problem occurs in calm conditions, on soundings, there is the option of anchoring,
or using the dinghy as a tug, lashed alongside or pushing from astern.
If you think this is impractical, consider that the enormous engineless windjammers based in tiny Camden Harbor use their dinghies for just this purpose (as shown above) day in and day out.
On passage the simple answer is a get home sail and adjusting course to an off the wind destination.
The get home sail is relatively easy to rig and has the advantage of working if you have a bad load of fuel which would also disable the back up auxiliary.
But the sail is limited in usable angle and wind range. We think of it more as a long distance, off the wind rig.
And if it is blowing hard and there is a lee shore toward which the boat is drifting? None of our prior solutions had the ability to deal with this scenario. The concept of using a genset PTO with a hydraulic drive to a “leg” behind the rudder offered the least impact on the engine room. But when we got into the hydraulic losses, and potential disruption of water flow behind the rudder when the rudder is turned, hydraulics did not look like a solution that would work in stronger breezes trying to maintain offing from a lee shore.
Circa kept examining different approaches, keeping in mind that we wanted to maintain a beautiful engine room with good access. Then a few weeks ago we got a drawing that showed a new approach. A compact 100 HP Yanmar diesel would be used with a remote mounted V-drive. The diesel would live in the space formerly occupied by the diesel heater, with the heater moving above the Yanmar, onto a platform hung from the deck.
The lead drawing and that above show in detail how it all fits together.
This approach has many benefits:
- With 100HP at the ready there is enough power to maintain an eight knot cruising speed.
- The engine has a mechanical fuel injection pump and manual shift/throttle. This improves the odds of coming out of a lightning strike with propulsion capability.
- There is sufficient power and prop efficiency so that if the main engine were lost the option of continuing to cruise could be viable in many cases.
- The prop is operating in relatively clean water flow with moderate shaft line to hull bottom angle indicating reasonable prop efficiency.
- Bottom line, we expect the FPB 64 running on its get home engine to perform as well or better than most trawlers running on their main engines.
Over the past three weeks Circa has refined the design, and it is now scheduled to be installed on FPB 64-5 and retrofitted on FPB 64-1. When you consider that these boats use 100HP or less for propulsion at a cruising speed (9.7 knots) you can see where this is a get home system with muscle.
There are some penalties, of course. You have the initial cost to consider. There is a visual impact on the engine room appearance. And the added skeg and prop will create additional wetted surface drag. But overall we now feel the benefits, functionally and emotionally, make this system worth serious consideration.
Posted by Steve Dashew (August 12, 2011)
August 12th, 2011 at 11:53 am
It seems to me (from a gut feeling rather than engineering numbers) that the added drag, weight and cost of this system would really close the gap between single and twin main propulsion engines. If one were considering adding this system I would look very hard at twin main propulsion engines. Can you show us the difference with numbers?
August 13th, 2011 at 2:00 pm
Hi Daryl:
There are man issues involved in the single vs. twin decision besides efficiency. But on that item alone you are looking at between five and ten percent gain. You h ave to consider shaft to buttock angle, wake fraction (which varies with where on the hull you are), the hull shape, water flow in rough water, and many other items. There is also the aft starboard cabin, which goes away with twins.
August 13th, 2011 at 8:49 am
Steve,
Interesting. Will there be another skeg to support shaft coming from the second v-Drive?
I assume with prop folder behind a new skeg, drag would be minimal. I would also assume the second skeg, assuming its there, would have some effect on walking the boat through a zero speed turn.
Will the second engine be used for anything else? How often would it need to be run to maintain its viability in an emergency. I assume battery would be completely isolated?
so many questions…inquiring minds are hungry for the latest thinking:)
August 13th, 2011 at 2:03 pm
Howdy Scott:
Full skeg to the bottom of the get home prop for protection. Gori folding prop. Drag on the order of a two/three percent increase. Second engine will help turning clockwise. We wold want to run it weekly, or at least twice a month, under load of course.
August 13th, 2011 at 10:58 am
Steve,
Great approach, and with a well designed skeg, certainly appears more efficient/less drag than the sail drives normally used for get home. All mechanical
A couple of questions:
– what’s your guess at drag impact, and what folding prop type are you considering?
– 100 HP – largest I’ve seen, but I guess you could run both engines if necessary to put distance between yourself and a storm front – is that practical?, or alternate between the 2 on a regular interval, to ensure all systems are “go”?
– lastly, does this get-home system change your main prop design considerations, ie do different main prop profiles exhibit less drag, and do you intend to lock or freewheel the main prop, when the main is shut down?
Regards, great work as usual!
John
August 13th, 2011 at 2:08 pm
Hello John:
Won’t repeat some answers already given. For increased top end, if the boat is light on liquids, you might pick up 1/2 to 3/4s of a knot. Heavy not much at all. No change in the main prop. The engine is 100HP because we want it to work in adverse conditions, not just calm seas and light winds. If you are going to do this, might as well do it right!
August 13th, 2011 at 5:57 pm
steve – my apologies if you received an incomplete message just now – strange keyboard / strange mouse…
sounds like the all of above respondents had similar questions – one remains – what’s the expected helm response, when the main prop shadows the rudder?
regards again.
John
August 14th, 2011 at 6:31 am
Hi John:
Maneuvering on just the wing engine will be compromised, of course. But the FPB 64 has such a large rudder that she should still be more easily controlled than yachts with smaller rudders on their mains, or at least comparable in maneuverability. There is the thruster to assist when stopped or at dead slow. There will also be a difference in how the FPB 64 handles direction of turn, clockwise being favored.
September 7th, 2011 at 5:07 am
How about drag from the main prop when just running the Yanmar? Will the mainprop be freewheeling to avoid drag? What about lubrication of the main shaft in that case?
I have a dual I/O drive on my small boat and if I shut down one engine the drag from the drive not running is formidable.
September 7th, 2011 at 12:44 pm
Excellent question Torbjorn:
Ideally the prop will be locked, which will substantially reduce its drag compared to free wheeling. There is a shaft brake, which we are hopeful will lock the prop if clamped when the boat is not moving. However, we will have to see how this w orks in the real world.
If the prop is spinning, then the engine needs to be started every four hours to lube the transmission, or the tranny should be overfilled so the oil splashes the gears (both suggestions from ZF engineers).
The free wheeling prop will easily increase drag by 30% or more.
September 7th, 2011 at 5:16 pm
I think you will find this link interesting. Locked prop induces far more drag than a free wheeling one acording to this test.
http://www.sailnet.com/forums/gear-maintenance/53628-fixed-vs-free-wheeling-prop-test-data.html
Perhaps the best way to get a decent speed and lower fuel flow, in case of failure of the main engine/prop, would be to “loose” the main propellar in some way.
September 7th, 2011 at 7:58 pm
Hello Again Torbjorn:
Years ago, before Maxi feathering or Martec folding props, most yachts were fitted with two or three bladed fixed propellers. There was lots of discussion about free wheeling vs fixed, but when racing everyone we knew fixed their props.
The second direct experience I can sight is the dead stick glide ratio of a small power plane. If you take a Cessna 150 and test its glide ratio with the engine off with and without the prop turning and then with it turning, you will find glide ratio gains of between ten and twenty percent with the prop stopped.
We should have one of the FPB 64s retrofitted with the get home engine early next year at which time we can conduct a bit of testing.
September 16th, 2011 at 10:16 pm
@Torbjørn Nordmo…
I have had similar results on my souped up Banjer 37 during a couple Atlantic crossings. Once, this heavy displacement boat reached 12 knots under heavy spinnaker with force 6/7 on its stern quarter with the shaft free wheeling and driving an alternator belted on the flexible coupling – its 3 to 1 Paragon transmission was designed to take it. I was amazed, but once the old Benmar auto pilot showed signs of stress, I took the spi down…
The big bonus was that the normal prop wash over the rudder – instead of a disturbed water flow caused by a locked prop – helped with steering and made the autopilot’s work much easier…
The only drawbacks: the belted alternator burnt up due to the high speed and the stainless steel rings where the spinnaker sheets were bent became oval…
September 16th, 2011 at 10:52 pm
Wny not get rid of the AC generator, hang some Electodyne alternators off of the wing engine, and turn it into a D.C. boat. You might want to add some synchronizing inverters as well. The engine would be exercised and always ready to start and you would not really need the AC generator at all. You might even come close on the costs and engine room clutter.
September 17th, 2011 at 12:19 am
Good thinking Steve:
We have built our own DC genser using Yanmars in the past, and this may be another case where it makes sense. We are looking into this now.