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.