outboards in nacelles

Hello. I have just moved over to a Banshee Catamaran from a monohull and am really enjoying the sailing but don't like the transom mounted outboard (9.9 Yamaha HT) which cavitates and can't push against wind in a chop (recently spent a night in the Cook Strait covering just a few miles).

The boat originally had two 9.9s in pods situated in the cockpit lockers. Apparently this arrangement was not satisfactory and the engines died due to corrosion. The bridge deck openings were glassed over and a sled mounted engine fitted to the transom.

I am considering revisiting nacelles in the boat and took some pictures from the forum (http://www.boatdesign.net/forums/attachments/propulsion/67015d1328748229-ganging-outboards-does-work-twin-engine-eclipse.jpg) to a local boat builder. He has limited experience with this type of design and his first concern was about water being forced upwards in a following sea. He also felt that corrosion might be an issue.

I would very much appreciate insights from any of you that have experience with cats driven by outboards located in nacelles. If you think that they are a good idea I would also value any pointers you have with regard to design (e.g. of the outboard mount).

Many thanks, Alex

 

honestly, in my experience with them on a 42' cat that had 2 sleds that dropped from the cockpit, they were terrible. I sailed over 1000 miles on that boat and any time we had to motor in over 2-3ft seas, it was a disaster as the props would hop clear of the water when the boat would pitch. Maybe they needed longer shafts? who knows.

But, Im a bit of a purist so it forced me to sail more and longer before I would resort to motoring.

I would be scared of following seas as well with the picture you posted. Sleds will rise and fall with the waves and in worse case they will snap off but leave the boat totally fine.

just my .02

 

My first line of enquiry would be to Richard Woods ?

 

There is a conversation about this in the propulsion section:

http://www.boatdesign.net/forums/gas-engines/catamarans-nacelle-pod-propulsion-51148.html

I agree with 2far about the corrosion and following sea, I believe these sleds should be retractable. Being just aft of the aft crossarm they are closer to the pitch axis and there should less cavitation than transom mounted engines.

By being adjustable you can lower them until they stop cavitating in most conditions. Gold Coast Yachts used this arrangement for many of their day charter boats that operate commercially. They are not perfect but for OB power on a catamaran I believe a hinged sled to be the best compromise.

 

Alex did email me and this was my reply

"Good to hear from you and to hear you bought the Banshee.

Putting the outboard on a transom is a horrible idea (although the Auckland based Flica has them). How big an engine do you now have? Can you turn to port and to starboard equally well?

And obviously you want to stop the fuel leaks. Can you not just drill holes though the bridgedeck so the lockers vent out underneath?

The corrosion issue is down to Yamaha, not the installation. The second Banshee launched is on its third set of Yamahas. The new ones started to corrode within 6 months of use. Yamaha tested everything and agreed it was their fault. Apparently the anodes they were using were sub standard. I think they will be replacing the engines under warranty. I have since heard of 3 other Yamaha engines with the same fault.

I would suggest fitting them in nacelles in the cockpit lockers. Originally there was only one outboard fitted, but many owners changed to twin engines. The alternative is to use a 15hp mounted centrally. That way you free up the cockpit lockers, and have a lighter installation

However I would not use the wide dish nacelle we used on the Banshee (we used it because we already had that mould) Instead I would use a deeper, more Veed nacelle. Something like the bow of the hull in fact. It can be quite small and narrow and still give the engine enough protection

I attach some photos of my Eclipse installation to guide you. You will see that I took off the hold down catch, which was really awkward to use, and replaced it with a "hold down" rope. I also had a raising rope. That meant I did not need to open the outboard locker to start, stop, raise or lower the engine. The other lines connect to the tillers so the engine turns with the rudders (they are released when sailing)

I also fitted the engine controls low down, so they would not catch on sheets and ropes. I adjusted the controls with my foot. Some one once commented "Uh??" I said, "what do you do in a car"

I hope that helps"

Richard Woods of Woods Designs
sailing catamaran designers

www.sailingcatamarans.com

 

Go the vee

I have a 38ft cat with a pivoting nacelle. It is about 300mm off the water when raised but has very full sections. It makes too much noise and I built this one because I could get a mould for it. I would recommend as Richard says to build the sharpest most veed nacelle you can. I will be rebuilding this nacelle when I return from this years cruise

Phil

 

The bob oram style outboard pods seem to be neat , recessed into the chamfer panel area so they're protected that way...




View looking aft under the bridge deck, outboard is tilted up.



Finished product

 

Thank you for your replies. I will get a price for the building of nacelles designed as Richard suggests.

We have a single Yamaha 9.9 which is not steered. I have not noticed a difference in manoeuvrability between port and starboard although I am still getting to grips with berthing in a confined space.

Richard, have you experienced issues with following seas being channeled up the nacelles? Also if you have an outboard on each side is there a reason to have these steered? Are the new Yamaha 9.9s better with respect to corrosion?

Alex

 

The Yamahas with corrosion problems were (I think) 2011 or 2012 models. It took Yamaha 18 months to agree to check out the corrosion, which they did in May

I used the steerable outboard on my Sagitta and Eclipse (same system on both) but not on my Romany, even though it was fitted. Even so we could turn tightly. obviously with twin engines you can turn on the spot, so much depends on where you keep the boat. It makes sense to have a single engine centrally mounted. usually turning to port is harder because of the prop kick. that applies to monohulls and powerboats as well of course.

The owner of the motorboat in our marina next to our Skoota asked us to move as far from his boat as possible as he said he was very un-manouverable at slow speeds, as he didn't have a bow thruster and couldn't easily turn to port

Richard Woods of Woods Designs

www.sailingcatamarans.com

 

Thank you Richard, Do you think we could base the nacelle design on the hull of an Canadian style canoe?

Alex

 

Here is a canoe mould which I hope to base the nacelles on

 

A lot of Malcolm Tennant's cats use central engine pods, my own boat is Vorpal Blade, a very close match in size to your Woods Banshee and we have a High thrust 25hp four stroke Yamaha in a central pod - see pic. It works very well and we get no cavitation even when motoring upwind in a decent chop. This motor replaced a 2 stroke yamaha 60hp with an XXL shaft and while that did very we'll in dead flat water as soon as there was any chop it was completely useless and actually quite unreliably dangerous in a lee-shore situation.
Our pod is based on a pod from a GBE albeit with a lot more reinforcing.
This same setup is used on the Tennant Turissimo 10m and 11M cats very effectively.
Here is one pic showing the stern view, motor fully tucked up I will get some more when I a at the boat today. Feel free to PM me if you want some measurements etc

 

Thanks Luff Tension. Some more info would be very helpful. I could PM you but feel that your information might be helpful to others on this forum. I have found a real lack of specific information on this issue. Cheers, Alex

 

Here you go, the pod/engine set up.
The lowest part of the pod sits about 100mm from the LWL at rest, it hits bigger waves when sailing upwind but that isn't really an issue, downwind we have never had an issue with it scooping water.
The only time the pod gets any water in it is when motoring directly upwind as the spray travels up the motor leg and deposits itself in the lowest point, solved of course with some 25mm aft facing drains.
Our bridge deck clearance at that point is about 850mm.
Overall the setup works very well, the Yam 25hp high thrust is an awesome motor, I wouldn't think a 10hp high thrust would be up to the job particularly trying to get upwind in a 25knot Hauraki Gulf chop.
You could probably get a reasonable result making a strong "semi-transom/ring frame" to attach the outboard to and them making a stressed ply or foam core fairing around it to stiffen it up and keep the water out.
Excuse the mess in the pics, the boat is in maintenance mode so there is crap everywhere, you are welcome to come and have a look PM me for details.
Cheers
Bill

 

Has anyone mentioned yet that whatever attachment method is chosen, you really need to cut to the chase and go for extra long shaft motors. Long shafts are just a WAFTAM. the extra 5 inches makes all the difference when the weather turns.