Aux power for the Multihull crowd.

[w32honu]

I imagine that this has been tossed around a bit. Sorry if it seems repetitive. But trying to wrap my head around this question......... I may be off to the next big project.

Curious about dual outboard aux power mounted on sleds. I know there are a ton of cats out there with dual outboards. And I have seen a handful of Tris with similar mounts along the Vakka.......port and starboard.

Interested less in theory and more of real world experience. And I realize the virtues of an inboard diesel. Rather not debate that angle.

So if you have done the dual outboard aux thing......What did you think? Lots of cavitation? Only a bit?

Asymmetric thrust. Effective for close in maneuvering?

Power? Speed? Maintenance? Overall cost vs. and inboard?

Anything else you might think of.............

Thanks.

[Boatguy30]

Works great for well designed boats of the owners plan to SAIL. I just returned from a 4 month cruise covering roughly 1500 miles including over 300 miles on ICW. Used about 40 gallons of gas total on my 34' cat.

[Franziska]

Take those with the longest legs.
Mount them as close to the longitudinal center of rotation.
Mount as far apart as feasible.
Add wave reflector in front.
Either sliding mounts or rotating mounts pulling the legs up and inwards.
Mounts pivoting longitudinal need a very good stop or will indeed tilt up in reverse.

Works great for me.

Two engine of the same type give superb handling on a cat.

When I bought the boat I had one only and that one was offset to port.
That was terrible handling.
Adding the second is improving things dramatically (no surprise!)

I have two Yamaha 9.9 high thrust. Brillant engines with a power to weight ratio no Diesel engine installation can match.

Drawback. No hot water from them, very small alternators.

www.ladyrover.jimdo.com

[w32honu]

Thanks guys.

Asymmetric thrust and close in maneuvering? What has been your experience?

Curious if in hard reverse whether a pivoting sled might tend to rise up?

[44'cruisingcat]

We have Honda 20 XL shaft with "Powerthrust" props. T.hey're not on sleds, they're mounted on permanent brackets, and have power tilt

Close manoeuvring is fine, we have plenty of reverse thrust. Motoring is fine, maybe noisier than diesels though. I expect they won't last as long as diesels would, due to their exposure to the environment. Still, friends got 15 years out of their Honda 20's and that seems comparable to the lifespan of a lot of diesels.

Compared to diesels, they were much cheaper, they're much lighter, easier to install, take up no room in the boat, allow us to sail better due to having no prop drag and less weight. Maintenance is cheaper and easier, props are always spotless with zero maintenance, anodes last for years, they allow extremely shallow draught. The boat never smells of diesel.

On the negative side, they don't produce anywhere near as many amps as diesels can. And fitting bigger alternators isn't an option. They don't make hot water. They probably won't motor quite as well into rough stuff as inboard diesels would. maybe a bit noisier. Probably won't last as long. You have to put up with uninformed opinion....

[w32honu]

Thanks for the input. I have done a modification to our current monohull that has been highly successful. Honda 15 in a well. Fully retractable for an absolutely clean underbody. The old aperture filled in as well. The gain in performance is measurable. Especially in light air. Motor pivots to act as a "stern thruster" of sorts. Excellent for close in maneuvering. Not bad for a 12,500 pound mono. Over all it has been a big success...........despite the critics.

You do not see too many tris out there with dual outboards. That is my curiosity. I would worry a bit because the lower units would be exposed mounted along side the hull at the aft aka. Also the issue of reverse with rotating sleds.

Almost all Cats are dual powered. Many with outboards. And I figured your experiences would be relevant. Tx.

[cruisersfarm]

Originally Posted by w32honu

Thanks guys.

Asymmetric thrust and close in maneuvering? What has been your experience?

Curious if in hard reverse whether a pivoting sled might tend to rise up?

I have twin outboards in special wells, so cannot comment on sleds. Wells were easier for me to build and added no additional weight, so I chose them.

Sleds need to be buoyant so the outboards cannot be submerged even in the roughest of conditions.

I often run a single outboard on long motoring trips, lifting the other clear of the water. That's as asymmetric as it gets.

When running this way, the boat steers like a monohull, essentially. As you maneuver, you have to rely on motion through the water to get steerage from the rudders. It will also turn more sharply in one direction than the other.

When running both engines, you need no motion of water over the rudders and in fact, don't need the wheel. You can drive it like a tank or a Bobcat loader.

Seems like reverse would be the biggest issue with sleds. Do they lift and cause aerated props in reverse? Something to check out.

Don't for a minute think the power from an outboard is any different from an inboard either.

On a single 30hp outboard, I get a consistent 7.2 knots, towing a RIB, on a 50' cat. It's a performance cat, but still. I could get more out of it, but my props are pitched low for higher thrust in low speed docking type situations. I think I could get 9 knots with a proper pitch. I run out of RPMs well before I hit any wall with hull speed.

Note: My outboard props are precisely in the same location the saildrives are in my boat's design, so they behave exactly the same, except they are lighter and retractable.

[Rapanui]

I have not had twin outboards, but on our FP Maldives we had a single Yamaha 9.9 high thrust. Initially, it was difficult to manoeauvre, but then added steering lines to the engine linked to the rudders and voila, the boat was a joy to handle in tight situations. If you do not need twin engines for other reasons, steering a single outboard could be a really viable option

[w32honu]

Originally Posted by cruisersfarm

Don't for a minute think the power from an outboard is any different from an inboard either.

On a single 30hp outboard, I get a consistent 7.2 knots, towing a RIB, on a 50' cat. It's a performance cat, but still. I could get more out of it, but my props are pitched low for higher thrust in low speed docking type situations. I think I could get 9 knots with a proper pitch. I run out of RPMs well before I hit any wall with hull speed.

Cannot agree more. That has been my experience as well.

When I originally was considering the mod mentioned above I had questions about prop theory and the production of thrust.......etc. Well the slide rules came out the the engineers pounced..... in a good way. There was theoretical talk of prop travel, disc rotation, tip speeds, and torque............... you name it. It was all very interesting. And that is exactly what the Forum is so good for.

In the end the theory was that it was probably not going to work as well as the old desiel. I ended up using a high thrust prop and a basic "thrust to weight " ratio for the Horsepower. Worked so well I think I went too far with the power. Could have used a 9.9 And that is for a 12,500 pound mono.

In the end I attribute a lot of the increased performance to a clean prop. Tilted up and living clear of the water my prop is always clean. That has made up the difference and more.

[thinwater]

While wide spacing is nice, there is a limit.

* Many sailor motor on one engine to save fuel and noise; in calm conditions the speed difference is often very small. However, if the engine spacing is too wide, you have to use a lot of rudder.

* If you have 1 engine out and they are farther than ~ 6' apart, docking is a royal pain. All the boat wants to do is go in circles, since the rudders have little affect at low speeds.

* If crossing breaking bars is in your future, the wider the better. This may be your only effective steering. That is the only time I would like wider spacing.

Mine are about 6 feet apart, and I feel that is a good compromise. One engine docking is still painful, but doable. At speed, the affect on balance is tiny.

The main point, as others have mentioned, is to keep the motors near the center of pitching; so placed, they work nearly as well as inboards. Transom--Bad.

[admiralslater]

We have a Pdq 36 which uses the factory standard 9.9 yamahas . when we first started building the boats we engineered a far to complicated rotating lift mechanism. Later on we used a fixed false transom with a shaped wave deflector . The motors are about 7 feet forward of the transoms . We sail in lake Ontario which has a lot more chop than bidg waves and in the 13 years we have owned the boat Cavitation has never been and issue. as for the one engine operation we generally don't do it ,but on an ICW run I would, as the second engine provided only a couple of knots speed . As for coming into a dock with only one that would be out of necessity not choice so you would have to just do your best .
If the sled is long enough it may not lift in reverse or it you could create a simple lock . Our first cat was a Warrham 27 with a single out board on a sled that was hinged 3 feet forward ,if a big wave hit it the deflector would lift the unit up . It never swamped

[Colin A]

Originally Posted by w32honu

Thanks for the input. I have done a modification to our current monohull that has been highly successful. Honda 15 in a well. Fully retractable for an absolutely clean underbody. The old aperture filled in as well. The gain in performance is measurable. Especially in light air. Motor pivots to act as a "stern thruster" of sorts. Excellent for close in maneuvering. Not bad for a 12,500 pound mono. Over all it has been a big success...........despite the critics.

You do not see too many tris out there with dual outboards. That is my curiosity. I would worry a bit because the lower units would be exposed mounted along side the hull at the aft aka. Also the issue of reverse with rotating sleds.

Almost all Cats are dual powered. Many with outboards. And I figured your experiences would be relevant. Tx.

Would love to see some pics of your outboard setup if you have some. Thanks

[w32honu]

Hope this works....... Have not uploaded any pics onto a thread in awhile. These are older photos of the boat.

Much of the set up is similar to the work that James Baldwin has done. Check out Atomvoyages.com

[FSMike]

Originally Posted by w32honu

Thanks for the input. I have done a modification to our current monohull that has been highly successful. Honda 15 in a well. Fully retractable for an absolutely clean underbody. The old aperture filled in as well. The gain in performance is measurable. Especially in light air. Motor pivots to act as a "stern thruster" of sorts. Excellent for close in maneuvering. -----

Had a similar setup on my 35' Lodestar trimaran. It worked very well although mine did not have room enough to pivot very much.
Removing the engine and putting in the plug resulted in a speed gain of about a knot or so in most cases, maybe more, it's been a long time.

[KJThomas]

I had a single 15hp mounted under the starboard cockpit seat with a fixed oval section tapered wave deflector/motor mount under the bridgedeck. Just inboard of the hull.

Now replacing with a 9.8 high thrust in the same position. (The 15hp has died).

Maneuvering can be a problem as you have to get some speed up to steer with the rudders, and at slow speeds the motor tends to push the boat always to port, both ahead and astern.
Two motors in the same position. Under the cockpit seats just inboard of the hulls would be excellent on a larger cat.
Mine is too small (9.5m cat) to take the weight of two motors unless they were below 6hp, and then I cannot get them with electric start and remotes, which are a requirement of the First Mates.

Long term plan is to either shift the mount to the middle of the bridgedeck. (This works fine in another similar cat I have been driving). And/Or. an electric outboard on the back of the port hull, (On the side of the rudder?) for a slow speed thruster?

My experience with sleds is they are more trouble than they are worth. A tapered wave deflector mount and a long or extra long shaft makes for a nice simple set up.

Under the cockpit seats keeps the motors out the way.

Two motors with remotes are ideal if the boat is big enough. Boats with that set up are a dream to maneuver.

About 1/3 of the boats length from the stern works fine for keeping the props in the water when it is rough.

The main disadvantage of outboards is the amount of fuel you have to carry if you are cruising to a time limit. (Offsets some of the weight advantage) Have to get back for work. This summer in NZ was a lot of big swells with no wind. 15hp was about 7 litres/hour at 7 to 8 knots. 60miles = 60 litres.