houseboat propulsion-- outboard?

Spouse- with occasional 'hold-this' help from me, is building a very oddball sort of homemade houseboat, which we hope will take us exploring various rivers and inland waterways around our coastal NC home. When finished, it'll be 16'x56', with flatbottomed catamaran hulls, made of 8' treated-plywood, copper-clad sections, bolted end to end. The whole thing will weigh about 11 tons. We were figuring all along, on hanging a pair of high-thrust 60 hp outboards on the back, with the idea that this isn't a planing hull, and we didn't need to go very fast. Also were considering adding a bow thruster in the form of a sideways-mounted, perhaps retractable, trolling motor installed forward, between the two hulls, for added control while docking, if necessary.

Now we've had several people--including the kid who answered the phone at Mercury, tell us this plan won't work at all, that we'll never be able to control anything that big and heavy with outboards, that we'd need much more power, much bigger, flatter props than they have. Why, his chart didn't go past 10,000 lbs.

Help. Surely somebody here has some experience and thoughts on how to decide on propulsion- horsepower, prop diameter and pitch, and who sells them.

Thanks for any thoughts.

ATR

 

56' and 11 tons is WAY beyond outboard power. A pair of small diesels, or gas inboards with lots of reduction gear swinging big props is what you want. Outboard props are way too small, and will just beat the water to a froth. As will the trolling motor at the bow. A 56' cat houseboat will have LOTS of windage, making maneuvering difficult at best. Outboards just won't cut it. Twin inboards will probably handle o.k., but you may still want a "real" bow thruster. Just my humble opinion, but probably the same as many others around here. Good luck!

 

I too would suggest a pair of small diesel inboards. You may be able to pick up a used pair; also, many of the major diesel makers (Yanmar, Deere, Volvo, Lugger, etc) offer smaller models.

An outboard's prop is generally designed to push a lightweight boat very fast. To push a heavy boat slowly, you need a prop that is larger diameter and shallower pitch. Inboards give you the flexibility to pick the right prop. And they'll probably use a lot less gas as wellas be much quieter.

Bow thrusters can get expensive, so I'd really think about how much you'd need one. I've seen them in boats as small as 35 feet, and there are also well-designed 80+ footers that dock just fine without. As with the main engines, a bow thruster for a boat this size would need a large diameter, shallow pitch prop.

 

Thanks, both, for the input. We're new at this oversized retirement project, so we're feeling our way as we go. Does seem as if it would be nice to have quiet, efficient diesel inboards, designed just for what we have. On the other hand,....

The reasons we hoped to make do with outboards are several: 1. We live near a little marina, where it seems like there's always somebody crouched down in his bilge swearing, sweating, and hitting something with a wrench. We don't seem to notice that the outboard guys spend much time tinkering. (Are today's outboards really more reliable, or do people haul their boat onto a trailer and take it to the outboard repairman?) 2. This boat isn't going to be very easily hauled out, should we encounter major problems with shaft or props. To be able to back a truck up to the dock and lift the whole unit out might be valuable. And outboard repairmen seem to be fairly easy to come by.
3. The water around here is awfully shoally-- seems as if it would be nice to be able to to push a button and lift the props out of the muck if we got stuck. 4. Finally, to be truthful, 4-stroke bigfoots are what the chief boatbuilder has been thinking of all along; going with inboards is going to require a major paradigm shift, though that may end up being the best alternative.

Am I right in concluding that our major problem will be finding props that are large enough and flat enough to take advantage of the horsepower? Or am I missing something?

If we find that the boat handles well enough in our sheltered locale and then perhaps out in the Albemarle Sound on a breezy afternoon, we really would like to venture farther afield, maybe wandering down the ICW for a couple of months, assuming by then, the nursing home will give us a furlough.

 

Yes, you are right in concluding that the main problem with outboards would be fitting an appropriate prop.
The Merc BigFoot, in the 60hp size, is intended for single engine pontoon or deck boats of about 1 tonne or less.
In general, an inboard diesel is more reliable than most outboards (most modern OBs are pretty good, though). Fixing a modern outboard often requires a computer, and at the minimum a lot of skill with a specialized engine type that isn't found in many other applications. Mechanical diesels are everywhere, as are their repairmen; their electronic counterparts are indeed a bit harder to fix but are also more reliable and efficient.
If you could find an outboard with a lower unit capable of swinging a prop of appropriate size, this would work. The key is to fit a prop that is wide enough for your boat; I'm not aware of any such stock engines although there may be aftermarket lower units that can fit a 16, 18, 20" prop.

 

Those outboards will push your houseboat, but you'll be lucky to get a few knots of speed out of them in calm water. They are designed to push much lighter boats at much higher speeds. This why they have small diameter props with fat pitch, they have to eat lots of water quickly to be efficent. A boat of this size and weight will be stuck at hull speed, which I'm guessing is around 10 knots for a 52' - 54' LWL. This needs a much slower prop speed, which isn't available in an outboard designed to push a 2,000 lb.bow rider to 40 knots. The plans should be pretty clear about what type of engines should be used. If you've designed it yourself, I wonder about the rest of the boat. I know a guy that actually does design boats and he said it's a calulation that happens pretty early in the process, not during the build. He's the one that had me concerned about "other ellements of the structure" as he put it, when I mentioned it.

 

As you noticed matching the props to this boart will be difficult.
Most 60 hp engine mfg plan on a tiny lightweight boat that goes like the dickens (plaining) and build props to suit.

NO problem , look at the suitable engines and see which can swing the largest DIAMETER prop . The highest gear reduction (3-1 rather than 2.5 -1) would be the second thing to look at.

When you have that figured out somply get a CUSTOM set of props made. Won't cost very much extra and will allow slow powering.

I would figure with moderate power the boat will cruise at 7 or 8K and not eat too much fuel. The ability to ust the shallow draft with the OB kicking up is worth any extra maint.

Be advised the engine MFG sell "commercial" versions of their engines that are better built

 

Before we scrap the original plan, we'll investigate Fred's recommendations, to see if there's an OB that will do what we want done.

Hunter, you're right, I'm afraid: It's not only a home-built, it's a home-imagined and -planned, with nothing on paper, which means most experts simply shudder, cross themselves, and resolve to stay out of our way. Husband DID build a 1:12 model beforehand, though, which worked wonderfully on the lake with a little RC motor. The boat is 50'LWL, 56'LOA. Draft is about 12", and freeboard about 3'. Or will be. Someday. It's a slow project. And yes, it's weird. But the parts that are in the water now, and some of them have been for a year, seem sturdy and dry. (Curious? Here's the saga, so far: http://reiheld.home.mchsi.com , then click on 'boat.' We take comfort in seeing what else manages to stay afloat-- like 'Son of Townhall', the collection of detritus that got all the way to Marseilles.
Husband needed a retirement project, didn't see plans that seemed like what he wanted, didn't want to fool with fiberglass, felt comfortable with plywood, already had the tools, etc, etc. So far, it seems to be going well enough. And yes, we're both learning a lot, some the hard way, and much from good people like y'all, who are willing to share the expertise. You're all invited for a plastic cup of mediocre champagne when we finally cast off the mooring lines.)

Thank you all for the advice! Keep it coming-- we can use all the brains we can borrow.
ATR

 

I just read through your site, ATR. Looks like quite a fun project. The concept of joining together prefab modules is, in fact, the same technique used to build 90,000-tonne cruise ships! This method also gives you a substantial extra safety factor in the event of a collision, since you have two separate full watertight bulkheads at each module junction.

Looking at your hull shape, I think if you can find an outboard that can swing a good wide prop, you will probably be able to get by with outboards. IF you can find one to swing such a prop, that is. I'd look for something with a gear ration of 3:1 or more and that can handle a 15" or bigger prop. Since shallow draft seems to be a big concern, I'd also suggest looking for a hydraulic or electric jack plate that can lift the motors at least 8". If you have the budget, though, a pair of four-cylinder inboards (rebuilt if you like, starting at $2500 or so each) coupled to sterndrives (also rebuilt, starting at about $1500 for Merc Alpha, perhaps $2000-2500 for a good rebuilt Bravo II which I would consider a good bet for this boat) would give you the ability to get your props out of harm's way and would be quieter and more efficient.

Modern 60hp outboards with the gearcases you need would probably run you at least $6,000 each; add a jackplate and rigging and your powertrain will probably come to about $15k. I would be very tempted to find a pair of rebuilt inboard-sterndrive units, which will be a lot more fun to install but I think you can probably come in under $12k with rebuilt sterndrives. And these engines are readily repairable by the handyman as they are very similar to carburetted car engines. (All prices USD approximate.)

 

Indeed they do. In many cases the "commercial" 60hp is actually the same model as the "consumer-grade" 80hp, detuned for longer life and with some of the more commonly replaced parts beefed up. If you're running motors for hours on end, day after day, the extra $$ for the commercial model is well worth the reduced maintenance costs.

 

The last big houseboat show, that I went to, had several large, 16' beam, 50' to 70' long, that were powered by outboards. One had twin 60s mounted about 8' apart. These boats were all aluminum and didn't have as much weight as yours but, were still heavy. They claimed to handle well, and didn,t require a thruster to dock. Good luck with your project.

 

That's encouraging! The simpler we can keep this thing, the better. George, you don't happen to remember the manufacturer of those houseboats, do you?

 

Propco (propcopropellers.com) makes some very good big blade workboat style props in low pitch/max diameters and they even have some designed specifically for houseboats with reverse hook that improves manueverability and backing down. Also, with the engines as far apart as yours you will likely turn in place fairly easily when one is in forward and the other in reverse so a bowthruster likely will not be needed IMO. I think I would go for more power, however, say Yamaha 115's with 14x11 props. Interesting project BTW.

 

A lot of big houseboats have outboard power. Look around at houseboats for sale sites and see what they are powered with. Here's one with sizable single hull houseboats and you'll notice they all have single outboards. Only one is a commercial outboard, I don't believe any of the others are anything but regular outboards. They might not be the most effecient motor for what you're doing, but they work. Sam
http://buyaboat.reachlocal.net/Houseboats.aspx

 

WOW! Too cool, I want one!