outboard vs inboard

For the purposes of this thread let’s assume a boat of 25’- 35’, 4-6K pounds displacement, simidisplacement hull. I hope that doesn’t take all the fun out of it. While toying with ideas for a boat like this, I was convinced that an inboard was the best way to go. I had worked out weight reduction and a simplified drive for a gas or diesel inboard. When it came down to the details, inboards need air intake, exhaust, and cooling water systems. These systems require room and careful placement for them to work correctly. You can make an incredible amount of compromises to allow for these systems. This I think is a very strong reason to use an outboard on a boat of this type.
Gary

 

On a 25' boat, I would most likely have to go with a single diesel for cost considerations (and weight considerations), whereas I would have twin outboards. I much prefer the idea of redundancy. Even if diesels are very reliable, failures still happen.

 

I think that a very important factor in a small boat is LCG, the longitudinal position of the center of gravity. Now, two powerful outboards (~200 HP) weights ~500 lb each, don't they? (I'm not so keen on US units) This is a significant part of the total displacement of 4-6k, and that weight is put after the transom, so the LCG is shifted towards the stern, compared with an inboard solution. Having LCG after is good for fast planing hulls, but I don't think it's so good for semi-displacement ones.

Gary, cool you've been in Italy for so long, how did you like it? actually, I wasn't even born in '76

 

Those who follow these forums will have a pretty good idea of where I stand here - The pros of outboards generally outweigh the alternatives.
However, Gary has put a cat among the pigeons a little by posing the question in regards to a semi-displacement hullform. Outboards usually come into their own once a boat is up on plane and able to run freely. Heavily loaded outboards, or propped down so as to run @ around 4000 rpm but at semi-displacement speeds, will have the ability to use a fair bit of fuel. The solution, I think, is to use a hullform which remains essentially on plane down to quite low speeds - say 12 - 13k. This way, a planing hull - which, when required can still go faster - will not be loading up its powerplant whilst running at the speeds usually associated with "climbing the hump". (It's this kind of hullform I would propose for options 1 & 4 of our poll).

Further, there's a lot more that needs to be considered in choosing the powerplant. Is the boat to be home-built, for instance ( installation is much easier with o/b's ). Is it to used commercially (diesel's will tend to live longer and MAY use less fuel, but will have longer down time as they can't simply be "swapped" over).

As far as LCG goes, this can be taken care of with the location of other onboard weights.

 

Will,

You hit on the original goal for my little cruiser. That is, the ability to cruise economically in the ICW at double sailboat speed. I tended to get bored running up and down the ditch and rivers at 6kts for very long. There are only two ways to get this speed in a monohull. A semi-displacement hull or try to slow down a planing hull. The weight and economy of the semi-displacement hull made that choice less attractive to me. The other avenue has been virtually unexplored in over 50 years since engines became more powerful and lighter. The definition of "planing" tends to get argumentative but I will settle for a hull that will run level, economically and comfortably at any speed from the top end down to about 10kts. If you look around, there are some who claim that but when you look at the photos of them running at low speed, they still look bow high with the forward view restricted. For example, the fancy runabouts featured in a recent Woodenboat magazine.

Anyway, that is the goal I set and it has been achieved. I don't believe in "no compromise" boats. They just don't exist. Some may look a bit like that until you get a whiff of the price tag or some other hidden problem. Whatever design we arrive at (if it is a single design), there will be compromises of some features in favor of others.

LCG, or balance, is an overdone topic. As you say, it can be changed, and is, every time someone moves their body fore or aft. The design has to be insensitive to some weight shifts or it will be a problem. For instance if you get the boat to "plane" at 12kts and a crew walks to the stern or you hit the wake of a passing boat and the boat drops off plane and plows, that is unacceptable in my view. The boat need to have some reserve longitudinal stability to take care of that kind of thing. Most small boats don't.

Nemo, you are corrrect about the weight of those big 200hp outboards. It is not an unsolvable problem but, in my view, no lightweight cruising boat needs anything like 400hp to meet a reasonable speed. I'm happy with 50hp and can't believe that adding 6 or 8 feet to the length and 6 inches to the beam requires 8 times the power.

 

Tom,
I guess it really depends on you your target cruising speed. For the purposes of option 4, in our poll, I suggested a cruise in the 20 - 30 k region. The twin 200's may indeed a produce a cruise more in the 35 - 40k area. Option 1 is a whole different kettle of fish - a pair of 200hp o/b's is unsuitable for any number or reasons - not least of all that they'd almost swallow most homebuilders budget before they'd built a boat to hang them on!

The design that I have been working up should be equally capable of running much smaller engines - a single or twin istallation totalling say 150 - 200hp. This would obviously slow things up considerably and may (but not necesarily) lead to a more fuel efficient rig. There is some argument for this - it is not always practical, comfortable or safe to travel at 40k for any length of time - but it certainly is fun! (As soon as it's a little more presentable - & I figure out how to do it I'll post some images...)

However, we digress from the original question....... inboard or outboard. There will always be a number of other things to consider apart from the engine(s) themselves. Hot water systems, for instance are a simple installation when coupled to the vessels engine cooling sysytem. It is a slightly more complicated issue with an outboard (open to idea's here...)
And then there are the issues of aesthetics and noise - some don't like the look of a boat, particularly if it is of a traditional nature, if it has an o/b hanging off the stern (not me..). Inboards tend to sound better (?) than outboards and are often more quiet (though this is an area which can surely be addressed better than it has been generally)

 

We can put auxiliary systems in the disadvantage column for outboards. Hot water and cabin heat are high on my list as must have. It is cold here! Some spring days are the best time to be on the water. When at anchor in a quiet bay you won’t want the engine running just for heat so you still need an alternative.

As for looks, the topsides and cabin need to look like they belong with an outboard. The engine case on most outboards is one area that I don’t like. I won’t wear a shirt with someone else’s name on it, and I don’t want my boat to either. You can get costume graphics of your own choosing to go right over the manufactures logo.

Gary

 

Gary,
Not only can you address the graphics issue, but the level of noise can be addressed by having soft, sound insulating covers made to go over each motor

 

Don't have much time this morning but I'll address the noise thing just a bit. My boat has a sound-insulated, open- back box over the Yamaha T50 that totally obscures view of the engine from inside the boat. Of course, it is visible from the side and rear when outside the boat. Most people familiar with outboards remark on how quiet the engine is.

To me it is still louder than I'd like, although, at idle or slow speed, it is very quiet compared to a 2-stroke jerking and banging away and it starts and purrs with the lightest flick of the key. Now here is a negative to my boat - at speed, the engine is probably noiser in the pilothouse than when sitting in the aft seats on either side of the engine. My wife and a guest normally sit there and hold a conversation over the top of the engine box with no trouble. We do the same thing inside but vibration/noise is transmitted through the lightweight, monocoque scantlings more than I'd like. It is a negative but acceptable result of the lightweight/performance/trailerable equation. Another compromise, to be sure. Compared to the interior of most sailboats that I've known under power, it is much quieter.

 

Oops - that last post obout engine covers was from me - must remember to log in...

There was an interesting article in a recent edition of professional boatbuilder about eliminating noise, which suggested that a major proportion is as a result of transfer through the boats structure. Outboards generally employ soft mounts, but I wonder about the possibilty of soft mounting the section of transom which supports the motor.....

 

I wanted to make sure that all reading this post saw this.

www.rotarypowermarine.com

From this thread.

http://www.boatdesign.net/forums/showthread.php?s=&threadid=526


Gary

 

Here I sit with two ways to go. The rotary engine makes the I/B very attractive, at least for the US market. I'll just work in both directions until I hit a snag, or the group puts me on the right path.
Gary

 

Well for my money, there is no other choice but to go for outboard(s).
Rotaries have a well founded reputation for being frightfully thirsty for one thing.
There is also the question about reliability and parts availablility. The major manufacturers have worldwide support for their products, they are all well sorted, and if something does ever go wrong, there are plenty of people able to fix them.
There is far less maintenance to be done on an outboards leg. Not so with a sterndrive.
And of course installation for an outboard is a snip compared to any of the other options.

 

In the thread duluthboats linked to above, some fuel figures are listed that don't seem that bad:

What do you think? What would you think the GPH range would be for an similar hp outboard?

The price is higher, but not that much: ~$6900 for the engine + ~$3000 for the stern drive...

 

We operate a 26 foot deep-v which weighs around 3 tons. It has 2 x 200hp Optimax merc o/b's.
At 35 knots (about 3600 rpm) we use a total of 15 gph - 7.5 gph per motor
At 45 knots (about 4600 rpm) it rises to 20 gph total. In both instances that equates to 2 litres per n mile ( 2.3 nm/gal ).
Just as importantly, at around 6 knots, we can better 6 nm/gal.

These are figures based on my own experiences, in a relatively heavy boat, fully laden with fuel, water etc. - not manufacturers claims which have a habit of being a little optimistic......
However, if we take the figures quoted, and use the Pacemaker 26 as a comparison as it is closest in configuration, weight etc to my own:

Top speed 29K, consumption at WOT 32gph - works out to 0.9 nm/gal - 2 1/2 time as much as the outboards.

Not only that but for our project design, we have yet to establish exaclty how fast we want to be capable of cruising at. It may well turn out that we only need 50 hp (as with "Liz") to achieve our goals, in which case outboards essentially become the only option