outboard vs inboard

[duluthboats]

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

[David Dobbs]

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.

[nemo]

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 :-)

[Willallison]

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.

[tom28571]

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.

[Willallison]

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)

[duluthboats]

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

[tom28571]

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.

[Willallison]

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.....

[duluthboats]

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

[duluthboats]

Here I sit with two ways to go. :confused: 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

[Willallison]

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.

Rotaries have a well founded reputation for being frightfully thirsty for one thing.In the thread duluthboats linked to above, some fuel figures are listed that don't seem that bad:175c cruise data we have includes:
3.5 GPH (a large wooden displacement hull)
5.5 GPH (Kenner Vision 18 bay boat with PSI drive, top speed 50mph)
7.0 GPH (per engine, Pacemaker Wahoo 26 twin. Top speed 29kts)
6.5 GPH Ebbitide RX-19 jetboat
WOT fuel consumption is about 16 - 17 GPH.
http://www.boatdesign.net/forums/showthread.php?s=&threadid=526

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...

[Willallison]

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

[duluthboats]

:) Will,
Thanks for putting the numbers down so we can understand them. It's a good illustration of how difficult it is to do comparisons.

Also how easy it is for me to get side tracked. The boat I started this thread with, would be overpowered with this rotary engine.

Are there any other thoughts on O/B vs I/B?
Gary

[tom28571]

Will, Gary & all,

Your posts bring up another important point. It is not just the top speed that we need to be choosing. The performance over the entire range may be even more important to most people. This point is well illustrated by Gary's boat. It is very fast, probably much faster than is needed for a boat intended for cruising and the fuel use figures look very good to me for such a heavy boat. My question is, how does it perform in the transition speed range between displacement and full planing and what is the extent of that range? Gary can answer that question easily.

Although "Liz" only needs 50hp to go 22mph and runs and rides easily at ANY speed in between, some compromise was made to do this. I do not know how much power she would accept or how fast she would go with more power. The compromise in design that was made in the design of the hull bottom makes it possible to run very level with no stern squating as planing is achieved. Unlike all the books predict, the trim angle increases throughout the speed range and is greatest at top speed. I know that top speed would be increased if I could force the trim a bit higher to decrease friction drag, but the longitudinal stability of the hull is too high to allow that with the current setup.

What I have done is to spread the lift further longitudinally than is normal by the chine flats which are at a positive angle of attack relative to the rest of the monohedron planing surface. This positive angle also caused the flats to get progressively wider toward the stern so that their maximum lift is at the the stern. This is my one original fundamental contribution to the design. Actually, I got the idea from thoughts of Weston Farmer from over 50 years ago. Anyway, I compromised some top speed in favor of a better cruising speed range. These chine flats are intended to work like trim tabs to help the boat get onto plane early. The difference is that they don't have the high drag of trim tabs and are, of course, fixed so they can not be lifted as speed is increased.

Apparently, the hydrodynamic lift of the hull does not overcome the lift of the chine flats until a much higher than normal speed is reached, thus the trim angle increase comes much later in the speed range than for "normal" boats.

I don't know how fast "Liz" would safely run with more power but I expect that at some point the tendency for the chine flats to depress the bow would cause handling problems. By using an overpitched prop, I did get up to 25mph and had no problems in smooth water. Probably 30mph or so would not be a problem in relatively smooth water. The chine flats also cause the boat to turn flatter than a normal V hull. Higher power might also cause problems with quick, high-speed turning. To test the safety, I've run through tight high-speed turns but as a caution would not try to treat her like a sport boat.

Designing planing boats is much more demanding than displacement or semi-displacement types and I have much to learn.

[duluthboats]

Ok, to continue this, I’ll be more specific.
LOA 9m
Beam 2.5m
Displacement (using Tom’s definition) 2000 kg
(Displacement = full cruising weight including all cruising stores, gear, fuel, water and crew.)
Cruising speed 16 kts
Top speed 20 kts
These are targets for my boat. Not to be confused with Option 1, which is still up for debate. :D

Gary

[Willallison]

Tom,
I assume you were actually referring to my boat, not Gary's, but the answer to your question - as I'm sure you already know;) - is it doesn't perform at all well in the lower speed range. Below about 22 knots the boat wants to fall off plane. Even with 400hp on the back, it struggles even to semi-plane. That is as a result of the way the the whole rig is set up.
But then, that isn't what the boat was designed for - the hull is an adaptation of an early Formula deep-vee, it was built by a somewhat infamous abalone poacher (not me:D ) so was built in order to carry a heavy load of illegally caught fish as fast as was possible. When we bought the boat (with two older carburetted o/b's which incidentally used almost twice the fuel..) it would do 60mph but would struggle to remain on the plane below about 30. We've altered the engine set up somewhat, so in spite of having much bigger engines (3Litre as opposed to 2.5) we still run a top of around 60mph, but can maintain slightly lower speeds as well. The boat is now under propped in order to achieve this - we estimate that with a light fuel load (it can carry 800Litres), the motors lifted up a bit and higher pitch props we could pull closer to 75mph.

As I said before, I think we need to consider some more basic design objectives before we can decide on what form of propulsion we use. I'll start a new thread on this

[tom28571]

Thanks Will,

Yes I made a mistake about who's boat I was talking about. It's not a knock on your boat but it does illustrate really well the other end of the performance envelope from where we are interested for a cruising boat. A ride in your boat would be exciting, especially for one just recently lured away (at least partly) from sailboats. I wish we had more input from the other guys on just what speed range we should be interested in. No serious discussion of hull design can be started without that nailed down, or, at least, a desired range which might have to be modified by reality. In many areas where we would like to cruise, your boat would be restricted to low displacement speed by no-wake rules.

While the beam limit of 2.5m has been established, the most important hull dimension for a planing boat is the waterline beam at the hull CG. For one thing it's about the only waterline dimension that does not change at different speeds.

[Willallison]

Couldn't agree with you more Tom.
I'm not suggesting we should adopt anything like "Force Eleven" for our coastal cruiser - it makes an excellent coastal cruiser so long as your happy zip from one spot to the next when it's relatively calm. The boat's capable of belting along at 40 knots in just about any weather - its occupants are not!

[Portager]

I read this entire thread, but I still don't know what the conclusion is.

I think the speed selection drives the engine selection. If it is a high speed planing hull it favors outboards, but displacement and semi-displacement favor inboard.

On the topic of single versus dual engines. I have read that 80% of engine problems (at least for diesels) are caused by fuel problems. So having dual engines may be a false sense of security unless you have separate fuel systems and you refuel at separate docks.

Does anyone have data comparing the reliability of inboard versus outboards? While I on the subject what about weight or power to weight ratio and specific fuel conversion efficiency?

The ability to connect auxiliary systems favors inboards, but back-ups are also necessary. I know of diesel heaters/water heaters that provide space heat and hot water directly from diesel fuel. Is there an equivalent gas system?

One option I have seen is to use electric auxiliary heat. This provides heat from shore power when your connected. You can also provide some heat from the battery bank and delay starting the main engine. In addition when you do start the engine it can provide heat and recharge the battery bank.

Cheers;
Mike Schooley

[Willallison]

We haven't actually reached a conclusion on this thread yet Mike, discussion will no doubt resume once we have resolved the issue of Option One's speed profile, which is up for review. As you suggest - by changing the desired speed range, we may come up with new requirements for our propulsion system.
As far as auxhillary systems are concerned, there are a multitude of possibilities regardless of which engine package is incorporated. You mention heat or hot water, for instance. I know of a number of local boats - both petrol and diesel - that use gas (as in LPG - do you guys call that butane in the US?) for their heating and hot water. Just an example, but one of many.

[Portager]

Here is a FAQ page on LPG. http://www.e-lpg.com/product.asp

LPG stands for Liquefied Petroleum Gas and is generally a mixture of Propane (chemical formula C3H8) and Butane (chemical formula C4H10). In some cold areas they adjust the mixture ratio of propane and butane from winter to summer since the boiling point of butane is very close to 0 degrees C and the vapor pressure drops off drastically at low temperatures.

The problem with LPG, propane and butane on boats is the gas is heavier than air so it will settle in the bilge and 1 liter of liquid will form approximately 250 liters of vapor, so it doesn't take a very large leak to create a lot of vapor.

If LPG, propane or butane are stored and/or used on boats then they should be stored in an external locker per ABYC requirements and gas detectors should be provided.

It makes much more sense to me to use the waste heat from the main engine to heat water and the boat and eliminate the propain storage (yes I meant to misspell it) requirement.

Cheers;
Mike Schooley

[Willallison]

This is true for an inboard engined boat - indeed it is for that very reason that I use a methylated spirit cooker and heater on my boat. It is particularly important on petrol (gas) powered vessel where the risk of spark is much greater than diesel engined boats.
But given all this, then how do you go about heating water aboard an outboard powered cruiser? With the high tech nature of modern outboards, I wouldn't have thought that modifying the route of the raw water cooling would be an option.......:confused:

[willibuch3]

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
I am now building Ted Brewer's 34 ft Quiet Times and plan to power with one of the 4-stroke outboards. Power plans aren't firm but I am certainly 75% convinced that for the reasons you raise it makes good sense. One of the more important factors for me is what I hope will be much quieter operation. I believe the noise reduction will be real for the following:

1. Less vibration because of 6 cylinders on a 100 to 150 hp engine.
2. The engine will be on the transom far from the enclosed cabin where the steering station will be located.
3. The manufacturer's hype indicates reasonable fuel economy but I will have to gain some experience to be convinced.

[Willallison]

hmm....this is a tough one.
The new generation of 4-stroke o/b's are pretty bloody good - quiet, efficient and light. OTH, so are the new lightweight diesel sterndrives that are just coming on to the market - mercruiser make a terriffic little one - about 120hp I think, and the new volvo-penta's look even better. You can also swing a bigger prop, which is something that always concerns me when running o/b's on larger boats. Don't know the weight of the "Quiet Times", or whether Brewer designed it with o/b's in mind - but it would be well worth your while to consult with him on any changes you plan to make to the original plans.

As for noise, I think an inboard (or sterndrive) in a well insulated engineroom will always be quieter than an o/b - though a 'sound board can go a long way to reducing the difference.

[tom28571]

Will,

I see that this thread did not die completely. I think the best solution to a quiet inbard engine is use CV joints in the drive shaft so that the engine can be isolated on cushioned mounts. On a big heavy boat with huge engine bed stringers it is not as important but, on a lighter boat, it is difficult to absorb the vibrations and they move easily through the whole boat. I like outboards too but would not think them best for a large boat. There must be some point where the advantages of the inboard overcome the disadvantages as size and weight goes up and area of intended use expands.

[willibuch3]

Ted brewer has provided a supplemental drawing for the outboard installation. I must soon make a decision for or against the outboard since spring is approaching and I will begin setting up the frames soon. The outboard transom frame is slightly different from the inboard frame in that the frame timbers are about 1/2 inch thinner. However, the planking(plywood) is double the thickness of the inboard version.
It seems like so many important things to worry about when building a boat but maybe that is what we enjoy.

Bill(willibuch3)

[Portager]

Ted Brewer's Quite times http://www.tedbrewer.com/power/quiettimes.htm# is a nice looking boat with a full head and nice sleeping accommodations. In a boat that is intended for over night stays, I would want a battery bank to run reading lights and maybe a fan and hot water for the shower. That would lead me to the inboard. On the inboard you can mount any size alternator you want to recharge the battery bank and you can use engine cooling water to heat a hot water tank via a heat exchanger. There is no good way to get hot water from an outboard and the alternator is very limited. If you went with the outboards, then you would also need a generator to heat water and recharge the battery bank. Better to go with a single inboard that does it all.

Regards;
Mike Schooley