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!

[tom4660]

I BUILT A 29 FOOT SEMI DISPLACEMENT HULL WITH TWIN OMC v6 CROSSFLOWS THE TOTAL FUEL CONSUMPTION AT 3000 RPM IS 11GPH @ 23KTS .TO USE OUTBOARDS EFFECTIVELY YOU MUST MODIFY THE HULL BY ADDING A HOOK OR FIXED TRIM TABS.THIS ALLOWS HIGHER SPEEDS WITHOUT OVER POWERING THE HULL AND TRYING TO Plane. ALSO THE ENGINES TIMING AND FUEL CURVES MUST BE MODIFIED. WITH OUTBOARDS PROPS,AND WIEGHT ARE CRITICAL.DONT TRY TO LUG THE ENGINES THEY WONT LIVE LONG LIKE THAT.

[Willallison]

Tom,
Is the 11gph you quoted per motor? If not then you are getting good economy given the technology of your engines. I assume that the boat is fairly light......
I'm not sure that I agree with about the hook in the hull, or fixed trim tabs. There aren't too many outboard powered raceboats running around with this configuration. I have a 19 foot tournament ski boat which has a slight hook. It helps keep the boat on plane to quite low speed and has a very low angle of attack - both good for water skiing, but its top end is limited as a result.
Trim tabs are designed to be adjustable in order to accomodate differing conditions - whether they be alterations in onboard weights or changes is sea state. For instance, when running with a sea, it is desirable to have more bow up trim. A hook in the running surface - whether it be as a result of hull shape or fixed tabs - would prevent this.
I'm a little confused with your remarks about preventing the boat from being overpowered and planing, yet at anything over about 7knots, your 29 footer is on the plane (1.34 x sq. root WL length).
One thing I definitely agree with you about is overloading. This applies to engines of all variety, but outboards do seem to like it the least. It is not so much a problem of over weight as it is about over propping.

[tom4660]

will you are right the boat is on plane ,what i meant was that the hull needs a bow down approach . without the tab or hook the boat ran bow up even with the outboards trimmed down.This was more evident at high speeds.The 11 gph is at cruse (total between engines) this goes up dramaticaly with rpm. At 5500 the motors burn 24gph according to flowscans.I dont run the motors above 3400 anyway, the hull doesnt like the speed and i dont like buying the gas.

[mitch]

Hi Guys! I do like the disscusion. Have you thought about small diesel i/o s. Check out the new issue of Boating. They have an artical about Reggie Fountain s new boat. With tripple Merc 500HP I/O S verses Tripple Yanmar Diesels running suface drives. It is surprising as for top speed and for higher planing speed as well as for fuel economy Neal