two engines or one plus kicker

[arthor]

Hello all,

I am intending to build a cruiser in the 28'-30' region. The ultimate aim is to take her out into coastal waters, across the English Channel up in to the Baltic and Norwegian fjords. She will also spend a good deal of time on rivers/canals. As you can imagine, this gives me quite a wide range of power requirements. I have spoken to the designer and it can be built with engine beds for a sterndrive to be dropped in later but there is nothing to say I can't start off with an outboard well. I have pondered the benefits of outboard versus inboard and to be honest, for simplicity, interior space reasons and initial outlay, I quite like the idea of outboard power. (why is there so much panic about the dangers of petrol when we quite happily drive around in potential firebombs anyway??). It is semi displacement and the designer says 25 HP would happily get me home in an emergency.

If I was going to spend my time blasting about at sea then engine choice would be easier but I will be bimbling about at low speed a lot of the time and just want the power on tap if I need it or want it. I have heard and read that outboards don't like low speed operation. Is this the case? Is it possible to change gearing or props to offset this?
Would my best layout be something like a 200hp and a 25 aux or would a twin 90/115 layout be my best option?
I was thinking that the 25 could also be my river engine but maybe a slightly bigger one would be better. Perhaps a 200/40 hp layout??

Are most big/little layouts rigged up so that both are hooked up to steering and throttle regardless of whether they are tilted up or running?

regards

arthor

[gonzo]

The steering will be together, but the throttles and shifters separate. If you need 25 HP minimum, a bit extra wouldn't hurt, say 40. Usually you tilt the motor not in use.

[apex1]

Think about a inboard Diesel and a Controllable Pitch Propeller and the problem is fixed! Low speed, low consumption on rivers (without killing the engine), higher speed, power when demand at sea!

Regards
Richard

[arthor]

Thanks for those replies gents.

The main reason for the little engine was as an auxiliary. I just thought that running it on rivers would avoid the scenario where a spare engine gets ignored until you need it, at which point it sulks and refuses to go.
I also figure that it seems daft to have two fuels on. So a big and little outboard seemed to be a fine idea. If I have already put the beds in for an inboard then I have the option of changing later. I anticipate that I would have to get into the realms of a lot of big power usgae to justify the expense of a diesel outdrive set up. Even then, unless I go for a twin, I would still need aux power, ie the little outboard.
I am still pondering the question of whether outboards like low revs working and whether a change of gearing or prop is possible to offset this.

thanks again

arthor

[apex1]

Understand, sorry no clue about outboards! But I, for example, am fine with one engine only.

Regards
Richard

[tonyr]

I built a 24 foot lobster-style boat 4 years ago, and have run it at displacement speeds (say < 6 knots) since then using a single 50 h.p. high thrust Yamaha outboard . Rev speeds typically run in the 1400 to 2200 range, with 1800 typical. The WOT speed quoted for the engine by Yamaha is in the 5000 rpm range.

So essentially I have been running the unit at very slow speeds for years, with NO adverse affects at all. Maintenance has been as specified (oil changes, plugs, grease etc.).

It really does seem that if you are running a modern computer controlled, fuel injected gas engine that it will not foul up at extended low revs like a Diesel might.

For what it is worth, but I hope this helps.

Regards, Tony.

PS I use a 2.5 h.p. on a towed 10 foot hard dinghy as my get-you-home device. If I was running at sea regularly instead of mostly inland, I would go with two 25 h.p. engines instead of a single bigger one. If the latest Yamaha 25 h.p. high thrust had been available four years ago, that would have been plenty of power for my application, and more economical too.

[apex1]

You´re dead right tony,
the outboard (4stroke) does´nt do wrong with low revving for years. The Diesel gets killed after a while.

so you did something like this?http://www.boatdesign.net/gallery/data/500/Rumpf_1.jpg

Regards
Richard

[tonyr]

Richard, yes, it looks quite familiar. I even ballasted mine (650 pounds) to get more of a shippy feel to it.

Tony.

[marshmat]

Most outboards don't really mind being run at lower RPM. They tend to be happiest around 1/2 to 3/4 throttle, but modern 4-stroke or 2-stroke DI types will loaf along near idle quite nicely. The older, carburetor-type 2-strokes often don't handle low revs as well, though.

I have a suspicion, Arthor, that your boat is a little too large and heavy for efficient operation with outboards- you might not be able to swing a large enough diameter prop at low enough RPM. Without more data on the boat, it's hard to say.

By far the biggest killer of outboard engines is dirty/wet fuel, which usually kills twin engines within a minute or so of each other. Twins (or a kicker) as a "get home" backup only works if they have independent fuel systems, filters and tanks. Catastrophic mechanical failure of the engine itself is, by comparison, quite rare.

Being in England, you probably see quite a difference in price (and perhaps more importantly, waterside availability) between petrol and diesel fuels, which certainly strengthens the case for a single inboard diesel despite its greater up-front cost. However, your use profile would leave the diesel underloaded a lot of the time.... hence Richard's suggestion of a CPP system.

[apex1]

Richard, yes, it looks quite familiar. I even ballasted mine (650 pounds) to get more of a shippy feel to it.

Tony.

We do´nt ballast, there is a Diesel in her.http://www.boatdesign.net/gallery/data/500/Heck_2.jpg

[mark775]

You designed and built these beautiful craft, Richard? What is the notch in the transom?
I just clicked on your profile and I think the answer to the first question is "yes". Right up my alley, your work.
Oh, to the thread...I concur with Richard on that, as well - the single diesel inboard can be trusted with no need for a kicker. My personal (single engine) boat since 1984 with about 4,000 days on her has not failed to find the barn under her own power.

[apex1]

Too many flowers Mark.

Build yes, though not with my own hands. Design, no, we have NA´s and designers engaged to do the "paperwork", before the shipwrights get their hands dirty.

Nice to hear you enyoyed the Gallery.

The notch is the step from the "swim platform". There will be a door to close that.

Regards
Richard

[arthor]

Many thanks again gents. Much food for thought.
The main points I have from this are that a decent modern outboard will quite happily run at low revs for some of the time.
People tend to regard a diesel as the engine of choice for a boat in the 28-30 foot range.
It was my intention to have seperate fuel tanks and try to fill up at different pumps.
I was intending that the 40hp would run me quite happily up and down rivers and probably quite high revs and the bigger engine would be for coastal work when it would also be running at quite high revs. It would be on low revs when used on the river as I would be going slow but would mainly be used just to keep it "on the go" if we went a long time without going offshore. Each would be a spare for each other with both being the ideal engine for its prime use. Lots of 30' grp cruisers (and even bigger) only have 40-60 hp outboards over here. There is one parked up next dor at our marina.
Your points are taken regarding the diesel but if they don't like low revs either, I will still be best of getting a little outboard.
We went down to our current boat last weekend and it started when we got there but we were delayed getting on the river. Went to start it an hour later and it wouldn't start (inboard diesel). It turned out to be an electrical fault and I finally got it fixed the next day. Glad it didn't happen upstream as we don't have a little outboard on. I reckon you can now understand my paranoia.

thanks again gents

arthor

[Willallison]

Arthor - can you give us a little more info on the boat you are intending to build? The weight, in particular.
The idea of allowing for an inboard for later installation seems like a rather odd solution to me. Are you suddenly going to change your cruising habits down the track, whereby the 'current' set up is no longer a sensible approach? Also, one advantage of an outboard installation is that the cockpit sole can be lower, lowering everything else in the process. With an outboard (or 2) on a pod, you can gain a great deal of additional cockpit space (though the pro's and cons of pod installations are somewhat complicated too...).

As others have said, modern outboards - whether direct injection 2-strokes, or 4-strokes - are quite happy pottering along slowly all day long. Furthermore, they don't spit half the fuel out the exhaust like the old ones used to, so are pretty economical at low revs too.
It's impossible to make a sensible call one way or the other without a great deal more info.... if you can give it to us, we'll enjoy helping you to make the decision though!

[Willallison]

Oh - one other thing.
A single engine is pretty much always the most efficient solution. Commercial vessels, large and small, manage to safely ply the oceans of the world with just one day in, day out.
However, the single / twin engine debate is (generally) an emotional one. If you are not going to be comfortable setting off with only one propulsion source on board, then it's best to eliminate it as an option and move on to finding the best alternative

[apex1]

Re reading the posts above I just noticed, that the engine bed is thought for a "sterndrive"!!! In this case a CPP is not available. That makes the outboards a more sensible alternative.
In addition to the facts Will added.

If the intended use is a very intense sort of cruising, the common single Diesel, shaft, CPP, would be by far the best choice in every aspect. Even (no especially) in a price comparison.
Otherwise, see above....

In the mono vs twin dispute we should never forget, that commercial Diesel installations in general have much beef. (displ. to power ratio) A common yacht arrangement has´nt. But the outboards are neither the best examples in longevity! The average boat Diesel lives min. five times longer than the average OB. (make a cost comparison over the intended lifespan)

Regards
Richard

[marshmat]

Re reading the posts above I just noticed, that the engine bed is thought for a "sterndrive"!!! In this case a CPP is not available. That makes the outboards a more sensible alternative.
In addition to the facts Will added.
Yes.... the idea of designing a boat with the intent to re-power it in future, with a different engine setup than the original, seems a bit odd. And, I've only heard of one sterndrive with a CPP, and it's not entirely clear whether that particular model is real or just some engineer/marketer's dream.

In the mono vs twin dispute we should never forget, that commercial Diesel installations in general have much beef. (displ. to power ratio) A common yacht arrangement has´nt. But the outboards are neither the best examples in longevity! The average boat Diesel lives min. five times longer than the average OB. (make a cost comparison over the intended lifespan)

A valid point, Richard. However, I'm not sure how applicable this argument is for the case of a typical pleasure cruiser, which may only see 100 hours a year of running time. A good outboard can easily live for 20+ years without major servicing, used this lightly. A diesel in similar use- despite having rung up nowhere near its achievable running hours- is probably going to need major servicing due to corroded heat exchangers, deteriorating gaskets, etc. at about the same time the outboard craps out for good.

Of course, if you run the thing hard and fast 14 hours a day year-round, your outboard will probably be toast in two or three years, while a good diesel (despite costing at least twice as much up front) might get to ten years or so before needing a serious overhaul.

Having said that, I tend to come out strongly in favour of inboard-mounted diesel or gasoline engines once you get over about 180 hp. The outboard's advantages of light weight, low cost and simplicity are largely defeated when you make them too big, and while all that weight aft is OK for a fast boat, a couple of quarter-tonne engines hanging a metre off the transom can seriously hamper low- to mid-speed performance in some boats. Outboard lines are redesigned on a fairly regular basis, and some specialized parts can get pretty scarce after a while. But there are dozens (if not hundreds) of aftermarket dealers and suppliers that can get you OEM-quality or better parts for your 20-year-old 4.3 V6.

Whether you can find a sterndrive that has a chance of surviving extended saltwater use, well, that's another story. I've heard good things about the Volvo composite-casing unit, but it hasn't been around long enough to say how long it'll live.

[apex1]

Matt
>>>A valid point, Richard. However, I'm not sure how applicable this argument is for the case of a typical pleasure cruiser, which may only see 100 hours a year of running time.<<<

For exactly that reason I asked for the intended use, in the post above!

[arthor]

The boat in question is the Power 3 at the following link.

http://www.selway-fisher.com/Mc2130.htm

Hope it comes up ok. I have spoken to the designer about extending and widening it (30'x9'6" ish) and he will do the necessary to adapt the plans for a suitable renumeration as would be expected. As you will note, an option for it does include an outboard well so there is scope and some intention for outboard power there although inboard is anticipated.
My thinking thus far has been to build it to incorporate two full length LVL stringers (have spoken to the designer about this also and he has been very patient and helpful). If I build in an outboard well designed to take a 'big and little', I can get it on the river with just the little to start with. Once we venture further afield, perhaps move mooring downriver on the estuary, there will be a need for more power. That is when the big outboard comes in. If by then, we have decided to go for a diesel sterndrive, I have the necessary strength in the stringers to sit it on with modifications to the transom and a bracket for the little outboard. If we decide to go with an outboard, she is practically ready to have one fitted on.

Our usage will evolve from occasional river trips with scope for occasional estuary/coastal trips to more extended cruising much further afield when we retire (I am already 50). I realise that in 3-5 years time, I may still have a lump of plywood upside down but it will still have inbuilt options for us. By then, we may have outgrown the capability of our current boat and will have a better idea of what we will want to do with the new one and what power set up we want. I have this vision of it all but it does come out on here very well and the reality will change no doubt. I don't see us motoring flat out for long distance at sea, more of the occasional hop from one estuary to the next. This is why the big outboard seems more logical with the initial smaller outlay for something that may not get 100s of hours notched up on it. The list price of a Tohatsu 115hp TLDI is less than £9000.

You have all been extremely helpful, for which I am very grateful and I hope the above gives all a clearer idea of what is in my head.

arthor

[apex1]

So you have a design drawn for a single INBOARD! A perfect choice. Why complicating it?
A sterndrive combined with a aft cabin is at least not very clever!
A outboard well in that position is´nt a perfect solution either.
WHY?

A proper Diesel with a CPP will be about the same price range as your two outboards! But it will live for ever and another year. And (only with a CPP) you have never any hassle going slow or fast, right as you want.
All the Scandinavian fishing fleet is equipped with CPP´s. They need trolling at very low speed for long times, but then they have to go home to land the catch, pretty fast. All with the same single engine. Thats proven since ages.
And Fishermen do´nt waste money. Nor do they rely on unproven stuff.

Lookup "frenchmarine.com" and "EP Barrus" both in Britain. Both have prices on their site! Look for something in the 4,5 liter class, like John Deere or Deutz. These engines at 100 - 120 hp live forever, when combined with a CPP. That will add another 6.000 quid to you bill. Choose a "Hundested", worth it´s weight in gold. That may come out a tad over your arrangement with two OB´s. But you, and your children, will never have to replace it, and sure you´ll notice a substantial difference when bunkering.

Give it a thought! You´ll end up happier, believe me.

Regards
Richard

[marshmat]

Arthor,

You're looking at a relatively heavy boat, drawn for a single inboard, double-chine hull (sort of semi-displacement, probably won't plane easily) and with a relatively low cruising speed.

If you're really set on outboards, you can probably make them work. But I suspect that this boat will run better (and be better balanced) with a single inboard diesel, and given the way you're expecting to use it, a CPP is probably a sensible drivetrain choice. Outboards are generally meant for light, fast boats- a look at their RPM ratings, gear reductions and maximum prop diameters will make that painfully clear.

I'll probably take a bit of flak for saying this, but you don't really have to drop twenty grand on a "proper marine engine"- I suspect you could find a used tractor or industrial engine, have a good mechanic overhaul it, and buy a suitable controllable-pitch prop and gear, while spending less than (or comparable to) what you'd shell out for a suitable outboard. If you're really cheap, pick up a second- or third-hand F/N/R transmission and fixed prop, but be aware that such a setup won't do low speeds as well as the CPP.

[apex1]

No Flak from my side Matt.............

I just compared his OB at 90 horses with a inboard at 100 and found both are 100€ per pony. So, the second OB should pay half of the CPP already.
Sure there are second hand engines on the market, some unused at 3.500€ and less.
here:http://www.dieselenginetrader.com/index.cfm
The CPP though has to be taylored, means new.
But the real smile comes when bunkering! or better said when not....

[tom28571]

Another factor in the one or two set up is that in most all installations, there is only one fuel supply. Often there is only one electrical installation for both. Many, if not most problems with any engine installation is with the fuel or electric equipment and these are not usually separated for twin installations. A failure in fuel or electric affects both engines and kills the reliability factor. With two engines, there is redundancy but also twice the failure rate.

If you've got to have both belt and suspenders, I'd prefer the one main engine with a kicker solution with the kicker having its own small fuel tank. I'd probably not even mount the kicker for routine use. One friend has such a set up with the steering not coupled between the engines. When trolling or just running slow, he drops the kicker in and steers with the big one, using about 1/3 of the fuel that the big 150hp would. Not an ideal set up but, it works so well he is not inclined to change it.

[Willallison]

Ok - having looked at the website, I would have to concur with the others.
This boat is set up to operate with a small inboard engine. A little diesel would be ideal. Whilst it's true that diesel's don't like to operate underloaded for extended periods, I'd be inclined to go for a straight shaft installation. CPP is an expense I would do without - just give the engine a decent run under load every now and again.
Whilst the flat run aft suggests that the boat would sort of plane ok, the double chine is certainly not ideal for doing so - it would be wet and very likely suffer somewhat from dynamic instability. The sections also look very flat throughout, so it would likely pound quite badly too.
If you want a boat that goes fast, choose a different design.
On this basis, an aux. outboard of around 25 hp, with a lowish pitch prop, would be plenty and could be mounted on a simple bolt-on bracket. Steering can be done using the existing rudder, though obviously a separate throttle would be required. To simplify the refuelling, you could opt for one of the Tohatsu diesel outboards, though I would agree with Tom that you should serve the two engines from separate tanks.
Initially, you could indeed leave the main engine out altogether and just use the outboard. You would probably have to add some ballast to account for the shifts in weight and to offset any lost stability, but otherwise it should be no real problem

[CDK]

My Croatian neighbor is a professional fisherman. In his case that means he has 3 big trawlers rummaging the Adriatic seabed 6 days a week unless there are repairs or maintenance to be done. He does that for as long as I live here, so at least 15 years.

For his very limited spare time he uses a 30 ft Italian built open sports boat moored next to mine. It is powered by a Merc Optimax 225 2-stroke engine and since he once damaged the prop, a 10 hp for emergencies.
Before that he had a big Yamaha 4-stroke but he wrecked it when leaving the bay at full throttle a few years ago. He claims the Merc uses less fuel and is more reliable because it has fewer parts.

He watches me changing oil, cleaning props, replacing zinc anodes etc. and shakes his head: he does none of these things because his engine doesn't even touch the water so it looks as if it was installed yesterday. There is of course the fuel bill: where I fill up once in spring he starts every weekend trip at the fuel station.

But all things considered he is not the fool I though he was.

[arthor]

Just when I was thinking that I am indeed a looney, CDK comes up with that response that indicates that someone somewhere sort of thinks the way I do.
I have a feeling that if the boat gets built, it will end up with an inboard diesel and maybe a little kicker.
Of course another option is to stick with my current boat and set it up the way I want. Because of a big 6 cylinder volvo, I would not be able to lower the pilot house(??) floor so I was thinking that setting it further back and using a V box, I could free up space forward. I know that will shift weight backwards ( I am assuming with a vee box, you effectively move the engine back one full length and a bit more). The batteries and water tank is aft at the minute so shifting them forward might go some way to distributing weight.
My current boat is a Fjord 27 and some have been fitted with up to 200hp engines which give it very passable coastal performance. This means that I already own a very good hull but just need to shift the engine backwards and beef it up.
All the responses I have had seem to indicate that my build plan is not such a good idea. This forum is excellent thanks to people like yourselves. I was not looking for a favourable or otherwise response but a response and that is what I have and will adapt my plans accordingly. Does anyone think that moving the engine location on my Fjord is feasible then??

regards

arthor

[Willallison]

Does anyone think that moving the engine location on my Fjord is feasible then??


Feasible maybe.
Sensible - no.

There are a myriad of different boats out there, new & 2nd hand. One or more will almost certainly fit your needs. Go buy one.
Pouring money into changing an older boat into something that its not meant to be will see that money taking a one-way trip.

[CDK]

Just when I was thinking that I am indeed a looney, CDK comes up with that response that indicates that someone somewhere sort of thinks the way I do.



There is however a major difference (at least one) between you and my neighbor. I used the phrase "his very limited spare time" where you in post #1 are dwelling the northern hemisphere. My Croatian fisherman jumps aboard, picks up his wife and kids from the pier and leaves the bay like a bat out of hell, heading for the fuel station. When he returns a few hours later, he tilts the engine while answering a call from his cell phone and a few minutes later I hear him start his car and leave. Sometimes he doesn't get near his boat for months.

I concur with Willallison but at the same time know that there are other factors playing a role. A decade ago I took a potential buyer to our boat because I thought we didn't need it anymore, but my wife whispered "don't do it, I love this boat". So I asked a ridiculous price, the guy said no and I started making plans for replacing the twin stern drives with something completely unique that still isn't finished.

You have a sturdy Fjord 27 with a prop shaft. Newer boats look better but have an eggshell hull compared to your boat. If you really intend to use it like you describe in post #1 and have time/money to complete the conversion, installing a proper diesel with a V-drive is worth considering.
Your retirement is still several years away and in the meantime gasoline will become a still more expensive commodity. Imagine how long you can travel on a tank of red diesel!

[apex1]

CDK

this will be the first time I contradict you (as far as I remember).

But I have my doubts the Fjord will do good with shifting the weight 2 meters aft. And we should not forget, that some people enjoy the task of re- engineering and redoing everything on their boat, to a lesser extend than you.

When the original plan of building and cruising was´nt completely misunderstood, the boat will see quite a bit of travelling, and is not thought to sit in the marina for ages.
If so, a beefy (not high power) Diesel and a controllable pitch prop, is the way to go. The fuel savings alone will pay quite soon. Let alone the longevity of the engine.
When a possible engine failure is a real concern (of course it can happen, and if only a net blocks the prop), a decent kicker OB will do whats needed, and does´nt cost a fortune when replaced after 15 years of rusting away.

Regards
Richard

[liki]

Just a simple question whose answer somewhat interests me:

Which manufacturer would offer a CPP in a suitable size for a 27' power boat? Aren't the minimum propeller diameters somewhere around 70cm?

[apex1]

Hundested, Westmekan, etc.

There is no min. diameter. Though below 50cm there are´nt any off the shelve, as far as I know. But it is hard to find a CPP quotation. The manufacturers do´nt deal with end users.

Regards
Richard

[CDK]

CDK

this will be the first time I contradict you (as far as I remember).



I think you're right Richard, promise me not to make a habit of it.

Before I bow for the weight shift argument I wish to remark that I recall seeing 27 ft Fjords with Volvo-Penta stern drives. When we bought the Draco in 1986 we visited a lot of similar boats on sale and we might have bought a Fjord back then, but the Draco had twin engines and the interior was quite spectacular because the previous owner was an interior decorator and had all brand new upholstery and carpeting.
But it may also be that my mind is playing tricks on me and that the Fjords with a stern drive were different models in the same class; 1986 is a long time ago. They all shared the strange gray-greenish color: I sprayed mine white when I was fed up with the many comments about having a navy boat without a gun on the foredeck.

[apex1]

Is´nt the Fjord a double ender?

[CDK]

Arthor should know. I quickly looked on the net: what is offered in the UK has a prop shaft. I've never seen a Fjord 27 here, just the smaller open ones, although they must have been produced in serious quantities.

[mark775]

I concur with most all of the above, tho much variable proportional weight should be at the center of stability which takes the engine back and V-driven, fuel and fish and passengers amidships. as much as possible, IMO.

[CDK]

Weight shift, the simplified math:

Assume a simple rectangular hull (pontoon) is 8 m. long, 2 m. wide and has an equally distributed load of 500 kg @ m. or 4000 kg total weight.

(A).
The center of gravity is of course in the middle.
The force on each side of the CoG can be expressed as a torque of 2000x2 = 4000 kg/m.
Immersion is 4:16 = 0.25 m.

Now we move the 200 kg. engine 2 m. towards the stern. This shifts the CoG 0,1 m. to the left (B):
0.4x500 + 1.4x300 + 2.4x500 + 3.4x700 = 4200 kg/m (stern) and 0.6x500 + 1.6x500 + 2.6x500 + 3.6x500 = 4200 kg/m (bow), so equilibrium is obtained.
Average immersion of the stern side is now 2:7.8 = 0.258 m.

This calculation gives an increase of the stern side depth by 8 mm caused by moving the engine 2 m. towards the stern. For a real hull, narrow at the bow, wide at the stern the effect will even be smaller.

[Easy Rider]

If I recall correctly you guys have been talking about outboards on a strut, outboards, IOs, Vee drives, inboards, gas and diesel both except the strut example. That seems nuts to me givin a boat whose displacement is rather low with a large part of it in the powerplant. Seems to me if it was designed as an inboard all the other systems would not be responsible choices. And I know professional boat builders produce boats with the same hull with inboard and IO or OB power. That dosn't mean it's right. We amateurs are in a position to be more professional that professionals. I hear a lot of bias in these posts (especially pro diesel). I think Arthor would do well to install a nice smooth and quiet gasoline engine. There's some balancing bias and if more's needed I'd go twins also.

Easy Rider

[mark775]

My 13 meter semi-displacement boat, as an example, has a 1,300KG engine at the center of gravity. I carry 985 liters of fuel. If I use half of it, I removed 445 kilos from a point 1.5 meters from the stern (the longitudinal center of the tanks). If ten passengers decide to go below (forward), their weight, 800 kilos, shifts from, let's say, an average of amidships to 3 meters forward of amidship. The combined transfer then becomes profound, particularly in a following sea when 3 meters forward of amidships is already starting to be in the "not as much buoyancy" zone. I can very much feel how much fuel I have, if there is any free water in the bilges, and where the passengers are. I'm suggesting that to remove a variable from needing it in the stern, to where it won't matter as much, amidships, seems better design, all things being equal. To solve my dilemma, I have water tankage aft but, at times, carry more weight than if my fuel tanks were amidships. Also, a V-drive would put the mechanical noises furthe from the living space BUT that puts the engine(s) aft where there tends to be less vertical clearance - the drawback.

[CDK]

We amateurs are in a position to be more professional that professionals.


Do we have a statement of the year contest?
This beats "holier than the Pope".

[CDK]

My 13 meter semi-displacement boat, as an example, has a 1,300KG engine at the center of gravity. I carry 985 liters of fuel. If I use half of it, I removed 445 kilos from a point 1.5 meters from the stern (the longitudinal center of the tanks). If ten passengers decide to go below (forward), their weight, 800 kilos, shifts from, let's say, an average of amidships to 3 meters forward of amidship. The combined transfer then becomes profound, particularly in a following sea when 3 meters forward of amidships is already starting to be in the "not as much buoyancy" zone. I can very much feel how much fuel I have, if there is any free water in the bilges, and where the passengers are. I'm suggesting that to remove a variable from needing it in the stern, to where it won't matter as much, amidships, seems better design, all things being equal. To solve my dilemma, I have water tankage aft but, at times, carry more weight than if my fuel tanks were amidships. Also, a V-drive would put the mechanical noises furthe from the living space BUT that puts the engine(s) aft where there tends to be less vertical clearance - the drawback.

Two questions:
-What is the total weight/displacement?
-Where is the cog ?

[Easy Rider]

CDK, Im honored . ha ha. In the past (I've got a lot of past) I remember a typical style hard chine 34' FG boat w a 6cyl diesel and an I/O. The engine was mounted amidships with a long shaft back to the I/O. If it was originally a standard inboard boat (and I think it was) and the engine wasn't moved fwd a bit the CG was still aft of where it was as an inboard, but I'll bet it was still a very well balanced boat and better than original with the rudder effect from the I/O and the steering power to cope with following seas not to mention harbor maneuverability. CDK, many "professionals" probably use the same hull with inboard and/or outboard,and/or I/O power and I'm saying it's not very professional. A knowledgeable amateur (and there are many) may not need to kow tow to the bottom line and build correctly. All kinds of bad design comes out of marketing pressures or demands.

Easy Rider

[mark775]

Total weight without passengers, half fuel, is (By hydraulic pressure on Travelift) 7,300 kilos. COG by same means, 42% forward (do you mean "center of stability? - then I don't know but slightly behind that). Displacement, 18 gross, 12 net.

[CDK]

Total weight without passengers, half fuel, is (By hydraulic pressure on Travelift) 7,300 kilos. COG by same means, 42% forward (do you mean "center of stability? - then I don't know but slightly behind that). Displacement, 18 gross, 12 net.

Did I understand it correctly?

Center of gravity I define as the imaginary point where the sum of all gravitational forces act. If a small block is placed under that point, there is equilibrium. You gave that as 42% forward.
I am not sure about a center of stability. That may sound strange coming from an engineer, but I mainly studied in Holland and only finished in the UK. The terminology in the two languages is very different.

[mark775]

No, the other way. 42% of length overall forward from the stern. This is where the travelift load balances, as I recall. Sorry for the confusion. Before, I rounded the metric length - it is actually 12.19 meters. The center of Travelift then would be 4.9 m foward from the transom.
Nominal
38800

[CDK]

That's better. I had some remarks lined up for the weird weight distribution, but I'm glad I don't need them. People give you negative points for less.

The calculation for the loss of fuel weight is not so easy because it doesn't only changes the CoG but also lowers the total. The best way would be to draw a vector diagram, but the last time I made one was 45 years ago. But I can still handle a pocket calculator.

I left the data as you originally supplied them, except for the CoG of course. Substituting the new values will only marginally change the results.
The total weight of 7300 kg in the CoG at 5.46 m from the stern and 7.54 m from the bow can also be expressed as two forces at half the distances, so 2.73 m and 3.77 m. I call the bow section A and the stern B.

From 3.77A = 2.73B and A= 7300 - B you can calculate them as 4234 kg for the stern section (B) and 3066 kg for A. Multiplying each with the distance to the CoG gives 11558 kgm as the torque (moment) around the CoG. Add a zero for Newtons, I'm an old man so I don't.

The fuel tank, half filled, represents 3.96 x 500 = 1980 kgm (3.96 is the average distance from the CoG), which is approx. 17% of 11558. Emptying the tank will move the CoG forward by a little over 0.23 m, filling it completely will shift in backward 0.21 m. If it was the pontoon I drew, the angular
change would already be noticeable, but with a hull like yours the horizon really comes up several degrees when the tank(s) gets empty.

Your 800 kg baseball team, huddling around the engine, do not change the equation significantly. The total weight becomes 8100 kg, A = 3432 and B = 4698, torque around the CoG = 12938. Actually the CoG then already shifts a bit forward, but only marginally. But their contribution changes significantly if
they are placed 3 m towards the stern: it represents 3 x 800 = 2400 kgm or almost 19% of 12938 so shifts the CoG more than 0.35 m towards the bow. You'd have to open the throttle to compensate for it, more if the tank would be nearly empty (but you wouldn't do that of course).

So my advice is to tell the boys to sit aft in such a situation so they compensate for the loss of fuel weight.

It would be better to have fuel tanks located more towards the bow instead of near the stern, but that would mean sacrificing cabin space.

I wish you a happy New Year!

[TollyWally]

This last bit has turned into a very interesting thread.. I kind of wish it were on it's own. A friend of mine has a gillnetter with a sliding reel. Originally designed to provide more picking space he started using it to adjust trim. The end result, the reel is placed for optimum trim because the picking space is "good enough" regardless of reel position.

On a completely different note.

"Stupidity must be a virtue, whole industries, governments, even economies depend on it...... "

I wish I'd said that!

[mark775]

A lot of the "area M" gillnetters use sliding reels...Him ignoring it's designed purpose is hilarious! I imagine that they mostly all do that.

[arthor]

I seem to have missed the last few responses to this thread which seems to have evolved somewhat.
I have seen Fjord 27s advertised with sterndrive power up to 180hp but I think these are conversions. I have seen marinising kits for 1.9l Peugeot TD rated at 115. The kits include adaptor plates for Volvo 280 legs. A couple of those shouldn't be too heavy with the stern drives. Enough power and, with seperate fuel (and electrics?) aux enginability. I have to admit that this setup seems quite attractive to me actually though I am sure there are bags of reasons why it shouldn't.
My boat is actually a Selco, the building of which was taken on later by Fjord. They had their own 27' which was a bit different.
I have to admit that one of my thoughts when considering shifting the volvo out was the weight distribution. I have also pondered the fact that anything I put in would likely be a deal lighter and would have less of an impact on this issue. I have looked at the Yanmar and Steyr straight 4 common rails (160-180hp ish) which are a bit pricey but if it is the only major expense then I can live with that. Using a four would bring the front end of the engine rearwards leaving a bit more leeway forward for my rejigging.
What we have is big enough if the space is used more efficiently and we like the idea of the heart and soul of this boat living on with us. If I went out and got another boat, I would probably end up with something around the 40' mark and spend far too much. I am hoping that this intended project will partly cure my building/tinkering lust.
Option 1 - The Peugeots on sterndrives
Option 2 - Yanmar/Steyr 4 cyl direct replacement
Option 3 - An outboard set up.

thanks once again for all your help and advice.

[apex1]

If I were you, I would go with the Steyr.

Regards
Richard

[mark775]

We've got a few of those here, Richard. It's a strange little thing, with split block. Have they proven reliable?

[apex1]

We've got a few of those here, Richard. It's a strange little thing, with split block. Have they proven reliable?

Yes Mark, very reliable.