two engines or one plus kicker

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

 

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.

 

Is´nt the Fjord a double ender?

 

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.

 

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.

 

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.

 

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

 

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.

 

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

 

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

 

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

 

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.

 

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

 

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!