semi-displacement posibilities for open water

[RAVENNA]

My last question on this topic has to do with stability.lets see if I understand what you( my peers) are telegraphing.A bulbous bow helps very little ( up to 10%)on resistance.Its purpose is to reduce wave resistance.Minor improvements in fuel consumtion( far under 10%) at full ahead.Erics comment had me considering the addition to the bow of this 138' vessel for a moment but only for a moment.so lets recap( 1 )So with smoother performance( i.e. less bow resistance),( 2 )lower r.p.m.'s (less drag) at higher speeds ( 3 ) less reverberation ( lower r.p.m.'s),lower r.p.m.'s= less noise=quieter ( 4 )less wake.( 5 )brake the head on wave sooner = canceling a portion of the abutment energy=less pitch ..my question is what is your definition of stability? two people have commented that there is little to no effect on stability.so I do humbly inquire what it is that I am not understanding about stability.

[RAVENNA]

yes this is the vessel in question.I am not computer savy enough to do links,thank you.As my first comment stated it would be a live aboard.

[groper]

what you are not understanding is the words little effect and the emphasis on this meaning... people tote in marketing blurbs that this or that feature gives them better performance here and there.... the effect you will get will be barely, if at all noticeable in most cases, with regard to the bulb and stability...

But if your keen and dont mind spending a dollar, then go ahead and get a naval architect to obtain a plans for this hull or failing that, redraw the hull and do the appropriate calculations so that you may quantify more precisely how much more stability you will get and more importantly, how much better the diesel fuel consumption will be - this is the most important thing you should be considering in a yacht of this size with respect to the hull resistance and the effect of bulbs etc...

On the other hand, if your interested in more in stability, then you should consider other options such as stabilizing foils etc. these are much more effective at controlling stability as its primary purpose...

[kerosene]

And read some basic book like 'nature if boats' - right now you seem to mix concepts and terms.

[Eric Sponberg]

Originally Posted by RAVENNA

My last question on this topic has to do with stability.lets see if I understand what you( my peers) are telegraphing.A bulbous bow helps very little ( up to 10%)on resistance.Its purpose is to reduce wave resistance.Minor improvements in fuel consumtion( far under 10%) at full ahead.Erics comment had me considering the addition to the bow of this 138' vessel for a moment but only for a moment.so lets recap( 1 )So with smoother performance( i.e. less bow resistance),( 2 )lower r.p.m.'s (less drag) at higher speeds ( 3 ) less reverberation ( lower r.p.m.'s),lower r.p.m.'s= less noise=quieter ( 4 )less wake.( 5 )brake the head on wave sooner = canceling a portion of the abutment energy=less pitch ..my question is what is your definition of stability? two people have commented that there is little to no effect on stability.so I do humbly inquire what it is that I am not understanding about stability.

Stability has to do with the rolling characteristics of a boat, and its tendency to tip over sideways. The primary factors that govern stability are length on the waterline, beam^3, displacement, and the height of the center of gravity. You can see that beam^3 (i.e. beam x beam x beam) is the big factor here. A bulbous bow does not add any beam, and since it is totally submerged, it adds only a tiny bit to displacement, and a tiny bit to center of gravity, and nothing to length on the waterline because it is all below the waterline. Therefore, its effects on stability are miniscule.

A bulbous bow, if properly designed, may reduce the necessary horsepower required for top end speed slightly, but as I mentioned before, it can be just as easy to get the shape and size wrong as it is to get it right. Without doing model testing, it is a crap shoot. A bulbous bow can also increase required horsepower instead of decrease required horsepower. At slow speeds, the added wetted surface and displacement created by the bulbous bow can increase power requirements. Also, if shaped properly, a bulbous bow can reduce pitching motion, but again, if not properly designed, it can aggravate pitching motion by slamming harder and creating more noise and discomfort.

So, all in all, as a retrofit to this vessel, I would vote no, the risk is too great for wasting money on something that has little chance of working they way you would like it to work.

The link to the brokerage listing on the vessel indicates the gross tonnage as 80 tons. This has nothing to do with the vessel displacement, but rather it is a measure of its passenger carrying capacity. We don't know the length on the waterline, but let's take 10' off LOA to estimate it = 128'. It's a wall-sided vessel, so the waterline beam would be close to the max. beam, we'll keep it at 30'. Draft is 12'. And guessing at a prismatic coefficient of 0.5, we can estimate displacement = 128 x 30 x 12 x 0.5 = 23,040 cubic feet. Multiply by 64 lbs/cu.ft. seawater = 1,474,560 lbs displacement. Then divide that by 2,240 lbs per long ton = 658.2 long tons displacement.

Next, we can put this result in the displacement speed formula developed by Dave Gerr: Speed/Length ratio = V/Lwl^0.5 = 10.665/(displacement in lbs/Horsepower)^0.333 = 10.665/(1,474,560/800)^0.333 = 0.87. This is well below hull speed which is 1.34. So this vessel travels at less than hull speed, so she is not making too many waves. There would be nothing for a bulbous bow to do. If V/Lwl^0.5 = 0.87, we solve for vessel speed = V = 0.87 x Lwl^0.5 = 0.87 x 128^).5 = 9.8 knots. That's the maximum speed for the installed 800 horsepower.

As for positioning bow thrusters further forward, this is already a long vessel for its beam: L/B = 138/30 = 4.6. Just putting them down low in the bow will give you plenty of thrusting advantage; you don't need to add vessel length any further forward, really, in my opinion.

I hope that helps.

Eric

[RAVENNA]

thank you for clarifing.I still have no intention of adding a bulbous to this vessel. . never did.your info has been helpful in directing me towards the correct vessel thank all of you for taking the time to answer my question.I know I can be alittle dense ( or alot )

[charlierossiter]

No one has mentioned and the ship details don't seem to mention, whether the ship is/was classed. For many applications and areas of operation this ship, or any other that you buy, would have to comply with LY2 or equivalent. Since it comes from the Great Lakes, achieving sea-going certification may be somewhere between difficult and expensive. Good advice on these aspects should take precedence over niceties like bulbous bow, stabilisers, speed etc etc.

[gonzo]

The seller claims the vessel needs structural repairs. You need a survey.

[MechaNik]

Thanks Paul. A picture says more than words can. Ravenna you need more advice than a forum can provide if that is the vessel you want to take on let alone take offshore.

[Mr Efficiency]

The price should tell anyone, it ain't an up-to-scratch proposition.