semi-displacement posibilities for open water

[RAVENNA]

the vessel I'm interested in is 138' with 12' draft steel hull,weighting in at 80 tons with a beam of 30'.Not sure if this is classified as a semi or full -displacement? I'm hopeful to convert this to a live aboard yacht.Hull needs bow trusters.its a twin engine detriot.The question I have (first of many )is it recommended to have a bulbous bow for open water? I understand The benefits of the bulbous bow in reguards to fuel and speed but overall stability is where I get alittle grey.I haven't committed to the purchase yet .My overall plan is mission trips to Africa with enjoyable stops in protected waters.

[kerosene]

I am a total amateur - however...
I seriously doubt bulbous bow is going to work well as an after thought in a hull that was designed to have it.
Secondly those work in narrow speed ranges.

Mission trips to Africa!? I hope you burning your own money.

[RAVENNA]

my concern was more along the line of stability.this vessel is well within my price range so some refit is also.

[Mr Efficiency]

Must displace a lot more than 80 tons with those dimensions ?

[Eric Sponberg]

Ravenna,

You don't give us much information to go on, so we cannot answer your questions. Whether it is a semi-displacement boat or a displacement boat depends on the shape of the hull and the total horsepower installed in comparison to the vessel weight. If you can provide those, we would be better able to judge.

At 138' long, chances are it is a displacement hull--generally there is little need to make such a large vessel go so fast as to reach semi-displacement (or semi-planing)--the fuel consumption would be very high.

A bulbous bow is not going to affect stability much at all--that should be the least of your worries. And if the boat does not have one now, to design one and build it onto the boat is going to be a major investment on which you will likely see little return. If you are lucky, you may get 5% to 10% less resistance to the hull when running at top speed, but that will maybe translate into only a fraction of fuel savings. It will take you a long time to recoup that investment. And that is if you are lucky with your bulbous bow design. Bulbous bows are tricky--you can get them exactly wrong rather than exactly right. Bulbous bows are very sensitive to shape and proportions, and the only way to get them exactly right is to conduct extensive model tests that encompass a number of different design parameters. The testing alone is expensive.

If the engines are in good shape and running well, then maybe that is the best you can expect. If you have to replace the engines, money would be well spent to replace them with newer and more efficient models. If you want to save more on fuel and be more efficient, then you may want to consider some hybrid options. And before this brings out the detractors against hybrids, let me qualify this recommendation: You have to look at your mission. Besides going across the Atlantic to get to Africa, will you be spending a lot of time at slower speeds and maneuvering? Full diesel engine power will get you across the ocean in the most efficient way. But if you will spend a lot of time motoring slowly in and out from port to port or in out of the way areas, then having a parallel electric drive might be a suitable "greener" option. Money spent directly on a new power package will be far more effective than money spent on a bulbous bow.

If you would like to learn more about my experience with steel vessels and bulbous bows, you can go to my website and look at the Moloka'i Strait vessels:

Moloka'i Strait 65: http://sponbergyachtdesign.com/Molokai65.htm

Moloka'i Strait 72: http://sponbergyachtdesign.com/Molokai72.htm

Moloka'i Strait 79 http://sponbergyachtdesign.com/Molokai79.htm

I hope that helps. Give us some more information about this vessel that you are considering buying, and we'll try to offer more advice.

Eric

[Wavewacker]

Originally Posted by kerosene

Mission trips to Africa!? I hope you burning your own money.

If it's a religious mission.....LOL, probably not. I have never seen any religious mission run off the pockets of one person. I'd bet there is a non-profit behind it and contirubtions paid. The most economical way to get to Africa would be to fly commercial or charter, sounds like a way to "net out" a yacht in the end.

[RAVENNA]

Thank you Eric your link was very helpful.As of yet getting info on the vessel has been difficult as it is moored 2500 miles away and I'm dealing with a middle person.there is many things to consider in my purchase but I definetly want to use/refit an older vessel.I love to see them recommissioned as yachts.

[RAVENNA]

I believe it to be a soft chine and the power out put is 800 hp

[groper]

for a vessel that size, 138' length 12' draft, soft chine, 800hp puts it well and truly in the full displacement hull category. Adding more horsepower will net you little/zero gain, unlike a planning hull the speed of a displacement hull is governed by the hull shape and length rather than the horsepower applied- adding a bulb will also net you little as eric already mentioned and has nothing to do with stability.

Spend your money on the refit and bringing it all up to sound physical and mechanical condition if the hulls performance, as it is now, is satisfactory to your needs. If the hulls performance DOES NOT meet your needs, find another hull... You will never modify the hull enough to achieve any real gains unless you change the entire hull shape - which is the same as building a new hull from scratch - you get the idea?

[gonzo]

Are you looking for a yacht or a "mission vessel". They are totally different. There are lots of people claiming to have missions or some kind of non-profit organization but only want a tax break. You need an on site surveyor to get a proper opinion.

[kerosene]

Originally Posted by Wavewacker

If it's a religious mission.....LOL, probably not. I have never seen any religious mission run off the pockets of one person. I'd bet there is a non-profit behind it and contirubtions paid. The most economical way to get to Africa would be to fly commercial or charter, sounds like a way to "net out" a yacht in the end.

Exactly my point.

[RAVENNA]

I thank all for their input.I was to the understanding that the bulbous bow reduced pitching in heavy seas.With fuel economy and less wake put aside the wider hull in front would allow for the trusters to be placed futher forward maximizing their efficency.I truly appreciate the structual advice and links.(as for ethical comments this is a free world and you are allowed your opinion)

[RAVENNA]

P.S. I never said anything about adding to this vessel( bulbous bow) merely what direction(ship to suit my needs) to look toward.

[gonzo]

The bulb's main function is to reduce wave making resistance.

[pbury]

I assume this is the vessel:

http://au.yachtworld.com/core/listin..._id=26664&url=

Looks like a project... and a half.

Paul