Propulsion system selection

[amamini]

Hi everybody,
Can you help me for selection the best propulsion system for the following high speed planning boat?
L=12 m
B=3.5 m
T= 0.7 m
V= 20-25 Knot
Power = 2 x 450 HP (calculated with 50% efficiency)
Different preposition of propulsion system is as follow:
-Inboard engine + Shaft and propeller
-Inboard engine + water jet system
-Inboard engine + Z drive
-Out board system.
For more information this boat was engaged for a research laboratory to study on the marine environment and will working in Persian Gulf and Oman sea area.
I am looking for your respond

Regards

Ahmad

[CDK]

Persian Gulf and Oman sounds to me like an area where good mechanics and spare parts are scarce.

Ignoring the data you supplied, for maximum dependability I'd go for 3 big outboards plus a spare one if it is adamant that the boat stays operational. With a hoist and a basic toolbox an engine can be exchanged by the members of the crew.

[amamini]

Hi CDK,
Thank you so much for your reply. Ok, what you think about water jet system?

Ahmad

[CDK]

Originally Posted by amamini

Hi CDK,
Thank you so much for your reply. Ok, what you think about water jet system?

Ahmad

Frankly, I try not to think about water jets. But now that you've asked, here is a picture of my two brand new Berkeley jets after being submerged in seawater for nearly 3 months.
If you must be able to navigate in very shallow waters, can find someone who builds you jets from stainless steel and poor efficiency is no concern at all, then you might consider such a propulsion.

[marshmat]

Ahmad,

Waterjets only really come into their own at speeds higher than 25 knots, and when the boat will have a very large variation in load conditions. They're commonly selected for shallow-water use as well. But you pay a substantial premium for good jet units, and in many cases- especially if you tend to run at less than 25 knots- you'll take a bit of an efficiency hit.

Jet units can often be lumped into one of three categories. The first is the tiny, high-revving axial flow pumps used in jet-skis and the sport boats derived from them. The next is the Berkeley, Aggressor, Jacuzzi, etc. pumps such as CDK's, that are meant for relatively light, very fast boats. The last category, and the one you'd be looking at, is also the most expensive- industrial grade pumps designed for heavy use in salt water at moderate planing speeds; Hamilton, KaMeWa, UltraJet are examples in this category.

Three or four honkin' big outboards would be a good way to ensure that you can easily swap things out if something breaks.

But I'm not a huge fan of outboards from about 180 hp on up- mainly because outboard lines are redesigned so frequently, thus making spare parts hard to come by ten years later. A family member recently replaced a boat with a 21-year-old 4.3L OMC Cobra, when that drive finally wore out. That block is still in production and parts are readily available. Parts for a large outboard that may only be in production for seven or eight years can be hard to find, and expensive when they are found.

In the boat you're talking about, Ahmad, it'll be really important to do a long-term economic analysis of a few options. Look at a trio of big outboards- figure out how much they cost, how much fuel they'll drink, how much it'll cost to fix them. Now do the same for a pair of high-performance diesels, with props in one case, sterndrives in a second, jets in a third. Maybe give the IPS or Zeus systems a look. Figure out whether you're willing to pay more for an extended operating range. Then the answers will start to become clearer.

[amamini]

Dear Matt Marsh,
Thank you very much for your comments. That was very interesting. I will really try to do a "long-term economic analysis" for selection of propulsion system. For your information according to my study a challenge to select inboard system is the weight of the propulsion system !

Regards

Ahmad

[mydauphin]

Personally, I would try to fit a pair of diesels, straight shaft inboard. It can be done, more reliable, open area in back of boat for work. More economical, no gasoline to deal with. All depends on the mechanics you have in area. I notice in Caribbean islands, very large planning hulls 50 feet, with 4 and 5 200hp 2 stroke yamahas. They get them cheap and can fix them cheap.

[amamini]

Dear Mydauphin,
Thank you for your comment. The requirements of owner of this boat are to have two diesels straight shaft inboard. The problem is the weight and the dimension of propulsion system!

I think the "Inboard engine + Shaft and propeller" is not a good recommendation, I search the motivations to change that!

Regards

Ahmad

[Ad Hoc]

Waterjet would be a good solution. Your froude number is over 1 so a nice self contained package would suit, maybe the
FF jets
http://www.rolls-royce.com/marine/pr.../kamewa_ff.jsp
or
Ultrajets
http://www.ultradynamics.com/

[liki]

A local magazine tested stern drive, jet and shaft systems, same engines on a same boat. Yanmar diesels, can't recall exact model and 27' hull.

The outcome was that stern drive was the clear winner giving clearly best economy, quickest acceleration and earliest (engine rpm) planing. Jet reached the highest top speed and started to top stern drive in economy at around +30kn.

Shaft drive (V transmission) had no winning points in this comparison which strongly emphasized economic points.