Displacement hull?

In all my reading and research I haven't seen anything about the use of gas engines in larger displacement hulls, (60 to 65 ft). Is this due to the torque curve difference between gas and diesel engines? Would lower gearing help?

 

No, Its Due To Cost Of Fuel

 

Yes I understand that, but at the moment where I live, diesel is twice what gas is, and has been for over 5 years now. I do believe it is a trend for the future.

 

Thank you for your responce Cliff. I do indeed see the difference in fuel usage between the two types of engines.

 

Gas engines have a thermodynamic efficiency of about 28%, diesels arrive to 40% and more. you can find more here (don't want to repeat myself ):
http://www.boatdesign.net/forums/diesel-engines/diesel-efficiency-vs-gas-efficiency-25951.html

While the difference in fuel usage might be acceptable for small boats intended for very occasional leisure use, it becomes a huge ammount of fuel for bigger vessels.
Less fuel-efficient engines require more space for tanks (or smaller range with equal tank volume), which means more weight. More weight means yet more engine power required, which means still more fuel... and so on till you realize that your yacht has become a very nicely styled mini-tanker.

 

In a 65 footer, a diesel engine will be substantially heavier than an equal powered gas engine. This could offset any additional fuel weight, depending on the type of boat involved.
Diesels have further advantages over gas. Gasoline vapors sink and while this is no problem in a truck where there's open air under the vehicle, a boat hull traps those vapors at a low point, where they can't escape. This presents an expolsion risk diesels don't have to contend with.
Also, a marine environment (damp, salt air) is hard on electric connections and while ignition systems can be protected the diesel at least dispenses with a distributor, ignition wires, coil, and so forth.

 

Thank all of you very much for your input.

 

The biggest hassle is todays diesel will last in a good , maintained tank for years.

Gas will last perhaps a month if alcohol is in its makeup.

Gas is fine on boats up to about 40 ft where the light weight will allow plaining , and as in the Great Lakes , the season is short , the distances between marinas small.

FF

 

the obvious problem, as well is gas is a bomb, diesel is safer

 

Dino...Where in Wyoming do you have water enough for a 65 Foot boat?

 

Great question....

Having had many gas boats, and several diesel... The only gas boats I will ever have are outboards... Period

 

"as well is gas is a bomb, diesel is safer"

This is true in theory but with a couple of million gas boats for almost 100 years , many installed in inboards up to about 40 ft ,
it is a personal demon , a problem only for those with that personal demon.

The engine power a boat requires can be calculated with a very simple few rules of thumb.

Cheap displacement travel is usually at 1.15 times the sq rt of the LWL.

Every 2200 lbs of actual displacement WEIGHT! (not the USCG "document" tons , they are volume) will require 2 hp for cruising .

A 100,000 lb displacement boat will require less than 50 hp to cruise.

You should be able to get 10hp in the water from every gallon of gasoline , perhaps 16hp from every gallon of diesel.
Operating Efficiency is harder from the diesel as they don't handle light loads very efficiently , gas is easier to set up.

Plug in the LWL and actual weight of your dream boat and see if you can live with the fuel cost of gas.

Remember the maint and replacement costs on gas will be 1/4 to 1/10 of diesel , so if you only do 200-300 hrs a year (3X the US national use))

gas may be just right for your displacement boat.

And remember gas has 1/10 the mechanical noise of many diesels, so may work out better for relaxed cruising.

FF

 

Wow...that was more rules of thumb in one post than I think I've ever seen. I'll say that a good rule of thumb is that if a post has more than 5 rules of thumb, it is in danger of being over ruled.




I know, terrible pun. I couldn't help myself.

 

Hear, hear
that was a bold entry..............................so to say

 

But for the dreamer , simple concepts can help decide what is and is Not suitable.

FF