Displacement hull?

[Dino55]

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?

[peter radclyffe]

No, Its Due To Cost Of Fuel

[Dino55]

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.

[Dino55]

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

[daiquiri]

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.

[alan white]

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.

[Dino55]

Thank all of you very much for your input.

[FAST FRED]

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

[peter radclyffe]

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

[thudpucker]

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

[mydauphin]

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

[FAST FRED]

"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

[dmilamj]

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


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.

[apex1]

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.


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

[FAST FRED]

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

FF

[dmilamj]

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

FF

Oh I agree, I cut and pasted your rules of thumb into a spreadsheet...cause I am definitely a simple dreamer.

It was more a chance to drop a silly pun than anything else....

[powerabout]

The only thing about gas v diesel and safety is in OZ if you ask an insurance company which power is used in boats that go up in flames they will always say diesel.
Why, the boats usually have huge engines in small compartments so the engine box/room is way hotter. Many dont have water cooled turbos just covers over them, if an oil line or injector line cracks you have instant bonfire as the spontaneous combustion temp of diesel and oil is way lower than the exhaust temp.
Remember the old fill your motor bike up and drip fuel on the the hot exhaust..never goes up as gas needs about 750f to go up...
Now if you add a spark to the game its a different story....

[powerabout]

The rule of thumb that works (ed) in the US was that diesels were never really cost efficiant for the average user as gas engines were so cheap the cheaper fuel cost never factored in unless you were a commercial operator.
I thinks thats why the US never really had small marinised diesels.
What US made diesel engine under 100hp is there for a boat?
No market for a truck either

[Submarine Tom]

Perkins, I believe, are US made are they not?

[Ad Hoc]

No, Perkins is UK company
http://www.perkins.com/home

[Fanie]

Compromises compromises compromises. Can't you guys just come up with a solid answer for once :D

Some pretty neat answers there, I could not think of anything clever to add, and if Richard Apex has nothing to add then you know it is REALLY good :D

I have never been a diesel fan, but then I cannot imagine a big boat with a petrol engine.

[powerabout]

A boat is a compromise isnt it?

Perkins was owned by US GM from late 80's not sure now

[daiquiri]

Compromises compromises compromises.

That could be a perfect title for the first lesson in any course in engineering.

[FAST FRED]

"What US made diesel engine under 100hp is there for a boat?
No market for a truck either"

However anyone wanting a great US diesel can simply install an International DT 360 or Int DT 466.

These are a over 100hp rated , but are really cheap at any truck wreckers, have std SAE bell housings (Twin Disc can be fitted) and are really cheap to keep.

AS truck, or skool bus engines they take light loading and prolong ideling better than an industrial engine.
Either would do 5-6gph worth of work for a long time!! As you should expect from 1300lbs of engine.

The same engines come in mechanical injection and electric ,boat use would be safer with the older mechanical setup.

A complete rebuild kit is on sale a couple of times a year at about $700.00 US so its not like Volvo or Yanmar with gold plated parts. And the International dealers actually stock parts! Unlike V or Y who fly them in at YOUR expense.

FF

[powerabout]

Fast Fred
Sound good, I have always like IH engines.
Who makes the marinising kits?
Regards
Powerabout

[powerabout]

Perkins is ownd by CAT I see...
Cat also owns MAK in Germany so it has quiet a range of engines now.
And perkins had worked with the Japanese for years so...
parts made in Japan,( with iron ore from Australia smelted with Australian Gas?) assembled in the UK, painted yellow and sold in the USA?

[FAST FRED]

Who makes the marinising kits?

YOU DO!

Getting the flexplate and marrying the SAE2 bellhousing to a Twin Disc is all off the shelf., at the tranny store.

The only question then is what to do with cooling and the exhaust.

On a slow (non plaining ) boat keel cooling is the answer , as it has all the advantages .Can even be home made from 1 1/2 water pipe .

Se were left with the exhaust , dry stack with 2 inline truck mufflers would be first choice , cheap and quiet."Hospital Grade Silencers" is quieter but heavy 200lbs and expensive new. Used??

A sea water pump (belt or electric driven ) with an electric backup would handle cooling the exhaust enough to use rubber hose to the transom.

The truck exhaust manifold would have custom blankets made to keep the exhaust heat out of the engine room,

OR there are wet exhaust manifolds made , but at about $3000 are sorta pri$y for what you gain.

FF