Diesel Vs Gas Engines

[wooden head]

I would go on the high side on hp for that Connie. My 1957 youngquist is 34 feet with 300 hp bb Chrysler sometimes that seems a bit light. Mostly I run half throttle or a bit more. I can jump up on plane and run from weather at 19 knots but I wouldn't want to push that hard for long. With a 45 you have more hull speed but you also have a lot more wood to haul. I would rather have a little too much power rather than too little when I need it. Given my druthers I would run with the torque and economy of diesels unless this is a waterfront condo that won't go far.

[Squanderbucks]

Responding to pcmm, I would be surprised if the 185 HP would give you the performance that you want. My 38 ft CC Commander is all glass and I'm sure much lighter in weight than your 45 ft and it tops out at 25 MPH with twin 250 hp Detroits - cruises at 20 MPH.
If yours maxed out at 20 MPH while on plane I wonder if you would still be fully on plane at normal cruise RPM.
Mine gets on plane at about 2400 , stays on plane solidly at 2500 but normally is cruised at 2600 - 2700 RPM. Full throttle is 3000 RPM for my installation.

[rwidman]

Originally Posted by jeremiason

We are pretty much done with sailing and will convert to a Cruising Powerboat when the sailboat sells. I guess in SW Florida it becomes inevitable to own a power boat :-D

We plan on doing the Circle Loop and so gas mileage will be a consideration.

Also since we will be on the ICW and some of the canals, I believe the boat should be no bigger than 35-37 feet.

So far we have looked at a number of power boats and found that there are very few Power Boats under 40 feet with with diesel engines. When you do find diesels, the POwerboats are priced much higher (20-30%) more than gas Powerboats.

Gas Engines also have other problems besides worse gas mileage.

Gas engines don't like salt water... Around here owners of Mercruiser Engines are replacing their Exhaust Risers every 4-5 years (About $4000 each engine). Also if you get an I/O Drive, you will need to replace seals more often and again fight rust.

An option to this is looking at the new Power Cruising Boats with Out Board Engines.... The outboards are more fuel efficient and last longer than inboard gas engines.

There are several brands making boats over 30 feet with outboards, including Sea Ray's 37' Venture, which is powered by twin outboards

As I posted above, my 28' trawler has a diesel engine. It's not the only one, there are others.

Your friends with the Mercruisers are changing their manifolds and risers because they didn't get the optional closed cooling system that isolates them from salt water. Outdrives are a problem if left long term in salt water but that has nothing to do with diesel vs. gas.

I doubt outboards last longer than inboards because they typically run at a higher RPM. They also don't have the torque for heavy cruising boats, they are better at powering high speed lighter boats.

Many folks who live on their boats full time or for extended periods (like cruising the Great Loop) feel more comfortable sleeping with 200 gallons of diesel fuel rather than the more explosive gasoline. Gasoline is pretty safe if installed and maintained correctly, but diesel is safer.

Yes, diesel costs more when you buy the boat, but it will bring a higher price when you sell it.

In the end, you weigh all the options and make your own choice. Either will get you there.

[wooden head]

"Your friends with the Mercruisers are changing their manifolds and risers because they didn't get the optional closed cooling system that isolates them from salt water. Outdrives are a problem if left long term in salt water but that has nothing to do with diesel vs. gas."

This is only partly correct. The risers are going to have to be raw water cooled unless you run a dry stack exhaust. That means they will have to be replaced regularly. Most of what I see says every 5 years at least in salt. The manifolds themselves should last longer.

[tkeithlu]

CC45 must be totally confused by now. The problem is that you are getting contradictory responses that are based on different assumptions. You can move my 44' 60,000 pound steel trawler AT HULL SPEED with about 85 horsepower (I have 140; remember to leave reserve for alternators and the like). Hull speed is 1.34 times the square length in feet at the waterline, answer in knots. That's about 8 1/2 knots for both of us. That's very econominal, and great for long range cruising. The other answers assume that you want to go faster than that, and will be either climbing your bow wave (plowing, huge wake, huge fuel bill) or actually planing, which may not require quite so much power once you get there, but bunches to get past plowing and up on a plane. I don't know your hull shape, but my suspicion is that you've got a cruiser that won't plane. So, if you want to go diesel, do you want the power and cost of going 20 knots or are you willing to stick to hull speed? Then we can talk engines.

[wooden head]

Tkeithlu, what is that formula again ? I'm coming up with a hullspeed around 120knots. I'm guessing that should be the square ROOT of lwl? Otherwise I think it was a very astute post.

[tkeithlu]

I apologize. The formula for hull speed is 1.34 times the square root of length on the waterline, with length in feet and hull speed in knots. As you pick up speed, the distance between your bow and the next crest in your wake gradually lengthens. Hull speed is the last point at which your stern is supported by that wave. After that, the crest is behind your stern, the transom in down in the valley behind the bow wave, and your boat is pitched upward, bow up stern down. You are climbing your bow wave rather than motoring on the level, commonly called "plowing." It takes a lot of thrust to continually climb that hill. Trasition to planing involves climbing on top of the bow wave, and requires enough flat surface on the bottom to supply the necessary lift. Deep bottomed and keeled boats don't have that flat surface area, and do not plane, but may be much more seaworthy.

[ranger58sb]

Originally Posted by pcmm

because they're not! I haven't seen a new cruiser built in the last 30+ years that came with a gas engine! Where would you buy one even if you wanted one! no one makes gas engines for boats any longer.

???

Gas inboards are quite common in new powerboats under 36-38' or so (give or take).

Review product offerings from Sea Ray, Meridian, Cruisers, Carver, Four Winns, various offshore brands, etc etc etc. Lots of "pre-owned" Silvertons in that size range on the market with gas engines.

Volvo and Mercruiser have both just introduced new inboard gas engines, and Crusader has been a big supplier over the last 30 (at least) years...

-Chris

[ranger58sb]

Originally Posted by rwidman

Your friends with the Mercruisers are changing their manifolds and risers because they didn't get the optional closed cooling system that isolates them from salt water. Outdrives are a problem if left long term in salt water but that has nothing to do with diesel vs. gas.

Originally Posted by wooden head

This is only partly correct. The risers are going to have to be raw water cooled unless you run a dry stack exhaust. That means they will have to be replaced regularly. Most of what I see says every 5 years at least in salt. The manifolds themselves should last longer.

I'm told another reason is because the manifolds/risers/elbows are typically cast components, so raw water in at least the mixing elbows -- and probably salt-laden air in everything else nearby -- is more corrosive in that kind of system than it would be with stainless exhaust components. (Although apparently there are some after-market stainless replacement components.)

The gas owners in our club typically change their elbows -- and sometimes risers at the same time -- every 5 years in salt water. 7 years max. And even if they don't change manifols and risers, they usually dismount those and inspect closely when they're messing with the elbows... since any corrosion isn't necessarily visible from the outside.

That stuff apparently lasts longer in fresh water, but I haven't heard a definitive rule of thumb for that.

-Chris

[a64pilot]

Originally Posted by tkeithlu

I apologize. The formula for hull speed is 1.34 times the square root of length on the waterline, with length in feet and hull speed in knots. As you pick up speed, the distance between your bow and the next crest in your wake gradually lengthens. Hull speed is the last point at which your stern is supported by that wave. After that, the crest is behind your stern, the transom in down in the valley behind the bow wave, and your boat is pitched upward, bow up stern down. You are climbing your bow wave rather than motoring on the level, commonly called "plowing." It takes a lot of thrust to continually climb that hill. Trasition to planing involves climbing on top of the bow wave, and requires enough flat surface on the bottom to supply the necessary lift. Deep bottomed and keeled boats don't have that flat surface area, and do not plane, but may be much more seaworthy.

That is one of the best explanations I have ever read on the subject, well written

[a64pilot]

A simplistic way to look at cruising speed and fuel consumption is to look at your wake.
If you have any kind of significant wake, there is tremendous energy in those waves, look at how they will rock boats that weight tens of thousands of lbs and see how they crash ashore.
Well, 100% of the energy in those waves in your wake, came from your fuel tank, your burning a lot of fuel to generate waves

[kiwiben]

According to me , Gas Engine is good for small Vessel(for it's low maintenance cost), beside Diesel engine is good for mid to large vessel. Some people also have different opinion too
Cheers

[tkeithlu]

I just looked (with a friend who's on the market) at a 25 and a 27 foot Ranger Tug, both powered by small diesels, one a Cummins and the other a Yanmar. They were perfect little Looper and coastal cruisers, capable of being trailered but comfortable for two, with showers and toilets, and low enough to get under most bridges. They were called "semi displacement" which means that they'll cruise below hull speed (about 6 1/2 knots) very economically, but also plow at up to maybe three times that - but at considerable cost in fuel/mile.