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Old 29-03-2012, 10:15   #31
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Re: Which powerboat to cruise the Caribbean?

"So ... it sounds like we have a consensus that 'Running your diesel engine at far less than maximum power is bad for the motor'. When I'm at a marina or boat show kicking tires, what questions do I need ask to come up with the answers I need (i.e. maximum RPM, maximum torque, etc)? How do I use these numbers (i.e. it's always bad to run at less than 25% for extended periods)?"

I'm not sure we do have that concensus. It has been widely debated on this forum in the past. Commercial fisherman, truckers etc even run their engines at idle extensively. The big engine manufacturers derate their engines top RPM depending on the severity of the service. (ie: a continuous duty rating will decrease the max rated RPM extensively and a Pleasure Craft rating will increase the rated RPM on the same engine) Sure you dont want the engine running at 25% rpm under load most the time.....
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Old 29-03-2012, 10:17   #32
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Re: Which powerboat to cruise the Caribbean?

If you are trying to maintain a course with a twin you will actually burn more fuel at x speed running on 1 engine vs two. As for RPM as long as you are maintaining sufficient water temp and not blowing unburned fuel out the exhaust you are not hurting them.

The boats you have mentioned seem to indicate a budget of around a half million. There is no wrong or right answer for the type of boat that is right for you considering where you wish to use it.

I only have 2 suggestions for your search. STABILIZERS!!! Second, will you ever want to fish? Think this one over real hard as a 40ft cat with the dink on an arch will make it impossible to back down that marlin and get it in the boat.

I would contact a broker in Florida and tell him the dollar range you are looking at. Then fly to Florida and spend a week looking at boats. The right one will pick you. What a wonderful predicament you have! Enjoy
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Old 29-03-2012, 10:42   #33
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Re: Which powerboat to cruise the Caribbean?

Have a look there:

http://www.mayrick.com/

designed & built in the Caribbean (St Marteen)

Otherwise:

Welcome to Nordhavn.com - Power Thats Oceans Apart
Passagemaker Lite - Tad Roberts' fast, seaworthy, fuel-efficient long-range passagemaker

All of them are long range & sober cruisers, but prices are not the same
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Old 29-03-2012, 11:16   #34
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Re: Which powerboat to cruise the Caribbean?

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Originally Posted by Cheechako View Post
... Sure you dont want the engine running at 25% rpm under load most the time.....
Percent of maximum fuel consumption is a better estimate as to how hard an engine is working rather than using RPM. Input (fuel) roughly equates to output (work). Diesels typically produce about 20 horsepower per hour for each gallon of fuel.
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Old 29-03-2012, 11:21   #35
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Re: Which powerboat to cruise the Caribbean?

Leopard 47 powercat?
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Old 29-03-2012, 13:03   #36
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Re: Which powerboat to cruise the Caribbean?

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Percent of maximum fuel consumption is a better estimate as to how hard an engine is working rather than using RPM. Input (fuel) roughly equates to output (work). Diesels typically produce about 20 horsepower per hour for each gallon of fuel.
Ok ... now I'm confused. So RPMs aren't really the appropriate measure, maximum fuel consumption is. When we're talking about diesel engines, aren't maximum RPMs and maximum fuel consumption the same thing? In my research, fuel consumption and RPMs tend to increase in a linear fashion for diesel engines. How does all this mesh with Cheechako's comments about fuel consumption and engine wear?

I assume your rationale here is that you really don't want your twin 300HP diesels running at 1250 RPMs for very long because it's taxing on the motors. This is exactly the scenario with the Beneteau Swift Trawler 44. Maximum RPM (and fuel consumption) is 3600 RPMs for those engines. In the case of the Moorings 474 Powercat, your contention is the twin 150HP motors running at 1500 RPMs with about the same speed and comparable fuel consumption is better because maximum speed and fuel consumption occurs at 2800 RPMs on these engines.

Is that an accurate assessment?
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Old 29-03-2012, 13:17   #37
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Re: Which powerboat to cruise the Caribbean?

No it is not a linear scale. A modern diesel engine will burn 1 gallon per hour for each 19 to 20 hp used. If we need 100 hp to move at 10 mph motor a may make 100 hp at 1300 rpm and motor b at 3100 rpm. Gallons per hour remain the same.
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Old 29-03-2012, 13:27   #38
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Re: Which powerboat to cruise the Caribbean?

[QUOTE=msdj69;918988]Ok ... now I'm confused. So RPMs aren't really the appropriate measure, maximum fuel consumption is. When we're talking about diesel engines, aren't maximum RPMs and maximum fuel consumption the same thing? ...
[QUOTE]

No, it's not a linear relationship, it is more parabolic. Yes, maximum RPMs have the highest fuel consumption because the most work is produced.

I don't know the relevant particulars for those boats/engines
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Old 29-03-2012, 14:31   #39
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Re: Which powerboat to cruise the Caribbean?

My opinion, a little informed, is that one needs to operate the engine withing an rpm range that does not overload the engine (lugg) or cause the exhaust gas temperature to be too low. It is possible to run the engine with the coolant water at the correct temperature but not be generatating the combustion temperatures needed to keep the valves clean, rings seated and clean, etc.....
That said, for the type of useage you're planning, find a boat you like and you'll learn to work around the minor stuff. If a flatbottomed twin engine boat that can semiplane is what you're looking for and will be happy with go for it, just learn to run up the engines on occasion to make them happy.
It's a tradeoff one way or another it's a boat.
Unless it's a power cat.......I think those are just about perfect.....if you aren't going to sail.
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Old 29-03-2012, 14:51   #40
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Re: Which powerboat to cruise the Caribbean?

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Originally Posted by Sailmonkey View Post
My opinion, a little informed, is that one needs to operate the engine withing an rpm range that does not overload the engine (lugg) or cause the exhaust gas temperature to be too low. It is possible to run the engine with the coolant water at the correct temperature but not be generatating the combustion temperatures needed to keep the valves clean, rings seated and clean, etc.....
That said, for the type of useage you're planning, find a boat you like and you'll learn to work around the minor stuff. If a flatbottomed twin engine boat that can semiplane is what you're looking for and will be happy with go for it, just learn to run up the engines on occasion to make them happy.
It's a tradeoff one way or another it's a boat.
Unless it's a power cat.......I think those are just about perfect.....if you aren't going to sail.
That's the conclusion I'm coming to as well. Go Cat or Go Home.

In all seriousness, it does seem like the power cats are well suited for this purpose. I've been perusing literature about the Leopard (Moorings) units as well as the Fountaine Pajot power cats. They're all pricey but very nice platforms for this type of cruising.
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Old 29-03-2012, 15:12   #41
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Re: Which powerboat to cruise the Caribbean?

A suggestion different from those you've received so far, you might want to look at a motorsailer. I'm refering to a boat with more room than the typical sailboat, typically shallower draft, with a bigger engine and prop and smaller sails. A boat like this, a true displacement hull, will be significantly more economical to run, within the limits of its hull speed. It will typically be much more heavily built than a semi-planing powerboat, and it will be more seaworthy. Tankage, especially defined as range, is typically a lot better. It will almost certainly be more seakindly, and this is significant if you're going to be living on it in unprotected anchorages. A last point is that being able to set some sail will really steady the boat down while making passages, and it gives you the flexibility of being able to move the boat under sail if you need to due to breakdown, etc. Flexibility and back up systems are good things.

Good luck in your search.

Paul
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Old 29-03-2012, 15:20   #42
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Re: Which powerboat to cruise the Caribbean?

Quote:
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A suggestion different from those you've received so far, you might want to look at a motorsailer.
or something like this "Diesel Duck" if I had the financial resources:

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Old 30-03-2012, 07:38   #43
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Re: Which powerboat to cruise the Caribbean?

Quote:
Originally Posted by Pauls View Post
A suggestion different from those you've received so far, you might want to look at a motorsailer. I'm refering to a boat with more room than the typical sailboat, typically shallower draft, with a bigger engine and prop and smaller sails. A boat like this, a true displacement hull, will be significantly more economical to run, within the limits of its hull speed. It will typically be much more heavily built than a semi-planing powerboat, and it will be more seaworthy. Tankage, especially defined as range, is typically a lot better. It will almost certainly be more seakindly, and this is significant if you're going to be living on it in unprotected anchorages. A last point is that being able to set some sail will really steady the boat down while making passages, and it gives you the flexibility of being able to move the boat under sail if you need to due to breakdown, etc. Flexibility and back up systems are good things.

Good luck in your search.

Paul
Thanks Paul. I'll do some research on motorsailers.
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Old 31-03-2012, 07:15   #44
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Re: Which Powerboat to Cruise the Caribbean?

During your motorsailer research drop by:

RunningTideYachts, Ltd. - Multihulled expeditionary yachts of distinction and a wonderful combination of sail & power to take you anywhere in the world, swiftly, economically, and in style.
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Old 31-03-2012, 09:14   #45
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Re: Which powerboat to cruise the Caribbean?

Quote:
Originally Posted by Sailmonkey View Post
... as a general rule, faster boats usually have a less comfortable motion caused by bottom profiles. Slow is a hole in the water, fast is a rock on top of it.
The amount of V shape in the bottom of a hull is known as deadrise. A flat-bottomed boat tends to rise onto a plane quickly and provides a comfortable, stable ride in calm water, but may pound heavily in rough water. A deep-V hull tends to provide a softer ride in those conditions, but may be more reluctant to rise onto a plane. A round (trawler style) hull tends to roll at slower speeds and at anchor (where a cruiser might spend most of his/her time).

An excellent two-part article discussing cruising power boat design:
1 ➥ Duckworks - Projects
2 ➥ Duckworks Magazine - Designing the Great Alaskan 26 - Part 2

And another article:
1 ➥ Power Boats
2 ➥ Power Boats: Boat Design Parameters (cont.)
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