Lobsterboat characteristics.

I'm wondering if a lot of this stuff about slow running diesels is urban myth?

I'm thinking of grenny's that spend their whole life constantly running at 1500revs here in Europe for an output of 230vac/440. They can do tens of thousands of hours and are left running under all load conditions 24/7.

Must trucks cruise at 90km/hr 55mph at 1500 revs, and a lot of marine engines like the Cummins 6bt come straight out of trucks.

I would agree with you once you get down below 1500 revs at very low loads where the bores get glazed and won't retain lubrication on the cylinder surfaces causing severe wear and blow by into the crankcase.

 

You have remarkably good taste in production cruisers Rusty. The vessel pictured in my previous post was mine for 11 years, a great cruising boat.

Easy to maintain, easy to service, great fuel economy, roomy interior, awesome flybridge, covered cockpit. For a first try Mainship and Cherubini hit a home run IMHO.

And so yes, I believe a lobster boat hull can make an excellent platform for a good cruising boat.

 

The efficiency of the lobster boat is suspect IMO. I've never seen one that didn't leave a huge wake. Look more at the stern and aft than at the bow. Wakes don't lie.

 

There is considerable variation in the shape of "lobster boat" hulls. Many cruising boats have been built on the same hulls used for commercial lobstering in Maine. Also many commercial lobster boats are happy at over 20 knots.

 

David has a point. Conversions tend to be what they are, well burdened work boats with mahogany interiors. Some boats look to be lobster boat designs, but really are quite pared down "in the tradition of" designs, with just the aesthetic considerations employed.

 

All this talk about lobster boats only makes me hungry !

 

>Must trucks cruise at 90km/hr 55mph at 1500 revs,<

And as they go UP hills the load is varied from full throttle to nothing.

Its not the RPM that determines under loading its the percentage of load AT THAT RPM.

Loafing at 1500 for cruise with an engine and prop that can run 3000 does the engine no favors.

 

Yes! Because your still missing the point ive been trying to get through to you... that is, displacement to length ratio!!!

One thing you need to be careful of, that is, weight vs displacement... 2 different things. Weight is what the empty boat weighs. Displacement is the quanity of water displaced when the boat sits on her designed waterline and is *usually* what its weight is at full payload. You cannot compare apples with apples unless you know the dry weight of each boat your comparing. In the case of your comparison, the merry fisher weighs 4.5t empty, and sits on a sub 29ft waterline thanks to its overhanging bow. It also has a fly bridge (weight + windage). Compared to the rangeboat, which has a 39ft waterline, empty weight is less than its 4 tonne displacement, and has no fly bridge burdening it. Therefore, the displacement to length ratio is much better, and thus is more efficient, especially in the semi displacement or hump speed regime. Like i said before, there is no magic in any of this, but you need to learn what to look for as ive pointed out above. Sometimes its alot more obvious, as others have noted, the presence of a large wake behind many boats is a dead giveaway that they are inefficient at that speed...

there is some video of the rangeboat under power and where the wake can be seen here - http://www.allboatsavenue.com/le-range-boat-un-cruiser-bi-cabine-revolutionnaire/

and some more here - https://www.youtube.com/watch?v=56bcFjYyNNY

 

Having been out in Lobster boats In the UK (on the Atlantic side of the Outer Hebrides) they appear to be 2/3 the size of the US ones, with a much smaller cabin in proportion to the deck space.
They are single engined, and I have never seen one plane, 5-6 Knots would appear to be their normal maximum speed. Normally manned by one or two men.
This is a somewhat distant shot of one, http://en.wikipedia.org/wiki/File:Pabbay_and_boat.jpg

 

Rusty:

I think you could be deliriously happy with one of Rueul Parker's Commuters.

See: http://www.parker-marine.com/commuterpage.htm

 

If the GB was the daddy of them all, the Mainship 34' was the peoples choice of economical trawler that a normal middle class person could aspire to own.

Pretty looking too.

 

I think you've hit the nail on the head; deadrise, draft and a big skeg are going to produce a big wake=drag.

 

The photo I posted in post 1 shows a modified lobster boat hull with hard chines, not round bilges like a normal design. I guess this will produce more dynamic stability across the speed range.

I imagine it's these modifications that attract leisure boaters and sport fishermen.

 

+1

When we were kids we used to have a neighbour down in the west of Ireland that went fishing for lobsters in his curragh, and he used to drop in fresh ones into our holiday house every week.....

Hot melted butter on grilled lobsters.

 

It's weird that engines don't like running at constant speeds; you would imagine that varying the speed and load would be harder on a Diesel engine.

This probably why there are very few diesel engines that have successfully transferred from cars to boats.