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Old 01-16-2016, 08:06 AM   #2841
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Al,


No, never saw this one before. I can see why you are saying that, but in that size (46') I would have half the displacement and a 10+ knots cruising speed.


There is an interesting view of it on a related blog:





At least it has a reasonably fine entry and it is not dragging the transom in the water, but hull shapes like this have a very steep rise in the resistance curve approaching hull speed.
They can appear economical at slow speeds, but you need to be happy with what you get there and 7.5 knots cruising for a boat that size is pretty slow really.


For any given type of displacement hull, resistance and displacement are almost directly correlated. If you want long range and economy, the first thing you do is get rid of dead weight. Then you suddenly need less fuel, so smaller tanks, which makes the boat lighter again. Next, the engine can be smaller because it has become easier to push, and the boat gets lighter again...


With Wild South, we had decided to draw the line at 4500kg / 10000lbs, because we were concerned she would bounce around too much. These days she steams around with low tanks most of the time, because the issue never materialised and the weight saving makes her faster and more economical.
Also she had started with a 3-53 GM and later changed to a marinised Nissan LD28. Not only it made the boat quite a bit lighter, but it actually improved the motion.


Eric
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Old 01-16-2016, 08:55 AM   #2842
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This boat sold awhile back I think. It was an interesting conversion. John Deere main. No generator. Small window AC in the master ran off the inverter. Kind of cool in a utilitarian way.
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Old 01-16-2016, 11:45 AM   #2843
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Winnie the Pooh is kind of "famous" to those of us that are/were on the T&T email server. It is a sailboat conversion, hence the hull shape.
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Old 01-16-2016, 11:59 AM   #2844
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Interesting boats

I went to look at it. I thought it was cool, but the wife didn't share my enthusiasm. 😁

I thought it was well done, and the owner was very knowledgable IMO. Why is it "famous"?
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Old 01-16-2016, 04:14 PM   #2845
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Seaspray wrote;
"For any given type of displacement hull, resistance and displacement are almost directly correlated. If you want long range and economy, the first thing you do is get rid of dead weight. Then you suddenly need less fuel, so smaller tanks, which makes the boat lighter again. Next, the engine can be smaller because it has become easier to push, and the boat gets lighter again..."

Thanks for bringing that up ...
Many focus on props, engines and their fuel consumption curves whereas a reduction in vessel weight will probably give better results. Loose 10% disp. loose 10% fuel burn. And the best place to trim weight is while searching for a boat. Just pass up the heavy dogs. And w a smaller boat one looses speed but gains big in fuel economically.
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Old 01-16-2016, 05:28 PM   #2846
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Seaspray wrote;
"For any given type of displacement hull, resistance and displacement are almost directly correlated. If you want long range and economy, the first thing you do is get rid of dead weight. Then you suddenly need less fuel, so smaller tanks, which makes the boat lighter again. Next, the engine can be smaller because it has become easier to push, and the boat gets lighter again..."

Thanks for bringing that up ...
Many focus on props, engines and their fuel consumption curves whereas a reduction in vessel weight will probably give better results. Loose 10% disp. loose 10% fuel burn. And the best place to trim weight is while searching for a boat. Just pass up the heavy dogs. And w a smaller boat one looses speed but gains big in fuel economically.
I think what you're talking about is Semi-displacement or planing hull boats. A displacement hull boat efficiency changes only slightly based on weight. Most true displacement hull boats are about as efficient with full fuel as they are nearly empty.
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Old 01-16-2016, 05:51 PM   #2847
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No .. Talking about full displacement boats/hulls.

When I think of how much power power is required to drive a FD hull I'm think'in how much hp per ton of disp will I need. Willy (my boat) has 5 and 3.5 to 4.5 is about right. Trawlers are largely SD boats and I'm quite sure they are not linear in this reguard.
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Old 01-16-2016, 07:13 PM   #2848
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Anyone with a 50+ ton full displacement vessel will tell you that there is hardly any difference in fuel economy or speed between light load and full load.

On Semi-displacement / planing hulls, weight is critical, since you have to transition from displacement to on top of the water and it takes horsepower to do that.
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Old 01-16-2016, 07:34 PM   #2849
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Winnie the Pooh is kind of "famous" to those of us that are/were on the T&T email server. It is a sailboat conversion, hence the hull shape.
Fast Freds neighbors conversion. Very nicely done. The new owner is probably very happy.
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Old 01-16-2016, 09:00 PM   #2850
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Originally Posted by cardude01 View Post
I went to look at it. I thought it was cool, but the wife didn't share my enthusiasm. 😁

I thought it was well done, and the owner was very knowledgable IMO. Why is it "famous"?

Mark was a prolific poster on the server.
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Old 01-16-2016, 09:32 PM   #2851
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Anyone with a 50+ ton full displacement vessel will tell you that there is hardly any difference in fuel economy or speed between light load and full load.

On Semi-displacement / planing hulls, weight is critical, since you have to transition from displacement to on top of the water and it takes horsepower to do that.
"hardley any difference" in a boats gros weight means hardly any difference on any size boat. It's basically directly porportional.
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Old 01-16-2016, 10:34 PM   #2852
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Here's an interesting fishing boat in Alaska. Looks like she's ready for quite big water. Must be one of those guys that fishes 20 miles offshore. Visibility out those pilothouse ports must be fairly limited but that's what my dad had on his Navy "baby freighter".
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Old 01-16-2016, 11:01 PM   #2853
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Nice little salmon troller Eric. Not much view out the portholes but those portholes are likely to survive repeated breaking green water incidents. Only view he cares about is filling that fish hold.
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Old 01-17-2016, 01:47 AM   #2854
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I'm thinking he chose those ports because they were available at the time. Served the purpose. Nothing more, nothing less. "Get-er-done" type of thing. win-win on all fronts. +1, why not?
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Old 01-18-2016, 01:13 AM   #2855
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No .. Talking about full displacement boats/hulls.

When I think of how much power power is required to drive a FD hull I'm think'in how much hp per ton of disp will I need. Willy (my boat) has 5 and 3.5 to 4.5 is about right. Trawlers are largely SD boats and I'm quite sure they are not linear in this regard.
Exactly, purely proportional for FD hulls as long as the hydrodynamics don't change, like when the transom starts getting wet etc.
Wild South is light, but still pure displacement. It is a matter of hull shape first and foremost, it really relates to the way the hull works.

A quantity of interest when choosing a boat is the length/displacement (L/D^0.33) ratio if speed (and economy at speed) is any consideration. The rise in the resistance curve is softened/deferred, at least initially, when the L/D ratio is high enough.
It is nice to get at least 5.5 or so, preferably 6+. The best length to use in the calculation is the wetted length of the hull at speed. Displacement is volume in cubic metres.

SD hulls lose their top end a lot more quickly when loaded and a lot of the time they are not as good at lower speeds. I have come to think that trying to generate hydrodynamic lift is more a mistake than anything else, unless the objective is planing speeds. Who says lift says induced drag and the penalty is very significant.
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Old 01-18-2016, 06:38 AM   #2856
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A full displacement boat will go cheapest at about the SQ rt of the LWL.

As it takes 2-3 HP per ton (2240lbs) dumping 10% of a 50,000 lb boat is just over 2 tons.

With a diesel creating at best 16hp per gallon the removal of a 4 to 6 hp requirement is not measurable , unless on a long ocean passage where 1/4 GPH might get noticed in 50 or 100 hours of steady motoring..

The advantage of the true full displacement hull is constant ,
hull shaped for the task, no dragging transom, proper sized engine and gearing.

6K or 7K is where most folks seem to travel when its a 500, 1,000= miles of trip.
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Old 01-18-2016, 04:17 PM   #2857
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A full displacement boat will go cheapest at about the SQ rt of the LWL.

As it takes 2-3 HP per ton (2240lbs) dumping 10% of a 50,000 lb boat is just over 2 tons.

With a diesel creating at best 16hp per gallon the removal of a 4 to 6 hp requirement is not measurable , unless on a long ocean passage where 1/4 GPH might get noticed in 50 or 100 hours of steady motoring..

The advantage of the true full displacement hull is constant ,
hull shaped for the task, no dragging transom, proper sized engine and gearing.

6K or 7K is where most folks seem to travel when its a 500, 1,000= miles of trip.
Yes, but the question of the most economical speed is a little more involved. In terms of hull, L/D has a lot to do with it, the canned formula is only going to be relevant around a given value of L/D (not a very favourable one, I would add).

Because power requirements always increase faster than speed, we could naively offer that slower must be better. In fact it is not, because the engine has a fixed fuel consumption just to "run itself" for a start and there is a point where slow speed becomes more wasteful. Low load makes engines less efficient.

Same about the propeller, its efficiency at low thrust is poor: too much torque expended to overcome friction over the blades without producing enough useful work. This is also why a larger propeller is not automatically better for cruising - well on the contrary. It needs enough pitch to get good efficiency at design cruising speed and the ability to slip instead of overloading closer to the top and this creates an optimum combination of diameter and pitch.

Lighter pure displacement boats are in fact very different in many ways and they need to be analysed specifically.
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Old 01-23-2016, 01:40 AM   #2858
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Big Tollycraft

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Old 01-23-2016, 02:14 AM   #2859
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One from Aust, for the Tollycraft enthusiasts. Used Tollycraft 74 for Sale | Boats For Sale | Yachthub
Very nice BIG Tolly!! Thanks for link. - Art
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Old 01-23-2016, 10:24 AM   #2860
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Banshee;

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