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Old 12-26-2013, 02:10 PM   #41
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....Sooooo, instead of 1.34 x Square root of LWL = full displacement Hull Speed.

I have chosen to cruise at 1.1 x Square Root of 40.2 which is my LWL (6.97 almost 7 knots correct?)
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Old 12-26-2013, 05:07 PM   #42
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Portuguese,
A big yes in my opinion. Or as so many are saying these days "perfect".
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Old 12-26-2013, 07:49 PM   #43
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....Sooooo, instead of 1.34 x Square root of LWL = full displacement Hull Speed.

I have chosen to cruise at 1.1 x Square Root of 40.2 which is my LWL (6.97 almost 7 knots correct?)
I'd have to go back and look at Voyaging Under Power to confirm, but I think it's a little different than what you said.

I've always understood it to mean 1.1x the "Displacement speed", where the displacement speed is 1.34 x sqrt of LWL. I'm not saying it's the right or wrong way to look at things, but they calculate the Displacement speed, then look at going 0.8 x displacement speed, 0.9x 1.1x, etc.
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Old 12-26-2013, 08:54 PM   #44
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I've always applied the "not so constant" right in the formula...1.1 times the square root of the waterline length.
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Old 12-26-2013, 10:06 PM   #45
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Thank you Fernando, you clarify a question
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Old 12-26-2013, 10:06 PM   #46
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Well.......Ebbtide is happy at 1650 rpm and varies between 7 and 8 knots depending on the conditions. We average 7.5 kts most of the time and burn an average of 1.75 gallons per hour. 120 Lehman, 4 blade prop 24x16 LH, full fuel and water tanks.
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Old 12-27-2013, 05:52 AM   #47
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A DD 8-71 will burn significantly more fuel to generate 70 HP as compared to a John Deere 4045T (4 cyl).

The use of a DD 3-71 would have the fuel burn near identical. DD requires operation over 60% of rated power for efficiency.

The SQ rt of the moving LWL is known as SL.

SL times 1.34 or so is where only sailboats go with displacement boats , as a big down wind is free.

Most marine motorists will refuse to pay for the fuel needed to dig a big hole in the water and attempt to climb over the bow wave.

Depending on how FAT and HEAVY the boat is the rule of thumb for Long Range Cruise is SL x .9 to SL x 1.15.

Somewhere with a normal boat the best compromise of speed and range will be found there .

Skinny very light boats with a Length to beam ratio of at least 6-1 can travel a bit faster with the same fuel burn .

8-1 seems the limit unless high expensive speeds are going to be the normal operation , like a ferry.
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Old 12-27-2013, 01:07 PM   #48
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A DD 8-71 will burn significantly more fuel to generate 70 HP as compared to a John Deere 4045T (4 cyl).

The use of a DD 3-71 would have the fuel burn near identical. DD requires operation over 60% of rated power for efficiency.

The SQ rt of the moving LWL is known as SL.

SL times 1.34 or so is where only sailboats go with displacement boats , as a big down wind is free.

Most marine motorists will refuse to pay for the fuel needed to dig a big hole in the water and attempt to climb over the bow wave.

Depending on how FAT and HEAVY the boat is the rule of thumb for Long Range Cruise is SL x .9 to SL x 1.15.

Somewhere with a normal boat the best compromise of speed and range will be found there .

Skinny very light boats with a Length to beam ratio of at least 6-1 can travel a bit faster with the same fuel burn .

8-1 seems the limit unless high expensive speeds are going to be the normal operation , like a ferry.
Fred - Happy Holidays!

With your in-depth calculation methods how would you recommend to calc our Tollycraft's "hull speed". Also, what do you feel our boat's hull speed actually is (before it begins pushing water to climb up onto plane)?

Boat Stats: Loaded weight 21K lbs +/-, OAL 34', moving LWL 32', OAH from WL 12' 6", Deck Beam 12' 6", Bottom beam 10' 6", 2' 9" draft, dead rise 11 degrees, short keel with 9" max depth 5' from stern, full length hard chine...

With our Tolly's (off plane) moving LWL 32' I mathematically calc "hull speed" at 7.58 knots. Having run different speed-to-fuel use tests I've found that while using twin gassres both running an economical fuel-use speed is 6.5 to 7 knots... 2 +/- nmpg With only one screw running most economical speed is 5 to 5.5 knots... 2.75 +/- nmpg. On full plane at 16 to 17 knots 1 +/- nmpg.

Your marine knowledge always interests me. I look forward to learn what you feel our Tolly's hull speed is, and how you calc it.
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Old 12-27-2013, 01:22 PM   #49
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Fred - Happy Holidays!

With your in-depth calculation methods how would you recommend to calc our Tollycraft's "hull speed". Also, what do you feel our boat's hull speed actually is (before it begins pushing water to climb up onto plane)?

Boat Stats: Loaded weight 21K lbs +/-, OAL 34', moving LWL 32', OAH from WL 12' 6", Deck Beam 12' 6", Bottom beam 10' 6", 2' 9" draft, dead rise 11 degrees, short keel with 9" max depth 5' from stern, full length hard chine...

With our Tolly's (off plane) moving LWL 32' I mathematically calc "hull speed" at 7.58 knots. Having run different speed-to-fuel use tests I've found that while using twin gassres both running an economical fuel-use speed is 6.5 to 7 knots... 2 +/- nmpg With only one screw running most economical speed is 5 to 5.5 knots... 2.75 +/- nmpg. On full plane at 16 to 17 knots 1 +/- nmpg.

Your marine knowledge always interests me. I look forward to learn what you feel our Tolly's hull speed is, and how you calc it.
Unless you know your hull's constant at something between 0.5 and 2.0 at those lower speeds...using 1.34 is as good a guess as anyone can make (per the one of thousands of supporting articles like the one I linked) about your vessel.

What I think Fred, myself and a bunch of others are always referring to the 1.0 (plus/minus) number...is just running well enough below your hull speed for economy reasons....but knowing exactly where that number is for your boat would take many test runs and a careful plotting of fuel consumption versus speed to see if there even is and what that magic number is. It's the point where that professor in my linked article says

" Typically the energy required to speed up a displacement hull then becomes exponential in speed rather than quadratic"
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Old 12-27-2013, 01:37 PM   #50
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Art,
I think HS has little to do w boat performance on a hull that is not FD. Faster burns more fuel ... slower burns less.

With a SD hull at hull speed the stern will be right on top of it's following wave and the transom is deep in the water creating so much drag that the usual thought of benefits of HS aren't realized. It's just another speed w no special benefits. A FD hull has a stern shape that fits the face of the following wave and it surfs a bit and the following wave (to some degree) pushes the boat fwd. And the transom is high (or even out of the water) creating little drag. It's a bit like the Bart using it's electric motors as generators and brakes to recap some of it's energy spent. The FD hull reclaims some of it's energy lost making the bow wave.
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Old 12-27-2013, 02:09 PM   #51
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The only reason I track fuel use is for planning fuel stops while traveling. When I boated on a inland lake I never worried about it. I can guess within 5 gallons when I go to the fuel dock how much it'll take to top off the tanks. I'am not going to stop boating because of the cost of fuel, as a matter of fact I wish I was able to be out burning some right now. So the way I figure "It is what it is" & I'll live with it.
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Old 12-27-2013, 02:27 PM   #52
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The only reason I track fuel use is for planning fuel stops while traveling. When I boated on a inland lake I never worried about it. I can guess within 5 gallons when I go to the fuel dock how much it'll take to top off the tanks. I'am not going to stop boating because of the cost of fuel, as a matter of fact I wish I was able to be out burning some right now. So the way I figure "It is what it is" & I'll live with it.
It's not always about money...sometimes it's just plain old what do I have to do to stretch it to the next fuel stop...or make it some place safe on just a few gallons because you got sold bad fuel and can't use most of it...I guess I could keep going...but......it's just nice to know your options....

If money was no object, I'd travel by Learjet.....and boat in several boats strategically placed all over the world, of all different kinds for all different kinds of boating....
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Old 12-27-2013, 02:29 PM   #53
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so just back a hair off your bow wave a bit, shut down your engine and surf till your next stop.....

these discussions get better and better....
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Old 12-27-2013, 02:32 PM   #54
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so just back a hair off your bow wave a bit, shut down your engine and surf till your next stop..... these discussions get better and better....
"Blow-boaters stuck in a trawlersman shell"
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Old 12-27-2013, 06:31 PM   #55
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Ron I agree and I'd like to be burning too. But mine's on the hard.

However as we change boats or otherwise buy boats we have a huge say in how heavy and hull efficient our next boat is going to be.

But the value here on TF is how good the conversation gets and how much we can learn.
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Old 12-27-2013, 06:42 PM   #56
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We're copacetic running at 1.1 of the square root of our waterline, meaning roughly 6.3 knots with almost 32-foot waterline using 1800 RPM of the John Deere 4045's maximum of 2400.

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Old 12-27-2013, 06:54 PM   #57
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But the value here on TF is how good the conversation gets and how much we can learn.
The conversation is a great way to spend these winter months when some of us can't be on our boats & on a cruise somewhere. The amount of daylight is increasing each day now as is my attitude.
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Old 12-27-2013, 07:07 PM   #58
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Fred - Happy Holidays!

With your in-depth calculation methods how would you recommend to calc our Tollycraft's "hull speed". Also, what do you feel our boat's hull speed actually is (before it begins pushing water to climb up onto plane)?

Boat Stats: Loaded weight 21K lbs +/-, OAL 34', moving LWL 32', OAH from WL 12' 6", Deck Beam 12' 6", Bottom beam 10' 6", 2' 9" draft, dead rise 11 degrees, short keel with 9" max depth 5' from stern, full length hard chine...

With our Tolly's (off plane) moving LWL 32' I mathematically calc "hull speed" at 7.58 knots. Having run different speed-to-fuel use tests I've found that while using twin gassres both running an economical fuel-use speed is 6.5 to 7 knots... 2 +/- nmpg With only one screw running most economical speed is 5 to 5.5 knots... 2.75 +/- nmpg. On full plane at 16 to 17 knots 1 +/- nmpg.

Your marine knowledge always interests me. I look forward to learn what you feel our Tolly's hull speed is, and how you calc it.
I'll be interested in Fred's views as well. His rules of thumb are pretty good.

For what its worth, I have found the Hawaii Marine Company spreadsheet for Displacement and SemiDisplacement hulls pretty good also.

Displacement and Semi-Displacement (or Semi-Planing) Hull Powering Calculations, Description

It uses material from Gerr's Propeller Handbook amongst other sources. It has a small fee and user licence so I cant post the xls file I played around with using some of Art's info. I had to guess a few things, but for Art's Tolly I got a figure for SLR of 1.37 from it, giving 'hull speed' of 7.7 kn.

The xls file seems to manage the FD versus SD hull type issue reasonably well. For SD hulls, like my own, I find that SLR of 1.2 is still pretty economical. For Art, 1.2 is a speed of 6.8 kn so it seems to fit for his Tolly as well.
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Old 12-27-2013, 07:44 PM   #59
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I'll be interested in Fred's views as well. His rules of thumb are pretty good.

For what its worth, I have found the Hawaii Marine Company spreadsheet for Displacement and SemiDisplacement hulls pretty good also.

Displacement and Semi-Displacement (or Semi-Planing) Hull Powering Calculations, Description

It uses material from Gerr's Propeller Handbook amongst other sources. It has a small fee and user licence so I cant post the xls file I played around with using some of Art's info. I had to guess a few things, but for Art's Tolly I got a figure for SLR of 1.37 from it, giving 'hull speed' of 7.7 kn.

The xls file seems to manage the FD versus SD hull type issue reasonably well. For SD hulls, like my own, I find that SLR of 1.2 is still pretty economical. For Art, 1.2 is a speed of 6.8 kn so it seems to fit for his Tolly as well.
I highly doubt that a planning or semi-displacement hull has a SLR of 1.37 as that is above the average that is usually reserved for full displacement hulls with a slippery prismatic coefficient...

While I'm no NA or even self proclaimed expert.....just sounds high for a boat that can plane.

1.2 sounds a lot better.
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Old 12-27-2013, 08:41 PM   #60
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I would like to see some real world figures. How about a 40 foot lwl boat weighing 50,000 lbs with a 14 foot beam doing 7 knots. No b.s. real figures. You guys with Krogen 42s will know this best. My opinion is that ANY boat that fits this criteria will get about 2 to 3 mpg. regardless of hull configuration, diesel that is, subtract 1/3 for gas. But gas is cheaper, so we could do cost per mile. New wrinkle.
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