GT27 Sea Keeping ability

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Evan_Gatehouse
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Re: GT27 Sea Keeping ability

Post by Evan_Gatehouse »

"I would take her across the Gulf Stream or across the British Channel in good weather . ."

The good weather part is the essential bit to understand. The English Channel or crossing to the Bahamas from Florida is about 40-50 miles. On a good day no big deal.

But in a blow, the GT27 shouldn't be out there.
Is there a function that I can enter SLR and get Cp?
. To change the Cp you have to change the hull form. Not something most folks would worry. The difference is not huge for different speeds in terms of efficiency. Few of us motor or sail at just 1 speed.
the St. Lawrence seaway, Erie canal, Hudson River and South possibly to the the Chesapeake. This area includes open water of Lake Champlain, Lower Hudson River, Raritan Bay, Barnegat Bay, etc. for which it is not suitable to cruise in even moderate weather conditions.
Most of those areas are fine because the fetch is small. Barnegat and Raritan Bay and parts of the St. Lawrence can be much more exposed so you'd be restricted in how rough you'd want to take it those areas.
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mizzenman
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Re: GT27 Sea Keeping ability

Post by mizzenman »

Fallguy, being that the USCG has approved Dave Gerr's Solaris Sal for commercial use carrying passengers and it is the basis of my design, I'm not too concerned. The 500lbs of panels on a 6,000lb boat, raises the VCG by about 6 inches, not 24 inches. I've seen plenty of fly-bridges installed on boats this small and that would be a lot more weight up higher than 500lbs of solar will be. 20 watts per sq foot is typical of today's efficient panels. 8ft x 20 ft x 20w/sq.ft = 3,200. not sure why you would consider this proven design 'not practical'

For fun I did an Angle of Vanishing stability. I don't know what to put in as the ballast ratio on a powerboat without a lead/iron keel. I used .2 and got an AVS of 110 deg, which puts it between inland and offshore. Even lowering the BR to 1%, the AVS is still about 100 deg, fine for inland... though i kinda doubt these results

Evan_Gatehouse
Going 40-50 miles offshore with a 25hp outboard on a vessel that goes 7 knts is not seaman like. Weather changes and a nice sunny day at 10am may be blowing and storming 5 hours later! It's irresponsible for a designer to suggest that it's okay to do that... sure you might, probably will, get away with it, but if not... :help:

Changing Cp is a very easy transform function in software. I found what I was looking for regarding optimizing Cp to SLR. And for a solar 'trawler' boat I do want to optimize my hull for a SLR of 1.1 or 1.2. I have a Cp of .55 now and that's optimum for about 5.7knts on my 26', SHP = 6.2hp. Many large power boats do run at one of the 2 optimum speeds, displacement or planing modes, to save fuel.
Skene's Elements of Yacht Design (by F Kinney, 5th ed):
SLR Cp
1.0 0.52
1.1 0.54
1.2 0.58
1.3 0.62
1.4 0.64
1.5 0.66
1.6 0.68
1.7 0.69
1.8 0.69
1.9 0.70
2,0 0.70

rick berrey
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Re: GT27 Sea Keeping ability

Post by rick berrey »

Now I am confused , you want a boat that can go 7 kt with a 25 hp , take a large breaking wave to beam and ride out rough weather , and you want the designer to tell you it will do just that . Then you say it is irresponsible for the designer to suggest it when he tells you what the design will do offshore . Dave Gerr has some nice designs , no prices on the site , but I bet you could get what you want from his 25' river cruiser power boat . J is retired , so we will be relying more and more on Evan for designs like the Great Loop Cruiser 32' x 8.5' ( foam ) . And as he has sailed many offshore miles in a boat that might have averaged 7 kt on the long haul , maybe had more than 25 hp , referbed his boat before his trip , and is a Designer , not only is respected on these forums , but deserves respect on these forums . Insulting the designers here and then continuing on with your conversation on their forms is like breaking in a house , sitting down to the table and telling the homeowner what to cook for supper .

fallguy1000
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Re: GT27 Sea Keeping ability

Post by fallguy1000 »

I think you'll find 3200 watts very difficult. The high efficiency 20% plus panels are very size specific. Panel mounting on a small boat no margin will be hard. Even a bath fan or vhf antenna requires space. And the thing zi ran into was cost. The cost for a decent panel was $500, because they all doubled panel costs to freight. So 8 high efficiency panels is hard to fit and 4-6k. Capitalize a 10 year life and $500 a year becomes impractical for many. Sorry, riding a rough road....
My boat build is here -------->

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mizzenman
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Re: GT27 Sea Keeping ability

Post by mizzenman »

rick berrey wrote: Sat Dec 03, 2022 11:29 pm Now I am confused , you want a boat that can go 7 kt with a 25 hp , take a large breaking wave to beam and ride out rough weather , and you want the designer to tell you it will do just that . Then you say it is irresponsible for the designer to suggest it when he tells you what the design will do offshore.
My intent was not to insult the designer. You see, Jacques NEVER said what the GT27 was designed for or capable of. He said that operation of the vessel was up to the discretion of the skipper (almost sounds like a lawyer wrote that) and that he would take the GT27 across the Gulf stream or British Channel. In my experience, this is not the way a Naval Architect speaks about their designs. Fallguy, put a realistic operating envelope on the boat, up to 15mph winds and 2ft seas, which is something Jacques as the designer should know and should have said IMO. I assume he did an analyses of the safe operating envelope for all his designs. So why not share that information? My opinion of the responsible operation of the GT27, while contrary to Jacques, is not intended to insult Jacques even though it differs to his opinion. BUT I don't want Jacques's opinion! That's my point. I don't want any opinion. I want to know what the results of the analysis that he performed as the Naval Architect who calculated/designed the boats safe operating envelope.

mizzenman
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Re: GT27 Sea Keeping ability

Post by mizzenman »

fallguy1000 wrote: Sat Dec 03, 2022 11:54 pm I think you'll find 3200 watts very difficult. The high efficiency 20% plus panels are very size specific. Panel mounting on a small boat no margin will be hard. Even a bath fan or vhf antenna requires space. And the thing zi ran into was cost. The cost for a decent panel was $500, because they all doubled panel costs to freight. So 8 high efficiency panels is hard to fit and 4-6k. Capitalize a 10 year life and $500 a year becomes impractical for many. Sorry, riding a rough road....
$500 per year is about 100 gallons of fuel. I think a lot of boaters spend that on fuel, so I'm happy to buy panels and not (much) fuel. The altEstore is a 2 hour drive, so I won't pay freight, I'll pickup. Paradea Series 10 540W panel, is 90 inch by 45 inch. 6 of these would fit a 6'8" by 22.5', 3,240 Watt rating and costs $1,980 plus tax.

I will stick a VHF antenna off the very back of the roof along with a stern light. I don't plan to vent through the roof. I will have opening port lights / windows along the cabin walls.

fallguy1000
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Re: GT27 Sea Keeping ability

Post by fallguy1000 »

More power to you if you can get panels that cheap..not here
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BarraMan
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Re: GT27 Sea Keeping ability

Post by BarraMan »

The "sea keeping ability" of this boat seems pretty obvious to me! 8O

Image

The designer says, "This is a river cruiser, ideal for protected waters: rivers, lakes, bays or the ICW."

Seems pretty clear! :doh:

mizzenman
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Re: GT27 Sea Keeping ability

Post by mizzenman »

BarraMan wrote: Sun Dec 04, 2022 10:42 pm The "sea keeping ability" of this boat seems pretty obvious to me!

The designer says, "This is a river cruiser, ideal for protected waters: rivers, lakes, bays or the ICW."

Seems pretty clear! :doh:
I'm glad you are satisfied with such a vague description. Clear? It is Not. The designer also said "I would take her across the Gulf Stream or across the British Channel". There are areas of bays and the ICW that build short 5 foot seas that would certainly stop this boat in her tracks (going to weather), maybe send her to the bottom (if on the beam). "A river cruiser, ideal for protected waters: rivers, lakes, bays or the ICW" could mean less <1 foot waves, <15mph winds to one person, or capable of 5ft seas and 25knts to another (especially considering these design are aimed at amateurs who may not know better). So is it really that clear?

As you know BarraMan, There are specifications that govern how to design boats. Since Jacques' designs are not for commercial production, those specs are not legally invoked, but they certainly should be a guide, if not satisfied to be comprehensive. Regarding the design principles, DNV-Loyd States "The design loads will be based on operational parameters that give restrictions to operating speed versus significant wave height. The operational restrictions shall be given by the designer." So, the designer should know what the operational parameters (operating speed versus wave height) are. I'm quite surprised that I'm getting flack for asking for this. It should be basic info the designer has, like GM or Cp.

IMO, A Naval Architect should be able to provide numerical evaluation for the operating envelope, as required by all major boat design certification bodies. For what its worth, my intended use is 100% limited to rivers, lakes, bays and the ICW, BUT this GT27 vessel will most certainly NOT satisfy that need for me. It simply would not make way to weather in relatively common conditions in many lakes and bays that have several miles of fetch along my intended use area. Perhaps "several miles" is not "protected" enough? I don't know because the description is vague. show me the numbers! hahaha

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VT_Jeff
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Re: GT27 Sea Keeping ability

Post by VT_Jeff »

mizzenman wrote: Mon Dec 05, 2022 1:25 am The designer also said "I would take her across the Gulf Stream or across the British Channel".
I can't locate that in the description or study plans, interested where you saw that.

Edit: If it was in a thread that JM commented on, that would explain why I was not seeing it, I had assumed it was from the plans/description.
Last edited by VT_Jeff on Mon Dec 05, 2022 10:53 am, edited 1 time in total.
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