Boat Builder Central - Builder Forums

I have read the forum and I respect the stance Designer Mr. J Mertens takes in stating "The decision to leave the dock with a specific boat in a certain type of weather and area is up to the skipper, not the designer." But he has also stated that the skipper "knows his boat". As a customer looking to buy a design, I want/expect to know the boat's designed capability before I buy it. A Naval Architect should be able to define the safe operating envelope for a boat they design such as C.E. class C or D, etc.

So, I ask will the GT27, with the recommended 25hp outboard, be able to motor off a lee shore if a blow kicks up? Specifically, what wind a sea state can the GT27 make way to weather in? If it is blowing 15knts and there's a 1 foot sea (CE D), what speed can be made motoring to weather? What about making way to weather in a 25knt wind and 3 foot seas? Will she take a 3.5 ft breaking wave on the beam without risk of rolling, swamping or significant damage (blowing out windows)?

thanks

No houseboat ever leaves its anchorage in a 25 knot wind. A 13.5kt or 15mph wind is the limit of almost any commercial company I've ever used.

These boats are designed to stay ashore under bad conditions, so the questions asked are already outside the acceptable operating conditions.

A 3.5 foot wave breaking over the bow is simply not good helsmanship and the boat should have been at anchor.

These boats are designed for winds below 15 mph or 13.5 knots and sea states below 2'.

They are difficult to manage abeam and should not he operated in 2' seas abeam. But rather, one would quarter. And this means extending the trip.

I will not speak for Jacques, but really the questions are out of order, except leaving a shore in 1 footers. The expected speed would be at or jist above hull speeds as the boat would probably pound a bit in heavier chop. The hull speed of the GT27 is about 7mph.

For commercial houseboat operators; all I have used require an emergency vessel. The emergency vessel is typically a 14-18' skiff that can manage heavier winds. A hip tied or towed skiff in bad conditions is also a problem as hip tied becomes easy to break lines.

I have been on wind pounded anchorages and the boat cannot remain there. The way this is managed is to leave the day prior. If you are counting on departing 4 foot rollers that have driven you up on the beach; you are also mistaken because the boat will be aground hard and the water will be shallower and shallower still at trough bottoms.

I do apologize for the tone, but safe operating is even more important for houseboats. Even knowing most summer storm directions is a key to safe operation.

I agree with Fallguy on most points , but the GT27 is a riverboat , a bit more than a houseboat . She is designed to take chop , I would have no problem being in 2' chop bow or quarter , 3.5' breaking wave to the beam and you are into that " up to the Shipper part " . It is clear in the notes that this is an inshore boat , even with a 125 hp or larger it would be near shore on a very good day , no reason to put a ABC or D on it .

Thank you for the responses. The reason I want to put a C.E. classification C or D is for discussion purposes to clearly understand the limitations of this boat before I buy/build it. I find the information from the designer to be lacking and certainly contradictory to Fallguy1000 assertion, stating "These boats are designed for winds below 15 mph or 13.5 knots and sea states below 2' " which is a C.E. Class D vessel. This does not jive with "I would take her across the Gulf Stream or across the British Channel in good weather . . I think she would be a good boat for the Puget Sound" (from the designer). There's a huge difference between a "House Boat" and a "river cruiser, ideal for . . rivers, lakes, bays or the ICW" which I would assume exceed class D and perhaps meets CE class C requirements. A boat that may swap from a large boat wake (a 3ft breaking wave) is not suitable for open bays such as the Puget sound or Chesapeake. And while a houseboat won't leave anchorage in 25 knots of wind, the weather changes fast and a calm sunny day can throw a Thunderstorm at you with 25+ knts. I have zero interest in making an emergency mayday call because a pop-up thunderstorm hits or some jerk in a 70ft power boat sends a big wake at me. Given Fallguy's description of this boat, it would be extremely irresponsible to take this boat across the gulf stream or British Channel, in my opinion.

In conclusion, I find this boat unsuitable for my needs. I'm building this for my father who will be using it as a live a board cruising 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.

I will pursue a DE25 like hull, but with a GT27 like cabin. My father has bad knees and 1 level living is needed.

The second reply is much easier to understand the direction of the SOR.

I am not Jacques. I will not get into an argument with him either. Can the GT27 do the crossings? Sure, in a boat parade with calm seas. But taking a 25 horse single engine outboard boat 50-80 miles offshore is certainly not advisable.

The boat can certainly handle a large wake, which is not really what was stated earlier. A 3.5 foot breaking wave implies going through a surf, generally, not a boat wake. This boat is not designed to go through a surf, again, my opinion. In fact, that comment/question was what inspired my comment.

This website has a bunch of people eager to support you from plan purchase to launch. We do not enjoy killing dreams either. The DE25 would be a good alternative for you and capable of much more seakeeping with an accomodation tradeoff. All boats have tradeoffs. This is why honing the SOR is so important.

Mizzenman , my only problem with the GT27 and what your intended use is would be the cabin height and wind if caught out in a bad storm . With a 125 hp or greater the GT27 will plane and get you across open water quickly , so a 30 to 40 mile run would be no problem , and use a kicker to push it at displacement speed . I would not take it out in the Gulf of Mexico very far because of the windage issues in a storm , and with the same cabin you will have the same issues with a DE25 type hull . As far as getting swamped by big wake you turn in and quarter it in any boat . We had a 24' work boat get swamped and sank a few years back , a large yacht came up the river and the wake was so high the driver got scared and threw it in reverse trying to back into the bank . The first thing CG asked was did he have the bow into the wake I don't think you should rule it out yet if it the interior suits your father,s needs .

I would like to point out one other issue.

The classification system, itself, is problematic.

The leap from seakeeping class D to class C is massive.

How unfair that the ce designation for D is seas under one foot and the next step is for seas under 2 meters or just over 6'.

And this is why I landed somewhere in between. The boat would manage 2' seas fine, but if you run onto Lake Erie into 6 footers abeam, you would be rather unhappy.

I agree Fallguy, the step from CE class D to C is a big one. Rick, I'm not going to stick 100+ hp on her to run from a storm, see below.

SOR: Trailerable, live aboard for 2, canal/bay cruiser. Cruise at almost 6knt with 5kw shaft power (calm conditions) Per Gerr. 8hour with 40kWh battery for ~50mile range w/o sun. Primary use on canals, but capable of large bay crossings in 20knts of wind. Single level deck and cabin (1 step okay). Solar roof, e-drive, back-up gen.

I drew up a boat, but don't see how to attach files... 6,000lb design disp, LOA = 26, BOA = 8.7, LWL = 25.4, BWL = 7.5, Draft = 1.5, Cp = 0.55, S = 149 ft^2

Power Details: I intend to use a 10kw or 14kw electric motor and cover the roof in solar similar to the Solar Sal 24 ( www.solarsal.solar ), but I would stretch to 26 or 27 feet and enclose the sides for a livable cabin. Use a 48VDC household solar setup likely from EG4, which has a 6kW MPPT charger/inverter for $1250 and a 31kWhr LiPO4 battery bank for $9k (this is 6 batteries, I will likely add a few more). I can fit about 3,200 watts of panels at $1/watt ( I expect about 20kwh of daily production, good for 25miles every day on Sun only & farther if I go slower). 6kW Gas generator for back up power $900. The motor, controller, throttle, contactor, etc. is about $3k. Add another $2k for shafting, prop, stuffing box and $1,000 misc and that's $15,000. Not much more than an inboard diesel.

How do I optimize this Cp for 6.0knts or maybe 5.5knts would be better? I wonder how much the optimum changes for half a knot? Not much I'd guess. I think I read that an SLR of 1.2 had an optimum Cp of 0.58. Is there a function that I can enter SLR and get Cp?

Worried about seakeeping, then you won't be putting 500 pounds of solar on the roof of a light boat and raising vcg by 2 feet, maybe you can put the batteries in the bilge, but 3200watts is like 10 of my panels which are 39" wide or 390" or 32 feet of end to end panels.

Simply not practical afaic.

"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.

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...

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

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 .

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....

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.

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.

More power to you if you can get panels that cheap..not here

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!

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!

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

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.

Here is a quote from Jacques.

Be good to newcomers on the site please. Our goal needs to be to help people build boats. If Jacques said, "I", that means himself as a seasoned operator with 50 or 60 years of helmsmanship. He did not advise this boat be used that way.

jacquesmm wrote: ↑Thu Mar 27, 2008 3:04 pm The GT27 should be kept near to shore in good weather only.
The boat will perform well on rivers, ICW, in the Keys etc. but not to cross to the Bahamas or cruise the Gulf of Mexico.

You could do all what you describe with the GT27 except crossing the Gulf or the Gulf Stream. You can go from the Mississippi to the Keys through the ICW with occasional small trips outside.

I am prejudiced and do not consider any outboard boat as an all weather boat. Fast planing outboard boats can go offshore because they can get in protected waters in a short time if the weather changes.
And they will take bad weather much better than the GT27.
Consider the GT27 to be an houseboat that will take a chop much better than most other houseboats but it's still in the houseboat category.

These plan designs are intended for the home builder and not a commercial shipyard. I would guess a commercial shipyard would have a naval architect on hand and engineers all along the way making sure all commercial standards are met on any designs. As far as giving specifics about a boat and what it can handle, that again would be out of the hands of the designer, since they are in no way supervising anything being done to the build or if the design plans are followed. And to much exact information in what seas a boat can handle would open the designer to a lot of liability. And with society's mindset now days of "put the blame on someone else for my actions", that would hold true as blaming the designer for any mishappens the builder or the helmsman may have with the boat. The op of this thread is expecting information a commercial shipyard would expect from a naval architect with a firm understanding "contract" that the boat be build to very exact design specs and there will be documented proof, again just guessing, that the design was followed.

I just watched a show of a ship being build, there were a lot of engineers involved in supervising its construction and a no one was allowed to go onto the next part of the build till an inspector signed off on the construction. We are talking one bulkhead to the next. From the description the OP gave of his career, he is from that type of environment. Welcome to the private sector! No one supervises or inspects anything you do or if you are following any plans to the letter. The plans you buy from these designs will give you where to put frames, strings, skin and the thickness of the wood needed to do so. Also let you know where and how to reinforce those same things. If the plans show a cabin, it's possible it will show a few different outside shell of said cabins, framing and recommendations on where to put the helm station, head and so on. But it is still up to you to do it and figure out how. You may decide to use a lot cheaper glue than epoxy to hold everything together, that is your choice even though the plans say otherwise. You may want to move a frame a few inches from the plans and again, your choice. In the end, you take on all liability on making it seaworthy.

If you are not only the builder but the skipper as well, then you make the decisions on where and what conditions YOUR built boat will do. How many times have you seen an ocean going vessel have structure failure in rough seas, even though the vessel should be able to manage the conditions the Capt. put it through? Who was a t fault? The designer, the shipyard or the Capt.?

Also, a skipper can take a boat into rough seas and capsize or get swamped. Another skipper in the same exact boat, exact same conditions and the boat may not take on too much water. Who would be at blame for the first skipper losing the boat? Could it be a design flaw because it was said, on paper and by science, it could manage those conditions.

Remember, we live in a time where if you order hot coffee, there HAS to be a label on it that it is hot and even then, someone will want to sue someone else if they spill it and they get burned.

Mizzzenman, Jacques in most plans asks professionals to contact him for design adjustments for production boats. He also attaches that the boat is designed to ABYC standards. Don't know why he did not on the GT27 other than that it is an older plan. I would also put a little larger motor on that houseboat due to its windage. Maybe a 40HP, not much difference in fuel use at 7-8 mph but more power if needed.

Good luck on your build!

Tom

To expect that a small boat like this would have an operating envelope any more detailed than the description is just not accurate. Maybe you could point me to boat plans for amateurs that have such a design envelope stated.

Not sure why you're comparing it to a class operating envelope that is used for design of classed boats. These vessels are so far outside of what a class rule would consider that it's just not useful to even mention them in the same breath.

Ian Farrier said about his F-27 that it wasn't designed to cross oceans - but people have done so and while he doesn't recommend it, they usually do OK. That's about the same level of accuracy as Jacques comments. It's more about the operator than the boat. It's designed for the hull pressures of planing speeds with a 25 HP motor but the low bow restricts its seakeeping in waves. It isn't going to capsize in 3' beam seas unless you do something stupid. Other than that I can guarantee there is no secret design envelope that Jacques used - I sure don't use one for the little boats I've designed for this site either.

as required by all major boat design certification bodies

- which would those be? Other than CE and that is for production boats. And they don't certify the design like a class society.

I've spent a bit of time trawling through Jacques comments on the GT27 and other various boats. He does say once (back in 2006) that he might take it across the channel in good weather or the gulf stream in good weather. He's also pretty clear that the boat is not designed for this and should not be built for this. But if you know the boat very well and are an experienced skipper the boat won't kill you. The GT27 can be built with a bigger than 25hp outboard, which might reduce the crossing time. He also states in the same post that people take kayaks across oceans, but nobody calls them ocean capable boats.

In every other post I have seen he reiterates that the boat is designed for nearshore use only in good weather. I find that clear, as someone that has taken a D category boat 10km offshore an exposed coast multiple times. I am still around though once or twice I was beginning to wonder.

I would agree with the choice of DS23 or 25.

OWT I totally agree with you except I believe, you meant the DE23-25. We all have to remember that Jacques is an accomplished open ocean sailor and what he can do most of the rest of us would not attempt. Evan has made a circumnavigation of the globe. If you do build the GT27 follow their wisdom and stay close to shore. The Gulf of St. Lawrence down can be dangerous so have an anchorage planned that you can get too quickly.

I don't know what the step down is in the DE25 but you could make the deck flat for your dad and make the head room 6'2" in the cabin without the boat looking ugly. It would be a much better handling boat in rougher conditions and wind states.

Good luck on your build.

Tom

In Belize I took our FB11 with a tiny outboard out in 6 seas to guide somebody through a pass in the reef. I wasn't worried in the slightest though the folks in the sailboat were slightly worried for me. I think it's mostly all about knowing your boat and how it will deal with whatever conditions you might face.

I wouldn't take a GT27 across the English Channel in anything stronger forecast than a F3 (7-10 knots wind) AND the weather pattern was stable. The English Channel is not known for afternoon thunderstorms that suddenly arise . 25 land miles is only a few hours at 7 mph. I'd do it in a sea kayak with the same sort of forecast (well stronger winds for the kayak if they were astern!)

He has worked as an Engineer in the marine industry for 10 years , worked on yachts for the past two , and has a design he tried to post here and couldn't . When designing my house i did all the design work and engineering math myself for a concrete structure and wanted someone to back check me . I have never had a problem getting an Engineer or other Professional I have worked around in my industry to lend a helping hand , as was the case on my house design . So I am sure in the position he is in he can find someone to help him finish up his design , and provide him with the numbers he wants if he is not capable as an Engineer in generating the numbers himself . Most people understand what inshore means

rick berrey wrote: ↑Fri Dec 09, 2022 10:57 pm He has worked as an Engineer in the marine industry for 10 years , worked on yachts for the past two , and has a design he tried to post here and couldn't . When designing my house i did all the design work and engineering math myself for a concrete structure and wanted someone to back check me . I have never had a problem getting an Engineer or other Professional I have worked around in my industry to lend a helping hand , as was the case on my house design . So I am sure in the position he is in he can find someone to help him finish up his design , and provide him with the numbers he wants if he is not capable as an Engineer in generating the numbers himself . Most people understand what inshore means

Huh?

Evan , I have the plans for the sk21 Sea Kayak , I might get around to building it one day , I'm glad to know you would cross the English channel with it ,

The huh is he didn't intend to build a design from Bateau , as an Engineer in the marine industry he should be able to either complete his own design or get help from someone in the industry to complete it .

Sorry for the delayed reply: I had Covid and could not think straight for two weeks.

The GT27 is a houseboat.
She is not designed to go offshore. She should operate only in protected waters. An experienced skipper can take her out in good weather but he has to understand her limits.
I know about the different CE classifications: I started designing boats in Europe, many years ago but I don't design to the CE or ISO specs.
Except for few character boats like some of my very small runabouts, all my designs respect the ABYC and USCG specifications. In most cases, those specs match the ISO ones.
To work through the ISO specs is a major job. Specialized European design offices charge about $ 5,000.00 per design to check things like stanchions height, portholes location, stability and more.
That is why some European plans for small boats cost much more.
If you have to worry about the offshore stability of the GT27, you are looking at the wrong boat for that program.
The GT27 will be safe and comfortable in rivers, lakes and here in the US, in the ICW. I would go to the Florida Keys or maybe the Bahamas in very good weather but not more.
Where I live (Florida), the GT27 would be a great boat. It also should be ideal for many lakes and rivers, for the inshore parts of the Gulf of Mexico and others places of the world, for example, Friesland, Zuyderzee, Constanz Lake etc. but not offshore.

I know that I wrote about the skippers ability to judge the capability. It is self evident: once you have the experience, you get a feel for the boat and the weather. You will not have to ask. It is not measurable with bureaucratic rules. I remember my daughter rowing safely in her D12 from her anchored boat to the shore, with her children, in windy weather. It was safe, she knew the boat and had the experience but in the same location, same type of boat, same day, an inexperienced man capsized and almost drowned. He did not know the limits of his boat or himself and did not understand the risk. No amount of calculations would have prevented that accident.