GT27 Sea Keeping ability

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mizzenman

GT27 Sea Keeping ability

Post by mizzenman »

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



fallguy1000
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Re: GT27 Sea Keeping ability

Post by fallguy1000 »

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.
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rick berrey
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Re: GT27 Sea Keeping ability

Post by rick berrey »

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 .

mizzenman
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Re: GT27 Sea Keeping ability

Post by mizzenman »

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.

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Re: GT27 Sea Keeping ability

Post by fallguy1000 »

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.
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rick berrey
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Re: GT27 Sea Keeping ability

Post by rick berrey »

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 .

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Re: GT27 Sea Keeping ability

Post by fallguy1000 »

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.
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mizzenman
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Re: GT27 Sea Keeping ability

Post by mizzenman »

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.

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Re: GT27 Sea Keeping ability

Post by mizzenman »

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?

fallguy1000
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Re: GT27 Sea Keeping ability

Post by fallguy1000 »

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.
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