GT27 Build (Wes K)

To help other builders, please list the boat you are building in the Thread Subject -- and to conserve space, please limit your posting to one thread per boat.

Please feel free to use the gallery to display multiple images of your progress.
wkisting
Active Poster
Active Poster
Posts: 377
Joined: Tue Jul 10, 2018 11:24 am
Location: Augusta, GA

Re: GT27 Build (Wes K)

Post by wkisting »

Hi all,

Long time no post. After the brutally hot summer turned into a maddeningly cold, wet winter, we got delayed by many months. However, in February, we got back to our GT27 build and have been plugging away ever since. As of yesterday, the topsides are all painted and the hardware is starting to go on! Borrowed my neighbor's carport to shield us from the blazing sun...
20220522_165941.jpg
I'm back to thinking of motors... I was settled on a 40hp outboard to ensure extra thrust for muscling the boat around in a strong wind or current. However, I notice there are "high thrust" versions of smaller outboards like the Yamaha 25. I'm wondering if that's a better choice, since it's lighter weight (~143 lbs. vs. 223 lbs.) and probably would sip less fuel, potentially extending range (?). We'll be running at displacement speeds. I don't know how much the "high thrust" makes a difference, having had no experience with these types.

The 25 has an alternator that outputs 16A current for charging. The 40 is at 17A, so not a huge difference. Either one would charge our onboard systems, I think.

The 40 doesn't come in a high thrust version. The 50 and 60 do, but those are getting pretty heavy (~290 lbs.)... Wish I had more experience with midsize outboards to know how much of a difference the power is likely to make for a displacement cruiser. I know some people push a much heavier sailboat with a tiny 5hp or 10hp, so maybe the 40 is overkill?
Wes
---------------------------------------------------------------------------------------------------------
My build thread is here --> viewtopic.php?f=12&t=63644

fallguy1000
* Bateau Builder - Expert *
* Bateau Builder - Expert *
Posts: 10198
Joined: Tue Jan 12, 2010 12:25 am

Re: GT27 Build (Wes K)

Post by fallguy1000 »

TomW ought to be able to do some calcs for you on those engines.

I have been on quite a few houseboats. You need to avoid high winds, generally. That is anything over about 15 knots. So, a 25 high thrust would be okay. The only time I'd like more power was exiting a beach. If you drive up onto a shoreline and get pushed up or hang a bit; you want to be able to get plenty of reversing power..

Richard Woods, the designer of my boat said to expect 1 knot of speed per 5 hp limited to hull speed or so, ftmp.

Based on that and hull speed assumption for the GT27 of 8mph, I would anticipate the 25 to be enough unless you get stuck.

Despite your worries about weight; the 40 would help you exit a beach landing with the weight aft as well. I have had to fill coolers with water on the aft deck when stuck bad a couple of times. Usually this happens when winds or wakes drive the boat up over a few days.

Another thing to consider is engine life. I'd prefer to run a 40 at half throttle than a 25 at 80% throttle.

It is a big boat, but lightish; I'd lean heavy to the 40 for beach exits. See what TomW says for props.

He's gonna want engine specs for each, rpms, etc and he'll want a boat weight estimate..
My boat build is here -------->

viewtopic.php?f=12&t=62495

Fuzz
* Bateau Builder - Expert *
* Bateau Builder - Expert *
Posts: 8920
Joined: Tue Aug 02, 2011 4:23 am
Location: Kasilof, Alaska

Re: GT27 Build (Wes K)

Post by Fuzz »

My best guess is Dan's speeds are about right. I would think 10-15hp would get you close to 8mph. Lets see what Tom comes up with. As for hp I would want any motor chosen to be a high thrust model. They swing a larger prop and are built for pushing large loads at displacement speeds. Other outboards are more suited for planing speeds. In any case I would want the motor to be fuel injected. Fuel injection makes motors run so much better I would have a hard time buying any new motor without it.

User avatar
cape man
* Bateau Builder - Expert *
* Bateau Builder - Expert *
Posts: 8267
Joined: Sun Jan 20, 2008 12:02 pm
Location: Lithia, Florida

Re: GT27 Build (Wes K)

Post by cape man »

I'd go with the 25 high thrust.
The world always seems brighter when you've just made something that wasn't there before - Neil Gaiman

wkisting
Active Poster
Active Poster
Posts: 377
Joined: Tue Jul 10, 2018 11:24 am
Location: Augusta, GA

Re: GT27 Build (Wes K)

Post by wkisting »

I wish there was an in-between option besides the ~130lbs. 25hp high thrust models and the ~260lbs. 50-60hp high thrust models. I guess 260 lbs. isn't all that much to hang on the transom for a boat this size though, and if I'm running most of the time at half throttle, the fuel consumption probably isn't all that bad either.

Of course, it's pretty hard to find any of these motors in stock right now. I contacted a few places about Yamaha and Suzuki motors and they all said 7- to 8-month wait if they order today (!). Ugh. I think the Suzuki DF50AV is looking like the best match, as it's got a better gear ratio (2.42:1) compared to anything else I can find in mid-range outboards and can swing up to a 14" prop for good "bite"/maneuverability at slow speeds. At 253 lbs. it's also lighter and more affordable than the competing Yamaha T50/60 high thrust model (262 lbs.), which has a 2.33:1 gear ratio. Cheaper and lighter still is a standard DF40A (no "high thrust" option), which weighs in at 229 lbs. These are the motors I keep coming back to.
Wes
---------------------------------------------------------------------------------------------------------
My build thread is here --> viewtopic.php?f=12&t=63644

TomW1
Very Active Poster
Very Active Poster
Posts: 5844
Joined: Sun Apr 19, 2015 10:57 pm
Location: Bryson City, NC

Re: GT27 Build (Wes K)

Post by TomW1 »

Wes if you wish I can run the motors through my calculators and give you a top speed and a prop recommendation for them. Both the Suzuki and Yamaha's are quality motors.

I will need:

Weight of boat as finished:
Weight of passenger:
Fuel @ 6lbs per gallon
Gear taken aboard LBS.
Any water carried at 8.35lbs per gallon
I willl put in the motor specs as we are doing a comparison. I get them right off the MFG's specs table.

Well let me know.

Tom

Corrected weight of gallon of water.
Last edited by TomW1 on Thu May 26, 2022 12:21 am, edited 1 time in total.
Restored Mirror Dinghy, Bought OD18 built by CL, Westlawn School of Yacht Design courses. LT US Navy 1970-1978

fallguy1000
* Bateau Builder - Expert *
* Bateau Builder - Expert *
Posts: 10198
Joined: Tue Jan 12, 2010 12:25 am

Re: GT27 Build (Wes K)

Post by fallguy1000 »

TomW1 wrote: Wed May 25, 2022 6:30 pm Wes if you wish I can run the motors through my calculators and give you a top speed and a prop recommendation for them. Both the Suzuki and Yamaha's are quality motors.

I will need:

Weight of boat as finished:
Weight of passenger:
Fuel @ 6lbs per gallon
Gear taken aboard LBS.
Any water carried at 10lbs per gallon
I willl put in the motor specs as we are doing a comparison. I get them right off the MFG's specs table.

Well let me know.

Tom
Okay, Tom! Nice offer. A gallon of water is 8.35 pounds. Tanks weigh a bit I suppose.

Just curious, this is a planing hull, no?

So, isn't the 40hp gonna be getting closer to the hump.
My boat build is here -------->

viewtopic.php?f=12&t=62495

TomW1
Very Active Poster
Very Active Poster
Posts: 5844
Joined: Sun Apr 19, 2015 10:57 pm
Location: Bryson City, NC

Re: GT27 Build (Wes K)

Post by TomW1 »

Wes now that I am back at the cabin on the Mississippi, we are on vacation, I looked at the Study Plans and your motor choices again. I am a little concerned about what you want your GT27 to do. Is it to be a displacement cruiser, or a semi-displacement cruiser, or do you want a full planning cruiser? The 25hp will only give you displacement speeds for sure. The calculators will give you speeds of the other motors. If you want, I can calculate a motor that will keep you just above planning speed. But then anything over 50 means you cannot use the plans for the motor Bracket that Jacques includes. So, lets stick to motors under 50HP for now.

Just some late night thoughts.

Tom
Restored Mirror Dinghy, Bought OD18 built by CL, Westlawn School of Yacht Design courses. LT US Navy 1970-1978

fallguy1000
* Bateau Builder - Expert *
* Bateau Builder - Expert *
Posts: 10198
Joined: Tue Jan 12, 2010 12:25 am

Re: GT27 Build (Wes K)

Post by fallguy1000 »

TomW1 wrote: Thu May 26, 2022 12:15 am Wes now that I am back at the cabin on the Mississippi, we are on vacation, I looked at the Study Plans and your motor choices again. I am a little concerned about what you want your GT27 to do. Is it to be a displacement cruiser, or a semi-displacement cruiser, or do you want a full planning cruiser? The 25hp will only give you displacement speeds for sure. The calculators will give you speeds of the other motors. If you want, I can calculate a motor that will keep you just above planning speed. But then anything over 50 means you cannot use the plans for the motor Bracket that Jacques includes. So, lets stick to motors under 50HP for now.

Just some late night thoughts.

Tom
I just wanted to point out that the 40hp is probably going to be pushing the boat at hump speed and getting terrible fuel economy. Can you verify my assumtion?
My boat build is here -------->

viewtopic.php?f=12&t=62495

TomW1
Very Active Poster
Very Active Poster
Posts: 5844
Joined: Sun Apr 19, 2015 10:57 pm
Location: Bryson City, NC

Re: GT27 Build (Wes K)

Post by TomW1 »

Thanks, fallguy I corrected the water weight. I don't know what the 40 or 50 will do until I get his numbers. I don't like to speculate as every boat is so different but probably for the 40HP you're right. Weight is so important in these speed calculations. On some of Jacque's boats I have seen a 10HP increase give only 3mph boost, on others a 5mph. I will start with the 25 as he says it should have a displacement speed of 8-10 mph to make sure that I'm agreeing to his numbers before moving on, normally we are right on. Again, it depends on weight and if Wes's weight is heavier or lighter it may not match.

Tom
Restored Mirror Dinghy, Bought OD18 built by CL, Westlawn School of Yacht Design courses. LT US Navy 1970-1978

Post Reply

Who is online

Users browsing this forum: No registered users and 9 guests