GT27 Build (Wes K)

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TomW1
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Re: GT27 Build (Wes K)

Post by TomW1 »

Wes try to put your thoughts together on what you want this boat to do for you. The motors you are looking at may or may not get you on plane and then only at wide open throttle. If you just want a displacement cruising boat then a 25-30HP motor will work fine. If you want to get on plane and make time at times you will want a 75-115HP motor. Perhaps with a smaller kicker to cruise with. Well some things to think over tomorrow.

Tom
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cape man
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Re: GT27 Build (Wes K)

Post by cape man »

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?
We'll be running at displacement speeds.
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.
Twice in his first message he says displacement speeds... 8) His question is what will he gain if he goes from a 25 to a 40, 50, or 60.
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Re: GT27 Build (Wes K)

Post by fallguy1000 »

cape man wrote: Thu May 26, 2022 9:43 am
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?
We'll be running at displacement speeds.
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.
Twice in his first message he says displacement speeds... 8) His question is what will he gain if he goes from a 25 to a 40, 50, or 60.
Right.

Wes has been clear that he wanted displacement speeds, but it would be good to know what the 40 does around the hump.
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Re: GT27 Build (Wes K)

Post by OrangeQuest »

fallguy1000 wrote: Thu May 26, 2022 10:43 am
cape man wrote: Thu May 26, 2022 9:43 am
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?
We'll be running at displacement speeds.
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.
Twice in his first message he says displacement speeds... 8) His question is what will he gain if he goes from a 25 to a 40, 50, or 60.
Right.

Wes has been clear that he wanted displacement speeds, but it would be good to know what the 40 does around the hump.
If the 40HP comes in a high thrust then he would gain more push against current and wind. And if needs to go faster than "cruise" speeds then have the throttle reserve to do so. Like Dan has said, 80% throttle to "cruise" or 40% to 50% throttle. And how much of a difference on fuel consumption verses wear on a smaller engine. There is a lot of freeboard and cabin that could catch the wind very easily, would fighting that be enough if just to cruise you are using 80% of the throttle? Not much of a safety net on windy days getting back to a safe port.
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Re: GT27 Build (Wes K)

Post by Jaysen »

Unpopular and likely not an option… 2x25 dual throttle at helm. Allows for motor trim for windage as well as economy at cruise. Likely not much heavier than 50hp.
My already completed 'Lil Bit'. A Martens Goosen V12 set up to sail me to the fishing holes.
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Re: GT27 Build (Wes K)

Post by TomW1 »

Displacement it is. Let me run the numbers and see what his bes option is. The 40 is not a HT but not bad with a 2.27 gear ratio instead of around 2.0 you see on other brands in that HP range. You guys have me curious now, so I am going to run Jacques 5000lb imaginary boat through my calculators using a 25, 40 and the 2 50 HT motors. I wisll do the same for Wee if he wishes and he is not far off the 5000lbs, then he can use theses.

Till later today.

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

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Re: GT27 Build (Wes K)

Post by TomW1 »

Wes here we go starting with the Yamaha 25HT, Yamaha 50HT, Susuki 30, Suzuki 40, Suzuki 50HT. I have put cruise at 60% of top speed as that is where I have found it to be in most boats, but you will just have to determine it for yours, once the motor is mounted.

Yamaha 25HT, 2.08 gear ratio, with a 9.25 x 7 prop, top speed 13, cruise 7.5-9.5, 141lbs 20inch shaft
Yamaha 50HT, 2.33 gear ratio, with a12.25 x 8 prop, top speed, cruise 18.5, 11-13, 262lbs 20inch shaft

Suzuki 30HP 2.09 gear ratio with controls, with a 10 x 7 prop, top speed 14, cruise 9-11, 158lbs 20inch shaft. I thru this one in as it was similar to the Yamaha 25HT.
Suzuki 40HP 2.27 gear ratio, with a 11 x 8 prop, top speed 16.7, cruise 10-12, 229lbs with 20inch shaft
Suzuki 50AV 2.42 gear ratio with a 12 x 9 prop, top speed 18.7, cruise 11-13, 253lbs with a 20inch shaft

Notes: the Suzuki 30HP and 50AV have an rpm range of 5300-6300rpms, all the others have a 5000-6000rpm range. All are 20-inch shafts. Weights are per MFG listing which is without fluids. Just for fun I checked what the top speed would be if run through the calculator for a 90HP motor. It came out the same as Jacques 25mph. I used the Yamaha 90 specs and added 100lbs to the boat weight as the 90 weighs 100 more than the 50. That is all I changed.

Wes, I hope this helps give you an insight to what the various motors can do for you.

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

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Re: GT27 Build (Wes K)

Post by wkisting »

Sorry for the delayed reply. We've been working long hours on the boat. Part 8 of the build videos is now up if you'd like to see the latest progress: https://youtu.be/Wp0nX7SgZxk

I really don't know what the finished weight will be, but I will estimate 5500 lbs. That could be way off. Hopefully not.

We're only looking for displacement speeds, which I assume is around 8mph--although Tom's numbers suggest maybe cruising speed will be higher (?). Jacques had said a 25hp would push it to hull speed, as Tom's numbers seem to bear out. However, I'm worried about having reserve thrust for windy conditions. The sides are high and it will catch a lot of wind, so I'd like extra horsepower and a larger prop to help muscle it around under better control. Mainly, I'm wondering if it's worth the doubling of weight for the higher thrust.

I have a professionally made aluminum motor bracket (flotation style) on the back, rated to handle/support up to 150hp. I won't go that large on the motor, of course, but the weight of the motor should not be a problem in terms of bracket strength. The Suzuki 60hp and 50hp high thrust models weigh the same (~251 lbs.). My understanding is they can swing up to a 14 inch prop... Am I correct that, if I have that option, it's better to go with the largest diameter available (14") for max responsiveness and "grip" on the water at slower speeds?
Wes
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fallguy1000
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Re: GT27 Build (Wes K)

Post by fallguy1000 »

The largest prop may not be very efficient because props are generally sized so the motor can achieve rated rpms. If the prop is too large, the engine cannot achieve that and the engine will not rev up all the way. Had it happen..

Consider your desire to battle an unexpected wind and the consider what happens when you can only take the engine to 65% of rated rpms.

So prop is sized to achieve rated rpms, not for 'grip'.

I think I'd go suzu 40hp or to 50hp and HT if poss, despite wandering a bit and wondering if this gets you just below the hump. I have been on a lot of houseboats and they never want you to run them above 3000 rpms. They shake a bit if u go higher because hull speeds are made there. Most of our speeds are around 7-10knots. If we travel too far away for return day; we'd split the return trip into two days.

I wish Tom would be able to explain where the hump is better, but perhaps he can't. Personally, I'd be terrified going 18mph in this boat because there is so much top.
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Re: GT27 Build (Wes K)

Post by Fuzz »

I have an 8hp high thrust Yamaha and a Suzuki high thrust 9.9hp. The Suzuki uses a larger 4 blade prop. The Yamaha has an even larger 3 blade prop. This is how they came from the factory as they are built for push and not speed. I have run both of them on my 22 foot boat. The boat all up in fishing trim with 1000# of people onboard weighs about 6000#. Both motors will push the boat over 5 knots. I am not sure of the max speed as I have never had a reason to check. I do know after 5 knots or so about all that happens is more noise is made. About props the simple answer is large and flat for push and smaller with lots of pitch for speed.

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