GS 28 - Behavior at High Speed

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Johnston
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GS 28 - Behavior at High Speed

Post by Johnston »

I keep coming back to the GS 28 ...


1) I have been reading the study plans and am curious what sort of speeds the GS 28 would obtain with her maximum designer horsepower rating of 300HP (in a single engine configuration)...

2) I know it is not recommended - but I am curious to know how the performance would be if larger engines were added - lets say the Mercury 450 R - which only weighs 689lbs?

3) I know the boat is designed for "cruising between 20-25 mph" - so I understand she is not designed to be efficient at much higher speeds - but I am still curious how she would behave at say 40 or 50mph? Would she handle ok? And can the hull handle this?

I guess we all know it is sometimes nice just to open it up a little - even if only for a short while!

fallguy1000
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Re: GS 28 - Behavior at High Speed

Post by fallguy1000 »

Hulls are designed with a maximum horsepower. There is almost no way you can add 33% more horsepower without modifications to the design.

The core and skins would be more. The boat would get heavier and fuel economy is lost.

That said, I am not the designer. However, I know enough about boat designs to know this is not possible, nor practical.

The boat would weigh more to add structural elements for the added power. Then she'd not make her lines and sink deeper into the sea, creating more drag at low speeds. If you did it without lines study; the engine height would be wrong even.

The GS28 is a lightweight, high performance machine.

Offshore boats especially need to heed power constraints. The G forces can be tremendous flying off a swell and chop and coming down into a trough. The boat, overpowered, would likely suffer strucutural failures.

Even the load paths for higher horsepower must be reconsidered. Beefier knees, deeper stringers are all part of the calculation, for example.

Build her as drawn; it would be a fine boat. The designer has drawn many high speed boats.
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Johnston
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Re: GS 28 - Behavior at High Speed

Post by Johnston »

@Fallguy - thank you for your response.

Hopefully the designer can answer what her maximum speed would be with the stated 300 hp (max).

It is my understanding from this forum that he is conservative on his HP recommendations - for various reasons - but that doesn’t necessarily mean it could not hold up to more power as designed?!

The proposed Verado 450 weighs 689 pounds and the max allowable weight for twin outboards is 800 pounds - so I would certainly not be violating any weight and balance criteria with this.

I am mostly interested in how the hull would perform on a flat calm stretch of inland water.

I would not plan to jump waves with it at 50 miles per hour… I understand that this is not a Cigarette racing boat and would not use it as such… I am a reasonable seaman and aside from everything else, do not enjoy crashing and pounding around the ocean when a little more careful captaincy would get you there slower but safer!

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Re: GS 28 - Behavior at High Speed

Post by fallguy1000 »

Johnston wrote: Thu Jan 20, 2022 10:34 am @Fallguy - thank you for your response.

Hopefully the designer can answer what her maximum speed would be with the stated 300 hp (max).

It is my understanding from this forum that he is conservative on his HP recommendations - for various reasons - but that doesn’t necessarily mean it could not hold up to more power as designed?!

The proposed Verado 450 weighs 689 pounds and the max allowable weight for twin outboards is 800 pounds - so I would certainly not be violating any weight and balance criteria with this.

I am mostly interested in how the hull would perform on a flat calm stretch of inland water.

I would not plan to jump waves with it at 50 miles per hour… I understand that this is not a Cigarette racing boat and would not use it as such… I am a reasonable seaman and aside from everything else, do not enjoy crashing and pounding around the ocean when a little more careful captaincy would get you there slower but safer!
Your seamanship is not in question. The boat's ability to handle higher horsepower is...

Generally, designers build with certain safety margins. This means going from 300-325 might not be the end of the world, but the designer is certainly not going to lose in a civil matter. The case gets tossed if you say, my boat broke.

The weight is not the issue.

David Gerr wrote a book on the Elements of Boat Strength. In a nutshell, horsepower is a factor in determining skin or hull thickness. You cannot go above the designer's spec because the designer used the horsepower rating to determine the hull.

The designer could go back and plug in the next logical horsepower step 300 to say 325 and see if the skins and stringers and load paths are going to be okay, but it isn't going to work for 450 versus 300. Even the transom for a 30% increase in hp may need to be beefier, for example. The changes become dramatic. The hull bottom thicker; then the boat weighs more and misses her lines. It is not engine weight that is the concern. It is added weight and modification requirements for such a dramatic increase to HP.
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Re: GS 28 - Behavior at High Speed

Post by Fuzz »

I think I saw that JM is off some place with little or no internet so it might take a little while to get your answer.

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Jeff
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Re: GS 28 - Behavior at High Speed

Post by Jeff »

I have sent a note to Jacques to respond/check this question. He is abroad on a vacation spo I don't expect a quick response!!! Jeff

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Re: GS 28 - Behavior at High Speed

Post by TomTom »

Will be interesting to see what the boss says - I know he has sometimes said that a boat can take more horsepower than what he recommends/ prefers.

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Re: GS 28 - Behavior at High Speed

Post by jacquesmm »

Hulls are designed for a certain speed and will become less efficient above or below that.
It is not only a matter of strength but also maneuverability and safety. That overpowered hull could be dangerous.
I worked for Cigarette and I am familiar with very fast boats. The GS28 hull is a planing hull but has a deadrise that varies progressively, compare the profile view of the chine with that of a deep vee hull, the difference is visible.
I don't want to give specs for extreme HP increase, it would imply that I support it.
Sorry.
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Re: GS 28 - Behavior at High Speed

Post by Johnston »

This is understood. Is it possible to give a speed estimate for a single 300 HP … that I believe would be within design limits?

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Re: GS 28 - Behavior at High Speed

Post by BarraMan »

Johnston wrote: Fri Jan 21, 2022 2:07 am This is understood. Is it possible to give a speed estimate for a single 300 HP … that I believe would be within design limits?
How fast will a GS28 go wil 300 ponies on the back?
I’ll venture an answer - fast enough! :lol:

How do I know? :doh:

Cause the recommended max hp for my Hull is 250, I upscaled it 10% but still put 250 on the back.

It’s a big boat (22 ft) and built like a tank! It’s not prop’d for speed - but will nudge 50 kts (call it 60 mph). :help:

Now that I have built it and driven it a lot, I know it would handle 300+ hp with ease - but why? It is nice, comfortable and safe at 25 - 30 kts, sometimes 35 kts, occasionally 40 kts!

Why would I want to do 50 kts and burn 3x the gas I do at 25 kts - it’s a fishing boat ! 8O

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