GS 28 - Behavior at High Speed

Ask questions before buying our plans or request a new design. Anybody can post here
Johnston
Frequent Poster
Frequent Poster
Posts: 44
Joined: Wed Dec 23, 2020 4:00 am

Re: GS 28 - Behavior at High Speed

Post by Johnston »

We are based in Gabon.

bklake
Active Poster
Active Poster
Posts: 267
Joined: Mon Aug 04, 2008 9:33 am
Location: Virginia
Location: Virginia

Re: GS 28 - Behavior at High Speed

Post by bklake »

I must be getting old because the picture of the twin engine GS 28 on plane with one engine gets me more excited than going 50-60 kts.

It seems to me that the GS 28 is designed to do a certain task. You are asking for it to do something it wasn't designed to do. I am a huge fan of excess power. A little excess power, not a lot. If 150hp will work, 200 will be better if it is within the design specs. 300hp is a bridge too far (for me).

Johnston
Frequent Poster
Frequent Poster
Posts: 44
Joined: Wed Dec 23, 2020 4:00 am

Re: GS 28 - Behavior at High Speed

Post by Johnston »

bklake wrote: Sun Jan 30, 2022 1:49 pm I must be getting old because the picture of the twin engine GS 28 on plane with one engine gets me more excited than going 50-60 kts.

It seems to me that the GS 28 is designed to do a certain task. You are asking for it to do something it wasn't designed to do. I am a huge fan of excess power. A little excess power, not a lot. If 150hp will work, 200 will be better if it is within the design specs. 300hp is a bridge too far (for me).
I too was very impressed with an image I saw on this forum of the boat on plane with a single 85Hp I believe it was.

But 300 HP is listed in the study plans as being acceptable and so I don't really understand why I feel like I am being chastised for asking a question about what the top speed of an engine that is within the designers limits?!

I also believe many engines share the same blocks/ weights - so it would makes sense to get a 300 that weighs the same as say a 225 - all else being equal?

User avatar
BarraMan
* Bateau Builder - Expert *
* Bateau Builder - Expert *
Posts: 2164
Joined: Tue Dec 11, 2012 3:44 pm
Location: Australia

Re: GS 28 - Behavior at High Speed

Post by BarraMan »

But 300 HP is listed in the study plans as being acceptable and so I don't really understand why I feel like I am being chastised for asking a question about what the top speed of an engine that is within the designers limits?!
I think it’s pretty obvious which way I lean, given that I built a fishing boat that does 50 kts! :help:

The fun thing about building a boat is that you can do stuff that you want to - within reason! I did lots of stuff to my boat, the result being that it looks nothing like the Mangusta 20. My main concern was that it be legal and therefore insurable, so I put the maximum recommended hp on it even though it’s upscale 10%. On Jacques advice I put an additional layer of glass cloth on the outside.

If the plan says 300 hp and that appeals to you, then I say go for it! :D I will be watching with interest! :)

User avatar
cape man
* Bateau Builder - Expert *
* Bateau Builder - Expert *
Posts: 8268
Joined: Sun Jan 20, 2008 12:02 pm
Location: Lithia, Florida

Re: GS 28 - Behavior at High Speed

Post by cape man »

You are right on the weight issue - a 300 and 225 weigh the same, so that's not an issue. I think you got your answer on top speed (upper 40s, maybe hit 50). As to handling at those speeds a pair of trim tabs and a decent skeg is also a good suggestion. Go for it!

As to feeling chastised, don't take it that way. Many here, including myself, are in love with how most of the designs and building methods allow you to power the boats with much lower hp than the same boat built in fiberglass and therefore will question why you want to go to the max. I think your question was a good one and made at the right time - before you started the build. Again, go for it. I would love to see video of a GS28 hitting 50 mph!
The world always seems brighter when you've just made something that wasn't there before - Neil Gaiman

Johnston
Frequent Poster
Frequent Poster
Posts: 44
Joined: Wed Dec 23, 2020 4:00 am

Re: GS 28 - Behavior at High Speed

Post by Johnston »

cape man wrote: Thu Feb 03, 2022 7:25 am You are right on the weight issue - a 300 and 225 weigh the same, so that's not an issue. I think you got your answer on top speed (upper 40s, maybe hit 50). As to handling at those speeds a pair of trim tabs and a decent skeg is also a good suggestion. Go for it!

As to feeling chastised, don't take it that way. Many here, including myself, are in love with how most of the designs and building methods allow you to power the boats with much lower hp than the same boat built in fiberglass and therefore will question why you want to go to the max. I think your question was a good one and made at the right time - before you started the build. Again, go for it. I would love to see video of a GS28 hitting 50 mph!
If it does 50 mph I swear I will only run it at that speed for a very short time ... just long enough to make a video for you! I don't want people to be offended that I am operating it above its most efficient cruising speed :D :D

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

Re: GS 28 - Behavior at High Speed

Post by fallguy1000 »

I was nice to you and you told me to shut it. Let's get that behind us. I don't care.

I'd love to see the boat built, and I don't care how fast you go with it. All I did was offer you help and I still would.

The reasons I offered you help was the designer was away and we have seen him answer the overhorse questions before and I understand the mechanics well.

But I figured it would be fun to know top speed, too.

If you build the boat, you can expect a lot of support on the forum.
My boat build is here -------->

viewtopic.php?f=12&t=62495

bklake
Active Poster
Active Poster
Posts: 267
Joined: Mon Aug 04, 2008 9:33 am
Location: Virginia
Location: Virginia

Re: GS 28 - Behavior at High Speed

Post by bklake »

I like horsepower and a little more is always better. Consider this. A Sprinter van is a great vehicle for what it does. Hold a lot of stuff and move quietly and smoothly from place to place. It has no business competing on a race course and would look silly being out there. I have no doubt that it could run a pretty good time, better than most panel vans, but that is not what it was designed to do.

Many general aviation planes have had modifications made to aerodynamics and more horsepower added. You still can't exceed their max speed or weight. Bad things happen. You can go at max speed using less power for a savings in fuel cost. You can climb out of shorter airfields with the maximum load with more power but you still can't take more than maximum weight.

So a 300hp outboard weighs the same as a 225. Go for the 300. Keep the extra power in your pocket for when your fat friends want to go for a ride. Still able to do 30 knots when lesser HP tops out at 25. Trim tabs seem like a small concession to use the extra power. They have other benefits like leveling out the boat in a crosswind.

The GS28 is my dream boat. I really want to build one.

Johnston
Frequent Poster
Frequent Poster
Posts: 44
Joined: Wed Dec 23, 2020 4:00 am

Re: GS 28 - Behavior at High Speed

Post by Johnston »

fallguy1000 wrote: Thu Feb 03, 2022 9:34 am I was nice to you and you told me to shut it. Let's get that behind us. I don't care.

I'd love to see the boat built, and I don't care how fast you go with it. All I did was offer you help and I still would.

The reasons I offered you help was the designer was away and we have seen him answer the overhorse questions before and I understand the mechanics well.

But I figured it would be fun to know top speed, too.

If you build the boat, you can expect a lot of support on the forum.
Fallguy - I appreciate all the time and effort that you took to answering my question about totally overpower the boat. You are clearly with good knowledge and intentions.

But, this was a question by me to try and understand better how this hull shape might perform at very high speeds. Maybe I should have made that clearer. In future I wont ask theoretical questions without clarity!

Nonetheless - my question about whether the designer was comfortable with the speeds this hull could attain with the listed max of 300 HP (within the limits on the study plans) still have not been answered. I felt like this important question got overlooked.

Forgive me if I have offended you - but this question for the designer about whether they are comfortable with a 300 HP engine at full throttle on this hull design - still remains to be answered. I appreciate he is away and will gladly wait for his input.

Many thanks for everyone's input and support.

jacquesmm
Site Admin
Site Admin
Posts: 28215
Joined: Wed Aug 14, 2002 1:00 am
Location: Florida USA
Contact:

Re: GS 28 - Behavior at High Speed

Post by jacquesmm »

Johnston wrote: Fri Feb 04, 2022 4:51 am . . . but this question for the designer about whether they are comfortable with a 300 HP engine at full throttle on this hull design - still remains to be answered.
A 300 HP at full throttle would, at designed weight, push this hull close too or above 50 mph. 300 is the limit I list on the study plans in part for liability reasons. the transom and structure can take 300 HP or more with very little modifications.
See this on the study plans:
. Keep in mind that while we show a top speed of 38 mph, the hull is designed for cruising between 20 and 25 mph. 125 HP is the minimum to maintain
planing, 150 to 200 is ideal.
For more speed, ideally, the hull should be slightly different, with the parallel buttocks lines extending further forward. Some porpoising should be expected at very high speed, trim tabs will be a must. If the builder insist on more than 300 HP, I will give some tips about increased fiberglass scantlings. The bottom core thickness would stay the same.
Jacques Mertens - Designer
http://boatbuildercentral.com

Post Reply

Who is online

Users browsing this forum: No registered users and 8 guests