Break out the Calculator!

Anyone interested in helping out again with these mind-numbing calculations? I recently posted a thread concerning the position of the fuel tank on a 28 Topaz we are converting from inboards to bracket mounted outboards. I found the measurements I needed to plug into the formula some of the guys provided, but I must have missed something,(the tank can't sit 18 ft. from the transom!).
I'll put out the measurements I came up with and maybe the brains out there can help me out....again .. Thanks!!


Old motors each 1100# = 2200#
Distance from transom to center of motor = 10'-0"
length of old motors w/ gears = 4'-0"

Old fuel tank = 235#
Distance from transom to center of fuel tank = 40"
length of fuel tank = 48"


New motors each 525# = 1050# total (225 yamahas)

Transom bracket = 235#
Transom bracket length = 30"

New fuel tank length = 7'-0"
Tank weight = 255#


I think thats it . Many THANKS in advance!

Capt. Chris

 

I guess I might be wrong,but i think it's important to know where the CG is to make this kind of calculations.

 

Chris,

I think your calculation is correct, the new engines are half the weight of the old ones and 8 ft closer to the transom. Weight of the fuel tanks are about the same, so you have to compensate for a lot of momentum from the old engines!
Try this calculation again, but add a ballast weight. Move the tank to the position you want it to be and figure out how far the ballast has to go and how much it has to be.

 

Ballancing act

Hi Captn Chris.

My approach to this would be to start at the transome as before. Next, we have to calculate the moment of the new engines and mounting system. To do this, we must estimate the horizontal center of gravity of each component and multiply that times each components weight. And since we are now working behind the transome, we will use negetive numbers. So here goes:

engines = 1050lbs x -2.5ft (the end of the 30in brackets) = -2625ftlbs
brackets= 235lbs x -1.25ft (half of 30in) = -294
-2625ftlbs - 294ftlbs = -2919ftlbs for your new engine system.

Next we subtract the moment of the old engine system which we are removing.

-2200lbs x 10ft = -22000ftlbs.

Now we 'add' the two moments together.

-22000ftlbs -2919ftlbs = -24919ftlbs

This is how many ftlbs your boat is now out of trim. To correct this, you will most certainly need ballast. How much you will need depends on:

1.) How far you can move the tank x its weight half full of fuel, and
2.) How far forward you are willing to put the cg of your ballast.

Let say you can move the tank 2ft forward. The tank wieghs about 235lbs and it will have about 700lbs of fuel in it when it is half full. This give us:

(235lbs + 700lbs) x 2ft or 1875ftlbs of positive moment.

Next, we add this to our original moment of 24919ftlbs.

1875ftlbs -24919ftlbs = -23044ftlbs of remaining moment.

Now we divide that by how far forward we are willing to put he cg of the ballast. Since we want to use as little ballast as posible, we will but its cg as far forward as possible. Let's say we can put it 18ft forward of the transome.
We now get:

23044ftlbs / 18ft = 1280lbs of ballast that we will need.

It is interesting to note that by using the lighter engines that must be transome mounted, we have actually had to make the boat heavier.

-2200lbs (old engines) +1285 (new engines and mounts) + 1280lbs (ballast) = 365lbs of additional weight. Not only that, but these new weights are much further out in the ends of the boat which can contribute to pitching.

Ahhh. The wages of retro design.

Best of luck.

Bob

 

Why do you have to add weight to make it correct? First off that assumes it was correct when new Most importantly why not use large trim tabs for added lift at the stern if needed?

 

tale of two buddys

Hi Woodboat

In answer to your question, imagine a 13ft runabout with a 220 lb man on board. It just so happens that where he is sitting makes the boat sit perfectly level. Now enter his buddy who wieghs in at a more modest 129lbs. He is out swimming and he just happens to decide to climb on board over the transome. The only trouble is that he didn't tell his freind this. And his freind, at this precise moment, decides to jump in for a swim. Even though the boat is loaded with picnic, camping, and fishing gear. The bow goes up, way up, and the stern goes down. The boat looks comical in this trim but is in no danger of sinking. Even so, no one would consider trying to power in this trim. Not even with a trolling motor.

Trim tabs can do a lot. But they can't work miracles. And what happens if the boat sudenly stops?

As to your other question. I sure hope the original designer knew his business (if not, I want his job!). But, just in case, the ballast can be temporary. Otherwise, it is my guestimate that the boat will trim down by the stern about 12in and up by the bow by about 18in. In this conditon would this 10,000lb boat even be able to get going fast enough for the trim tabs to work? At the very least, it would look like s**t.

Hope this covers it.

Bob

 

Well, not really. This is what I am thinking. The man jumping on board really doesn't apply to this boat at rest as it is 28FT and is lighter since removing the inboards. Additionally equipment such as the fuel tanks are being move so to compensate. So at rest the boat should be just fine. Also inboards tend to be front heavy. Every inboard I have owned has the bilge forward. Every outboard has the bilge aft. So the boat may have been a compromise with inboards and actually is in better trim with outboards. My current inboard boat has Vdrives with the engines very close to the stern. This boat planes extremly easy and runs well for it's size. So... I suspect with water tanks and fuel tanks forward and trim tabs that trim will be very good without adding weight.

 

Woodboat,

Just read all posts and look at the equations, maybe you'll see the light then!!

 

Thanks for the quick reply to my questions. I wish I haven't learned so much about boat design recently!! All those numbers!!... but seriously I think the weight situation is very important thats why I reach out to you folks for your imput. Thanks. The bracket we are building I found out will have almost 600# of bouyancy. This should be a big help with the motors aft. I also have had two of these Topazs (28'-29') in the past and they both were very bow heavy with light fuel. They ran ok without much tab too. This project will get a set of Lenco tabs from the start. We also just bought a new 20 gal water tank to mount under the cabin sole. Don't worry there will plenty of (ballast) foward, as the cabin is the first place for all crap to collect Getting some pictures together soon .. promise not to say "are you nuts!" and I'll post them
Still dazed but mostly confused,
Chris :?:

 

Hi Capn Chris

This time I will learn from you. I am very unfamiliar with large planing power boats. The only thing I know is that they have two dominate bottom designs.
One is the so called constant deadrise like you see with Sea Ray, Cigarrets, Donzis, personal water craft, and other high performance mono powerboats designed for moderate to rough conditions.
Two is the so called warped bottom, like you see with lobster boats, small aluminum fishing skiffs, and other work boat types.
The warped bottom is best at being able to be driven at a variety of speeds, from trolling to planing, but can pound and slam when driven fast in rough water.
The constant deadrise is best at being driven fast in rough water and can tolerate much worse conditions than its warped bottom couzen, but really hates being driven at slow speeds. There, it is inordinately fuel thirsty and hard to keep on a straight course.
The reason I bring this up is to point out that one has its bilge, the lowest point in its hull, in the bow (the warped bottom) and the other has its bilge toward the stern (the constant deadrise).
I do not know which of the two types your boat is. My concern is that it is the warped bottom type. If it is, having it trim too far aft can accentuate its bad qualities (the pounding and slamming) and minimize its good points (the wider range of speeds it can operate in).
Both bottom types have deep 'V' sections in the forward part of the boat, but, in the warped bottom's case, these sections flatten out until, in some cases, the bottom is dead flat at the transome. With the constant deadrise type, The 'V' sections flatten out only a little then continue all the way to the transome without flattening any further.
Hopefully, this discription will help you determine which hull type your boat is. Perhaps Wooden Boat is correct in saying my dire trim warnings may be a bit extreme.

Also, the 600lbs of exta bouyancy will certainly help, but I think the real question is is how far out of trim the other way the original configuration was.

Perhaps its best to 'split the difference'. Why not provide for half the ballast moment recommendation I made and make that all removeable. Sand bags, perhaps. Then, when the boat is completed, do sea trials with and without the ballast. But take your time about it. Make sure you do them in a wide range of sea conditons in both the with ballast and with no ballast configuration before making your final decision.

On a more personal note. Back in the day, when I was studying boat design, I was assigned to design a 'sport fisherman'. This is a large planing powerboat with a low stern cockpit for fighting, then landing large game fish. It needed engines that were big enough to get it on a plane, but were also far enough forward to be convenietly out of the way. You can imagine the trim problems that had (I still have nightmares). Being, as usual, more interested in inventing than copying, I tried to do it 'my way'. What I got was a top heavy monster that had to have ballast tanks to keep it upright (next time I will copy).

Best regards.

Bob

 

Excellent advice. I think this is the boat. The review talks about it being very nose heavy.

http://www.yachtsurvey.com/boatreviews/topaz_28.htm

 

It's a Topaz thing.

Thanks for all the interest in the mighty Topaz guys!. For the record the boat is a modified V bottom (15 deg.) . It has a beam of 10' and is a planing hull. They were a bit nose heavy because the engine placement was foward to give it the giant cockpit size they were famous for. I appreciate your concern for the stability of the craft, but in the past I have owned or ran many different types of powerboats and one boat in paricular comes to mind . The 30' wellcraft scarab I owned had a deep v hull (24 deg.) and twin 454 big blocks right against the transom with mercruiser sterndrives, the fuel tank was only 18" in front of the motors and it held about 185 gals. Yeah it was *** heavy for sure, but it never felt unstable. We ran it hard in the ocean on many ocassions. That was in younger years, now I have a 35' Viking flybridge converible (sportfisherman). It weighs 12 tons and has a modified v bottom like the Topaz, which for me I like the best because it's best of both worlds. Stable at rest (drift fishing) not too bad in a head sea, maybe pounds abit. Thats what prompted us to rebuild the Topaz. Fast, stable and hopefully more fuel efficient. As for adding weight, ballast .. wow .. thats a no no in the speed game . The weight , the more horsepower, the more fuel .. the vicious cycle begins! I'm with Bob the next boat show I go to I'm going to do some copying !!

Thanks
Chris

 

Hi Capn

Good luck to you on your project. I will ask you one more time to reconsider the ballast issue.

Vince Lombardi once said: "Winning's not the main thing. It's the only thing."

He later expressed regret for making that statement.

Sacrificing a little speed for more comfort and more safety may be a good bargain indeed. What good is a faster boat if you're affraid to use it.
And if it slams too hard on its flattish bottom on seas its bow was supposed to part, that's exactly what's going to happen.
I hate being so forceful and blunt, but it's only because I'm concerned.

Bob

 

You have too much faith in the original designer and their powerful marketing. The reviews I have read and by Capt. Chris's personal observations the original design was a nose heavy compromise done to increase cockpit area. A quote:

Also the transom bracket adds 600# of positive flotation. If your right it should be easy to tell on it's first trip around the block Capt. please come back and post how it rides so we will all be better informed no matter which way it goes.

 

Not to worry Bob!

Thanks for your genuine concern Bob. I'm a long way from the water with this project and have many more days ahead of reseach on this project. I plan on hitting many boat shows to look at boats like Contender,Hydrasport,Century,Yellowfin,Intrepid,Regulator and many others that run big outboards on brackets or transom extensions. This is the new trend in inshore and near offshore fishing because of todays outboard technology,more range less fuel and a 30 kt cruise. As far as safety is concerned , the prudent skipper plays a big part of the picture. I am a Coast Guard licensed captain and my first responsibility is to the safety of my passengers and my second is to GET THE FISH!!!

Thanks
Chris