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Rick
12-20-2001, 08:11 AM
I am referring to my twin GT. The boat is aft heavy. When running with full power it takes significant trim to keep it bow down and to stop porposing, inducing huge drag. Like flying a lear jet with speed breaks. What should the balance be on this hull? Is fore ballast, or fuel tank movement, battery relocation a realistic or good solution. Does antone know what the optimum weight distribution for this hull design?

Woodsy
12-20-2001, 08:57 AM
rick,

I think the reason the 21 GT w/twins is so rare is because of the handling issues of an additional 850lbs of motor & outdrive on the back of the such a small boat. Donzi probably figured this out, thats why there were only 2 made.

I would start to look at a two pronged approach. I do not know what your budget is, but try to lose some weight in the engine compartment... aluminum heads, aluminum intake, aluminum exhaust manifolds etc... you can lose about 300+ lbs on your stern. Thats a pretty signifigant reduction. But it also results in a pretty signifigant reduction in your wallet! But, as we say in the space industry, "lightness is rightness"!

I guess my first approach on a budget would be adding ballast to the nose. I would look at putting one of those water bladders wakeboarders use in the nose. That way you can experiment with how much weight you need and where you need to put it. The lower in the boat the better. You need to be aware of the center of gravity. Water weighs about 8lbs a gallon. Because it is such a rare boat, I would not do anything permanent to the setup of the boat itself. Moving the gas tanks, could produce unexpected handling and possibly disastrous results. Fuel isn't light. Moving the batteries forward will result in a signifigant voltage drop unless you run some serious heavy gauge wire. Not to mention possible ventilation issues.

You also might want to look at a bigger set of trim tabs. The longer ones will work better and reduce the drag required to keep the nose down. It's a leverage thing.

Adding weight to the nose, will certainly slow the boat down, but your ride quality might improve. Your speed might improve as well if you don't have to drag so much tab. You will need to do alot of testing to figure out what the best setup is for you. Test with full gas tanks & 1/2 full & empty. Try to do your testing on the same day. Write down you results and the test conditions. This will help you analyze the problem better.

Just my 1.50 worth!

Woodsy :D

Scott Pearson
12-20-2001, 09:57 AM
Rick,
I have a great solution to that problem...Just sell the boat to me and you wont have to worry about a thing.

....These rocket guys are scaring the hell out of me..... :eek: :eek:


(NJ)Scott

Danny
12-20-2001, 11:48 AM
Woodsey is right on the money with his suggestions. My GT has 350 GM block with the Volvo 280 and my friend has a gt with 454 and Merc Tr drive with the transmission. His boat sits lower and rides/ handles differently than mine. His boat sits low enough in the stern that his trim is not as effective as it should be, almost useless and he has to tab to keep the boat from porpoising as soon as he raises the trim at all. Defeats the purpose. I think tabs are for getting on plane quicker and for levelling and that the ideal scenario is no tab at normal running speeds, however this may not always be the reality.
In the spring we are going to try some different props, hoping that a cupped prop may give the stern a little more lift.
The main intension is to try the bladder scenario as well, allowing you to adjust the weight without doing anything permanent.
Good luck I'd be very interested in learning about your progress on the weight distribution.
Danny

jwright
12-20-2001, 12:03 PM
My 21GT has a small block and I need to use just a bit of tab at WOT to eliminate porpoising. When I do use a little tab I can actully trim the drive up farther to reduce the wetted surface. The boat certainly does not need much positive trim. When I broke a trim line at our Donzi get togther in Michigan, I was amazed how well I ran with the drive all the way down! I can imgaine that mroe weight in the aft might hurt my boat, it is very nice right now.
I think another important factor is that the 21GT (at least mine does) has significant rocer in the bottom. This gives the boat the ability to have "natural" bow lift and is probably hurting you. I hate to suggest changing the hull, but maybe a little hook or wedge in bottom may help too. Maybe you could screw some temporary wedges to bottom for a test to see what happens.
Keep us posted.

turbo2256
12-20-2001, 12:43 PM
I agree weight reduction in the rear would help. last year I replaced heads, exhaust manifolds and intake with aluminum. The weight savings was almost 150 lbs for a carberated motor. Try mounting anything mounted to the stern to the back of the seat. Also you can use sand bags and/or weight plates and move them around to feel out changes in distribution of weight

Rick
12-20-2001, 05:14 PM
Thanks for the input. I will keep you posted on the progress. Scott, I know you would make it right! Thanks Rick

Scott Pearson
12-20-2001, 10:15 PM
Rick,
I'm sure you will work it out and solve the problem. I was just interested in the boat because it is so rare. If you ever want to sell it please let me know. Your gonna have to fix that problem first though.... :D

Thanks!
(NJ)Scott

turbo2256
12-20-2001, 11:18 PM
Forgot a 4 blade prop might help to they givea little lift in the rear. Tats if you dont already have them.

Formula Jr
12-21-2001, 05:35 AM
I like Woodsy's approach. I can only add that finding the proper weight distribution is fairly easy. The water bladder in the nose is a good idea, but I would probably use 1 gallon milk jugs filled with water. These weight 8.3 Lbs each. Put as many as you need as far forward as you can till the boat rides the way you want it to. You will have changed the weight distribution to the proper center of the boat by doing this. Then weight any of the parts you replace with lighter parts on the engines. And get some lighter Batteries. Find the difference in weight of the exchanged parts and remove an equal amount of water weight from the bow.
This will just lighen up the boat and not move this weight distributuion. Once it is all sorted out, go buy some lead ingots that will equal the weight of your residual water jugs and silcone them in the bow. This is really the best you can do. Unless you want to do some grinding work on the hull to remove some of the rocker. Or make a custom hatch out of Kevlar.

Woodsy
12-21-2001, 05:48 AM
Formula has got it right... I didn't even think of milk jugs.... My only concern would be them shifting around or tipping over in a hard turn...those people from the northwest are truly crafty...

Woodsy :D

DonziDreamin
12-21-2001, 12:24 PM
This is not only for a boat that is aft heavy, its for anyone who wants to increase the handling on their boat. Move waight to the Center of Gravity on your boat, on a Vee-style boat it is approximatley 29% from the stern.

GEOO
12-21-2001, 06:47 PM
Rick, Approx. 1/3 CG sounds right. I would try some temporary weight for testing before moving tanks, batteries etc. Try sand bags up front. Weight aft may make the boat run faster at WOT, maybe hook up some type of water balast in the bow??? GEOO