18 Center Of Gravity Shift ?

[silverghost]

As many of you are aware I am involved with a Donzi "Benchseat 18" conversion project. "YellowJacket"

The converted boat's new center of gravity now being shifted aft is an issue that has been on my mind for some time now.

Since the driver's helm seat position is now located at the original aft benchseat location I need to think about shifting some weight foreward.

Just how much weight to shift is the BIG question ?

I plan on installing a new bigger gas tank foreward towards the bow of the boat.

I am also trying to decide if I should keep the old Volvo~Penta 270 unit & add a Volvo "T" trim assembly or go with another new outdrive system.
The old 270 has some rust issues and is now frozen in place and does not move from side to side to steer; nor does it tilt up !

I am planning a new higher horse-power smallblock installation; possibly switching to Chevy power.
Chevy gives you more bolt-on performance options.
I have not ruled out Ford all together however !

My question to the forum members~~~

Does anyone know the old balance point for the older 1970s Donzi 18 with a full tank of gas. Basically a fully rigged & loaded ready to launch boat ?

Does anyone have a measurement from the stern trnsom of the older 18 boat to the center balance point on the hull ! The point where this boat would balance equally fore & aft ?

ALSO~~~

What was the average passenger load expected by the original designers in the old two front bucket seats ? (mine are now long gone~~~but I must properly account for this lost weight also )

If I had the information above I could then compute the amount of new weight shift needed foreward to properly maintain this same designed-in original balance point.

After this project is finally completed I want this boat to ride & plane properly like the
original Donzi 18 2+3 we all know & love so much !

I don't want this boat to ride with her bow sticking in the air & be unable to plane at speed .

I need to get the balance right the first time~~~
I do not wish to depend on heavy trim tab down angles to get level trim on this finished project .

Any Center of Gravity ideas or balance information greatly appreciated !

Thank's

[MOP]

You need to try to contact Geo! When I did my 22 over I asked his advice, which lead me to move as much weight aft as possible. I now have a new tank just ahead of the engine and just about everything in the engine compartment is back as far as I could get it. The results have proven what he told me! The boat did 61 it had a Tempest 300 package with an Alpha, I build a very mild 383 it makes about 370 HP going from 300 to 370 with no CG change would have netted maybe 63. I also added a Bravo which steals about 3MPH over the Alpha! The boat on a good day runs 65.5, take close notice of go fast V hulls they sit very low in the stern. These hull need the CG aft, you have to realize our boats are built for the masses. They need to be commodious not strictly setup for speed, keep the weight aft and you will go faster. These hulls to get the nose up, only two ways to achieve this weight transfer aft or drive trim. Getting the nose up and keeping the drives thrust parallel to the water surface provides the best overall performance. Sure you can get great gains in speed with a lot ponies, but why not think about getting the CG in a better place. Don Tamm said it best you can overcome a lot with HP, I think you can achieve even more using you head.

Phil

[MOP]

I needed to add a little more! When I did my boat over there was a bit of skepticism about what I was doing, several said it would not plane off. Initially is had a Blackhawk drive which a few said would not work, in only one respect were they right top end. Using the BH on my hull was thought to be a total bust, I proved that wrong ferrying a boat load of people to my buddies island up a t 1K. It planed off great with 5 people, the BH which is a harder drive to plane off with than a standaard leg. I do not have long tabs just standard sized Bennets, with the Bravo I still need a fair amount of trim a WOT but less than a stock CG boat. If I had it to do over again it would still have the Alpha, trouble was I listened to some of the BS thast cost me!

[maddad]

Silverghost, I don't think your moving enough weight far enough to make the nose stay in the air or have trouble planing off. A 400+ ci smallblock and an old Volvo drive is a cheap, fast and dependable set up. Steering gets important as you get faster.

[mrfixxall]

i dont think you will have any problems with the nose being to lite..remember the added weight in fiberglass and wood that is added to the deck to make the boat a bench seat,worse case scenario, you could always add a ballast tank in the nose like fountain did on their 24' boat..

[silverghost]

Fixx~~~
You, and my late father, are on the same wavelength with the ballast tank idea.

Before he passed-away early last Summer Dad bought a 15 gallon Grey Poly-plastic tapered & V bottom shaped & bow-shaped gas tank at a new marine parts surplus house complete with an electric float level sender assembly .
It would tuck up nicely in the 18's bow just ahead of the new bigger fuel tank between the stringers !

His idea, like yours, was to use this as a ballast trim tank with water or extra reserve gasoline to help trim the benchseat 18 boat.
The empty tank itself weighs very little ~~~possibly 25 lbs or less !

The tank seems to fit in place in the foreward hull like it was actually made for the Donzi 18's bow !

I will have to post a digital shot of this tank~~~
I never saw another like it !
Cost~~~ $ 35. !

I will install it in my project~~~just in case I find I really do need it~~~
I will install a through hull gas tank vent & deck fill & hose assembly; along with running a pair of wires from the tank's level gauge to the back of my dash.
I also will install a drain line in it's 3/8" drain plug fitting; and run it aft also along with an extra tank top fuel pick-up line !

Better to do this before I put her repaired & re-cored deck back on !

If I don't find I really need it for ballast~~~

So what~~~
Out of sight~out of mind !

[mrfixxall]

Fixx~~~
You, and my late father, are on the same wavelength with the ballast tank idea.

Before he passed-away early last Summer Dad bought a 15 gallon Grey Poly-plastic tapered & V bottom shaped & bow-shaped gas tank at a new marine parts surplus house complete with an electric float level sender assembly .
It would tuck up nicely in the 18's bow just ahead of the new bigger fuel tank between the stringers !

His idea, like yours, was to use this as a ballast trim tank with water or extra reserve gasoline to help trim the benchseat 18 boat.
The empty tank itself weighs very little ~~~possibly 25 lbs or less !

The tank seems to fit in place in the foreward hull like it was actually made for the Donzi 18's bow !

I will have to post a digital shot of this tank~~~
I never saw another like it !
Cost~~~ $ 35. !

I will install it in my project~~~just in case I find I really do need it~~~
I will install a through hull gas tank vent & deck fill & hose assembly; along with running a pair of wires from the tank's level gauge to the back of my dash.
I also will install a drain line in it's 3/8" drain plug fitting; and run it aft also along with an extra tank top fuel pick-up line !

Better to do this before I put her repaired & re-cored deck back on !

If I don't find I really need it for ballast~~~

So what~~~
Out of sight~out of mind !

you wont need a deck fill,you can use the engine's water pump to fill the tank but you will have to reduce it so water can still go to the engine to cool it amd use a electric water pump to empty it to a through hull fitting..

[Greg Guimond]

Before you drive yourself nutty guessing, get a real reading of where the Center of Gravity really is. The sling is the easiest but there are several ways to get the data.

[silverghost]

Greg:
Great idea !

Here is my problem~~~

There is no engine in my project boat~~~
&
The old fuel tank will not hold any liquid ! It's rusted-out !

Any balance measurements that I make now would be pointless without an engine & a full fuel tank.

I have no idea where the center of gravity SHOULD be on a properly balanced & 1/2 fueled stock Donzi 18.

I need this proper balance point measurement in the future when I have the new fully rigged engine, (unknown at present), & new 1/2 full fuel tank in place.

I will then be able to adjust my converted benchseat 18 boat's internal weight fore & aft to get the boat's center of gravity balance point back to where it really should be on a stock fast Donzi !

Perhapps the Donzi factory made a trade-off between fastest possible hull speed & the best balance position for the old 2+3 passenger set-up ?

I am building this boat for best possible realistic speed & balance from the new rear benchseat helm area .

I have a sling , chain hoist & I-beam to do this balance test here later when the engne & 1/2 full fuel tank are finally in place !

I just need to know what the old factory stock 2+3 Donzi 18 balance point is ?

Does anyone know what the old factory stock balance point measurement is from the transom?
I suspect some person on the forum has measured this balance point before ?

[Greg Guimond]

We had the same issue and used sandbags to replicate the engine weights. That helped to make a final selection of tank size and location. If you read through "Building Surface Tension" you'll see the details.

[MOP]

Get it done then use movable ballast to tune it, any other way is just guess work.

[joseph m. hahnl]

I agree with Mop. "Giter dun" then if necessary add ballast.

We use to sit all the way in the back of the 13 whaler and drive with our feet. With the 1973 40hp long shaft it kept up to a MFG with a 70hp.

Now with a 50hp short shaft, I moved the interior aft by 6". never had any problems getting on plane or at speed. The only time I ever notice the fulcrum point, is when it's just floating or at headway speed.

The only thing I would be worried about is the height of the exhaust tips,
when it's just floating, waves , wakes, bilge water ,and so on.

[Greg Guimond]

Two different approaches to a common problem. The gas tank is mounted 13" forward of the transom in this weedeater. When filled with fuel she runs fully "planted" at top end and re-entry is very square with no drama. If its me, I go the strap route first to get the best shot at proper CG, then you can fine tune with a little ballast if it is even needed. I just hate carrying any extra weight when benchracing

[MOP]

You may or may not need weight, adding prior to having the know how of how it sits and handles is foolish. I have been working on and building boats for longer than most on this board are old, just like all other facets only wet testing will tel the rest is just conjecture!

Phil

[silverghost]

Phil:
I know you are right~~~and I respect your opinion as I know you have much more hands-on performance boat building & repair + rigging experience !

I guess it's just the old Mechanical Engineer in me wanting to sort out all possible potential problem issues on paper~~~First~~~ before I build-up this project !

I know too well that this is not always possible in the real world .