Fuel tank placement with bracket

captmattson · #1 07-13-2013, 08:33 PM

I'm currently laying out my '69 20 sf. It was nearing gutted when I received the boat. I have a bracket on the way from hermco. I'm planning on a 200 etech or the new lighter 4 cylinder 200 Yamaha. I'm building the boat with the live well under a leaning post and battery in the console. What would be a recommended distance from the transom or placement of a 75 gallon fuel tank? I'm thinking centered or a foot forward of center. ?? The hull didn't have a floor or fuel tank on board when I purchased it. I'm also raising the floor 2". The shearline has been cut down 2.5" toward the stern and the new transom will be enclosed. The boat will be laid out in a hybrid bay boat design.

Ill be starting a photo journal on the site this week. Thanks for all the help.

CHANCE1234 · #2 07-13-2013, 08:56 PM

I don't know about the fuel tank but with the modified lower back end and raising the deck 2 inches you will not have much freeboard at all. Which may be fine if your just using in calm waters. Good choice with the hermco, popular around this website.

captmattson · #3 07-13-2013, 10:24 PM

no worry with the freeboard, it will be a bay boat style boat

RUSTYNTABATHA · #4 07-14-2013, 07:21 AM

the cut sheer gives her some realy nice curves...lol .. and with the floatation bracket i would think moving the tank forward could be an issue unless you run some calculations based on the weight of the motor you plan to use.

flyingfrizzle · #5 07-14-2013, 11:38 AM

I love the way a broken sheer line looks. I have done something simular to a 26' Silverton hull I have. It has become a lost project at this time. I might get back on it once the 27' sc is done.

FishStretcher · #6 07-14-2013, 01:02 PM

You might want the fuel tank under the bow cleat. (only sort of kidding)

There are many detailed posts (Bushwacker has one, I think) that talk about how a bracket can hurt the ride and handling of a 20 footer by putting all that weight back there that isn't floating when you are on plane, but is floating when you aren't on plane.

Bushwacker · #7 07-14-2013, 01:39 PM

Quote:

Originally Posted by captmattson

. . . What would be a recommended distance from the transom or placement of a 75 gallon fuel tank? I'm thinking centered or a foot forward of center. ??

Moesly generally tried to center the fuel tank right on the boat's CG so trim wouldn't change as you burned off fuel. (As a pilot with something like 13,000 hrs in military cargo planes, he understood the importance of CG location better than most folks!) Of course the original design location of the tank was based on a motor weight of about 300 lbs and no bracket! I do know that after installing a 429 lb motor on a 30" setback Hermco bracket, I had to move the axle on my trailer aft 11.5" to keep enough weight on the tongue, so you would have to move the tank at least that far forward from the stock location, to keep the same CG.

I realize that the Seafari is a different animal than the CC model with more weight up front, but just to give you one data point, the CG on my boat currently appears to be about 6' forward of the transom. Since aft cross member on my EZ loader roller trailer is free to rotate or pivot on the bolts attaching it to the trailer frame, and most of the weight of the boat is carried by the bank of 16 rollers on the rear cross member, the whole boat rotates during launching or loading when it's CG crosses that pivot point (the bolt just forward of the light/license plate). The first picture below shows the boat being winched up on the trailer just before it pivots, so the CG is aft of the rear cross member at this point. In the next picture the bow had dropped down at least a foot onto the front bank of rollers, so the CG has passed the pivot point. The side windshield ends 6'4' forward of the transom, and the pictures indicate that the CG is aft of that, so I'm guessing that it's roughly 6' forward of the transom.

The optimum CG location depends on how you plan to use the boat. I run offshore a lot, often in seas of 2-3'+, so I'd move the CG forward on my boat at least a foot if I could, because I'm having to run a stern lifting 4B prop and a Doelfin to get my planing speed down to 12 mph like it used to be with a 300 lb motor and no bracket. That prop and fin have limited my top speed to about 39 mph per GPS. If I run a smaller prop with more pitch and no fin, I can run almost 50 mph with a light load, but that combo won't plane below about 22-23 mph! (A friend of mine just repowered his Seafari (no bracket) with a 150HO E-TEC and he said it pegged his 50 mph spedo on the first run, before he even started playing with props and engine height!)

With the low freeboard you'll have with the cut down sides and raised deck, I'm guessing you're not building an offshore boat, but want something that will run fast in flat water. In that case, you'll probably want your CG about where mine is. Maybe you could find the approximate CG of the bare hull by putting a 2x4 and floor jack under the keel and lifting it slightly off the trailer bunks to see which way it tilts when you lift it, then keep moving the jack around until it doesn't tilt. Once you have that info, you could set up a spreadsheet to calculate the approximate CG location with various gas tank and console locations. If you PM me with a snail mail address, I could mail you some sketches showing how to calculate the CG location. Denny