Too Stern Heavy?
 

OK, I have a 28’ Checkmate with twin 502s with Stellings Boxes and Hydraulic Steering and am looking at putting side mount Whipples on the engines. The boat has an empty cabin (just facing couches and the V Berth cushions) so it is light in the bow. This combination is very stern heavy and with the addition of the Whipples, it will be worse. The question is, is there an ideal center of gravity for a standard deep V hull? Is it possible to have the bow too light and the stern too heavy for maximum speed and good handling? If yes is there a formula for proper hull balance? I know that some cats are set up with the engines forward to balance the air entrapment hull but do not know if this is important for a deep V hull with no steps or a pad.
Thanks.

If you are going to want to go faster, then moving the CG aft is actually a good thing, and will help carry the bow. It is going to take more tab to get it on plane, and the tradeoff is that you won't be able to stay on plane as slow as before. Fuel consumption running at cruising speeds will go up due to having to use more tab to keep from porpoising, but that is the trade-off on a non-step hull.

Now, twin 502's & boxes is quite a load for a 28. If the setup was good before, though, you should have a good starting point once the blowers are added. You can always bolt some ballast into the nose if it needs it.

A thing to consider. If you installed alu heads and performance exhaust manifolds or headers, you would lose quite a bit of weight right there. Heads could net you about 70lbs pr. engine, but I'm not sure on the weight savings on something like EMI or KE manifolds and Hardin, Stainless ect. headers. If going SC, alu heads and a good exhaust would not be a bad idea anyway.

I went thru this a few years ago on my 272 Baja, except I had the opposite problem, too light in the rear and it wanted to bow steer/not air out unless I had 4 passengers (300-400 lbs in the back seat), I bolted some removable ballast in the very rear of my molded swim platform, to the tune of about 50 lbs. It was just a starting point and turned out to be too much (which will be similar to your possible problem). When I would plane out the nose was then in the sky and boat acted REAL stern heavy as if I had 500 lbs in the back seat. I cut the weight down to about 20 lbs and it works about the best its going to. When I was reasearching the boat balancing answer I talked to several people, I was basically told you could sling your boat on a single sling and see where the balancing point is but really that is not necessary, when you get your boat done try adding a little weight in very tip of the bow if it acts stern heavy. Remember , leverage is on your side too, thats why 50 lbs in my very stern acted like 400 lbs in my back seat (approx 8 feet FURTHER BACK) so it won't neccesarily take all that much weight, Smitty

you can make an easy 550hp from those 502 mpi engines. valve job and some mild head work with those heads. new cam and rockers. VERY reliable setup.

your going to need some new drives and or drive work with bolting on SCs.

there is no magic CG number for your boat.

good luck on your winter project!

someone has to say it

get rid of the fat chicks.:party-smiley-004:

I think you may be on to something. I could toss a donut into the cabin and see if I like the way the boat handles with more weight in front?

Actually, I have been considering ways to change the balance of the boat for awhile. The simplest would be to move the batteries forward in the boat but I am not sure they could survive the additional travel / pounding. Other ideas are aluminum heads, aluminum flywheel, moving the engines forward (can be done fairly easy due to the extension box drive shafts but not cannot move very far due to the fuel tank in the floor). I am just trying to decide if any of this is worth the effort. It seems like I read a while ago that a 35' Fountain was faster than a 29' model with the same power because of the hull balance? Don't recall if that was with stepped hulls or the previous models.

Sounds like a sweet boat you have! Don't forget to upgrade your drives. SCX upper and lower/ SCX upper sc lower/ b-max/ etc will work great.

BTW I love my prochargers.

buy some 25 lbs lead shot bags at cabela's, gander mountain, bass pro shops or outdoor world...u get the idea. the bags will stay put where ya place them up front. the bags can be moved around the boat to make drastic changes without going back to the docks. i would buy a 150 to start with.

removing weight from the back will have very little effect. adding weigh to the front will have far greater effects. adding weight further away from the CG with also increase your moment of inertia too.

Since the boat will naturally carry the bow, use stern lifting props like Hydro QIV's

Center of balance is important in a boat and stern heavy boats cause some interesting problems. Yes, a 29' Fountain with the exact same power as a 27' Fountain (both singles) is faster and handles better. (I work for a Fountain dealer.) As for a 29' Fountain (w/ twins) compared to a 35' Fountain, same deal. And the 29' looks like it's going to sink by the stern at the dock. This has to do with both balance and a hydrodynamic formula having to do with beam vs length. If you keep the beam the same, adding length (to a point) adds speed.

Getting back to your Checkmate (which we also used to sell), twin 502s is a good deal of weight in the stern. Adding weight to the bow to balance it will help it handle much better, but may take a tick off the top end. But, if you want speed you can actually use, you may have to add ballast. Why do you think many race boats can add or remove ballast at the bow?

How fast is the boat now ? a friend of mine had a 29 fountain w/twin 454 efi's 80mph boat if you had the balls and all the planets were aligned..that boat with that much weight sounds like could be a handfull....

My 29 Fountain with my Supercharged 502s is on rails at 86-87 GPS mph. I have never run the boat un loaded and low on fuel but I am failry sure I could see 90+ before it got scary.

Stepped (and which generation) or non-stepped though? The 28' Checkmate is a non-stepped deep V.

Oops, just saw you sig. If memory serves me correctly, '97 is either non-stepped or the very first generation with a single step. Yep, it sould run like it's on rails.

My Checkmate would run 80 on GPS when I purchased it all stock. The boat never handled well above @ 75 mph. It liked to chine walk on smooth water and the bow went skyward in rough water. I blamed some of this on the chincy drive gauges not giving an accurate reading of drive position and the rather small (12 X13.5) Insta Trim Tabs on the boat along with cable steering. I then updated the boat with stellings boxes, imco hyd steering 12 X 16 tab plates and engine improvements. The first time out with this combination the boat hit 83 on lake MI but was again chine walking and bouncing a lot. I only made one pass and the input shaft on one outdrive broke. This was the end of the season so the drive repair was completed over the winter. Also after the one trip out the transom was covered with soot so I contacted the person who updated the PCMs for the engine upgrades and was told to install adjustable fuel pressure regulators and set them to 36 psi. Over the winter I also added 30" labbed Bravo One props. The next year the boat never ran as fast as I expected which I believe was due to the fuel pressure being too low. Now I am going through the engines and plan to add the whipples for next year. I don’t like adding weight without function so I would prefer to relocate weight to balance the boat but do not know if there is enough benefit to justify the effort.

hey inspector have you tried swapping prop rotations i am having similar issues on my boat and also have the stellings boxes i went from spinning out to in and found that the boat had much better manners at speeds the nose stayed down instead of riding sky high i even picked up about 5 mph

Single step

u have a 28 foot boat with two big blocks sitting in the back of it. moving stuff around will not change your CG. people that think so have never actually taken CG measurements let alone know how to calculate them. changing the CG in a 40 foot boat is difficult to do! you will have to add weight to accomplish this. big changes in weight on a 40 foot boat results in moving the CG an inch!

the bottom of that checkmate is a mess like all other mass produced boats that are not blueprinted. you should focus a bit there.

many racers talk about changing CG when all they really did was change the moment of inertia. which they have no idea what that actually is or how it works.

to get your boat to run better(more stable at speed) you will have to do bottom work and add weight to it!

Where can I read about the hull blue printing process? Is there more too it than just making the running surface true / flat? Are the size and shape of the lifting strakes modified? Is there an ideal hull shape (convex vs. flat) on the running surface? I am interested in finding out more about this process.

I have planned on trying changing the prop rotations but never got around too it. Maybe next year if the engine work gets completed...

yes flat is the ideal. strakes, chines, and transom 90 deg SHARP...this is way easier said than done!!! you will need somewhere where the boat can be lifted.
i'm sure there are plenty of places that you could do it in your area.

if u do end up fixing the bottom. b sure to add larger bunks to your trailer and extend them past the transom. a trailer can twist the hell out of a boat. trailers can do more damage to a hull than the inaccuracies of the original production.

prop rotation is easy...swap the props and run the sticks in reverse to go forward.

I had this picture laying around that might give you an idea what 90 degree breaks look like on the strakes, chines and transom. Race boats have even sharper edges.

You'll find that your Checkmate has beveled edges which make it much easier to pull from the mold. Simply squaring them up makes a huge difference in the way the boat handles. In fact when we squared off just the transom on a friend's 25' Checkmate many years ago, it just about cured his porpoise problem.

http://i37.photobucket.com/albums/e6.../PadBottom.jpg

Lots of good ideas put out so far. Ballast in the nose is the cheapest to start with. I would look into different props, something with more bite and stern lift. I'm surprized no ones asked about your prop depth yet. If it's still chine walking maybe trying to space the lower down a little might help. You changed alot of variables when you added the boxes, I did the same thing last season and I'm still working out the bugs. Changed too much at once. Stellings boxes even in the neutral slot raise the props 7/8". I'm a cat hull but when I added the boxes I was at 3" above the bottom and the azz got real loose over 100. Spaced it down 1 1/2" and it bites hard now. We also moved the engines forward 2 ft. Just a touch too much. Boat flies flat but this season we'll take out 1/2" of spacer and add a few pounds of ballast in the tail and regear the lowers. With the changes so far we picked up 9mph with no power changes, just weight distribution/ballance and prop depth. With some more tweeking on the balance and prop depth plus the new gears hopefully we can squeeze out aonther 10 but the drives aren't holding up so SCX's are in the plan too.

Sorry for the run on, I'd start with ballast and prop depth tweeks. Good Luck!