Fiberform with OMC won't plane--HELP!

We have a 1975 Fiberform with twin 292s and OMC outdrives. We have had it for about six years and still cannot get it up on plane. Have changed the gearing in the outdrive, changed props, changed ignitions to electronic, and more. Does anyone know what the solution might be? JP

 

I can help you diagnose your problem. Is your boat the 28' flybridge? If not, I'll need the length, beam and underbody type (deep vee, modified vee, etc.). A picture of the bottom would help me understand its condition and shape. I'll also need to know the actual weight of the boat as-is. Many travelifts have scales or you can weigh it on a truck scale and then weigh the trailer alone. I'll also need to know the rated engine output. I assume those are the Ford V8's, and I'm guessing they make about 200-210 hp. If you know the final drive ratio and prop geometry, that would be nice to have too. Finally, if you could share some more detail as to what is actually happening when you try to get it on plane it would be helpful. For instance, does the boat just squat deeper and deeper and never get over the "hump?" What rpm are the engines reaching? Do the engines seem to be running okay? Do you notice anything unusual about their behavior? Anything else strange?

 

Yes, it is a 28' flybridge! The bottom was just pressure washed down to the gel coat to get rid of the barnacles (it had been in salt water before we bought it) and completely resurfaced and repainted with epoxy. The boat weighs about 10,000 pounds with everything on it.

Just changed the OMC outdrive gearing from four cylinder to six cylinder. The final drive ratio is 1.62 to 1, I think. The props are three blades, 14x12 but I may change to 14x10. Yes, it does squat deeper (it is a faster slug!)!!! I moved the outdrives up 1 1/2", putting the cavitation plates even with the bottom of the hull.

We have two Chevy 292s that are gas fueled. Their horsepower is about 195 per engine with about 375 ft lbs of tourque on each. The engines were bought from Motorworks as rebuilds. The rpms will only go to 3500, though they should go to 4200-4500. They engines will turn 5300 without any load. The starboard engine temperature overheats at full rpm--and has since the engines were put in, for some reason--pull it back to 3000 and it comes back down. The starboard engine also goes through a water heater every year. Port engine temperature runs about 160 degrees, where it should be. Have 160 degree thermostats in both engines. Installed V8 heat exchangers, along with the 6 cylinder ones that came with the 250 engines that came with the boat (doubled up).

Having some type of carbuerator problems with the starboard engine also. I'll look into that tomorrow. A picture of the boat (with our Christmas Ships decoration) is attached. Thanks for the help! JP

 

Correction--the ration is actually 1.39 to 1. Tomorrow I am having the props changed to 14/10. JP

 

Hi JP:

Let me know what happens with the 14x10's. Clearly, if you're not developing full engine rpms you have an overload situation. The question is from what? Your bottom is clean and more or less fair, correct? (Can't really tell from your pic, although Santa and the reindeer are mighty cool). My first thought is that you are somehow overweight. It's not terribly unusual for boats of that vintage to have osmosis/waterlogging issues. Plus, with cruisers, people move stuff aboard, but hardly ever move it off. Have you checked your weight with a scale of some sort? Alternatively, can you tell if your actual waterline more or less matches the design waterline? (Sometimes the DWL will be scribed into the gelcoat so that you have a line to run the bottom paint to. If this is an overweight issue, note that the previous owner(s) may have just painted right on past the scribed line to an inch or two above the scum line. Be sure to check a few inches down from the top of the bottom paint.) Or, does anyone else nearby have the same boat without this failure to plane issue? Does your actual flotation waterline match his?

Does the boat have trim tabs? I have seen circumstances where relatively heavy cruisers with an aft weight bias and outdrives could barely get over the hump without tabs. Throw in a couple of extra passengers or some waterlogging or lots of gear, and they wouldn't have gotten on plane either. If you don't have tabs, can you try it with a couple of people (3-4 maybe) way up on the bow?

I suppose it's also possible that you have some restriction in the various drive linkages somewhere due to an error in installation geometry, but that doesn't seem very likely since the outdrives have apparently not been self-destructing on a regular basis. I'm not all that up-to-date on my OMC drive history, by the way. Are those the old non-trimmable under load type? Or the newer Cobra type? I think moving them up a little was probably fine (so that the anti-ventilation plates, aka cavitation plates are even with the bottom), but I wouldn't go any higher until this is resolved. You don't want them ventilating once you do get on plane. Obviously, if they are trimmable you want them all the way in (down) when trying to get on plane.

I'm usually working on larger, inboard boats, so the outdrive thing is not particularly familiar territory. I take it the largest diameter prop you can swing is 14". I calculate that to be a little small relative to optimum for your application. Also the 1.39 final drive ratio is a little shallow. I would prefer the 1.62's or maybe even a little deeper. You can't go much deeper though, because then you'll need more prop diameter (or blades) to develop enough surface area to absorb the power.

By the way, I suspect the overheat issue you're having is due to inadequate cooling water flow. There is almost certainly a restriction somewhere in your cooling water loop -- a piece of plastic got sucked up, a hose is kinked, some refuse got left in there during assembly, something. Are the engines freshwater cooled? If so, maybe something is partially clogging the heat exchanger.

Anyway ... at something like 390 total horsepower and 10,000 pounds, with a normal, clean modified vee bottom shape, you should be doing at least 23 knots. Let's see if we can get you there.

Geoff

 

How fast are you going @3500 rpm? 10-12 MPH? I ask because although you do have an issue from the 3500 rpm VS where you should be @4200RPM deep V boats "feel" like they are in a hole even when on plane sometimes.
I have twin 350s in my 50 footer. I have a little problem but even so current RPM is 4150 starboard and 3900 port. Top speed is around 17 MPH. Swinging 19X16s Your boat should run circles around me.