Performance question

[Sports Car Guy]

I know this is one of those un answerable questions, but worth asking.
I have a 1988 245 Vista OMC 260 Cobra. Main question is what speed should I expect WOT and cruise. Info I can find says the boat tops out at 39mph, which I'm guessing is optomistic, but I'm not even close, although RPM is in range. Looking for some real world speed numbers of similar boats.
First time owning a boat this heavy, had 21 ft Sylvan 140 I/O and 22 foot Trophy 260 I/O in the past. I know it's not a speed boat, its a big pointy floating camper, so I'm not expecting to pop skiers up. Had it in the water this summer, fix as I go. No real issues except vcan't get more than 10 gallons of gas in the 70 gallon tank, but that's another story
So engine runs well, runs up to 4300 rpm WOT, but I cant get over about 25 mph (GPS and speedo ) flat water with 2 people, minimal fuel and not much stuff onboard. Fresh cold water, 450 ft above sea level. (Seneca Lake NY) Needs the tabs to get out of the hole, not a surprise. If I trim drive up once it tops out and tabs are up, doesn't add much speed or RPM, starts to porpise, then prop ventilates. Trimmed out, it falls off plane if I drop below 3700. If I add 3 more people in the back, can barley get up on plane at all.
I'm using a Hustler 15x15 4 blade, worlds better than the 15.5x15 omc 3 blade which was listed as stock.


What might be wrong and what I verified. Didn't get alot of time to work on it.
1. I had to change the outdrive with a rebuilt unit to get it in the water, its from a 5.0 engine, slightly different gear ratio, 1.5:1 vs 1.41:1 original as far as I can tell.
2. It has an automotive MSD HEI distributor, curve unknown. I did get a proper marine unit, didn't install yet.
3. I do get full wide open throttle. Flame arrestor is clean, and I do get correct RPM range
4. Bottom is not fouled, no visable damage or hook, handles well on plane.
Head gaskets had been changed before I bought it, compression unknown.

Any input appreciated.

Out of the water for the winter, but may as well start thinking about it. Big problem is a friend, who is an engine builder, is offering to sell me a 375 hp marine smallblock 383, very tempting. Would cruise nice at 25

 

[tpenfield]

You might want to consider that the boat may be a bit water logged and heavier than normal.

Maybe take the boat/trailer to a scale and hen just the trailer to net out the weight of the boat.

 

[Scott Danforth]

being a 1988, I guarantee your heavier than you are supposed to be. check for moisture.

a better ratio drive is not a bad thing, it would allow your motor to spin up higher

get rid of the automotive distributor

only getting 4300 is either you are over-propped (probably not the case), or the motor has issues, or you are holding 1500# of water.

I would get the boat weighed.

 

[Sports Car Guy]

Thanks, didn't think weight. I do alot wth old cars, so used to getting someone elses problem. It feels to me like the engine is short on power, but that's seat of pants. Cars don't mysteriously gain weight like boats can Distributor goes in in spring and then can check compression and properly tune it. Boat is 250 miles from home, won't get time til spring, but the ramps a mile from the lake house, so easy test drives.
Once they fixed the shift tube leak, it really takes on zero bilge water at the dock. Reputable marina did the work, they pulled engine and whole drive and checked the transom, no leak or rot issues. It does seem to be dragging its ass and seems to sit low at the dock and under power.
I've seen boat project videos where the side areas are soaked, under the water tanks/battery area, but hard to tell without cutting holes. Those boats were swamped for a while. Wonder if I drilled a weep hole at the bottom of the closed box section if I'd get seepage from the foam if its wet.

 

[racerone]

Run it on a ---Marine dynamometer-----That will check true engine power output.

 

[Lou C]

There's a lot not known here. The original engine would have been a 350 with a 4bbl Quadrajet, with a Prestolite points & mechanical advance distributor. I'd start with a compression test, that will help track down lost power, what carb does it have? Get rid of the auto distributor as Scott said. You can find the Prestolites on ebay if you want a points distributor, because it will work with the OE ESA module so you can shift to neutral. Or you can convert it to the Delco EST but will have to get an ESA that will work with it, CDI electronics in Alabama makes these.
Ideally you'd want your max RPM to be higher, like 4600-4800, to reduce the strain of pulling that heavy boat up on plane with the little 350. To me a 24' Cruiser should have come with a big block or in modern times the top end small block like a 383. A 260 HP small block is fine for a 20-22 foot bowrider or cuddy.

Hopefully this boat was stored dry, these boats had a LOT of wood in them and are very prone to rot and absorbing moisture. All that horrible carpet on the deck is a prime cause, with unsealed holes everywhere. I wound up doing a deck replacement on mine about 15 years ago.