imq707s

I finally got my 1985 21' Scarab back in the water this weekend after working on it all winter. I had a local prop shop add some extra cup to the blades of my prop a few weeks ago to give the bow a little more lift. Well the cupping worked, but it reduced my max rpms from 4800 to around 4500 but the top speed stayed the same, 58 MPH. What are some simple mods I can to the the motor (Jasper Marine 350) to get the RPM's back around 4800 and gain some more speed? I've been thinking about changeing to a comp cams extreme marine cam, and adding roller rockers. What do you think?

blue thunder

I believe the WOT max rpm for your engine is 4200-4600 rpm. If that is the case you are close to as good as it gets with that engine. Have you maximized your trim settings? Try trimming for max speed. Labbed prop may help as well.

BT

imq707s

Yep, It was trimmed out as much as it would go without the prop starting to cavitate. At what RPM should I start worrying about the motor having some problems (5000rpms)? Thanks for the advise.

blue thunder

I think max speed is achieved at max trim at max rpm. Your speed will start to drop at some point and your rpm will continue to rise. Just before that happens is your best speed with that prop. If the rpm at that point is above your max recommended rpm for the engine, you should prop down 1 inch in pitch for every two hundred rpm needed to reach max recommended rpm. In my opinion, when you have obtained max speed at max recommended engine rpm, with the proper trim, you have max speed for your combo. Anything more will take less weight, more hp or less hull/od friction. Each different prop will be a new animal because it will change wetted surface (lift) of the hull. I have learned these ideas through trial and error. I am always trying to optimize what I have. I'm sure others could offer much more on hp improvements and best props.

Have fun!!

BT

Rambunctious

my understanding is the peak power of the SBC is 4800,
4200-4600 is for the 330 horse 454, and 4800 for the 454mag

I would look into labbing THAT prop, you are pretty dialed in. converging onto the best point. I am considering labbing which will supposedly get you 100-200rpm gain through the improved prop efficiency, at that time I want them to "slightly" cup to put some of that rpm back into the pitch, hopefully going from 4500 and 65mph to 4600 AND 67-68mph

John at York propellor labs props very reasonably and has received alot of good press here on the boat. you may see 60+ right back at the peak power at 4800 with a totally optimized prop. cupped and labbed.

Of course I would NEVER talk a fellow gearhead out of adding more power ."argggg arggggg argggg" in the Tim Allen voice...