Will a 350ci do it?

[doc_cody_blue]

Hi ppl,

a buddy of mine just bought this old Glastron as a winterproject. We are now looking for a matching engine. The only thing this baby should is run at least 50mph, more would be welcome

Should all be done with "lowbudget" so maybe an old 350 mercruiser may come into consideration? I'm talking about the carburetted models, not sure which generation that is, since I'm from Germany and these Chevy engines are not too common here

I'm sorry but I don't have more details about the boat here right now. I only know that there was a V8 in there before.

So I would really like to hear your recommendation about the engine.

Thanks,
Andy

 

[oops!]

Re: Will a 350ci do it?

speed depends on a number of factors.....

rpm, gear ratio, weight , prop pitch, hull design.

however, a three fifty in that rig will do nicely......

the 50 mph will depend on what the motor is attached to (drive ratio and prop pitch).....i can see it happen easy

 

[doc_cody_blue]

Re: Will a 350ci do it?

hi ppl

the baby is running for a while now with the 350, alpha 1, 23" high five prop.

actually we are running 55mph (gps) at 5100rpm

the only bad thing is, that if you hit the gas from low speeds, cavitation or something accours. like no grip. you got to bring it back to low rpm a couple of times an then hit it again. any ideas how to fix that? i don't know where the slip is coming from. there is no roost or something. rpm just goes up and speed doesn't increase. shall we try a different prop? more pitch? less pitch? 25"? i don't even know if the high five is a good choice. we just got a good price for it second hand

however, this boat is a lot of fun already

thanks to all here in the forum who helped us already

regards,
andy

 

[Bob_VT]

Re: Will a 350ci do it?

Place a question in the prop section about it.

 

[MikDee]

Re: Will a 350ci do it?

Hey, Are you the "Flying Dutchman?" Looking good!
I knew you could do it! As to your problem, maybe the prop is slipping on the hub a little?
To test the prop, put an indelible (permanent) mark on the shaft, & hub, of the prop, and see if it moves (goes out of alignment) after you use it.

 

[erikgreen]

Re: Will a 350ci do it?

I'm gonna take a wild guess and say that if you've got a prop that'll do the 55 mph speed it's high pitch, and probably you're going to have a lousy hole shot with lots of cavitation, especially if you rev the engine.

A prop that pushes water enough to run efficiently at 55 without lots of problems up there is going to need to be babied through the lower RPM ranges until it gets up into the revs where it runs well.

FYI, 4600 is generally the recommended max RPM for the mercruiser 350s, and then not all the time. If you are making extended runs at speed using that RPM, expect to shorten the life of the motor, and watch your engine temps, because a brief interruption in water flow from the lower unit will melt quite a lot of stuff....

Erik

 

[n2ostroker]

Re: Will a 350ci do it?

Reminds me of my old Thompson cuddy. Too fast for its own good. Running my old 21p prop I got a spin a couple of times and the prop hub was fine. When I pulled the motor last I had spun the coupler a few times. It was still hooking up but once in a while from a dead stop it would slip. Just put a new/used one on and haven't had it happen again.

What gear ratio is the outdrive? My 21p and 1.5 drive takes me to 58-59mph and 5300rpm.

 

[doc_cody_blue]

Re: Will a 350ci do it?

so far thanks to all for your answers.

the outdrive ratio is 1.84:1 which is actually taken from the v6. maybe possibly thats the reason for the high rev of 5200rpm. changing the ratio to the original of 1.5:1 would change rpm to 4200rpm at the same prop rev. i guess that wouldn't work that way, would it? thats just from the mathematical point of view

if i change to longer gear, do i have to change to lower pitch?

@erikgreen yea true, lousy hole shot is what we got here got to take back a couple of times. never do WOT while boat is not planing, no grip
thanks for the info, we never do long runs at wot, just for measurements actually. cooling works pretty well and engine stays cool under all circumstances

@mikdee i don't see a chance for the prop to slip on the hub cause there are a couple of really hard teeth, correct me if i'm wrong on that,please? i will put such a mark anyway

thanks,
andy

 

[MikDee]

Re: Will a 350ci do it?

@mikdee i don't see a chance for the prop to slip on the hub cause there are a couple of really hard teeth, correct me if i'm wrong on that,please? i will put such a mark anyway

thanks,
andy

Between the prop splined socket, and the prop hub is a bushing, made of rubber, or some kind of synthetic plastic, that is pressed in and is held together by friction, to act as a cushion in case you hit something underwater. It can start to wear out, burn out (from being overstressed), or if you hit something, losing it's grip, and start to slip,,, either gradually, or all at once, then it needs to be rehubbed, or replaced.

 

[erikgreen]

Re: Will a 350ci do it?

so far thanks to all for your answers.

the outdrive ratio is 1.84:1 which is actually taken from the v6. maybe possibly thats the reason for the high rev of 5200rpm. changing the ratio to the original of 1.5:1 would change rpm to 4200rpm at the same prop rev. i guess that wouldn't work that way, would it? thats just from the mathematical point of view

Yeah, RPM is measured at the engine, not the prop, so you're definitely turning that prop outside its optimal rev range. It's great that you have the speed you want, but because of the gearing from the V6 you're supplying more torque than you need while getting lower rpms than you think. I suppose as an alternative you could put a bigger diameter prop on, since you have the torque to turn it, and/or switch to a four blade, and that would improve your hole shot. It might cut your top end a few MPH though.

if i change to longer gear, do i have to change to lower pitch?

Maybe, you'll have to find out what prop the boat "likes" with the diff. gearing. By the way, it's expensive and annoying to change gears in a lower unit without lots of special tools, so you'd have to trade/buy a new lower unit.

@erikgreen yea true, lousy hole shot is what we got here got to take back a couple of times. never do WOT while boat is not planing, no grip
thanks for the info, we never do long runs at wot, just for measurements actually. cooling works pretty well and engine stays cool under all circumstances

Sounds good, I'd keep an eye on it.

Erik

 

[MikDee]

Re: Will a 350ci do it?

I'm gonna take a wild guess and say that if you've got a prop that'll do the 55 mph speed it's high pitch, and probably you're going to have a lousy hole shot with lots of cavitation, especially if you rev the engine.

A prop that pushes water enough to run efficiently at 55 without lots of problems up there is going to need to be babied through the lower RPM ranges until it gets up into the revs where it runs well.

FYI, 4600 is generally the recommended max RPM for the mercruiser 350s, and then not all the time. If you are making extended runs at speed using that RPM, expect to shorten the life of the motor, and watch your engine temps, because a brief interruption in water flow from the lower unit will melt quite a lot of stuff....

Erik

This sounds logical Erik, Do ya think there could possibly be "prop blowout" by having too much power, & too low a gear ratio, with a high pitch, high rake prop causing cavitation at the start? This thought had crossed my mind as well Somethin like laying down rubber in a car?,,, lol