How many more RPM's should I expect to see?

loadnet · Oct 16, 2003

1976 24' Searay 240 Fly Bridge Sedan 351 V8 Switching my mercruiser over from 1.32r to 1.5 Can anyone tell me how many RPM's I should increase after changing my gears over? Boat won't go over 4200rpm with the 1.32ratio Got a 12P prop Talked to a guy with the same exact boat except he has a 305cu and he claims he's doing 4500rpm and getting 32 to 35 knots at WOT Is that possible for a 305cu to get that kind of speed out of this heavy 4200 to 4700lb boat? or is he yanking my chain? Thank you,

The Marine Doctor · Oct 17, 2003

Re: How many more RPM's should I expect to see?

Find out what HP his engine is compared to yours. Just because you have a larger displacement block does not necessarily mean you have more power. TMD

tylerin · Oct 17, 2003

Re: How many more RPM's should I expect to see?

Don't know the Fords real well (not at all) but your friend's is running good rpm's at wot and I don't think yours is especially considering you have a 12p? prop. I don't think that 32 knotts out of that 305 is out of the question

loadnet · Oct 17, 2003

Re: How many more RPM's should I expect to see?

Thank you for the replies back on this.. Answer to reply 1) the 305 is kicking 185hp My 351 is kicking 233hp. Answer to reply 2) I am happy to hear that 32 knots is not out of the question once I am able to increase my RPM at WOT to: 4500. My question is: How many RPM's do you guys think I'll pick up at WOT if I switch from a 1.32 to a 1.5r My outdrive is 1.32r now, and I'm changing it to 1.5 asap. I posted a request to anyone out there willing to swap mine for a 1.5 but no one replied back so I guess I'll have to tear her down.

tylerin · Oct 17, 2003

Re: How many more RPM's should I expect to see?

Don't quote me on the 32 knotts but my 21 ft Sea ray w/ 5.7 does 50mph w/ 21 high five prop at 4500 rpm's, so I'm just speculating

loadnet · Oct 17, 2003

Re: How many more RPM's should I expect to see?

Thanks Tylerin.. I know for sure I'll have to change my prop pitch after changing over from 1.32 to a 1.5 but once that's figured out, I'm thinking I'm going to be able to pick up at least 300 RPM at WOT which would increase my WOT RPM to 4600 - which would be right on the money! at least that's what my current plan is unless someone out there wants to swap outdrives even.

tylerin · Oct 17, 2003

Re: How many more RPM's should I expect to see?

Capthooknfinger, I don't know the gear ratio of my drive off hand but I if that is the original ratio in the boat I wouldn't change it. That's my opinion. I think you have another issue why your motor won't achieve its recommended Rpm's

loadnet · Oct 17, 2003

Re: How many more RPM's should I expect to see?

It's not the original outdrive, which is what sparked me to check out the teeth count after downsizing prop after prop after prop.. When I picked up the boat it had no outdrive on it. It was stolen off the boat in the lot where it was being stored, prior to my purchasing it. The guy that installed my exisiting drive said he was pretty sure the one he was installing was a 1.5 which is what is required for all or should I say.. (most) v-8 engine applications. I've contacted him and he's agreed to install the 1.5 gears to make the thing right... for a fee of coarse..

tylerin · Oct 17, 2003

Re: How many more RPM's should I expect to see?

Well than, You're doing right by getting the correct ratio back in her. I was just puzzled why you were changing ratios. Sorry

loadnet · Oct 17, 2003

Re: How many more RPM's should I expect to see?

No problem.. I appreciate your feedback on the matter.. Now if I could just get this guy over here to do the job I'd be all set. He claims he's up to his keeZKas rapping up boats for the winter, so it's like pulling teeth to get em to respond. One Day at a Time I guess..

Capt. T · Oct 20, 2003

Re: How many more RPM's should I expect to see?

I expect that your RPM increase should match the difference in the gear ratios. e.g. 1.5/1.32 = 1.14. If your running 4200 rpm @ WOT now then I would expect to see around 4700-4800 rpm after the change. Anybody see anything wrong with this thinking? As you said, you will definately need a prop change. Of course you could just go up in pitch now and see what kind of performance you get w/o changing the gears. Might save you $. Just a thought here, but why the rush? You're going in the right direction, but end of season is coming (even if you don't live in WI). I'd wait till January or February when the guy is looking for something to do in his shop. Doing stuff 'off-season' is usually a good way to save a few bills.

loadnet · Oct 20, 2003

Re: How many more RPM's should I expect to see?

Thanks for the response Buck.. There really isn't any rush now. I just went out on what may be my final trip this season, so I've pulled the plug on the project till the guy has plenty of time on his hands. The price has already been negotiated. Only thing that could change my not using this guy is if I fall into a better deal elsewhere. 4700 to 4800 will be perfect. I'm down to a 12P prop right now and can't go any lower than that. I've thought about going up a couple numbers yet decrease my diameter but when I think about it long enough I just say.. why bother. I have the wrong gear ratio. No sense messing around with props till the ratio is 1.5 for V8's like it should be. PB

Bondo · Oct 20, 2003

Re: How many more RPM's should I expect to see?

Well, I Barely Slipped thru Math...... But, If I take the difference between the ratios..... 1.50 1.32 ____ .18 Times the 4200rpms you're running Now..... My calculator says you'll see 756rpms increase, if all else is equal....

loadnet · Oct 21, 2003

Re: How many more RPM's should I expect to see?

Well if I pick up only 500 RPM's more I'll be the happiest little camper in Rhode Island. I'll be back to the board here to let everyone know exactly what I had to do to increase my knots, after installing the new ratio gears. I'm sure I'll be turning in my 12P prop for one much higher Pitch... like a 19 or 20 hopefully. My engine at: 3000RPM after the switch should be kicking the boat out to around 22 to 25 knots, and at WOT throttle it should do 35knots minimum... and I'm hoping for more. I'm ripping out cabinets, refrigerator, stove, in order to lighten up the boat as well. Thanks for the response you guys. Appreciate it. I didn't know how to do the Math in order to determine the RPM increase at all. Big help there on this.

PB

Bondo · Oct 21, 2003

Re: How many more RPM's should I expect to see?

I think you'll be spinning a 15" at 4600rpms.... Good Luck..... like a 19 or 20 hopefully. I Think this is Wishful Thinking........

loadnet · Oct 21, 2003

Re: How many more RPM's should I expect to see?

Remember what you said Bondo because next season I'll let you know for sure ok.. The main thing I'm trying to accomplish is to let me engine run easier at 3500 to 4000 than it is now. It runs fine.. and kicks out about 15 to 18 knots at 3000 to 3300 for a heavy boat.. that's not too, too bad, but I know she'll run easier and go a bit faster once the switch is completed. Thanks for your information Bondo. PB

JasonB · Oct 21, 2003

Re: How many more RPM's should I expect to see?

Capn, be careful with the RPM's on that engine. The 1.5 is the factory ratio, but the Ford engines aren't rated to turn the same as the chevys. My 302 in factory for was rated at 4000-4200 and I think the 351 was rated the same. I currently turn it about 4400 on my 2400lb boat with a 19 pitch prop at nearly 50mph. The higher R's don't bother me as this engine was rebuilt last year by my brother to our specs

That 351 is a workhorse.

loadnet · Oct 21, 2003

Re: How many more RPM's should I expect to see?

Thank you, I'll double check my manual, but last I knew my WOT was supposed to be right between 4400 to 4600 RPM. PB

How many more RPM's should I expect to see?

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