johnny25 come in.............over.

i'll try to make this short as possible....

I was reading the thread(forgot users name) about the 1980 35 rude that he stripped apart and rebuilt/repainted and you two were discussing ways to get more hp. about the intakes, his motor(and mine 1980 j25rcsa) have 0396788 and you told him that from 1984 on they started using 0391054 and 0395626 for intakes on the 20/25/30/35 motor(wich I assume mine applies also 31.8 block) with the replacement number for those two being 0396790 and that those 3 intakes are slightly taller when laid flat and the middle of the inside was thicker hence another 1.5hp in theory. I already am 99.9999% sure that newer intake will bolt right on to my motor but you have way more knowledge on these and want to hear it come from you before I go hunting parts sites/ebay...... thanks in advance.


and one more quick question....you said they stopped using the 0396788 in 84 and started using the above 3. long story short..... I saw the 0396788 being used on the J28ESLCEA and the J25RLCUR and was wondering if it was a misprint or if there was a reason for it. seen it on ME thanks again.....

Im not exactly sure what I said in that post but I believe what I meant was that 396788 was not used anymore after 1984 on the 30hp motors. It was used on the 20's and 25s but when OMC went to prop rated HP the 396788 was not allowing enough air flow/volume to get the 30hp rating. That is where the 396790 came into place. Now you have to remember OMC didn't ever come out and say this, I have had to try and figure out through research and parts I have seen personally and measured to come to this conclusion of how they made the 30hp.

In 1984 the intake on the 35hp (which became the 30hp in 1985) had an intake number of 391054 which was replaced by 395626 and then again by 396790 so I assume that any of these three numbers and intakes will bolt right on and make your motor into a 30hp. There are other numbers also that will also do this but it takes some digging and research.

[url=https://www.tinboats.net/forum/viewtopic.php?p=328164#p328164 said:

Johny25 » 48 minutes ago[/url]"]Im not exactly sure what I said in that post but I believe what I meant was that 396788 was not used anymore after 1984 on the 30hp motors. It was used on the 20's and 25s but when OMC went to prop rated HP the 396788 was not allowing enough air flow/volume to get the 30hp rating. That is where the 396790 came into place. Now you have to remember OMC didn't ever come out and say this, I have had to try and figure out through research and parts I have seen personally and measured to come to this conclusion of how they made the 30hp.

In 1984 the intake on the 35hp (which became the 30hp in 1985) had an intake number of 391054 which was replaced by 395626 and then again by 396790 so I assume that any of these three numbers and intakes will bolt right on and make your motor into a 30hp. There are other numbers also that will also do this but it takes some digging and research.

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cool man that's what I wanted to hear before I checked any further. to me that's the only logical explanation for switching it when they started the prop rating. alright im gonna pour over some numbers and maybe start thinking about trying it. but before that I still wanna find the right pitch prop for the carb switch. do you think I could put a 12 or a 13 on it? or I should say do you think a 13 would fit? or is that too big? one more thing(sorry) what do you think an average prop slip would be for a 12 or 13 on my motor? 15%? 20%? thanks again man...

You have the split gearcase that has the 12:21 ratio I believe correct? Sorry my memory is not as good as i wish it was. Assuming that then you likely are not going to be able to turn anything over a 12" pitch. And a good slip number for motors like these is 12-18% from my experience.

what prop are you running now and what RPM are you turning? I am sure you have told me this before but I have way to many numbers rolling around in my head to recall

[url=https://www.tinboats.net/forum/viewtopic.php?p=328177#p328177 said:

Johny25 » Today, 11:46[/url]"]You have the split gearcase that has the 12:21 ratio I believe correct? Sorry my memory is not as good as i wish it was. Assuming that then you likely are not going to be able to turn anything over a 12" pitch. And a good slip number for motors like these is 12-18% from my experience.

what prop are you running now and what RPM are you turning? I am sure you have told me this before but I have way to many numbers rolling around in my head to recall

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split case 1.75 correct. 11 pitch, to be honest I cant remember what the rpm's and speed were when I told you but im thinkin 5300-5400 28mph??? but im not sure and I don't think I had the 35 carb on it then either but I may have.

but I have lightened my weight in the boat quite a bit and when I was out the other day I didn't have my tach hooked up and the hummingbird got forgot at home so I don't know what rpm and speed was but I did notice that at WOT it sounded like I was going faster than before but I don't think I was going any faster than with the weight still in the boat that I removed prior to this. could removing that weight (few hundred pounds) be making my prop slip and raise my rpm's? that's all I can think of...... I have the tach hooked up now and am planning on putting on the 25 carb and seeing what I get then swappin the 35 on so I know just what I gained. I know wrote it down the first time but I lost it and my memory when it comes to that kinda stuff is not good at all. so I hope in the next day or two I can get the info.

Removing weight should raise RPM and slip should go down. The slip won't go down very much mind you but it should just a bit. Just think of having your motor on a steel barge that weighs 10,000lbs.......never going to plane it out and slip is going to be astronomical, likely in the 70% or higher range. Less weight=less drag, which=less slip......theoretically. Same reason hydroplanes usually have really good slip numbers......barely anything touching the water which makes for less resistance/drag. If memory serves me hydros run around 7% or less sometimes.

[url=https://www.tinboats.net/forum/viewtopic.php?p=328235#p328235 said:

Johny25 » Yesterday, 22:00[/url]"]Removing weight should raise RPM and slip should go down. The slip won't go down very much mind you but it should just a bit. Just think of having your motor on a steel barge that weighs 10,000lbs.......never going to plane it out and slip is going to be astronomical, likely in the 70% or higher range. Less weight=less drag, which=less slip......theoretically. Same reason hydroplanes usually have really good slip numbers......barely anything touching the water which makes for less resistance/drag. If memory serves me hydros run around 7% or less sometimes.

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I gotchya... like I said it sounded like I was going faster(high pitch at WOT) than normal. you don't think that with the extra hp from the 35 carb and the boat being lighter and the 25hp 9 1/4x11 prop it could maybe be spinning it too fast and its slipping more? before when the weight was added and still had the 25 carb I was running around 49-50 000 rpms or close to that and after the 35 carb was put on im pretty sure I was real close to 5500. im gonna check rpm and speed today or tomorrow with both carbs and see what I get..

well I justr got back from the lake and im shocked because what I thought actually is whats going on.... I think :-k

ok I did not switch carbs yet but at WOT with the 9 1/4x11 prop and the 35 on the motor I was between 5800-5900 for the most part, it went below 58 and above 59 but onl

WTF???? lol!! whoops y a couple times and right away it went back between 58-59 and bounced around that 100rpm range, gps had me going 27mph. when I said earlier it sounded like I was going faster than normal but I didn't think I was the last time out.

I went WOT then backed down the throttle and kept it right under 5500 rpm's and got 27mph on the gps. if I gave it the rest of the throttle there was no speed change and the rpm's rose to 5800-5900. so has to be starting to slip above 5500 rpm's right? that's why the increase in rpm's, sound but not in mph? im baffled....wish like hell I had a 12 or a 13 pitch prop...

and I keep thinking now that I was a bit over 31 or so mph last year when I put the carb on... wish I could remember what thread we were in so I could read through it and see........

With numbers like that I would go up to a 13 pitch.....unless you plan on running with more weight than you did today on a regular basis. And even then you could always throw the 11 on when running real heavy.

I had the same issue when I tried to run an 11 pitch SS prop......at about 5700rpm I gained no more speed but the motor spun up and over 6100rpm. Only ran that prop once and then got a 14 pitch and now I run 5250-5450 depending on load

[url=https://www.tinboats.net/forum/viewtopic.php?p=328282#p328282 said:

Johny25 » 42 minutes ago[/url]"]With numbers like that I would go up to a 13 pitch.....unless you plan on running with more weight than you did today on a regular basis. And even then you could always throw the 11 on when running real heavy.

I had the same issue when I tried to run an 11 pitch SS prop......at about 5700rpm I gained no more speed but the motor spun up and over 6100rpm. Only ran that prop once and then got a 14 pitch and now I run 5250-5450 depending on load

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you don't think a 13 is too big for my motor? or I should say will I have clearance from the cav plate? I mean the biggest they sell ( or recommend ) is 12 and ive never thought to measure between prop shaft and cav plate. but I suppose the 35 carbs not meant to be on there either lol! actually it don't matter if its a 12 or 13 if I can keep it just below 6000 rpm's at wot that would be sweet. the 35's max operating range is 5000-6000 and mine is 4500-5500 and the blocks are the same im sure its not gonna hurt nothing and the fact that I have only one lake and the Missouri river where I can go above 5 mph no wake within about 20 miles it don't get out and run often unless theres nobody 8 blocks away at the local 5mph lake like there was today :evil: lol!

hmmmm this is tough... do I keep the 11 and the tach on and just ease back on the throttle to stay around 5500 or do I go higher pitch and hope I can do full throttle and gain a bit of speed without it being spun????? I am putting back about a 1/3 of the weight I removed...im gonna have to think before I spend money on a prop and take the chance of having to put it on CL or ebay lol!

Johny25 said:

With numbers like that I would go up to a 13 pitch.....unless you plan on running with more weight than you did today on a regular basis. And even then you could always throw the 11 on when running real heavy.

I had the same issue when I tried to run an 11 pitch SS prop......at about 5700rpm I gained no more speed but the motor spun up and over 6100rpm. Only ran that prop once and then got a 14 pitch and now I run 5250-5450 depending on load

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how fast are you going with them rpm's? with your blue boat you built the kids seat it? 88 25 mod 30?

That's true, it is hard to find a 13 pitch for that gear case huh. Well you could easily turn a 12 around 5500-5700 pm probably and with more weight in it you like drop another 100-150rpm so that may be the prop you want. Yes you should get more speed out of it than the 11 and you likely won't get that slip at top RPM. You should be able to hit 29-30mph with a nice prop.

I run around 28.5-30mph depending on the load in my boat. That is with a 14 pitch and 2.15 gear case. With just me in the boat and all my gear out can turn almost 5700 and go just over 30mph. With the fishing gear, cooler, misc and my 2 boys I run around 28-28.5mph at 5300-5450rpm. I am running a stainless Solas prop

that's scootin for a 26 yr old johnny, and them ole boys aren't no good either :---) lol!! yeah im gonna start hunting for a 12. I hate to buy a brand new one and have it not work. gonna check ebay but even then you cant believe what some people list some of them parts as on there. if I find one i'll email the seller if theres no pic with the dia and pitch... I shouldn't even ask cuz I can feel my money flying away already but what did you pay for your prop?

I think after shipping I paid about $160 and I got it off iboats

well I found a prop, 10x13 and at 5300 rpm's I can get 33 mph. that's WOT with just me and my fishing gear without my MK PD50,both batteries (my sonar battery is about 2lbs maybe) no toolbag, and my floor out(4x8 sheet of 3/8 ply). old prop was 9 1/4x11. gonna put the floor,MK,tools and batteries back in and try it today if my 9 1/4x12 shows up wich it is suppose to. I got the 13 prop off CL for 50 bucks... I think the 12's gonna be the best one... im thinkin I can turn that one over 5500 a bit but we'll see...