Over-reving issue

[NW Redneck]

Off with the Merc hat, on with the OMC hat! LOL
Once again your supplicant comes to the great guru Walleyehead humbly requesting advice.


I took my 18' Cal-glass with OMC 175 out for a spin today to check my RPM's and see if my tach is working correctly now. I found out 2 things:

1: Tach is definitely hooped. Only reads 2600-2800 max, and I just checked the wires and re-made all the connections from the back of the tach to the coil.

2: My WOT rpm's are WAY too high! With the Blue Point DMM I got 4800 rpm out of that little Ford 302!

According to the speedo I'm doing 42-43 mph @ WOT with a light load. I have been cruising at 35mph as that's where the engine 'sounded happy'. At that speed I was getting 4200 rpm which is my recomended max rpm. (3800-4200) Currently running a 14 X 16 prop.

The boat is quite a$$ heavy, and has ALOT of bow rise getting on plane. Am I right in thinking I need to go up in pitch, and if so how much?

BTW, I'm planning on getting smart-tabs as soon as the funds are available to help with the bow rise issue.

Once again, Thank you!
Colin

 

[walleyehed]

Re: Over-reving issue

If we follow the rule of 200 per inch, a 19P should put you real close to the max RPM.
The best way to set this up is to test the prop you have with a normal load.....never test alone or light...this will cause hole-shot issues if you prop for these results..always go with a normal load...when lite, you just refrain from holding it at WOT.

 

[NW Redneck]

Re: Over-reving issue

I wasn't far off from what will be a 'normal' load today. Me, 2 kids, safety gear, asstd. 'stuff', 3/4 tank fuel, kicker+full cruise-a-day. Only thing missing would be the Admiral and a cooler.8) I'll try to lay my hands on a 19P and have them both on board when I test it! I'll post back here with the results. You are a gentleman and a schollar! :$d

 

[NW Redneck]

Re: Over-reving issue

Got an update! Guy at work has an old omc languishing in the weeds that had a 14X18P prop! Went out with kids again (sorree, no wifee!) and what a difference! Time to climb out of the hole was cut by almost half! Still with enormous bow rise though.

Here's what I got with this prop:

WOT: 4400 (+/-25) Almost exactly 200 per inch!
Speed: 45-46 mph - up 3-4 mph!

Tried cruising at 35mph and got 3480 rpm - down from 4200 . I was figuring 3800 to match the decrease at WOT but I guess it's not a linear progression. Now the 'sweet sounding' spot is about 4000rpm @ 38 mph.

My question now, is do I look for a 19P to hit the 4200 mark, or since I was a little light just stick with the 18? Is the extra 200rpm over max a big deal? Will I ever get my bow-rise down? Will our hero escape certain death and stop the evil Dr. Zorg in time????

Sorry, got carried away there.8)
Thanks again W!

 

[walleyehed]

Re: Over-reving issue

Wow, I wish I had your energy!!!
Hey, if the 18 does that good of a job, yet leaves you a bit long on RPM, I'd say you'll be fine...
When you do have those heavy loads, you have the prop for it. You may gain a tad of speed and some fuel economy by going with a 19P, but it must be close to the same design to work right.
As for the nose coming up so far, I'd look real seriously at a set of Nauticus Smart Tabs....I've tested them and I run them now without any question. They take care of themselves and are self adjusting, pressure-wise, and can be fine tuned for more or less pressure depending on your needs...I'd bet your bow-rise will "almost" disappear.

 

[NW Redneck]

Re: Over-reving issue

I wish I had it too! I am only ten min. from a ramp so from changing the prop to backing the boat back into the driveway was 1hr 15min. Then I had to make dinner and check iBoats! 8)

I think I'll just stay with the 18 then. I'm not going to be running WOT alot anyway, so the extra mile or 2 doesn't matter that much to me. It's plenty fast for what I need and it will be nice to have the prop for the occasional 'big' load.

I have already sold myself on the Smart-Tabs. (See OP. Getting old-timers? LOL) Just a matter of fundage now. Thanks again for your advice! I just hope i get the FIL's Campion up to revs as easily as this! Gonna re-check the hull and carb. Gotta wait for payday friday for more gas tho! :/

 

[blues]

Re: Over-reving issue

[whisper uid=83045]

Enter private whisper here

[/whisper]

hi, read your post about the 302 ford-
I have a 1979 23 ft. day cruiser,merc 260 sterndrive- the 350 chev motor needed replacing, I got a good deal on a ford 302 in ex cellent shape.due to financial issues had to go with this.
less power, from a top of 50 mph with the 350 to now a high of 40 mph.wot rpm is around 4100
with a 14.5x19 prop
I dont know the factory spec on wot rpm.

I d like get another 5 mph- up to 45 mph, and cruise around 35 mph.

I dont know alot about boats, not as much as I should-
if I change the prop to a 14.5x17 I would get more rpms,
maybe up to 4500
and maybe get the extra 5 mph?

another option, I was thinking of was , to upgrade to a 4 barrel carb, as they have one for this 302 from edelbrock marine,.
would anything help,.?

 

[walleyehed]

Re: Over-reving issue

The design of your prop will have more to do with speed and RPM than just pitch alone. There may be a 19P that would offer more bow-lift and give less wetted area, basically an "over-all" lifter, which might increase your speed and may raise the RPM a tad. What model of prop are you running now?

 

[blues]

Re: Over-reving issue

ok,, scratch the four barrel carb, I just canceled that order, as today I tried a stainless prop, marked 14x20
and this boat really took off, fast to plane, and a top speed of 45 mph, @ 4000 rpm,
I never expected anything like this,..
maybe a differnt stainless prop would be even better,.
14.5x17
this little ford 302 in a 23 foot sleek craft, almost equals that chev 350
all the expert s told me this cant be done.
from 260 HP with the 350 down to 188 HP with the ford 302. and only 5 MPH difference.
the prop and maybe the boat design, as the sleekcrafts were very sleek,

 

[walleyehed]

Re: Over-reving issue

Still can't tell ya much if you don't have the prop manufacture...the model and type. They all differ in their intended use.
As for gaining speed and being 5mph different, considering the HP, that's about right...it takes 17-18HP to gain 1 mph and if that 20 runs anywhere near your top of your RPM range, I wouldn't be looking at a 17 for anything but pulling tubes/skis.

 

[Texasmark]

Re: Over-reving issue

Along the lines of what Kenny is saying, go to the BAM site and look at their props. On some of them they tell you what differences you can expect between the props. Only a few mentioned there, but the idea is the same.

Mark