prop diameter difference

houdini1966 · May 5, 2006

i have a 90 hp evinrude with 13 3/4 15" alum 3 blade prop that runs 4500 to 4800 rpm WOT and i am looking at replaceing it with a composite 4 blade adjustable pitch ( 13 16"-21") prop i know that each adjustment up will decrease rpm by about 200 rpm but what will decreasing the diameter do and will going to the lighter prop incerase the over all rpm base line??

walleyehed · May 5, 2006

Re: prop diameter difference

First of all, we need to verify that tach. You're ATLEAST 1000RPM short of where you should be at WOT. A 16 4-bld would be simular to a 17 3-bld....not the direction you need to go. It's standard for a 4-bld to have less diameter than a typical 3-bld. Tell us a bit about your boat type, and yr model of engine...if your engine is a newer looper, you're more than a 1000RPM short and it WILL cause engine problems. Let's hope the tach is not accurate in this case, unless you're using this engine on a larger pontoon-type or we may already have some engine issues.

rickdb1boat · May 5, 2006

Re: prop diameter difference

The first problem I see is that your WOT RPM is way low already. You should be pulling at least 5800 with that motor. And that's running a 15" pitch prop right now. Going to a 4 Blade 16"-21" is only going to make the problem worse. The diameter will not affect RPM's much. You need to figure out if your Tach. is off or if you are really running that low of RPM before you do any prop switching. Might be that your motor set-up needs some work. What type and size boat are you running and where is the cavitation plate in relation to the bottom of the hull? (Typing slow today Walleye, but at least we're on the same track)

houdini1966 · May 5, 2006

Re: prop diameter difference

i am running on a 1984 170 tourney grady white hull ( about 17 ft) tach is good and specs for this 1992 e90tlend evinrude say power rated at 5000 rpm and WOT range is 4500 to 5500 the cavitation plate is about 1/2in above the bottom of the hull ( between the drain plug and the lowest point of the hull) i may have some power pac problems as from time to time it will surge up to the 5000 to 5500 rpm range but that is being worked on right now and the prop will not be changed untill that is fixed... the reason for the possible prop change is that i tend to run a little heavy and am looking for a little lower planeing speed on the outbound trip as i carry 24 gal extra fuel ( go out on the aux tanks come back on the main 30 gal) btw rpm specs are from the shop manual

walleyehed · May 5, 2006

Re: prop diameter difference

Understand the "rated" RPM quite well...that 4500-5500 was great on 1984 fuel. Todays fuel is CRAP compared to what it was then and most of us that run, rebuild, test, rebuild, etc..have found that increasing that original figure (5500 for your crossflow)to 5800 or even a bit more decreases the coking and carbon build-up substantially. If you've been running this engine at 45-4800RPM for very long, I'd bet money you have rings stuck in the piston ring lands and probably some light scoring in the cylinders...your combustion temps would be very, very high. How do you know the tach is accurate???has it been verified by an electronic shop tach??? On that boat, I'd say you have engine issues if it will only spin 45-4800 with the 15. I would guess it close to 6000 with that prop and a healthy engine.

houdini1966 · May 5, 2006

Re: prop diameter difference

yes tach was verifed by a shop tach ( boat is in the shop now getting tuned up and ready for this season i called and asked them to verify my tach after reading your last postes.... so far the every thing is normal ie compression, spark plugs, operating temp, ect and the power pac will be in on monday.) i know it will run up to 7000 rpm when not under a load and i am not sure what the WOT rpm will be after the new power pac is there any thing you could suggest i do to get up on plane as a little slower speed not to worried about top end speed just want to plane a little sooner and or a little longer/ slower

moderator1 · May 5, 2006

Re: prop diameter difference

I have to ask....how do you know it will go to 7000RPM, no-load???? What spark plugs are in use, and can you call and find out what the actual numbers were for compression??? And, if you call, just to make a point, ask them what the RPM should be at WOT. Is your shop a JohnRude dealer?? I think I know where walleyehed is going with this....

houdini1966 · May 5, 2006

Re: prop diameter difference

um... i kinda goofed one day and went to wot while warming up .... no it is not a johnrude dealer but they have done all the work on this boat scince my grandfather bought it back in 1984 and they put this engine on it in 1992 as to the rpm i confirmed the numbers with them when i asked them to check the tach.... dont get me wrong... i am open to any ideas and i post here to get the insites of other folks who have been doing this for longer than me and tend to look in more places than just the shop i feel that they have to stay where the factory tells them and wont push it with someoneelses toys...and as i dont have the mony to experment i hope to learn from folks who have been there and done that...... the spark plugs in use are ql77jc4 champiom... and i will get the compression numbers later (just called and phone was busy will try again when i get home) anyother info i can get just tell me and i will get it as soon as i can..... so whe do you think he is going as i have know idea at 3 buck a gallon i would like to get the most out of my fuel and as i said i run heavy... mostly due to fuel and my size... i estamate about 1400 lbs total people fuel equptment and motor.... i know if i left the extra fuel at the dock it would be better but then the cost of running would go up almost to far (fuel is so much cheaper on land than on the water or so i have found at least it was last year...)

brownies · May 5, 2006

Re: prop diameter difference

I've got a problem understanding two "sort of" prop laws. (sorry, knew of no other way to describe it). 1st...If you are replacing a three blade with a four, drop 1 or 2" in pitch. (figured the same would apply when going from three blade to FIVE blade?) 2nd...diameter doesn't affect rpm all that much. Here is what I'm running into. I have a 25" High Five 5 blade prop and a three blade 25" Yamaha prop. The high five is smaller diameter and turns 5800 rpm at WOT. The three blade prop will only get me to 5400 rpm. ????

walleyehed · May 5, 2006

Re: prop diameter difference

houdini, #1, be very cautious not to exceed 2000RPM no-load...the engine can actually run away and turning the key off won't stop it. I'm surprised the flywheel didn't depart the area, and most times on a run-away, the engine craters before you can disconnect the fuel supply. The V4 crossflows run best at 5800RPM... less work, less fuel used at any RPM. Now, let's take into account I know you're thinking that the manual says other-wise, and why is this guy telling me different...well, we found out the engines started lasting MUCH longer, decarbs weren't revealing massive carbon build-up(coking is not the same, but the higher RPM helps prevent it due to the quality of todays gas not handling higher combustion temps)cylinders look better, and of course better fuel economy. In all honesty, I would prop for 5800 with a normal load and consider 5500 as the minimum WOT. I've ran my V6 crossflow to 6700RPM and it's rated at 4750-5750. I prop for 5800 minimum when I set up for everyday use. Now, if your engine checks out, we can raise your RPM some by raisng the engine on the transom...lifting a bolt-hole or 2 can make quite a difference on most boats, some respond better than others...then, when we find the heigth it runs best with the prop you have, we can then go a step farther and change out props to a higher quality wheel designed to be run higher and maybe be able to go another hole up. Keep us posted on what your shop tells you. OK, brownies, we're not apples to apples on the H5 vs. the 3-bld. The yammy prop has alot more blade area "per blade", and likely has more cup that the H5. remember rule #1...the lower the blade count, the higher the efficiency. The 25 yammy was built for speed and the H5 was built for hole-shot. Your rules DO apply if we stay with the same blade design.

houdini1966 · May 5, 2006

Re: prop diameter difference

ok i understand... when i lift the engine how high up are we talking... cav plate to the water line??? i ask so that when i put it in the water next week i will check and see how close i can get with the adjustment room i have avalble now (looks like about 1 1/2 inch by the sloted hole in the bottom motor mount cant see the top ones as they are covered but i bet i have at least 2 of the 4 avable maybe 3..... dont realy want to drill holes in the transom if i can avoid it... its still grandpa's even if my name is the only one on the title thanks for all your help...

houdini1966 · May 5, 2006

Re: prop diameter difference

ps oh ya and you made my wife happy too now that i am not going out and getting a new prop this week..... grin

walleyehed · May 5, 2006

Re: prop diameter difference

wait 'till next week when I recommend a prop and see how she feels about me then...........

houdini1966 · May 8, 2006

Re: prop diameter difference

ok motor check out good had to replace the power pak and now its running at 5200 to 5500, 35 to 40 mph loaded, and i think i am ready to play with it i have 3 holes to work with... how high is too high and do i need to block it up while i am testing ( something between the motor and transom top )or will it be safe to wait untill i have the motor set at the final hight to do that or is adding a hydrolic jack plate like i see on some of the bass boats a better way to go?? i had never heard of this untill a few days before i posted here... thanks in advance

prop diameter difference

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