changing 15x17 to 4 blade or ss

omc cobra
351 ford 5.8l
15x17 michigan wheel aluminum its thick and dull no sharpness
wot 4000-4100 rpm speed 38-40mph
15x17 michigan wheel prop. 011007 stamped on it
1991 four winns sundowner 225
trailer and boat wiegh 6200-6300lbs so about 5000 lbs boat
bernett hydralic trim tabs

maximum wot is 4000-4400 so i cant go much higher.

i pulled the engine housing tonite 4, 1 inch ss screws, and i do not see any stickers on the block or heads and ect. stating horsepower. i found a patent sticker and an omc sticker with model and serial numbers nothing else but some random stamped numbers on the block.

to me the boat feels sluggish at 40 mph with a 351, the hull is dry and prop is free of major damage i know this is more of a heavy cruise around the lake boat more then a 70-80 mph speed demon but i would think 50,mph would be obtainable wouldnt it?

not sure if this will help any seen some one else post the gear ratio
gear ratio overall 1.41:1[.71]
upper 21:16
lower 14:26
from my booklet that i got when i bought the boat.

Re: how to find horsepower rating on i/o

Re: how to find horsepower rating on i/o

deleated post

Re: how to find horsepower rating on i/o

Re: how to find horsepower rating on i/o

Maybe I am blind but what kind of boat is it .How long,weight,load would help.A dinged prop is not good.Vibration on drive shaft=bad seals,bearings over time and small loss of mph,mpg's. Those three small blades are what is propelling that craft,any variance is magnified. It isn't like you have three good tires and one bad one on your car, more like a 3 cylinder engine and one cyl. has a misfire.Your rpms are right on so may want to try you a 14.25-75x19 SS cupped prop which might gain 2-3mph's as the aluminum is flexing quite a bit a WOT.Any local prop shops?

Re: how to find horsepower rating on i/o

Re: how to find horsepower rating on i/o

four winns sundowner 225 so 23 feet give or take, boat and trailer wiegh in at 6200-6300lbs so about 5k lbs for the boat. i have bernett hydralic trim tabs dont know if that matters at all. prop looks like a year old mower blade that hasnt hit any thing but grass its dull and has a thick edge no dings or bends that would put off to much.

updated first post with all that info.

as for prop shops no one around me lets me just slap it on and take a drive around there watering hole, i've heard some place down south does that like alabama?. i have 2 local places that i could pick one up off the wall {used}, check the shelves, or order. i am not so hot for spending money on a prop that may or may not work.

my current prop is as big as i think i can go, i've only got 1/2 inch or 5/8s before i'd hit the lower unit[cavatation plate?]

Re: how to find horsepower rating on i/o

Re: how to find horsepower rating on i/o

HP does not come into play as it is all about mfg's rpm's rating.Trim tabs are all about leveling boat while under way and hole shots,not top end.You mention vibration and small ding in prop,not a good thing for outdrive and overall performance.To the best of my knowledge you have a well built,heavier hull and are close to max speed.As I stated a SS prop will not flex and give you a couple more mph other than that your only option is a larger displacment engine .It is what it is..you are not going to gain 4 +mph with any sort of prop adjustment.

Re: how to find horsepower rating on i/o

Re: how to find horsepower rating on i/o

The OMC Cobra 351 engine was rated @ 235 horsepower. I don't know what the redline is, however I doubt it's much more than you're presently turning. Are you trimming out the outdrive completely? If so, you might want to have the outdrive checked to see if there are any bearing problems--holding down rpm's. Be wary of strange vibrations.

A 4 Winns is a pretty substantial boat. I'm thinking performance is marginal due to the boat's weight.

Aluminum props are cheap, and you don't get hurt too badly experimenting with pitches. At least you'll have a spare if a purchased prop is not exactly right. The 17 pitch prop sounds right to me. As mentioned above, you may see a 4+ mph gain switching over to a stainless prop. I look at a stainless prop as a good investment, as I'm no longer tearing up aluminum props every year--12 years w/o problems.

Hydraulic trim tabs work coming out of the hole, and at low to medium speeds. It'll pull like you've got 50 hp more. At high speeds, you retract them--unless you have a couple of fat people sittin' on one side--then you level the boat out. Best $450 I ever spent.

Good luck.

Re: how to find horsepower rating on i/o

Re: how to find horsepower rating on i/o

i do trim it up when i get up to speed i will gain about 2 mph more doing that. at wot i can only go for a minuite or 2 and it will lower the rpm and backfire, is my current prop to much pitch or is that another problem?

what is the 75 in 14.25 75x19? 19 is pitch and 14.25 is how long the prop is from end to end.

i found 1 prop i iboats its the solas hr titan 14 1/8x19 ss 4 blade it said it was cupped the only other ss 4 blade didnt say that it was cupped. would this be the prop i should buy?

Re: how to find horsepower rating on i/o

Re: how to find horsepower rating on i/o

I disagree with recommendation for going to a stainless 19 pitch.

You are turning 4,000 to 4,100 with your current 17 pitch aluminum. Stainless is more efficient...will bite the water better. All things being equal a Stainless prop will therefore spin at a LOWER rpm than an aluminum would. You are already at the very bottom end of your RPM range.

Going to a 19 will drop your RPMS over the 17. And going to a stainless will drop you even more. Going to a cupped prop drops it more. And now you're talking about a 4 blade...which drops you even more. So that's a whopping FOUR prop characterisitics that ALL reduce RPM. All together I will toss out a wild guess that a 19 stainless 4 blade might only spin 3,500-ish on your boat, and that would definitely be bad.

If you're going to stainless, you might be able to stick with a 17 but I doubt it. Frankly I wouldn't be surprised if you might even need to drop to a 15 if you're seriously considering an aggressively cupped/raked 4 blade stainless like the HR Titan...

A test drive is the only answer. Find a place that will let you try a prop and exchange it. Alternately you can buy a prop, try it, if you don't like it, sell it on Craigslist or Ebay for a partial loss. It's not ideal and costs a few bucks, but sometimes it's the only way.

Obviously I don't know for certain, but I really strongly believe you would very much regret buying ANY 19 pitch stainless prop for your boat...it sounds slightly overpropped as it is with your 17.

As for your HP question, I do not know the answer. But you have a 20 year old motor. When was the last time it was treated to a truly/fully comprehensive tune-up? Set/check base timing AND advance timing, new plugs/wires/cap/rotor, points/dwell if equipped, carb tune, compression check, etc. etc. etc....

If it's been a while, a solid tuneup has potential to get you more performance than any prop change will.

Re: how to find horsepower rating on i/o

Re: how to find horsepower rating on i/o

craz1ecars, thank you, as ive only has 1 person tell me what prop pitch might work well. i wanted a 4-blade to tow easyer with a 3 blade i cant even plane the boat out, i've also been thinking that it was over propped when i bought the boat. the last owners i think had yaht races on lake michigan[ i found maps and gps coords on the gps when i started looking.

i dont want to spend $300 for a ss prop and sell it for $150 when it doesnt work. i also dont need a spare as i have 2 15x 17's aluminum.

i did a tune up this year plugs, wires, cap, rotor, timing set, ect, ect, ect.. no compression check being omc with the manafolds i'd have to remove them to do that. almost removed them to replace plugs.

ok heres the new list,
solas rubex 14 1/2 x 15 4 blade ss
solas hr titan 14 1/2 x 15 4 blade ss

hr titan has the bigger cupping correct? and which one would be better to start with.

Re: changing 15x17 to 4 blade or ss

Those two props are identical in blade design and dimensions, and will perforrm exactly the same. Only the hub is different between Rubex and Titan. Titan has a fixed hub, Rubex has an interchangeable hub. I say shop by price alone if those are the two you are deciding between.

Beware that a 4 blade has potential to be a hair slower than your current 3 blade. You did mention that higher speed may be a priority. Shouldn't be much different though.

It certainly will improve your planing substantially. Hopefully the 15 won't over-rev. I think you will be fine with it. But I only THINK that to be true as an educated guess, and I cannot guarantee it.

I still suggest a test drive if at all possible.

Please report back with results of whatever you try.

Re: changing 15x17 to 4 blade or ss

FYI to craze1car the OP did not have rec rpm's for engine. He went back and edited it.Thats where the 19" came in. I agree that a 17" 3 blade or better yet a 16" SS if you can find one would be the way to go. A 15" will pop it out of the hole quick but you will lose top end and probably be way over the red line. I just had a 15" 3 blade bronze on a 15' tunnel hull boat with a 110hp outboard and hit 6,000.Then I put a 19" 2 blade bronze and hit 4100.Switched to a 17"SS cupped and hit 5200+ 1 to 200 depending on chop and wind.The motor is rated for 48-5200.My point is that the 1"=100 rpm does not always hold true.Sometimes a motor can really get lugged down and vice versa,with only a 2" diff in pitch.Look at the diff rpms between the 19 and 17. I really gained a lot more rpm's than what is the normal formula written on here for hitting your redline

Re: changing 15x17 to 4 blade or ss

my biggest thing is i am playing with not just rpm, not just aluminum to ss, and not just 3 blade to 4, and size from 14-15, but all 4 at the same time. there is no place near mid michigan that i know of that lets you borrow a $300+ 4 bladed ss prop for a test around the lake. i feel comfertable that i wont hit anything in my usual lakes but there is no returns on a prop once it hits the grease on the drive shaft, and i've seen the same ss prop on craigslist for 3 months now that still hasnt sold[isnt compatable with my omc].

Re: changing 15x17 to 4 blade or ss

sorry for all the posts, guess i want speed and faster planing and to not mess with my rpm which is not possible. at this point what i've been researching is that "rake" helps with speed and planing out so what company/style prop, offers the most effective rake for a 3 blade ss. throwing out the idea of a 4 blade for now maybe next year after i get a good 3 blade ss.

as far as i can find on iboats my only choice is,
apollo 14 3/8 x 15 ss
solas new saturn 14 1/2 x 15 ss
solas rubex 14 1/2 x 15 ss
stiletto advantage 14 1/4 ss

i know little to nothing about prop companies and design, and with my guess id probably end up with the solas brand because it has a larger diameter. cant seem to track down a 15x15 or 15x16 3 blade ss.

Re: changing 15x17 to 4 blade or ss

ok i got a new prop from the shop down the road and i did a test on it today, the 15x17 is my previous prop.

15x17 aluminum michigan wheel, 4,100-4,200 rpm, 40-42mph wot.

15x15 ss, i think this is a michigan wheel, 4,300-4,400 rpm about 35-36mph wot.

i didn't notice much help planing out and it seemed to put the bow down further even after i trimed up. it also seemed to put off a better wake to tube on, i will post my results after i tube this weekend.

if the graph stayed the same would a 15x19 put out about 3,900-4k rpm and 45-46mph? and would that be hard on the engine not running 4,400 rpm at wot? just wondering because i may have the need to feel some speed.

Re: changing 15x17 to 4 blade or ss

This is why I suggest a test drive when possible.

I'm a little surprised at that much drop in speed with the 15 pitch, and yet no noticable improvement in holeshot? Using prop slip calculator, that stainless you are borrowing is less efficient (creating more slip) than the original aluminum. This is the reason you are losing a bit of speed.

As for whether your theory on going to a 19 pitch would hold true? I doubt it. I believe you would indeed run in the neighborhood of 3900 RPM, or likely even less...more like 3800. But assuming similar 15%-ish slip that will calculate out to about 42 mph...the same as your 17. And it would be lugging your engine a bit.

But it seems you have found a local shop that may be willing to exchange and/or loan you props for testing? If yes, there is no better way to find the perfect prop for your boat than to test. You have nothing to lose by getting ahold of a 19 and seeing what it does. Just maybe those 351 Fords have massive low-end grunt and prefer to run in the 4,000 range??? I dunno...no experience with that motor.

Theories on paper and on calculators are nothing more than theories....they do not always prove true in the field.

Between the 2 props you have right now, it seems your original 17 is the better choice.